It is sometimes said that the present is the key to the past. It is equally true, however, that the past is the key to the present and, perhaps to a limited degree, to the future. To a degree, Arizona, New Mexico, and Trans-Pecos Texas form a natural unit, united by warm and near-warm desert conditions interrupted by mountain ranges that often allow cooler, more mesic conditions. The modern character of the flora and fauna of Arizona, New Mexico, and Trans-Pecos Texas has been set largely by climatic and biological events of the Pleistocene and Holocene geologic epochs. Without an understanding of these events, the present day constitution and distribution of our regional flora and fauna make little sense. To the degree that future climatic change and disturbance may be similar to occurrences in the past, knowledge of that past may allow anticipation of future problems.

A major aim of this work is to bring together Pleistocene records and information about the vertebrate fauna for the region in a manner useful to both the interested lay person and scientists. I am well aware that such efforts often end up serving neither audiences well, but if the person with a general interest can manage to overlook some of the more esoteric material and the lists of fauna and sites, and the scientist some of what might seem elementary, perhaps both may find it not too bitter a pill. A major reason for presenting this work on the web is to allow easy updating as new, missed, or corrected data become available.

Dating the Pleistocene

In earlier days, geologists were stuck with relative dating—that is, they often were able to determine by stratigraphy whether a given geologic bed was older, the same age, or younger than another bed. In general, a stratigraphic bed lying above another was younger than the second, etc., and physical attributes and fossils often allowed correlation of beds not in direct contact. The system was not perfect, but generally workable. What such a system could not do was to assign a date, other than extremely rough estimates based on such things as the thickness of a stratum.

More recently, dates have been assignable based on the decay of radioactive elements into different isotopes of the element or into a different element. The decay rate is invariant for the elements used in dating, and the proportion of decay products to the original element or isotope allows calculation of the time passed. For the most part, the "clock" is set when molten rock solidifies, capturing the element, and as they are formed, its decay products. Thus most radiometric dates are associated with igneous rock, and older fossils usually are dated by whether they are above, between, or below dated igneous beds, or are correlated with such a bed occurring elsewhere.

An exception to the once-molten rock requirement occurs with 14C (carbon 14). This radioactive isotope of carbon is usable for dating organic material up to about 50 ky. Unlike the other radioactive elements commonly used, 14C is formed by the action of cosmic radiation changing nitrogen atoms to the radioactive carbon isotope. Some of this 14C ends up in the CO2 used by plants for photosynthesis and is incorporated into the tissues of the plants. From the plants, the radioactive carbon is passed up the food chain to herbivores and carnivores. At the death of an organism, new 14C no longer enters the body and the "clock" is started; the radioactive carbon decays back into nitrogen with a half life of about 5730 years. For an expanded exposition of radiocarbon dating, see the web pages of the Radiocarbon Laboratory, University of Waikato, New Zealand.

Because of the way that 14C is created, the amount in the atmosphere (and thus in organisms) varies through time. In order to determine a calendar date, the 14C date must be calibrated so that higher levels of 14C don't give dates too recent and low levels dates too old. Somewhat simplified, this is done by radiocarbon dating substances that have known ages, such as tree rings, and comparing the radiocarbon dates with the known calendar dates.

During much of the time since 14C dating began in the middle of the last century, only raw 14C dates were available and as such reported in the scientific literature as year BP (Before Present, which has been standardized to mean before 1950). In the present work, uncalibrated 14C dates are given as appearing in the literature; calendar dates are given with the addition of "cal" (e.g., 22,522 ± 737 cal kya). When calendar years are given without a reference, the calendar date has been calculated through CalPal-2007online (Danzeglocke et al. 2012).

Both raw 14C dates and calibrated dates usually are reported with ± a standard deviation. This denotes a statistical uncertainty such that we would expect the true date to fall into the range indicated about 68% of the time; with the standard deviation doubled, about 95% of the time. It is usual to accept such a 95% chance with the knowledge that about 5% of the time the true date will fall outside of the range.

