On the Origins of New Forms of Life
9.10: Pinnipeds
EUGENE M. MCCARTHY, PHD GENETICS
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In addition to the terrestrial carnivores there are the pinnipeds (seals, sea lions, and walruses). In recent taxonomies, these forms are often categorized as a family of the Order Carnivora. But in past years they were usually placed separately in Order Pinnipedia. This indecision reflects the fact that seals do not seem to fit very well with the largely terrestrial (with the exception of otters) members of Order Carnivora. Nevertheless, they show certain similarities to land carnivores in their anatomy and behavior.
Typical reconstruction of Plesiosaurus
Accepted theory says pinnipeds, also, are descended from a "small, primitive, generalized" placental mammal living in the late Cretaceous by way of a terrestrial carnivore intermediate. But once again, this seems to be an unnecessary assumption inasmuch as seallike animals were already in existence in the late Cretaceous. These animals are to be found among the smaller, supposedly reptile, Mesozoic marine predators, the plesiosaurs. Their build was seallike and they fed on fish. In particular, the remains of such creatures as Plesiosaurus (Figure 9.6) and Peloneustes are reminiscent of modern pinnipeds. Plesiosaurs are known even from early Mesozoic formations. The nothosaurids, creatures somewhat less suited to an aquatic environment and widely considered to be ancestral to the plesiosaurs, date back to the early Triassic. Why, then, should we embrace the “primitive placental” of the late Cretaceous?
8.1: Dubious Assumptions - © Macroevolution.net