Any casual fossil
hunter will know his invertebrates. There are perennial classics like the
trilobites and ammonites, familiar yet still beautiful forms like clamshells
and conches, true artforms of nature such as crinoids and sea-urchins and
strange patterns in stone, like corals and sponges. There however exists a type
of creature which you are unlikely to have ever seen alive in person, you can
commonly find in Paleozoic and Mesozoic strata, is older than dirt and most
often you can even find completely intact. These creatures are the brachiopods
and, despite their many charms, most people interested in fossils or fossil
collecting usually give them nothing but disregard, either because they seem
unassuming, boring or people have little idea what they are really dealing with
when they find them. With this post I want to highlight some aspects of these
creatures, which in my eyes make them attractive little critters, in some ways
more than other invertebrates, both from an intellectual view and a practical
one for fossil collecting.
One of a kind
Fig. 2: The
generalized anatomy of a brachiopod
What are
brachiopods? Most people, when they first see them, assume they are some type
of clam, but the reality could not be further. Clams, oysters, scallops and so
on belong to a group called Bivalvia, a class of the mollusc-phylum, alongside
other classes like the cephalopods and snails. As the name says, they have two
valves, one on the left and right side enclosing their body. This means when
you find a living clam lying flat on the beach it is actually reclining on its
side. Brachiopods are not molluscs and while they too have two shells, these
are arranged completely differently. Whereas in most clams the shells are
wing-like mirror-images of each other, in brachiopods they are differentiated
and correspond to the top and bottom of the animal. In other words when you see
a living brachiopod lying on the beach, it really is lying on its
“belly” or “back”. The back-half of those two shells encompasses the soft body,
the rest of the cavity makes room for the lophophores, coiling tentacle-like
organs which the animals use for filtering the water for food. These tentacles
are actually supported by an internal skeleton made of cartilage, similar to
your ears and nose, which grows from the smaller dorsal shell at the top. In
many groups (though not all), the bottom/ventral shell also has a hole at the
back, where a tail-like organ, called the pedicle, grows. The brachiopods can
use this as a sort of stalk to anchor themselves to the ground, though compared
to the muscular foot of clams its movement is limited.
Fig. 3: The difference
in symmetry between brachiopods and bivalves. As you can see, the main line of
symmetry in brachiopods goes through the shells, while in bivalves it
goes between the shells.
Surprisingly or
unsurprisingly, no other group of animals on Earth follows this body plan,
which is why brachiopods are grouped into their very own phylum Brachiopoda.
This makes them anatomically and evolutionarily as distinct as are the whole
of Mollusca, Arthropoda and Chordata. Their closest living relatives are
animals called phoronids
(horseshoe worms) and moss-animals/bryozoans, the latter of which
is also commonly found in fossil form. Together these three form the clade
Lophophorata, its members are united through the same type of filter-feeding
tentacles, the lophophore. Indeed, phoronids are perhaps best imagined as
brachiopods without shells (they instead form a chitinous tube) and bryozoans
as miniature phoronids who exclusively live in honeycomb-patterend colonial
shells. Where exactly the Lophophorata fit in the tree of life is a point of
contention. Bilaterally symmetric animals are usually split into two groups:
Deuterostomes are animals such as echinoderms (starfish, crinoids,
sea-cucumbers) in which the anus is the first orifice to develop in the embryo,
while in protostomes (to which animals such as molluscs and arthropods belong) it
is the mouth. Brachiopods oddly follow neither of these models, the first body
orifice that develops in the embryo closes up later and the mouth and anus
develop out of new cavities, which is very weird. This makes it difficult to
classify them on an embryological basis. Due to some developmental and
anatomical details it had long been suspected that brachiopods and other
lophophorates were inside or somewhere adjacent to the deuterostomes, which
would actually make them not too distant relatives of ours. Yes, chordates,
including you and me, are deuterostomes, meaning every human was at one point
of his development nothing but an anus (and some apparently never grow out of
this phase). Most genetic tests however put lophophorates in the protostomes, inside a
mega-clade called Lophotrochozoa, which also includes molluscs and annelid
worms (such as the earthworms in your garden). Not all however, a few other
molecular studies still support the deuterostome-hypothesis. I imagine this whole discussion to only get more interesting, as there have been recent finds that Deuterostomia may not actually form a monophyletic clade.
