Friday, 13 March 2020

Solved and Unsolved Fossil Enigmas - Part 1

Fig. 1 : Tribrachidium heraldicum, an over 550 million year old organism form the Australian Flinders Range. What it was or how it relates to other creatures is not known and may never be. Its entire phylum, the Trilobozoa, likely went extinct before higher animal life even took the stage.

About 99% of all species that ever existed on this planet are now extinct. Of those, the great majority has never left traces in the fossil record, as fossilisation is a rare process that requires special conditions. Of those that have left fossil traces only a small percentage will be found and studied by paleontologists. What this means is that there simply are things about prehistory, or really the past in general, that will forever be lost in time, often without us ever having a clue of what we have lost. This becomes the most obvious when we actually do find things, but cannot easily put them into an understandable context anymore, because the context they once existed in is no more. Lifeforms and artefacts that are so far removed from us in time that we are unable to even properly place them into the tree of life. For this and the next post I wanted to show and discuss the history of some of the greatest fossil mysteries (I recommend listening to these tracks in the background while reading). Some of these were eventually able to be solved, but most are still enigmas to us. We begin in the Archean Eon and work our way up from there. 

The Klerksdorp Spheres

Oldest and first up on our list is something that is not a fossil and has actually been solved, but it is simply too fascinating to not be mentioned here. The Klerksdorp Spheres are geologic artefacts found by miners in Ottosdal, South Africa. At first glance they look like spheres, disks, bowls and vases, ornamented with simple straight lines. Finding something like that under the ground at first does not sound remarkable as it simply seems like ordinary pottery of some ancient human culture. You will however probably raise an eyebrow if I tell you that these objects have been found in rocks that are 3 billion years old. This has of course led to all sorts of out-there speculations about their origin. The most bizarre one has to come from an episode of Ancient Aliens, which claimed that extraterrestrial visitors used these spheres as microbe vessels with which to seed Earth with life (despite the fact that the oldest signs of life on Earth are significantly older than 3 Gya). 
Fig. 2: One of the Klerksdorp spheres.

On closer inspection by geologists and mineralogists however it became clear that these objects are of natural origin. They were found inside pyrophyllite deposits and consist of either wollastonite or pyrite. They were most likely formed from a mix of volcanic ash and silicates that, during underground metamorphosis, crystalized or fused into symmetric concretions through extreme pressure and heat. Similar, though far less odd-looking concretions are known from other sites, such as the Moqui Marbles from the Navajo Sandstone. On closer inspection the Klerksdorp Spheres themselves are not perfectly shaped, as is often claimed, and have no obvious sign of having been made by an intelligent being with any purpose. Though, then again, would we be even capable of recognizing an intelligence unlike our own? 
Fig. 3: Concretions similar to the Klerksdorp spheres, found in Schoharie County, New York.

The Francevillian biota

The first on our list that is still largely mysterious is the Francevillian biota, sometimes also called Gabonionta. These are a group of fossils found by French geologist Abderrazak El Albani in Gabon and they date to about 2.1 Billion years ago, making them Paleoproterozoic in age. Fossils from this and far older times were already known, but they were all of unicellular organisms, such stromatolites produced by cyanobacteria. Gabonionta stand out because they are the oldest possible evidence of multicellular, eukaryotic life. The shale they come from apparently used to be a shallow delta with oxygenated water, with the Gabonionta living on top of the sediment. The fossils could grow up to 17 centimeters wide and most were of a circular or oval shape with a bulbous center and a flat fan radiating out of it. In short, they look a bit like nipples. In 2014 El Albani also identified other types, such as ones that look like elongated pearl-strings that ended in a “flower”. The fossil status of the Gabonionta has often been questioned. German paleontologist Adolf Seilacher for example claimed they were actually pseudofossils, more precisely inorganic pyrite crystals that formed during metamorphosis. Similar, though a lot younger structures were also reported from Michigan, USA, but the discoverers interpreted these also as inorganic concretions. In his 2014 paper El Albani refuted Seilacher’s claims. While parts of the fossils were pyritized, not all of them were. The structures were also formed at the same time as the sediment and could therefore not have grown into the rock after the fact. Pollen-like structures containing organic material were also associated with the fossils. It therefore seems likely that the Gabonionta represent real fossils of actual organisms. But what were they?
Fig. 4: Francevillian fossils identified by El Albani and his team.

