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.
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.
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.
At this point we’re into Mammalia proper, and here the first major split happens between the Yinotheria (the lineage including monotremes and their relatives) and the Theriiformes (the lineage that includes everything else).
Although once widespread, with fossils known from Europe, Asia, Madagascar, and South America as well as Australasia, the remains of yinotherians are unfortunately rare and very fragmentary. Most specimens consist only of teeth and pieces of jaw, so we know very little about their ecological diversity or what they looked like.
The oldest known yinotherian fossils date to around 170 million years ago in the mid-Jurassic, but genetic studies suggest their last common ancestor with the theriiformes could have lived as far back as the Late Triassic 220 million years ago.
Kollikodon is known from a couple of jaw fragments from the Early Cretaceous of New South Wales, Australia (112-100 mya). It was a close cousin to the monotremes, but unlike its living relatives it had well-developed teeth which appear to have been highly adapted for crushing hard food – possibly insects, shellfish, or even tough plant matter.
Its full size is very uncertain, with estimates ranging from 50cm up to 1m in length (1′8″-3′3″). If accurate, the upper estimate would make it one of the largest of all known Mesozoic mammals.
The image above is therefore very very speculative. Most reconstructions seem to depict Kollikodon as little more than a large platypus, despite it lacking evidence of a bill, so I decided to go for something more visually different. Less aquatic-specialist, more pig-like omnivorous generalist eating whatever it can crunch in its jaws.
Falling evolutionarily between the docodonts and Mammalia itself, Hadrocodium is an important transitional form in the early mammal family tree. Something very similar to it would have been the common ancestor of all modern mammals.
Living in China during the Early Jurassic (196-189 mya), it was one of the first known mammals to have both an enlarged brain cavity and the characteristic middle ear bones of modern mammals, about 45 million years earlier than such traits were previously thought to have evolved.
It was also one of the smallest mammals of all time, measuring only about 3cm long (1.2″) – similar in size to the smallest mammals alive today, the bumblebee bat and Etruscan shrew.
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.