Thunder beasts in pictures

Yesterday's brief look at brontotheres was such a rip-roaring success I thought I'd do a little bit more on them (on members of Brontotheriidae that is, not on rip-roaring successes). No time for a proper article: all I've done here is to take screen-shots of various powerpoint slides (from a perissodactyl lecture I give), and throw in a few words where appropriate.

To begin with, we mostly know of brontotheres thanks to the gigantic, Late Eocene North American taxon Megacerops Leidy, 1870, now thought to include both Brontotherium Marsh, 1873 and Brontops Marsh, 1887 (the slides shown here were produced before Brontops was sunk into Megacerops). It was previously argued (e.g., Mader 1999) that Brontops differed from Megacerops in the cross-sectional shape, size and orientation of its horns, but it has since been argued that a continuous range of variation seen among Megacerops specimens negates any effort to split them into separate genera. Megacerops was the last North American brontothere, and the only one present in North America during the Late Eocene. It's the best known member of Brontotheriita, a clade that also includes Protitanops, Eubrontotherium, Notiotitanops and Duchesneodus, all of which are from late Middle Eocene (Mihlbachler, cited in Janis et al. 2008). In the slide above, the life restoration at left is of course by Burian; the silhouetted reconstruction is by Greg Paul.

Henry Fairfield Osborn famously used brontotheres to illustrate his idea of orthogenesis: essentially, 'straight line evolution', where animals evolve in a single direction (typically, toward greater body size and, in brontotheres, toward ever larger horns). None of the animals used to illustrate orthogenesis support it, as newer discoveries have shown that they are merely twigs on a far more complex tree (the other branches of which do not form the tidy series required for the orthogenetic hypothesis). Eotitanops, shown here from a painting by Graham Allen, was a sheep-sized, long-lived Early and Middle Eocene basal brontotheriid. Like other basal brontotheres (some of which, like Pakotitanops, were Asian), it lacked horns. More derived brontotheres (the Brontotheriinae) include the central Asian Embolotheriina (see below) and Brontotheriina, the clade that includes Protitanotherium, Diplacodon and Brontotheriita (Megacerops and close kin).

Typically associated with North America, brontotheres were a constant and important presence in Eocene Asia. Basal brontotheriids, and both basal and derived brontotheriines, inhabited the continent, and some inhabited Europe as well. The presence of brontotheres in the high-latitude Arctic faunas such as those of Ellesmere Island (Eberle & Storer 1999, Eberle 2006), is interesting.

One of the most interesting groups - hey, here's a whole slide devoted to them - are the embolotheriines. The body shapes and limb proportions of at least some of these animals indicate that they were hippo-like and amphibious (Mihlbachler et al. 2004). Sauropod workers might be interested to see all those '-titan' names. Some embolotheriines (labelled on the slide as 'Embolotheriinae' because I followed the taxonomy used by McKenna & Bell (1997)) are best known for possessing gigantic horns often described as resembling battering rams. These horns are apparently hollow and quite fragile, so a role in vigorous combat might be unlikely. Broken and healed ribs and facial injuries have been reported in other brontotheres, so they almost certainly did fight, and sexual dimorphism in horn size and shape supports the idea that males battled for territory and/or mates. And soooo much more to say, but that'll have to do.

Refs - -

Eberle, J. J. 2006. Early Eocene Brontotheriidae (Perissodactyla) from the Eureka Sound Group, Ellesmere Island, Canadian High Arctic: implications for brontothere origins and high-latitude dispersal. Journal of Vertebrate Paleontology 26, 381-386.

- . & Storer, J. E. 1999. Northernmost record of brontotheres, Axel Heiberg Island, Canada - implications for age of the Buchanan Lake Formation and brontothere paleobiology. Journal of Paleontology 73, 979-983.

Janis, C. M., Hulbert, R. C. & Mihlbachler, M. C. 2008. Addendum. In Janis, C. M., Gunnell, G. F. & Uhen, M. D. (eds). Evolution of Tertiary Mammals of North America, Volume 2: Small Mammals, Xenarthrans, and Marine Mammals. Cambridge University Press (Cambridge), pp. 645-693.

Mader, B. J. 1998. Brontotheriidae. In Janis, C. M., Scott, K. M. & Jacobs, L. L. (eds) Evolution of Tertiary Mammals of North America. Volume 1: Terrestrial Carnivores, Ungulates, and Ungulatelike Mammals. Cambridge University Press, pp. 525-536.

McKenna, M. C. & Bell, S. K. 1997. Classification of Mammals: Above the Species Level. Columbia University Press, New York.

Mihlbachler, M. C., Lucas, S. G., Emry, R. J. & Bayshashov, B. 2004. A new brontothere (Brontotheriidae, Perissodactyla, Mammalia) from the Eocene of the Ily Basin of Kazakstan and a phylogeny of Asian "horned" brontotheres. American Museum Novitates 3439, 1-43.