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The Brontotheres: "Thunder Beasts"

Skull of a Brontotherium (Megacerops coloradensis) on display at Zurich natural history museum.  Image by Rama. (CC BY-SA 2.0 FR)

Skull of a Brontotherium (Megacerops coloradensis) on display at Zurich natural history museum. Image by Rama. (CC BY-SA 2.0 FR)


The Brontotheres - Facts about the Thunder Beasts


Fast Facts

A little too rhino-like lifesize model of a group of Megacerops at the Canadian Museum of Nature, Ottawa, Ontario. Photo by: D. Gordon E. Robertson. CC BY-SA 3.0


Name: Brontothere (pronunciation: "Bron-toe-Thir") "Thunder Beast"
The Sioux Nation thought these fossils were from giant Thunder Horses. Marsh gave them the scientific name Brontotherium "Thunder Beast" in honor of the Thunder Horse.

Taxonomy:
Mammalia (Mammal Class) - Perissodactyla [Odd-Toed Ungulates] (Order) - Brontotheriidae (Family)

Generally accepted Genera:
Brontotheres were a very successful family that lived throughout the Eocene. As a result, there are quite a few genera. The largest and most well known genera is Megacerops, which use to be called Titanotherium, Brontops, and Brontotherium.


Age/Extinction: Eocene ~ 53 to 34 myo
Contrary to popular beleif, Brontotheres did not live in the Oligocene, they went extinct in the late Eocene.
The problem stems from the fact that the last fossils come from the Chadronian age, which was originally thought to be early Oligocene, but is now considered to be late Eocene.

Distribution: North America and Asia
Brontotheres are well known from North America. They are also found in Asia and a few places in Eastern Europe.

Body Size:
Brontotheres were the largest mammals in North America during the Eocene.
Megacerops could reach a height around 8 feet tall at the shoulders and a length of 15 feet.

Description:
Brontotheres superficialy resembled rhinoceroses with very large heads. Later genera had large boney horns on its snout.

Ecology/Diet: Herbivore:
Brontotheres lived in social groups browsing on vegetation in forested and woodland environments. They were the sort of like the elephants of the Eocene as they were the largest browsers of their time before elephants diversified and evolved into their large sizes.

Fun Facts:
Although some Brontotheres reached sizes of small elephants and occupied the ecological niche of elephants, they were not related. They were more closely related to tapirs, rhinos, and horses.




Introduction / Physical Description


Assistant Curator James W. Gidley stands with a newly mounted skeleton of a Brontotherium hatcheri now known as Megacerops coloradensis. From the Smithsonian Institution Archives

Historical Photo of the Assistant Curator James W. Gidley standing with a newly mounted skeleton of a Brontotherium hatcheri now known as Megacerops coloradensis. From the Smithsonian Institution Archives (SIA). (Public Domain)



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.



Lower jaw of Megacerops (Specimen USNM V 8782) from the White River Group in South Dakota. Specimen
from the National Museum of Natural History

Lower jaw of Megacerops (Specimen USNM V 8782) from the White River Group in South Dakota. Notice the large, distinctive molars. Specimen from the National Museum of Natural History - Smithsonian Online Collections. CC0






Origins, Distribution, and Species


Megacerops coloradensis (Specimen USNM V 4262) from the White River Group in Nebraska. Specimen
from the National Museum of Natural History

Megacerops coloradensis (Specimen USNM V 4262) from the White River Group in Nebraska. Specimen from the National Museum of Natural History - Smithsonian Online Collections. CC0


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.

