Mammalian Explosion 293As we move up the cladogram through the diverse groups of synapsids (fig. 13.3), more
and more mammalian characters appear in a piecemeal fashion. The next grade up from the
pelycosaurs is the “therapsids,” which dominated the Late Permian landscape and evolved
into a number of large wolf-sized creatures with big saber-toothed canines (fig. 13.2C), as
well as lineages of huge herbivorous synapsids, some with beaks and almost no teeth, and
others with thickened skulls and ugly bony knobs on their faces for display and head-to-
head butting. In short, these therapsids dominated many different niches in the Late Permian
terrestrial ecosystem. They were also considerably more advanced than typical pelycosaurs
like Dimetrodon (fig. 13.2B). The temporal opening on the side of the skull was now much
larger, presumably for expansion of jaw muscles, therefore enabling them to have a stronger
bite force and even some chewing motion. If you look at the palate of the skull, there is the
beginning of a secondary palate, a roof of bone that grows out from the edge of the upper
jaws and encloses the original reptilian palate in a tube. This enables advanced synapsids
and mammals to breathe and eat at the same time, something that reptiles (except crocodil-
ians, which independently evolved a secondary palate) cannot do. Reptiles, with their slow
metabolism, can hold their breath for a long time while they swallow a large prey item, but
the presence of a secondary palate shows that synapsids must have been developing an
active, “warm-blooded” metabolism and needed to process their food quickly to survive. In
addition, the old single ball joint that hinged the skull to the first neck vertebra (known as
the occipital condyle) is now split into a double ball joint, presumably for more flexibility in
moving the head.
In therapsids, the canines are much larger, and some of the rest of the teeth are now more
specialized as well, with serrated edges like steak knives. There are also striking differences
in the limbs (fig. 13.4), with a much stronger more flexible shoulder girdle and an increased
number of lower back vertebrae fused to the hip bones. Finally, they no longer have the
(D)
FIGURE 13.2. (Continued )