Reptiles with waterproof skin and eggs are colonising the land. They are not dinosaurs, but synapsids: a group defined by the single hole in the skull behind each eye where jaw muscles attach. Mammals are synapsids too, so these creatures are more closely related to us than to dinosaurs.
Sail-backed Synapsids
Sail-backed synapsids (above), like the plant-eating Edaphosaurus on the right, are common. They can grow up to 3.5 metres long. The carnivorous Dimetrodon, at back left, is a little longer, reaching up to 4.6 metres. The sails on these species may have heated and cooled the body. Skulking in the left foreground is the massive-skulled Ophiacodon. These early synapsids are known as pelicosaurs.
The first therapsids
By the mid-Permian, pelicosaurs are being displaced by therapsids (above). This group was becoming more mammal-like: their legs were positioned vertically under their body and they had three types of teeth – incisors, canines and molars. (A reptile’s teeth may be different sizes but they are all the same shape). Some were also thought to have fur and be warm- blooded.
Dinocephalians, a sub-group distinguished by their interlocking incisors, dominated the mid-Permian. They weighed up to two tonnes. Dinocephalians included herbivores such as this herd of Estemmenosuchus or Ulemosaurus, represented by the fossil, and the carnivorous Eotitanosuchus, emerging from the water, which could reach a length of five metres. The whole group mysteriously disappeared around 270 million years ago.
Gorgonopsids
Gorgonopsids, a later group of therapsids, were fearsome carnivores. The name refers to the Greek monster the Gorgon. Some of the largest examples include the three-metre-long Inostrancevia (below), and the similarly sized Dinogorgons (above), shown here fighting over a carcass.
Gorgonopsids were characterised by their large, powerful jaws and sabre-teeth. But their mighty incisors could not save them from the biggest mass extinction event in Earth’s history. Thought to have been triggered by a series of massive volcanic eruptions in what is now Siberia, 80-90% of plant and animal species disappeared in what is known as “The Great Dying”. It marked the end of the Permian and the start of the Triassic.
Cynodont survivors
The therapsids were almost wiped out in the Great Dying, clearing the way for dinosaurs. They were diapsids – distinguishable by two holes in the skull behind each eye socket, like modern-day birds and lizards.
A handful of therapsids survived. Among them were the herds of herbivorous Lystrosaurus, shown at the water’s edge and in fossilised form. And most importantly for us, the cynodonts: the ancestors of mammals. One is shown here edging out onto the finger of rock (above).
Little holes in the fossilised snouts of cynodonts suggest they had whiskers, which means they probably had fur and were warm-blooded.
The cynodonts lived in the dinosaurs’ shadow for 200 million years, until a mass extinction triggered by a crashing comet favoured this ancient lineage once again.