BRONTOTHERES: EOCENE RHINO-LIKE MAMMALS

Brontotheres are perissodactyl land mammals, a large group that includes tapirs, rhino-like animals, rhinoceroses, and horses. According to Mihlbachler and Prothero (2021), bronthotheres were the dominant herbivore land mammals of the middle and late Eocene of North America (South Dakota, Nebraska, Wyoming, Colorado) and a few places in Asia.

Brontotheres [“bront” is Greek, meaning thunder, and “ther” is Greek, meaning beast]. Brontotheres have been referred to, therefore, as “thunder beasts” and less commonly as  “thunder horses.” They have also been referred to as “titanotheres” (also derived from Greek, meaning giant beasts).

The rhino-like brontotheres first “appeared” during the early Eocene in both western North America and Pakistan (Asia) during the Eocene. Contrary to what early workers believed, brontotheres did not live during the Oligocene. It has been difficult to determine which country they actually originated in, but the general consensus is that they likely originated in North America. Brontotheres then migrated via the Bergingia1 land bridge (see one of my recent previous posts) to western Asia and eventually to eastern Europe. There were, however, some migrations brontotheres from Asia into North America (Mihlbachler, 2008). 

The early Eocene brontotheres were small size, but by middle to late Eocene, they reached their largest size. Most of the Eocene genera and species were hornless, but horns appeared during the late Eocene. Horns were highly variable in size and shape within single species of brontotheres, thereby confusing early workers who overnamed the species of these fossils.

Fig. 1. The late Eocene Megacerops gigas  had two large nasal horns. 

Fig. 2. The skeleton of the skull of Megacerops gigas a late Eocene brontothere from Colorado. This species, which is endemic to western North America, was 8 feet tall at the shoulder and 15’ in length, which would have been larger than any living rhinoceros. This exhibit is in the Mammal Hall of the Los Angeles County Museum of Natural History.

Fig. 3. Closeup of the skull of the same skull of Megacerops gigas shown in Figure 2.

Figure 3: Fragments of late Eocene brontothere teeth from the nonmarine Sespe Formation in Simi Valley, southern California. The largest fragment is 2 mm in height. Brontotheres have been found also elsewhere in Southern California Eocene deposits (e.g., San Diego area). 

The worldwide cooling event that began near the end of the Eocene (and continued during the Oligocene) was responsible for grasslands replacing the forests. This dramatic change in vegetation was undoubtedly responsible for the demise of the brontotheres, which were browsing herbivores that lived in forests. 

Fig. 4. A modern-day African rhinoceras (5 feet tall at the shoulder = substantially smaller than Megacerops) housed at the Los Angeles County Museum many years ago. True rhinoceroses first evolved during Miocene time. They occurred in both North America and Asia. They went extinct in North America during the Pliocene, but they continued to thrive in Africa, Asia, and Europe. Today, only five species are left and they live in tropical climates. Two species live in Africa, and three are found variously in India, Sumatra, and Java. 

Further information:

Prothero, D.R. 1994. The Eocene-Oligocene transition: Paradise lost. Cambridge University Press, New York, 291 pp. (paperback). See pp. 34, 153–164. 

 www.fossilguy.com

An excellent overview of the subject.

References Cited:

Mihlbachler, M.C. 2008. Species taxonomy, phylogeny, and biogeography of the Brontotheridae (Mammalia: Perissodactyla). Bulletin of The American Museum of Natural History 311:1–145.

Mihlbachler, M.C. and D. Prothero. 2021. Eocene (Duchesnean and earliest Chadronian brontotheres (Brontotheriidae), Protitanops curryi and cf. Parvicornus occidentalis, from west Texas and Mexico. Palaeontologia Electronica. DOI: 10.26879/944 

  

GEOLOGIC HISTORY OF TAPIRS

Tapirs belong to the three-toed perissodactyl group of ungulate land mammals. Other members of this group are rhinoceroses, horses, brontotheres, and chalicotheres. Tapirs resemble pigs, but tapirs have a prehensile (flexible) nose.

Tapirs are tropical browsing mammals. There are four living species (Tapirus terrestris, T. bairdii, T. pinchaque, and T. indicus), and they live in forests and woodland near water. Tapirs survive today in Central America (Honduras and Panama), northern South America (i.e., Brazilian Amazon, Brazilian Pantanal, and Peruvian Amazon), and southeast Asia (i.e., southern Thailand, south Myanmar through the Malayan Peninsula, and Sumatra in Indonesia).

