Miomancalla

The mancallines were a lineage of flightless semi-aquatic birds closely related to auks. Known from the Pacific coasts of what are now California and Mexico, between about 7.5 and 0.5 million years ago, they convergently evolved a close resemblance and similar lifestyle to both the recently-extinct North Atlantic great auk and the southern penguins.

Miomancalla howardi here lived in offshore waters around southern California during the late Miocene (~7-5 million years ago). The largest of the mancallines, it just slightly beat out the great auk in size – standing around 90cm tall (~3′) and weighing an estimated 5kg (11lbs).

Like great auks and penguins it would have been a specialized wing-propelled diver, swimming using “underwater flight” to feed on small bait fish. It probably spent much of its life out at sea, probably only returning to land to molt and breed.

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.

Triopus

Although Triopus draboviensis here might look like an isopod or a trilobite, this small arthropod was actually part of a rather rare group called cheloniellids.

Known from the early Ordovician to the early Devonian (~480-408 million years ago), only about 7 different species of cheloniellid have been described so far. Their evolutionary relationships were uncertain and controversial for a long time, but currently they’re thought to be distant cousins of trilobites within the Artiopoda.

Living in what is now Czechia during the late Ordovician, about 460-450 million years ago, Triopus is only known from two partial fossils. It was around 4cm long (~1.6″), and like other cheloniellids it had a body made up of wide radiating exoskeleton segments that fully covered its legs, and probably also a pair of whip-like appendages at the rear.

Its body was more domed than those of its relatives, who were generally very flattened, suggesting it was specialized for a slightly different lifestyle or habitat. Without any preserved appendages it’s not clear what its ecological role was, but since other cheloniellids had horseshoe-crab-like feeding structures it may have been a similar sort of generalist, preying on small invertebrates and scavenging carrion on the seafloor.

Wapitisaurus

Back in the 1980s, a fossil of a partial reptile skull was discovered in British Columbia, Canada, dating to the Early Triassic about 250 million years ago. Its triangular skull shape, large eye sockets, and what seemed to be distinctive spiky frills on the back of its head initially caused it to be identified as a relative of the gliding weigeltisaurids.

But the aptly-named Wapitisaurus problematicus would have had to be a very unusual member of this group. With an estimated length of up to 2m (6’6″) it was much larger than any other known weigeltisaurid, it was the only one known from the Triassic side of the “Great Dying” mass extinction event, it was found in a completely different part of the world, and its teeth seemed more like those of marine reptiles like thalattosaurs.

In recent years new discoveries and re-analysis of weigeltisaurid fossil material have resulted in much better modern understanding of their skull structure – and with that came the realization that Wapitisaurus really didn’t seem to match with them after all.

So a new study has finally identified what this problematic reptile really was… and it turns out the teeth didn’t lie! It was a marine thalattosaur all along!

Wapitisaurus had rather large eyes compared to most other North American thalattosaurs, and although the front parts of its jaws are missing it probably had a long slightly hooked snout similar to its close relative Thalattosaurus. It’s also now one of the oldest known members of the thalattosaur lineage, showing that some of their specialized skull features like retracted nostrils had actually appeared very quickly during their evolutionary history.

…Oh, and those “spiky frills” on the back of Wapitisaurus’ skull? They were actually all teeth from both the upper jaw and the palate, on broken shards of bone that had been displaced to just the right spot to muddle up its identity for over three decades.

Glossoceras

Although the only nautiloids living today have characteristic tightly coiled shells, earlier in their evolutionary history these cephalopods were much more diverse.

And Glossoceras gracile here is an example of one of the more unusual groups of nautloids: the ascocerids.

Living during the Late Silurian, about 422 million years ago, in wheat is now Gotland, Sweden, Glossoceras was only around 5cm long as an adult (~2″). Like other ascocerids it started out its life looking like a fairly standard early nautiloid, with a long straight shell that curved slightly upwards, but as it approached maturity things got weird – the front part of the shell grew out into a much more bulbous flask-like shape, and the old juvenile section broke off entirely.

The gas-filled buoyancy chambers of its adult shell were positioned directly above its body chamber rather than behind like in other nautiloids, giving it very good stability in the water. The shell walls were also very thin and lightweight, which would have made it a much more maneuverable swimmer.

Phosphatherium

Phosphatherium escuillei was one of the very earliest known members of the proboscideans, a lineage today represented only by the three living species of elephants.

Living in what is now Morocco during the late Paleocene and early Eocene, around 56 million years ago, it would have been about the size of a cat, roughly 30cm at the shoulder (~1′) and 60cm long (~2′). It had a fairly low flat head with a proportionally short snout, while the back end of its skull behind it eyes was elongated, supporting large powerful jaw muscles.

Wear patterns on its teeth suggest it ate a lot of tough vegetation, and it may have been a semiaquatic animal behaving somewhat like modern tapirs or pygmy hippos – spending a lot of the daytime lounging in water, and emerging onto land to forage during the night.

Spectember/Spectober 2023 #11: A Large Spider

An anonymous submission requested a “spider the size of a coconut crab”:

A shaded sketch of a speculative giant herbivorous descendant of jumping spiders. It's a big stocky spider covered in fuzz, with thick legs ending in cloven-hoof-like claws. It has the characteristic large main pair of eyes of jumping spiders, with the other three pairs more spread out around the front and sides of its wide head, and it also has two pairs of "horns". Its abdomen is wide and round, somewhat flattened on the top, with several spiky structures around the edge.

Ceratohispidus aspectus is a distant descendant of jumping spiders living on an Aotearoa-like landmass, isolated with no mammalian predators.

This particular lineage is notable for both their extreme gigantism (with their larger size and weight causing them to lose the ability to jump) and for having taken up herbivory in a similar manner to one modern species. Most of these big plant-eating spiders are around the size of wētāpunga, and occupy a similar ecological niche, but Ceratohispidus is the largest of them by far – rivalling the modern coconut crab with a body length of up to 40cm (~1’4″) and a legspan of almost 1m (~3’3″).

After reaching sexual maturity at 5-10 years old, adults grow very slowly, molting only once every year or two and taking several decades to actually get anywhere close to their maximum size.

Ceratohispidus’ thick legs end in hoof-like claws, and it selectively browses on vegetation by snipping off pieces with its pincer-like palps. A gizzard-like structure in its digestive system helps to grind up fibrous plant material with small gastroliths, and its wide abdomen houses both large book lungs and a tracheal system with air sacs that can contract and expand to provide a small amount of active ventilation.

While the “horns” and spikes ornamenting its body may provide some defense from the few avian and reptilian predators in its habitat, they’re mainly used as part of highly elaborate visual displays between individuals.