Why Are Shark Teeth Everywhere in the Fossil Record? Thousands Per Animal

Sharks grow teeth in rows that advance forward as older teeth are shed, so a functional tooth is replaced within days or weeks and a single animal produces teeth continuously for its whole life, with lifetime totals running to tens of thousands in many species. Meanwhile the rest of the skeleton is cartilage, which is far less likely to fossilise than bone and which usually disintegrates, so a shark leaves teeth and little else. The teeth themselves are coated in a mineral harder and denser than ordinary bone, which resists both wear in life and destruction after death. The result is an enormous supply of durable objects entering the sediment record continuously across hundreds of millions of years.

The teeth carry more information than their abundance suggests:

• •Species identity in many cases, since tooth form is characteristic
• •Diet, since tearing, cutting, crushing and grasping teeth have distinct shapes
• •Position in the jaw, which varies systematically and lets a set be reconstructed
• •Body size, estimated from tooth dimensions using relationships measured in living species
• •Growth, since tooth size changes with age in ways that can be sampled across a population
• •Isotopes recording temperature and diet from the water the animal lived in

The best-known extinct shark is known almost entirely from teeth and the limits that imposes are worth stating. Teeth exceeding seventeen centimetres in slant height establish an enormous animal, and body length estimates derived by scaling from living great whites have ranged from around fifteen to over twenty metres, with the wide range reflecting genuine uncertainty about which living species is the right reference and about how proportions scale. A small number of associated vertebral columns exist and constrain the estimates further. Reconstructions of the whole animal are therefore extrapolations, and depictions of it as a scaled-up great white have been challenged, with recent work arguing for a different body shape based on the vertebrae and on comparison with a wider range of living sharks.

Teeth dominate the record and a few other structures survive in particular conditions. Vertebrae calcify in many species and preserve as discs with growth rings that can be counted, which is how ages and growth rates are estimated. Dermal denticles, the tooth-like scales covering the skin, are microscopic and abundant and are recovered by processing sediment, which gives a record of shark presence where no teeth are found. Fin spines preserve in the groups that have them. Exceptional deposits with unusual chemistry preserve whole animals as flattened outlines, including body shape, fins and occasionally stomach contents, and those few specimens do most of the work of establishing what extinct sharks actually looked like.

Collecting is unusually accessible, which is why these are frequently somebody's first fossil. Marine sediments of the right age outcrop widely, and teeth concentrate where erosion removes the finer material and leaves the dense fragments behind, including on beaches below eroding cliffs, in river gravels cutting through marine deposits, and in phosphate deposits worked commercially. Certain localities are famous and productive enough to support organised collecting. Identification is straightforward to genus in many cases using published guides. The usual cautions apply, since collecting from protected sites, from private land without permission and from unstable cliffs are each a problem, and a tooth without a recorded locality loses most of its scientific value while retaining its appeal.

Continuous replacement means one animal sheds tens of thousands of teeth, and the rest of a shark is cartilage that rarely survives, so teeth dominate the record. They record species, diet, jaw position and body size. The largest known extinct shark is reconstructed almost entirely from teeth, which is why its size estimates range from fifteen to over twenty metres.