Thursday, July 16, 2015

A Late Cretaceous early mammal tooth

Multituberculates were early rodent-like mammals (see my sketch below) that lived from the Late Jurassic to early Oligocene. They have the longest mammalian lineage (approximately 125 million years) and are the only branch of mammals to have become extinct. They did survive, however, the mass extinction at the end of the Cretaceous Period 65 million years ago. In the early Oligocene, multituberculates were most likely out-competed by true rodents.

They were rat- to squirrel-size and were most likely the first herbivorous mammals. They were widespread, but most are known from Europe and North America.

An early multituberculate. P4 refers to its most forward molar tooth
 (see remarks given below).
Their fossil record consists of minute teeth, which are scarce. As shown below in the plaster replica, they had rodent-like front teeth (incisors), then a gap, followed by a series of gnawing cheek teeth (premolars and molars). The cheek teeth possess many cusps, or tubercles; thus, the derivation of the name “multituberculates.” The first premolar (P4) in the lower jaw was commonly of large size and used as a massive, blade-like tooth for shearing purposes.

Side view of plaster replica of jaws and teeth of Ptilolus? sp., an early multituberculate. This replica is about 15 times larger than the actual jaws and teeth.

                                                                top view of the lower jaw

An actual P4 molar tooth (3 mm width, 3.75 mm length) of  Ptilodus? sp., a Late Cretaceous squirrel-like multituberculate is shown below. This Cretaeous tooth was found in modern-day anthills in the Bug Creek  area of Montana. Paleontologists search these anthills for fossils because ants burrow into Upper Cretaceous rocks and bring the microscopic-sized fossils to the Earth's surface.

   side view of a P4 molar (3 mm width)

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