The Oligocene Evolutionary quiescence

The global climate grew even cooler and more seasonal at the transition to the Oligocene (35 My ago). Tropical vegetation and the polar deciduous forests continued to shrink, with the latter eventually disappearing. Antarctica became ice-capped, but ice was yet to appear at the northern pole. Yet, the world was still somewhat wetter than at present, with lesser differences between the tropics and the poles. After the late Eocene extinctions, the Oligocene seems to have been a period of evolutionary stability; thus, this epoch does not attract the attention of many paleontologists.

Primitive Eocene proboscideans such as the moeritheres and barytheres seem to have lived to the end of the Oligocene. At the same time, there appeared in the early Oligocene at the Fayum site at least two types of palaeomastodontids that are closer to the elephants than are the moeritheres or latter-day deinotheres. The palaeomastodontids may actually have originated during the late Eocene and given rise to two major proboscidean branches, the mammutids and the gomphotheres, the latter being the ancestors of the true elephants. One type of palaeomastodontid, Palaeomastodon beadnelli, had oval-shaped (in cross section) tusks in both upper and lower jaws. The absence of canines and anterior premo-lars produced a marked diastema (gap) between the tusks and the cheek teeth. The molars were bilophodont, with cusps arranged transversely in pairs. Standing about 2 m high—the size of a subadult elephant—Palaeomastodon is believed to have been a dweller of forest and open woodland.

Another type—Phiomia serridens—stood a little taller than Palaeomastodon and had more specialized dentition. Molars were trilophodont, with three transverse pairs of conical cusps. Its horizontally projecting lower tusks were shorter and flattened, perhaps aiding the animal in feeding. Phiomia is believed to have been a creature of moister lowlands. Both the palaeomastodonts had short trunks, a bit longer than those of tapirs.

Except at the beginning, the quiet, equable Oligocene did not witness any great evolutionary upheavals. Many of the abundant mammals of the Oligocene belong to orders that still exist, although the families they represent are now extinct. The near absence of any proboscidean radiation (at least this is what the evidence suggests) could simply be a reflection of this evolutionary quiescence among mammals. But, 10 million years was too long a vacation for biological evolution, and the stage was already set by the end of the Oligocene for the next explosive radiation of mammals.

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