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.
While some docodonts like Agilodocodon were going up into the trees, another branch of the group was specializing into semi-aquatic habits instead.
Castorocauda is known from the Middle Jurassic of China (165-161 mya), represented by an exceptionally preserved fossil showing soft tissue and hair impressions. About 40cm long (1′4″), it would have lived in a wetland environment and was well-adapted for swimming, with a flattened scaly beaver-like tail, webbed toes, and a coat of dense fur very similar to that of modern mammals, made up of both guard hairs and underfur.
Its strong forelimbs suggest it was capable of digging burrows, like modern platypus, and its sharp backwards-pointing teeth indicate a diet of slippery prey such as fish and worms.
It was also one of the earliest known mammals with (possibly venomous) spurs on its ankles. This feature is only seen today in monotremes, but seems to have been an ancestral trait common to all early mammals that was later lost in the lineage leading to marsupials and placentals.
Before we get to the actual-Mammalia-mammals, there’s one more group of mammaliaformes who deserve some attention – the docodonts.
Falling evolutionarily just outside of Mammalia itself, docodonts first appeared in the mid-Jurassic and lasted until the Early Cretaceous. They used to only be known from teeth and jaw fragments and were thought to have been fairly generic shrew-like terrestrial insectivores, but more recent discoveries have shown them to have actually been some of the earliest mammals to specialize into diverse habitats.
Agilodocodon was adapted for climbing around in trees, making it one of the earliest known arboreal mammals (although not the first climbing synapsid). Living in China during the Middle Jurassic (165-161 mya), it measured about 13cm long (5″) and had sharp gripping claws and flexible wrists and ankles similar to modern climbing mammals like tree squirrels.
When it was first described in 2015 it was suggested that its spade-like front teeth were specialized for gnawing bark and feeding on tree sap – but a later study found that its teeth didn’t really resemble those of any modern sap-eating mammals, and in fact were closer in shape to those of insectivorous marsupials and elephant shrews.
If there’s one Mesozoic mammal that’s been relatively well-mentioned in dinosaur books and popular media for many years, it’s undoubtedly Megazostrodon. Often depicted as “the first mammal”, it actually occupies a point in the mammal evolutionary tree somewhere between the earliest mammaliaformes and the common ancestor of all modern groups.
Megazostrodon lived during the very end of the Triassic and the Early Jurassic of South Africa (201-189 mya), and is represented by some near-complete fossil material – a rarity for this sort of small ancient mammal, most of which are only known from teeth and other fragments.
About the size of a mouse, only about 12cm long (5″), it was an insectivore with teeth adapted for chewing and crunching through hard arthropod shells. Enlarged regions of its brain associated with the senses of hearing and smell show it was likely nocturnal, occupying an ecological niche similar to modern shrews.
It probably reproduced similarly to modern monotremes, laying small parchment-shelled eggs and lactating from patches of skin. Fossils of the closely related and similar-looking Morganacudon show evidence of toothless infants and juveniles with a single set of milk teeth, suggesting these were some of the first mammals whose young were entirely dependent on milk during the earliest stages of life.
The exact line between “highly mammal-like cynodonts” and “actual mammals” is very blurry. The transition was gradual and the fossil record is incomplete, and even the definition of “mammal” varies depending on who you ask. Do we take the strictest possible route and only include everything coming after the most recent common ancestor of all living mammals – the “crown group” Mammalia itself? Or do we go broader and also include the closely related Mammaliaformes, which already had some of the defining anatomical features of mammals?
(For the purposes of this theme month I’m considering mammaliaformes to count as mammals, but if you prefer the crown group definition then it’ll be a few more days before we reach Mammalia-proper.)
The earliest ancestral mammaliaformes would have looked something like Adelobasileus, a transitional form from the Late Triassic of Texas, USA (221-205 mya). About 10-15cm long (4-6″), it was probably a shrew-like insectivore and may have been close to the start of the hypothetical “nocturnal bottleneck” in mammal evolution – a point where mammal ancestors are thought to have taken up nighttime activity patterns to avoid competition and predation from early dinosaurs.
Sinoconodon is known from the Early Jurassic of China (196-189 mya). Unlike later mammals it seems to have experienced reptile-like continuous slow growth throughout its lifespan, and had multiple replacements of some of its teeth.
Fossils of several different life stages have been found, averaging at similar sizes to Adelobasileus, but the biggest and longest-lived specimens are estimated to have reached the size of a large brown rat at around 35cm long (1′2″) and 500g in weight (~18oz) – big enough to be a weasel-like carnivore feeding on small vertebrate prey.