At least the flower-like type resembles modern dictyostelids, a type of slime mold. However, Gabonionta lived in saltwater, while no marine dictyostelids are known. Further compounding is the fact that the Gabonionta are very isolated in time. No structures similar to them are known from earlier times and the next closest-looking organisms would not appear for another 1.5 billion years in the Ediacaran. It is therefore possible that the Gabonionta were an independent experiment in multicellularity that evolved in response to a temporary oxygenation event and died out again in a subsequent anoxic event. This possibility of course gets the imagination going. Hypothetically, what if they did not go extinct but would have been able to further proliferate? What might Earth look like today if something like the Cambrian radiation had happened as far back as 2 billion years ago? Look how far animal life evolved in “just” 600 million years and imagine what would happen if it had an extra 1.5 billion years of time. How would multicellular life have adapted to the Snowball Earth phases of the Late Proterozoic? All questions we will never have an answer to, but speculation does not hurt. There is also another interesting question the Francevillian biota opens up, one which might have a definite answer one day: Were there similar experiments, perhaps even ones that were more successful, appearing more than once throughout the Precambian Eons? Have we so far simply not found their remains because they did not easily fossilize or all these rocks are so ancient that few survive until modern day? How many geologic and fossil records might have been destroyed during the Huronian and Sturtian glaciation events, forever to be lost to us? 
Fig. 5: Close-up of a gaboniont (or maybe it's two?)

The Precambrian Nuclear Reactor

Here is another thing that is not a fossil, but simply too fascinating to not include, curiously also from Gabon and not even far away from where the Francevillian biota was found. Back in 1956, when the country was still a French colony, an uranium-mine was founded in the Oklo region by the CEA to provide material for nuclear power plants across France and other parts of Europe (as was typical, Gabon profited very little from this and most of the reserves were scavenged before the country could gain independence). In 1972 it was noticed in an uranium-enrichment facility in Pierrelatte that the concentration of Uranium-235 in the probes from Gabon was unnaturally low when compared to other natural uranium-reserves. This prompted an investigation, as it was feared that some of the material might have been illegally sold to other parties for the development of nuclear weapons. While the investigation did not find this to be the case, it found something strange at the Oklo mine. The Uranium-235 concentration in the deposits themselves was already significantly lower than what is usual in all other known uranium mines and there were also anomalies with the neodymium and ruthenium elements at the sites. All the measurements were in accordance with what would be expected if the uranium had already undergone nuclear fission inside a reactor… 1.7 billion years ago. Was there someone or something back then on ancient Earth using our natural uranium-reserves to create nuclear power or even weapons? The answer is arguably even stranger. It was found that back in Precambrian times the conditions at Oklo were just right to create what is called a natural nuclear fission reactor, a hypothetical structure that was already proposed to be possible in 1956 by chemist Paul Kazuo Kuroda. 1.7 billion years ago the uranium-ore became inundated with oxygenated groundwater. This allowed the Uranium-235 to undergo a nuclear chain reaction. The heat generated by this evaporated the groundwater, which stopped the process and allowed the reactor to cool down until new water could flow back, restarting the process. This mechanism may have repeated for hundreds of thousands of years until the Uranium-235 was too depleted to naturally undergo fission.
If you are worried of the possibility that you might be living on top of such a natural reactor without noticing, do not worry. The reaction was only able to take place in the Precambrian because the natural concentration of Uranium-235 was a lot higher back then and fission is not possible anymore nowadays outside of an artificial reactor. At least that’s what we think…

The Ediacaran biota

From a few fossils and artefacts we now come to nearly an entire fauna and time period that consists of taxonomic headaches. The Ediacaran was the last period of the Proterozoic Eon and saw the first major radiation of multicellular creatures. Many of the lifeforms from this time were, however, so strange that we cannot tell anymore if they were animals, plants, fungi or something completely different. More than one paleontologist has joked that the Ediacarans are actually aliens. 

