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.
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.
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 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.