The last eutriconodont featured this month specialized for a semi-aquatic lifestyle very similar to modern otters.
Known from the Early Cretaceous of China (125-112 mya), Liaoconodon was about 35cm long (1′2″) and had a long streamlined body and paddle-like limbs. Like other eutriconodonts it was carnivorous, likely feeding on fish and aquatic invertebrates in its wetland habitat.
Its ears show a transitional state between those of earlier mammaliaformes and modern mammals, with the inner ear bones almost fully separated from the jaw aside from a thin rod of cartilage. While this cartilage disappears during embryonic development in modern mammals, in Liaoconodon it was ossified (turned to bone) and appears to have helped to support the eardrum – although it’s not clear whether this was the ancestral state for Mammalia and fully separated ear bones convergently evolved multiple times in different lineages, or whether this was an evolutionary reversal within the eutriconodonts.
Spinolestes was another gobiconodontideutriconodont, closely related to Repenomamus but not quite so large. About 25cm long (10″), it’s known from an incredibly well-preserved fossil that includes fine microscopic details of fur, skin, and internal organs. Notably even its external ears were preserved, the earliest known in the fossil record, showing a broad mouse-like shape.
Its coat was made up of both underfur and guard hairs, with a longer mane along its neck and back. There were around a dozen keratinous scales on its rump, under the fur, along with numerous “protospines” – stiff spiky hairs similar to those of modern spiny mice. Some hairs even show damage that matches symptoms of a fungal infection.
It also had strong forelimbs and a reinforced spine similar to both modern xenarthrans and hero shrews. It was likely an insectivore, and may have used its strong back much like hero shrews are thought to do, pushing under heavy logs and rocks and levering them up to find invertebrate prey underneath.
Living during the Early Cretaceous of China (125-122 mya), Repenomamus was part of a branch of the eutriconodonts known as gobiconodontids. These relatively big mammals were specialized carnivores, with strong bone-crushing jaws and their incisor teeth modified into long fang-like shapes.
Repenomamus giganticus was roughly the size of a modern wolverine, about 1m long (3′3″). A second species in the same genus, Repenomamus robustus, was about two-thirds that size but still among some of the largest known Mesozoic mammals.
Since it was larger than some of the dinosaurs it lived alongside, it’s likely to have eaten some of them – and one specimen of R. robustus was actually found with the bones of a juvenile Psittacosaurus in its stomach.
One of the earliest major branches of the theriiformes were the eutriconodonts. First appearing in the fossil record in the Early Jurassic, about 190 million years ago, these mammals were a highly successful group that adapted to a variety of different niches and lasted up until nearly the end of the Cretaceous.
Their exact relationships to other theriiformes are a little uncertain, with it being unclear whether they split off before or after the multituberculates (another major group featured later this month).
Volaticotherium is known from the Middle Jurassic of China (165-161 mya), and was the first gliding Mesozoic mammal to be discovered (although we now know about quite a few more). It was part of a branch of the eutriconodonts known as the volaticotherians, a widespread lineage which ranged through most of the Jurassic period and into the mid-Cretaceous.
Measuring about 26cm long (10″), or about 14cm (5.5″) excluding the tail, it’s known from a mostly complete skeleton with impressions of fur and skin. A gliding membrane extended from its hands to its hindlimbs and the base of its tail, its feet had grasping toes, and its tail was flattened to create an airfoil-like shape.
It had sharp slicing teeth, indicating a carnivorous or insectivorous diet – unusual since most other known gliding mammals are predominantly herbivores.
Based on the measurements of the teeth it was probably one of the larger eutriconodonts, close in size to Jugulator. I can’t find any body size estimates, but it may have have a total length of around 50-60cm (20-24″).
Plenty of fossil mammals are known solely from teeth, but what’s most interesting about this one is that its remains were found in coastal marine deposits without any signs of degradation or transport damage by water currents. This indicates the animal probably died at sea very close to the location where it was preserved.
A fewother eutriconodonts are now known to have been semi-aquatic, so Ichthyocondon might have been adapted to a similar lifestyle, making it one of the earliest known marine mammals. Another potential explanation is that it was a Volaticotherium-like glider that got blown out to sea.
As with many of this month’s paleontological mysteries, we need some more substantial fossil remains to know for sure. I’ve reconstructed it here as both main possibilities, as an otter-like semi-aquatic animal and as a patagium-bearing glider.