Paleontologist-friendly
Despite their
lacking popularity, brachiopods make for almost perfect specimens to collect, analyse
and exhibit, largely thanks to their anatomy. The primary layer of a
brachiopod’s shell is made up of calcite, which is the most stable and durable
of the carbonate minerals, and also used by echinoderms for their internal
skeleton. By default, this makes them more likely to be preserved than the
shells of ammonites and some bivalves, which often incorporate the more likely
to disintegrate mineral aragonite. More importantly, when you find a brachiopod
you will almost always find the complete, three-dimensional animal, in complete
contrast to most bivalves. Ever wonder why at the beach or the fossil record,
you often find only one half of a clam’s shell? It is because bivalves possess
ligaments at the hinge, which open up the shell when in resting position, while
they need muscles to actually close it. When a bivalve dies, the ligaments
shorten and naturally split apart the two shell-halves only to be finally
separated with further decomposition. This condition is almost completely
reversed in brachiopods. They need muscles to open the shell and to close it.
Thus, when a brachiopod dies, the shell will stay closed and the animal is very
likely to fossilize with both shells still attached to each other. This also
helps shield and preserve the otherwise delicate arm-skeleton on the inside of
the shell, which you could then theoretically access by carefully sawing the
specimen open. This all makes it very easy to identify the proper classification.
The often smooth, bulbous shapes of brachiopods also significantly help with
preparating the fossils, as it makes it easy to split them apart from the
surrounding rock. Depending on the condition, nature may have already done the
job for you, the most beautiful brachiopods I found at the eroding Holderbank quarry
I could pick up from the rubble like pebbles, many intact and with only a bit
of dirt attached.
Fig. 4: Just a few
of the Late-Jurassic brachiopod-fossils I found myself in the Tafeljura, some
more intact than others. The smooth-shelled forms are terebratulids (probably
of the genus Argovithyris), the rilled ones rhynchonellids (probably Lacunosella).
The only common name that brachiopods have is “lamp-shells” as their form
vaguely resembles the shape of an ancient oil-lamp. Out of experience I can
tell you that rubbing a brachiopod will not grant you three wishes, just odd
looks from the other people at the quarry.
Outside of pure
fossil-hunting and collecting, brachiopods are also of significant use in
biostratigraphy. The fact they are so easily and often preserved, that they are
preserved in such good conditions and therefore are easy to identify and the
fact they used to be so widespread all combine to make individual
brachiopod-species the perfect index-fossils. Though not for relative dating, like ammonites, as brachiopod species tend to be quite long-lived. Instead they are used as great indicators paleo-facies and environments.
Mysterious
Origins
Brachiopods are
older than dirt. This is a statement one can technically make on a factual
level, since the first stable paleosoils only start appearing in the Devonian.
The earliest fossils of definitive crown-group brachiopods of the genus Aldanotreta
first start appearing in the early Cambrian, at the beginning of the Tommotian (or
Cambrian Stage 2) 529 million years ago as part of the Small Shelly Fauna. This
makes them older than the appearance of the first trilobites(!) and the famous
lifeforms found in the Sirius Passet and Burgess Shale Lagerstätten. The Small
Shelly Fauna is often little known to the general public, as it is not as weird
as the Ediacaran biota which preceded it and not as spectacular as the Burgess
Shale organisms which came after it, but it is of vital importance for
understanding the origin of various animal groups. As the name suggests, the
SSF can be generally described as an assemblage of various small animal-shells,
sometimes complete but most often fragmentary. While correctly classifying the
members of this fauna is sometimes difficult, confident assessments can still
be made and thanks to its surprisingly continuous record, studying the SSF
allows one to accurately time the pace at which the Cambrian radiation occurred.
