Month of Mesozoic Mammals #03: Almost-Mammals

Adelobasileus & Sinoconodon

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.)

An illustration of an extinct early mammal. It's a rat-likeor shrew-like animal with a long snout, small ears, and a long furry tail.
Adelobasileus cromptoni

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.

An illustration of an extinct early mammal. It resembles a mixture of a rat and a weasel, with a long low blunt snout, small ears, short legs, and a long bushy tail.
Sinoconodon rigneyi

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.

Month of Mesozoic Mammals #02: Swimming Cousins

Kayentatherium

Known from the Early Jurassic of Arizona (196-183 mya), Kayentatherium was part of a group of cynodonts called tritylodontids – very close cousins of the true mammals, specialized for herbivory. They had strong jaw muscles, large incisors, and grinding cheek teeth, an arrangement convergently similar to modern rodents, and were some of the latest-surviving non-mammalian synapsids, persisting into the Early Cretaceous.

Kayentatherium wellesi skull by 5of7 || CC BY 2.0

Kayentatherium was one of the larger tritylodontids at just over 1m long (3′3″), and appears to have been semi-aquatic, with oar-shaped hindlimbs and a flattened beaver-like tail. Although not the first non-mammalian synapsid to be interpreted as a swimmer, it was the earliest close relative of the true mammals to develop these sorts of adaptations.

Month of Mesozoic Mammals #01: Cynodont Ancestors

Welcome to March, and the Month of Mesozoic Mammals!

Although traditionally depicted as tiny “boring” shrew-like animals completely overshadowed by the dinosaurs they lived alongside, in the last decade or two we’ve discovered that Mesozoic mammals were actually incredibly diverse. They ranged in size from only a few centimeters to over a meter long, adapted to a wide range of ecological niches, and developed into some of the most successful and long-lived mammal groups of all time.

So this month we’ll be looking at how mammals evolved and experimented during the Age of Dinosaurs, from their earliest Triassic ancestors all the way to the end-Cretaceous extinction.

Starting with…

Thrinaxodon & Trucidocynodon

All mammals are synapsids (related to animals like Dimetrodon), and are descended from a group known as the cynodonts.

Cynodonts originated in the Late Permian, about 260 million years ago, and were one of the few synapsid lineages to survive through the devastating Permian-Triassic extinction. Although not quite mammals themselves, their skeletons were already fairly mammal-like, with semi-upright postures, differentiated thoracic and lumbar vertebrae, a secondary palate that allowed them to eat and breathe at the same time, and pits on their snouts suggesting they had well-developed whiskers – which would also imply the presence of a coat of fur, since whiskers are modified hairs.

An illustration of an extinct cynodont, a close relative of early mammals. It's a vaguely badger-like animal with small ears and a long tail, pictured curled up asleep.
Thrinaxodon liorhinus

Thrinaxodon was an early cynodont about 50cm long (1′8″), living in the Early Triassic of South Africa and Antarctica shortly after the mass extinction (~252-247 mya).

It was capable of digging, with complete specimens found curled up inside their burrows, including pairs that may indicate some form of social behavior and one instance of sharing with a temnospondyl amphibian.

An illustration of an extinct cynodont, a relative of early mammals. It's a somewhat badger-like animal with small ears and a long tail.
Trucidocynodon riograndensis

Trucidocynodon lived later during the Triassic in Brazil (~220 mya) and was one of the biggest known non-mammalian cynodonts at around 1.2m long (4′).

It had more upright limbs than some of its other relatives, and a semi-digitigrade stance that seems to have been adapted for running, suggesting it was an active predator. Considering it was living at a time when predatory crocodile-relatives and early dinosaurs were thought to be the dominant carnivores, its large size is especially surprising.

Caviramus

Caviramus schesaplanensis, a pterosaur from the Late Triassic of Switzerland (~205 mya). Known from two fossil specimens – a partial jaw and a much more complete skull and skeleton – it was about the size of a modern raven, with a length of around 60cm (2′) and a wingspan of 1.35m (4′5″).

(The more complete fossil is also sometimes considered to be a separate genus and species, Raeticodactylus filisurensis, depending on which pterosaur specialist you ask. If it was a different animal it still would have been very closely related to Caviramus, though, and the two would likely have looked very similar to each other.)

