The Paleocene Ancient origins

Contrary to popular belief, the mammals were not the last vertebrate class to evolve. They appear in the fossil record during the late Triassic Period over 200 My ago, about 60 My before the birds and only a short time after the first

Figure 1.2

A simplified evolutionary tree of the Proboscidea. (Based on Shoshani 1992 with the addition of Phosphatherium.)

Figure 1.2

A simplified evolutionary tree of the Proboscidea. (Based on Shoshani 1992 with the addition of Phosphatherium.)

Figure 1.3

A reconstruction of the global distribution of the proboscideans beginning with the ancestral forms along the shores of the ancient Tethys Sea in present-day northern Africa. (From Shoshani and Tassy 1996b with the addition of Phosphatherium.) The modern distributional ranges of Elephas and Loxodonta are more accurately depicted in appendix 1.

Figure 1.3

A reconstruction of the global distribution of the proboscideans beginning with the ancestral forms along the shores of the ancient Tethys Sea in present-day northern Africa. (From Shoshani and Tassy 1996b with the addition of Phosphatherium.) The modern distributional ranges of Elephas and Loxodonta are more accurately depicted in appendix 1.

dinosaurs. Mammals, though small, nocturnal, and mostly omnivorous, were very much in evidence during the Jurassic and Cretaceous Periods, the golden age of the dinosaurs. By the end of the Cretaceous, 65 My ago (the so-called K-T boundary), the dinosaurs made their exit in one of the most catastrophic events of the planet. Whether the dinosaur extinctions were triggered by an asteroid impact or prolonged volcanic activity in the Indian Deccan, the stage was now set for the succession of new dynasties—"victors by default" in the words of Christine Janis.

The Tertiary Period dawned on a world in which the upper range of niches, in terms of body size, had been vacated. This was an opportunity for mammals to emerge from their tiny, nocturnal lairs. Yet, they took several million years to reach even moderately large body sizes.

One possible reason could have been that, during the early Tertiary Period (the Paleocene Epoch, 65-57 My ago), the forests were denser—a consequence of a warmer, wetter world with no dinosaurs to trample the vegetation. Dense forests favor small, tree-dwelling animals over large, ground-dwelling ones. The Paleocene seems to have enjoyed a more equable climate than today, with much less contrast between the equator and the poles. The higher rainfall was more evenly spread over the year. Deciduous forests extended into the polar regions, while tropical type forests covered much of the rest of the globe. There is evidence that forests in North America, for instance, were dense and swampy.

It is in the Paleocene world, not long after the exit of the dinosaurs, that there is now evidence for the existence of a proboscideanlike creature. Dental remains of a small "ungulate," weighing perhaps 10-15 kg, were described in the leading scientific journal Nature in 1996 by Emmanuel Gheerbrant, Jean Sudre, and Henri Cappetta. The material was originally obtained by F. Escuillie from a fossil dealer. Although the discoverer and exact origin of the fossil are not known, the fossil was unambiguously traced to the phosphate deposits of Ouled Abdoun Basin in Morocco. The material was named Phosphatherium escuilliei. At this time, no inferences have been drawn about the habitat in which the creature lived. The teeth characters indicate a close relationship to another early proboscidean, Numidotherium koholense, from northern Africa that lived about 7 My later.

There must be more discoveries and analysis by paleontologists for a better understanding of Phosphatherium and its significance not only to proboscidean evolution, but also to the early radiation of the placental mammals.

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