#dryolestida

Month of Mesozoic Mammals #23: Toothy Jaws Cronopio

Cronopio was one of the earliest South American dryolestoids, living during the start of the Late Cretaceous of Argentina (100-94 mya). Known only from partial skull material, it had a long snout and enlarged canine teeth which gave it a superficial resemblance to the fictional “sabertoothed squirrel” Scrat from the Ice Age film series.

(from fig 4 in Rougier GW, Apesteguía S, Gaetano LC (2011) Highly specialized mammalian skulls from the Late Cretaceous of South America. Nature 479(7371):98–102 doi: 10.1038/nature10591)

Although its full size and appearance is unknown, it’s estimated to have measured about 15cm long (6″). It was probably an insectivore or an omnivore, but the rather delicate nature of its snout and fangs suggest it had a weak bite and relied on strong jaw muscles to chew up its food with a specialized rotating motion. It’s also not clear whether its saberteeth had a function for feeding, or if they were used for display and fighting like in some modern deer.

And while I’ve reconstructed Cronopio here with ankle spurs, it’s actually unknown whether dryolestoids had this feature. They occupied an evolutionary position between mammals that definitely had spurs (symmetrodonts) and ones that definitely didn’t (therians), but fossil remains of dryolestoid ankles are poorly-preserved and incomplete.

(I also wanted to do a depiction that wasn’t so blatantly Scrat-like, because that’s become sort of a paleoart meme. Soft-tissue and long fur can really change the outward appearance of mammals, so enjoy this weird tiny pig-rat version of Cronopio.)

Month of Mesozoic Mammals #22: Mole-Mimics Paurodon

Some of the closest cousins to the therians, or perhaps even their direct ancestors, dryolestoids first appeared in the mid-Jurassic (~168 mya) and were found throughout North America, Eurasia, and North Africa up until the Early Cretaceous (~125 mya). But despite mostly disappearing from the northern hemisphere fossil record at that point, they moved into South America and flourished, becoming some of the most diverse and common mammals there during the Late Cretaceous.

A few even survived the end-Cretaceous mass extinction, with one of the largest members of the group known from the start of the Cenozoic. Then they disappeared again, only for a final survivor to turn up in the Miocene - just 17.5 million years ago.

(There’s also been a proposal that modern marsupial moles might even be living dryolestoids, but that paper hasn’t been peer-reviewed and genetic analyses place them firmly as true marsupials.)

Paurodon lived during the Late Jurassic of western North America (155-145 mya). Although known only from jaws and teeth, the fossil material seems to represent a series of different growth stages, and it was probably a mouse-sized animal growing to about 10-15cm long (4-6″)

Although some of its close relatives appear to have been tree-climbers, Paurodon’s jaws strongly resemble those of modern golden moles -- suggesting it was similarly specialized for a diet of earthworms, and may even have had a subterranean mole-like lifestyle.

Month of Mesozoic Mammals #24: Big Herbivores Coloniatherium

Most dryolestoids seem to have been insectivores and omnivores, but one group known as the mesungulatids specialized for herbivory. Although mostly known only from teeth and skull fragments, the size of these pieces indicates that they were surprisingly large compared to their other close relatives -- making them some of the biggest mammals around at the time.

They had fairly long blunt snouts with strong jaws and teeth adapted for crushing and grinding plants, and may even have been able to chew with a side-to-side motion similar to placental ungulates. In fact, mesungulatids were initially mistaken for early ungulates based on how convergently similar their teeth looked, hence their group’s name.

Known from the Late Cretaceous of Argentina (72-66 mya), Coloniatherium was one of the bigger mesungulatids (although not as large as the dog-sized Cenozoic Peligrotherium). Its full body size is uncertain, but it might have been similar to a modern marmot at around 50cm in length (1′8″).

It lived in a coastal plain or estuary environment and was one of the most common mammals in its ecosystem, suggesting mesungulatids were a particularly successful lineage despite reaching sizes where they would potentially have been directly competing with small herbivorous dinosaurs.

The allochiropterid Chimil kaslem, from the Oligocene of Antarctica. Here depicted with speculative structural colours like those of mandrill faces, exhibitting for a potential mate. By hodarinundu

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