Marine amniotes II: synapsids

Mammalia (Cretaceous - Recent) The most recent common ancestor of modern mammals and all of its descendants. In broad strokes, its phylogeny is very clear:

As with Aves, you have many sources of a huge variety of information on mammalian biology, paleo- and otherwise. This lecture concentrates on one biomechanical issue: The synapsid return to the oceans.

Although many synapsids have been fresh-water aquatic (E.G. the Jurassic mammal-relative Castorocauda), only one group, the placental mammals, have ever returned to the oceans as exclusively marine creatures. This probably has a lot to do with reproductive physiology.

The diversity of marine placentals: Just as the extinction of large theropods on land created an ecological vacuum into which mammals, birds, and crocodylians attmepted to radiate; the extinction of predatory marine reptiles in the oceans opened up the "marine amniote" niche. During the Paleogene, placental mammals aggressively invaded this niche. Their diversity includes:

Of these, the earliest group to get their feet wet, and the most transformed for aquatic life are Cetacea, so this lecture concentrates on them:

Cetacean phylogeny: The phylogeny of living Cetacea is straightforward:

Derived features of living whales:

These are remarkable adaptations, but all the more so when one considers the starting point for this evolutionary trend: a primitive even-toed ungulate. Today, we think of these creatures as swift herbivores, but during the early Paleogene, artiodactyls experimented with a variety of life styles including:

Competing hypotheses of phylogeny: