10 Facts About Reptiles

Reptiles have gotten a raw deal in the modern era—they're nowhere near as populous and diverse as they were 100 or 200 million years ago, and many people are creeped out by their sharp teeth, forked tongues, and/or scaly skin. 

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Reptiles Evolved From Amphibians

Hylonomus, the first true reptile. Wikimedia Commons

Yes, it's a gross simplification, but it's fair to say that fish evolved into tetrapods, tetrapods evolved into amphibians, and amphibians evolved into reptiles—all of these events taking place between 400 and 300 million years ago. And that's not the end of the story: about 200 million years ago, the reptiles we know as therapsids evolved into mammals (at the same time the reptiles we know as archosaurs evolved into dinosaurs), and another 50 million years after that, the reptiles we know as dinosaurs evolved into birds. This "in-betweeness" of reptiles may help to explain their relative scarcity today, as their more evolved descendants out-compete them in various ecological niches.

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There Are Four Main Reptile Groups

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You can count the varieties of reptile alive today on one hand: turtles, which are characterized by their slow metabolisms and protective shells; squamates, including snakes and lizards, which shed their skins and have wide-opening jaws; crocodilians, which are the closest living relatives of both modern birds and extinct dinosaurs; and the strange creatures known as tuataras, which today are restricted to a few remote islands of New Zealand. (Just to show how far reptiles have fallen, pterosaurs, which once ruled the skies, and marine reptiles, which once ruled the oceans, went extinct along with the dinosaurs 65 million years ago.)

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Reptiles Are Cold-Blooded Animals

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One of the main characteristics that distinguishes reptiles from mammals and birds is that they're ectothermic, or "cold-blooded," relying on external weather conditions to power their internal physiology. Snakes and crocodiles literally "fuel up" by basking in the sun during the day, and are especially sluggish at night, when there's no available energy source. The advantage of ectothermic metabolisms is that reptiles need to eat much less than comparably sized birds and mammals; the disadvantage is that they're unable to sustain a consistently high level of activity, especially when it's dark.

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All Reptiles Have Scaly Skin

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The rough, vaguely alien quality of reptilian skin makes some people uneasy, but the fact is that these scales represent a major evolutionary leap: for the first time, thanks to this layer of protection, vertebrate animals could move away from bodies of water without risk of drying out. As they grow, some reptiles, like snakes, shed their skin all in one piece, while others do it a few flakes at a time. As tough as it is, the skin of reptiles is fairly thin, which is why snake leather (for example) is strictly decorative when used for cowboy boots, and is much less useful than multi-purpose cowhide!

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There Are Very Few Plant-Eating Reptiles

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During the Mesozoic Era, some of the biggest reptiles on earth were devoted plant-eaters—witness the multi-ton likes of Triceratops and Diplodocus. Today, oddly enough, the only herbivorous reptiles are turtles and iguanas (both of which are only remotely related to their dinosaur forebears), while crocodiles, snakes, lizards, and tuataras subsist on vertebrate and invertebrate animals. Some marine reptiles (like saltwater crocodiles) have also been known to swallow rocks, which weigh down their bodies and act as ballast, so they can surprise prey by leaping out of the water.

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Most Reptiles Have Three-Chambered Hearts

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The hearts of snakes, lizards, turtles and tortoises contain three chambers—which is an advance over the two-chambered hearts of fish and amphibians, but a marked disadvantage compared to the four-chambered hearts of birds and mammals. The problem is that three-chambered hearts allow for the mixing of oxygenated and deoxygenated blood, a relatively inefficient way to deliver oxygen to body tissues. (Crocodilians, the reptile family most closely related to birds, have four-chambered hearts, which presumably gives them a much-needed metabolic edge when snapping at prey.)

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Reptiles Aren't the Smartest Animals on Earth

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With some exceptions, reptiles are about as smart as you'd expect: more cognitively advanced than fish and amphibians, about on an intellectual par with birds, but way down on the charts compared to the average mammal. As a general rule, the "encephalization quotient" of reptiles--that is, the size of their brains compared to the rest of their bodies—is about one-tenth of what you'd find in rats, cats, and hedgehogs. The exception here, again, is crocodilians, which have rudimentary social skills and were at least smart enough to survive the K/T Extinction that rendered their dinosaur cousins extinct.

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Reptiles Were the World's First Amniotes

A clutch of turtle eggs. Getty Images

The appearance of amniotes—vertebrate animals that lay their eggs on land or incubate their fetuses in the female's body—was a key transition in the evolution of life on earth. The amphibians that preceded the reptiles had to lay their eggs in water, and thus couldn't venture far inland to colonize the earth's continents. In this respect, once again, it's natural to treat reptiles as an intermediate stage between fish and amphibians (which were once referred to by naturalists as the "lower vertebrates") and birds and mammals (the "higher vertebrates," with more derived amniotic reproductive systems).

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In Some Reptiles, Sex Is Determined by Temperature

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As far as we know, reptiles are the only vertebrates to exhibit "temperature-depend sex determination": the ambient temperature outside the egg, during the development of the embryo, can determine a hatchling's sex. What is the adaptive advantage of TDSD for the turtles and crocodiles that experience it? No one knows for sure; certain species may benefit by having more of one sex than another at certain stages of their life cycles, or TDSD may simply be a (relatively harmless) evolutionary holdover from when reptiles rose to global dominance 300 million years ago.

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Reptiles Can Be Classified by the Openings in Their Skulls

The skull of an anapsid reptile. Wikimedia Commons

It's not often invoked when dealing with living species, but the evolution of reptiles can be understood by the number of openings, or "fenestrae," in their skulls. Turtles and tortoises are anapsid reptiles, with no openings in their skulls; the pelycosaurs and therapsids of the later Paleozoic Era were synapsids, with one opening; and all other reptiles, including dinosaurs, pterosaurs and marine reptiles, are diapsids, with two openings. (Among other things, the number of fenestrae provides an important clue about the evolution of mammals, which share key characteristics of their skulls with ancient therapsids.)

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Strauss, Bob. "10 Facts About Reptiles." ThoughtCo, Oct. 24, 2017, thoughtco.com/facts-about-reptiles-4090030. Strauss, Bob. (2017, October 24). 10 Facts About Reptiles. Retrieved from https://www.thoughtco.com/facts-about-reptiles-4090030 Strauss, Bob. "10 Facts About Reptiles." ThoughtCo. https://www.thoughtco.com/facts-about-reptiles-4090030 (accessed January 20, 2018).