The Great Eocene Extinction?

[Polycarp]

If you look at any book on historical paleontology, you see quite a few groups that made it successfully through the K-T extinction, only to die off in the Eocene 20-25 million years later.

Check out the following list:
[ol][li]Champsosaurs, aquatic reptiles looking much like crocodiles (their distant relatives, but with nostrils high on the snout[/li][li]Sebecosuchids, long-legged narrow-headed landliving crocodiles which were apparently excellent land carnivores[/li][li]Multituberculates, a surviving group of Mesozoic “primitive” (non-marsuiopal, non-placental) mammals convergent on Sciuromorph rodent niches (squirrels, groundhogts, beavers, etc.)[/li][li]Mesonychids, large-headed hoofed carnivores closely allied to pigs and whales[/li][li]Dinocerata, such as Uintatherium[/li][li]Brontotheres, rhinoceros relatives with large bulky bodies and massive knobs on the snout where rhinos have horns[/li][li]Pantodonts, sheeplike and tapirlike primitive herbivores[/ol][/li]
That’s nowhere near exhaustive – there are a lot of groups that died off at the end of the Eocene. It’s merely a representative sample, with some of the higher taxons from the dieofff emphasized.

Was this a true mass extinction event? Does it just happen that the Eocene covered a period when surviving “living fossils” and “experimental” forms from a post K-T radiation died off? Are there other significant examples of taxons going extinct at the end of the Eocene outside the tetrapods?

[Colibri]

The Eocene-Oligocene transition, while not recognized as among the “Big Five” mass extinction events, was the biggest faunal turnover since the end of the Cretaceous. Most recent evidence suggest it occurred in the earliest Oligocene rather than right at the boundary.

It is known as the Grande Coupure, or “great break” in European mammal faunas. Although some have attributed it to some sudden cause such as an impact event, it seems to have been fairly drawn out and the extinctions mostly due to a cooling drying climate and other factors. It involved plants and marine invertebrates as well as mammals.

From here:

Neither these data, nor evidence from coeval fossil plants and soils in central Oregon, support the notion of a “Terminal Eocene Event,” nor any other single, abrupt paleoclimatic shift or extinction. The Eocene-Oligocene biotic and climatic transition was drawn out over some 6 m.y. Abrupt forcings such as meteorite impacts or volcanic eruptions are less likely explanations for cooling and diversity decline than long-term processes such as mountain building, changing ocean currents, or reorganization of the carbon cycle by coevolution of grasses and grazers.

[wevets]

I haven’t seen the Eocene-Oligocene extinction listed as one of the handful of “Mass Extinctions,” although it does get (dis)honorable mention here:

An extinction at the Eocene-Oligocene boundary affected marine organisms more than terrestrial animals. It occurred at a time of major climatic change, not obviously linked with any single major impact or any major volcanic event. There were impacts around this time. One struck the east coast of the United States, scattering tektites from a crater that now lies under Chesapeake Bay. Another struck central Siberia, with debris scattered perhaps to Europe. These were clearly separate events. Although it is premature to link them to the extinctions, impacts are not ruled out as contributing to the E-O event.

From an essay by Richard Cowen at the UC Museum of Paleontology.

There’s only a tiny blurb about it in Cox & Moore’s Biogeography (you can find it on pages 198-199,) that mentions a shift in mean annual temperatures from about 27°C to 12°C, and a shift in mean annual temperature range from 5°C to almost 25°C in North America’s Pacific Northwest and a large, but vague, change is mentioned in sea temperatures from warm seas to cold.

It’s not mentioned at all in Biodiversity by Lévêque and Mounolou except that a graph with poor scaling shows that transition similar in size to the Ordovician-Silurian and Devonian-Carboniferous transitions. I think that’s an artifact of the graph, though, since the text mentions loss of about 85% of all species at the end of the Ordovician and loss of about 75% of all marine species at the end of the Devonian.

Sorry the above is probably not all that helpful.