Therapsids the what

By about 265 million years ago (round about your knuckles) most pelycosaurs had died out. Replacing them were a new group of mammal-like reptiles, called the therapsids (not a word to say first thing in the morning with a mouth full of muesli). These little beauties would have been favourites to win first prize in any ugly competition. Some had a bulky body, stubby legs, short tail and oversized head, often with

protruding fangs that probably dripped litres of drool. Many looked like a cross between a hippopotamus and a crocodile.

The therapsids were much more mammal-like than the pelycosaurs. The world in which they lived was warmer, so they didn't need a built-in radiator to warm them up in the mornings. The first therapsids were small, lightly built animals, and almost certainly efficient killers. Many had long snouts packed with a fearsome array of teeth that allowed them to neatly slice flesh from their prey. Unlike their lumbering pelycosaur relatives, who had slowly waddled along, these therapsids had limbs set beneath their bodies, rather than outwards. Because they stood more upright they could run fast.

Not all therapsids were meat-eaters. It wasn't long before plant-eaters evolved. Some, like Estemmenosuchus from Russia, were ox-like, with horns protruding from their heads. One, called Moschops, was the size of a small elephant. Instead of a trunk it had a large, parrot-like beak. Later ones that lived about 220 million years ago had much smaller heads.

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The most common were the dicynodonts (which means they had a pair of tusk-like teeth). Their other teeth were really tiny and useless. Some may have lived in the water, like hippos. Others may have burrowed in the ground, like wombats.

The most mammal-like of the later meat-eating therapsids were the cynodonts. These were dog-like animals, that had long legs and short tails, and bodies that may well have been covered in fur. If you could

have peered into their mouth, without having your nose bitten off in the process (or fainting at the smell, like a four-week-old meat pie), you would have seen that their teeth were quite like yours. They had cheek teeth for chewing, and canines (your Dracula teeth) and incisors (the ones you use, but shouldn't, for biting your nails). The tooth fairy of the time would have been extremely overworked, because cynodonts shed teeth and replaced them with new ones throughout their lives. This is something reptiles do. We change our teeth only once, like other mammals.

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