How are different groups of mammals genetically related to one another? Where did each group originate and evolve? How did the various groups of mammals become distributed throughout different parts of the world? Such questions have been the subject of a great deal of investigation by both paleontologists and, more recently, molecular biologists. Paleontologists have an important perspective on these questions because they routinely deal with the only direct evidence we have for the history of Life, namely the fossil record.
The twenty major living lineages that we call the orders of mammals vary in their diversity, from a single living species (e.g., the aardvark, the lone member of the Tubulidentata) to more than a thousand (mice, rats, squirrels, etc., in the Rodentia). Hundreds more mammal species of all living orders, and of a number of extinct orders, are known from the fossil record. For those orders that have lots of living and fossil species, understanding their evolution is no small task.
The order Perissodactyla, the "odd-toed hoofed mammals," includes some familiar animals, namely horses and rhinoceroses. The only other living perissodactyls are the tapirs of Central and South America and Malaysia. Living perissodactyls are not very diverse, but fossil perissodactyls include as many as fourteen extinct families and dozens of genera, tracing back as much as 55 million years in time and spanning every continent except Australia and Antarctica. The abundance and diversity of fossil perissodactyls has made them attractive to paleontologists both as indicators of the age of the rocks that contain them and as models for examining evolution through time.
My research investigates the phylogeny of perissodactyls. I'm specifically interested in the relationships among the earliest members of the Perissodactyla, especially those that lived during the period known as the Eocene, between about 35 and 55 million years ago. Early perissodactyls are found at this time in North America, Europe, and Asia.
Phylogenetic analysis, or cladistics, is the method by which evolutionary relationships among organisms or groups of organisms (taxa) are determined. Which organisms are more closely related to each other than to other organisms? This is the basic question of phylogenetic analysis, and the answer is an evolutionary tree, or cladogram.
The evidence for a close relationship between two or more taxa comes from shared derived characters. A character is any aspect of an organism; in my work, they are usually features of the anatomy of the skeleton and of teeth, since these are the body parts that fossilize. Derived characters refer to those features that are more specialized, or that have evolved from an ancestral condition. For instance, the possession of hair is a derived feature among vertebrates; the fact that all living mammals share this feature in some form is evidence of the unique common ancestry of mammals.
The basic data for my work come from specimens of fossils held in numerous museum collections throughout the world. In the course of my work, I've visited numerous museums throughout the U.S.A., as well as in several European countries and China. Because fossils are rare, a paleontologist has to go to where the fossils are in order to study them, and that means a lot of travel to museums.
When I examine fossils, I have two goals. One is to check all of the characters that I've observed in other fossils and determine whether this fossil (or, more correctly, this fossil species) has the ancestral or derived condition for any given character. The second goal is to see if I can identify new characters to use in my analyses. Since I'll often want to re-check what I examined at the museum, I do a number of things to record what I observe at a museum. The most basic thing I do is take notes on each specimen. I also take high-resolution digital photographs, which are also useful when I publish my results. I even take video of specimens while giving myself a "lecture" on what I'm looking at, so I can effectively match my notes with the anatomy I saw. Finally, for teeth, I often make molds using a substance called polyvinyl siloxane, and back at my lab I make casts from the molds, so I can examine the shape of the teeth in three dimensions even after I've left the museum.
All of this information is then used to construct what is essentially a table of what conditions different perissodactyl species possess for different characters. I then use a computer algorithm to find the cladogram that fits the data best.