Towards the end of the Cretaceous, about 69 million years ago, the most diverse and numerous mammals in the northern hemisphere were the metatherians, close relatives of modern marsupials.
And Unnuakomys hutchisoni was the most northern-living of all these metatherians.
About the size of a modern mouse, around 10-15cm long (4-6″), and with teeth that suggest it was a shrew-like insectivore, this little metatherian lived in northern Alaska in what’s known as the Paaŋaqtat Province – a region with a distinctive population of endemic polar animals. At the time this area was located at an even higher latitude than it is today, around 80-85ºN, but due to a greenhouse climate it was also warmer, with no permanent ice and the average temperatures staying above freezing.
Unnuakomys was by far the most common mammal species in the Paaŋaqtat Province, represented by numerous fossil teeth and a few jaw fragments, and it also seems to have been the only metatherian living in the whole region. This may just be a preservation bias in the fossil record, but it might also indicate that Unnuakomys was uniquely specialized to endure the several months of continuous darkness each winter in its polar woodland environment, while other North American metatherians were restricted to more southerly latitudes.
Cabarzia trostheidei here lived during the early Permian in what is now Germany, about 295 million years ago.
Despite its very lizard-like appearance it was actually part of the varanopid lineage, a group of scaly amniotes traditionally classified as early synapsids (distant relatives of modern mammals), but which more recently have been proposed to instead be sauropsid reptiles closer related to early diapsids.
It was around 50cm long (1’8″), and its short arms, long legs, slender body, and long tail suggest it was capable of shifting into a bipedal posture when running at high speeds, similarly to some modern lizards – probably mainly to escape from larger predators, but possibly also used to pursue fast-moving prey like flying insects.
And whether varanopids were actually synapsids or sauropsids, this makes Cabarzia the earliest known example of an animal running on two legs.
Halszkaraptorines were a group of small dromaeosaurids known only from the Late Cretaceous of Mongolia. They were odd little raptors with flattened snouts, long necks, and flipper-like arms – features that suggest they were specialized for swimming, making them the second known lineage of semi-aquatic non-avian dinosaurs after the spinosaurids.
This “duck-raptor” interpretation has been a little controversial since it was first proposed in 2017, but we’ve just gotten some more evidence for it in the form of an entirely new halszkaraptorine.
Natovenator polydontus lived in what is now the Gobi Desert in southern Mongolia, around 72 million years ago. The size of a small duck, about 45cm long (18″), it had jaws full of many needle-like teeth, a long flexible goose-like neck, and a streamlined body with a wide flattened ribcage convergently shaped like those of modern diving birds.
Although it had long strong legs, these don’t show much in the way of aquatic specializations and would have been used more for walking and running on land. Instead it may have used its flipper-like arms to propel itself through the water, like modern penguins or auks.
It probably had a lifestyle similar to modern mergansers, swimming and diving in lakes and rivers, and preying on fish, amphibians, and aquatic invertebrates.
The pycnodonts were a diverse group of ray-finned fish that were found in shallow coastal waters from the late Triassic to the late Eocene (~215-37 million years ago). They usually had deep but very narrow body shapes with a disc-like appearance, convergently similar to modern reef fish like marine angelfish or butterflyfish – but some looked much weirder, with elaborate horns and spines, long snouts, or vertically-stretched bodies.
Most of them also had jaws full of round flat teeth used to crush hard-shelled prey, but some may instead have been herbivorous grazers similar to parrotfish.
And a couple of lineages even became carnivores.
Serrasalmimus secans lived in what is now Morocco during the late Paleocene, about 59 million years ago. Although only known from its jaws, the size of the fossil material suggests it was fairly large for a pycnodont, possibly around 80cm long (~2’8″).
It had sharp flesh-cutting teeth similar to those of modern piranha, but with a surprising evolutionary twist. Unlike any other known ray-finned fish, Serrasalmimus‘ teeth were true shearing carnassials anchored into bony sockets, with new replacement teeth forming directly below each current tooth – a very specific arrangement of features previously only known in mammals.
This is an especially remarkable example of convergent evolution because on land placental carnivorans were developing their own carnassials at the same time, just a few million years after the K-Pg mass extinction. Both mammals and pycnodonts were simultaneously taking advantage of the vacant predatory roles in their respective ecosystems, and ended up with incredibly similar tooth adaptations as a result.