Dimetrodon

Name:

Dimetrodon (Greek for "two measures of teeth"); pronounced die-MET-roe-don

Habitat:

Swamps of North America

Historical Period:

Permian (280 million years ago)

Size and Weight:

About 11 feet long and 500 pounds

Diet :

Meat

Distinguishing Characteristics:

Big head; narrow body; large, thin sail on back

About Dimetrodon:

It's often mistaken for a dinosaur, but Dimetrodon lived long, long before the first dinosaurs evolved--280 million years ago, during the Permian period. An example of a pelycosaur--one of the first families of reptiles to live entirely on land--it went extinct about 250 million years ago, along with countless other species. (See 10 Facts About Dimetrodon and a gallery of Dimetrodon pictures.)

Dimetrodon's most spectacular feature was its large sail, which ran along the entire length of its back. Paleontologists speculate that the main purpose of this sail was to regulate its owner's temperature: it soaked up sunshine during the day (much like a solar panel), and also helped dissipate energy if Dimetrodon became overheated. Secondarily, this sail may have served as a mating display, and it doesn't hurt that it helped Dimetrodon look a lot bigger in the eyes of any hungry carnivores lurking nearby.

archaeothyris

Name:

Archaeothyris; pronounced ARE-kay-oh-THIGH-riss

Habitat:

Swamps of North America

Historical Period:

Late Carboniferous (305 million years ago)

Size and Weight:

About 1-2 feet long and a few pounds

Diet:
Probably carnivorous

Distinguishing Characteristics:

Small size; powerful jaws with sharp teeth

About Archaeothyris:

To the modern eye, Archaeothyris looks like pretty much any other small, scurrying lizard, but this reptile has an important place in the evolutionary family tree: it's the first known synapsid, a family of reptiles characterized by the unique openings in their skulls.

As such, this late Carboniferous creature is believed to have been ancestral to all subsequent pelycosaurs and therapsids, not to mention the early mammals that evolved from therapsids during the Triassic period.

Hylonomus

Name: 

Hylonomus (Greek for "forest mouse"); pronounced high-LON-oh-muss

Habitat:

Forests of North America

Historical Period:

Carboniferous (315 million years ago)

Size and Weight:

About one foot long and one pound

Diet:

Insects

Distinguishing Characteristics:

Tiny size; sharp teeth

About Hylonomus:

It's always possible that a more ancient candidate will be discovered, but as of now, Hylonomus is the oldest true reptile known to paleontologists: this tiny critter scuttled around the forests of the Carboniferous period over 300 million years ago. Based on reconstructions, Hylonomus certainly looked distinctly reptilian, with its quadrupedal, splay-footed posture, long tail, and sharp teeth.

Hylonomus is also a good object lesson in how evolution works. You might be surprised to learn that the oldest ancestor of the mighty dinosaurs (not to mention modern crocodiles andbirds) was about the size of a small gecko, but new life forms have a way of "radiating" from very small, simple progenitors. For example, all mammals alive today--including humans and sperm whales--are ultimately descended from a mouse-sized ancestor that scurried beneath the feet of huge dinosaurs more than 200 million years ago.

Family tree of the reptiles and their descendants

Family tree of the reptiles and their descendants

The reptile family began when the first hard shelled egg was laid about 310 million years ago. Since then reptiles have invaded the land, sea and air by evolving into a variety of fascinating and bizarre types. Only a few of them survive to this day. Mammals and birds can also trace their ancestry to early reptiles.

This chart illustrates the major branches of the reptile family tree, showing how the groups relate to one another and approximately when they evolved. The relative size of each group is not indicated. The bars of color that reach the top indicate the groups that are still living today.