The World at Their Feet

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Science  20 Feb 2004:
Vol. 303, Issue 5661, pp. 1107
DOI: 10.1126/science.303.5661.1107a

Although mammals and dinosaurs evolved at about the same time about 200 million years ago, the fossil record of mammals is scant. Because mammals tended to be small in numbers and stature, their delicate bones are rarely preserved and the record consists mostly of teeth.

One way to sidestep this gap in the fossil record is to look for tracks made by mammals. Lockley and Foster have discovered several sets of mammal tracks from the latest Cretaceous (about 75 to 65 million years ago) in Colorado. The three- to five-toed footprints resemble those of modern rodents; however, rodents did not evolve until later. The footprints probably belong to a multituberculate or a marsupial. From the tracks it is clear that these mammals were small and agile and lived among birds, dinosaurs, and other reptiles on a fertile coastal plain. More work will be needed to connect the partial record from head to toe to determine more about how mammals evolved in the Mesozoic. — LR

Ichnos 10, 269 (2003).

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