Being Slothful at Hagerman: The South American Connection
While many people may be aware that Hagerman Fossil Beds has a great variety ofdifferent types of animals they may not be aware how "international" this variety is. One can almost think of Hagerman as a crossroads where animals from different continents have met. Many of the animals found at Hagerman originated in North America and later dispersed to other continents. Certainly horses and camels come to mind as two classic examples of groups that were present only in North America during most of their history, but later crossed the Bering Land Bridge into Asia and Europe. If it hadn't been for this dispersal we might only know about horses and camels from their fossils, since their descendants survived in the Old World after both groups became extinct in their "homeland" at the end of the Pleistocene Epoch or Ice Age. While horses and camels, along with other animals such as beavers, went west, other groups such as the proboscideans, (which includes living elephants and their extinct relatives like mammoths and mastodons) moved east and entered North America. We have a primitive mastodon-like animal at Hagerman, whose ancestors originated in Africa long before this animal showed up in North America.
This exchange of animals didn't happen all at the same time. We know from the fossil record that during different times in Earth's history, conditions were right for certain groups of animals to become widespread. Occasionally this spreading out coincided with the disappearance of certain barriers, such as a drop in sea level, and resulted in animals reaching other continents . Because different animals have different ecological requirements and different limits on where they can live not all animals on either side of a barrier can cross. When we do see certain groups dispersing into new areas it tells us something about their ecology. The first appearance of a new group of animals in the fossil record on a continent is often used by paleontologists to define periods of geological time.
Besides an international connection with Asia, Europe and Africa to the west, the fauna at Hagerman has a southern connection as well. Previous Critter Corners have discussed the otter, Satherium, and the grison, Trigonictis, at Hagerman and how their closest living relatives are found in South America today.
These animals, along with llamas and peccaries, also found as fossils at Hagerman, are among a diverse array of animals from North America that did not enter South America until about 2.5 million years ago. The reason for the timing of their first appearance in South America is that prior to this date the Isthmus of Panama, which today connects the two continents, was submerged and South America was an island continent, much like Australia today. Only after the movement of the continents, due to plate tectonics, pushed up the area we now know as the Isthmus of Panama, were many animals able to disperse from North America to South America and vice versa.
When we think of Australia as an island continent, we immediately think of its strange animals, such as kangaroos, wombats and the duck-billed platypus. Like Australia, South America had its own strange (at least to most people) assortment of animals, many of which unfortunately we only know about from their fossils. One of these groups are the Xenarthrans (the name means strange joint and refers to extra connections between some of the bones of the back) which today includes armadillos, anteaters and tree sloths. While these three groups of animals don't look anything alike.
Superficially, they share many features of the skeleton and other aspects of their anatomy that shows they share a common ancestor. As their name implies, tree sloths are very much restricted to living in trees. While today there are only two types of sloths, the two-toed and the three- toed, in the fossil record there are literally dozens of different types of sloths that are popularly called ground sloths. These extinct relatives of the tree sloths ranged in size from animals as large as an elephant down to about a medium sized dog. As the popular name suggests, most were too large to live in trees and lived full time on the ground. Like their living relatives, the extinct sloths were herbivores - some were browsers while others were grazers.
While there were many types of ground sloths in South America only a few different types moved north into North America. What is interesting is that one type, called the megalonychids, a long word that merely means "great claw" in reference to the large claws on the hands and feet, actually entered North America about nine million years ago. This was long before the formation of the Panamanian land bridge that let the North American animals move south. In addition to reaching North America, members of this group of sloths were also able to reach some of the islands in the Caribbean and their fossils have been found in Cuba, Hispaniola and Puerto Rico. Not a bad trick for a group of animals that are supposed to be "slothful". The best known member of this group is Megalonyx, an animal first described by our third president, Thomas Jefferson, who coined the name which later was used to form the family name Megalonychidae for this group of related animals.
