296 Evolution? The Fossils Say YES!
sprawling limb posture seen in primitive synapsids but instead held their limbs vertically
and walked fully upright. Consequently, there are many subtle changes in the shape and
musculature of all the limb bones, and the finger bones are much shorter because these ani-
mals no longer sprawled and walked flat-footed with toes splayed out like a lizard’s but
instead began to walk on the tips of their toes like most mammals do.
The next grade up is a group called the “cynodonts,” which originated in the late Perm-
ian, survived the greatest mass extinction event in earth history at the end of the Permian,
and then dominated the world of the Early Triassic. Cynodonts (the name means “dog tooth”
in Greek) are very much like mammals in lots of ways (figs. 13.2D and 13.3). The temporal
opening at the back of the skull is even more enlarged, allowing for several jaw muscles
to develop, which in turn made it possible for cynodont jaws to chew as well as bite. The
canines were large, and the postcanine teeth (premolars and molars) are now specialized
and multicusped for chewing, not the simple conical piercing teeth of reptiles and primitive
synapsids. The secondary palate was now almost fully developed, with the internal air pas-
sage opening not in the mouth (like in primitive synapsids) but in the back of the throat (as in
mammals). Instead of sprawling, the limbs (fig. 13.4) are even more upright and specialized
for rapid running, with a lightweight shoulder girdle and a reduced set of hip bones (except
for the expansion of the iliac blade of the hip along the spine). In the heel, the ankle bone
known as the calcaneum develops a long extension for the attachment of the Achilles tendon,
a sign of much more efficient and faster running. The tail is shorter, too. The rib cage disap-
pears from the lower back, and in some specimens, the ribs are locked together with flanges
of bone. This suggests that they did not breathe by expanding the ribs in and out (like reptiles
do) but instead had a solid rib cage and used a muscular wall between the lung cavity and
abdominal cavity, the diaphragm, to pump air in and out of the lungs. Some advanced cyn-
odonts, such as Thrinaxodon (fig. 13.2D) actually have small pits on their snouts that suggest
the presence of whiskers. (Hair does not normally fossilize, so it is hard to know whether
any given fossil had hair or not.) If so, then cynodonts probably had hair elsewhere on their
bodies as well, and hair probably appeared early in synapsid evolution with the evolution
of the smaller therapsids.
The most advanced cynodonts, such as the tritylodonts and the trithelodonts of the Early
Triassic (fig. 13.3), were small weasel-like forms with highly specialized skulls, large tempo-
ral openings and several sets of jaw muscles, highly specialized molars and premolars, and
very doglike skeletons with almost all the typical mammalian features. In these and many
other features, they are so mammal-like that they have often been called mammals. Indeed,
the transition from the most primitive synapsids all the way to mammals is so smooth that it
is rather arbitrary where to break the continuous sequence and begin calling advanced syn-
apsids mammals. However, most paleontologists agree that creatures like Adelobasileus, Sino-
conodon, Megazostrodon (fig. 13.4), and Morganucodon, from the Late Triassic of New Mexico,
China, and South Africa, are bona fide mammals, because they are dramatically smaller in
body size (rat-sized or smaller), had lost the ribs from their neck vertebrae, had reduced the
reptilian elements in their shoulder girdle (clavicles, interclavicles, and coracoids), and had
reduced the ilium portion of the hip bones to a simple rod running along the spine. Most
importantly, they had a jaw joint between the dentary bone of the jaw and the squamosal
bone of the skull, the defining character of Mammalia.
Through this entire gradual transition is an even more amazing story in the jaws and ears
of these animals (fig. 13.5). Early synapsids like Dimetrodon had a classic primitive amniote jaw.