The first forms of the SSF already start appearing in the late Ediacaran with
creatures such as Namacalathus, Sinotubulites and Cloudina,
whose exact affinities are unknown, though they likely belong to cnidarians and
diploblastic worms. Fascinatingly, Cloudina’s nested shells often preserve
boreholes, which appear to have been selectively made by other animals to feed
on the organism’s insides. This makes it possibly the oldest record of one
animal predating on another and this first occurrence of predators is the likely
reason why so many small shelled animals suddenly started appearing in the
first place. As the arms-race continued, the Cambrian began and the enigmatic
forms of the Ediacaran died out, we find various new sclerites and partial
shells appearing in the SSF, including those of echinoderms but also weird
forms like halkieriids, chancelloriids, hyoliths and tommotiids. As the Early
Cambrian goes on, we also find small shells and sclerites of early molluscs, brachiopods,
stem-arthropods and other modern groups. By the beginning of the Middle
Cambrian most enigmatic forms die out and the fauna is superseded by the first
appearance of trilobites and other famous Cambrian lifeforms of larger size. By
the Ordovician most of what used to be the SFF is now only made up of the small
shells of gastropods (snails).
Fig. 5: The
strange Cambrian organism Halkieria. Take special notice of the odd,
brachiopod-like shells imbedded at the front and after.
Like with many of
the Cambrian invertebrates, the discussion around what sort of organism the
brachiopods evolved out of is an interesting one. Traditionally brachiopods had
been linked to an organism named Halkieria and its close relatives. Halkieriids
can be best imagined as flat slugs covered in a chain-mail-armor made up of multiple
mineralized scales called sclerites. These already appeared at the very start
of the Cambrian and survived even past the Middle Cambrian extinction of most
of the SSF. Perhaps they were related to the slug-like Kimberella from
the Ediacaran. In addition to their sclerites, halkieriids also possessed two
shell-plates at both the head- and tail-end of the body, which may have been
used for muscle-attachment or for protecting gills. In their structure these
shell-plates look suspiciously, and I mean very suspiciously, like the two
valves of brachiopods, in addition to also having been made of calcite. Some halkieriids
are curiously also found in a protective rolled-up position, similar to
pillbugs. The classic idea of brachiopod-origins, the “brachiopod-fold-hypothesis”
has therefore been that some halkieriids became sessile by permanently folding
their bodies along the mid-section into this sort of pillbug-shape and then grew
compacter until the two shells at the end of the body met in the middle to form
a hinge. What may support this is that the larvae of some modern brachiopods go
through a stage which resembles such a fold. Unfortunately, this hypothesis
does not entirely explain where many key-features of brachiopods come from,
such as the lophophores or the stalk. Furthermore, the halkieriids possessed
various features not found in brachiopods, such as a muscular foot and in one
member possibly even a radula, which would put them a lot closer to true molluscs.
Though even more bizarrely, the sclerites of halkieriids are nearly identical
to those found in chancelloriids, which are radially symmetric lifeforms often
interpreted as being early Cambrian sponges.
Fig. 6:
Shell-parts of Eccentrotheca, a possible close relative of phoronids and
brachiopods.
A better
alternative opens up if one assumes that brachiopods evolved out of an animal
similar to other lophophorates. In this light their most obvious ancestors
become the tommotiids, which basically looked like phoronids with tube-shaped
shells. Among these animals one can even find fossils which show the
transitional stages from a phoronid-like creature towards the typical brachiopod
body-plan. Eccentrotheca from Australia is an early Cambrian tommotiid
found with a cone-shaped shell. The pointy end had a hole through which a stalk
probably grew to attach the organism to the ground, while the other end was
broad and open to allow the lophophores to filter the water. At this end the
animal curiously had additionally sclerites, which may have been the beginning
of the closing shell. After this came Micrina and Paterimitra,
considered stem-group brachiopods, which already had the two shells, though where
still somewhat cone-shaped and could not fully enclose their bodies.
Fig. 7: Paterimitra,
a stem-brachiopod.
