Tag: therapsid

Month of Mesozoic Mammals #05: Climbing Trees

Agilodocodon

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.

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.

Bulbasaurus

Who’s that synapsid?

It’s Bulbasaurus phylloxyron!

This creature was a member of the dicynodonts, a group of herbivorous mammal-relatives with beaks and protruding tusks. Its fossils are known from the Late Permian of South Africa, about 259-254 million years ago, and it would have been roughly the size of a cat, around 60cm long (2′).

It wasn’t officially named after the pokémon character Bulbasaur, but instead in reference to the bulbous bosses on its snout. But combined with how the species name “phylloxyron” means “leaf razor”, it doesn’t seem to entirely be a coincidence.

Anteosaurus

Anteosaurus magnificus, a dinocephalian from the Middle Permian of South Africa (~266-260 mya). Known from several skulls and fragments of the rest of the skeleton, it was one of the largest carnivorous non-mammalian synapsids with an estimated body length of at least 5m (16′4″).

The skull of Anteosaurus [image source]

It had patches of thickened bone above its eyes forming a pair of short “horns”, as well as heavily reinforced areas around its skull roof and the sides of its lower jaw. These were probably used for head-butting behaviors, and similar adaptations are seen in other groups of dinocephalians.

The front part of its mouth was also prominently upturned, and it had enlarged “sabretooth” fangs – although these features are covered by lips in my reconstruction.

Geikia

Geikia elginensis, a dicynodont synapsid from the Late Permian of Scotland (~254-252 mya). Known only from a single skull discovered in the 1890s, it would have measured around 50cm long and was closely related to South African forms like Bulbasaurus.

It had an unusually shortened snout and forward-facing eyes – sort of like a pug with a beak – and a pair of protruding nasal bosses on its snout. It was probably a selective browser, biting off small pieces of vegetation at a time, and its large eyes and stereoscopic vision suggest it may have been nocturnal.

Unsolved Paleo Mysteries Month #08 – Everything Dies Except Lystrosaurus

The extinction event that wiped out the non-avian dinosaurs is probably the most “famous” mass extinction, but it wasn’t the worst one in Earth’s history. That morbid honor goes to the Permian-Triassic extinction 252 million years ago – also aptly known as the Great Dying.

A truly massive amount of biodiversity was lost in this event, with 96% of marine species and 70% of terrestrial species disappearing. Some marine ecosystems seemed to rebound fairly quickly, but overall it may have taken at least 5-10 million years for anything close to full recovery. Terrestrial vertebrates may even have taken up to 30 million years to regain previous levels of diversity.

And… we’re not sure why it happened.

One of the main potential culprits is the massive eruption of the Siberian Traps – one of the largest known volcanic events on Earth – but other explanations include an asteroid impact, methane-producing microbes, ocean anoxia, the formation of Pangaea, a nearby supernova destroying the ozone layer, and even dark matter.

Or it might have been a result of multiple causes at once, events that wouldn’t have been so severe individually but became disastrous in combination. This is known as the “Murder on the Orient Express Model”: maybe they all did it.

But there’s also a secondary element to today’s mystery. In the aftermath of the Great Dying, a small dicynodont synapsid briefly took over the world. For the first few million years of the Triassic, around 95% of the Earth’s population of terrestrial vertebrates were all Lystrosaurus – no other genus or species of animal has ever dominated to such a degree.

Why did these squat little dog-sized animals survive and thrive when everything else was struggling? They might have been opportunistic generalists able to deal with changing conditions better than other groups, the extinction of most large predators may have allowed their population to explode, or it might simply have been a matter of luck.

We just don’t know.