The haramiyidan featured yesterday was a ground-dwelling animal, but most others in the group were actually highly adapted for tree-climbing. They were very squirrel-like in appearance, with grasping hands and feet and tails that may have been prehensile – and some took this lifestyle even further, becoming specialized gliders.
Living during the Late Jurassic of China (157-163 mya), Maiopatagium is one of at least four known gliding haramiyidans. It was about 25cm long (10″), around half of which was its long tail, and had a gliding membrane extending between its wrists and ankles. The proportions of its hands and feet were very similar to modern colugos and the feet of bats, which has been interpreted as evidence of the same sort of upside-down roosting behavior.
Its close relative Vilevolodon had rodent-like teeth highly adapted for crushing and grinding, suggesting these haramiyidans were herbivores feeding mainly on seeds and soft plant matter.
Now we come to a group known as the haramiyidans, and things get a little… confusing.
First appearing in the Late Triassic, haramiyidans are known from Europe, Greenland, Asia, and North Africa, and lasted up until the mass extinction at the end of the Cretaceous – making them one of the longest-lived mammal lineages ever.
The problem is that we don’t know exactly what they were. For a long time they were known only from teeth and jaw fragments, and recent discoveries of more complete fossils show a mix of anatomical features that make it difficult to definitively place them in the mammal evolutionary tree. Different studies come up with different answers depending on which haramiyidans are included in their analyses.
The two main competing ideas are that they’re either close relatives of the multituberculates, or not even part of Mammalia itself at all, belonging somewhere in the mammaliaform cynodonts instead. It’s an issue that probably won’t be cleared up without more fossils and more comprehensive studies of the group as a whole.
Megaconus lived during the Middle Jurassic of China (165-161 mya). About 30cm long (1′), its known from a complete skeleton with some “primitive” features in its ear bones, vertebrae, and heels, giving support to the haramiyidans-are-mammaliaformes hypothesis.
It had teeth similar to rodents, with long incisors and large molars, and was either an omnivore or a herbivore, comparable to modern ground squirrels. The structure of its limbs suggest it would have walked with a gait like armadillos or rock hyraxes.
Fur impressions on the fossil show a mix of guard hairs and underfur – but surprisingly the underside of its belly seems to have been only sparsely haired, or even naked.