During the
Paleozoic brachiopods were the most successful and widespread filter-feeders
and reef-builders, reaching their peak in the Devonian, and could be found in a
variety of marine ecosystems. Today they are but a shadow of their former
selves, most of them only live in deep-sea environments and thus are rarely
ever seen by humans unless they go directly looking for them. The lack of post-Paleozoic
success is largely linked to the success of bivalve molluscs, as well as the Permian-Triassic
mass extinction event. During said event large parts of Earth’s ocean (back
then there was only one due to the supercontinent Pangea) not only became
anoxic but also highly acidic due to carbon-emissions by massive volcanism,
causing death and decline to almost all shelled animals, since their calcareous
shells easily dissolved under these conditions (thanks to human carbon
emissions the same process is unfortunately repeating today). While bivalves
were hit just as hard as brachiopods at the time, they managed to steadily
increase in abundance in the subsequent periods, while brachiopods were never
able to recover their former diversity. Why exactly it happened that way is not
clear, though it seems increasingly unlikely that bivalves directly outcompeted
brachiopods, as the two did not share exact same lifestyles and trace-fossils
attest that Mesozoic shell-crackers such as placodont reptiles, heavy-toothed
fish and starfish actually preferred eating clams over brachiopods. Even today
most marine predators tend to avoid eating brachiopods, either because they
have chemical defences, taste awfully or just do not have enough meat to them
to justify the effort (Cohen et al. 2006).
Hidden Depths
A probable reason
why many people think of brachiopods as boring is that they seemingly lack in
diversity. Once they have seen one, they think they have seen all of them. Bivalves
occupy many niches and possess various body-shapes adapted to digging, filter-feeding,
reef-building, microcarnivory and in the case of scallops even swimming. In
contrast most brachiopods seem to only be sessile filter-feeders. However even
they have developed a truly fascinating variety of morphologies and lifestyles,
which one can see both in the fossil record and modern day.
Fig. 8: A Cambrian
hyolith, illustrated by Emiliano Troco, standing on its stilts in order to filter
the water free from the murky filth on the ground.
The oldest example
of aberrant groups may be the hyoliths, which are either stem-brachiopods or
basal lophophorates. Hyoliths were truly alien-looking animals which also first
appeared in the Cambrian and survived all the way to the Permian mass
extinction. They might be best described as brachiopods trying to be… well,
actually I cannot really find a good analogy for them. Like crown-group
brachiopods they had a main ventral shell in which the body sat and an
operculum, a smaller shell attached with a hinge, on which the lophophores grew
and with which they could close the shell. The big difference is that the
ventral shell did not have a pedicle, but was instead much larger than the
operculum and cone-shaped like the shell of an orthocone, likely meaning that
the animal lied flat on the seafloor. Most bizarrely, out of the mantle grew two
large spikes or stilts, which could elevate the body a bit from the ground. It
is thought that through muscles the hyoliths could rotate and move these spikes
and that way actually crawl across the seafloor if they wanted to.
Fig. 9: An
aquarium with a few Lingula anatina. The name derives from the tongue-shape
of the shell. Take note of the sensory setae at the shell-margins.
Higher up on the
family tree are the inarticulate brachiopods, which unlike all other
brachiopods do not have a mechanical shell-hinge. Inarticulates are still alive
today in the form of Lingula and its close relatives, which have used
this to their advantage. The lack of a hinge allows Lingula to horizontally scrape its shells and its setae against each other in a sort of shearing-motion,
which the brachiopod can then use to dig itself underground like many bivalves
do, but most other brachiopods cannot. Various other quirks also set them apart
from regular brachiopods. Their pedicle is muscular, dextrous and flexible,
acting more like a tail or tentacle than the static stalks of the other brachiopods.
Because humans can and will eat anything that is not immediately toxic to them,
their fleshy stalks have made Lingula the only brachiopod to be
commercially fished and consumed. Linguloids are also the only brachiopods
which use oxygen-carrying proteins in their blood, though it is haemerythrin
instead of the hemoglobin that we use. Instead of calcite,
linguloids build their shells out of apatite, which is the same material vertebrates use for our bones. Under observed aquarium conditions, Lingula
has actually shown quite complex behaviour: They will use their pedicle like
the tentacle of an octopus to probe for a suitable spot, then use it to arch
their whole body until the shell-tips point directly downward onto the spot,
then use the aforementioned shearing-motion to dig themselves a burrow
head-first into the ground, then lift themselves out again with their pedicle,
rotate and then finally settle tail-first into their new home (Cohen et al.
2006). That is quite calculated for an animal which does not have a brain. Actually,
many aspects of how brachiopods sense the world are mysterious to us, in part because
researchers simply lack interest to investigate them. For example, adult
brachiopods lack eyes or any similar sensory organs, but, for reasons we cannot
understand so far, are able to sense when a dark shadow (for example of a large
fish or human diver) looms over them, causing them to instinctively close their
shells.
