Skip to main content Accessibility help
×
Home

Evolutionary patterns at the terrestrial Eocene-Oligocene boundary in North America

  • Donald R. Prothero (a1)

Abstract

Recent breakthroughs in magnetostratigraphy and 40Ar/39Ar dating have shown that the boundary between the Chadronian and Orellan land mammal “ages” (long thought to be mid-Oligocene) correlates with the Eocene-Oligocene boundary (about 33.9 Ma). This boundary gives an exceptionally dense, detailed record of faunal, floral, and climatic changes, well constrained by magnetostratigraphy and radiometric dating.

Evidence from paleosols and land floras (Retallack, 1992) document a striking cooling and drying event across this boundary, with a woodland vegetation (greater than 1000 mm annual precipitation) replaced by a wooded grassland (500 mm annual precipitation). According to Wolfe (1992), mean annual temperature declined almost 13°, and the annual range of temperature increased dramatically from 5° to about 25°. Sedimentological evidence from eastern Wyoming (Evanoff et al., 1992) show an abrupt transition from moist floodplains to semi-arid landscapes with abundant wind-blown volcaniclastic dust. Most of these events took place over a few thousand years. This is certainly one of the most severe climatic events in the Cenozoic.

Late Eocene land snails (Evanoff et al., 1992) are large-shelled subtropical taxa now typical of central Mexico, indicating a mean annual range of temperature of 16.5° and annual precipitation of about 450 mm. In the early Oligocene, these were replaced by drought-tolerant small-shelled taxa indicative of a warm-temperate open woodland with a pronounced dry season. Reptiles and amphibians (Hutchison, 1992) show a trend toward cooling and drying, with aquatic forms (crocodilians, freshwater turtles, and salamanders) replaced by land tortoises; size reduction in turtles also indicates increased aridity. Mammals show only minor changes across this boundary. A few archaic groups which may have depended on woodland browsing (such as the rhino-like brontotheres, the camel-like oromerycids, and several archaic rodent groups) are the only taxa to go extinct. Minor speciation events occur in horses, the deer-like leptomerycids, and camels. The oreodont Miniochoerus shows a gradual dwarfing of about 30% over about 100,000 years. Most other species show no significant changes across this climatic crisis, although some change in relative abundance. Most mammals show stasis spanning millions of years before and after this transition, and some of the land snails are virtually indistinguishable from modern taxa.

Traditional Neo-Darwinian theory would predict that animals should evolve rapidly in response to such strong climatic selection. Instead, most animals respond by going extinct and being replaced by unrelated forms, or do not change at all. A few show punctuated speciation events at the boundary, and only one shows prolonged gradual dwarfing. This suggests that animals are not infinitely flexible “balls” on an adaptive landscape, but have some kind of internal homeostasis that prevents gradual change in response to selection. Extinction, emigration, or punctuated speciation events seem to be the preferred response.

    • Send article to Kindle

      To send this article to your Kindle, first ensure no-reply@cambridge.org is added to your Approved Personal Document E-mail List under your Personal Document Settings on the Manage Your Content and Devices page of your Amazon account. Then enter the ‘name’ part of your Kindle email address below. Find out more about sending to your Kindle. Find out more about sending to your Kindle.

      Note you can select to send to either the @free.kindle.com or @kindle.com variations. ‘@free.kindle.com’ emails are free but can only be sent to your device when it is connected to wi-fi. ‘@kindle.com’ emails can be delivered even when you are not connected to wi-fi, but note that service fees apply.

      Find out more about the Kindle Personal Document Service.

      Evolutionary patterns at the terrestrial Eocene-Oligocene boundary in North America
      Available formats
      ×

      Send article to Dropbox

      To send this article to your Dropbox account, please select one or more formats and confirm that you agree to abide by our usage policies. If this is the first time you use this feature, you will be asked to authorise Cambridge Core to connect with your <service> account. Find out more about sending content to Dropbox.

      Evolutionary patterns at the terrestrial Eocene-Oligocene boundary in North America
      Available formats
      ×

      Send article to Google Drive

      To send this article to your Google Drive account, please select one or more formats and confirm that you agree to abide by our usage policies. If this is the first time you use this feature, you will be asked to authorise Cambridge Core to connect with your <service> account. Find out more about sending content to Google Drive.

      Evolutionary patterns at the terrestrial Eocene-Oligocene boundary in North America
      Available formats
      ×

Copyright

References

Hide All
All citations from Prothero, D.R., and Berggren, W. A., (eds.), 1992, Eocene-Oligocene Climatic and Biottc Evolution, Princeton Univ. Press, Princeton, N.J., 568 pp.

Metrics

Full text views

Total number of HTML views: 0
Total number of PDF views: 0 *
Loading metrics...

Abstract views

Total abstract views: 0 *
Loading metrics...

* Views captured on Cambridge Core between <date>. This data will be updated every 24 hours.

Usage data cannot currently be displayed