This illustration is Figure 3 from Burger and Tackett (2014). It shows the Stratigraphic occurrence and phylogeny of North American brontotheres starting at the early Eocene(1)Lambdotherium popoagicum and ending at the late Eocene (20)Megacerops sp.  CC BY 3.0

This illustration is Figure 3 from Burger and Tackett (2014). It shows the Stratigraphic occurrence and phylogeny of 20 genera of North American brontotheres starting at the early Eocene (1) Lambdotherium popoagicum and ending at the late Eocene (20) Megacerops sp. Diplacodon elatus (15) is shown reconstructed in the lower corner. CC BY 3.0



Figure 201 from Mihlbachler, 2008 showing a modern cladogram (evolutionary tree) he produced from an extensive phylogenetic analysis of brontotheres from North America, Asia, and Easern Europe.  This also shows the numerous dispersal events between North America and Asia.  Notice the tree has numerous branches at many different intervals.  This modern tree shows no simple linear progression that is often used.

Figure 201 from Mihlbachler, 2008 showing a modern cladogram (evolutionary tree) he produced from an extensive phylogenetic analysis of brontotheres from North America, Asia, and Easern Europe. This also shows the numerous dispersal events between North America and Asia. Notice the tree has numerous branches at many different intervals. This modern tree shows no simple linear progression that is often used.




Ecology - Habitat, Diet, and Horns


Restoration of a group of Megacerops. The largest one is around 8.6 feet (2.6) tall.  From the Field Museum of Natural History, Report Series Volume IX, 1931. Public Domain.

Restoration of a group of Megacerops. The largest one is around 8.6 feet (2.6) tall. From the Field Museum of Natural History, Report Series Volume IX, 1931. Public Domain. In reality, these animals would have prefered to be in the more forested envrionment shown off in the distance.



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.


This is the skull of Embolotherium andrewsi on display at the Tianjin Natural History Museum.  The nasal protrusion (horn) is very elongated and nearly hollow. Photo by: Jonathan Chen [background removed] (CC BY-SA 4.0)

This is the skull of Embolotherium andrewsi on display at the Tianjin Natural History Museum. The nasal protrusion (horn) is very elongated and nearly hollow. Photo by: Jonathan Chen [background removed] (CC BY-SA 4.0)



Figure 5-1 from Mader (2019) showing the left side of the skull of Sphenocoelus specimen AMNH 1851. Sphenocoelus had an odd internal nasal cavities, but no bony protrustions, or horns, from the nasal area. (CC BY-NC-SA 4.0)

Figure 5-1 from Mader (2019). showing the left side of the skull of Sphenocoelus specimen AMNH 1851. Sphenocoelus had an odd internal nasal cavities, but no bony protrustions, or horns, from the nasal area. (CC BY-NC-SA 4.0)



These are four reconstructed heads of Megacerops.  These were originally thought to be four different genera, but are now grouped under the Megacerops genus. The differences are due to different aged animals and individual variation.  From the 1929 monograph “The titanotheres of ancient Wyoming, Dakota, and Nebraska.

These are four reconstructed heads of Megacerops. These were originally thought to be four different genera, but are now grouped under the Megacerops genus. The differences are due to different aged animals and individual variation. From the 1929 monograph “The titanotheres of ancient Wyoming, Dakota, and Nebraska." by Osborn. (Public Domain)




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.




Discovery and Native American Mythology


Othniel Charles Marsh with the Sioux chief Red Cloud in New Haven, Connecticut.  Red Cloud granted Marsh permission to fossil hunt in the Sioux Nation and eventually became friends.  This photo is from Red Clouds visit to Marsh's home in Connecticut. (Public Domain)

Othniel Charles Marsh with the Sioux chief Red Cloud in New Haven, Connecticut. Red Cloud granted Marsh permission to fossil hunt in the Sioux Nation and eventually became friends. This photo is from Red Clouds visit to Marsh's home in Connecticut. (Public Domain)



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.


Lower jaw referred to as 'Prout's Palaeotherium Specimen' collected by fur trader Alexander Culbertson from the White River Badlands in 1843. USNM 21820. (Photo-Vincent Santucci) Figure 3 from Santucci (2017))

Lower jaw referred to as 'Prout's Palaeotherium Specimen' collected by fur trader Alexander Culbertson from the White River Badlands in 1843. USNM 21820. (Photo-Vincent Santucci) Figure 3 from Santucci (2017). This brontothere specimen was the first fossil mammal collected in the American West which spurred a fossil rush.