Tapirs first appeared about 55 million years ago during early Eocene time in North America, with seven genera during in the Eocene. Bones/skulls of tapirs have been found in Wyoming, the Dakotas, and southern California. They have been found also in lower Eocene deposits on Ellsmere Island of Canada. There were also tapir-like rhinoceroses during the Eocene in North America. Tapirs have a good fossil record in Eurasia. There were also tapir-like horses during the Eocene in Europe. They persisted in the warmer areas of Europe, Asia, and North America (see next paragraph) until late Pleistocene time.

In North America, there were 3 genera of tapirs during the Oligocene, 4 in the Miocene [there was some overlap of these genera in the Oligocene and Miocene], 1 in the Pliocene, and 1 in the Pleistocene (De Santis and MacFadden, 2007). Early Miocene remains of tapirs have been found in Oregon (John Day fossil beds), South Dakota, Nebraska, and the Texas coastal plain. Remains of a Miocene tapir (Miotapirus sp.) have been found in the basal bone bed at Sharktooth Hill in south-central California. Rare remains of Tapirus merriami have been found in Pleistocene Saugus Formation north of Los Angeles, California. Rare remains of late Pleistocene tapirs have been found also at Rancho La Brea in Los Angeles, southern California.

North American species of tapirs migrated via the Great American Biotic Interchange (GABI) southward to Central America and South America. 

Very rare remains of tapirs have even been found in the late Pleistocene La Brea Tar Pits.

The dispersal route out of North America and the timing of this migration took place have not been documented yet for Asian tapirs, including the modern-day Malaysian tapir. Some paleontologists during the 1940s through 1980s dealt with these thorny questions by saying that the route was likely via a land bridge between Alaska and Siberia during Miocene time (= Beringia 2). Some subsequent researchers have been much more cautious and typically say “it is assumed that they reached eastern Asia during the middle Cenozoic.” Is it important not to confuse this early migration (Beringia 2) out of North America with Beringia 3 = the “Ice Age Beringia land bridge,” which took place during Pleistocene time.

HORSES

Overview

Horses (equids) are herbivore ungulates (hoofed mammals) belonging to order Perissodactyla, which also includes tapirs and rhinos. The fossil record of horses ranges from Paleocene?, and early Eocene to Recent. Their Cenozoic biodiversity (number of families, genera and species) was highest during the first half of the Cenozoic when they arose on northern continents and later spread southward into Africa and South America. During the second half of the Cenozoic, their biodiversity has gradually and significantly been reduced. 

Most of the early Cenozoic equids (like the other perissodactlys) lived in woodlands and were rooters. During the later Cenozoic, with the spread of grasses (tough to chew and digest), many perissodactyls adapted to living on prairie grasslands.

With the changes in where and how they lived, as well as in what they ate, horses adapted accordingly. Their bodies became larger with longer legs, and the number of toes on each foot decreased. These changes enabled horses to become stronger and faster runners, thus better fleeing their predators. Their skulls also got bigger, thereby allowing for bigger brains. Larger skulls allowed also for larger teeth that were more complex internally, with vertical layers (folds) of enamel growing down, deep inside the teeth. These kind of teeth are called hypsodont teeth (see one of my previous posts), and they are very effective in increasing the strength of the teeth, thereby resisting wear and tear when the horses eat tough grasses with a high content of abrasive silica. Like humans, horses have only two sets of teeth: milk and adult. The latter can last up to 15 years in modern horses.

 About 10,000 years ago, domestication of horses and other livestock began.

Earliest Horses

The earliest known perissodactyls are two genera (one species each) of early Eocene age. Both are small dog-size—see one of my previous posts: Eohippus in Wyoming, northwest New Mexico, and Colorado; and Hyracotherium in London, England and in Paris, France. The morphologic and biogeographic histories of horses are summarized in the following chart.

Horse Teeth

Like in most mammals, the teeth of a horse are differentiated into four types: incisors, canines, premolars, and molars. The premolars and molars are grouped together as the cheek teeth. The teeth are, furthermore, very characteristic of the orders of the mammals to which they belong, and the morphology of the teeth reflect the diets of the mammals.

The diagram above shows the right-side view of the skull/teeth of a modern horse.

