Sail of the century

These mammal-like reptiles spread right across a single great land mass called Pangaea, between 320 and 220 million years ago. (We've finally arrived at your fingers.) One of the first mammal-like reptiles was one of the most peculiar. Known as Dimetrodon, it was about the size of a dog, but with shorter, stubby legs, like a crocodile. The really odd thing was the huge, fin-like sail on its back. Extremely long, thin spines stuck up from the animal's backbones. Between them was a thin membrane, like the sails of a yacht. (If they ever got caught on an iced-over

Mammal-like reptiles appear

Fossils show that the bases of the spines were packed with a rich supply of blood vessels. The sail probably worked like a reverse-cycle air conditioner. If Dimetrodon stood sideways to the sun in the early mornings, it would quickly warm up. If it got too hot, it could turn away from the sun, or find some shade, and it would lose heat through these huge sails. Like lizards and snakes today, these animals were cold-blooded. That's why we still call them reptiles, even though they were in some ways similar to mammals.

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Dimetrodon was one of a group of animals called pelycosaurs. While some of the pelycosaurs were peaceful plant-eaters, others were ferocious meat-eaters. Sphenocodon, for instance, was as large as a lion, and had a mouth crammed full of sharp, vicious teeth. Other pelycosaurs, called ophiacodonts, seemed to think they were crocodiles, living for much of their time in the water. They had a long snout, studded with many sharp, peg-like teeth. Their eyes were set high on their head, so they could peek out of the water while being almost totally submerged.

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