5 - Enameloid Microstructure Diversity in Modern Shark Teeth

Abstract

Modern sharks (selachimorphs) are the sister group of the batomorphs, both forming the neoselachian clade (modern selachians). With around 500 species, modern sharks represent only a small percentage of the wide variety of “fish” (chondrichthyans and osteichthyans), but this group displays a great diversity in terms of morphological and ecological adaptations as well as reproductive strategies. Though the vast majority of them are found in the marine realm, sharks are present in all aquatic environments, from freshwater to the deepest ocean depths. The first proven selachimorph fossils date from the early Permian (296 mya) of Russia, with teeth belonging to Synechodontiformes, a group of sharks with uncertain affinities that vanished in the early Cenozoic (60 mya). Despite their ancient origins, it was not until the early Jurassic (201 mya) that the selachimorphs began to diversify and move toward the top of the oceanic food chain. The majority of selachimorph groups are high-level trophic predators, ranging from durophagous sharks to ambush predators and mega-predators. However, some species (basking sharks, whale sharks and megamouth sharks) have developed microphagous (filter-feeding) diets. A wide range of predation strategies found in sharks are also illustrated by their main predation tool: their teeth. No less than nine types of dentition have been defined to illustrate the different adaptive patterns seen in shark teeth, taking into account the morphology, arrangement and heterodonty of the various teeth of the lower and upper jaws. These diverse morpho-adaptive dental patterns make sharks a subject of study particularly helpful in understanding the evolution of enameloid microstructure in relation to adaptive and/or ecological stresses.