The First Reptiles

Ancestral Reptiles of the Carboniferous and Permian Periods

Despite its name, Tetraceratops wasn't related to the much later Triceratops
Dmitry Bogdanov

Everyone agrees how the old story goes: Fish evolved into tetrapods, tetrapods evolved into amphibians, and amphibians evolved into reptiles. It's a gross oversimplification, of course—for example, fish, tetrapods, amphibians, and reptiles coexisted for tens of millions of years—but it'll do for our purposes. For many students of prehistoric life, the last link in this chain is the most important, since the dinosaurs, pterosaurs, and marine reptiles of the Mesozoic Era all descended from ancestral reptiles.

Before proceeding, though, it's important to define what the word reptile means. According to biologists, the single defining characteristic of reptiles is that they lay hard-shelled eggs on dry land as opposed to amphibians, which must lay their softer, more permeable eggs in water. Secondarily, compared with amphibians, reptiles have armored or scaly skin, which protects them from dehydration in the open air; larger, more muscular legs; slightly bigger brains; and lung-powered respiration though no diaphragms, which were a later evolutionary development.

First Reptile

Depending on how strictly you define the term, there are two prime candidates for the first-ever reptile. One is the early Carboniferous Period (about 350 million years ago) Westlothiana, from Europe, which laid leathery eggs but otherwise had an amphibian anatomy, especially pertaining to its wrists and skull. The other, more widely accepted candidate is Hylonomus, which lived about 35 million years after Westlothiana and resembled the small, skittery lizard you run across in pet stores.

This is simple enough, as far as it goes, but once you get past Westlothiana and Hylonomus, the story of reptile evolution gets much more complicated. Three distinct reptilian families appeared during the Carboniferous and Permian periods. Anapsids such as Hylonomus had solid skulls, which provided little latitude for the attachment of robust jaw muscles; the skulls of synapsids sported single holes on either side; and the skulls of diapsids had two holes on each side. These lighter skulls, with their multiple attachment points, proved to be good templates for later evolutionary adaptations.

Why is this important? Anapsid, synapsid, and diapsid reptiles pursued very different paths toward the start of the Mesozoic Era. Today, the only living relatives of the anapsids are turtles and tortoises, though the exact nature of this relationship is hotly disputed by paleontologists. The synapsids spawned one extinct reptilian line, the pelycosaurs, the most famous example of which was Dimetrodon, and another line, the therapsids, which evolved into the first mammals of the Triassic Period. Finally, the diapsids evolved into the first archosaurs, which then split off into dinosaurs, pterosaurs, crocodiles, and probably marine reptiles such as plesiosaurs and ichthyosaurs.

Lifestyles

What's of interest here is the obscure group of lizard-like reptiles that succeeded Hylonomus and preceded these better-known and much larger beasts. It's not that solid evidence is lacking; plenty of obscure reptiles have been discovered in Permian and Carboniferous fossil beds, especially in Europe. But most of these reptiles look so similar that attempting to distinguish between them can be an eye-rolling exercise.

Classification of these animals is a matter of debate, but here's an attempt to simplify:

  • Captorhinids, exemplified by Captorhinus and Labidosaurus, are the most "basal," or primitive, reptile family yet identified, only recently evolved from amphibian ancestors such as Diadectes and Seymouria. As far as paleontologists can tell, these anapsid reptiles went on to spawn both synapsid therapsids and diapsid archosaurs.
  • Procolophonians were plant-eating anapsid reptiles that (as mentioned above) might have been ancestral to modern turtles and tortoises. Among the better-known genera are Owenetta and Procolophon.
  • Pareiasaurids were much larger anapsid reptiles that counted among the biggest land animals of the Permian Period, the two best-known genera being Pareiasaurus and Scutosaurus. Over their reign, Pareiasaurs evolved elaborate armor, which still didn't prevent them from going extinct 250 million years ago.
  • Millerettids were small, lizard-like reptiles that subsisted on insects and also went extinct at the end of the Permian Period. The two best-known terrestrial milleretids were Eunotosaurus and Milleretta; an ocean-dwelling variant, Mesosaurus, was one of the first reptiles to "de-evolve" to a marine lifestyle.

Finally, no discussion of ancient reptiles is complete without a shout-out to the "flying diapsids," a family of small Triassic reptiles that evolved butterfly-like wings and glided from tree to tree. True one-offs and well out of the mainstream of diapsid evolution, the likes of Longisquama and Hypuronector must have been a sight to see as they fluttered high overhead. These reptiles were closely related to another obscure diapsid branch, the tiny "monkey lizards" such as Megalancosaurus and Drepanosaurus that also lived high in trees but lacked the ability to fly.