a saga of supercontinents, sea-floor spreading, savannas, and sabertooth cats
DAVID D. GILLETTE, COLBERT CURATOR OF VERTEBRATE PALEONTOLOGY, MUSEUM OF NORTHERN ARIZONA
"The Great American Biotic Interchange" has not yet gained popular acclaim as a household expression in Arizona. Nor have many in our good state realized that Arizona played a critical role in this exchange. Still fewer know that a critical component of that exchange were animals called "glyptodonts, or that events at both poles influenced their immigration and evolution.
Glyptodonts were 4-legged tanks, pseudo-tortoises with fur, protected by a rigid shell composed of tightly interlocking plates an inch thick and more. The largest glyptodonts weighed a ton, ate plants, and probably spent a lot of time in water, along shores of lakes and streams. They resembled their distant relatives, the armadillos, but the fossil record of these two groups spans tens of millions of years indicating they diverged early in the history of their branch of placental mammals.
Nearly as big as a Volkswagen Beetle, and about the same shape, glyptodonts were abundant in South America where they originated. Among the first paleontologists to study glyptodonts was Charles Darwin. I often wonder what the influence of his observations of these strange armadillo-like beasts had on the formulation of his theories of evolution.
The glyptodont shell was so rigid that the coalesced armor plates are often found still connected, occasionally as a complete carapace like the one shown in main-image above. There are even records of indigenous South American people using a fossil carapace for shelter—these shells were large, to be sure, but they surely made for crowded if not solitary refuges! The carapace and tail in main-image (above) is the gigantic South American genus Doedicurus, famous for the bony tube that encased the vertebrae of the rear half of the tail, and the horrific mace that could have dealt a death blow to a would-be predator. The spikes are not mounted on this tail, but the attachment surfaces are easy to recognize.
The arrival of the "glyptodont fauna" of southeastern Arizona 2 million years ago heralded the mixing of faunas that till then had never come in contact: eventually mammoths, mastodons, horses, deer, camels, dogs, and cats spread populations from their North American homelands, to South America where their competitors were all marsupial mammals and odd placental mammals. At the same time, some of those odd placental mammals such as anteaters, armadillos, glyptodonts, and ground sloths from South America invaded the northern savannahs of that continent, then the coastal plains of Central America and soon thereafter, into Mexico and North America. Glyptodonts and ground sloths should be in the vocabulary of every native Arizonan, because these strange animals were among the Arizonans that lived beside mastodons, mammoths, saber-tooth cats, lions, extinct horses, camels, llamas and more in the North American Ice Age, right here in the desert Southwest. It's desert here now, but 2 million years ago, these animals lived in well-watered savannas and riparian forests that later dwindled in the Ice Ages and left little but these fossils as testament to wet times gone by. This was the "glyptodont fauna," so named because these armored tanks were at times the most abundant large animal in this bizarre landscape.
The story of these pilgrims from South America involves two supercontinents, three continents, two oceans, an island chain, an isthmus, glaciers, and sea level. It begins 200 million years ago early in the Mesozoic Era, during the time of dinosaurs. Earth's midsection was mostly ocean, and two huge landmasses occupied the northern hemisphere (Laurasia) and the southern hemisphere (Gondwana). The breakup of Gondwana began then, a process that produced the Gondwana continents: Antarctica, Australia, New Zealand, Africa, South America, India, and Madagascar. South America, the homeland of our armored immigrants, separated from Africa, Australia separated from one side of Antarctica and South America from the other side, and eventually Antarctica took its present, familiar polar position. The other land masses slipped away, in the process creating new ocean landscapes: the Atlantic, Pacific, and Indian Oceans, and with the final isolation of Antarctica, the Antarctic Sea. In broad perspective, the Antarctic Sea and Antarctica became Earth's refrigerator, affecting temperatures of ocean water through ocean currents that variously heated and cooled the seas and the atmosphere.
Following its isolation from Antarctica and other southern continents most vertebrate animals on the island continent of South America had evolved for at least 63 million years in isolation, more closely related to Australia's mammals than those in North America. (One marsupial, the opossum, invaded North America in the Ice Age and occupies most of Mexico and southern United States today.)
The key to the story here is the connection between the two continents, today called the Isthmus of Panama. Geologically speaking, the Isthmus is very young. Until only a few million years ago the oceans now called the Pacific and Atlantic were one. There was no land separation, so marine animals and plants could freely mix eastward and westward, with little isolation. But with movement of the Caribbean Plate in the late Cenozoic, the two continents became connected, forever isolating the two oceans but uniting the land areas where land dwelling animals and plants could mix. This was evolution at its finest: the sudden mixing of animals and plants in both directions, tests of their adaptations and survival. Some groups on both continents invaded the Isthmus beginning around 3 million years ago. A few of those groups managed to expand their populations through the Isthmus and into the other continent. Glyptodonts, as ungainly and awkward as they appear, were among the most adaptable from South America. Their populations shifted northward through the Isthmus and into Central America. When sea level fluctuations began about the same time, coastal savannas opened up as sea level dropped by as much as 100 meters, and then closed off when sea level rose again. One major cause of these sea level changes was fluctuation of glaciers in Antarctica and the arctic region of North America.
