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Unsolved Paleo Mysteries Month #09 – The Unknown Ugly Crocodile

In 1923, paleontologist Charles Camp recorded the discovery of an unusual-looking skull from the Late Triassic (~220 mya) of Arizona, USA. He made a field sketch before attempting to remove the fossil from the surrounding rock – only for it to completely fall apart, leaving just a couple of intact fragments covered in odd bony knobs.

Camp’s original drawing of the *Acallosuchus* fossil, and the remaining broken pieces. Scale bars equal 5cm for H and 1cm for I. [*source*]

The fossils were stored away at the University of California Museum of Paleontology and were left forgotten for the next sixty years. Eventually they were rediscovered by Robert Long and Phillip Murry, and described in 1989 with the name Acallosuchus rectori (meaning “Rector’s ugly crocodile”).

But what is it?

At first it was classified as a proterochampsid, a group of archosauriforms known from South America. But this classification was based on some additional skeletal remains that were thought to belong to it, which were later split off and named as the semi-aquatic Vancleavea instead. It’s also been compared to Doswellia and the pseudosuchian Revueltosaurus.

The material is just far too fragmentary to make a confident identification, and the original sketch is anatomically unclear. At best we can say that Acallosuchus was an “indeterminate diapsid” – some sort of reptile, but for now nobody knows what.

I’ve restored it here based mainly on proterochampsids, but any interpretation of this animal is going to be highly speculative until more fossil material is found.

Unsolved Paleo Mysteries Month #08 – Everything Dies Except Lystrosaurus

The extinction event that wiped out the non-avian dinosaurs is probably the most “famous” mass extinction, but it wasn’t the worst one in Earth’s history. That morbid honor goes to the Permian-Triassic extinction 252 million years ago – also aptly known as the Great Dying.

A truly massive amount of biodiversity was lost in this event, with 96% of marine species and 70% of terrestrial species disappearing. Some marine ecosystems seemed to rebound fairly quickly, but overall it may have taken at least 5-10 million years for anything close to full recovery. Terrestrial vertebrates may even have taken up to 30 million years to regain previous levels of diversity.

And… we’re not sure why it happened.

One of the main potential culprits is the massive eruption of the Siberian Traps – one of the largest known volcanic events on Earth – but other explanations include an asteroid impact, methane-producing microbes, ocean anoxia, the formation of Pangaea, a nearby supernova destroying the ozone layer, and even dark matter.

Or it might have been a result of multiple causes at once, events that wouldn’t have been so severe individually but became disastrous in combination. This is known as the “Murder on the Orient Express Model”: maybe they all did it.

But there’s also a secondary element to today’s mystery. In the aftermath of the Great Dying, a small dicynodont synapsid briefly took over the world. For the first few million years of the Triassic, around 95% of the Earth’s population of terrestrial vertebrates were all Lystrosaurus – no other genus or species of animal has ever dominated to such a degree.

Why did these squat little dog-sized animals survive and thrive when everything else was struggling? They might have been opportunistic generalists able to deal with changing conditions better than other groups, the extinction of most large predators may have allowed their population to explode, or it might simply have been a matter of luck.

We just don’t know.

Unsolved Paleo Mysteries Month #07 – Vexing Vetulicolians

Vetulicolians were a group of small marine animals best described as “problematic”, known from the Early Cambrian (~518-507 mya) of China, Greenland, Canada, and Australia. They had bulbous but streamlined bodies with a mouth opening at the front, no eyes, a thick exoskeleton-like cuticle, and a segmented swimming tail. Most also had five pairs of openings which may have been gill slits.

The image above depicts Vetulicola rectangulata, a 7cm long (2.75″) vetulicolian with a fairly “typical” body plan for the group, and the more unusual 14cm long (5.5″) Skeemella clavula.

