What Is A Reptile?We know what they look like, but exactly what is a reptile? How do we tell reptile fossils from those of amphibians? One defining feature of all reptiles is the number of bones in the wrist and ankle. This can be used to clearly define whether fossilized remains of ancient tetrapods are amphibians or reptiles, as in the case of the most primitive reptile yet discovered, Hylonomus. The HandHere the term "hand" refers to the front foot. The amphibian form has four central bones called centralia, shown below in pink. All reptiles, whether alive or fossilized from long ago, have only two of these bones.
Other things to note are the reduction of carpal bones from five to four or less. These are shown in light blue. Whether the fifth carpal bone from amphibians migrated out to the position of the green bone in the reptile hand (called the pisiform) is not known. In reptiles, that bone serves as an extra point of connection for muscle tendons which give the limb more maneuverability. The FootDifferences in the bone structure of the back feet (called just "feet" here) are just as noticeable as for the hands. The pattern for the primitive amphibian is much the same as that for the hand, and this supports the idea that these creatures represent an early effort to leave the water, using little more than slightly adapted fins. The reptile foot by comparison has a reduction in the number of bones and the fusing of some elements. The pattern of bones is also different from those seem in the hand, which is further evidence that these creatures were adapting to life on land.
Does It Really Define Reptiles?To say that all creatures that have these features are reptiles, and that it is a reptile because it has these features is a self defining statement, and is therefore not acceptable. So how does it help us decide what is a reptile? It is true that all known reptiles have no more than two central hand and foot bones, and that amphibians have more. There are other features that point to the likelihood of a fossil belonging to a reptile, and hence an amniote. The most noticeable differences lie in the skull. These add weight to claims about a specimen's being classed as a reptile or an amphibian. Return from What Is A Reptile to Reptile Evolution or return to the Reptiles Home Page.
|