Fig. 6: A selection of vendobionts. Paleoart of Ediacaran biota is often rare, as it is simply difficult to reconstruct these lifeforms from long ago. Many of the reconstructions that can be found are based more on what the organism is interpreted as rather than what is actually found in the fossil. The Spriggina here for example is too elongated and annelid-like than what is actually suggested by its fossils
The Ediacaran, formerly known as Vendian, began about 635 million years ago and ended 541 mya. Technically fossils were already known from this period since 1868, but for the longest time these were believed to be either abiotic pseudofossils or actually of Early Cambrian age. Reginald Sprigg for example named his 1947 paper on fossils he found in the Ediacara Hills of Australia “Early Cambrian(?) Jellyfishes”. It was finally realized that these organisms were a lot older when Charnia masoni was discovered in England in 1956 by Tina Negus. Ever since then, trying to figure out what the Ediacarans actually are has been a great challenge. The problem is that most are either too simple, too abstract or too surreal to be easily recognizable as any form of organism known to us. Almost all were soft-bodied organisms, with some seemingly building skeleton-like structures out of internalized sand. There are no recognizable organs, no heads or eyes and they seem to have grown in fractal shapes or exhibited glide-symmetry, a form of pseudo-bilateral symmetry barely seen in modern animals. The arguably most well-known Ediacaran organism, Tribrachidium, even exhibited tri-radial symmetry, resembling a wheel lying flat on the ground, while the fossil of Eoandromeda looks like a miniature spiral galaxy. Most of the organisms outwardly resemble plants, but true plants would not appear until considerably later and there are many Ediacaran formations, such as those of Mistaken Point in Newfoundland, that used to be deep-sea environments where photosynthesis is not feasible. An early attempt was to simply shoehorn these organisms into the ancestry to known animal-phyla. As examples, Charnia and similar organisms were interpreted as sea-pens (a type of cnidarian), Dickinsonia as an annelid, Arkarua as an early echinoderm and Spriggina and Parvancorina as some sort of proto-trilobites. There were however several problems with such interpretations. The oldest uncontested sea-pen fossils only come from the Early Cretaceous. Charnia’s morphology and growth also do not match those of a sea-pen. Arkarua has a pentaradial symmetry, but lacks literally every other trait known from echinoderms. Spriggina, Dickinsonia and similar fossils were traditionally grouped together into the Proarticulata (meaning “before Articulata”, an outdated hypothetical clade that includes annelids and arthropods) because it was thought they exhibited a segmented body-plan and bilateral symmetry. In reality however, their bodies exhibit glide-symmetry, meaning that the right side of the body is not an exact mirror of the left side, the segments are actually isomers. While a degree of chirality is known from many bilaterian animals, in none is it so central to the entire body-plan as in the “Proarticulata”, making it hard to actually classify these as bilaterian animals. This is further complicated by the fact that all Dickinsonia fossils show no visible internal organs, such as a gut, a mouth or an anus. This seems to be a genuine lack rather than an artefact of preservation, as the Ediacaran fossils are already soft-bodied imprints from Lagerst√§tten. Spriggina suffers from the same problems, but does at least have something that looks vaguely like an arthropod’s head-shield. However, several studies have failed to find any traces of eyes or a mouth in this “head”. There is no evidence of legs on Spriggina nor are there trace-fossils, instead the organism seems to have lied flat on the sea floor its entire life, perhaps using its “head” as a holdfast. It gets even more bizarre when we consider the fact that the obviously head-and-limbless Dickinsonia and its close relative Yorgia actually do have trace-fossils attributed to them. They seem to have crawled along the sea-floor like giant amoebas, perhaps with the use of cilia, soaking up bacterial mats with their entire ventral body-surface. I am forced to think of biological roombas.

Fig. 7: Up: A digitally enhanced image of a Spriggina fossil. Compare this with the slightly more fanciful reconstruction from Fig. 6 or other reconstructions that give the organism legs or eyes. Left: Dickinsonia, perhaps the most well-known Ediacaran organism. Right: Epibaion, a trace-fossil attributed to Yorgia, likely showing how the organism grazed on bacterial lawns that covered the sea-floor. The trace-maker itself is preserved on the lower far right. 
It is for reasons like this that many have suggested that the Ediacarans are actually more closely related to each other than to modern animal groups and that they represent a unique and unified branch on the tree of life that had its heyday but died out without leaving descendants. This hypothetical group has been either called Vendobionta or Petalonamae. Adolf Seilacher, whom we have mentioned earlier, interpreted them to be giant unicellular organisms, similar to modern Monothalamea/Xenophyophorea, that were able to grow to such considerably large sizes (the largest Ediacaran organisms could grow nearly 2 meters long) by hollowing their body out into multiple hollow “quilted pneus”. American geologist Mark McMenamin disagreed. He saw them as clearly multicellular organisms related to animals, although diverging so early from them that they should be considered their own Kingdom or at least phylum. In the 90s he also gave us the “Garden of Ediacara”-Hypothesis, which as far as I am aware is to this day still the most accepted explanation for how these organisms lived. It goes like this: Back in the Ediacaran there were no known predatory animals yet and no worm-like creatures that caused significant bio-turbation, meaning the majority of the sea-floor was covered in thick bacterial mats and lawns of algal scum. While some like Dickinsonia may have partially fed on the mats, the majority of the organisms seem to have lived sessile, autotrophic lifes growing on top of these mats. Likely through endosymbiosis with bacteria, the vendobionts were probably capable of gaining energy through absorbing chemicals (chemotrophy) or free nutrients (osmotrophy) from the water. Some species living in shallow water may have even been capable of photosynthesis, similar to corals. This would greatly explain the many plant-like shapes, fractal growth and lack of gut-systems in most of these organisms. McMenamin even found chemical evidence for photosynthesis in a fossil of Pteridinium (McMenamin 1998, p. 144). In the 1998 book The Garden of Ediacara he however also goes a bit off the deep end and proposes that while “proarticulates” like Spriggina and Parvancorina were such photosynthetic vendobionts, their superficial resemblance to arthropods came about because they, independently of metazoans, evolved a nervous system and a brain. He proposes that their cephalons were capable of chemical- and perhaps light-reception and would have helped the organism adapt to light-changes and environmental threats, like reorienting their bodies towards the wandering sun or hardening up their cuticles in case a storm arrived. To overly simplify it: he thought these creatures basically behaved like plants with brains and speculated that if they would not have died out, they may have been capable of evolving more complex nervous systems and even intelligence (McMenamin 1998, p. 241-242). That is pretty radical, considering the only evidence he was going off of was the outer shape of this organ. His only argument for why these cephalons were not just holdfasts was that they were simply too large for that function.
Fig. 8: McMenamin’s model to how the Ediacaran organisms are related to each other and to animals. His idea was that, instead of developing an internal cavity like animals, the vendobionts developed from conjoined cell-families growing and spreading away from the embryo. The different shapes and growth-modes of the Ediacarans then evolved by iterating these cell-families along bi- or unipolar directions. They thus were incapable of evolving internal organs and instead had to rely on their outer surface for efficient nutrient-exchange.