Fig. 10: An old
illustration of a dissected Lingula to give you a better understanding
of how the lophophores sit inside the body of an average brachiopod. Extinct
forms such as the spiriferids had in contrast rather ridiculous spiral shapes
resembling radiators.
Lingula is often regarded as a “living fossil”, as brachiopods
of pretty much the exact same morphology have existed since the Ordovician
period. In fact this animal was among those which made Darwin first coin the
term. It has however become doubtful if these really are of the same lineage or
that Lingula has remained unchanged since then, it appears more likely
that these are separate brachiopod-lineages which arrived on the same burrowing-adaptations,
as it is really not all that difficult to evolve the Lingula-shape from
the basic brachiopod body plan (Emig 2008). This is called parallelism and it describes
the phenomenon of two closely related lineages convergently arriving on the
same shape, because they inherited the same initial body plan and all the evolutionary
potential that came with it. Parallelism is arguably not a true example of
convergent evolution, as the ability itself to evolve into a certain form becomes
an inherited trait in these cases. Many things people commonly think of
as examples of convergent evolution, such as carcinization in crustaceans or
feliforms repeatedly evolving saber-teeth, are actually parallelisms.
Fig. 11:
Reconstruction of Late Paleozoic productids and their strange ventral spikes.
Among the
articulate brachiopods one of the most successful and strangest forms was the
order Strophomenida, who had their heyday in the Ordovician and later again in
the Permian, but are now unfortunately extinct. Whereas most modern brachiopods
are biconcave (both shells bow outward), the Ordovician strophomenids were
concavo-convex, meaning the shells nested into each other like a pair of stacked
bowls. Even stranger was the Permian sub-group Productida. Productids entirely
lacked the pedicle and instead attached themselves to the seafloor through a
unique method: The ventral shell grew long, curved spikes with which the organism
rooted itself into the sand, like the foundations of a beach-house. The
Devonian productid Gigantoproductus with a size of 30 cm is the largest
brachiopod known, but instead of spikes it used strange, wing-like protrusions
in its shell for more stability in the water-currents.
Fig. 12: Reconstruction
of Leptodus and close relatives.
Strangest among
the productids was probably the Middle Permian Leptodus. This animal’s dorsal
shell was highly reduced and fern-shaped, meaning that even when it closed onto
the ventral shell, the soft mantle of the animal inside was still exposed
through windows. Two interpretations for this exist. The first is that the
dorsal shell had, for unknown purposes, actually become incorporated into the
body, as a sort of internal skeleton, similar to what happened to the shells of
coeloid cephalopods. A One-Valved
Brachiopod, if you will. The more likely interpretation however is that Leptodus
was the brachiopod-answer to corals and rudist bivalves: The animal had managed
to symbiotically incorporate zooxanthellae (planktonic algae) into its skin and
was partially living off photosynthesis through its partners. The fan-shaped
shell offered light-exposure in exchange for minimized protection.
Fig. 13: Cretaceous
Pygope. The sediment-filled hole in the middle would have been hollow in
life.
Apart from the
inarticulate ones only a few brachiopod-orders survived into the Mesozoic and
modern day, but even among these can be found weird fossil forms. One of the
most outstanding are the Jurassic Pygopidae, in which both shells split in half
to separately house one of the lophophores. In many pygopids the shell thus
forms a triangular hole in the middle, just for the two halves to fuse again at
the end. The particular shape of this fusion is what gave the group its name,
Pygopidae can roughly be translated as “butt-faces”. Also worth mentioning is Early
Cretaceous Peregrinella. While it outwardly looks like typical
rhynchonellid brachiopods, it stands out due to its large size (10 cm), still
unattained by modern brachiopods, and its calcified arm-skeleton. The sediments
it is found in show that Peregrinella lived around deep-sea methane-vents
and it is theorized that it may have attained its weird quirks through living
in symbiosis with chemosynthetic bacteria. Similar explanations have also been
used for other giant brachiopods like Gigantoproductus mentioned
earlier.
