You can’t talk about fossils without talking about time, and mammalian paleontologists (also known as paleomammalogists) generally have two different ways to talk about how long ago an animal lived.
The first way is to discuss age in terms of absolute time: saying a fossil is 25 million years old, for instance. The only way to determine a fossil’s age in terms of millions of years is to directly date the rocks in which the specimen was found using radiometric dating techniques. A slightly less precise age can be determined by dating the rocks above and below a fossil, giving a range of possible ages: older than 25 million years but younger than 30 million years, for instance. The problem with dating rocks directly is that the method only works for certain types of rocks, and fossils are seldom found in those rock types. More frequently, a rock layer above and below a fossil can be dated, yielding an age range. Talking about age ranges can be a bit cumbersome, however, so paleontologists often instead say the fossil pertains to a specific part of the geologic time scale: the Late Jurassic Period or the early Oligocene Epoch, for instance. Most paleontologists have an idea of how these periods of time are arranged and their rough ages, so it makes for a convenient way to talk about time.
Also, since all paleontologists (not just paleomammalogists) use the same time scale, it’s easy to discuss the age of events with scientists from other parts of the world or those that are studying different types of organisms. In some instances, the resulting age range is so large as to be essentially meaningless. (A range of 5 to 25 million years old is not very helpful for a paleomammalogist.) At other times, the rocks above and below a fossil can’t be dated at all, or datable rocks are present, but the researchers don’t have the time, money, and/or expertise to do radiometric dating. So then what do you do?
What paleomammalogists often do is refer the fossil (or the fauna) to what is known as a land mammal “age”. Land mammal ages can be thought of as an alternate system for describing how old something is; the system is based on the observation that faunas of different ages are comprised of different species of mammals. Each land mammal age tends to be characterized by one or more species of mammals (or a combination of species of mammals) that are unique to that period of time. By examining a large number of faunas of different ages, an entire time scale can be erected based on the succession of mammal species. Each unit in the time scale is known as a land mammal age.
As an example, think about finding a deposit of fossil bones that includes an arm bone from a Columbian mammoth (Mammuthus columbi), the tooth from a sabertooth cat (Smilodon fatalis), and the skull of a dire wolf (Canis dirus). We know that these animals are extinct now, but that they were alive not too long ago. Without having to date these bones radiometrically, we can fairly confidently say that these belong to the Rancholabrean Land Mammal “Age” (about 250,000 to 11,000 years ago); this period of time is exemplified by the species of mammals that are preserved in the La Brea Tar Pits in California. Similarly, if we go out to the badlands of South Dakota and find the fossilized jaw of a big titanothere (Brontops sp., also known as a brontothere) we can be fairly confident it comes from the Chadronian Land Mammal “Age” (about 37 to 34 million years ago); this age is characterized by, among other things, the presence of some of the largest, most impressive species of these titanotheres.
The only problem with land mammal ages is that they aren’t applicable among all continents. Think about Australia, for instance. There isn’t one mammal native to Australia that is also native to North America, and it has been that way for tens of millions of years. Accordingly, there is no way to tell if two faunas from these continents are the same age based solely on the mammal species that are represented. Because of this geographic problem, a separate system of land mammal ages exists for each continent. The set of land mammal ages we’re most concerned with, of course, is the South American Land Mammal “Age” (SALMA) sequence.
In South America, the Cenozoic fossil record has been divided into approximately 20 of these land mammal “ages” (see figure at upper left). Most of these have traditionally been based on fossil localities in an area of southern South America known as Patagonia. The large latitudinal range of South America and the large proportion of it located in the equatorial tropics (about 70% of the continent’s area) have made integrating fossil localities from northern South America into the traditional SALMA sequence challenging; estimating time based on faunal similarity is difficult when different species of animals live in the northern and southern parts of a continent. This correlation problem has been compounded by a lack of radiometric or paleomagnetic dates (i.e., “absolute” dates) for most faunas and the presence of significant time gaps in the fossil record. Recent advances in radiometric dating techniques (e.g., laser fusion 40Ar/39Ar dating) and the discovery of important new Cenozoic fossil localities within datable rock layers (such as the Tinguiririca Fauna of Chile) have helped improve our understanding of the relationships among these SALMAs.
- Croft, D. A. 2016. Horned Armadillos and Rafting Monkeys: the Fascinating Fossil Mammals of South America. Indiana University Press, Bloomington, Indiana, 320 pp.