Classification-wise, methods and finds have considerably changed since then. A geobiological study from 2018 has shown that fossils of Dickinsonia contained cholesteroids, a tell-tale biological marker for animals. This was further complimented by a cladistic analysis from the same year, which found the Petalonamae (Dickinsonia, Swartpuntia, Pteridinium and related forms) to be animals that are more closely related to Eumetazoa than to sponges, though still more basal than even jellyfish. Interestingly, they included the Cambrian organism Stromatoveris in their Petalonamae. Stromatoveris has long been thought to be an ancestor to modern Ctenophora, better known as comb-jellies. These very alien-looking creatures were once thought to be the sister-group of cnidarians, however genetic analyses have found them to be far more basal, diverging from the rest of the animal kingdom only slightly after sponges… just like Petalonamae apparently. Reading the paper I was therefore very surprised that the authors do not at one point make the connection and consider the possibility that comb-jellies are actually descendants of the Ediacaran biota. The fact that the nervous system of ctenophores has a completely different biochemistry to that of all other animals lines up scarily well with McMenamin’s idea that vendobionts independently evolved a nervous system. Curiously many ctenophores can live in symbiosis with algae and they move with cilia, something Dickinsonia was likely capable of. 
Fig. 9: A less stylized reconstruction of Ediacaran biota. This time Spriggina (6) is shown as a sessile organism similar to Charnia, with the cephalon being a holdfast. Here's an interesting thought for paleoartists: If Ediacarans are indeed related to modern comb jellies, as I suspect, there is a slight possibiliy that some of these forms may have actually been bioluminescent. 

Other affinities for the Ediacarans have also been proposed in recent years, such as a relationship to the living microscopic organism Trichoplax adherens. If all these enigmatic forms really went extinct at the beginning of the Cambrian has also been questioned. The hexagonal fossil Vendoconularia, discovered in 2002, has been interpreted as the transitional form between the triradial Trilobozoa (of which Tribrachidium is a part of) and the quadradial conulariids of the Paleozoic. In the next part we will also look at Protonympha, which might be a Devonian descendant of Spriggina. Regardless of these possible survivors, fact is that the majority of Ediacaran organisms had gone extinct by the end of the period, for reasons that are just as mysterious as the victims. According to the Garden of Ediacara-Hypothesis, the vendobionts likely died out when at the beginning of the Cambrian true animals began to diversify, with burrowing organisms destroying the bacterial lawns that the Ediacarans depended on and the first predatory animals eating up the defenceless vendobionts. There are some problems with this, like the fact that the Ediacarans were worldwide in distribution and had likely already been living with other metazoans for quite some time. Therefore they likely would have had enough time to develop some forms of defence. Geological evidence is amounting that at the end of the Ediacaran a genuine mass-extinction occured, brought about by an environmental catastrophe like volcanism or perhaps even a cosmic impact, rather than an ecological turnover. How might life look like today if this catastrophe had not happened? Stephen Jay Gould writes:

"...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).

Now there's a thought for spec-evo enthusiasts. There is so much more to say about the Ediacaran biota and as they are a personal favorite of mine I definitely plan to write more about them in the future.