Their evolutionary affinities have been uncertain for a long time, and at different points they’ve been classified as arthropods, chordates, kinorhynchs, basal deuterostomes, or even given their own unique phylum. A genus named in 2014, Nesonektris, has been interpreted as having a possible notochord – making vetulicolians chordates, and potentially placing them close to the tunicates – but their exact relationships are still unresolved.

(Skeemella also complicates matters, having some features considered more arthropod-like than other vetulicolians. But since it’s only known from a single specimen, more fossil material is needed to figure out what’s going on with it.)

Unsolved Paleo Mysteries Month #06 – Tricky Trilobites

Trilobites are common and recognizable fossils, found around the world from the Early Cambrian to the Late Permian (521-250 mya), and ranging in size from 1mm to 72cm (0.03″ – 2′4″). They were some of the first organisms on Earth with complex eyes, and some groups also developed ornamentation like spines, horns, and tridents. The image above depicts a particularly elaborate genus known as Dicranurus.

Occasionally fossils have been found showing fine details of trilobite anatomy like antennae, legs, gills, and digestive organs, and we’ve even recently discovered their eggs.

And yet we don’t really know where they came from. Much like the pterosaurs we started the month off with, trilobites appear suddenly in the fossil record with no intermediate or ancestral forms to definitively link them to other groups. We know they were definitely arthropods, but which arthropods they were most closely related to is still uncertain.

They might be related to the chelicerates (arachnids, horseshoe crabs, and eurypterids), or they might be part of the mandibulates (crustaceans, insects, and myriapods). But the exact relationships of these major arthropod groups are still in dispute, too, and phylogenetic results can vary wildly depending on whether trilobites are included in the analysis or not.

It’s probably going to be some time before any sort of consensus is reached.

Unsolved Paleo Mysteries Month #05 – Confusing Chitinozoans

An illustration of an enigmatic microfossil. It's shaped like a round-bottomed flask. — Lagenochitina sp.

An illustration of an enigmatic microfossil. It's shaped like a tall conical flask, with its lower half covered in spines. — Lagenochitina sp.

Chitinozoans are tiny microfossils (50-2000µm in size) commonly found in marine deposits all around the world between the end of the Cambrian and the start of the Carboniferous (~489-358 mya). Often described as “flask-shaped”, they have a variety of external ornamentation, are sometimes found in linked chains, and are important as Paleozoic index fossils.

But we don’t know what sort of organism actually made them.

They’ve been proposed to originate from a wide range of creatures, but currently the main hypothesis seems to be that they were the egg cases of certain marine animals – such as annelid worms, polychaete worms, molluscs, or even conodonts. Or possibly they might be immature graptolites. Or relatives of living ciliates. So far, though, no single identification seems to have gained any widespread acceptance.

[Edit: As of 2020, some exceptionally well-preserved chitinozoan specimens suggest these organisms were actually protists.]

Unsolved Paleo Mysteries Month #04 – Who’s That Theropod?

In the early 1970s an opalized dinosaur leg bone in a South Australian gem shop came to the attention of paleontologist Neville Pledge. The specimen’s owner allowed it to be borrowed and studied, and it was eventually named as Kakuru kujani – Kakuru after the Rainbow Serpent of Australian Aboriginal mythology, and kujani after a variant spelling of the Guyani, the local indigenous people. Later the fossil was auctioned off to another private owner and lost to science for nearly 30 years, until finally being acquired by the South Australian Museum in 2004.

But all we really know about Kakuru is that it was some sort of theropod dinosaur. The 33cm (1′) tibia probably belonged to an animal up to about 2m long (6′6″), living during the Early Cretaceous (~125-112 mya), but any placement in a specific group is almost impossible. Based on particular features of the bone – such as a tall and narrow astragalar process – it’s been proposed to be either an oviraptorosaur or an abelisaur. But more recent examinations have concluded the bone’s preservation is too poor for those features to be confidently identified, and consider Kakuru to be a basal coelurosaur or even just a dubious name for an indeterminate theropod.

