(Disclaimer: This
is called a speculative essay for a reason. While an attempt was made at
accuracy, this was ultimately written for a speculative evolution
project, brand new but already in need of an overhaul, about what life
would be like on Earth if Ediacaran lifeforms had not gone extinct. Because of
that and because some aspects of these lifeforms’ biology is still genuinely
mysterious even to experts, a lot of things had to be guessed or speculated on
by myself and I am certainly not an authority on these matters. For
transparency, things based on actual scientific work will be marked with
citations, while highly speculative statements by me will be marked with an [S].
Any divulgence into the esoteric or philosophic is also primarily a stylistic
choice. This post and its follow-ups, in an altered form, will likely serve as
the project’s introductory text, but since this is a group project I will
mostly not be responsible for any future creations based off the stuff written
here. So don’t come to me to complain about things like Dickinsoniasaurus rex or whatever
not being hard-spec enough for you, that’s what Rhynia is already for.)
Fig. 1: Charnia masoni, one of the first members
of the Ediacaran biota ever scientifically recognized.
“Are those who
are out to deny the truth not aware that heaven and earth were once a single
entity, which We then split apart? And that We made every living thing out of
water? Will they then not begin to believe?”
“If He wills
it, He can make you, oh humanity, disappear and in your place bring forth other
beings; and ever is God able to do that.”
- The
Holy Quran, verses 21:30 and 4:133 (translation based off Muhammad Asad’s work)
The year is around 575’000’000 before the present. The exact date
or time of day is hard to determine using our modern timekeeping systems. The
moon is still much closer to the Earth than it is today and has not slowed our
planet’s rotation down as much, so a day is closer to twenty-two hours than
twenty-four. Let us just say it is morning. And while this morning was long ago
for us, for the Earth it was just one of many that had come before, for by this
point our planet was already 3’985’000’000 or more years old. The first times
the morning sun shone over the Earth was in the Hadean, when the air was but a wild
mix of carbon dioxide and methane and the seas were a boiling mess at the mercy
of the moon’s ever stronger tides. And yet in this chaos there already formed signs
of life. In the alkaline thermal vents on the ocean bottom formed through
serpentinization in an oxygenless environment complex molecules, eventually
capable of self-replicating and mutating. Once those two properties arose,
natural selection did the rest and out of these raw components the first cells
evolved. The Hadean was followed by the Archaean, in which these first lifeforms
divided into the domains Bacteria and Archaea. The former eventually evolved
photosynthesis, transforming the oceans and the atmosphere with oxygen, while
at the same time evolving forms that could use the new molecule for their
metabolism. Some of the archaeans made use of this, by enslaving the
oxygen-respirators, thus turning them into mitochondria and themselves evolving
into the first eukaryotes. Geochemical signs of steranes, a molecule only
produced by eukaryotes, shows that this already happened by 2.7 billion years
ago during the Late Archaean (Knoll 2003).
Fig. 2: Fossils from the Francevillian biota, around
2.1 billion years old (taken from El Albani et al. 2014). Time was not kind to
them.
Soon thereafter
the Proterozoic Eon began and the eukaryotes greatly diversified. And they did
not stay unicellular until suddenly animals appeared in the Cambrian, as many
people like to think. Already 2.4 billion years ago, in rocks from South
Africa, can be found fossils of multicellular mycelia from fungi (Bengtson et
al. 2017). This is a rather underappreciated find, as it shows that eukaryotes
were ready to experiment with multicellularity rather soon after they evolved
and that fungi are possibly a lot older than both plants and animals. These
first beginnings, which if unadulterated might have led to Earth becoming a
truly bizarre mushroom world, unfortunately faced great challenges, as at the
same time these mycelia grew, a grand catastrophe was already on its way. The
process of photosynthesis had now advanced so much that much of the former
methane in the atmosphere had fused into the much less potent greenhouse gas
carbon dioxide. As the sun’s luminosity back then was still much fainter than
it is today, the weakening of the greenhouse effect led to the Huronian
glaciation, the first Snowball Earth phase in which nearly the whole planet is
thought to have been covered in ice shields. It lasted about 300 million years.
The process may have significantly halted photosynthesis on a global scale,
allowing for the gradual build-up of new gases, leading again to the end of
this ice age. Something weird happened again immediately after the glaciers
receded. In rocks from Gabon can be found mysterious fossils from 2.1 billion
years ago. Both physiological and geochemical evidence indicates that these
were not only eukaryotes, but also, again, multicellular lifeforms (El Albani et
al. 2014), but unlike their predecessors from before the ice age they were now
large enough to be seen by a human with the naked eye. They came either in
strange, string-like shapes or as flat “nipples” with an egg-yolk-like core and
a surrounding frill. What exactly they were is hard to say. Broadly, they
resemble modern dictyostelids, a form of slime mold that can switch between
both unicellular and multicellular phases, but the Gabon fossils are not only
larger than slime molds, being able to grow up to 17 cm wide, but also more
complex and seem to have lived in a marine environment. They seem to have been
able of at least some coordinated growth, so they could have potentially
evolved higher grades of organisation if given the opportunity, but their time
was cut short as well. The Gabonionta arose during an unusual oxygenation spike
and went completely extinct after oxygen levels fell again (El Albani 2014).
Who knows what life today might be like today if things had been different?
Fig. 3: The typical body-types of sponges. Red are
filtering surfaces/chambers, while grey is the so-called mesohyl, a jelly-like
substance possibly related to the hydroskeleton of more derived animals.
Low oxygen
conditions were from then on the norm in the Proterozoic, but certain lineages still
experimented with multicellularity, some more successful than others. In 1.6
billion year old rocks from India we find a 3 cm large organism named Tawuia
or Chuaria that might have been another form of slime mold or a
multicellular acritarch. Then 1.2 billion years ago we find the oldest algae
(Knoll 2003), the ancestors of what would one day become our plants. Among
these oldest algal fossils was the red algae Bangiomorpha pubescens,
which is the oldest multicellular organism from which sexual reproduction is
known. About 400 million years after Bangiomorpha
another important development happened in a different branch of the eukaryotes.
Choanoflagellates are today a type of protist that filters the water for food
using a sort of ciliated-umbrella and a flagellum. Some of these cells can band
together to form tree-shaped colonies, perhaps to give each other a sturdier
holdfast and to increase filtering area. During the Tonian period, close
relatives of these organisms banded together and formed permanent colonies,
which then eventually reached enough specialization to act as a single
organism. In Namibian rocks from 760 million years ago we thus find Otavia
antiqua, the oldest known, sponge-like organism, if not proper sponge.
There are possibly even older sponge fossils from 890 million years ago (Turner
2021). These were the first members of the new clade Metazoa, better known as
animals. Their age, while being perfectly in line with molecular finds, is
rather amazing as they predate the Late Proterozoic Oxygenation. This shows
that animal-like life can arise even in low-oxygen environments. Even if it is
not all that complex. Sponges have simple shapes and simple lives. After their
mobile larva, the planula, settles down, they develop into vase-shaped
structures with perforated walls. These body walls are often supported by tiny
spicules of spongin or other minerals, forming a skeleton. Each pore or chamber
is lined with flagellate cells, resembling very much those of the choanoflagellates,
which by whipping the water create a current that the sponges use to filter
water. Some sponges might have a slightly more complex anatomy than simple
vase-shapes by building internal filtering tunnels and chambers inside their
walls. Sponges curiously also already possess various precursors to things that
would later evolve into nerve cells and muscle-based movement in later animals
(Dunn et al. 2021), but, despite having existed for over half a billion years,
have never evolved more complex body forms beyond those which we can already
find in these Proterozoic rocks. The reason is that, for all their potential,
sponges inherently lack organisation. While they have differentiated cells, all
sponges lack true tissues (except for the Homoscleromorpha, which seem to have
convergently evolved basement membranes similar to cnidarians). Each cell of
the body, when needed, can evolve into every other cell-type available and if you
put a sponge in a blender, every little surviving piece will develop into a new
one from scratch. While this sounds great on paper, it also means that no group
of cells in the sponge-body can specialize itself for a single task, but has to
always stay somewhat amorphous and simple for the case it has to change later.
Without true tissues, sponges cannot evolve dedicated organs, such as skin,
muscles, a brain or a digestive tract, and therefore are unable to achieve a
more complex degree of organisation. Greater changes in their world had to
happen for the next breakthrough.
Fig. 4: Trichoplax adhaerens, possibly one of
the simplest, free-moving animals one could ever find. Note how there appears
to be no body symmetry.