Wonderful Burgess Life

Ever since Darwin, the lifeforms of the Cambrian have been a particular challenge to interpret. At first the main problem was their seemingly sudden, fully formed appearance in the fossil record without any apparent ancestors. This problem was further compounded when earlier organisms, such as the Ediacaran biota, were eventually found but they barely had any resemblance to the Cambrian organisms. Furthermore, the Ediacaran biota is separated from the Cambrian fauna by an apparent mass extinction and the diversification event of the equally mysterious Small Shelly Fauna. At the very least however, we know some organisms from the Ediacaran that probably were ancestors or close relatives to later Cambrian animals. The organism Kimberella for example is often interpreted as an early mollusc (though its purported “radula” does not quite fit the bill) and fossil embryos of probable bilaterian animals are known from the Early Ediacaran Doushantuo Formation. It now seems that while there probably was a genuine diversification-event going on right at the beginning of the Cambrian, likely in response to the extinction of the Ediacarans, the true origin of most animal phyla likely is a lot older and we just have trouble finding these proto-animals because they were very small and/or lacked easily petrifiable skeletons.
Fig. 10: The Burgess Shale, as originally reconstructed by Charles Knight in the 1940s. Those shrimp-like animals in the back later turned out to actually be the mouth-parts of the much larger organism Anomalocaris.

Identifying what the Cambrian animals were was not actually a problem until the late twentieth century. Until then, most of the Cambrian fossils were simply interpreted as primitive forms of still existing or well-known animal-groups. When Charles Doolittle Walcott discovered the now famous Burgess Shale in 1909, he simply shoehorned all the enigmatic forms into early orders of trilobites and crustaceans. Beginning in the 70s however, a reassessment was undergone on the Burgess fossils by Harry Whittington, Derek Briggs and Simon Conway Morris. They found that the anatomy of many of the creatures found in the Burgess Shale was so bizarre, unique and disparate that most could not be classified into any known animal-groups. While the absolute number of species was of course lower, the disparity in basic body-types found in the small Burgess phyllopod-beds (which is smaller in area than your average houseblock) exceeded the anatomical range of all animals on Earth found today. By then current taxonomic laws, many of the fossils would have required to be grouped into their own phyla. Anecdotally, when Conway Morris found the fossil of Odontogriphus in one of Walcott’s drawers he exclaimed: “Oh fuck, another new phylum!” (Gould 1989, p. 143).
Fig. 11: A collection of Cambrian oddballs as originally drawn by Marianne Collins for Gould’s Wonderful Life. Upper row, left to right: Opabinia, Nectocaris and the adorable Odaraia. Lower row, left to right: Dinomischus, Marrella and Yohoia. Some of these reconstructions, like the one of Nectocaris, have aged considerably while others hold up surprisingly well.
By now many of these creatures have become famous staples in paleontological literature and media. The most famous is Anomalocaris, which was originally thought to have been three different creatures until Whittington and his students put the pieces together. It was a roughly one meter long predatory animal with a segmented body and compound eyes like an arthropod, however it lacked segmented legs and instead swam by undulating a set of gill-flippers. Its only limbs were a pair of flexible fanged arms at the front of the snout. Its mouth was circular and resembled a toothed pineapple-slice. Alongside it lived the previously mentioned Odontogriphus, which looked like a living band-aid and bore a strange tentaculate mouth. There were Nectocaris and the vetulicolians, which combined elements of arthropods, chordates and cephalopods. There was Amiskwia, which resembled a nudibranch with a whale’s tail-fluke and two fins. There was Wiwaxia, resembling a slug covered in armor-scales and long spikes, possessing a strange, serrated beak. The organism Hallucigenia was so weird that Conway Morris accidentally reconstructed it with the belly-side up, thinking the spikes on the back were legs and the two-clawed legs on the underside were multiple beaked mouths attached to necks. Taking the cake is probably Opabinia, looking like the result of a drunken playthrough of the videogame Spore. Its rough body-shape was similar to Anomalocaris, however its head bore five eyes and at the front grew a long, flexible proboscis which ended in a jaw-like pincer. Like an elephant it used this organ to transport food to the mouth, which was located underneath the head. When Whittington presented his reconstruction of the creature, the audience allegedly laughed. 
Fig. 12: Left: The original reconstruction of Hallucigenia by Conway Morris with the dorsal spines as legs and the true legs as mouths growing out of the back. Right: A reconstruction by Andrey Atuchin from 2016, turning the animal on the proper side. Nowadays Hallucigenia is classified as a distant relative to modern velvet worms. The bulbous "head" that Morris originally identified later turned out to be leaked gut-contents. His Hallucigenia was a literal shit-head.

