Tag: diapsida

Almost-Living Fossils Month #05 – Cryptic Choristoderes

The choristoderes were a group of aquatic reptiles that mostly inhabited freshwater environments. Known mainly from North America, Europe, and Asia, they first appeared in the fossil record in the Late Triassic (~205 mya) – although their lineage could potentially go back further than that – and they varied in appearance from large long-snouted croc-like creatures to more lizard-like and miniature plesiosaur-like forms.

Many them were fully aquatic and spent their entire lives in the water, with some developing the ability to give live birth and others returning to land only to lay eggs (with only females having well-developed enough limbs to be able to haul themselves out onto shore). In some places their fossils are incredibly common, with every life stage represented from babies to adults (even one with two heads!), and yet despite having such detailed knowledge of their lives we still don’t know exactly what type of reptile they actually were.

Their evolutionary origins and relationships are very unclear, with the only real certainly being that they’re at least diapsids. They’re often classified as either archosauromorphs (closer related to crocodilians and dinosaurs/birds) or as lepidosauromorphs (closer related to lizards), but they could also be a much earlier separate branch of the reptile family tree.

Some of the large crocodilian-like neochoristoderes survived into the Cenozoic and initially did quite well for themselves – even outcompeting actual crocodilians in the northern continents for a while – but then they seem to have fallen victim to the cooling and drying climate of the Eocene-Oligocene extinction about 33 million years ago.

But that wasn’t the end of the choristodere lineage just yet.

A small number of fossils of a little choristodere named Lazarussuchus have been found in a few different places around Europe, with the youngest specimens dating to as recently as the Early Miocene (~20-16 mya). Surprisingly it wasn’t closely related to the neochoristoderes at all, but instead seems to have been part of a much older and more “primitive” branch of the choristodere family tree that must have been surviving since at least the mid-Jurassic with very little presence in the fossil record.

At about 30cm long (1′) it was less aquatic than most other choristoderes, with large claws that would have given it good traction on land and a more generalized lizard-like body plan. One specimen even preserves soft tissue impressions, showing that its toes lacked webbing and that it had a low crest running along its tail.

Its lack of specialization may have been the reason for its longer survival, being able to adapt to a wider variety of environments compared to its more water-reliant cousins.

It’s unclear exactly how much closer to present day these rare last choristoderes survived. If they managed to make it through the mid-Miocene extinction then they might potentially have persisted until the onset of the Pleistocene Ice Age 2.5 million years ago – but their fossils are scarce enough that we’ll probably never know for certain.

Xinpusaurus

Thalattosaurs were a weird and rather mysterious group of Triassic marine reptiles. It’s not clear where they actually fit on the reptile evolutionary tree (we know they’re diapsids, but nobody can really agree on anything more definite than that), and they had some very strange skulls that seem to have been highly specialized for something, although their actual function is still unknown.

Xinpusaurus kohi here is known from the Late Triassic of China (~232-221 mya). About 1.3m long (4′3″), with half of that being its paddle-like tail, it had an elongated upper jaw that formed a protruding pointed spear-shaped snout.

It’s not clear whether this odd snoot was an adaptation for hunting similar to the long bills of swordfish – there’s quite a bit of variation in length and shape between different individual specimens – or if it was serving some other purpose like the sexually dimorphic noses of some modern lizards.

Ascendonanus

The recently-described Ascendonanus nestleri from the Early Permian of Germany (~290 mya). This 40cm long (1′4″) animal was a member of a group called varanopids – which may have been an early branch of the synapsid lineage and distantly related to modern mammals*.

Known from several near-complete fossils that include rare soft tissue impressions, it’s the first varanopid to show preserved skin details – revealing a pattern of very lizard-like rectangular scales. If it is a synapsid this is a pretty big deal, since early synapsids were previously thought to have had scale-less leathery skin.

It also had unusual mosaic-like patches of tiny osteoderms above its eyes, a feature previously known only in some temnospondyl amphibians. Whether this was the result of convergent evolution or the trait actually being ancestral to most tetrapods is unclear.

Its slender body, long digits, and highly curved claws indicate it was an agile climber. It probably mainly lived up in the treetops, feeding on insects, making it one of the earliest known tetrapods specialized for an arboreal lifestyle.

(*Maybe. There’s apparently an upcoming study that suggests varanopids might actually be sauropsids instead.)

Sclerocormus

Sclerocormus parviceps, an unusual ichthyosauriform from the Early Triassic of China (~248 mya).

Its short toothless snout suggests it was a suction feeder, using water pressure differences to pull small soft-bodied prey straight into its mouth like a syringe.  Along with a heavily built body similar to those of hupehsuchians, and a very long tail that made up over half of its 1.6m length (5′3″), it was probably a fairly slow swimmer living in shallow coastal waters.

It was a close relative of Cartorhynchus, and may have been similarly capable of hauling itself onto land like a modern pinniped.

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.

Spinoaequalis

Spinoaequalis schultzei, an early diapsid reptile from the late Carboniferous of Kansas, USA (~300 mya). Measuring about 30cm long (1′), it appears to have been semi-aquatic, with limbs like a typical terrestrial reptile but also possessing a long flexible paddle-like tail to aid in swimming – making it one of the earliest amniotes to experiment with “returning to the water”.

Drepanosaurus

One of my favorite palaeontological discoveries of 2016 also involved one of my favorite extinct animals: the odd little “monkey lizard” Drepanosaurus.

Living during the Late Triassic of Europe and North America (~218-212 mya), this reptile measured about 50cm long (20″). It was already known to have some particularly weird anatomy, featuring a humped back, grasping feet, a prehensile tail ending in vertebrae modified into a pseudo-claw, enormous claws on the second finger on each hand, and a bizarre arrangement of its forearm bones.

In 2016, new fossils gave us a better look at those arm bones, and they turned out to be even stranger than previously thought. As well as the radius and ulna being unusual, several wrist bones were also heavily modified and elongated, creating an arm setup unique among all known tetrapods. This may have been a specialization for “hook-and-pull” digging, ripping up tree bark to get at burrowing insects – but now the real mystery is why no other tetrapods have ever managed to modify their limb bones to this sort of extreme.