Was Hylonomus lyelli
The First Reptile?

Hylonomus lyelli (meaning Lyell's Wood Mouse) is the earliest known amniote in the fossil record. This is another way of saying it is the oldest reptile yet discovered. The rocks it was found in are 312 million years old. This little creature was a tree dweller and looked much the same as small lizards of today. It measured about 20 cm in length (9 inches) including the tail. It had sharp pointy teeth that were best suited to catching and eating insects, so it is assumed that its diet consisted of these creatures.

Some remains of this ancient reptile have been found in the hollowed out stumps of fossilized trees such as the giant lycopod Sigillaria shown below to the right. There are a couple of possible explanations for this. One is that the unfortunate creatures fell into pits made by the rotted underground tree stump, but this seems unlikely as these creatures lived in trees and were presumably good climbers.

What Made It A Reptile?

The big leap forward for reptiles over their amphibious cousins was the amniotic egg. However since the soft parts of bodies and other things like eggs are never fossilized, other features need to be used to tell whether Hylonomus was a reptile.

There are subtle differences in the skeleton between it and the very similar looking amphibian Microsaurs that were around at the same time. Chief among these is the presence of an ankle bone formed by the fusion of three separate foot bones. This is present in all fossil and modern reptiles, but is never found in fossil amphibians from the same time.

Where Was It Found?

Fossilized remains of H. lyelli were found at the Joggins formation in Nova Scotia, Canada by Sir William Dawson in the mid 1800s. The rock layers they were in are 312 million years old, placing this species in the late Carboniferous period of the Earth's past. Animal tracks from the same region point to this creature being around as far back as 315 million years ago. The species was more recently described extensively by Robert L. Carroll, who clarified its real status as a reptile.

The Original Reptile?

Specimens of Hylonomus also possess canine teeth which are a feature of the earliest known mammal-like reptiles (synapsids). This suggests that H. lyelli could be the ancestor of all reptile groups, or at least a very close relative of such an animal. More details about the exact position of H. lyelli in regard to the evolution of all reptile groups will hopefully come to light with more fossil discoveries in the future.

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