Opabinia was in fact so strange and so important for our understanding of evolutionary history that Stephen Jay Gould wanted to name a book after it. However, that book eventually came to be named Wonderful Life (named in homage to the 1946 movie It’s a Wonderful Life), released in 1989. In his book the evolutionary biologist took the finds from Whittington’s team and interpreted them into a radical new proposal: Nothing in evolution was set in stone and there is no strive towards progress. While many of the Cambrian animals were unique, they also lived alongside creatures that were clearly members of known phyla. Alongside Anomalocaris and Opabinia could also be found forms like Sanctacaris, an ancestor to chelicerate arthropods like spiders and scorpions, Aysheaia, a precursor to our modern velvet worms, Naraoia, an early trilobite, or Canadaspis, a genuine crustacean. Many of the Cambrian oddballs were also not primitive or simple, but showed fascinatingly sophisticated adaptations and specializations. The compound-eyes of Anomalocaris briggsi for example had a resolution 30 times higher than that of any trilobite and would have only been exceeded by those of modern dragonflies. Based on all this, Gould argued that there was nothing that made these enigmatic forms inherently inferior to the phyla known by us that existed at the same time and the reason why they went extinct and our phyla lived was by pure coincidence. If we were to rewind the tape of life (this now so famous phrase), changed just a few parameters and replayed it again, a descendant of Opabinia might be sitting at its laptop now and writing this blog instead of me. This he further exemplified by the fact that the only known Cambrian chordate at the time was Pikaia, a small, unassuming lancelet-like creature (we now know of a lot more Cambrian chordates, though most follow the same pattern as Pikaia in being rather unassuming, as vertebrates would not become major players until the Silurian). An extraterrestrial visitor coming to Earth 500 million years ago looking at the oceanic life would have never been able to predict that out of such an unimportant worm-like organism would one day arise the multitude of fish, the tetrapods, dinosaurs, birds, mammals and humans. Gould identified many other such chance-events in the history of life on Earth where things could have gone radically differently, such as the extinction of the dinosaurs, and concluded that history in general, including the history of life, is built on contingency and inherently unpredictable. The fact that you and me exist to philosophize about this is a gift of luck, rather than a guaranteed destiny.
Fig. 13: A reconstruction of Opabinia from 2016, showing the organism using its proboscis to probe the sediment. The identity of the spike-like structures you can see on the belly is disputed. Some interpret these as remnants of jointed legs, placing this organism closer to true arthropods, while others see these as unrelated structures.
Gould’s interpretation, probably causing many people a form of existential crisis, met with harsh opposition, most surprisingly from Simon Conway Morris. Even though Morris was originally the one who reclassified almost every Burges Shale fossil he could find into a new phylum and was a good friend of Gould, after the release of Wonderful Life he did a complete 180 degree turn and released The Crucible of Creation in 1998 as a direct response to Gould’s book. He now did the same thing as Walcott, shoehorning all the Cambrian oddballs as primitive forms into previously known phyla and argued that through the phenomenon of convergent evolution, whereby two largely unrelated organisms evolve the same body-shape through the same environmental pressures, was strong enough to make evolution predictable. He argued that there was something inherently goal-oriented in evolution and that even if the Cambrian ended differently, even if the dinosaurs never went extinct, evolution would in the end still produce humans or at the very least something very human-like (which is why he seems to have really liked Dale Russell's concept of the Dinosauroid). His argument ignored the fact that even if these animals were members of known phyla, their anatomy was still radically different enough that if they survived for longer, they would have most likely evolved into forms significantly different from those of their relatives. His appeal to convergent evolution also bears the problem that the ability to evolve into a certain shape is something inherited and therefore also dependent on contingency (something often forgotten by people who indulge into speculative evolution). The process of carcinisation in crustaceans is a good example of that. While at first it is remarkable that the placental wolves and the marsupial thylacines evolved into a very similar dog-like shape, it becomes less special once one considers that both descend from oppssum- or racoon-like mammals from which it is not difficult to evolve into a dog-shape. Animals such as dromaeosaurs or notosuchians likely occupied the same niches as canids or cats do today, but looked very different from them due to their own history. 
Fig. 14: Instead of the popular view of evolution as a widening cone of increasing complexity and diversity, Gould’s view, which he formed on the basis of the Burgess Shale, resembled a Christmas tree. After a rapid diversification event in which a multitude of adaptations are experimented with, a decimation event occurs that eliminates most lineages for unknown, seemingly random reasons. The survivors are left with a limited selection of body-types from which to evolve from.
The feud between Gould and Morris would become known as the Contingency vs. Convergency debate. How Morris’ change of heart came about has been a question for the ages, with some speculating that it resulted from Morris being embarrassed by his misinterpretation of Hallucigenia, which became widely known through Gould’s book. Personally I am firmly in the camp that it was because of Morris’ religious views. Morris is a devout Christian and has confirmed several times that he believes in a form of theistic evolution. Similarly to someone like Dale Russell, he seems to believe in an inherent, probably god-made plan in the evolutionary process that must inevitably result in humanoid organisms, making Gould’s arguments for purely contingent evolution very disturbing to him. It is likely for reasons like this why Morris is currently the only evolutionary biologist who still believes in orthogenesis and the Rare Earth Hypothesis. The latter - a hypothesis that proposes that complex life outside of Earth is exceedingly rare and that our planet is seemingly specially made to evolve humans on it - was first formed by Guillermo Gonzalez, an astronomer who believes in intelligent design (Darling 2001, p. 113-114.) and now works for the infamous Discovery Institute, a pseudoscientific organisation aimed at undermining evolutionary science through the use of disinformation campaigns. Please do not read this as an attack on Morris or on scientists who believe in God. I have no hard feelings towards the man and think he has done some incredibly good work as a paleontologist, just like Robert Bakker (a Pentecostal Christian) or Peter Dodson (a Catholic) have done (I myself am a muslim, though with slightly agnostic leanings). I think it is also a bit too harsh to call Morris a believer in intelligent design/creationism, he is currently working with the more amicable John Temple Foundation, which has rejected intelligent design several times. I just wanted to point out that there seems to be an apparent bias in how Morris sees evolutionary history and how he interprets fossils, as he seemingly works backwards from a pre-made conclusion. I should also mention my own bias, which is that I am a self-described “gouldist” (a bit similar to how Douglas Adams called himself a “dawkinsist”).

