 |
| 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 Lagerstätten. 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 Templeton 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”).
As a bit of an update to this paragraph, it is interesting to note that Morris' antics have apparently become more... questionable over the years. In his book From Extraterrestrials to Animal Minds, published in 2022 by the aforementioned Templeton Foundation, he for example proposes that UFOs are metaphysical objects from another reality bleeding over into our own. Make of this Strieberian view what you will, but there is a certain humor to a researcher who almost wholly rejects the existence of intelligent alien life coming up with an explanation for unidentified flying objects that is even more esoteric.
 |
| 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.
If you liked this and other articles, please consider supporting me on Patreon. I am thankful for any amount, even if it is just 1$, as it will help me at dedicating more time to this blog and related projects. Patrons also gain early access to the draft-versions of these posts.
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.
- Morris, Simon Conway: From Extraterrestrials to Animal Minds. Six Myths of Evolution, West Conshohocken, 2022.
- Palmer, Douglas/Barrett, Peter: Evolution, London 2009.
- White, Steve: Dinosaur Art II. The Cutting Edge of Paleoart, London 2017.
Papers:
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.
