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.
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.
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.
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.
Known from the Late Cretaceous of Mongolia (85-70 mya), Catopsbaatar was a fairly large multituberculate, similar in size to a modern chinchilla at about 40-50cm long (1′4″-1′8″) with about half of that length being its tail.
It was part of a group of Asian multis called the djadochtatheriids, which lived alongside famous dinosaurs like Velociraptor in a sandy desert environment. They were mostly jerboa-like animals capable of bipedal hopping – although one member of the group, Mangasbaatar, was a burrower instead.
Although Catopsbaatar had features in its vertebrae and hindlimbs convergently similar to those of modern hopping mammals, the somewhat more sprawling posture of multis mean it wouldn’t have jumped in quite the same way. It may have actually launched itself upwards at a steeper angle, in a manner a little more like a frog.
Djadochtatheriids weren’t the first hopping Mesozoic mammals, however, since fossilized footprints are known from both the Mid-Jurassic of South America and the Early Cretaceous of Korea. We don’t know what types of mammals made these tracks, or what they looked like, but they show that similar styles of locomotion may have evolved multiple times in early mammals.
Moving on to the next major group of the theriiform mammals, we have the multituberculates – or “multis” for short.
First appearing in the Early Jurassic, about 183 million years ago (possibly descending from haramiyidans), multis were one of the most successful and long-lived mammal lineages of all time, found throughout the world and making up more than half the known mammal species in some fossil deposits. They even made it through the end-Cretaceous mass extinction and became even more diverse in the Paleocene, although shifts in vegetation, climate change, and the rise of new predators seem to have sent them into decline by the mid-Cenozoic. The last surviving group of multis, the gondwanatheres, finally went extinct in the Miocene (~17.5 mya).
Multis had rodent-like teeth, except with huge blade-like lower premolars, and likely occupied similar ecological niches to their modern counterparts. The structure of their pelvises also suggests they were some of the earliest mammals to give live birth to tiny undeveloped young, similar to marsupials.
Rugosodon is known from the Late Jurassic of China (161-155 mya) and is one of the earliest multituberculates represented by near-complete fossil remains.
About 25cm long (10″), it was a ground-dwelling chipmunk-like animal with highly flexible ankle joints that would have made it very a fast and agile runner, capable of navigating uneven surfaces. These specialized ankles were a defining trait of multis, allowing later forms to adapt to lifestyles ranging from tree-climbing to burrowing to jerboa-like hopping.
And while many later multis were primarily herbivores, Rugosodon’s teeth show it was an omnivore, indicating that a more generalized diet was ancestral to the group.
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.
The last eutriconodont featured this month specialized for a semi-aquatic lifestyle very similar to modern otters.
Known from the Early Cretaceous of China (125-112 mya), Liaoconodon was about 35cm long (1′2″) and had a long streamlined body and paddle-like limbs. Like other eutriconodonts it was carnivorous, likely feeding on fish and aquatic invertebrates in its wetland habitat.
Its ears show a transitional state between those of earlier mammaliaformes and modern mammals, with the inner ear bones almost fully separated from the jaw aside from a thin rod of cartilage. While this cartilage disappears during embryonic development in modern mammals, in Liaoconodon it was ossified (turned to bone) and appears to have helped to support the eardrum – although it’s not clear whether this was the ancestral state for Mammalia and fully separated ear bones convergently evolved multiple times in different lineages, or whether this was an evolutionary reversal within the eutriconodonts.
Spinolestes was another gobiconodontideutriconodont, closely related to Repenomamus but not quite so large. About 25cm long (10″), it’s known from an incredibly well-preserved fossil that includes fine microscopic details of fur, skin, and internal organs. Notably even its external ears were preserved, the earliest known in the fossil record, showing a broad mouse-like shape.
Its coat was made up of both underfur and guard hairs, with a longer mane along its neck and back. There were around a dozen keratinous scales on its rump, under the fur, along with numerous “protospines” – stiff spiky hairs similar to those of modern spiny mice. Some hairs even show damage that matches symptoms of a fungal infection.
It also had strong forelimbs and a reinforced spine similar to both modern xenarthrans and hero shrews. It was likely an insectivore, and may have used its strong back much like hero shrews are thought to do, pushing under heavy logs and rocks and levering them up to find invertebrate prey underneath.
Living during the Early Cretaceous of China (125-122 mya), Repenomamus was part of a branch of the eutriconodonts known as gobiconodontids. These relatively big mammals were specialized carnivores, with strong bone-crushing jaws and their incisor teeth modified into long fang-like shapes.
Repenomamus giganticus was roughly the size of a modern wolverine, about 1m long (3′3″). A second species in the same genus, Repenomamus robustus, was about two-thirds that size but still among some of the largest known Mesozoic mammals.
Since it was larger than some of the dinosaurs it lived alongside, it’s likely to have eaten some of them – and one specimen of R. robustus was actually found with the bones of a juvenile Psittacosaurus in its stomach.