Cimolestans were one of the major mammal lineages that survived through the K-Pg mass extinction 66 million years ago. Closely related to early placentals, they had a burst of diversification during the first half of the Cenozoic and rapidly evolved into a wide range of specialized forms – some uniquely weird, and others convergently resembling more familiar modern animals like squirrels, bears, ground sloths, and hippos.

And one group known as the pantolestids were incredibly otter-like.

(Because synapsids love them some lutrinization.)

Palaeosinopa didelphoides here lived during the mid-Eocene, about 52 million years ago, in what is now the Mountain West region of the USA. It was similar in size to a small otter, about 1m long (3’3″), and had a streamlined body with a well-muscled neck, short powerful forelimbs, slightly longer hindlimbs, and a very long tail.

Inhabiting a subtropical lake ecosystem, it probably swam using both hindlimb paddling and otter-like tail undulations. Its strong jaws and teeth suggest it was specialized for crunching hard shellfish prey, but so far preserved gut contents have only shown fish bones and scales. Fairly large claws indicate it was also able to dig out burrows similarly to modern otters and beavers.

Although pantolestids were never particularly common animals they were quite widespread, expanding their range from their evolutionary origins in North America across to Europe and eventually into Asia. A cooling and drying climate at the end of the Eocene seems to have driven most of the group into extinction alongside all their other cimolestan relatives – but a few of the Asian species clung on slightly longer as the very last of their kind, with the last known fossils dating to about 28 million years ago in the early Oligocene.


In the early Cenozoic mammals were rapidly diversifying and evolving. And while it was the placental mammals that would end up being the most successful across much of the world, they weren’t the first mammal lineage to take advantage of all the ecological niches left vacant in the wake of the end-Cretaceous mass extinction.

The cimolestans were a group of non-placental eutherians – mammals closer related to modern placentals than to marsupials – that very quickly evolved into a wide range of niches during the Paleocene and Eocene, becoming some of the largest mammals of their time and producing forms as varied as squirrel-like, otter-like, ground sloth-like, and hippo-like.

But some of the weirdest of them all were the taeniodonts. Originating back in the late Cretaceous, these herbivorous cimolestans were characterized by short blunt snouts with large front teeth, and limbs with long claws.

Stylinodon mirus here was one of the largest taeniodonts, standing around 70cm tall at the shoulder (2’4″), and was also one of the last of its kind, living during the mid-Eocene about 50-40 million years ago in western North America.

It took the specializations of its lineage to the extreme, with a odd-looking boxy skull with enormous chisel-like ever-growing front teeth similar to those of a rodent – but derived from its canine teeth rather than its incisors.

Stylinodon skull | photograph by Yinan Chen | CC0

Its powerful front limbs and large claws were clearly specialized for digging, and for a long time it was thought to be obvious what its diet was – clearly it must have been unearthing roots and tubers from underground, right?

However, closer looks at its teeth raise a problem with that interpretation. That sort of food source should have left numerous telltale marks on the chewing surfaces of its teeth, scratches and gouges and abrasions from dirt and grit mixed in with the roots being eaten.

Yet Stylinodon barely shows any of those wear marks, suggesting that it rarely actually ate those food items. Its tooth surfaces were instead worn very smooth, indicating that it was eating something particularly tough that was constantly “polishing” them as it chewed — but what exactly that food source was is still unknown.

It may also have used its forelimbs to help pull down branches down towards its mouth, stripping off leaves and bark similar to ground sloths, chalicotheres, and therizinosaurs – but it probably did mostly use those big claws to actually dig, just perhaps mainly to construct large burrows rather than to find food.

Month of Mesozoic Mammals #31: The Survivors


Quite a few groups of Mezozoic mammals actually made it into the Cenozoic – including multituberculates, dryolestoids, various different metatherians, cimolestans, leptictidans, and possibly another unknown lineage in New Zealand – but most of them eventually declined and died out, and only monotremes, marsupials, and placentals still remain alive today.

We don’t know exactly when placentals originated. The first definitive fossils come from the start of the Cenozoic, but a few early ancestral forms probably already existed during the Late Cretaceous (estimated up to 90-75 mya) and only got their chance to rapidly diversify immediately after the mass extinction event.

One of the closest fossils we have to the earliest placentals is Purgatorius. Known mainly from teeth from the Early Paleocene of North America (66-63 mya), it’s not entirely clear whether it actually existed in the Mesozoic, but its remains have been found very close to the K-Pg boundary and one fossil might actually be from the end-Cretaceous.

A few foot bones have been associated with some of the fossil teeth, and if they do belong to Purgatorius then they show that it had very flexible ankles, a characteristic typical of tree-climbing animals. It would likely have been a squirrel-like creature, about 15-20cm long (6-8″), eating an omnivorous mixture of insects, seeds, and fruit. It may also have been capable of burrowing similar to modern chipmunks.

It’s often been interpreted as a placental mammal, specifically a very early type of primate, but more recent studies suggest it might not even be a true placental at all  – although it was probably still a very close relative of the common ancestor of all living placentals.

