GROUND SLOTHS

Ground sloths are extinct xenarthrans (“zee-nar-thrans”), a major group of land mammals mostly restricted to South and North America. Other xenarthrans are tree sloths, anteaters, armadillos, and the armadillo-like glyptodonts. The first three of these are still extant (living). The living ones have a variable body temperature that is lower than that for other mammals.

The word “xenarthrans” means “stange joints,” in reference to these animals usually having feet with large claws. Some ground sloths had ossicles in their skin.

The biology of ground sloths is well known because some exceptionally well-preserved specimens that have been found  in dry caves (in both South and North America still have their hair intact (in some cases reddish) and soft tissues preserved. Samples from these specimens have been most useful for DNA analyses by evolutionary biologists. There is no living animal quite like the ancient ground sloths. They were bipedal browsers of grasses and leaves, and are usually depicted as setting up on their haunches, feeding on the leaves of tall trees.

There were about 60 genera of ground sloths, and these animals ranged in size from cat-size to giants (Megatherium) over 6 m (18 ft.) long and probably 3 tons in weight. This large ground sloth ranged from South and Central America to the southern states of USA. Charles Darwin mentioned in his book “The Voyage of the Beagle,” that he had found Megatherium and other ground-sloth genera in South America.

Plastic model of Megatherium.

A sloth skull showing the peg-like teeth.

The early American politician Thomas Jefferson in 1797 described the Pleistocene ground sloth Megalonyx from West Virginia.This Pleistocene genus has been found also in Alaska and Canada and rarely at Rancho La Brea Tar Pits (in urban Los Angeles of Southern California.. 

Three types of late Pleistocene ground sloths have been found in the Rancho La Brea Tar Pits. One of these is the “Harlan ground sloth,” traditionally known as Glossotherium harlani. It stood a little over 6 feet tall (1.8 m) and weighed about 3,500 pounds. It lived in both South and North America. In recent years, vertebrate paleontologists have reported that the name Glossotherium harlani is problematic and, therefore, is now known as Paramylodon Harlan (Owen, 1840). Notice that Owen’s name is now in parentheses—which is an official nomenclatural requirement.

The immediately preceding photographs show the exterior and interior, respectively, of the skeleton of Paramylodon harlani from La Brea. 

Another ground sloth commonly found at Rancho La Brea Tar Pits is the “Shasta ground sloth,” Nothrotheriops shastense. This sloth was smaller than Paramylodon harlani and had a more elongate mouth, fewer teeth, and was probably a browser that ate leaves. Its skin had none of the ossicles found in Paramylodon harlani. 

Side view of the ground sloth Nothrotheriops shastense from Rancho La Brea.

As summarized in the following diagram, ground sloths originated in South America about 34 mya (million years ago) during the the early Oligocene. They migrated, with certainty, into North America near the Pliocene/Pleistocene boundary. This was about 2.7 mya, during the Great American Biotic Interchange (GABI), when the Isthmus of Panama emerged and connected, with certainty, North America with South America (see one of my previous blog posts).

Geologic overview of ground-sloth migration patterns through time. Literature search  and drafting by R. L. Squires (2023). 

Although there have been a few reports of earlier migrations of rare ground sloths into North America (during the late Miocene time, from 8 to 10 million years ago (mya), these need more study for confirmation.

They are two newly recognized genera:Thiniobadistes and Pliometanastes. Neither one, however, is known from South America. Both are endemic to North America. One genus occurs in Nuevo Leon in northern Mexico and Florida, and the other one occurs in Florida. A possible late Miocene arrival of ground sloths, coincident with the evolution of new genera of sloths, helps establish that South American mammals were moving, for the first time during that time, northward into North America. The exact migration route of this first pulse of northward migration out of South America has not been fully determined. Was it via Central America? Some researchers believe that it might have been via "island hopping" in the West Indies area, stretching north from the chain of Lesser Antilles Islands and then west along the chain of Greater Antilles Islands (including Puerto Rico, Haiti/Dominican Republic, and Cuba), and northward again into Florida. More research is needed to solve this unsettled dilemma.  

