Prehistoric Life 15

by Carl Strang

This year’s winter series is a review of the prehistoric life and geologic history of northeast Illinois. Each chapter will summarize current understanding, gleaned from the literature, of what was going on with life on Earth in a particular span of time, what we know about the local landscape, and what we can say about local life. I include some references, particularly to papers published in the journal Science which commonly is available at public libraries. Contact me if you need sources for other items. The Earth is so old that every imaginable environment was here at some point, from ocean depths to mountaintops, from equatorial tropics to tundra, and from wetlands to desert.

Eocene Epoch (56.5-35.4 million years ago)

The Eocene Epoch, literally “dawn of the recent,” originally was defined by Lyell in 1833 according to the percentage of fossil marine mollusks in France’s Paris Basin still living today (3.5%).

Life on Earth. Reefs reappeared in the oceans 48 million years ago, as new species of reef-building corals evolved among the coelenterates (Science 312:857).

The greatest diversity of Cenozoic Era terrestrial plants in the New World tropics (greater even than today) occurred in the early to middle Eocene. Diversity across the era has correlated fairly well with climate, temperature in particular (Science 311:1893). In the Arctic, temperatures 55-50 million years ago were so mild that even in the darkness of winter they ranged from just above freezing to 70F. Large animals could live there without migrating or hibernating, subsisting on browse and evergreen material. Species in that region included brontotheres, early rhinoceroses, tapirs, alligators, turtles, snakes and flying lemurs.

Our area in most of the Eocene was a diverse tropical forest.

In the early Eocene, order Artiodactyla (even-toed ungulates) evolved, part of a large number of appearances of new mammalian groups and extinctions of older ones, associated with the global warming spike. This time produced the first North American true primates (2 families), didelphids (opossum relatives), perissodactyls and artiodactyls, and their numbers rapidly increased. The lagomorphs (the order containing today’s rabbits) began to diverge from the rodents early in the Eocene. A group called entelodonts, ancestors of pigs, lasted from mid-Eocene to late Oligocene. Camels originated in North America in the late Eocene. The perissodactyl rhinoceroses trace their ancestry to the early Eocene, as do the horses. The latter began with Protorohippus (= the less correct name Eohippus), with fossils in North America. (Europe’s Hyracotherium has been determined to be a different, non-horse, perissodactyl mammal). Perissodactyl brontotheres, which were rhinoceros-like ungulates, underwent much of their evolution through the Eocene. Uintatheres were primitive, Eocene ungulate-like animals, neither Perissodactyla nor Artiodactyla, in several genera, with tusks, horns and tiny brains. Mesonychids were still around, carnivorous species including the enormous Andrewsarchus of Asia and smaller Synoplotherium of North America. That group may have been the source of the whales, though recent evidence suggests that Asian artiodactyls were a more likely origin.

The opening environments of the late Eocene began the trend to increased herding and larger body sizes. The great diversification of artiodactyls and rodents began at the end of the Eocene. The squirrel family (Sciuridae) appeared in western North America at this time (Science 299:1568). The order Proboscidea (ultimately to include the elephants) appeared in the late Eocene in Africa. Whales, rabbits and hares appeared in the Eocene, and the late Eocene produced the first moles, and modern carnivore families. An impressive skeleton of a primate from the middle Eocene was reported in 2009. This individual was lemur-like, but may have been close to the line that led to us (Science 324:1124).

The first rabbits had evolved by the end of the Eocene.

Until the modern carnivore groups expanded, for most of the Eocene the creodonts remained the dominant predators. Hyaenodon was a diverse genus from late Eocene to the early Miocene, ranging from western North America through Europe to Asia and Africa, and with a size from small coyote to large grizzly bear. Sabertoothed carnivores of family Nimravidae appeared in the late Eocene, emigrating from Europe to North America. Felids appear at the Eocene-Oligocene boundary in North America and Eurasia. Other modern carnivore groups were represented by small, generalized forms.

Modern bird families did not exist through the Eocene, though many of the aquatic orders (e.g., Gruiformes, the order containing cranes) diversified. Diatryma was a 7-foot-tall North American flightless gruiform predator.

Local landscape. North America was isolated by the middle of the Eocene (and never again was connected to the east, though the western Bering Sea land bridge was to come and go several times). North America had no tundra, coniferous forest, grassland, desert, shrublands or savannas through the Eocene. There was a brief but intense global warming peak in the early Eocene, with much rain, then a cooling and drying to subtropical conditions in the late Eocene. The local environment made the transition from tropical forest to subtropical forest. The cause of the global warming peak is thought to be an episode of flood basalt eruptions (with attendant release of greenhouse gases) in the North Atlantic as North America and Europe split apart (Science 316: 587).

Local life. Bats probably have been here from the Eocene on, except during glacial times. Bats may have originated in North America, and they underwent a huge diversification in the early Eocene with all present-day families represented by the end of the period (Science 307:580). Primitive true primates likely were here in the Eocene, and possibly into the Oligocene (families Adapidae [lemur-like] and Omomyidae [tarsier-like]).

Bats diversified in the Eocene.

In the early Eocene, the first horse Protorohippus (“eohippus”) very likely was here. Coryphodon, the largest land mammal of its time, probably was here, as was the river-dwelling, hippo-like Metamynodon. The early browsing, running rhinoceroses, like Hyrachyus, were widespread in North America, Europe and Asia. Entelodonts, the predatory ungulates, had a likely representative in Archaeotherium. Condylarths still were around, with the rat-sized, squirrel-like Hyopsodus found all over the Northern hemisphere and lasting well into the Eocene, and the small, generalized Haplomylus also a likely local candidate. We probably had some artiodactyls and rodents by the end of the Eocene, as well as early carnivores (Miacis, Hesperocyon), and creodonts such as Prototomus or Hyaenodon. Crocodilians and turtles were on Ellesmere Island into the early Eocene, so presumably were here as well. The huge predatory bird Diatryma is another good possibility.

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