Clades
Now, this is a correct phylogeny, so far as we’ve been able to determine. That is, by the data available, the chart correctly shows the evolutionary relationships between various members of the subphylum Vertebrata. But notice that it shows the birds and crocodiles to be closely-related animals. So what?
In a phylogeny, a clade is a group that includes the common ancestor of that group and all of its descendents. All the members of a clade share a set of unique characteristics because they all inherited them from their common ancestor. A clade is referred to as a monophyletic group, meaning that all the members of the clade are descended from a single common ancestor, and that the taxon contains all the descendants of that common ancestor.
If you look at the chart on the previous page, you can easily see that the archosaurs are a monophyletic group and, therefore, comprise a true clade. Similarly, the diapsids are a monophyletic group. But what about the reptiles?
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