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Unsolved Paleo Mysteries Month #14 – Thousands of Tiny Butts

Gluteus minimus are small fossils, sometimes referred to as “horse collars”, up to 11mm across (0.4″). Always slightly asymmetrical in the same direction, with growth lines on one surface and a solid internal structure, they’ve been found in large numbers from the Late Devonian of Iowa, USA (~385 mya).

Although they were first discovered in 1902, they weren’t formally described until 1975 – and ended up being named after a butt muscle.

What sort of creature they represent, however, is completely unknown. They’ve been suggested to be fish scales, fish teeth, parts of various molluscs, or brachiopods. And, more recently, to perhaps be otoliths.

They still can’t be confidently placed in any one phylum.

(Due to scarce reference images of G. minimus, this post’s illustration is directly based on the figure of the holotype specimen in the original description paper.)

Unsolved Paleo Mysteries Month #13 – The Case of the Absent Archaeopteryx

One of the most famous of all fossil organisms, and a classic example of a transitional form, Archaeopteryx is currently known from 12 body fossil specimens.

Except one of them is missing.

The Maxberg specimen was part of the private collection of Eduard Opitsch, the owner of the Bavarian quarry where it was originally discovered in 1956. Despite being partially disintegrated, and missing its head and tail, it was still an immensely important discovery – at the time, it was only the third recognized Archaeopteryx ever found.

After briefly attempting to sell the new Archaeopteryx, Opitsch eventually allowed it to be held at the local Maxberg Museum. In 1974 he permitted casts to be made from it – but then suddenly removed it from public display and refused all further requests to access or study it.

(This may have been a reaction to the 1973 announcement of the more complete Eichstätt specimen. Opitsch, who was described as having “a difficult personality”, became increasingly defensive about the fossil, seeming to feel this new discovery was getting more attention and was deliberately devaluing his own.)

From then on the Maxberg specimen was lost to science.

When Opitsch died in 1991 his heir attempted to locate the fossil – it was rumored to be kept under his bed – but it was nowhere to be found. There’s some speculation that he was buried with it, literally taking his prized Archaeopteryx to the grave as a final act of spite. Another possibility is that it was stolen and sold in secret, perhaps to this day hidden away in a wealthy owner’s private collection.

It’s been missing for over 25 years, but there’s still lingering hope that the missing Maxberg specimen will one day resurface.

For now, though, all we have left are a few casts, photographs, and x-rays.

Unsolved Paleo Mysteries Month #12 – Muddled Mosasaurs

Numerous groups of reptiles have “returned to the water” and become aquatic over the last three hundred million years, but tracing their direct ancestry can be surprisingly difficult. Highly modified and specialized anatomy, lack of transitional forms, and similar features convergently evolving multiple times can all obscure relationships, making it hard to properly classify them.

We’re only just starting to figure out the true origin of turtles (they’re probably archosauromorphs), and they’re a marine reptile group with living members.

Some of the completely extinct ones are even more uncertain. For example: mosasaurs. (Represented here by the eponymous Mosasaurus.)

While some semi-aquatic early mosasaurs are known, and they seem to be closely related to aigialosaurs and dolichosaurs, their exact placement within the squamates is a lot less clear. Traditionally they were regarded as the sister group to snakes, but some studies have found them to be closer to monitor lizards instead, and others have even placed them as much more basal scleroglossans. Their classification in phylogenetic analyses is “highly unstable”, changing depending on what other reptile groups are included, so there’s no real current consensus.

(And even if they are most closely related to snakes, that doesn’t necessarily help much – the exact origin and evolution of snakes is still very poorly known, too!)

Unsolved Paleo Mysteries Month #11 – Strange Snoots: Equid Edition

Horse evolution is often represented as a simple progression from Eohippus* to modern Equus, but it was actually a lot more complicated than that – and some ancient horses had some very odd things going with their snouts…

(* For a long time Eohippus was considered synonymous with Hyracotherium, but more recently has been split back off as its own genus again.)

An illustration of the skull of an extinct horse, showing the unusually large holes in the bones in front of the eye sockets. Below is a reconstruction of the horse's head in life. — *Pliohippus sp.* skull and head reconstruction

Pliohippus, from the Middle Miocene of North America (~15-12 mya), and several of its other close relatives had especially large, deep recesses in their skulls, usually referred to as “preorbital fossae”.

And the purpose of these holes is still unknown. Although superficially similar depressions are seen in various other perissodactyl groups, they vary in position and structure and probably weren’t all homologous.

Ideas have included resonating chambers, some sort of glands, inflatable sacs, or attachment sites for complex lip musculature.

An illustration of the skull of an extinct horse, showing the unusually large nasal cavity. Below is a reconstruction of the horse's head in life. — *Hippidion sp.* skull and head reconstruction

Meanwhile Hippidion from the Pleistocene of South America (2 million – 10,000 years ago) had especially long and domed nasal bones. This must have supported an enormous nasal area – possibly giving it a saiga-like air-conditioning system, a highly sensitive sense of smell, or perhaps even some sort of prehensile proboscis-like snout.

Unless we find some exceptional soft-tissue preservation, the facial anatomy of these equids is going to remain enigmatic.

Unsolved Paleo Mysteries Month #10 – Ambiguous Amiskwia

Amiskwia was a tiny soft-bodied creature from the Middle Cambrian, known from a fairly small number of fossils – about 18 specimens from the Burgess Shale in Canada (505 mya) and an additional one from the Maotianshan Shales in China (515 mya).

Despite only measuring about 2.5cm long (1”), it was one of the larger animals alive at the time. Its body features a head with two tentacles and a small mouth, a pair of stubby fins, and a flattened paddle-shaped tail, suggesting it was an active swimmer. Its internal anatomy has been well-preserved in some specimens, revealing a brain, gut, and traces of what may be blood vessels and a nerve cord.

But we don’t know what type of animal it is. At all.

It was initially thought to be an early arrow worm. However, fossils of Cambrian representatives of that group have since been found, and Amiskwia lacks their characteristic spines and teeth. A relationship to ribbon worms or molluscs has also been suggested, but these hypotheses have the same problems with missing key features.

So, for now, Amiskwia remains one of the “weird wonders” of the Cambrian Explosion with no obvious affinities to any other known group.

[EDIT: As of 2019, Amiskwia seems to have finally been identified as a gnathiferan!]

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.]