Onto the Land and Back to the Sea: The Amniotes 247anatomical characters will diagnose a fossil as an amniote. Once again, we have a long series
of transitional forms (the paraphyletic group known as “anthracosaurs”) that show pro-
gressively more and more amniote-like characteristics, and where to draw the line in this
continuous series is problematic (fig. 11.3). We can see several trends in the evolution of
anthracosaurs into amniotes. One is the fact that anthracosaurs have skulls that are very
deep vertically, with a narrow snout and a shortened region behind the eyes. Their skulls
eventually lose the notch in the back for the eardrum. By contrast, most other primitive tet-
rapods (such as the temnospondyls and lepospondyls) have relatively flat skulls and bodies,
with a wide snout, a large region of skull roof behind the eyes, and a well-developed notch
for the eardrum. Correlated with this is evidence that limbs of the anthracosaurs were robust
and held upright, so they frequently walked with their bellies off the ground. Temnospon-
dyls and lepospondyls had a much more sprawling posture and could rarely move without
dragging their bellies. The wrist and ankle bones of anthracosaurs were also modified for
much more active motion than the comparable bones in other primitive tetrapods. The neck
vertebrae of anthracosaurs were specialized into two bones: the atlas (which holds up the
skull, as Atlas once held up the world in Greek mythology) and the second neck vertebra, the
axis (the pivot joint on which the skull turns in relation to the neck). These are features found
in all later amniotes and allowed anthracosaurs to swivel their heads rapidly to catch prey.
Finally, the muscles and bones of the anthracosaur palate were modified so it had a much
stronger bite force. By contrast, the palate of temnospondyls and lepospondyls had a much
weaker “snapping” motion, with nowhere near as much bite force.
FIGURE 11.3. Skeletons of some of the many “anthracosaurs,” which are transitional fossils between
“amphibians” and the most primitive amniotes. At the top is the primitive gephyrostegid Bruktererpeton. In
the middle is the more advanced form Seymouria. At the bottom is the highly tetrapod-like Limnoscelis. Each
one is a mosaic of primitive tetrapod and advanced amniote characters, making it very difficult to decide
where the amphibians end and the amniotes begin. (From Carroll 1988: fig. 9-22; courtesy W. H. Freeman and
R. L. Carroll)
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