Evolution of a Super Predator
Sharks are one of evolution's most enduring success stories. Although they have few hard parts that can survive the insults of geologic time, sharks have left a long and rich fossil record. Some 2,000 to 3,000 species of fossil shark have been described. In contrast, the total cast of the dinosaur dynasty comprises only about 650 to 800 species. The ancestry of sharks dates back more than 200 million years before the earliest known dinosaur. The dinosaurs are long gone now, canceled despite enthusiastic ratings (especially among the 8-year-old demographic). But we still have sharks - about 1,100 species of them. To understand the sharks as they are today, it is helpful to understand how their ancestors were in the distant past - where they came from, so to speak. The ancestry of modern sharks is an epic tale, as full of mystery and intrigue as any first date.
Fossil Shark Teeth
A 'fossil' is any remains, trace, or impression of ancient life, preserved in rock. Fossils include not only mineralized bones and shells, but also casts of footprints, skin, and even coprolites (fossilized dung). Unfortunately for paleontologists, the conditions that favor fossilization are very stringent. Fossilization requires both insulation from the destructive effects of microbes and a supply of soluble minerals to replace organic tissue. If these conditions are met, once-living tissue may be replaced, atom by atom, to create a cast in stone of the original structure. However, because these conditions are rarely met, most organic material simply dissolves and thus the fossil record is vexingly incomplete. Many clues are not preserved at all, and others only under the most extraordinary of circumstances. Most clues do not survive the insults of geologic time. Or they are so poorly preserved that virtually every feature that may be used to trace ancestry is lost.
Shark teeth provide paleontologists an unusual suite of advantages and drawbacks. Collectively, sharks' teeth are among the most abundant of vertebrate fossils. They are mineralogically stable and are often preserved under circumstances that do not ordinarily favor fossilization. The structure of shark tooth blades consists of a matrix of minerals (hydroxyapetite) embedded in a protein (collagen). The protein portion of shark teeth is most commonly replaced with phosphates, often preserving fine details useful for identification and tracing ancestry.
If a fossil tooth is well-preserved, it affords numerous characters that can be used to identify the species (or at least the basic taxonomic group, such as genus, family, or order) to which it belongs. But, just as glass may be worn smooth by wave and sand, fossil shark teeth may have taxonomically useful features (such as serrations or secondary cusplets) eroded away by geological activity. Another complication stems from the fact that, unlike glass, fossil shark teeth were once part of living creatures. From studies of modern sharks, we know that the teeth of a given species can vary with such factors as position in the jaw, growth stage, sex, reproductive state, geographic population, individual peculiarities, and malformities. It seems very likely that ancient sharks also displayed such dental variability.
No matter how well preserved, individual teeth cannot display the full range of variability in a given shark species. Very few collections of fossil shark teeth include a 'natural set' - that is, a relatively complete assemblage of teeth from a single individual.
Most fossil shark teeth are found as isolated specimens. From these, 'artificial sets' can be constructed, but the practice is a less-than-exact science, relying on subjective decisions based on many years' experience examining recent and fossil shark teeth.
Because the rate of evolutionary change in sharks is very slow and gradual, it can be frustratingly difficult to determine where one species stops and another begins. There is no evidence of punctuated equilibrium (sudden 'jumps' in form) in the shark fossil record. Without sharp discontinuities, boundaries between named species are often made rather arbitrarily along a continuum of variation. Thus distinctions among some fossil sharks may best be considered 'chronomorphs' (forms within an evolving lineage) rather than biologically discrete species.
Given all these difficulties, it is very much to the credit of paleontologists that we know as much about the fossil history of sharks as we do.