Ground sloths went extinct approximately 10,000 years ago (we just missed them!). They were most likely hunted to extinction by humans. They lasted somewhat longer in Central America, possibly because of their smaller size there. 

Macrauchenia: The Interesting Story of a Very Strange South American Mammal

 In 1839, Charles Darwin mentioned his discovery of a half of a skeleton of a very strange fossil in Patagonia, Argentina. He described it as a “huge beast with a long neck like a camel.” In 1838, the British anatomist Richard Owen, named the fossil Macrauchenia patachonica. The genus name is derived from Greek and means “robust neck,” which reminded Owen of the neck of a llama. He believed furthermore that this fossil belonged to the same division as that of the rhinoceros, tapir, and paleotherium (a genus of early horse).

Subsequent experts referred to this strange fossil as belonging to a group of mammals called litopterns; in fact, it turns out that Macrauchenia patachonica was the last of the litopterns. 

Litopterns have been regarded by most vertebrate paleontologists as being herbivore (plant-eating) placental mammals that lived only in South America from Paleocene to Pleistocene time. Macrauchenia patachonica lived only during the Pleistocene.

Macrauchenia patachonica had a long body (about 10 feet long) with a moderately narrow head, a long neck and sturdy legs. This animal weighed about 2,300 pounds and was about the size of modern-day "black" rhinoceros. Furthermore, M. patachonica has three toes on each leg, with the middle toe the largest. Its skull has 44 durable teeth, used for grazing on grasses. Its skull also has a nasal opening high on the roof of its skull above but between its eyes. It is very unusual for a Pleistocene herbivore to have its nasal opening there. The ankle joints are such that it might have had the ability to rapidly change direction when it ran at high speed. Whether or not Macruchenia patachonica had a proboscis is controversial. The following sketches show both possibilities:

In the 2020’s, researchers using DNA-based studies of bone material determined that Macrauchenis patachonica can now be more accurately classified as closest (i.e., a sister group) in its evolutionary history to tapirs and horses. Furthermore, this fossil is not too far from the rhinoceros group, but is, in fact, far from the camel group.

A brief chronology of Charles Darwin’s life and how the extinct Macrauchenia influenced it:

1809-born in England

1831-employed as a naturalist for a voyage aboard the HMS Beagle (a ship of the British Royal Navy); the main purpose was to survey the coastline of South America and to chart its harbors; the studies by Darwin were largely a bonus—but what a bonus!

1836-returned to England and shared the fossils he collected with researchers at museums.

1838-Richard Owen, a specialist in anatomy, wrote about and named the fossil Macrauchenia.

1839-Darwin published his own findings about Macrauchenia.

1859-Darwin published his “On the Origin of Species.” Macrauchenis influenced his thinking about his theory of natural selection (= a process of how species form and change over time).

1882-Darwin died at age 73. He is buried in Westminster Abbey in London, England, and visitors can visit his burial site.

CAMELS

Camels are land mammals that belong to the order Artiodacytla and the family Camelidae. Camels originated in North America during the late Eocene and were restricted to that continent until the end of the Miocene. They earliest camels were rabbit sized. During the Oligocene to early Miocene time, they spread to open woodlands in places like South Dakota and reached goat size. During the Miocene, camels expanded into steppe and grasslands in North America. In order to be able to eat/grind coarse, gritty grass, camels developed high-crowned cheek teeth. Their necks and legs lengthened. Some species were llama-size, and other species began to resemble modern-day camels and giraffes.

Procamelus of late Miocene age (modified after Savage and Long, p. 217). Procamelus has very distinctive splayed feet.

Pliocene-age camels were biologically diverse (with eight different forms of camels)a in southern California (Ana-Borrego Park area). Also during Pliocene time, camels migrated for the first time into Asia, Europe, and Africa.