Dating the Pleistocene beyond the range of radiocarbon has been largely through radioactive elements with greater half lives than that of 14C. Time zero for such material is usually set when melted rock solidifies, preventing the addition of more of the radioactive element or the loss of it other than by radioactive decay. Since fossils are usually found in sedimentary rather than igneous settings, dating usually is by bracketing such sediments by lava flows or volcanic ash. In many cases, a dated sediment can be correlated with sedimentary beds that are not so bracketed, extending the dating to those beds.

Another dating technique involves minerals that are susceptible to magnetic fields. Minerals free to orient themselves to the magnetic field of the earth may do so in melted rock (such as lava) or during deposition as sediments. The magnetic field of the earth undergoes reversals erratically, with the north and south magnetic poles switching places. These reversals are preserved in the rocks, allowing knowledge as to the polarity at the time of solidification or deposition. Unfortunately, one reversal looks like another, but if other data can restrict the possible time span somewhat, then the magnetic data may allow pin-pointing the date of a specific horizon. For example, the Brunhes-Matuyama Reversal occurred approximately 780 kya, providing a fixed reference point.

Until recently, the Pleistocene Epoch, often known as the Ice Age, officially began about 1.8 million years ago (1.8 mya) and ended about 10 thousand years ago (10 kya), followed by our present epoch, the Holocene. A different definition now has been accepted by the International Union of Geological Sciences: The Pleistocene now begins at approximately 2.58 mya, when northern hemisphere glacial activity ramped up. The geologic epoch immediately before the Pleistocene is the Pliocene. With the change in definition of the Pleistocene, 0.8 my has been transferred from the last portion of the Pliocene to the Pleistocene (and to the Quaternary Period, consisting of the Pleistocene and Holocene epochs). This action also has transferred the later part of the Blancan North American Land Mammal Age (NALMA—see below) from the Pliocene into the Pleistocene.

More than one system of referring to a segment of geologic time is in use. One commonly used system is division of the Pleistocene into early (2.58-0.78 my), medial 0.78-0.126 my), and late Pleistocene (0.126-0.010 my). Another system often used by mammalian paleontologists depends on associations of mammalian taxa. These are the North American Land Mammal Ages (NALMA), and for the Pleistocene, consist of the later portion of the Blancan NALMA; the Irvingtonian NALMA, encompassing most of the Pleistocene; and the Rancholabrean NALMA, ending with the megafaunal extinction at the end of the Pleistocene. Morgan and Harris (2015) used subdivisions of the Pleistocene NALMAs based on regional data as follows: early late Blancan, 2.7-2.2 mya; latest Blancan, 2.2-1.6 mya; early Irvingtonian, 1.6-1.0 mya; medial Irvingtonian, 1.0-0.4 mya; late Irvingtonian, 0.4-0.250 mya; Rancholabrean, 0.250-0.010 mya. All dates are approximate. These subdivisions are used here.

With the late Blancan not lining up nicely with the boundaries of the geologic epochs, it becomes somewhat of a problem to determine which faunas to include in the Pleistocene. Relying heavily on dates from Morgan and White (2005), I am omitting the late Blancan Wolf Ranch and Pearson Mesa faunas as being pre-Pleistocene. It appears that the lower 30 m or so of the 111 Ranch fauna also is Pre-Pleistocene (Morgan and White 2005), but the upper portion is not; since it is not clear which taxa are in the Pleistocene portion and which in latest Pliocene, the 111 Ranch fauna is included in its entirety.

