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The Brontotheres - Facts about the Thunder Beasts

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Introduction / Physical Description

Brontotheres were the largest mammals in North America and Asia throughout the Eocene from around 53 to 34 million years ago. By the middle to late Eocene, Brontotheres had developed long skulls with bony hornlike protrusions. Their bodies were massive, and superficially looked like a rhino, although they had more elephant like legs. The largest genera in the late Eocene was the dual horned Megacerops, which reached heights around 8 feet at the shoulder and 15 feet in length. The teeth of Brontotheres are very large and rectangular with a distinct chewing surface. Some larger Brontothere teeth are over 4 inches wide which made them ideal for chewing vegetation.

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Origins, Distribution, and Species

Origins, Evolution, Distribution, and Species

Brontotheres belong to the Perissodacyla order, or Odd-toed Ungulates. These are hoofed animals that support their weight mainly on one toe. Today, there are 3 families of Perissodacyla, the horses, tapirs, and rhinos. However, in the past the diversity was much higher. In the Eocene, there were 10 families of Perissodacyla which made them the most abundant herbivores. One of the better-known extinct families are the brontotherids. Because of their large size, robust bones, and abundance, they have a very good fossil record (Prothero, 2009) and we can trace their evolution from their origins to their extinction.

The earliest brontotheres appear as the genus Eotitanops in the early Eocene around 53-51 million years ago. This genus is a very small, unassuming animal that is barely distinguishable from other early types of Perissodacyla (Prothero, 2009). After Eotitanops appears in the fossil record, it quickly diversifies into numerous genera in both Asia and North America. Today, we recognize 3 subfamilies of brontothere that contain over 40 genera with over 50 species.

Generally speaking, as time goes on, brontotheres become increasingly larger and gain head ornamentation in the form of bony protrusions toward the front of their skulls. The final genus that lived in North America is Megacerops which occurs just above the Chadronian-Orellan boundary of the White River Badlands (Benton et al., 2015). With shoulder heights of 8 feet and lengths over 15 feet, these are the largest of the brontotheres.

One problem with brontothere origins is the original Eotitanops genus nearly simultaneously appears both in North America and in Pakistan. In North America, Eotitanops borealis is found in the Wind River formation which is around 53 - 51 myo. In Pakistan, Eotitanops pakistanensis is found in the upper Ghazii Formation which is around 52.5 - 51.5 myo (Missiaen et al, 2011). Because of this, it's unclear on which continent brontotheres first evolved. They may have appeared in Asia and migrated into North America, or vice versa.

An extensive phylogenetic analysis of the brontothere taxa was done by Mihlbachler (2008). He determined there were between 9 and 12 separate brontothere dispersal events between North America and Asia. His data suggests brontotheres originated in North America and migrated into Asia. Regardless of which continent they first appeared on, by the middle Eocene, it seems they moved back and forth between continents numerous times.

Besides for North American and Asian brontotheres, there have also been a few specimens from Eastern Europe. One specimen named Brachydiastematherium transylvanicum was originally found in the Transylvania region of Romania, and two others of uncertain genera were found in Bulgaria.

Two brontotherid family trees are shown below:

The first is from Burger and Tackett (2014) and shows North American genera only. This one shows the ages as well as the skulls of the various North American Genera. Notice the gradual increase in size and head ornamentation as time goes on. Also notice the numerous branches at many different time intervals. Again, there is no simple linear progression. Also note this starts with a genus called Lambdotherium. Lambdotherium is problematic and might not be a basal brontothere. Remember, the early brontotheres are difficult to tell apart from other basal animals.

The second is a more complete tree from one of Michlbachler's family trees and includes both North American and Asian brontotheres and shows the numerous dispersal events. Again, notice the tree has numerous branches at many different intervals. This modern tree shows no simple linear progression that is often used.

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Ecology - Habitat, Diet, and Horns

Ecology - Forested Browsers

Brontotheres lived throughout the Eocene. The Eocene was a VERY warm time for planet Earth. It began with a thermal spike (the Paleocene Eocene Thermal Maximum) and stayed hot for much of the Eocene (the Eocene Early Optimum). The climate was much warmer and wetter than today. What this means is Brontotheres in Asia and North America did not live in dry grasslands or savannahs, but instead sediments show they lived in semi-tropical forested environments with lots of river systems.

Their diets based from studies of teeth also validate this. Isotopic tooth studies by Boardman and Secord (2013) show brontotheres fed in wet forested environments and dental microwear patterns indicate they were leaf-dominated browsers (Mihlbachler, 2002), feeding on foliage and fruits in their forested environments.

Horns and Social Behavior

Many genera of brontotheres toward the end of the Eocene had developed some type of horn structure. These horns were more like bony protrusions on their nasal bones which varied greatly by shape and size. Both male and female brontotheres often had them, although they were larger in males. The largest protrusions were on the late Eocene Megacerops of North America and Embolotherium of Mongolia.

Since the entire family of borntotheres are extinct, we have no modern equivalent to compare them to as comparisons with rhinos only go so far. As a result, no one is quite sure what the horns were used for. Since they are sexually dimorphic, they may have been used for mating and display purposes. One megacerops specimen was even found with a fractured rib. The only animal large enough to cause that damage would have been another megacerops. This means horns could have been used in combat between males during the mating season.