Let us have
some fun
An
underappreciated way of highlighting and generating interest in the often
strange anatomy of living animals is to show their potential through little
exercises in speculative biology, which, if you know me, I am a big enthusiast
of. Now I will present you a few excerpts from a small project of mine I
started working on and off a few years ago. The creatures here are not direct
descendants of brachiopods, but extraterrestrials that came from an ancestor with
very similar anatomy. While I have already thought of a scientific name for
them, for simplicity’s sake we will just call them xenobrachiopods for now.
Please excuse my very limited drawing- and photoshopping-skills, I have never
been a good artist and just hope the sketches get the ideas across I want to convey.
The atmosphere of
planet [REDACTED] is unnaturally dense thanks to multiple periods of constant volcanic
outgassing. Despite being quite a stretch removed from its parent sun, the
intense greenhouse gases create balmy temperatures and the air is constantly laden
with water vapor and misty clouds. This, combined with a Mars-like gravity, has
created perfect conditions which allow small to microscopic animals to freely float
in the atmosphere as aeroplankton. Part of this plankton are the larvae of the Shell-Tree,
a primitive xenobrachiopod. Hatching from their airborne eggs as free-floating
organisms, the tree’s larvae eventually grow too large to stay in the air and
sink to the ground. Should they happen to land in a suitable spot, the pedicle
takes deep roots into the ground and permanently hardens into a biomineralized,
armored stalk, which grows until resembling a large tree trunk. The top this
trunk holds the only lightly mineralized valves. The lophophores of the
shell-tree have grown into long, feather-like fans, which it uses to pick up
moisture as well as to feed on the aeroplankton that it once used to be a part
of. Statocysts alert the shell-tree of large trembling in the ground and
changes in air-pressure, indicating large predators or incoming storms, making
it retract its fans and close the shell. The individual here is a still
juvenile pioneer, growing at the outskirts of a desert which is gradually
becoming more habitable as the climate of the planet changes. Soon it will be
joined by other shell-trees and vendobiont-like photosynthesizers, eventually
forming a dense forest.
A few hundred
million years before the young shell-tree hatched, back in the primordial soup,
a close cousin took a very different route to life. Back then a new phylum of
predatory animals arose on [REDACTED], the trichordates, starfish-like animals
who with their three strong mesoskeletal arms were able to pry open the shells
of any valved animal. The ancestors of the Blimpshell first reacted to
this new threat by evolving a more flexible, muscular pedicle and adapting to a
burrowing lifestyle similar to Lingula. Unfortunately, the trichordates
eventually learned how to burrow themselves, so this was not of much use anymore.
A few of these xenobrachiopods responded by making it impossible to pry their
shells open: Both valves became permanently fused to each other like the
plastron and carapace of a tortoise. The adductor muscles became repurposed to control
the pedicle and the lophophores. Only three to four hole-shaped openings
remained at the front of the shell to allow the animal to stick out its
lophophores into the water. Eventually even stranger things happened. To probe
the ground for food, the arm-skeleton of the front-most lophophore fused with
the mouth to form a mobile, bony proboscis, which outwardly looked similar to
the siphon of clams but functioned more like the neck of vertebrates. Part of
the former lophophore became repurposed into an efficient gill-set, with four
to six spiracles at the front of the proboscis pumping water onto them. In a
herbivorous subgroup two hardened plates developed at the front of the
proboscis to scrape algae off rocks. These eventually evolved into a beak with
a very cephalopod-like jaw-joint. Some could grow beaks large enough to make
piecemeal out of the once dreaded trichordates. As the proboscis increasingly
took over the role of eating and breathing, the other lophophores could adapt
to new roles, in one clade evolving into fleshy fins. While these were very
flimsy, in structure similar to the fins of a lungfish, they could get the job
done of heaving the xenobrachiopod across the seafloor. The pedicle meanwhile
became a tail, with comb-like setae to swim away from predators in quick bursts.
