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Month of Mesozoic Mammals #28: Hop To It

Zalambdalestes

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: http://hdl.handle.net/2246/449

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

Juramaia

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.

Month of Mesozoic Mammals #26: Crunch Time

Didelphodon

While true marsupials didn’t appear in the fossil record until the early Cenozoic, some of their closest relatives in the Mesozoic were a group called stagodontids. These metatherians evolved in North America during the Late Cretaceous, and much like the deltatheroideans seem to have taken over some of the ecological niches left vacant after the extinction of most eutriconodonts.

Their strong jaws and large blunt premolar teeth were adapted for crushing hard-shelled food, and they seem to have been semi-aquatic swimming animals (somewhat similar to modern water opossums) specialized for eating freshwater invertebrates such as snails and crustaceans.

Most stagodontids went extinct during the end-Cretaceous mass extinction, but if the early Cenozoic genus Eobrasilia is a member of the group then at least some survived in South America up until about 52 million years ago.

Other metatherians persisted in Europe, Africa, and Asia for most of the Cenozoic, with some fossils dating to just 11 million years ago, and the sparassodonts were successful in South America until about 3 million years ago – but today the only living members of this branch of the therian lineage are the marsupials in the Americas and Australasia.

Didelphodon was a stagodontid living during the Late Cretaceous of North America (70-66 mya), and is known from skull and jaw remains, along with a fairly complete skeleton that hasn’t been officially described yet.

It had a long otter-like body and flexible feet, adaptations for efficient swimming, and was one of the largest known mammals at the time, measuring around 1m in length (3′3″).

Its sturdy jaws had an especially powerful bite force, one of the strongest relative to body size known for any mammal, and some of its molars were blade-like and similar in appearance to carnassials. Along with crunching on shellfish it would also have been capable of crushing bones and tough plants, and its diet was probably an omnivorous mixture of invertebrates, vegetation, carrion, and small vertebrates – potentially including other mammals and small dinosaurs.

Month of Mesozoic Mammals #25: Sabertooths

Lotheridium

In modern times the therian mammals are represented by just two surviving groups – placentals and marsupials. But both of these lineages contained many other extinct close relatives, and this final week of the month will focus on a few of them.

Modern marsupials are part of a larger grouping known as metatherians which split off from their common ancestor with placentals during the Jurassic period, at least 160 million years ago. Probably originating in Asia, they spread to Europe and the Americas during the Cretaceous, and diversified into several different groups – including some which would go on to become the dominant mammalian predators in South America during its long period of isolation in the Cenozoic.

One of the earliest branches of the metatherians were the deltatheroideans. These mammals are known from Asia and North America, evolving to fill vacant small carnivore niches after the disappearance of most of the eutriconodonts. Most of them went extinct at the end of the Cretaceous, but a few did survive for at least 10 million years afterwards.

Lotheridium was a deltatheroidean living during the Late Cretaceous of China (72-66 mya), and is known from a nearly complete skull. It’s likely to have had a head-and-body length of about 20cm (8″), and may have had a full length including the tail of up to 30-40cm (1′-1′4″).

It had elongated canine teeth, convergently similar to the saberteeth of many later mammal groups. It was probably a highly specialized predator, and may even have been capable of preying on small dinosaurs – its close relative Deltatheridium is known to have eaten theropods like Archaeornithoides.

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.

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.

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 a fringe proposal that modern marsupial moles might even be living dryolestoids – but that paper hasn’t been peer-reviewed and genetic analyses still 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 #21: Small Climbers

Zhangheotherium

Now we move further along the theriiform branch of the mammal family tree, into a group known as the trechnotherians. This is the lineage that contains modern marsupials and placentals (therians) along with their closest relatives – including today’s subject, the symmetrodonts.

Symmetrodonts are known throughout most of the Cretaceous period, with one possible late-surviving member in the early Cenozoic. They were small mammals with distinctively-shaped teeth specialized for carnivorous and insectivorous diets, and their skeletons show an odd mix of therian-like and monotreme-like anatomy – although the more “primitive” features are thought to be due to either convergent evolution or an evolutionary reversion.

One species, Spalacotheridium noblei, is known from some especially tiny teeth, and may have been one of the smallest mammals to ever live.

At first symmetrodonts were known only from fossil teeth and jaws, but Zhangheotherium was the first to be discovered with a complete skeleton. Living during the Early Cretaceous of China (130-122 mya), it had a body length of 15cm (6″) – similar in size to a hamster – and a total length including the tail of about 25cm (10″).

It had spurs on its ankles that may have been venomous, and a more sprawling posture than therian mammals, along with limb proportions that suggest it was adapted for climbing.

It was sometimes preyed upon by theropod dinosaurs, as a specimen of Sinosauropteryx is known with Zhangheotherium bones in its stomach.

Month of Mesozoic Mammals #20: Hidden Herbivores

Vintana

For a long time very little was known about a group of mammals called gondwanatheres. Named for their occurrence in the southern continents that made up Gondwana, they were represented only by fossil teeth and jaw fragments, and it wasn’t even clear what type of mammal they actually were. But recent discoveries of more complete skulls (and a currently undescribed full skeleton*) are starting to reveal more information, and we now know they were actually part of the multituberculates, or at least very closely related to them.

* described and named as Adalatherium in 2020

They were also the latest-surviving of the multis, lasting well into the Cenozoic with the youngest known fossils dating to just 17.5 million years ago.

Vintana lived during the Late Cretaceous of Madagascar (70-66 mya), and is known from a single skull. It was a specialized herbivore with rodent-like teeth adapted for chewing tough plants, seeds, and roots, and huge powerful jaw muscles attached to downward-extended cheekbones – a feature convergently seen in a few other mammals.

It was one of the largest known mammals of its time, estimated to have been at least 60cm long (2′). It had relatively large eyes, well-developed inner ears, and an expanded area of its brain associated with processing scents, all features that indicate it had very keen senses and may have been quite a fast and agile animal.

Month of Mesozoic Mammals #19: Red In Tooth

Barbatodon

Living during the Late Cretaceous of Romania (70-66 mya), Barbatodon was part of a group of European multituberculates known as the kogaionids. These multis originated on the then-isolated Hațeg Island alongside dwarf dinosaurs and giant pterosaurs, and adapted to a highly insectivorous diet. They even managed to survive through the end-Cretaceous mass extinction, and spread across Europe for a further 10 million years before going extinct in the early Eocene about 56 million years ago.

Barbatodon is mainly known from teeth and partial skull material, so its full size is uncertain, but it was likely rat-sized at around 25-30cm long (10-12″). In one specimen its teeth were also preserved with their original coloration – a distinctive “blood red”. This feature is seen in some modern rodents and shrews, and is caused by iron minerals in the enamel that are thought to add extra strength. Since multis didn’t have ever-growing teeth like rodents, this added durability would have been especially important to them.

Another group of multis, the taeniolabidoids, also had red teeth, but since fossil enamel is rarely so well-preserved and unaltered we don’t know whether this was a shared ancestral feature or due to convergent evolution.