The First Reptiles and Their Evolution

The First Reptiles and Their Evolution We all know how the old story goes: Fish evolved into tetrapods, tetrapods evolved into amphibians, and amphibians evolved into reptiles. Its a gross oversimplification, of course- for example, fish, tetrapods, amphibians,  and reptiles all coexisted with each other for tens of millions of years- but itll do for our purposes. And for many fans of prehistoric life, the last link in this chain is the most important, since it was the dinosaurs, pterosaurs and marine reptiles of the Mesozoic Era that all descended from ancestral reptiles. Before we proceed any further, though, we need to define what the word reptile means. As far as biologists are concerned, the single defining characteristic of reptiles is that they lay hard-shelled eggs on dry land (as opposed to amphibians, which are constrained to lay their softer, more permeable eggs in water). Secondarily, compared to 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). Depending on how strictly you define the term, there are two prime candidates for the first-ever reptile. The first is the early Carboniferous (about 350 million years ago) Westlothiana, from Europe, which laid leathery eggs but otherwise had a distinctly amphibian anatomy, especially pertaining to its wrists and skull. The second (and more widely accepted) candidate is Hylonomus, which lived about 35 million years after Westlothiana and resembled the kind of small, skittery lizard you run across all the time in modern pet stores. This is all 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 course of the Carboniferous and Permian periods. Anapsids like 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 both the left and right sides. These lighter skulls, with their multiple attachment points, proved to be a good template for later evolutionary adaptations. Why is this important? Well, 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, 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 like plesiosaurs and ichthyosaurs. Lifestyles of the First Reptiles But were getting ahead of ourselves; much of this information is discussed in  Before the Dinosaurs - Pelycosaurs, Archosaurs, and Therapsids. What were interested in here is the obscure group of lizard-like reptiles that succeeded Hylonomus and preceded these better-known (and much larger) beasts. Its not that solid evidence is lacking; plenty of obscure reptiles have been discovered in Permian and Carboniferous fossil beds, especially in Europe. Its that most of these reptiles look so similar that it can be an eye-rolling exercise to attempt to distinguish between them. The exact classification of these animals is a matter of continuing debate, but heres our attempt to cut through the froth: Captorhinids, exemplified by Captorhinus and Labidosaurus, are the most basal, or primitive, reptile family yet identified, only recently evolved from amphibian ancestors like 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) may or may not 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 the course of their reign, the pareiasaurs evolved elaborate armor, which still didnt prevent them from going extinct 250 million years ago! Millerettids were small, lizardy-looking reptiles that subsisted on insects, and also went extinct at the end of the Permian period. The two most well-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 would be 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 like Megalancosaurus and Drepanosaurus that also lived high up in trees, but lacked the ability to fly.