Fig. 15: Upper row, left to right: Anomalocaris, Leanchoilia and Wiwaxia. Lower row, left to right: The adorable Sarctocercus, Amiskwia, Odontogriphus.
Let’s go back to Cambrian classification, shall we? A lot has changed since the Contingency vs. Convergency debate started, the most significant change being that phylogenetics and cladistics have made rigid ranked taxonomy superfluous. Instead of creating new phyla or shoehorning them into existing ones, the Cambrian oddballs are now able to be viewed as their own unique forms while still fitting them into existing branches. For example, the Dinocaridida, the clade that contains Anomalocaris and Opabinia, do not have enough traits to be considered proper arthropods, however they are more closely related to arthropods than to anything else, hence why they are nowadays referred to as stem-arthropods. Together with other groups, both comprise the larger clade Panarthropoda. The usage of stem-groups and crown-groups is helpful in understanding relationships, though it is arguably a tad bit subjective (from a platypus’ point of view, all other mammals, including humans, could be called stem-monotremes, while both Pteranodon and Diplodocus are arguably stem-birds). I am of the firm belief that if velvet worms (Onychophora), currently considered their own phylum, were not alive today we would also just call them stem-arthropods, while if Dinocaridida were to have survived into historic times, Linnaeus would have given them their own phylum. Many Cambrian forms, such as Dinomischus, also still remain so unclassifiable that they cannot even be grouped into stem-groups. Compared to Morris’, Gould’s argument still largely holds up in that these creatures were distinct and specialized and would have gone on to produce radically different forms if they survived beyond the Cambrian. In fact, they did. At fossil sites such as the Fezouta Formation in Morocco it was discovered that some of these bizarre Cambrian forms had survived into the Ordovician (and in parts even into the Devonian). Instead of converging on their arthropod-relatives, as someone like Morris may have predicted, the descendants of Anomalocaris and its kin evolved into a radically different direction. The Moroccan dinocaridid Aegirocassis was over two meters long and used the two head-appendages of its ancestors to now filter-feed on plankton. This essentially made it the Ordovician equivalent of a whale or whale-shark. 
Fig. 16: A 3D reconstruction of the Ordovician filter-feeder Aegirocassis benmoulai. Why its lineage did not live beyond the Ordovician is hard to say, as there were few animals at the time that contended for the same niche. Its kind likely just fell victim to the End-Ordovician mass extinction event, but the same was true for about 80% of species at the time, meaning this was hardly its own fault. At least one dinocaridid, Schinderhannes bartelsi is known from as recently as the Devonian.
Outside of the fossil-record, the Contingency vs. Convergency debate has also been tested further. Many studies have now been done on fast-evolving organisms, such as guppies, rodents, lizards and bacteria to see if any predictions or patterns can be made out. What they have shown is that in the short-term, evolution can indeed be predictable, however the longer the experiments wore on the more they showed that over longer periods rather random chance-events do indeed happen that significantly alter the trajectory of a population compared to its test-group. In short: Be thankful for your existence, do not take things for granted and have fun imagining all the possible alternate worlds. For more information on the current status of the debate, I greatly recommend the book Improbable Destinies by Jonathan B. Losos.