South America's isolation as an island continent had come to a close. Glyptodonts had become abundant, diverse, and widespread. Mixing of faunas and floras tested adaptability of large organisms and smaller ones alike: all parts of the habitats were affected, from decomposer organisms in soils and parasites that infected plants and animals, through the reigning ecosystems. Glyptodonts surely carried unique parasites, and plants they preferred were almost certainly different when they were isolated in South America. Glyptodont evolution north of South America took a new twist. The earliest population, known mainly from southeastern Arizona, became dominant members of the ecosystem. They were small as glyptodonts go, weighing perhaps a quarter to half a ton. These were the progenitors of at least two more species in the United States, and two other species known from single specimens in Mexico. But they all belong in the genus Glyptotherium (groove-toothed beast). Our early Arizona species bears the unfortunate (for us) name Glyptotherium texanum (named for glyptodonts found in west Texas with a population that extended from Florida to Arizona), but the descendant species Glyptotherium arizonae had adults nearly twice as large, and weights exceeding a ton. These two species occupied an ecological setting in Arizona that was at least seasonally wet, with abundant vegetation and little resemblance to the modern Sonoran Desert of the American Southwest.
A third species had evolved from the earlier ancestors by the end of the Pleistocene and occupied only the Gulf Coast and southern Atlantic Coastal Plain. This was Glyptotherium floridanum, named for fossils found in Florida. Two others are known from Mexico: Glyptotherum mexicanum and Glyptotherium cylindricum, the only species with a name that lacks a geographic connotation.
Glyptodonts had a rigid shell composed of very thick armor plates of bone in the dermis. These plates interlocked during growth and protected the sides and back; the belly was not armored and was the most vulnerable part of the body. Some have suggested that glyptodonts could simply tuck the legs beneath the "shell" to protect the belly, much in the same fashion as their distant relatives, the armadillos (which instead have a flexible shell composed of rings of armor that allow them to curl up for protection). In adult glyptodonts the only flexible parts of the shell were on the sides toward the front. Otherwise the shell was as rigid as a tortoise shell, and just as cumbersome.
The armor plates were covered in skin and scales that had a rosette pattern formed by grooves in the bone. The general pattern for Glyptotherium was one central figure surrounded by six to eight peripheral figures. Details of that general pattern allow a fair measure of confidence in identification of the five species when skulls and skeletons are not available. Two or three deep pits in the grooves on each plate housed follicles of modified hairs, or bristles that served a sensory function, quite a valuable asset for an animal that could not see its sides or back. The tail was covered in plates, too, in a series of flexible rings that completely enclosed the tail vertebrae, muscles, and blood vessels. In Glyptotherium the tail ended in a terminal set of rings with only moderate ornamentation; in one South American group, the tail was armored with a defensive mace that would have deterred attackers like saber-tooth cats.
Additional armor plates protected the top of the head, but they were not impenetrable. One skull of a juvenile Glyptotherium texanum from southern Arizona has a pair of elliptical holes in the top of the cranium. These holes are the right size and spacing to lead to the conclusion that this individual was killed by a large cat with a sudden blow to the head that punctured the armor plates and skull roof. That skull is presently on display at the American Museum of Natural History in New York City.
In the interpretation here by artist Victor Leshyk, two possible attack modes are presented: one with the glyptodont's head tucked under, the other with the head in a normal standing position. In either case, the attacker had to have a large gape, and probably attacked from the front. We cannot determine which fossil cat produced these holes, but it's clear that this glyptodont met a quick personal extinction.
Glyptodont teeth were simple three-lobed cylindrical prisms without enamel. Grinding motions of the jaws flattened the crowns of the teeth, probably worn down by silica from grasses. They had neither canines nor incisors. The shortened front of the head might have had a flexible proboscis or trunk, like that of sea cows or tapirs. My interpretation of the head is a short proboscis, like the interpretation here by artist Victor Leshyk, but others have proposed a longer one, and still others have claimed the face was blunt and lacked any prehensile anatomy like the inset in the Leshyk artwork. These armored herbivores likely grazed on grasses and soft plants, perhaps even along shorelines or in water.
Glyptodont legs were short, heavily-built, and squat. Glyptodonts could probably outrun a tortoise, but few other animals in its ecosystem. Although running to escape predators might not have been very effective, swimming might have been an avoidance strategy. Most paleontologists consider the glyptodont habitat as savanna, but most glyptodont faunas include the South American capybara, a mainly aquatic rodent somewhat larger than a beaver, indicating proximity to water. I have often imagined glyptodonts "snorkeling" in water, their buoyant shells partly submerged and the only part of the head breaking water the upturned proboscis.
Glyptodonts disappeared from Arizona by the middle of the Pleistocene, and their populations became restricted to the Gulf Coast at the end of the Pleistocene, around 11,000 years ago. Their extinction coincides roughly with the extinction of most of the megafauna: mammoths, mastodons, camels, llamas, horses, ground sloths, sabertooth cats, giant bison, giant armadillos, and many smaller species. Paleontologists debate the cause of this seemingly abrupt extinction, which happened about the same time that humans entered North America from Asia. Most fall into one of two camps: overkill by the earliest Americans, or climate deterioration. A recent, highly popularized version of the deteriorating climate hypothesis is the assertion that a bolide from space (part of a comet or an asteroid) impacted the earth somewhere with such force that it affected climate and habitats, and soon thereafter caused the extinction of these giants from the Ice Age.
The glyptodont saga came to an end with their extinction, but research continues on these beasts from the past. Paleontologists in South America and North America alike have resurrected these poorly studied mammals in the past decade to discover a fruitful line of study. In Arizona, some real fossils of glyptodonts are on display at the Arizona Museum of Natural History in Mesa. These bones are from the early, small species, Glyptotherium texanum, from southeastern Arizona.
When I travel in southern Arizona, in my mind's eye I see glyptodonts, mammoths, mastodonts, camels, horses, ground sloths, saber-tooth cats, armadillos, anteaters, and tortoises. I also imagine early people here, and in South America where a direct association has been documented, living beside these armored marvels, and even hunting them.
Also, in this issue, there is a photographic tour of the Superstition Mtns. Click here to see it.
(Gars O'Higgins Station penguins)
(Twilight Saga commentary)
(Coming soon---Volcano Watch!)