Some Late Pleistocene, Now-extinct Fauna of the Southwest
Notes on Sources: These all make clear that the geographical distributions of the animals discussed below were highly variable, but all below were found in the American Southwest at least to some extent. A major site yielding dateable fossils was Rancho Labrea Tarpits, Los Angeles, from which the name "Rancholabrean fauna" (see below) derives.
The main source for the text of the first part of this essay -- an overview of the extinct fauna -- is the paper by Anderson, Elaine, "Who's Who in the Pleistocene: a Mammalian Bestiary", pp. 40-89 in Martin, Paul S. & Richard G. Klein, eds., Quaternary Extinctions (1984), U. of Arizona Press, Tucson, plus some materials drawn from other chapters in this same volume. For accounts of the flora of the time, I (RNH) used Spaulding, W.G., E.B. Leopold, & T.R. Van Devender, "Late Wisconsin paleoecology of the American Southwest", pp. 259-93, in H.E. Wright, Jr., ed., Late-Quaternary Environments of the United States, Vol. 1 (1983), Minneapolis: U. Minnesota Press. Most of the illustrations come from the fine book edited by Amman, Andrew William, Jr., Bezy, John V., Ratkevich, Ron, & Witkind, W. Max, Ice Age Mammals of the San Pedro River Valley, Southeastern Arizona, Arizona Geological Survey, Tucson, Down-to-Earth Series 6 (1998). The illustrators were John C. Dawson and Robert B. Horsfall.
The second part of the essay summarizes more recent research on the timing of the extinctions. Sources for that will be indicated further below.
Part One: Overview of the Flora and Fauna
Envision a San Pedro River Valley sometime prior to 11,000 B.P. ("Before the Present", set as 1950), looking not like today but like this:
an open woodland dominated by pinyon pines, junipers, and live oaks stretched from near the rivers edge to the altitude of Sierra Blanca, where, probably, some trees representative of higher, Montane forest also appeared. Extensive grasslands intermixed with sagebrush and perhaps Joshua trees as well as these stands of woodland trees. Summers being cool and dry, winters only moderately colder but much wetter, the paloverdes, mesquites, saguaros, ocotillos and creosote bush now characteristic of these foothills were nowhere to be seen. The now-extinct megafauna overviewed below probably roamed here sometimes, before the end of North Americas Pleistocene glaciation.
Late-Pleistocene large mammals became extinct in much higher numbers than did small mammals. Extinctions included the following. Some dates indicate where remains were last found.
Nothrotheriops: a large ground sloth, range from northern Mexico to southern Alberta; weight to 400 lbs. A browser, it fed on roots/stems/flowers etc of desert plants including, prominently, Mormon Tea [ephedra]. Latest dated sample from Rampart Cave, AZ is about 9,000 B.C.E.-- "Before the Common Era".
Glossotherium: a powerfully built sloth with huge forelimbs, claws. Lived in open country, dug roots, fed on grass/shrubs. Central Mexico to Idaho.
Equus: horses, several species, swift runners of plains and steppes. Last specimen 6,000 B.C.E. Alberta.
Platygonus: A flat-headed peccary, larger than living peccaries, almost pan-North American in its range. Last dated find about 10,000 B.C.E.
Camelops: like the living dromedary camel but larger, a grazer, traveled in large herds. Last found in Casper, Wyoming, from around 8,000 B.C.E.
Hemiauchenia: Long-legged llama. A fast-moving grazer, found from coast to coast.
Navahoceros: A mountain deer, larger than a mule deer, a climber; from Wyoming to Mexico. Last date from N. Mexico about 9,500 B.C.E.
Bison: Short-grass prairies, formerly more than one species; major known protein source for Clovis hunters. The main Holocene Bison survivors -- Bison bison -- are now in state/national parks. At left is Bison latifrons, a giant among this genus, its horns spreading over 10 feet.
Euceratherium: Shrub Oxen. Remains from northern California To Mexico; 4/5 the size of a modern bison, foothills grazers. At Burnet Cave, northern Mexico, found in the same level as human artifacts.
Antilocapridae (pronghorn Antelope-like)
Capromerix: Pronghorn antelope. small 4-horned pronghorns; plains grazers, weight 22 lbs.
Tetramerix: large 4-horned pronghorns; also California to Mexico. Weight to 130 lbs, similar in size to the contemporary pronghorns also found in this region.
Stockoceros: Intermediate between the first two above in size. Remains found in artifact-bearing levels at Burnet Cave, northern Mexico, 9,500 B.C.E.
Proboscideans of various kinds (elephant-like):
Mammutus: Mammoth. M. jeffersonii inhabited open prairies especially in the West, with populations in Arizona, feeding mainly on grasses. Last dates about 9,000 B.C.E.
