Month: December 2023

Lessiniabatis

Lessiniabatis aenigmatica was a rather strange stingray.

It lived around 50-48 million years ago during the early Eocene, in a shallow warm sea covering what is now Italy, with its three known fossil specimens all coming from the fish-rich Monte Bolca fossil beds.

About 60cm long (~2′), it had a round pancake-like body similar to many modern seafloor-dwelling stingrays – but uniquely it was also almost tailless, with only a tiny, slender, stingless tail.

It wasn’t a particularly strong swimmer, instead probably spending most of its time buried in the muddy seafloor sediment. When on the move it likely swam along just above the surface of the seafloor using undulations of its fins, foraging for smaller bottom-dwelling animals like worms, molluscs, crustaceans, and fish.

Megapterygius

Most mosasaurs all had very similar body plans: they were streamlined scaly monitor-lizard-like marine reptiles with four rounded paddle-shaped flippers, and many of them also had large shark-like tail fins.

But Megapterygius wakayamaensis here seems to have been doing something a bit different.

Living towards the end of the Cretaceous, about 72 million years ago, in the waters covering what is now western Japan, this mosasaur was around the size of a modern orca, roughly 6m long (~20′).

Unlike other known mosasaurs its flippers were huge, bigger than its own head and distinctively wing-shaped, with the back pair being larger than the front. This is an arrangement oddly reminiscent of the unrelated plesiosaurs, and may suggest a convergent sort of highly maneuverable “underwater flight” swimming ability – but unlike plesiosaurs Megapterygius also still had a powerful fluked tail, so how exactly all of its fins worked together is still unknown.

It’s also the first mosasaur known to preserve potential evidence of a dorsal fin. Some of its back vertebrae show a change in orientation at the point where a fin base would be expected to be, closely resembling the vertebrae shape of cetaceans like the modern harbor porpoise.

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.