Likely due to the
growing success of large algae, the aforementioned oxygenation event took
place. Either due to that or an increase in erosion thanks to the breakup of
the supercontinent Rodinia (probably both), greenhouse-gas-levels in Earth’s
atmosphere crashed again. During the aptly named Cryogenian, this began another
Snowball Earth phase between 720 and 635 million years ago, consisting of the
Sturtian ice age shortly followed by the Marinoan one. Right underneath the ice
masses may have evolved the Placozoa, possibly through neoteny out of the
planula-larvae of sponges if those are not a monophyletic group [S].
Placozoans, such as the modern Trichoplax, are tiny animals, just 1 mm
across, which resemble multicellular amoeba. They crawl along surfaces using
cilia and digest tiny food particles by engulfing them with their underside
like a carpet. This behaviour may have very well evolved first through the
grazing of algae growing on the underside of thin Cryogenian icesheets. While
in many ways placozoans seem more primitive than sponges, they have a few
things going for them. For one, they stay mobile throughout their lives, but
perhaps more important is the fact that they have actual tissues. Placozoans
are fully covered by an epithelium (skin), which surrounds a syncytium
(connective tissue). If current phylogenetic models are correct, the placozoan
epithelium and connective tissue might be homologous to the germ layers of
later animals: The epithelium may be related to both the ectoderm (out of which
in our embryos the skin and nervous system develop) and the endoderm (which in
us develops into the digestive tract and associated organs). The syncytium, likely
evolving out of the mesohyl of sponges, may also be related to the mesenchyme
of diploblasts (cnidarians and comb jellies) and later the mesoderm of
bilaterian animals like us (which develops into our muscles and bones, among
other things). Regardless, placozoans could not and still cannot play out their
potential. Like their forefathers, they lack any clear axes of symmetry.
Despite living most of their lives pressed flat on the seafloor, their body
does not actually have a top- or bottom-side, the epithelium is structurally
the same in all directions. If you flip a placozoan on its non-existent back,
it will carry on and crawl away like nothing happened. Neither do they have any
internal cavities, out of which internal organs could evolve. Unlike both later
animals and even the earlier sponges, placozoans do not go through a
gastrulation phase in their development, where the outer epithelium folds into
the body to become the endoderm. This might actually be a secondary loss of a trait that their ancestor did have, as nowadays placozoans mostly reproduce
asexually by splitting apart like large amoebas. Sexual reproduction has become
diminished to two individuals just swapping genes on occasion, like more basal
protists do. Without internal organisation, placozoans are actually at the size
limit at which multicellular cell balls can exist without suffocating
(Butterfield 2025).
Fig. 5: The simple life and diet of a placozoan. They
have no such thing as a gut and instead have to eat with their body surfaces
like a multicellular amoeba.
After the Marinoan
glaciation ended 635 million years ago, a new period began, now known as the
Ediacaran. For the first 55 million years, most animal fossils remained
microscopic (though not unimportant as we will later see). Around 580 million years ago another short ice
age, the Gaskiers glaciation followed. This age thankfully lasted only for a
few hundred thousand years, but when it ended, things got weird.
Growth and
Structure of a Charnia
So we are finally
back to the starting point 575 million years ago, with our actual protagonist:
a young specimen of the organism Charnia masoni. Charnia is named
after the Charnwood Forest in Leicestershire, England, where its fossils were
first discovered in 1956 by a little girl named Tina Negus. However, Charnia
was a widespread genus and can also be found in Russia, Australia and North
America. The individual we are observing today lived in what would one day
become Newfoundland in Canada. Charnia’s identity had long been a mystery
and the group it belongs to has likewise gone through many names, such as
Petalonamae, Frondomorpha, Rangeomorpha, Vendobionta or Vendozoa (the Vend-
prefix comes from the Vendian, an older name for the Ediacaran). I will settle
here for the first one, Petalonamae (as per Cuthill & Han 2018). Ideas on
the identities of these lifeforms have ranged from algae and giant lichen to
stingerless jellyfish and, in some cases, even deuterostomes (the group we
belong to). A growing consensus can now thankfully be discerned. Geochemical
evidence, in the form of fossilized cholesterol, shows that these creatures
were indeed animals (Bobrovskiy et al. 2018), but their morphology does not
relate them to anything known today. Unlike a sponge, Charnia, clearly
has tissues and unlike a placozoan, Charnia actually has style
structure: It grows along two axes of symmetry (topward and outward) and its
body is compartmentalized into distinctive sections. But Charnia still cannot
be confidently placed inside the crown group of the Eumetazoa. Eumetazoa are
the animals which possess, among other things, a mouth connected to an internal
system of guts. Their crown group (meaning all living members) consists of the
ctenophores (comb jellies), cnidarians (jellyfish, sea anemones, corals and
such) and bilaterians (every other animal you can think of, including
yourself). Ctenophores and cnidarians, due to being generally jellyfish-like,
were once grouped together in the larger clade Coelenterata, but many of their
similarities are actually convergent. Ctenophores seem to have diverged first
from the Eumetazoan family tree, while cnidarians are more closely related to
us. Anyway, Charnia and its fellow petalonams lacked any sign of a mouth
or similar body opening. The conclusion most cladistic analyses therefore
arrive at today is that these were stem-eumetazoans (Cuthill & Han 2018,
Dunn et al. 2021). Close to us but not quite there yet.
Fig. 6: A broad-scaled family tree of animals (taken
from Dunn et al. 2021). As you can see, Charnia and its relatives are
sandwiched in a rather crucial part of the cladogram, between the more
amorphous animals and the more structured ones.
But of course,
viewing Charnia and its kin as just our precursors or as a missing link
between the more primitive metazoans and the eumetazoans would do them a
disservice. They were unique and a whole group, dare I say phylum, onto their
own, with a bodyplan and mode of growth that had not been seen before or after,
having been totally lost to time. Our little Charnia, growing there on
its piece of Canadian seafloor, is made up of two parts, a holdfast and a frond.
Making up the frond are the so-called first-order branches, which are the
individual “arms” you see on the fossil. Each first-order branch is then itself
made up of ring-like segments called second-to-fourth-order branches (Dunn et
al. 2021). Each of these follow-up rings then seems to also be made up of
rectangular-looking units, which I will call chambers.
Fig. 7: The labelling of Charnia’s body-parts,
as per Dunn et al. 2021. Top left shows an older and less helpful approach
which tried to give each branch its own number. Now each branch is called a
first-order branch and each first-order branch is made up of a second-order
(and following) branches.
At the beginning
of its life (ignoring reproduction and embryonic stages for a moment) our Charnia
is just a little holdfast, without the frond. To form the frond, the individual
arms do not grow from a central stalk (Dunn et al. 2021), like with a
fern-leaf, but instead something rather remarkable happens. Out of the sides of
the holdfast first grow two lateral branches, which maybe exist to give the
future frond better support [S]. Then out of the center of the holdfast grows a
single first-order branch towards one side of its body. Out of one of that
first-order branch’s second-order branches then grows the next first-order
branch into the opposing direction. Out of that first-order branch’s
second-order branch then grows another first-order branch into the previous
direction. Each new first-order branch has slightly less second-to-fourth-order
branches, meaning they get shorter (Dunn et al. 2021). This growth-process
repeats itself until the classic, feather-like shape is reached and the
resulting midline of the body ends up being zig-zagged. The left side and the
right side of the body are not exact mirror images, but slightly shifted in
relation to each other. This is called glide symmetry and it is a type of
symmetry that really does not exist anymore in the modern animal kingdom. A few
extinct worms have elements of this in their armor, but there is no animal
anymore in which glide symmetry fundamentally controls the body. The exception
might be sea pens, a type of cnidarian to whom Charnia has often been
compared. Sea pens, however, are actually colonial organisms, like corals and
Portuguese man o’ wars, with each arm of the frond technically being its own
animal, called a zooid. The mode of growth of Charnia is incompatible
with it being a colonial organism made up of multiple semi-independent zooids (Dunn
et al. 2021), it really was a single, solitary animal that just grew this way.
Fig. 8: The growth and two life stages of a Charnia
(taken from Dunn et al. 2021). The multi-colored parts on the right show
how each branch is connected to the previous one at the base. What a Charnia
would look like before it grows the frond remains mysterious and we will get
back to that.
Knowing the
fundamentals, it is interesting to observe how Charnia grew across its
life. It grew apically, meaning the branches at the base of the frond were the
oldest, while those at the tip were the youngest (Dunn et al. 2021). Once a
certain size was reached, Charnia stopped growing new first-order branches and
instead began “inflating” the existing branches either by growing them bigger or
adding new higher-order branches (Dunn et al. 2021). Some Charnia-specimens
could grow over 65 cm tall this way, making them among the largest organisms that had existed on Earth up until that time. The splitting-up of its life into two
different growth-phases, differentiation with subsequent inflation, is a sign
of some already quite complex control over its anatomy. At least complex compared
with everything that came before.
