By | September 9, 2016

Mammals evolved from a group of reptilelike animals called pelycosaurs. The pelycosaurs lived long before even the dinosaurs and for a while were the largest animals on land. They looked more like lizards than mammals, but their link with mammals is clear from a special hole in the skull behind each eye. As in mammals, the jaw muscles passed through this hole, giving these animals a killer bite.


Key features
■ Cold-blooded
■ Reproduced by laying eggs
■ Small brains
■ Lizardlike, sprawling legs
■ Short claws on toes
■ Holes in the skull behind the eyes
■ Varied teeth

Pelycosaurs first appeared in the Late Carboniferous, 320 million years ago. They died out in the Late Permian, 251 million years ago.

Dimetrodon (die-MET-roe-don)


When: 280 million years ago (Early Permian)

Fossil location: Germany, USA

Habitat: Swamps

Length: 10 ft (3 m)

Diet: Meat

Dimetrodon was the most fearsome predator of its time. It was built like a Komodo dragon, but with a huge “sail” on its back formed from skin wrapped over rods of bone. Dimetrodon means “two-sized tooth”—unlike most reptiles, which have teeth that are similar to each other, Dimetrodon had teeth of several types, as mammals have. At the front of the mouth were long, daggerlike canines for piercing and grabbing flesh; at the back were smaller, sharp-edged teeth for slicing flesh.



Dimetrodon’s footprint

This five-toed footprint may have been left by Dimetrodon, one of the most common animals of its time.


Dimetrodon had a hole in the skull behind each eye socket. Strong jaw muscles went through these holes, giving it a powerful bite. Humans share the same feature.

Varanops (VA-ran-ops)


When: 260 million years ago (Late Permian)

Fossil location: USA, Russia

Habitat: Swamps

Length: 3 ft (1 m)

Diet: Small animals

Varanops looked like a modern monitor lizard. Compared to other pelycosaurs it was a fast-moving hunter with long legs, well suited to scampering after small animals, which it caught and killed with strong jaws lined with dozens of backward-curved teeth. Varanops lived in the Late Permian and was one of the last of the pelycosaurs.

Ophiacodon (oh-fee-ACK-oh-don)


When: 310–290 million years ago (Late Carboniferous–Early Permian)

Fossil location: USA

Habitat: Swamps

Length: 10 ft (3 m)

Diet: Fish and small animals

This large predator had a very long skull and huge jaws packed with 170 sharp, pointed teeth. It was built like a crocodile and may even have hunted like a crocodile, lurking in swamps or rivers ready to ambush passing prey. However, Ophiacodon is unlikely to have been an underwater hunter, since its tall skull would have been difficult to swing sideways in water, making prey such as fish difficult to catch. On land, Ophiacodon walked with its limbs sprawled like a lizard, dragging its tail behind it.

Eothyris (ee-oh-THY-riss)


When: 280 million years ago (Early Permian)

Fossil location: USA

Habitat: Swamps

Length: Skull 2.1/2 in (6 cm)

Diet: Meat

Only a single fossil of Eothyris exists: a broad, flat skull that was discovered in 1937. It shows that Eothyris probably had a quick, snapping bite. On each side of the upper jaw were two large fangs. The remaining teeth were smaller but sharply pointed, so Eothyris was a flesh-eater. A small animal, it perhaps hunted insects or reptiles smaller than itself.



Dimetrodon’s skeleton

Running along Dimetrodon’s back was a spectacular “sail” supported by tall rods of bone that grew from its spine. The sail might have been used to help this cold-blooded creature warm its body. In the early morning, Dimetrodon would have been cold and sluggish. Perhaps it basked in the sun, turning its body so the sail caught the sun’s rays. Blood flowing through the sail would have spread the warmth through the rest of the body, helping Dimetrodon become active.


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