Embolotherium's horns were extra bizarre. They were very elongated and had large nasal cavities in them, almost to the point that they were hollow. This would have enabled Embolotherium to make very loud resonating sounds for communication.

The images below show examples of horned and non-horned brontotheres.

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Extinction – The Eocene Oligocene Climatic Transition (EOCT)

The boundary between the Eocene and the Oligocene marked an extreme climate change event which disrupted ecosystems and caused extinctions all over the planet. Paleontologists even give this climate event a special name, the Eocene Oligocene Climatic Transition (EOCT). For the first time in over 100 million years, global temperatures cooled enough that glaciers and ice caps formed (Prothero 1994). This dropped sea levels and changed ocean circulation patterns. The cooler Earth became more arid. Semi-tropical forest environments that brontotheres lived in changed to open woodland and eventually semi-arid grasslands (Janis 1982). Unfortunately, brontotheres were highly adapted for eating vegetation in forested environments, so once their habitats were gone, they became extinct.

Brontotheres were not alone, 4 other orders of Perissodacyla also went extinct near the Eocene/Oligocene boundary. This climate shift rapidly dropped the diversity of life, including land mammals. The ones that survived were better suited for cooler and more arid grassland environments. These animals, such as horses and elephants, quickly diversified.

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Discovery and Native American Mythology

Brontotheres have been known by Native Americans long before the scientific community. The Great Sioux Nation was in the middle of Brontothere bone beds of the White River badlands. These fossils were so well known by the Sioux they became part of their mythology and today they are named after the Lakota word for them.

In an 1840's expedition, Captain James H. Cook recalls his visit to the Lakota chief Red Cloud in a memoir called "Sketches of the Life of Red Cloud." While there, he was shown a fossil jaw section containing a very large embedded tooth. They explained it was a tooth from a fallen "Thunder Horse", which would sometimes come down during storms and trample Buffalo. He was told stories of these Thunder Horses helping the Sioux on occasion by driving herds of Buffalo.

Later, Charles Marsh befriended Red Cloud and obtained permission to collect fossils of these giant animals in the Sioux territory. Marsh even named the largest fossil genus after the Thunder Horse, calling it Brontotherium, the "Thunder Beast". Today, this genus is invalid, but the name Brontothere is still used in taxonomy for this group of animals.

The very first mammal fossil described from the Western United states is also a Brontothere. This discovery sparked a frenzy of fossil expeditions well before the "Bone Wars" of the 1870's. In 1843, Dr. Prout, a paleontologist from St. Louis, obtained a jaw section with a giant tooth from a fur trader. Prout, with Joseph Leidy incorrectly identified it as a Palaeotherium. Later it was realized the specimen was a Brontotherium.

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Works Cited:

Benton, R., Terry, D., Evanoff, E., & McDonald, H. (2015). The White River Badlands: Geology and Paleontology. Bloomington; Indianapolis: Indiana University Press. Retrieved April 25, 2020.

Boardman, G.S., and Secord, R. (2013). Stable isotope paleoecology of White River ungulates during the Eocene–Oligocene climate transition in northwestern Nebraska: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 375, p. 38–49, DOI: 10.1016/j.palaeo.2013.02.010.

Burger, B. J. and Tackett II, L. (2014). The stratigraphic importance of the brontothere (cf. Diplacodon elatus) in the Brennan Basin Member of the Duchesne River Formation of Utah, Foss. Rec., 17, 69–74, DOI: 10.5194/fr-17-69-2014.

Janis, Christine (1982). Evolution of Horns in Ungulates: Ecology and Paleoecology. Biological Reviews 57: 261- 318. DOI: 10.1111/j.1469-185X.1982.tb00370.x

Mader, Bryn J. (2019). The narial morphology of Metarhinus and Sphenocoelus (Mammalia, Perissodactyla, Brontotheriidae). Palaeontologia Electronica 22.1.8A 1-15. palaeo-electronica.org/content/2019/2417-narial-morphology

Mihlbachler, Matthew. (2008). Species Taxonomy, Phylogeny, and Biogeography of the Brontotheriidae (Mammalia: Perissodactyla). Bulletin of The American Museum of Natural History. 311. 1-475. DOI: 10.1206/0003-0090(2008)501[1:STPABO]2.0.CO;2.

Mihlbachler, Matthew & Solounias, Nikos. (2002). Body size, dental microwear, and brontothere diets through the Eocene. Journal of Vertebrate Paleontology. 22 (Suppl.). 88.

Missiaen, P., G. F. Gunnell, and P. D. Gingerich. (2011). New Brontotheriidae (Mammalia, Perissodactyla) from the early and middle Eocene of Pakistan, with implications for mammalian paleobiogeography. Journal of Paleontology, 85: 665-677. DOI: 10.1666/10-087.1.

Prothero, Donald. (1994). The Late Eocene-Oligocene Extinctions. Annual Review of Earth and Planetary Sciences. 22. 145-165. DOI: 10.1146/annurev.ea.22.050194.001045.

Santucci, Vincent. (2017). Preserving fossils in the national parks: A history. Earth Sciences History. 36. 245-285. DOI: 10.17704/1944-6178-36.2.245.

Recommended Prehistoric Mammal Books:

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Additional Images of Brontotheriidae

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