Fig. 9: A sketch by me showing in blue what the
interior cavity of Charnia may have been like. The dotted lines are
speculative extensions of the channel into the branches and the holdfast.
The most
fascinating discovery about Charnia and its relatives is that their
internal anatomy was also more complex than previously thought. Many petalonams
have an outward appearance resembling quilted and inflated air mattresses. It
had long been thought that their interior was entirely filled with jelly or
cytoplasma and also tightly sealed from the outside world, with the skin being
the main organ of interaction with the surroundings, including digestion. It was also thought that
the individual sections of the body, such as the branches of the frond, were
virtually separate from each other, not interacting with the rest of the body. Newer examinations of Charnia and close relatives like Stromatoveris
show this to not have been the case. Shortly after death, these animals’ bodies
were filled in with sand and dirt, allowing for their three-dimensional
preservation (Cuthill & Han 2018). There were no obvious wounds from which
these sediments could have flowed into the body and the sediment-layers were
continuous. This shows us multiple things: 1) At least some natural opening(s)
must have existed that allowed these sediments to fill in Charnia’s body
2) Significant parts of Charnia were hollow and 3) Perhaps most
importantly, the different sections of its body were internally connected. In
fact, Charnia’s body seems to have had at least one interior channel
that connected the bases of all first-order branches, which makes sense, as
each branch descends from the one that came before (Dunn et al. 2021). It would
be easy to call this channel a form of gut, but again, Charnia still
does not have a mouth or anus to which this could connect. It is therefore
perhaps more comparable to the interior filtering-channels seen in leuconoid
sponges (see fig. 3, right). We will get back to this later.
What about the
finer detail, the nuts, bolts and material of Charnia? Charnia
was soft-bodied through and through, without hard parts, such as the spicule skeleton
that sponges have. But nonetheless, its body was capable of maintaining its
form even while being buried or filled with dirt. This strongly points towards Charnia
and its petalonam-friends having had a so-called hydroskeleton (Butterfield
2020). Instead of being filled with jelly-like tissue, as had previously been
thought, Charnia’s body was held erect by a skeleton of rings and layers
not made of bone, but by slightly pressurized seawater. Apart from support, a
hydroskeleton can also be used for respiration and, through hydraulics, even
movement. Many modern animals still have such a hydraulic skeleton, for example
annelid worms and echinoderms. If you are a human with a penis, your organ (or
in some of the men I’ve met, organelle), is also held erect by a hydroskeleton.
Foremost among the animals with a hydroskeleton are however the ctenophores and
cnidarians. In them, this organ is called a mesoglea and it is possible that Charnia’s
hydroskeleton was homologous with theirs. Assuming this, we can further deduce
that Charnia’s mesoglea was outwardly covered by an epidermis
(Butterfield 2020). This outer skin was then likely covered in a carpet of
microscopic cilias, as is still the case in placozoans and especially
ctenophores. The internal channels and chambers we discussed before were then
also likely covered in an endodermis, the same type of tissue which forms our
guts.
Fig. 10: The probable internal anatomy of Charnia’s
feeding chambers, to which we will get soon (taken from Butterfield 2020), as
well as how they likely filled with sediment and fossilized over time. Black
shows the hydroskeleton, surrounded by an ecto- and endoderm with cilia. DOC
and POC stand for dissolved and particulate organic carbon.
Did Charnia
have things beyond a skeleton and skin? After all, in addition to these
features, cnidarians and ctenophores also have muscles and nerves. The answer
is: most likely not. Not only is there no sign of this in the fossils, but this
can also be directly gleaned from how those fossils were preserved. We know
from both living sessile cnidarians and their fossils that when they run the
danger of being buried/have their guts filled with sediments, they actively
contract and try to move away from the danger (Butterfield 2020). Sea anemones
can in fact even detach from the ground and hop away from danger. We find no
such behaviour in Charnia and close relatives, the fossils show that
they let themselves be buried without any real resistance, which strongly
implies that they did not yet have muscles with which to actively move the body
or nerve cells with which to directly react to outside threats (Butterfield
2020 & Dunn et al. 2021). This of course does not mean that Charnia
was completely numb to the world surrounding it. After all, the general
awareness of animals, out of which eventually consciousness would evolve, did
not suddenly begin with the appearance of a nervous system, let alone brains,
but must itself have evolved out of earlier precursors (Godfrey-Smith 2017). Interaction
between the organism’s interior with the exterior environment is a fundamental
aspect of every known lifeform. Bacteria and plants already use chemical
pathways to react to changes from the outside and change their current
conditions accordingly, even if they are a lot slower at this than we are. They
have some form of awareness, even if it is just in a broad sense. The placozoans
preceding the petalonams furthermore had and still have fiber cells inside
their body that react to neuropeptides (Dunn et al. 2021), which seems to be
the direct precursor to a form of nerve cells. Sponges have similar precursors
and a few species are able to move their body by contracting the pinacoderm
cell-layer (Dunn et al. 2021). Even though generally thought of as sessile,
some sponges can even crawl across the seafloor at very slow speeds, using the
cilias of their filtering cells. Early petalonams like Charnia most
likely had access to such a repertoire of senses and actions and so were able
to react and perhaps even move, just at a speed way slower than what eumetazoans
are capable of. More importantly for later, petalonams would have had all the
necessary potential to evolve proper nerve- and muscle-cells themselves if
given the chance. We can actually say this with certainty because we eumetazoans
started out with the same basic tool-set as Charnia and went on to
evolve all these traits independently from each other: The ctenophore nervous
system evolved entirely independently from our own, while cnidarians and
bilaterians do not use the same muscle cells (Butterfield 2020 & Dunn et
al. 2021).
Since Charnia
would have had some form of awareness, one could maybe ask what would it have
felt like to be one. This is a question that we can philosophize about but will
never be able to answer. After all, some humans cannot even imagine what other
humans must feel like. The best assumption I can give is that it must have been
peaceful, like how you felt when you were just an embryo.
Ecology and
Lifestyle
Charnia’s environment was a strange one. As burrowing animals
were still very small and rare, most of the seafloor was not yet as well-stirred as it is today. This allowed microorganisms to carpet the ground in
thick films undisturbed. The Ediacaran was a period dominated by dense
bacterial mats and algal lawns, turning a former Snowball Earth into a
Slimeball Earth. This seems to have been crucial for most lifeforms of the
time. For Charnia it was the ground it grew on, allowing it to anchor
its holdfast onto a sticky film instead of needing to bury it into the soil
like plants have to do with their roots. Charnia shared its home with other
strange creatures. At its home of Mistaken Point can also be found other
petalonams, such as rangeomorphs, of which Charnia was likely itself a
basal member. These were other frond-shaped creatures, though sometimes of a
different configuration than Charnia. Fractofusus for example
lacked a holdfast and instead was made of two opposing fronds that lied flat on
the ground. Bradgatia is similarly also made up of multiple fronds, but
these all attached to each other at the base, making the organism look like a
cabbage. Among its neighbours, Charniodiscus resembles Charnia
the most. However, its holdfast was unnaturally elongated and at its end formed
a big, circular foot with which it stayed attached to the bacterial mats like a
plunger. Another form of life found in this so-called Avalon Assemblage (the
oldest known type of petalonam ecosystem, named after the Avalon peninsula on which
the Mistaken Point fossils were found) are medusoids, such as Aspidella.
The exact identification of these organisms is still mysterious. As the name
suggests, the medusoids were first thought to have been fossilized jellyfish,
however closer analysis revealed that they were instead disk-shaped, gelatinous
spheres growing on the seafloor. There is a distinct possibility that these
were not animals at all, but microbial colonies or giant protists, similar to
the modern xenophyophores (McMenamin 1998). At least some medusoid fossils
likely were just trace fossils left behind by the holdfast of petalonams, such
as the aforementioned Charniodiscus.
