Tag: avemetatarsalia

Lewisuchus

Last week I mentioned the one oddball dinosauriform that had crocodilian-like osteoderm armor, so let’s take a look at that one too.

Lewisuchus admixtus lived in what is now northwest Argentina during the late Triassic, around 236-234 million years ago. About 1m long (3’3″), it was an early member of the silesaurids – a group of dinosauriforms that weren’t quite dinosaurs themselves, but were very closely related to the earliest true dinosaurs.

(They’ve also been proposed as instead being early ornithisichians, but we’re not getting into that today.)

Much like its later silesaurid relatives Lewisuchus had a long neck and slender limbs, and was probably mainly quadrupedal, possibly with the ability to briefly run bipedally to escape from threats. Its serrated teeth suggest it was carnivorous, likely feeding on both smaller vertebrates and the abundant insects found in the same fossil beds.

Uniquely for an early dinosauriform it also had a single row of bony osteoderms running along its spine. Although it lived at close to the same time as the similarly-armored Mambachiton their last common ancestor was at least 10 million years earlier, and no other early dinosaur precursors with osteoderms are currently known – so this was probably a case of Lewisuchus independently re-evolving the same sort of feature.

Mambachiton

Mambachiton fiandohana lived during the mid-Triassic, about 237 million years ago, in what is now Madagascar – which at the time wasn’t yet an island, still being connected to both east Africa and India as part of southern Pangaea.

It represents the earliest known branch of the avemetatarsalians, or “bird-line archosaurs”, a major group of the archosaur reptiles that also includes pterosaurs and dinosaurs/birds. 

It’s only known from a few fragments but it was probably around 2m long (~6’6″), and would have been a carnivorous lizard-like animal with a long neck and semi-erect quadrupedal limb posture.

Unexpectedly for a bird-line archosaur it also had a staggered double row of bony osteoderms along its back, suggesting that the very earliest avemetatarsalians had some crocodilian-like armor. This seem to have very quickly been lost, though – there’s no sign of osteoderms in the next branches to split off after Mambachiton, the aphanosaurs and pterosauromorphs – and although they occur again later in one dinosauriform and various non-avian dinosaurs, this appears to be multiple cases of independent re-evolution rather than retaining the original ancestral trait.

Teleocrater

Teleocrater rhadinus from the Middle Triassic (~245 mya) of Tanzania. Measuring about 2.5m long (8′2″), it was one of the earliest known members of the avemetatarsalians – the dinosaur-and-pterosaur (or “bird-line”) branch of the archosaurs. Its fossils have been known for over 80 years, but it was only very recently given an official name and classification following the discovery of additional specimens in 2015.

It turned out to be rather different from what paleontologists had expected an early bird-line archosaur to look like. Instead of being a bipedal basal-dinosaur-like animal, Teleocrater was actually a quadruped with more crocodilian-like limbs and oddly elongated neck vertebrae.

I’ve done two variations of the image today – both with and without a little speculative proto-fuzz.

An illustration of an extinct reptile related to the ancestors of dinosaurs and pterosaurs. It's a quadrupedal animal with a lizard-like head, a long neck, crocodile-like limbs, and a long tail.
Teleocrater rhadinus (no fuzz version)

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.