Here we see a very
rough sketch of the internal skeleton. The arm bases of the former lophophores
now fill out the lateral opening to present a better site for muscle
attachment. The proboscis attaches to the inside of the dorsal shell and can be
retracted like the head of a turtle. The tail is omitted here, as it is not
actually endoskeletal. Like the real life Lingula, the pedicle of the blimpshell’s
ancestors had a chitinous cuticle, which in this form has evolved into a full-on
exoskeleton of chitin-rings controlled by muscles which attach to the inside of
the ventral shell. In some ways it resembles the tail of a crustacean or the
mouth-arms of dinocaridids. Due to this nature, the tail is rarely preserved in
fossils. The small bulges at the top and bottom of the front shells are viewing
platforms for primitive eyes, allowing it to see around the proboscis. Instead
of evolving its main sensory organs at the front of the proboscis as one would
expect, the blimpshell simply adapted pre-existing structures from its
ancestors. Said structures were aesthetes, similar to those of Earth’s chiton-snail,
which are complex mineral-eyes firmly placed in the shell. The nervous system
of the blimpshell is still quite decentralized, with multiple ganglia
controlling the movement of the arms, tail, eyes and proboscis. The blimpshell
and its relatives, despite their now gained mobility and ability, were still at
the bottom of the food-chain. The oceans of their primitive world were ruled by
large exoskeletal creatures looking like a mix between mythical sea-serpents
and legless centipedes.
A climate-induced
catastrophe came eventually for these sea monsters and the new mobile xenobrachiopods
could expand into new megafaunal niches, approximating perhaps the various
placoderms of Earth’s Devonian. Eventually one group adapted to swampy
mangroves and anoxic waters by adapting a buoyant system of pneumatic air sacs
inside the shell-wall (which gave the blimpshell its name) into a sort of
lung-system. The small bones of the fleshy fins fused to form long bones, joints,
fingers and eventually fully formed arms with seven digits, which these animals
could use to drag themselves across the landmasses. They came too late however,
the terrestrial abode was already occupied by another clade of animals, such as
the one eyeing our tree-climber here. This was a phylum of six-legged animals
with an internal skeleton and even a backbone, making them the most similar to Earth’s
vertebrates among the fauna of [REDACTED]. They were nonetheless strange in
other ways: Their jaw was a complex pair of spider-like cheliceres, derived
from former limbs, coupled with a long prehensile tongue. They had neither ears
nor nostrils, instead smelling and hearing was accomplished by an array of
antennae. Their eyes were multiple solid silicate discs. Breathing on land was
accomplished through a unidirectional book-lung derived from a former jet-propulsion
organ. Due to being the first lifeforms on the planet to evolve legs, the crustacean-like
ancestors of the six-leggers were able to colonize the land long before the
xenobrachiopods could. Smaller forms resembled arthropods and amphibians of
various kinds, the large megafauna consisted of six-legged and kangaroo-like
reptilian behemoths, which looked a bit like 1940s dinosaur reconstructions
with insect-heads attached. The new terrestrial xenobrachiopods, such as this Terecuda
could not compete and instead adapted to a life in the trees. The periostracum,
the skin growing over the shell in the brachiopodous ancestors, has now grown
thicker and a few muscles here and there now also attach to the outside of the
shell, making the shell’s appearance similar to that of soft-shell turtles. The
eyes on the shell have now become more complex and can move independently like
a chameleon’s and a more complex brain is developing underneath them. The beak at the end of the
proboscis has become a pseudo-skull, lacking eyes or a brain. What looks like a
depression for an eye is actually a tympanum, a primitive ear derived by
growing a membrane over one of the spiracles. The terecuda is an ectotherm that
lays leathery-shelled eggs through its cloaca, though a few of its relatives are
experimenting with elevated metabolisms.
Their time came
eventually, as an asteroid hit [REDACTED] and caused mass extinction. The
smaller six-leggers and some gracile, centaur-like grazers survived, but the
time of the behemoths was over. In their place came creatures like the Deinobrachion
above. Descended from a chameleon-like ancestor which already had an
elevated high-walk, deinobrachion and its relatives evolved an upright gait
with straight legs and in many ways converged on flightless birds and other theropods (though
if you look closely its equivalent to a knee is functionally an elbow). The
periostracum has grown even thicker and even developed a fuzzy pelt, similar to
the hair found on the shells of some gastropods, which now covers up the
sutures and fine details of the shell underneath. What the alien is doing here
is a sort of intimidating mating dance to a potential partner just offscreen.