Fig. 11: What the neighbourhood of Charnia would have
looked like (taken from Bottjer & Clapham 2006)
Now onto the most
pressing question of how Charnia and friends made a living. At first,
the very leaf-like shape of the body is of course very suggestive. Various
animals do still exist today which live in symbiosis with algae and other
microbes living in their bodies, such as corals and the very outlandish-looking
snail Elysia chlorotica. Especially in the latter example one can see
how an animal can assume plant-like shapes in order to profit from its
symbiotes. Placozoans can also have microbial endosymbionts (though not algae)
living inside them. Did Charnia therefore have algae or cyanobacteria living
in its body, who in exchange for shelter gave it sugar? Could the internal
channels of the branches have been there to house such symbiotes, making Charnia
a sort of appartement complex for microbes? Unfortunately, the sedimentology of
Mistaken Point indicates that it was actually a deep-sea environment. Charnia
and friends were living in total darkness, well below the depth where
photosynthesis would still be feasible. Of course, not all petalonams were
living in the deep ocean and especially those living in shallow waters could
have still employed this photosynthetic lifestyle. But Charnia was not
among them. The next best explanation for it could then be osmotrophy, which is
the passive absorption of nutrients suspended in water through osmosis. Certainly,
the Ediacaran seas would have already been full of free nutrients in the form
of waste products released from various bacterial mats, protists, giant algae
and early animals. Osmotrophy is also a form of feeding employed today by many
microbes, fungi and even some animals, such as sponges, corals and brachiopods.
This has been seen as the preferred feeding mode for petalonams by many workers
on the Ediacaran biota, most importantly Mark McMenamin (1998). I also endorsed
this view in an earlier post. Unfortunately, newer research shows that most
petalonams were too large for osmotrophy to still be feasible (Butterfield
2020). Furthermore, the animals who employ osmotrophy today only do it to supplement
their regular diet. Charnia could therefore not have exclusively lived
through osmotrophy alone.
Besides that, the
old idea of osmotrophy relied on the now outdated idea that Charnia
could only feed passively through its exterior skin. The discovery that
petalonams had interior chambers that could interact with the outside opens up
a whole lot of new questions and possibilities. First and foremost: Where were
the body openings? As said, petalonams, especially rangeomorphs like Charnia,
have no sign of a coelenteron (Dunn et al. 2021), the central mouth connected
to the gut of cnidarians and ctenophores, and not even something comparable to
the operculum of sponges (the vase-hole at the top). With these openings being
macroscopic and the grade of quality that these fossils are often preserved at,
this seems to have been a genuine absence. What we cannot exclude, however, is Charnia
having, instead of a single central mouth, a bunch of small to microscopic ones across
the body, perhaps something similar to the filtering pores of sponges. These
are such fine details that it is likely that they were simply not preserved
during fossilization (Dunn et al. 2021). Indeed, even in Cambrian Burgess
Shale-type fossil sites, which have a way higher grade of preservation than the
Ediacaran ones Charnia was deposited at, many sponge fossils do not have
their individual pores preserved (Butterfield 2020). Is it then possible that
each unit of each branch of the frond could have been a filtering chamber with
a small opening? We might actually have direct evidence of that (fig. 12). At
least two flattened but exceptionally well-preserved specimens of Charnia
actually do show a series of little, lense-shaped openings running across the
arm (Butterfield 2020).
Fig. 12: A flattened Charnia-fossil showing
possible openings of filtering chambers (taken from Butterfield 2020).
Underneath these
can also be found structures similar to mesenteries, which corals use to expand
the surface of their guts. The most likely explanation then for how Charnia
lived was that each branch of the frond was composed of a series of little
filtering chambers (as in fig. 10). The ciliated skin of Charnia transported dissolved
and particulate organic material from the water towards the openings of those
chambers, where they were then likely swallowed through cilial pumping. Inside
the chambers, more pumping by the mesenteries circulated the food until the
nutrients were absorbed by the chamber wall. This is very similar to how
sea-pens and other colonial cnidarians feed, but it needs to be stressed again
that current science shows that Charnia was a single animal and also not
a cnidarian (Dunn et al. 2021). In that aspect, the fact that it could achieve
such a feeding method without resorting to colonial growth makes it in some
ways actually more complex/impressive than sea pens. A sea pen is an army, a Charnia
was one John Rambo. The fact that all first-order branches of Charnia
were internally connected to each other through a sort of channel also becomes
even more intriguing. Possibly, like a strange mix of cnidarian, comb jelly and
sponge, all of the feeding-chambers in the arms were connected to each other
and through ciliary pumping transported food into the central channel [S]. From
there, food and other resources could then have been further distributed, again
through ciliary pumping, in any direction to where the body needed them [S].
Sponges, especially those with skeletons, can have pores which actually pump
water outside of the body instead of into it, in order to create better flow. Charnia
could have similarly coordinated some chambers to pump water in or pump water
out, to better direct the internal flow of water and nutrients [S]. This
hypothetical channel-system would in some ways be a perfect intermediate
between the internal chambering of leuconoid sponges and the coelenteron gut of
cnidarians/ctenophores, but at the same time it would be something wholly
unique. It has many hallmarks of a gut, but instead of one mouth and one anus,
it has many smaller openings and endings throughout the body. One wonders what
evolution could have achieved with such a thing.
Now that we (sort
of) know what Charnia ate, was there anything that in turn ate it? The
Ediacaran is generally characterized as a time in which predatory animals did
not exist yet, hence why ecosystems like Mistaken Point are often nicknamed as a
“Garden of Ediacara” in reference to the biblical Garden of Eden. Apart from
bore-holes in the shells of Cloudina, a very Late Ediacaran animal, and
a possible bite-mark in an Ernietta (McMenamin 1998), there is little
evidence that there were animals that could have fed on Charnia. At
least on a macroscopic level. Among microbes, predation has probably existed
since the very start of Eukarya, if not earlier. Just look at amoeba and flatworms
under the microscope. It has in fact been proposed once that petalonams grew to
such unprecedented sizes in order to escape microbial predators (McMenamin
1994), like how sauropod dinosaurs tried to become too big to eat for some
theropods. Charnia would have nonetheless likely still faced danger from
such micropredators in form of parasites. What possibly speaks against a world
entirely devoid of macropredators are fossils from the Mid-Ediacaran Doushantuo
Formation of China, which shows that essentially modern-looking jellyfish
already existed by this time. All modern jellyfish are carnivorous in one form
or another and, while hard to tell by fossils, it is not unreasonable to assume
that the Doushantuo ones already possessed the characteristic stinging cells,
as all jellyfish possess them today and the deepest split in their family tree
likely happened in this time. Their stinging cells are used both for hunting
and defence, but neither the typical prey nor the typical hunters of jellyfish
were around at the time (Godfrey-Smith 2017) and jellyfish do not prey on
large, sessile animals like Charnia… so what would they have used their
stinging cells for?! Some parts of the past unfortunately have to remain
mysterious. All we can say is that the Garden of Ediacara probably was not the
paradise we once imagined.
Reproduction?
Where did our Charnia
come from and how would it have made more of itself? The genus existed for
millions of years, so it certainly had some reproductive strategy that worked. Did
it simply bud-off copies of itself, did it release eggs in the water, did it go
through embryonal or larval stages? How
was its sex life? A study
done on its neighbour Fractofusus may shed some light on this.
Fig. 13: Fractofusus misrai, reconstructed here
reproducing itself through stolon-derived clones.
As it appears, Fractofusus
was actually sophisticated enough to have had two different modes of
reproduction it could switch between. The first one seems to have been asexual cloning
through a stolon (Mitchell et al. 2015). A stolon is a sort of stalk or root
which buds off from an organism to then grow clones of it along its length.
Plants and fungi use stolons to reproduce and a few modern animals do too. While some
polychaete worms use them to trail eggs and larvae behind their bodies while
swimming, the majority of stolon-users among the animals are colonial organisms
that clone themselves, such as bryozoans and corals. This might make you
scratch your head, as we previously established that Charnia was not a
colony of multiple zooids. However, Fractofusus may have been. If
you look very closely at its two (possibly three as per Gehling 2007) fronds,
it appears that these are not like the frond of Charnia. Rather, each
branch of each frond seems to have been made up of an individual Charnia!
This is actually a pattern seen in multiple rangeomorphs, such as Avalofractus
and Rangea. Some of these even have elaborate, plant-like central stalks
and bases connecting all of these little charns to each other, which is not
seen in Charnia (Dunn et al. 2021). What this means is that what makes
up individual branches inside the fronds of these more derived rangeomorphs are
equivalent to the whole body of Charnia (Dunn et al. 2021). While this
could be a form of highly advanced fractal growth of a single individual (basically
a rangeomorph iterating its whole self with each branching of the body),
combined with the stolon-reproduction in Fractofusus it seems more
likely that these derived rangeomorphs were actually colonial organisms produced
through cloning [S], with the connecting pieces/stalks between their branches
perhaps being analogous to the stalk in sea pens or the coenosarc of corals [S].
But again, this does not mean that rangeomorphs were just Ediacaran
representatives of sea pens. In sea pens, each zooid is equivalent to a tiny
anemone, whereas in Rangea and Fractofusus, each zooid was
basically a tiny Charnia, so there are fundamental anatomical differences.