The Conodonts

From the Cambrian tangent (which you probably did not predict from the post’s title *wink* *wink*) we come to a shorter example of a fossil enigma. It is short because it is now generally seen as solved. Conodonts are microfossils resembling comb-shaped, teeth-like structures that can be found in rocks from Cambrian to Triassic times. They were known since 1856 and are widely used as index-fossils to indicate the start and end of periods, however, until the latter part of the twentieth century it was a great mystery to whom these structures actually belonged to. At one point or another they were assigned to annelids, molluscs, nemerteans, chaetognaths and even plants. Because of vaguely similar structures found in its fossil, Simon Conway Morris once suggested that Odontogriphus may have been the conodont animal. Much of this speculation finally ended in 1983 when the fossil of Clydagnathus was found in the Carboniferous Granton Shrimp Beds of Edinburgh. It was preserved well enough to show the soft-tissue anatomy of the animal and it told us that the bearer of the conodont-teeth was, in fact, a chordate, one of our boys. Conodonts were apparently jawless fish with large eyes, superficially similar to the modern lampreys, that used their wide arrays of comb-like tooth structures like a Swiss army knife, either to shred away at meat or to filter-feed. Down below in the sources I linked an animation made by my own university’s paleontology department that shows how these teeth may have moved in the mouth. 
Fig. 17: Reconstruction of a conodont, with a zoom-in on the bizarrely elaborate jaw-apparatus. 
That’s it for now, I originally intended this to be just one post, but there are many more fossil enigmas to talk about. Next time we will look at more singular examples, such as Paleodictyon, Typhloesus, the Tully Monster and Chilesaurus

Related Posts:

Literary Sources:
  • Benton, Michael James: Vertebrate Paleontology, 1990 (3. Edition from 2005, German translation).
  • Darling, David: Life Everywhere. The Maverick Science of Astrobiology, New York 2001.
  • Godfrey-Smith, Peter: Other Minds. The Octopus and The Evolution of Intelligent Life, London 2017.
  • Gould, Stephen Jay: Wonderful Life. The Burgess Shale and the Nature of History, New York 1989.
  • Jenkins, Martin/Baker-Smith, Grahame: Life. The First Four Billion Years, London 2019.
  • Knight, Charles Robert: Life through the Ages, New York 1946 (Commemorative Edition).
  • Knoll, Andrew: Life on a Young Planet. The First Three Billion Years of Evolution on Earth, New Jersey 2003 (Second Paperback Edition).
  • Losos, Jonathan: Improbable Destinies. Fate, Chance, and the Future of Evolution, New York 2017.
  • McMenamin, Mark/Dianna: Hypersea. Life on Land, New York 1994.
  • McMenamin, Mark: The Garden of Ediacara. Discovering the First Complex Life, New York 1998.
  • Palmer, Douglas/Barrett, Peter: Evolution, London 2009.
  • White, Steve: Dinosaur Art II. The Cutting Edge of Paleoart, London 2017.
Online Sources:
Image Sources :

  • Fig. 1: Wikimedia
  • Fig. 2: Wikimedia
  • Fig. 3: Wikimedia
  • Fig. 4: El Albani et al. 2014.
  • Fig. 5: Wikimedia
  • Fig. 6: Jenkins 2019, p. 14-15.
  • Fig. 7 Up: Wikimedia
  • Fig. 7 Left: Wikimedia
  • Fig. 7 Right: Wikimedia
  • Fig. 8: McMenamin 1998, p. 234.
  • Fig. 9: Palmer 200, p. 42-43.
  • Fig. 10: Knight 1946, p. 3.
  • Fig. 11: Gould 1989, p. 126, 146, 175, 151, 114, 123.
  • Fig. 12 Left: Gould 1989, p. 155.
  • Fig. 12 Right: White 2017, p. 78.
  • Fig. 13: White 2017, p. 79.
  • Fig. 14 Left: Gould, p. 46.
  • Fig. 14 Right: Gould, p. 216.
  • Fig. 15: Gould 1989, p. 203, 185, 192, 180, 152, 148.
  • Fig. 16: Wikimedia
  • Fig. 17: Benton 1990, p. 59.


  1. Just wanted to say I love your blog and keep up the good work! I'm interested in paleontology but often find the discussions too technical to follow, so I like how accessible you make things while still providing detailed information. The Ediacaran biota are some of my favorites too. I would love to see your future articles about them, and of course the second part of this series too!