Due to a meiotic quirk going back all the way to the primordial soup, nearly
all animals on [REDACTED] are hermaphroditic and unable to develop
differentiated sexes. In most primitive lifeforms both parties become pregnant
after mating or they practice self-fertilization like plants do on Earth. More
complex animals like this one instead have a love-life more similar to snails
and flatworms in that the two partners have to determine (often violently) who
has to get impregnated by the other and therefore expend their precious energy
to produce and rear the offspring. Impressive horns, shields, domes and spikes
have developed in some forms for the purpose of such mating-fights, though the
deinobrachion keep it simple by just extending their “mating-tentacle” out of the
cloaca and showing off their colourful dewlap to each other. The one with the
most vibrant colours and the longest organ wins. Said organ is the little
dongle you see here underneath the base of the neck in front of the ventral
eyes. Like in their Earth-equivalents, the gut of the xenobrachiopods is U-shaped
and once the fused carapace evolved, the cloaca had to share the same bony
opening as the proboscis, though they are thankfully separated by skin and muscles.
Finally arriving back
to the time of the shell-tree, we here see its arch-nemesis, the Tree-Cracker,
which feeds on the insides and delicate fans of the former. Lizardine lips
behind the hooked beak hide what are massive crushing teeth similar to those of
placodont reptiles. The sharp beak is used to hack at any exposed muscles or
ligaments of shell-trees in order to get them to open and if that fails the
teeth simply crush through. With a fully elevated neck the tree-cracker stands
about as tall as a giraffe, often higher, though it pales in comparison to its
herbivorous relatives, which, thanks to the planet’s benevolent gravity and
perforated shells similar to sea turtles, can reach dinosaur-like proportions,
the largest species reaching a titanic size of [DATA EXPUNGED]. They are preyed
on by large, bipedal carnivores, such as Tyrannoproductus. If size is
not enough of a deterrent, the tree-cracker will present the predator with the
long setae growing on its chitinous tail, which can sting like hell. A smaller
relative is even able to absorb the toxins of the smaller shell-trees it eats
and store them in the setae, which then function like dangerous syringes.
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Related
Posts:
Literary
Sources:
- Ottiger, Ronald: Fossiliensuche
im Tafeljura. Eine Anleitung für den Sammler, München 2014.
- Prothero, Donald: Fantastic Fossils. A Guide to Finding
and Identifying Prehistoric Life, New York 2020.
Papers:
- Cohen, Bernard/Bitner,
Maria: Brachiopoda,
in: Encyclopedia of Life Sciences, 2013.
- Emig, Christian:
On the history of the names Lingula, anatine, and on the confusion of the forms
assigned them the Brachiopoda, in: Carnets de Géologie/Notebooks on Geology, 2008.
- Grant, Richard: Spine
Arrangement and Life Habits of the Productoid Brachiopod Waagenoconcha, in: Journal
of Paleontology, 40,1966, p. 1063-1069.
- Skovstead,
Christian, et al.: The scleritome of
Paterimitra: an Early Cambrian stem group brachiopod from South Australia,
in: Proceedings of the Royal Society B: Biological Sciences, 276, 2009, p. 1651
– 1656.
- Skovstead,
Christian, et al.: Scleritome
construction, biofacies, biostratigraphy and systematics of the tommotiid
Eccentrotheca helenia sp. nov. from the Early Cambrian of South Australia,
in: Paleontology, 54, 2011, p. 253 – 286.
- Williams, Alwyn: The
morphology and the classification of the oldhaminid brachiopods, in: Journal of
the Washington Academy of Sciences, 43, 1953, p. 279-287.
Image
Sources:
- Fig. 2: Cohen
2013, p. 3.
- Fig. 3: Prothero
2020, p. 93.
- Fig. 4: Image
taken by me.
- Fig. 5: Wikimedia
- Fig. 6: Skovstead
et al. 2011.
- Fig. 7: Skovstead
et al. 2009.
- Fig. 8: superterrane.tumblr.com
- Fig. 9: Wikimedia
- Fig. 10: Wikimedia
- Fig. 11: Grant
1966.
- Fig. 12: Williams 1953.
- Fig. 13: Wikimedia
- Rest taken or made
by be.
This article was not
written by alternate universe sapient brachiopod SCP-12969.
