Tag: stem-mammal

Month of Mesozoic Mammals #04: The Famous One

Megazostrodon

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.

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.