The Pleistocene climate consisted of a series of glacial ages separated by interglacials similar to the one we presently inhabit. Traditionally, it was thought that the Pleistocene was characterized by the initiation of glaciation in the northern hemisphere, and four glacial ages were recognized, named after the states where they were recognized. The oldest, the Nebraskan, was followed by the Kansan, the Illinoian, and the Wisconsin (or Wisconsinan). In more recent times, it's been recognized that many more than four glacial advances and retreats have occurred within the Pleistocene, with about 20 within the traditional 1.8 my. Events assigned to a particular glacial age when it was thought that there were only four in actuality may not have been contemporaneous and actually separated by appreciable spans of time. Only events of the most recent age, the Wisconsin, seems to generally be firmly based, thanks to radiocarbon dating techniques and the recency of the deposits; the Illinoian generally is still recognized by Quaternary workers, but Nebraskan and Kansan glacial ages have been abandoned, with glacial cycles preceding the Illinoian merely assigned to pre-Illinoian.

Information from deep-sea cores of sediments indicate the Wisconsin age was separated from an earlier major ice advance by a warm interglacial, the Sangamon (or Sangamonian), centered roughly at about 122 thousand years ago (Marine Isotope Stage [MIS] 5e). The earlier glacial is the Illinoian glaciation. Earlier substages of MIS 5 (5a, 5b, 5c, and 5d) often are considered as part of the Sangamon Interglacial (and this definition is followed here), but MIS 5a to 5d by some are considered as part of the Wisconsin (the Eowisconsin or Early Wisconsin) with 5b and 5d being stadials (relatively cold intervals) and 5a and 5c interstadials (relatively warm intervals). Following MIS 5, an early stadial (MIS 4) lasted roughly from 75 kya to 60 kya. This, rather than the MIS 5a-d span, sometimes is considered as the Early Wisconsin glacial, and is followed here. MIS 3 (Mid Wisconsin) is interstadial, though markedly cooler than MIS 5 and with significant climatic variability within the roughly 35-ky year span. The Mid Wisconsin ends at about 25 kya as the climatic deteriorates into the severe pleniglacial (full glacial, MIS 2). The most severe glacial climatic conditions of the Wisconsin centered around 20 kya. Starting at about 15 kya, the climate rapidly ameliorated (though with setbacks) and, depending on the author, the Pleistocene climate gave way to that of the Holocene around 14-10 kya.

A major setback in the return from glacial to interglacial conditions occurred around 10.9 kya BP (12.9 cal kya) with the onset of a cold interval known as the Younger Dryas; return to warmer conditions occurred about 1300 years later.

The Early Holocene of the Southwest still was cooler and moister than today, with these conditions replaced by more modern conditions around 8 kya. Fully modern vegetational conditions in southern New Mexico, however, may not have been established until about 4-5 kya.

During glacial conditions, much of northern portion of the continent was covered by ice sheets. At farthest advance, ice reached well into the northern parts of the United States. Glaciers occurred at high elevations in the northern part of our Southwestern region, with a small glacier occurring as far south as Sierra Blanca in southern New Mexico. Cooler temperatures and more effective precipitation strongly affected the biotic environment in the Southwest. Plants now limited to farther north or to higher elevations were able to move southward or descend, in many cases connecting patches of montane forest vegetation currently isolated by intervening lowland vegetation. Other moisture-loving plants found expanded habitat. Animals, so often reliant on vegetation as well as on direct climatic conditions, shifted their geographic ranges accordingly.

Fossil faunas document the presence of the various kinds of vertebrates at specific places during specific times. They thus provide biogeographic data for themselves and, through their connection with vegetation, for floral elements. Shifts in geographic ranges in turn cast light upon the climatic events ultimately largely responsible for the distribution of organisms.

The latter part of the Pleistocene record and that of the Holocene also clarify the environments under which man entered and adapted to the New World. Overlap of extinct vertebrates with humans and, in some cases, the evidence of interaction between them emphasizes the differences in ecological relationships between the period during which man was adapting to New World conditions and those of today.

Literature. Balter 2006; Danzeglocke et al. 2012; Fairbanks et al. 2005; Morgan and Harris 2015; Sanders et al. 2009; Scott 2010.


Last Update: 25 Jan 2016