Tag: convergent evolution

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 #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.

skull of Barbatodon || from fig 2 in Smith T, Codrea V (2015) Red iron-pigmented tooth enamel in a multituberculate mammal from the Late Cretaceous Transylvanian “Haţeg Island.” PLoS ONE 10(7): e0132550. doi: 10.1371/journal.pone.0132550 || CC-BY-4.0

Month of Mesozoic Mammals #18: Leaps and Bounds

Catopsbaatar

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.

Month of Mesozoic Mammals #16: So Many Gliders

Maiopatagium

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.

And these gliding haramiyidans also contribute to the confusing classification of haramiyidans – because although Megaconus’ anatomy suggested they might be mammaliaformes, studies of another glider, Arboroharamiya, give a very different result. Its ear bones and jaw show the characteristics of true members of Mammalia, supporting the hypothesis that haramiyidans were actually close relatives (or ancestral to) the multituberculates.

Month of Mesozoic Mammals #14: Otter-Mimics

Liaoconodon

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.

Month of Mesozoic Mammals #13: Looking Sharp

Spinolestes

Spinolestes was another gobiconodontid eutriconodont, 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.

Month of Mesozoic Mammals #11: Getting Airborne

Volaticotherium

One of the earliest major branches of the theriiformes were the eutriconodonts. First appearing in the fossil record in the Early Jurassic, about 190 million years ago, these mammals were a highly successful group that adapted to a variety of different niches and lasted up until nearly the end of the Cretaceous.

Their exact relationships to other theriiformes are a little uncertain, with it being unclear whether they split off before or after the multituberculates (another major group featured later this month).

Volaticotherium is known from the Middle Jurassic of China (165-161 mya), and was the first gliding Mesozoic mammal to be discovered (although we now know about quite a few more). It was part of a branch of the eutriconodonts known as the volaticotherians, a widespread lineage which ranged through most of the Jurassic period and into the mid-Cretaceous.

Measuring about 26cm long (10″), or about 14cm (5.5″) excluding the tail, it’s known from a mostly complete skeleton with impressions of fur and skin. A gliding membrane extended from its hands to its hindlimbs and the base of its tail, its feet had grasping toes, and its tail was flattened to create an airfoil-like shape.

It had sharp slicing teeth, indicating a carnivorous or insectivorous diet – unusual since most other known gliding mammals are predominantly herbivores.

Month of Mesozoic Mammals #10: Termite-Eaters

Fruitafossor

Living during the Late Jurassic of Colorado, USA (156-150 mya), Fruitafossor was one of the earliest known mammals specialized for feeding on colonial insects. It had peg-like enamel-less teeth and a reinforced spine surprisingly similar to those of modern armadillos and anteaters, and powerful digging forelimbs with only four fingers on each hand.

It’s known from an almost complete skeleton, about 15cm long (6″), but its highly modified features make figuring out its exact evolutionary relationships rather difficult. It may have been part of a very early offshoot of the theriimorph lineage, something with no close living relatives but still converging on the exact same adaptations as placental mammal groups that wouldn’t emerge until the Cenozoic 100 million years later.

Month of Mesozoic Mammals #07: Digging Down

Docofossor

The docodonts didn’t stop at exploiting ecological niches in the trees and water. Another branch of the group specialized into underground burrowing, developing convergent features remarkably similar to modern golden moles.

Docofossor is known from the Middle Jurassic of China (161-155 mya), and measured about 10cm long (4″). It had large shovel-like fingers, strong forelimbs, short sprawling hindlimbs, and pointed teeth adapted for capturing invertebrate prey. (I’ve also given it a patch of protective keratinized skin on its snout here, based on the related Haldanodon.)

It had a reduced number of bones in its fingers, a modification identical to some modern mammals – suggesting that these relatively “primitive” mammals were already using the exact same genes to regulate their anatomical development.