Reptile Characteristics:
Skull Structure

There are significant reptile characteristics in the skulls of early amniotes that make them clearly different from ancient amphibians of the same period. While these differences are somewhat of a "guess the origin" game in terms of when the amphibian became the reptile, they are none the less telling. Reptilian skulls share certain features regardless of their location in the fossil record.

The diagrams below show an ancestral amphibian (left) and an ancestral reptile (right). The bones are colored to show similarities and differences.

The grayed out areas are the eye and nose holes. Only one half of the skull is shown for clarity.

Differences Between The Two

1. The base of the skull is level in reptiles and notched in amphibians. This is only noticeable in the most ancient of both these groups. Later reptiles, such as turtles, took on very notched skull bases indeed.

2. Several of the skull bones from the amphibian ancestors are fused together into single bones or have been lost altogether in the reptiles. These are shown in red in the above diagrams.

3. The places where bones meet are quite different between the two in the back half of the skulls. It is the joining of certain bones, such as those on each side of the ones highlighted in red on the amphibian skull, that help scientists identify a skull as a reptile or amphibian.

4. One or more openings may be present in the outer skull. These are located between certain bones and are used to classify reptiles as belonging to particular groups. Position and number of openings determines whether a reptile is anapsid, euryapsid, synapsid or diapsid.

Summary

This is another example of using a feature to classify an animal simply because we know that similar animals share that feature. Obviously at some stage there had to be a transition between the amphibian and reptile skulls, and hopefully one day this intermediate form will be discovered.

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