This seems to instead be a genuine case of convergent evolution, which should
not be too surprising, as colonial, clonal growth has evolved multiple times
among animals (I already mentioned bryozoans). Note also that, since the oldest
definitive sea pen fossils only come from as recently as the Jurassic, they are
the actual (cheap) copy.
Fig. 14: The variation of rangeomorph bodyplans (taken
from Dunn et al. 2021). The grey areas show areas of homology, while the orange
ones are uncertain. As you can see, what in Charnia constitutes the whole body
is just a single leaf in other rangeomorphs. I personal pet theory therefore is
that some of these rangeomorphs were colonial organisms, with the orange areas
perhaps being similar to the coenosarc of corals or similar connecting pieces
in other colonial organisms.
Since Charnia
was, however, not a clonal colony, we might rule out stolon-based reproduction
for it. Here Fractofusus’ second mode of reproduction becomes
enlightening: on occasion it seems to have released little propagules into the
water, which then drifted off until settling on the seafloor to grow up
(Mitchell et al. 2015). For solitary petalonams like Charnia this would
have been the most feasible mode of reproduction. What exactly was the nature
of those propagules, or rather, how were they produced? Because they lack signs
of obvious gonads, general assumption about petalonams has been that they
produced them just through budding (see McMenamin 1998 for example). By that
logic, after a certain age the tip or the fringes of Charnia’s frond would have
simply shed off and regrown repeatedly, with the dispensed part floating in the
water until landing and growing into a new Charnia, sort of how plants
or starfish can grow clones of themselves from broken-off pieces. Like the
stolon-based approach of derived rangeomorphs, this idea, however, also assumes
that petalonams were only capable of asexual reproduction. At least I find
there to be a serious problem with that idea. Asexual reproduction is
successful for lifeforms in the short-term, like when recolonizing destroyed
habitats, but in the long-term only being able to clone yourself becomes a
serious detriment, because not only will your lineage evolve very slowly, once
a disease or parasite adapts to successfully infect and kill your body, it will
be able to infect all of your clones simultaneously as well, wiping your whole
kind off the map. Lineages of asexual eukaryotes are therefore usually very
short-lived in nature (Lane 2016) (prokaryotes circumvent these problems by
frequently exchanging plasmids with each other like trading cards). Looking at
the fossil record then, petalonams existed for at least 40 million years,
probably way longer, and during that time underwent some quite steady
evolutionary radiation. This both seems incompatible with purely asexual modes
of reproduction. Looking at sponges for reference again, it seems pretty
plausible for petalonams to have made use of sexual reproduction, at least
sometimes, without having gonads. Sponges do not have them either but are
hermaphrodites that can produce both sperm and eggs directly from their skin by
transforming individual choanocyte or archaeocyte cells into them. The
sperm-cells are usually spit into the water, until they meet another sponge,
which then sucks them up through its filtering system to combine it with its
own eggs inside. The fertilized eggs are then either released into the water or
kept inside the body until hatching. While petalonams likely did not have the
exact same cell types as sponges, there really is not much that would negate
such a style of reproduction and variations thereof in them and maybe that is
how the propagules of Fractofusus were produced [S]. Like sponges and
many other sessile animals, petalonams could then probably use sexual and
asexual reproduction selectively simply depending on the situation [S].
Fig. 15: A sketch by me showing embryological branch-off
points in the development of animals. Dark-shaded sections are equivalent to
the syncytium of placozoans, the mesohyl of sponges and the mesoglea of
cnidarians, but note that in reality these organs might not be 1:1 homologous.
I went for the idea that petalonam-development, like their cladistics, is
somewhere in-between sponges and “coelenterates”. Like sponges they might go
through a face where the blastopore (what would become the mouth in animals
like us) attaches to the seafloor and grows shut, but instead of becoming a
wide, open filtering cavity, the proto-gut becomes a series of interconnected
filtering-chambers.
Assuming then that
our particular Charnia was not the product of bud-cloning, but came from
a fertilized egg, what would its life stages have looked like before it grew
the frond? All metazoans begin life as a so-called blastula, which is basically
a hollowed-out cell-ball, and so would have Charnia. In eumetazoans like
us, the next step is then gastrulation: One side of the blastula dents into the
hollow interior to form the ball into a bowl (imagine it like kicking into an
airless basketball). The skin that still stays on the outside of the bowl
becomes the ectoderm, the one that is now on the inside becomes the endoderm.
In most animals, this dent in the blastula develops into the mouth (except for
deuterostomes like us where it becomes the anus). Would petalonams like Charnia
have also gone through such a gastrula-stage? The fact that they do not have
mouths would mean no and what possibly affirms this is that the older
placozoans do not go through one as well (though, again, this might be a
secondary loss). But sponges, as always, make the matter more complicated.
While it is controversial if it can actually be called that, the embryos of calcareous sponges and homoscleromorphs surprisingly also go through a phase that looks a lot like a gastrula, complete
with the beginnings of what looks to be a mouth-opening! But what would then
become a mouth in us actually attaches the sponge-embryo to the seafloor and
grows shut again, becoming a holdfast while the rest of the body develops into
its typical vase-shape with the osculum on top. What if the holdfast of Charnia
was homologous with the holdfast of sponges and the protostome (the ur-mouth)
of eumetazoans [S]? The embryo would have gastrulated and produced a
(proto-)protostome, which then attached the organism onto the bacterial mat
like a suction cup. Like in sponges, this opening then closes, but the body
above would follow an interior design more similar to that of
cnidarians/ctenophores with its mesoglea, epidermis and endodermis. The
proto-gut, now cut-off from the vanished proto-mouth, then branches into the developing
frond and builds up the individual filtering-chambers [S]. Such a development
would be a good reconciliation for how petalonams had an internal anatomy like
later animals without yet having a mouth.
The things
that came
So, there we have
everything, from the conception all the way to the adolescence of Charnia.
A part of every life however is death and for our Charnia it could have
surprisingly come in many ways. Obviously, we have fossils of individuals who
suffocated in sand-slides and I already mentioned that there must have been
diseases and parasites that befell these lifeforms from time to time. But our
particular Charnia could have gone out in a more bombastic way. Most of
the fossils at Mistaken Point were buried under ash after a nearby volcano
erupted and spilled a pyroclastic flow into the ocean, turning the site into a
Proterozoic Pompey. Not the nicest way to go, but certainly a memorable one.
Fig. 16: Tribrachidium heraldicum, a tri-radially
symmetrical animal perhaps related to the petalonams.
But just because
our Charnia is gone does not mean that we leave this chapter of life
behind. The petalonams continued to evolve long after it. Around 560 million
years ago, the Avalon faunas are followed by the Ediacara/White Sea
Assemblages, named after the Ediacara Hills digsite in Australia and the White
Sea region of Russia. Though note that the newer assemblage did not displace
the older one, but rather they continued to exist besides each other. This is
because the Avalon Assemblage represents deep-sea environments, whereas the
Ediacara Assemblage represents shallow marine environments and coastal lagoons.
What happened 560 million years ago is that some petalonams left the lightless
depths and wandered into the more lively and energetic shallows. While forms
like Charnia continued to exist, derived rangeomorphs, especially the
possibly colonial ones, greatly diversified. Among solitary forms also appeared
new organisms that experimented with new frond- and stalk-constructions. Swartpuntia’s
frond for example now had three or more fans constructed around a central
stalk. Other forms apart from the rangeomorphs began to appear as well:
Trilobozoa
Most iconic among
the new forms are likely the trilobozoans, with their leader Tribrachidium
heraldicum probably being the most iconic fossil from the Ediacaran. Tribrachidium
has long been puzzling, as its body is triradially symmetrical, something which
is not seen in modern animals anymore. It is made of three arms with raised
ridges, which curl around each other to form a disc. At the base of each arm
were little pits. This makes it resemble the triskeles of some ancient heraldic flags, hence the species name. A new study done on computer models of Tribrachidium
in simulated water currents has finally revealed why this animal was
constructed in such a way and how it likely fed: The curved arms caught the
water flow (and therefore the food particles suspended in it) and redirected it
towards the pits at the center of the body (Rahman et al. 2015). In the
process, the flow was also slowed down until it then came to a stop right above
the pits. Through simple gravity, the food particles would have then simply
sunk down into the pits, where they were then absorbed and digested (Rahman et
al. 2015). Possibly, the arms were lined with actively whisking cilia to
further help direct the flow [S]. This again rules out osmotrophy in an
Ediacaran creature, instead Tribrachidium was a suspension feeder like
many modern animals. The tri-radial design finally makes sense in this light,
as it allowed Tribrachidium to capture water currents from nearly all
directions. This does make one wonder why a design like this has not evolved
again ever since. Possibly, Tribrachidium’s hydrodynamics relied on the
rough surface of the microbial mats in lived on, as this would have created
more turbulent flow-dynamics immediately on the surface of the sea-floor
(Rahman et al. 2015). Whatever the case may be, this feeding style seems to
have been lucrative for a time, as many trilobozoans evolved that experimented
with slightly different arm-configurations, perhaps to specialize on specific
current speeds and particle sizes [S]. Close relatives included Anfesta,
whose ridges did not spiral but grew straight from the center.