It’s all a bit of a mess, really, and more and better material is needed to clear up this mysterious dinosaur’s identity.

I’ve restored Kakuru here in three different ways, to illustrate just how varied the interpretations are – on the left, an early oviraptorosaur; in the middle, a generic coelurosaur; and on the right, an abelisaur.

(Yes, the abelisaur is fluffy. South Australia was within the Antarctic Circle during the Early Cretaceous, and while the climate there wasn’t as cold as it is today it was still chilly enough for some floofy insulation to be useful.)

Unsolved Paleo Mysteries Month #03 – Ammonite Anatomy

Ammonites (or “ammonoids” in technical terms) are one of the most recognizable types of fossil, found in such high abundance that they’re frequently used to precisely date rock layers. They’re absolutely everywhere in fossil collections, and are even made into jewelry.

So we must already know everything we possibly could about them, right?

Except… we really don’t know what their soft parts looked like.

The fossil record for ammonite soft tissue is surprisingly empty for a group that existed for over 300 million years. A possible ink sac and a few organs have been found, but nothing else.

Based on their other cephalopod relatives, they probably had at least ten arms (the two longer tentacles shown on this Collignoniceras are a little speculative), along with a siphon for propulsion – but until we find that elusive exceptional preservation we just don’t know for sure.

[Edit: As of 2022, a few more traces of soft tissue have been found!]

Unsolved Paleo Mysteries Month #02 – The Paleodictyon Problem

Paleodictyon is the name for a net-like pattern found in the marine fossil record, starting in the Late Precambrian/Early Cambrian (~541 mya). Formed from thin tubes in seafloor sediment, each element of the mesh is around 1-3cm in diameter (0.4-1.1″), with entire networks covering areas of up to a square meter (10.7ft²). Some forms also have vertical tubes connecting the mesh to the surface.

And nobody knows what it is.

These patterns have even been found on the modern day seafloor at mid-ocean ridges. Samples have been taken, DNA tests have been performed… and nothing conclusive has yet been found.

Whatever makes these patterns is alive today, but we still don’t know what it is!

There are two main hypotheses about the mysterious identity of the mesh-maker. It might be some sort of small worm-like animal excavating burrows, engineering water flow through the tubes to collect food. Or the whole mesh might be the body imprint of a single creature – either a sponge or a giant foraminiferan.

Hopefully one day somebody will finally catch the Paleodictyon culprit in the act.

Unsolved Paleo Mysteries #01

Welcome to Unsolved Paleo Mysteries Month!

There’s a lot of things we now know about the distant past that seemed impossible only a few decades ago – discovering the colors of fossilized animals, fragments of collagen in dinosaur bones, and even finding near-complete remains of previously enigmatic animals like Deinocheirus.

But there’s also still a lot of things we don’t know. The fossil record is spotty and very incomplete, and even as we answer some questions others remain frustratingly unanswered.

So, every weekday during March I’ll be featuring a different paleontological mystery. Starting with…

Ptransitional Pterosaurs

We don’t really know where pterosaurs came from.

They appeared suddenly in the Late Triassic (~228 mya) with their anatomy already fully adapted for flight, and there are no traces of transitional forms before that point.

We at least know they were members of the archosaurs, and the sister group to dinosaurs, and their closest known relative seems to have been a small hopping creature named Scleromochlus. The complete lack of any other potential ancestors suggests that proto-pterosaurs must have evolved incredibly rapidly in an environment that just didn’t favor fossilizing their tiny fragile remains.

We might get lucky one day and finally find a pterosaur equivalent of Archaeopteryx, but for now all we have are hypothetical ideas of what such animals might have looked like.

Montealtosuchus

Montealtosuchus arrudacamposi, a crocodyliform from the Late Cretaceous of Brazil (~93-83 mya). About 1.8m in length (6′), it had slightly forward-facing eyes, giving it binocular vision, and long upright limbs – adaptations for active hunting on land.

Basically, it would have looked a little like a “crocodile dog”.