A Dinosaur In Your Home? Unearthing The Ancestry Of Living Reptiles
Practical Reptile Keeping|February 2017

So are reptiles the direct descendants of dinosaurs? They may look like them, but be prepared to be surprised as james brereton msc reveals the truth!

Many modern reptiles seen in collections today can show a striking resemblance to a more primitive group of animals – the dinosaurs. From the armoured horned lizard (Phryonosoma platyrhinos) to the raptoresque bearded dragon (Pogona vitticeps), dinosaurs and reptiles alike both fascinate us. Despite their perceived similarity in appearance however, many reptiles share only a distant genetic lineage with the prehistoric ‘terror lizards’, whereas others have a much closer relationship with the dinosaur lineage.

Indeed, some of the closest-living relatives of the dinosaur are surprising. This article will reveal the relationships between reptiles and their larger, extinct cousins (twice removed), as well as explaining how these relationships affect the biology and behaviour of our modern day charges.

Emergence of reptiles

Reptiles are not directly descended from dinosaurs. The first reptiles evolved from what was originally an amphibious ancestor, as early as 310 to 320 million years ago. The changes required to transform an amphibian into a reptile occurred gradually over the course of millions of years. Confined to wetlands and freshwater, amphibians were unable to take advantage of the scrubland, forests and desert habitats available.

The changes in amphibian biology that ultimately led to reptiles reflect essential adaptations toward conserving water, providing an ability to colonise terrestrial habitats in which water is relatively scarce. The characteristic amphibian skin, so important in absorbing oxygen, became replaced with a dry, scaled skin which is much more effective in retaining water. The moist eggs of amphibians, which need to be deposited in water or a high humidity environment, became encased in a hard shell, meaning that the entire reptile life cycle could occur outside of water.

Within a short space of time, reptiles were able to dominate a range of habitats, including forests and scrub land. New food sources became available, including invertebrates of a far greater size than those seen today: dragonflies, for example, with wingspans over 65cm (26in), existed at a time when there was more oxygen in the atmosphere.

The situation today

While many reptiles may look highly similar to dinosaurs, the genetic relationships between groups are very surprising. In reality, birds actually have more in common genetically with Tyrannosaurus rex than do our iguanas or monitor lizards!

Birds

Based on fossil evidence, it has become increasingly apparent that birds are descended from a saurian ancestor. The famous Archaeopteryx fossil from Germany, revealing exceptionally good detail, shows that Archaeopteryx shared traits with both dinosaurs and birds. These included hollow bones, feathers and talons, and, from its dinosaur side, teeth and a clearly saurian body shape. Described as the ‘missing link’, Archaeopteryx has helped scientist to uncover the relationships between dinosaurs and their living avian descendants.

Technically speaking, birds therefore represent a group of living dinosaurs. Given the emerging evidence that many large, well-known dinosaurs such as Velociraptor mongoliensis and Tianyulong confuciusi possessed feathers, it seems the gap between avian and saurian species is continuing to close.

Crocodiles

Despite their primitive and armoured appearance, crocodiles and alligators have a rather surprising family tree. The contemporary Order Crocodilia possesses a range of surprising traits hinting at their evolutionary heritage, which would be hard to guess at successfully, based on their lifestyle.

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