A Tale of Two Skulls: Part 1

The early colonisation of land by animals more complex than insects was lead by a group known as reptilomorphs, early fish-like animals that had developed four simple limbs to enable them to explore this new world. This began 370 million years ago1, and while these animals were initially aquatic, over the following tens of millions of years they evolved to spend increasing amounts of time outside the water, becoming increasingly amphibian-like.

It’s important to understand what kind of world they emerged into—the late Devonian mass extinction was ongoing, devastating sea life and culminating in the Hangenberg event which ended the Devonian2. On land, it was wet, warm, and completely alien—giant fungus trees up to eight metres tall dominated the landscape, with mats of liverwort covering the ground3. Ferns and the earliest trees dotted the land, creating the world’s first forests4.

By the start of the Carboniferous period 360 million years go, the amphibian-like reptilomorphs successfully colonised the land, but over the next fifty million they faced a new challenge. They were still semi-aquatic, and as such needed to lay their eggs in water, which was fine in the wet warmth of the early Carboniferous, but the world began to cool and dry. The trees of this world had evolved lignin in their wood, but no organism had evolved yet that was able to eat it, meaning that when a tree died it was buried, removing carbon from the atmosphere and laying down vast beds of coal5.

However, one group of reptilomorphs evolved to become increasingly lizard-like, and 310 million years ago they evolved the ability to lay eggs on land, without the need to return to the water6. The reptilomorphs were now two distinct groups: the amniotes, with their new-found ability to lay eggs on land, and the amphibians, who are still recognisable today.

The amniotes spread across the world, but there are two groups of particular interest to this story—the synapsids and diapsids. These two groups were named after a curious feature of their skulls, with the synapsids possessing one hole behind the eye7, and the diapsids two8, and these are the skulls that will form the basis of our story.

As the world continued to cool, the rainforests that allowed the ancestors of amniotes to colonise the world began to collapse, creating new niches that amniotes could inhabit but amphibians couldn’t, allowing them to spread and diversify. This marked the dying days of the Carboniferous, and as the ice age rolled in 300 million years ago, the Permian began.

For fifty million years, the Permian was dominated by two groups of large land animals: the amphibians and the synapsids. The diapsids were relegated to small, lizard-like organisms, failing to measure up against the several-metre long amphibians like the Diadectes9, or the pelycosaurs—synapsids that included the apex predator Dimetrodon10.

Dimetrodon NT2 small.jpg
Dimetrodon, an early-Permian syanpsid, by Nobu Tamura

Diadectes, an early-Permian amphibian, by Dmitry Bogdanov (CC BY-SA 3.0, Link)

Towards the end of the Permian, amphibians, synapsids, and diapsids continued to diversify, including two seemingly-unimportant groups—cynodonts, a diverse group of synapsids that could be readily described as ‘small-dog lizards’11; and the archosauriformes, diapsids that superficially represent ‘crocodile-lizards’12. At this time, nothing seemed capable of ending the dominance of amphibians and synapsids over the land.

Until the Great Dying.

Procynosuchus BW.jpg
P. delaharpeae, an early cynodont, by Nobu Tamura

Archosaurus ross1DB.jpg
Archosaurus, a late-Permian archosauriform, by Dmitry Bogdanov (CC BY-SA 3.0, Link)

Feature Picture: Thrinaxodon, an early-Triassic cynodont, by Nobu Tamura

A Tale of Two Skulls: Part 1

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