Tag: synapsid

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.