It had some odd anatomy for an early pterosaur, with proportionally long and slender limbs and a fairly heavily-built skull. There were bony crests on both its upper and lower jaws, with the upper crest probably supporting a much larger soft-tissue structure.

Powerful jaw muscles along with a combination of fang-like teeth at the front of its jaws and and serrated slicing-chewing teeth further back suggest it was specialized for eating particularly tough foods such as hard-shelled invertebrates – and it may even have been omnivorous, capable of eating plant matter as well.

Eospinus

Eospinus daniltshenkoi, a tetraodontiform fish from the early Eocene of Turkmenistan (~56-48 mya). Only about 5cm long (2″), it was a close relative of modern boxfish and triggerfish, as well as a completely extinct group called spinacanthids.

It was heavily armored, with large plate-like scales creating a boxfish-like carapace, but its most distinctive feature was its multiple long spines – three dorsal spines on its back, a fourth on its head resembling a “horn”, a pair of smaller spines on the sides of its body, and one on its underside formed from partially fused vestigial pelvic fins.

Boverisuchus

Boverisuchus magnifrons*, a crocodilian from the early Eocene of Germany (~50-40 mya). Reaching about 3m long (9′10″) it was much more heavily armored than its modern cousins, with an interlocking “exoskeleton” of bony osteoderms covering its body and limbs – leading to it being given the nickname “panzer croc”.

It was adapted for walking and running on land, with relatively long legs and surprisingly hoof-like claws. It may even have carried its weight directly on these hooves similar to mammalian ungulates.

And if that’s not unusual enough, its hind leg musculature suggests it also might have been capable of short bursts of bipedal sprinting.

[ * Originally known as Pristichampsus rollinatii before being reassigned in 2013.]

Phenacodus

Phenacodus primaevus, a mammal from the Late Paleocene to Middle Eocene of North America and Europe (~60-48 mya). About 1.5m long (5′), it’s thought to have been one of the earliest known odd-toed ungulates, walking on its middle three hoofed toes.

Its teeth were adapted for a diet of mostly plant matter, although it may also have been opportunistically omnivorous.

Another species in the same genus, Phenacodus intermedius, had a skull structure that suggests it might have had a muscular prehensile upper lip – or perhaps even a short tapir-like proboscis.

Westlothiana

Westlothiana lizziae from the Early Carboniferous of Scotland (~338 mya).

About 20cm in length (8″), this superficially lizard-like creature (nicknamed “Lizzie” by its discoverer) had a long slender body with relatively small legs, which may have been adaptations for burrowing similar to modern skinks.

Its anatomy shows a mixture of both amphibian and reptilian characteristics, suggesting it may have been a close relative of the first true amniotes. But exactly where it fits in that area of the evolutionary tree is still uncertain, with different paleontologists classifying it as either an early amniote, a reptilomorph, or a lepospondyl.

Longipteryx

Longipteryx chaoyangensis, an enantiornithine from the Early Cretaceous of China, about 120 million years ago. With a body length of only around 15cm (6″), it had a long snout tipped with a few hooked teeth and feet capable of perching – features that indicate it may have lived very similarly to modern kingfishers, feeding on fish and small invertebrates in its swampy forest habitat.

The enantiornithines were a sort of “cousin” lineage to modern birds. Most had toothy jaws and clawed wings, and the wide variety in their skull shapes suggests that they were specialized for many different dietary niches. The entire group went extinct during the K-Pg mass extinction and left no living descendants, but during the Cretaceous they were the most widespread and diverse group of birds*, with fossils currently known from every continent except Antarctica.

* Depending on how you define “bird”.

Syringocrinus

Syringocrinus paradoxicus from the Upper Ordovician of North America (~450 mya). Measuring up to around 6cm long (2.3″), it was part of an extinct group of marine animals known as solutes – characterized by irregularly-shaped bodies covered in calcite armor plates, the structure of which suggest they were echinoderms despite their complete lack of any proper symmetry.

It had two appendages, one a short “arm” that was probably used for feeding on food particles suspended in the water, and the other forming a longer stalk-like “tail” that may have served to propel it along the seafloor.

Solutes were once thought to be closely related to the equally weird-looking stylophorans, but some versions of the echinoderm family tree place them much further apart, suggesting their superficial similarities may have been due to convergent evolution instead.