Fig. 17: The hydrodynamics of Tribrachidium. It appears to have actually fed better in fast currents than in slow ones (Rahman et al. 2015).
Where exactly on
the family tree one might place trilobozoans is hard to say and they have
traditionally been considered their own phylum of stem-eumetazoans or stem-cnidarians. They can be
three-dimensionally preserved, but no trace fossils or other signs of movement
exist of Tribrachidium and friends, which likely means that, like Charnia,
they had a hydroskeleton but no muscles yet. But glide-symmetry, arguably the
hallmark of the petalonams/vendozoans, is not readily apparent in them, so they
might not be related to them as well. However, the pits at the base of each arm
are functionally likely similar to mouths… which would mean trilobozoans had at
least three of them, which is not really reconcilable with cnidarians and more
similar to the multiple, decentralized feeding-chambers of petalonams. The
segmented appearance of Tribrachidium’s arms might also point towards
them being constructed similarly to the fronds of rangeomorphs. For the
thought-experiment that is to follow, I therefore propose this origin story:
Trilobozoans might derive from Charnia-like rangeomorphs that lied flat
on the microbial mats and grew three fronds instead of one (perhaps again being
colonial forms?), radially splayed from each other from the holdfast [S]. With
time these fronds began to curl around each other to direct water-flow. The
filtering chambers along the arms gave up their feeding function, so the ridges
could function better hydrodynamically, while the chambers at the arm-bases
expanded their feeding-function to become proper “mouths” [S]. Channels likely
still existed inside the arms to transport food throughout the body into
digestive chambers inside the arms [S].
Proarticulata
Fig. 18: Some representative members of Proarticulata.
Top, left to right: Spriggina and Dickinsonia. Bottom, left to
right: Yorgia, Vendia and Archaeaspis. Before naming, the
last one used to go by the nickname “Soft-Bodied Trilobite”. The glide-symmetry
is the most obvious in Vendia, but can also be observed in all other
members depicted here.
Most intriguing
among the organisms appearing with the White Sea Assemblage are creatures such
as Yorgia, Dickinsonia and Spriggina. Outwardly they look
like bilaterally-symmetrical, worm- or trilobite-shaped creatures with
segmented bodies and in some forms even a head. This led early researchers to
propose that these were the ancestors of the Articulata (a hypothetical, now
largely polyphyletic clade that includes annelids an arthropods), making them
perfect precursors to the trilobites and other arthropods that were soon to
follow in the Cambrian. Unfortunately, on second look, this seems to be
untenable. On very close analysis, all proaticulatans are not perfectly bilateral,
but exhibit glide-symmetry, like petalonams. This is especially apparent in
forms like Yorgia, but can also be seen in Spriggina after
detailed study (Ivantsov 2001). The flanges of Dickinsonia are also
nearly identical to the fronds of rangeomorphs like Swartpuntia (Knoll
2003, Cuthill & Han 2018). The “heads” of proarticulatans are also not
reconcilable with the heads of bilaterian animals, with analyses frequently
failing to find evidence of eyes or a central mouth (McMenamin 1998). The same
goes for things like legs or a central through-gut. Any resemblance to
earthworms and trilobites is therefore purely superficial and proarticulatans
are best interpreted as petalonams of very similar construction to Charnia,
but lying flat on the sea-floor instead of hanging in the water. What was the
frond and holdfast in Charnia became the body and cephalon in Spriggina.
Fig. 19: The trace fossil Epibaion, showing the
crawling and feeding traces of Yorgia on the microbial mats. You can actually still
see the body of the trace-maker if you look to the right. It is the same fossil
as in fig. 18.
This surprisingly
makes proarticulatans all the more intriguing, for, you see, they actually have trace fossils associated with them. This is again seen very well
in Yorgia, where its trace fossil Epibaion is closely associated
with the body fossil (these traces also show that the former holdfast was now
the front of the body while crawling). The fossil Phyllozoon was likely
also a trace fossil of something like Dickinsonia. So, did petalonams
suddenly learn how to crawl? It certainly seems that way. As mentioned before,
even sessile sponges are able to crawl to some degree and likely the whole skin
of organisms like Charnia was covered in tiny cilia. While cilia are
mostly used for transporting food, animals as large as comb jellies can also
use them to swim, while placozoans use them to crawl (see fig. 5). The first
petalonams probably had at least some limited capacity for ciliated crawling
and the proarticulatans exploited this further to become full-time crawlers on
top of the microbial mats. This seems to have also come with a change in diet,
as the Epibaion traces show that Yorgia settled down on
the mat, fed on its contents and then crawled to a new spot once finished.
Proarticulatans were not filter-feeders anymore but had become active grazers.
Though note that this was probably not the case with all of them, as especially
the largest proarticulatans (who could grow up to 2 meters long) have no
crawling or feeding traces associated with them (Knoll 2003). At least when
they became adults, these giant petalonams probably were too large to still
move with cilia, so they likely settled down and began filtering the water
again like their ancestors.
Fig. 20: Ivantsov’s (2001) sketches of a larval Yorgia
(left) and a Vendia (right). The dotted lines show where he suggests
intestines or caeca could have run. Compare with my sketch from fig. 9.
This new lifestyle
represents an important step in the anatomical complexity of petalonams. Charnia
had mainly known two axes of symmetry (top-bottom and outward), whereas
proarticulatans, crawling on the sea-floor, would have known at least three:
Front-and-back (formerly top-bottom in Charnia), outward/sideways and a
new top-bottom (top now being the frond-surface facing the water, bottom being
the one facing the ground). Combined with them now crawling towards distinct
directions, this means that the body had to be further compartmentalized. The
bottom surface may have become specialized in crawling and feeding, while the
top surface filtered the water or specialized in respiration (with certain
filtering chambers perhaps becoming gills) [S]. Like the snail Elysia chlorotica,
some proarticulatans may have used the algae-cells they consumed as symbiotes
to do photosynthesis themselves [S]. This has indeed been put forward as an
explanation for the flattened edges of Spriggina (McMenamin 1998). The
need to streamline may also explain why these animals became outwardly more
bilaterally symmetrical, as the same hydrodynamics that are imposed on modern
animals forced them to adjust their ancestral glide-symmetry as good as
possible into a pseudo-bilateral symmetry [S]. What changes all this imposed on
the cephalon (the former holdfast) is especially fascinating. While crawling
forward, this would have been the first part of the body to come into contact
with fresh mat and therefore the first to feed. In forms like Yorgia, Vendia,
Spriggina and Archaeaspis it becomes very apparent that the
cephalon became wider and more shield-like compared to the holdfast of sessile
petalonams, likely to sweep up the bacterial mats like a lawnmower. While they
did not have a central mouth, internal casts of these fossils show that the
cephalon may have been full of tiny feeding channels, likely connected to
multiple feeding-pores at the bottom (Ivantsov 2001). These channels were then
likely connected to the internal channel at the base of the
frond-branches, which we discussed earlier with Charnia [S]. This could possibly
affirm my idea that the holdfast/cephalon develops from a sponge-like
protostome, which in proarticulatans, instead of growing shut or into a single
big mouth, divides into multiple micro-mouths [S]. In the big-headed forms the cephalon did possibly
most of the feeding, while the frond became specialized for crawling. In
contrast, forms like Dickinsonia, which have very small cephalons but
very large fronds, likely did the opposite and mostly fed with their frond. One
has to wonder though if the cephalon, especially in the former group, did more
than just eat. After all, this was the organ that would have come first into
contact with new food, new sediments, new currents and new threats. I had long
been doubtful, but realizing this I must say that McMenamin (1998) was
possibly, maybe even probably, right, when he proposed that these animals were
evolving cephalization independently of bilaterians like us (hence why they
happen to look vaguely similar to worms and arthropods). It is not unthinkable
that the top of the cephalon had an array of primitive sensory cells of the same
kind that were already present in sponges and placozoans. Chemical sensors that
detect anoxia or toxins, cilia that detect changes in water flow and pressure,
maybe even light-sensitive cells, the precursors to eyes. With independent
movement, clear body structure and possible cephalization, the proarticulatans
were well on their way towards developing a nervous system and muscles. Perhaps
some of them even already had primitive versions of those, as a few specimens of Dickinsonia may show signs of muscular retraction (Knoll 2003).