Month of Mesozoic Mammals #30: Strange Relations


A group known as the leptictidans were probably some of the weirdest early eutherians. With their tiny forelegs, big hindlegs, and long counterbalancing tails, they somewhat resembled jerboas or small kangaroos – except they also had long slender snouts that probably ended in sengi-like proboscises, and their feet were structured more like those of running animals than jumping ones. They’re also thought to have been mainly bipedal, convergently evolving a similar posture and movement style to non-avian theropod dinosaurs.

Leptictidium (Eocene, 50-35 mya) by Tim Bertelink || CC BY-SA 4.0

First appearing in the Late Cretaceous, they made it through the end-Cretaceous extinction and survived up until the mid-Cenozoic across the northern hemisphere, going extinct around 33 million years ago. They were probably omnivores, eating a mixture of insects, small vertebrates, and soft plant matter such as fruit and leaves.

Their mix of “primitive” skull features and highly specialized skeletons makes classifying them particularly difficult. They’ve been proposed to be placentals related to primates and rodents or afrotheres, a very early branch of the eutherians, or close to placentals but not quite true members themselves. The latter interpretation currently seems most likely, but they could also be a paraphyletic group at the base of placentals (suggesting that they could even be ancestral to placentals, and therefore all placentals would technically be leptictidans).

Gypsonictops was one of the earliest leptictidans, living during the Late Cretaceous of North America (70-66 mya). Known only from teeth and jaw fragments, we don’t know much about its appearance or full size – although it was probably smaller than its later relatives, perhaps about 35cm long (1′2″).

Any reconstruction of such fragmentary remains is going to be very speculative, but I’ve restored it here as a sort of transitional form, not yet quite as specialized. A more sengi-like animal, mainly quadrupedal but able to run and hop on its hind legs to flee from danger or chase after small fast-moving prey.

Month of Mesozoic Mammals #29: Rooting Around


First appearing in the Late Cretaceous, a widespread and diverse group of mammals known as cimolestans were once thought to be early members of placental groups like pangolins and carnivorans. But more recent studies have shown them to be part of a different branch of the eutherian family tree altogether, more like cousins to the earliest placentals and leaving no living descendants.

However, they did make it through the end-Cretaceous mass extinction and were quite successful during the early Cenozoic, evolving into forms ranging from giant herbivores to fanged squirrel-like climbers to otter-like swimmers, with the latter surviving until about 33 million years ago.

One group of North American cimolestans, the taeniodonts, were specialized for digging up tough roots and tubers, with large claws, strong blunt jaws, and big front teeth that became ever-growing in some species.

Schowalteria was the earliest known member of this group, living during the Late Cretaceous of Canada (70-66 mya). Only represented by partial skull material, its full size is unknown – some estimates put it at a similar size to giants like Repenomamus, but it was likely closer to half that size at around 50cm in length (1′8″). Still one of the larger Mesozoic mammals around, but not nearly as big as some of the Cenozoic taeniodonts would later become.

Month of Mesozoic Mammals #28: Hop To It


Living during the Late Cretaceous of Mongolia (85-70 mya), Zalambdalestes was part of a highly specialized group of mammals that it lends it name to – the zalambdalestids – which were an early branch of the eutherian evolutionary tree.

About 20-25cm in length (8-10″), it had relatively long limbs with especially strong hindlegs that show adaptations for rabbit-like hopping. Its long narrow snout may have ended in a flexible proboscis similar to those of modern sengi, and sharp interlocking teeth indicate a carnivorous or insectivorous diet.

Its long rodent-like incisors grew continuously throughout its life, suggesting it was gnawing on something tough enough to constantly wear down its front teeth.

Skull of Zalambdalestes || from fig 51 in Wible JR, Novacek MJ, Rougier GW (2004) New data on the skull and dentition in the Mongolian late Cretaceous eutherian mammal Zalambdalestes. Bulletin Of The American Museum Of Natural History 281:1-144 uri:

Some studies have proposed that zalambdalestids were actually very basal members of placental mammal groups such as rodents or rabbits, but the presence of epipubic bones in front of their pelvises (bones not found in placentals) shows they were a much earlier type of eutherian that still reproduced more like marsupials. Any anatomical similarities to later placentals were probably just the result of convergent evolution.

Month of Mesozoic Mammals #27: Treetop Origins


The other major branch of the therian mammals are the eutherians – represented today by only the placentals. But although the first eutherians split from their common ancestor with the metatherians all the way back in the mid-Jurassic, more then 160 million years ago, true placentals don’t seem to have actually appeared until right after the end-Cretaceous mass extinction.

There were many other side-branches of the eutherian lineage during the Mesozoic, however, and these last few days of the month will cover some of them.

(Some of these relationships are still under dispute, such as the exact position of leptictidans and Purgatorius and whether they’re actually placentals or not. Eutherian phylogeny is a bit of a mess.)

Juramaia was the earliest known eutherian, living during the Late Jurassic of China (161-156 mya). About 15cm long (6″), it was a shrew-like insectivore with limb anatomy that would have allowed it to climb up trees in a similar manner to modern rats.

Something very similar to it would have been the common ancestor of all later eutherians – suggesting that the earliest members of the group may have started out as tree-climbers before diversifying into different niches later on.

But despite it being closer related to living placentals than to marsupials, placental-style reproduction hadn’t actually evolved yet and it would have still given birth to tiny undeveloped young.