Mammal-dental formulae typically designate the number of teeth (upper/lower) on just one side of both jaws. For example, the formula for many modern horses is 3/3, 0/0, 3/3, 3/3, and it refers to the presence of three incisors, no canines, three premolars, and three molars on one side of the upper and lower jaws = 18 teeth. In order to obtain the total number of 36 teeth (excluding the variations noted in the next two paragraphs), one has to include also the teeth on the other side of the skull. Thus, the horse has a total of 36 teeth (18 + 18), when both sides of the skull are counted. This dentition formula is the same for donkeys, zebras, and the Przewalski wild horse. It is important to mention that in all of these animals, their teeth continue to erupt (grow) during the life of the animal. This is because they eat coarse/tough vegetation which wears down the teeth. The cheek teeth of horses erupt in a tightly packed unit (called an "arcade") that acts like a single grinding tooth. Cheek teeth are a composite of three hard materials: enamel, dentin, and cementum.

Male horses typically have 40 teeth because between ages four and five years old, they develop upper and lower canine teeth. The canines are positioned nearer the incisors than the premolars (on both sides of the jaws). Only about 25% of female horses develop canines and are smaller in size than those found in males.  

There is an additional potential of dental variability: between 13 and 32% of both male and female horses can have small-sized vestigial (non-useful) premolar (referred to as a “wolf tooth”) located in front of the first premolar (typically only on the upper jaw). 

Reference Used:

Carson, D.M. and S. W. Ricketts. Equine dentistry. vcahospitals.com

OVERVIEW OF MAMMAL TEETH

This post is the second one concerning background information about the ancient history of various mammal groups that I shall be focused on in my upcoming series of posts (see my immediately previous posts about land bridges),

Before proceeding into the paleontologic details of various groups of land mammals, I discuss here the various types of mammal teeth. The shapes and arrangements of teeth in mammals are extremely important for the classification and identification of fossil vertebrates (as they are for living ones).

Some vertebrates (fish [including sharks], amphibians, and reptiles) generally have continuous replacement of teeth, so that as one is shed or lost, another erupts to takes its place. Land mammals, however, are fundamentally different is this regard; they have a mostly pre-determined number of teeth and very limited replacement of teeth.

Mammal teeth are differentiated into four types: incisors, canines, premolars, and molars. The premolars and molars are grouped together as cheek teeth. The teeth are, furthermore, very characteristic of the orders of the mammals to which they belong, and the morphology of the teeth reflect the diets of the mammals.

The number of teeth varies greatly in different groups of mammals. Dental formulae (i.e., the “ratios”) designate the number of teeth (upper/lower) on just one half of one side of both jaws. For example, the formula for dogs is 3/3, 1/1, 4/4, 2/3 = 40. This means that there are at least 10 teeth (namely, three incisors, one canine, four premolars, and two molars) on each quadrant (= one quarter) of the skull. But, as there are quadrants, the number of teeth is 10 x four ] = a total of 40 teeth on the entire adult skull.

The above diagram shows the side view of each of the jaws in a skull of a dog and the types of teeth present. The canine teeth are large, whereas the incisors are small. The large and pointy shape of the most forward molar tooth in the lower jaw and the most backward placed premolar in the upper jaw move pass one other, like blades in a pair of scissors, in such a way to allow for a shearing action (allows for eating flesh and bone). These two teeth (present are both sides of the skull) are referred to as carnassial teeth.  

The formula for a typical adult human is 2/2, 1/1, 2/2, 3/3, and it refers to the presence of two incisors, one, canine, two premolars, and 3 molars; for a total of 8 x 4 = 32 teeth. In some individuals there is an additional molar in each quadrant of the skull. This molar is the most posterior one and is referred to as a “wisdom tooth”. If there are four wisdom teeth, then the formula is 2/2, 1/1, 2/2, 4/4 = total of 36 teeth. 

In many mammals, including humans, there are two sets of teeth: the deciduous (“baby” or “milk” teeth) and the adult teeth. In humans, the “baby” teeth (2/2, 1/1, 0/0, 2/2), for a total of 20 teeth, are replaced by the adult teeth, which erupt starting around 6 years old or so. Note: baby humans do not have any premolar teeth. 

A COMPILATION BY R. SQUIRES OF CENOZOIC LAND BRIDGES MENTIONED IN THE GEOLOGIC LITERATURE AND REFERRED TO IN MY UPCOMING BLOG POSTS:  

This post provides vital background information that concerns a series of subsequent posts by me on various Cenozoic land-mammals (for example, hippos, sloths, camels, mastodons, etc.). One of the major themes that concerns all these animals is how and when they migrated from one continent to another. As you will see, some migrated from continent to continent via land bridges (listed below). Other groups, however, were/are confined to a single continent. 

