Onto the Land and Back to the Sea: The Amniotes 255further reduced a lot of the bone to cartilage, another sign of a largely aquatic lifestyle. How-
ever, the shoulder girdle and hip bones are becoming much more robust and plate-like in
support of the limbs, a hallmark of later plesiosaurs.
Our final step into full-fledged plesiosaurs is Pistosaurus from the Middle Triassic of
Germany (fig.11.8C). This creature has a relatively primitive head with a slightly longer
FIGURE 11.8. A transitional sequence of fossils bridging the gap from primitive amniotes to highly derived
plesiosaurs. (A) Claudiosaurus from the Permian of Madagascar, with the primitive short neck, long tail, and
relatively large hands and feet not yet modified into flippers. (B) The Triassic nothosaur Pachypleurosaurus,
with longer neck, shorter more robust tail, and hands and feet more highly modified for swimming. (C) The
Triassic Pistosaurus, a relatively primitive plesiosaur, with longer limbs partially modified into paddles, a
longer neck, shorter tail, and longer skull. (D) The advanced plesiosaurs Cryptocleidus (top) and elasmosaurid
Hydrothecrosaurus (bottom), with much longer necks, smaller heads, shorter tails, and hands and feet fully
modified into flippers. (From Carroll 1988: figs. 12-2, 12-4, 12-10, and 12-12; courtesy W. H. Freeman and
R. L. Carroll)
(A)(B)(C)(D)