The things
that were to come
Around 548 million
years ago, the Ediacara-type assemblages are followed by the Nama Assemblage,
named after fossil localities in Namibia. Again, this does not mean a total
displacement, as the two communities did coexist for a short time. Whereas
Ediacara-types like the White Sea fauna were deposited on shallow continental
shelves, Nama assemblages seem to have been even closer to the shore, at the
mouths of river deltas. The world that the petalonams grew up in was now
beginning to markedly change. Trace fossils show that burrowing animals now
become more numerous, active and coordinated, with some now also beginning to
make vertical burrows. The makers of these traces in all likelihood were our
own ancestors, the first bilaterians. These of course did not come out of
nowhere. Only two to three million years after the end of the Marinoan ice age,
we find possible fossil embryos and eggs of such animals in the aforementioned Doushantuo
Formation (Yin et al. 2007), though this classification is controversial. In
Doushantuo strata dating between 600 and 590 (so even older than Charnia)
can also be found a curious, tiny fossil named Vernanimalcula, which has
been interpreted as an early adult bilaterian (Chen et al. 2004). This
identification is also controversial (others have proposed that the fossils are
in-filled bubbles inside the shells of acritarchs), but it is probably safe to
say that bilaterians had already evolved before petalonams gained their giant
(for the time) size. In the White Sea assemblage, we can already find in Kimberella
a possible relative of molluscs and with Ikaria wariootia from Australia
a similar, albeit more worm-like creature. These two likely grazed on the
bacterial mats like the proarticulatans, but not to a degree where the mats
could not regenerate quickly again. But the new, more complex burrowing
bilaterians showing up with the Nama Assemblage were now doing more fundamental
damage. Their stirring and mixing of the soil disturbed the placid soil that
the microbial films relied on and so the bacterial mats that the petalonams
evolved on were becoming gradually more broken up. For the very first time in earth history, armored animals evolved as well in the Nama assemblage, as we find tiny calcium carbonate shells from animals like Cloudina and Namacalathus, who may have been coral-like cnidarians or tubeworms. Regardless of all these changes, rangeomorphs,
trilobozoans and proarticulatans continued to exist and even in the Nama
Assemblage we see more evolutionary innovation with forms such as:
Erniettomorphs
and other “Sand-Anchorers”
Among the sessile
forms now started appearing creatures such as Ernietta. This was an
animal which looked like a plastic-bag constructed of palisades of tubes. The
lower part of the bag was buried by sand, both inside and outside the cavity,
likely to act as an anchor, while the frills stood erect in the water (Ivantsov
et al. 2015). At first it seems difficult to place this among petalonams or any
other animals for that matter. Looking at the “seam” on the bottom of the bag,
however, it appears that these tubes grew from each other through
glide-symmetry, something unique to Charnia and other rangeomorphs (Dunn
et al. 2021). Ernietta likely developed from a flat petalonam made of
two opposing fronds that intentionally buried itself under sand, either by the way
of a stolon or a burrowing larva [S]. The two fronds grow outward and upward
out of the sand and then meet again, sort of like the leaves of a
venus-fly-trap. The result is a partially sand-filled bag, whose fringes can
now filter the water for food while being safely rooted in the soil. Since this
was a shallow water organism, photosynthesis using symbiotes would have also
been possible.
Fig. 21: Reconstruction of African Ernietta plateauensis
(taken from Ivantsov et al. 2015). A bag-like creature where the bottom
half was burrowed in sand, while the top hung in water, likely to filter-feed.
Note how the seam at the bottom exhibits glide-symmetry.
Probably not
directly related, but functionally similar was Pteridinium, which
basically looked like a bathtub with a longitudinal wall cutting it in half. Pteridinium
likely also descends from a bipolar frond, but with three vanes instead of two,
sort of similar to Swartpuntia but without the stalk. The taphonomy
indicates that the bathtubs were filled with sand and that way could also
function like an anchor. Despite being mostly covered in shallow sand, there is
some tentative geochemical evidence that Pteridinium fed through
photosynthesis (McMenamin 1998), but since only the tips of the frond-vanes
looked out of the ground, it is perhaps more likely that it mostly fed through
filter-feeding.
Fig. 22: A cut through Pteridinium (taken from
McMenamin 1998). As you can see, the bathtub terminology does not come from me.
While potential
forms of sand-anchoring could already be observed in earlier medusoids, it
seems likely to me that in these petalonams, this strategy specifically evolved
to cope with the degradation of the microbial mats that earlier, holdfast-based
forms had to rely on. Who needs scum when you got gravel?
Conulariida:
Trilobozoans reloaded?
An interesting
fossil found from the Late Ediacaran of the White Sea region is Vendoconularia.
This animal greatly resembles the conulariids, a clade otherwise only known
from the Paleozoic. Conulariida were radially symmetrical animals with shells
that broadly resemble angular ice cream cones. Unlike the similarly shaped
shells of some cephalopods, the conulariid shell is, uniquely among animals,
constructed of multiple bony rods that were stacked on top of each other on all
sides as the organism grew in length, sort of like how one would build a brick
wall. These rods were made of calcium phosphate, the same kind of mineral that
vertebrates use to build up their bones. The cone most likely stood erect in
the water and its pointy tip was connected to the ground by a short stalk.
Because most Paleozoic conulariids had a four-fold symmetry, it was long
thought that they were part of or closely related to the Scyphozoa (true
jellyfish, see fig. 6), but Vendoconularia shows that these organisms
actually started out with a tri-radial symmetry (Ivantsov & Fedonkin 2002,
Van Iten et al. 2005). Together with Lower Cambrian fossils such as Anabarites,
this points towards conulariids having actually evolved from trilobozoans,
evolving their tri-radial symmetry into a six-fold symmetry and from there then
into a superficially jellyfish-like four-fold symmetry (Ivantsov
& Fedonkin 2002). Likely, conulariids would not descend from
spiral-armed trilobozoans like Tribrachidium, but the more
straight-armed ones like Anfesta [S]. At some point these might have
begun laying down a biomineralized skeleton beneath their arms/disk, gradually
building up the cone-shell as the organism aged, similar to how corals develop
[S]. While in Vendoconularia this skeleton is only very lightly
mineralized (Ivantsov & Fedonkin 2002), there are probably two reasons for
why this shell evolved: With the demise of microbial mats, trilobozoans could
not rely on the previous hydrodynamics of the sea-floor surface anymore, so
they began stacking themselves higher into the water column to catch better
currents to feed on [S]. The shell likely also provided protection from small
predatory animals, who, as the previously mentioned boreholes on Cloudina
indicate, were becoming ever bolder in attacking larger organisms [S].
Fig. 23: The shell
of Vendoconularia (taken from Ivantsov & Fedonkin 2002). Most life
reconstructions show an anemone-like animal (a single mouth circled by tentacles) sitting in the cone-opening, but in
my sketch I instead went with a version of Anfesta with three mouths and tentacles growing along the body-ridges. Yes, I know I am not a good artist, but you will probably get what I was trying to depict.
The possibly
direct linkage of trilobozoans and conulariids is significant for two reasons.
First and foremost is that Conulariida was an incredibly long-lived lineage,
which only went extinct during the Lower Triassic 245 million years ago. In
other words, their last members could have potentially met the direct ancestors
of the dinosaurs. This would make Trilobozoa one of the longest-lived animal
groups, beating out even the trilobites by a long shot. Second, Ordovician
fossils of conulariids preserve soft tissues, which show them having tentacles
similar to anemones. Those tentacles were then likely powered by muscles and
nerves. This could be proof that conulariids and in turn trilobozoans are not
petalonams, but stem-cnidarians or even proper scyphozoans. But no known
cnidarians build shells of the same construction and of the same material as
conulariids (Ivantsov & Fedonkin 2002). Furthermore, ctenophores obviously
evolved muscled tentacles independently of cnidarians and so could have the trilobozoans and other petalonams as well [S].
The End?
Around 541 million
years ago a major shift happened as the Ediacaran gave way to the Cambrian. The
microbial mats completely collapsed, as animal-burrows became ever more complex
and vertical. Predation also became a major issue for many animals, who now
began defending themselves with mineralized shells and skeletons. This led to
the major radiation of the so-called Small Shelly Fauna (which is elaborated more here), which likely saw the origin of typical shelled
animals such as clams, snails and brachiopods. But while these began to
flourish, many larger animals died out. The proarticulatans, despite being so
promising, vanished completely, alongside most rangeomorphs and the classic
trilobozoans. For the first 20 million years the SSF dominates, until the
appearance of the first trilobites. By the time of the famous Burgess Shale 508
million years ago, the world was a completely different one, dominated by
arthropods and close relatives, archaeocyathid sponges and worms of many kinds,
but seemingly with no petalonams to be found. What happened?