During the early Pleistocene, giant forms of camels evolved. The largest was Titantylops, which was 3.5 m at the shoulder. It lived in Nebraska. Also during this time, camels reached South America, where they evolved into the llama, alpaca, guanaco, and vicuña.

Skeleton of Titanotylopus nebraskensis on display at the Los Angeles County Museum of Natural History—Fossil Mammal Room.

The remains of the camel Camelops, a close relative of the llama, are found in the late Pleistocene Rancho La Brea tar deposits in Los Angeles, California. This camel went extinct about 11,000 years ago.

Side and top views of a camel tooth (2.75 in high, 1 in wide) from Rancho La Brea. 

Camels surviving today as the “camel” and dromedary. The “camel,” also known as the Bacrtrin has two humps (a sort of deposit of fatty tissue) and lives in the Gobi Desert of Mongolia. During the winter, Bactrins have long shaggy hair for protection from the cold weather. The dromedary has a single hump and lives in Africa and the Middle East. Domedaries have longer legs and shorter hair in comparison to the two-hump camels. They also have shorter tempers.

There are feral (escaped from captivity) camels in Australia today. 

Key references:

Jefferson, G.T. and L. Lindsay. 2006 Fossil treasures of the Anza-Borrego Desert. The  last seven million years. Sunbelt Publications. 394 pp.

Savage, R.J.C. and M.R. Long. 1986. Mammal evoution an illustrated guide. British Museum (Natural History). 299 pp.

MAMMOTHS AND ELEPHANTS

Family Elephantine = mammoths and elephants. Elephants today are represented by two genera/species: Loxodonta africana = the African elephant, and Elephas maximus = Indian or Asian elephant. Both of these genera range in geologic time from the late Miocene to Recent. 

Elephants are the largest living terrestrial mammals.

Their dental formula is 1/0, 0/0, 3/3, 3/3 = 26.

The six cheek teeth are replaced continuously from the rear throughout life; but they are only six replacements in the lifetime of about 50 years for an elephant. Only one (or one and part of another) cheek tooth is functional at a time. Each tooth is shed as it becomes worn.

They have no canines, and their tusks are incisors:

Loxodonta africana = African elephant has a pair of upper incisors elongated into tusks reaching over 3 m long in males.

 

Female Elephas maximus usually have no incisors, thus they have no tusks.

Early mammoths apparently evolved from gomphotheres (see one of my previous posts) in Europe during Miocene time. The fossil record of mammoths is imperfect, but they continued to be present in Europe during the Pliocene and Pleistocene.

Most mammoths belong to genus Mammuthus. Mammuthus meridonalis migrated out of Africa at the beginning of Pliocene time, about 5 mya. It traveled into Europe and then crossed the Beringia Land Bridge 3 into North America during the Pleistocene (see one of my earlier posts on fossil mammals). It eventually reached the west coast of the United States later during the Pleistocene. 

The biggest mammoth was the Europe/Asia Pleistocene Mammuthus trogontherii, which was about 15 feet high at the shoulder and had tusks up to 16.5 m long. [It evolved from Mammathus meridionalis.] Mammuthus trogontheril then migrated to North America (also via Beringia Land Bridge 3) and evolved into Mammuthus columbi. A rather famous locality for the remains of M. columbi is near Hot Springs, South Dakota. Apparently, an ancient sinkhole in this area “lured” many young mammoths to their death 26,000 years ago.

                 Literature research and drafting for this diagram by R. Squires 2022

Mammuthus exilis was a pygmy mammoth (only 4.5 to 7 ft. tall and 2000 pounds compared to its direct ancestor: the 14 ft. tall and 20,000 pound M. columbi) found on the Channel Islands (primarily Santa Rosa Island) off the coast near Los Angeles. The predecessor of M. exilis reached the islands 40,000 years ago by means of swimming (i.e., sea level was 300 ft. lower than today and the island was only 6 miles from the mainland. They evolved from normal-size M. columbi mammoths, but migrated to the islands because of increasing scarcity of food, increasing competition with other animals, and, most likely by predation from cats and wolves.

