Evolution of Shark Denticles
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Somewhere on the order of 420 million years ago there lived a group of fish known as the Thelodonti (click here to see one). A literal translation of the Latin name means nipple (thelo) scale (dont) (Pough 1999). To the best of anyone's knowledge, the Thelodonts were the first vertebrate group to develop numerous, tiny denticles on their skin. Their denticle-covered bodies appear in the fossil record at least 20 million years before the stem elasmobranchs, the earliest "sharks." Some Thelodonts even had internal denticles. This was probably an extremely useful adaptation in a group existing before the evolution of jaws. These internal denticles are thought to have been used primarily for crushing prey. However, it has also been speculated that they could have played a role analogous to gill-rakers in extant filter feeders (Pough 1999). Because there are no extant Thelodonts it is impossible to directly trace the advent of shark dermal denticles to this clade. However, using the techniques of cladistics (what are cladistics?) scientists can be relatively certain that sharks owe their rough, hydrodynamic skin to the the Thelodonts (Pough 1999). |
Did dermal denticles give rise to the first teeth?
| It is commonly held that dermal denticles of agnathous fish were the precursor to teeth in vertebrate evolution (Pough 1999). Histological studies of shark embryos show that while dermal denticles and teeth appear to be somewhat differentiated in extant Galeomorphs they share hard-tissues (shiny layer, enameloid, dentine, and acellular bone) and are formed by homologous tissue (Reif 1980). However, Sire et al. (1998) called this assumption into question based on their study of Denticeps clupeoides, a Teleost fish. This unusual fish, belonging to its own family (Denticipitidae), has extensive dermal dentition covering most of the dermal bones in its head. The investigators determined that the dermal dentition of D. clupeoides must be a secondary adaptation, not derived from Thelodonts. Assuming that dermal denticles can be easily derived, there is no necessary connection between the evolution of teeth and dermal scales. |
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