#docodonta

Here’s some of the amazing diversity of Mammaliaformes of Jurassic China. Volaticotherium glides in pursuit of a tasty bug. Agilodocodon, an excellent climber, already got one and brings it back to its young in the trees. Some Docofossors are digging up the lawn. Castorocauda, the largest mammaliaform of the Mesozoic, is going for a swim. With the exception of Volaticotherium, these are all Docodonts, a group closely related to the crown group mammals, having branched off a little before the monotremes did. I wanted to show off how much more diverse and interesting Mesozoic mammals were than we usually imagine. They did a lot more than cowering from dinosaurs and being eaten by them!

The docodonts were a group of mammaliaformes (close relatives of the earliest true mammals) which lived across North America, Europe, and Asia from the Middle Jurassic to the Early Cretaceous. Originally only known from teeth and jaw fragments they were traditionally thought to be fairly generic shrew-like insectivores, but more recent discoveries of better fossils have revealed they were actually much more diverse, occupying ecological niches ranging from squirrel-like tree-climbers to mole-like diggers to beaver-otter-like swimmers.

Most of the more complete fossil material of these animals comes from the mid-Jurassic of China, but one species from elsewhere is also known from a partial skeleton.

Haldanodon exspectatus here lived in central Portugal during the Late Jurassic, about 155 million years ago. Around 15-20cm long (6-8"), it had small eyes and short chunky well-muscled limbs with the front paws adapted for digging. Since it inhabited a very swampy environment it probably wasn't a pure mole-like burrower – extensive tunnels would have constantly flooded – but it may have instead been a similar sort of semi-aquatic animal to modern platypuses and desmans, foraging for invertebrates in the water and excavating burrows in the banks.

Roughened areas of bone on its snout may also have supported a patch of tough keratinous skin, which would have helped protect its face while digging.

———

Month of Mesozoic Mammals #05: Climbing Trees Agilodocodon

Before we get to the actual-Mammalia-mammals, there’s one more group of mammaliaformes who deserve some attention -- the docodonts.

Falling evolutionarily just outside of Mammalia itself, docodonts first appeared in the mid-Jurassic and lasted until the Early Cretaceous. They used to only be known from teeth and jaw fragments and were thought to have been fairly generic shrew-like terrestrial insectivores, but more recent discoveries have shown them to have actually been some of the earliest mammals to specialize into diverse habitats.

Agilodocodon was adapted for climbing around in trees, making it one of the earliest known arboreal mammals (although not the first climbing synapsid). Living in China during the Middle Jurassic (165-161 mya), it measured about 13cm long (5″) and had sharp gripping claws and flexible wrists and ankles similar to modern climbing mammals like tree squirrels.

When it was first described in 2015 it was suggested that its spade-like front teeth were specialized for gnawing bark and feeding on tree sap -- but a later study found that its teeth didn’t really resemble those of any modern sap-eating mammals, and in fact were closer in shape to those of insectivorous marsupials and elephant shrews.

Castorocauda lutrasimilis catching breakfast. I’m not sure about the hind leg, but I was too lazy to change it.

Castorocauda was a semi-aquatic docodont that lived about 164 million years ago, during the Jurassic period, in what today is Inner Mongolia. It was a piscivore, that also supplemented its diet with invertebrates.

It was mammaliaform, but not a mammal.

It could reach a little over 40cm in length, and between 500 to 800g in weight.

The name Castorocauda lutrasimilis means beaver-tail similar to otter.