One of the more famous mammoths in the North American Pleistocene rock record is Mammuthus columbi [also known as M. imperator (the "Imperial" Mammoth) or, according to some experts, as Mammuthus meridionalis]. It is best known from the La Brea Tar Pits at Los Angeles, southern California. This mammoth reached a maximum height of 13 feet (3.9 m). The average adult size was 12 feet (3.6 m) tall and was about 10,000 pounds (4,900 kilograms). 

On the left is Mammuthus columbi from the La Brea Tar Pits. On the right is a mastodon, also from the La Brea Tar Pits (see my previous post). 

Three views of Mammuthus columbi: upper image shows most of the skeleton, middle image shows the right side of the skull (including jaw bone and a molar); lower image shows the molar tooth more clearly. This skeleton of the late Pleistocene Mammuthus columbi [formerly known as M. imperator = Imperial Mammoth] is on display to the public at the La Brea Tar Pits Museum in the Los Angeles area of Southern California. 

Replica (cast) of a single molar tooth (2 in wide x 1 3/8 in high) from a Pleistocene juvenile mammoth. Mammoth molars have a flat chewing surface (in comparison to mastodons) because mammoths ate grasses.

The so-called "wooly mammoth" = Mammuthus primigenius (shown below) is also of late Pleistocene in age [to no younger than 10,000 years ago]. It was widespread  throughout the Arctic tundra-vegetation zone, in both the New World and Old World. It too migrated via Beringia 3 from Europe to North America (actually during the last Ice Age).

A plastic model of Mammuthus primigenius.

References Used:

Jefferson, G.T. and L. Lindsay. 2006 Fossil treasures of the Anza-Borrego Desert. The  last seven million years. Sunbelt Publications. 394 pp.

Savage, R.J.C. and M.R. Long. 1986. Mammal evoution an illustrated guide. British Museum (Natural History). 299 pp.

www.nps.gov  Channel Islands pygmy mammoths (this site has several interesting videos about these mammoths.)

MASTODONS

Mastodons are extinct proboscideans that range from Miocene age (about 15 million years ago) to about 10,000 years ago. Mastodons belong to family Mammutidae. They preferred to live in woodland savannahs areas and browsed primarily on leaves and other foliage. Their earliest fossil record is in Africa, Asia, and Europe. They were, in fact, the dominant megamammals in the world during the mid Miocene, and at this time, they migrated into North America, most likely via the Beringia 2 land bridge (see one of my previous posts).

The earliest “relatives” of mastodons are 1) the Eocene anthracobunids (no tusks) from northeast Pakistan and northwest India, and 2) the Oligocene paleomastodons from Africa. Some reports of co-called “Mastodon” fossils are actually gomphotheres (see one of my previous posts). 

The term Miomastodon” (in the diagram above) is in quotes because some workers consider it to be a mastodon, whereas others consider it to be a gompothere. “Miomastodon” is found in the Miocene-age basal bone bed at Sharktooth Hill, southern central California. 

In contrast to the controversy surrounding MIomastodon, Mammut americium was definitely a mastodon. Its most widespread distribution was during the Pleistocene, with occurrences in Alaska, Ontario (Canada), New England, Florida, New Mexico, California, and Honduras in Central America.

Mammut americanum is especially well known from latest Pleistocene (about 10,000 years old) asphalt deposits at Ranch La Brea tar pits in Los Angeles County, southern California.  Well preserved specimens of this elephant-like mammal are 6 feet tall at the shoulder. They have long elephant-like trunks. Their teeth consist of low-rounded cusps separated by wide gaps. They fed on leaves and twigs rather than grass.

Three views are shown here of “Simi mastodon” Mammut americanum, late Pleistocene (50,000 yrs), Simi Valley, CA. This specimen is now on display at the “Age of Mammals Hall,” Natural History Museum of Los Angeles County, southern California.