Fig. 24: A possible family tree constructed by me.
While I partially based this off Cuthill & Han 2018’s cladogram, most of
this is highly speculative, so do not take it as gospel.
The general idea
is that the End-Ediacaran extinction happened because the evolution of marine
ecosystems had reached a natural tipping point once bilaterian animals gained a
certain complexity and size. Petalonams, being muscle- and nerve-less bags of
fluids that grew up in a serene garden world, were simply too unadaptable to
cope with the major radiation of bilaterian animals. With the loss of bacterial
mats at the hands of burrowing worms, the sessile forms lost their footing and
the proarticulatans their food. The appearance of large predatory animals then
gave them the rest. In this view, the extinction of the petalonams was
inevitable as soon as our own ancestors evolved. In many ways this reminds one
of the old ideas about dinosaur extinction, wherein the terrible lizards were
too dumb, slow and reliant on swamps, so they were destined for extinction once
the climate changed and mammals evolved. And this should make everyone
immediately suspicious, because we all now know that was absolutely not the case. In
general, the idea of annihilation-by-worms leaves open a lot of questions and
problems, some of which eerily parallel the early debates about the
End-Cretaceous extinction:
- Erniettomorphs and
other sand-anchorers seem to have already anticipated the loss of microbial
mats, but went extinct anyway at the end of the Ediacaran. Why? Possibly they
were the victims of predators, but:
- Almost all of the
large Cambrian predators (as in larger than 5 cm), the classics like Anomalocaris,
Ottoia, Sidneya, trilobites, crustaceans and so on, only started
appearing long after the end of the Ediacaran (Knoll 2003). As the Cloudina
boreholes and the general size of the Small Shelly Fauna attests, the predators
along the Ediacaran/Cambrian boundary would have been miniscule priapulid- or
velvet-worm-like critters. For meter-long proarticulatans and rangeomorphs
these would at best have been a nuisance, not a serious threat.
- The idea that
newer groups must automatically displace older ones is awfully chauvinistic and
at odds with reality. After all, if the petalonams died out because they could
not compete with bilaterians, how could the even more basic sponges and
placozoans still exist today?
- Related to the
above, just as how the first mammals already appeared alongside the first
dinosaurs, bilaterian animals were already living alongside the petalonams
since at least the Avalon Explosion, if not earlier. Some like Kimberella
and Ikaria lived placidly alongside the likes of Dickinsonia and Spriggina
on the mats, so the petalonams would have had enough opportunity to adapt to
their neighbours.
- Conulariids, if
they are indeed part of the Trilobozoa and Petalonamae, would be proof that
petalonams had the capacity to evolve eumetazoan characteristics and persist
long past Ediacaran-like ecosystems. If it had not been for the worst mass
extinction in Earth’s history at the end of the Permian, they would likely still
exist today.
- Conulariids were
not the only survivors. With Stromatoveris psymoglena from the
Chengjiang biota we have a close relative of rangeomorphs such as Rangea
surviving all the way into the Cambrian Stage 3 without much anatomical change (Cuthill
& Han 2018). It lived there alongside typical Cambrian animals, such as
trilobites, priapulids, echinoderms, lobopods and even early chordates. This
shows that even petalonams of classic morphology could survive without the
microbe-mat environment and even alongside typical Paleozoic faunas.
It all appears
that petalonams were dominant and adaptable throughout their reign and maybe
even halted the rise of bilaterians, just as how the dinosaurs halted mammalian
evolution. What then did actually happen at the end of the Ediacaran?
Geochemical evidence shows that the isotopic signature of δ13C massively drops
right during this boundary, even harder than during the Marinoan or Gaskiers
ice ages, but curiously there are no sedimentological signs of global
glaciation (Knoll 2003). Furthermore, massive ocean anoxia occurred, especially
in shallow marine environments. Both these events happened in a relatively
short amount of time. These are all signs closely associated with mass
extinction events, including the Permian-Triassic boundary. The Proterozoic was
therefore most likely ended by a catastrophic event outside the power of both
petalonams and bilaterians. What could have been the cause? Using the
P-T-boundary for comparison, massive volcanic activity would seem to do the
trick. However, in such an event, the oceans would also strongly acidify, which
does not correlate well with the radiation of the Small Shelly Fauna at this
boundary, as acidic water would have attacked their shells. An asteroid impact
therefore seems perhaps more probable [S]. At the likely chance this asteroid
landed in the deep ocean, we might not be able to find its crater anymore, as
almost all of the oceanic crust recycles itself in cycles of 200 million years.
Like at the end of the Cretaceous, the ash thrown into the atmosphere by this
impact would have blocked out the sun for decades if not centuries, majorly
disrupting ecosystems as it made photosynthesis hard to maintain. Algae would
have suffered and the characteristic bacterial mats would have degraded even
faster. Assuming that at least some of the later, shallow-water petalonam forms
did indeed live by symbiosis with algae, even if just in addition to
filter-feeding, this would have spelled disaster for them as well. By direct
comparison, corals live much the same way and it is estimated that up to 98
percent of their species went extinct during the impact of the K-Pg-boundary. The
near-total breakdown of photosynthesis would then explain the observed anoxia.
Large organisms, especially the proarticulatans who were already suffering from
the loss of their food-source, would then have suffocated. Like mammals after
the extinction of the non-avian dinosaurs, the small bilaterians would then
have emerged from their burrows having to fill up the void. The few surviving
rangeomorphs such as Stromatoveris would then have been a classic
example of a dead clade walking, while conulariids were to petalonams what
birds are to other dinosaurs.
Fig. 25: In many ways the End-Ediacaran extinction
mirrors the End-Cretaceous extinction (taken from Knoll 2003). What this means
is that, like with the dinosaurs, the extinction of the petalonams was not
predetermined but rather bad luck.
What do we do now
with the growing knowledge that, in their end, Ediacaran animals really were
the “dinosaurs” of the Proterozoic, as Knoll (2003) puts it? Well, if all the
thought experiments on the internet and in the world of literature, which
explore worlds in which the asteroid at the end of the Cretaceous never hit Earth,
are anything to go by, it means that we can have a lot of fun by just asking: “What
if?” What if the Ediacaran-Cambrian extinction event never happened or,
alternatively, what if it did happen but wiped out Eumetazoa instead?
The things that
could have come
"...if Ediacara had won the replay, then I doubt that animal
life would ever have gained much complexity, or attained anything close to
self-consciousness. The developmental program of Ediacara creatures might have
foreclosed the evolution of internal organs, and animal life would then have
remained permanently in the rut of sheets and pancakes - a most unpropitious
shape for self-conscious complexity as we know it. If, on the other hand,
Ediacara survivors had been able to evolve internal complexity later on, then
the pathways from this radically different starting point would have produced a
world worthy of science fiction at its best." (Gould 1989, p. 314).
These were evolutionary
biologist Stephen Jay Gould’s few words on the Ediacaran biota in his list of
thought experiments towards the end of Wonderful Life. So far this
remains one of the few opinions given by a serious scientist on this
hypothetical scenario (most other scientists involved in spec-evo are, like everyone
else, obviously more interested in Holocene dinosaurs, the future or aliens).
Another one was had by geologist Mark McMenamin in his book Garden of
Ediacara, in which he, in direct contrast to Gould, argues that the
possible cephalization in proarticulatans points towards the beginnings of a
brain and therefore may have paved the path towards consciousness. Whose idea is more valid? Of course, because we do not have a time machine and therefore
cannot alter the timeline, we will never know with certainty. But our insight
into these lifeforms has improved well enough in the past two decades that we
can make some quite educated guesses. Especially Gould’s assessment, which
worked off the idea that petalonams were jelly-filled sacks that could only
interact with their environment through their outer skin, seems to be untenable
now. Petalonams did already possess internal organs in the form of a
hydroskeleton and feeding chambers and they likely could have evolved more than
that. But does this automatically mean that McMenamin was right and that they
could have one day gained consciousness, maybe even self-aware intelligence?
Join me in the
next part (hopefully soon to follow), where we explore some of the evolutionary
potential of many of the Late Ediacaran organisms as they did not enter the
Cambrian but instead Charles Walcott’s lost period of the Lipalian. One thing I
can already say is that Gould was right in at least one aspect: This truly
would have been a world worthy of science fiction.
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Related Posts:
Literary
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