De Geer Land Bridge

Connected various parts of the high Arctic in the early Cenozoic: Canadian mainland, Ellesmere Island, Greenland, and Svalbard to Norway. De Geer existed (intermittently?) at or near the end of the Cretaceous until the early Oligocene. See Brikiatis (2014).

Thulean Land Bridge

Extended from North America to Greenland, (intermittently?) from early Cenozoic until the early Oligocene. See Brikiatis (2014).

North Atlantic Land Bridge (NAtLB) or Greenland-Scotland Land Bridge

Extended from Greenland to Scotland during the middle Cenozoic.

Turgai Land Bridge

Extended between east and west Eurasia (intermittently in the Eocene?) during the Oligocene. See Brikiatis (2014).

Berginia 1 Land Bridge 

Extended from North America (Alaska) and Siberia. It occurred intermittently during Eocene and Oligocene time (between 55 and 25 million years ago (mya).

The following sequence of maps versus various intervals of geologic time (Eocene, Miocene, etc.)  provides summation of the major land bridges mentioned in my list:

Note: The general term “Beringia” has been used extensively in the published literature. The term is used mainly, however, for only for the most recent connection between North America and Sibera, but there have been at least two other similar land connections; hence  the overall term “Berginia” is misleading. That is why I labelled the first two "Berginias" as 1 and 2, and why I chose to label the youngest Berginia (the Ice Age one) as number 3.

Note: North America/Siberia land connection: It is possible that this connection occurred also during the latest Cretaceous (65.6 mya), thus explaining some dinosaur distributions. 

Beringia 2 occurred intermittently during the Miocene (between 25 and 5 million years ago, especially during late Miocene time. During that time interval, various groups of land mammals (many now extinct), as well as land plants, migrated primarily between North America and Eurasia. 

Greater Antilles-Aves Island (GAARlandia)

Latest Eocene/earliest Oligocene time only; shortly thereafter this island chain was dismantled and submerged. Today, only islands exist.

This very temporary land bridge allowed for some South American land mammals (ground sloths, capybaras) to migrate into Central America. Some current workers are hesitant to embrace this seemingly plausible migration route as not being all that viable. 

Great American Biotic Interchange (GABI) 

This acryonym refers to the greatest land animal exchange in Earth’s history = “Greatest American Biotic Interchange.” This exchange, which took place at the Isthmus of Panama portal, started at the Pliocene-Pleistocene boundary about 3 mya and is still going on. As shown in the following diagram, most of the animal migrations were southward out of North America into Central and South America (e.g., horses, mastodons, camels/llamas, tapirs, etc.); some animals migrated northward (capybaras, armadillos).

The "cats" arriving from North America were cougars, jaguars, and also saber-tooth cats. Other animal groups that migrated both ways were various insects, amphibians, snakes, alligators, turtles, tortoises?, and some flying birds.  

The large South American carnivorous land birds (phororhacoid ground birds----see one of my previous posts) reached also Texas and Florida during GABI.

Beringia 3

The latest high-latitude North Pacific connection between Alaska (North America) and Siberia (Asia) during the four Pleistocene Ice Ages (the most recent advance of ice was about 10,000 years ago). Glaciation caused the “Bering Strait” to be closed from time to time.

Note: The acronyonym “NALB” (North America Land Bridge), which has been used also for the Beringia 3-Pleistocene Ice Age connection, should be avoided because it can be confused with the North Atlantic Land Bridge (a.k.a. as the middle Cenozoic Greenland-Scotland land bridge during the Cenozoic).

OVERVIEW OF MAMMAL GROUPS THAT USED LAND BRIDGES FOR MIGRATION. All of the groups listed below (except humans) will be discussed in my upcoming posts.

Beringia 3 (Pleistocene time)

Mammoths

Humans

GABI (Pliocene-Pleistocene boundary)

Capybaras

Ground sloths

Glyptodonts

Cuvieronius gomphotheres

Beringia 2 (Miocene time)

Camels

Tapirs

Horses

Gomphotherium

Ambelodontidae (shovel-tusked gomphotheres)

Mastodons

Beringia 1 (Eocene time)

Unitatherium

Brontotheres

Horses