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 Books and Stuff!



Dinosaurs and Indians: Paleontology Resource Dispossession from Sioux Lands
Lawrence W. Bradley, 2014

Lawrence Bradley is a Professor at the University of Nebraska and was raised by Oglala Lakota tribe. He is the leading expert on vertebrate fossils taken from Native American lands. This book about fossils taken from the Great Sioux Nation is an interesting read from a Native American point of view. Available as paperback and Kindle.





The First Fossil Hunters: Dinosaurs, Mammoths, and Myth in Greek and Roman Times
by Adrienne Mayor, 2011

This book is a great merger of ancient Mythology and Paleontology. Fossils have been around since human beginnings. This book delves into what people thought fossils of long extinct animals were before modern science. It cleaverly pieces together origins of myth and reality in varous cultures, including the Greeks, Romans, and Egyptians. It's a great read! Available as paperback and Kindle.





National Geographic Prehistoric Mammals
Alan Turner, 2004

This is a nice childrens book that introduces the overlooked time after the dinosaurs when giant mammals ruled the earth. It's written by one of the worlds foremost paleontologists and brilliant illustrators. This book goes over numerous Cenozoic mammals in beautifully illustrated 2 page spreads. Some of the mammals include Gomphotherium, Deinotherium, Arsinoitherium, Mammoths, Giant Sloths, and Saber Toothed Cats!



Megacerops Toy Figure by MOJO
A great model for anyone that collects prehistoric beasts. It's a very acurate model of an adult male Megacerops. If your kid likes Dinosaurs, expand their horizons by trying out this Prehistoric Beast of the Cenozoic!





Additional Images of Brontotheriidae



Megacerops skeleton on display at the Museum of Natural History in Los Angeles..

Megacerops skeleton on display at the Museum of Natural History in Los Angeles.



Megacerops coloradensis skull in collections room at the Carnegie Museum of Natural History.

Megacerops coloradensis skull in collections room at the Carnegie Museum of Natural History.



An impresive display of Megacerops skulls at the Raymond M. Alf Museum of Paleontology near Los Angeles.

An impresive display of Megacerops coloradensis skulls at the Raymond M. Alf Museum of Paleontology near Los Angeles.



A layout of a megacerops lower jaws, vertebra, and other bones on display at the Raymond M. Alf Museum of Paleontology near Los Angeles.

A layout of a megacerops lower jaws, vertebra, and other bones on display at the Raymond M. Alf Museum of Paleontology near Los Angeles.



Skull of a large Megacerops coloradensis on display at the Raymond M. Alf Museum of Paleontology near Los Angeles.

Skull of a large Megacerops coloradensis on display at the Raymond M. Alf Museum of Paleontology near Los Angeles.



Historical photo of Smithsonian expedition members working on a Titanotherium (Megacerops) specimen.  Ribs and hind legs are visible. Image from the Smithsonian Instirution Archives.

Historical photo of Smithsonian expedition members working on a Titanotherium (Megacerops) specimen. Ribs and hind legs are visible. Image from the Smithsonian Institution Archives, Record Unit 46, Box 118, Folder Budget-Wetmore File 1929, After August 1



Skeletal reconstruction of a Brontothere after Marsh from the 1897 book

Skeletal reconstruction of a Brontothere after Marsh from the 1897 book "Extinct Monsters" by Henry Neville Hutchinson.(Public Domain)



A skeleton and restoration of an 11.8 ft (3.6 m) brontotheres in the 1929 monograph “The titanotheres of ancient Wyoming, Dakota, and Nebraska.

A skeleton and restoration of an 11.8 ft (3.6 m) brontotheres in the 1929 monograph “The titanotheres of ancient Wyoming, Dakota, and Nebraska." by Osborn. (Public Domain)




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