Mammut: Mastodont. Rare in western North America, preferred conifer woodlands; fed on these and swamp plants. Terminal dates 7,000 B.C.E. at latest.
Large carnivores also suffered disproportionate extinctions compared with smaller ones:
Canis dirus: The dire wolf scavenged from southern Alberta to Peru, larger than living gray wolf (Canis lupus), very powerful teeth. Terminal dates about 7,000
B.C.E. at La Brea tar pits, CA.
A variety of bears, including Arctodus (the short-faced bear), the most powerful predator of the American Pleistocene, was probably replaced by Ursus arctos -- the grizzly bear, which came from Asia at that time.
Homotherium: Scimitar cat. African lion-sized, mammoth-eater, rare.
Smilodon: Saber-toothed cat. Widespread; African lion-sized, preyed on large, slow-moving animals. Last date about 7,400 B.C.E. in Tennessee.
Most late Pleistocene bird extinctions involved large to very large flesh-eating birds, particularly the large-carrion feeders. Lost species of this type were numerous in western North America, but included most spectacularly the Teratornis incredibilis: The largest known North American flying bird, with a wingspan of 16 feet and a huge, deep, powerful bill. The surviving turkey vulture -- wingspan around 5 feet -- in contrast, specializes in small carrion, which no doubt explains its continuing survival in contrast to these much larger birds. Teratornis presumably became extinct along with the extinction of the other megafauna in North America. The decline of the California Condor throughout the Southwest is also probably due to the loss of large carrion in that region.
Click here for an illustration of a somewhat smaller species of Teratornis, taken from Steadman, DW and Martin, PS, "Extinction of Birds in the Late Pleistocene of North America" In Quaternary Extinctions: A Prehistoric Revolution. Martin PS and Klein RG. editors, Univ of Arizona press, Tucson, 1984. This remarkable image shows a reconstruction of Teratornis merriami, seated on the carcass of a dead horse. Its massive size -- though it was smaller than the amazing incredibilis-- may be estimated by comparing it with the images of the golden eagle, shown closer to the viewer, in front of the horse carcass.
Part Two: The Timing of the Extinctions
Main source: Haynes, C. Vance, Jr., 2008, "Younger Dryas 'black mats' and the Rancholabrean termination in North America", April 24 issue of the Proceedings of the National Academy of Sciences' Online Early Edition;
By around 11,000 B.P., the northern hemisphere was gradually warming and glaciers were in retreat. In our San Pedro Valley of Arizona, archaeological evidence indicates that animals under stress gathered at dwindling water sources, where they were killed Clovis hunters. Then abruptly the climate reverted to much colder conditions which lasted for about a thousand years, a dramatic and widespread cooling event that has been identified as the Younger Dryas Cold Spell, named after a tundra type of plant whose pollen appears in fossil assemblages at this time, now dated to approximately 10,900 to 9,800 B.P. Many of the megafauna extinctions date to or shortly following this time.
The causes of these extinctions have long been debated, and we will not presume competence to take a position on these matters (see the volume on Quaternary Extinctions cited above). However, geoarchaeologist and paleoIndian specialist Vance Haynes (see citation directly above) observes that the start of this Cold Spell is marked geologically by a "black mat" of organically rich soil, reflecting climatic conditions strikingly moister than either before or after this time. Climatic cooling probably caused reduced evapotranspiration, resulting in more effective recharge of water tables. Below the black mats, which are present at more than 70 geoarchaeological sites in the United States, evidence is present of the late Pleistocene flora and megafauna outlined above in Part One. Also present were the humans who hunted these large animals, hunters long identified by archaeologists as the Clovis Culture for the large fluted stone spear points they employed. But above the black mat, neither the megafauna nor Clovis points are anywhere to be found. As Haynes summarizes,
"Of the 97 geoarchaeological sites of this study that bridge the Pleistocene-Holocene transition (last deglaciation), approximately two thirds have a black organic-rich layer or black mat [of materials...] with radiocarbon ages suggesting they are stratigraphic manifestations of the Younger Dryas cooling episode 10,900 B.P. to 9,800 B.P. (radiocarbon years). This layer or mat covers the Clovis-age landscape or surface on which the last remnants of the terminal Pleistocene megafauna are recorded. Stratigraphically and chronologically the extinction appears to have been catastrophic, seemingly too sudden and extensive for either human predation or climate change to have been the primary cause. This sudden Rancholabrean termination at 10,900 +/-50 B.P. appears to have coincided with the sudden climatic switch [from] warming to Younger Dryas cooling. Recent evidence for extraterrestrial impact, although not yet compelling, needs further testing because a remarkable major perturbation occurred at 10,900 B.P. that needs to be explained."
For a detailed precis of Dr. Hayne's paper, see this link: C. Vance Haynes on the Rancholabrean Termination.