Given its long history in North America, a number of different species of Megalonyx have been described by paleontologists over the years. At Hagerman we have the species Megalonyx leptostomus. Leptostomus means narrow mouth and refers to the long and narrow skull and jaw of this animal. This species of Megalonyx was originally described from Mount Blanco Texas, the same location where the first specimen of our "Hagerman" horse, Equus simplicidens, was found. Specimens of Megalonyx were among the variety of fossils first collected at Hagerman by the Smithsonian field crews in the 1930's. In 1935 C.L. Gazin, the curator of vertebrate paleontology at the Smithsonian in charge of the last excavations at the Horse Quarry in 1934, wrote a scientific paper describing the fossil sloth material from Hagerman. He originally referred to the Hagerman sloth as Megalonyx leptonyx (can you figure out what leptonyx means? - The clues are in the other scientific names used in this article - answer at the end). This sloth was found in Castle Creek, Owyhee County in the 1870's but is now considered to be the same as Megalonyx leptostomus. While no complete skeletons of the sloth have been found at Hagerman, we have a good variety of bones for comparison with other species of Megalonyx. These include lower jaws, isolated teeth, various leg bones and a complete hind foot. Sloth teeth are very distinctive since they lack the outer enamel covering present in other mammals' teeth. Because the teeth of sloths grow throughout the animal's life, they have no roots and have a simple cylinder shape. As the names suggest, the claws are large and have a distinctive triangular shape that makes them easy to identify.
One of the more interesting aspects of the skeleton of Megalonyx is the shape of its heel-bone. The part of the heel where the tendon of the calf muscle, the Achilles tendon, attaches is large and expanded suggesting these animals had a strong and powerful calf muscle. This makes sense when you remember that these animals were one of the browsing sloths and probably sat upright on its haunches while feeding on leaves and twigs. Like in modern sloths, the large claws probably served to hook branches and bring them in range of the sloth's mouth but also probably provided defense against predators like the early saber-tooth cat or hyena-like dog. Megalonyx like all sloths lack the incisors, the front teeth in the skull. Instead, it had very large tusk-like teeth that could clamp down on leaves and twigs to help break them off branches. Megalonyx at Hagerman probably spent most of its time among the trees lining the various rivers and streams that flowed through the area and drained into Lake Idaho.
The narrow mouth Megalonyx was widely distributed and has been found not only at Hagerman but also to the west in Washington and as far east as Florida and of course, south through Texas into Mexico. The Megalonyx lineage continued in North America right up until the end of the Ice Age. One of the trends was that it got larger so that the last species, Megalonyx jeffersonii, was almost twice as large as the Hagerman species. Jefferson's Megalonyx has also been found in a wide range of localities and even as far north as the Yukon as well as south into Mexico. Unfortunately the success of Megalonyx along with all other sloths ended and all became extinct by 10,000 years ago at the end of the Ice Age. Why Megalonyx and the other sloths disappeared after surviving so long is one of the many mysteries that paleontologists are still puzzling over. Perhaps the first question to ask is what contributed to their long success. The sloth at Hagerman provides part of the story and further work and comparisons with other sites may eventually help us come up with the answer.
Gazin, C.L. 1935. Gravigrade sloth remains from the late Pliocene and Pleistocene of Idaho. Journal of Mammalogy 16(1):52-60.
Hirschfeld, S.E. and S.D. Webb. 1968. Plio-Pleistocene megalonychid sloths of North America. Bulletin of the Florida State Museum, Biological Sciences 12(5):213-296.
Jefferson, T. 1799. A memoir on the discovery of certain bones of a quadruped of the clawed kind in the western parts of Virginia. American Philosophical Society Transactions 4:246-250.
Kurtén, B. and E. Anderson. 1980. Pleistocene mammals of North America. Columbia University Press, New York, 442.
McDonald, H.G. 1998. The sloth, the president, and the airport. Washington Geology 26(1):40-42.