Karen Boswarva weighs up the science underpinning the dental weaponry of marine creatures
OCEANSCIENCE
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Karen Boswarva weighs up the science underpinning the dental weaponry of marine creatures
Fun fact – back before I first dived into the ocean or started my career as a marine scientist, I was a dental assistant. My days were spent looking into the mouths of anxious adults at two rows of pearly-white gnashers.
The anatomy of a tooth became common knowledge to me, a hard but porous white substance, called enamel, covering an inner layer of dentine that protects the nerves within the pulp cavity; each tooth then sits in a socket within the jawbones. Adult humans have evolved 36 teeth( give or take) made up of incisors, canines, premolars and molars, and as children we lose 20 milk teeth to the tooth-fairy. But that’ s our lot, and we work hard throughout our lives to keep those teeth in good shape.
When I finally hung up the saliva sucker, that interest in teeth moved with me to the underwater realm. You may have not considered it before, but a surprising number of marine animals have teeth. Less obvious species include sea urchins, sea slugs, and trigger fish! Sea urchins possess a set of strong calcium carbonate teeth within an organ called Aristotle’ s lantern, while the rasping tongue of a sea slug comprises many tiny sharp teeth made of chitin. Triggerfish teeth are made of dentine and enamel( just like ours) which can appear pointy or as plates for crunching and crushing coral reefs or hard-shelled molluscs.
One large group of marine animals well known for its teeth are the elasmobranchs – the cartilaginous group of fish containing sharks, skates, and rays. Elasmobranchs are literally all teeth! Not only exhibiting rows and rows of teeth that have evolved over hundreds of millions of years, their skin is composed of dentine and enamelrich tooth-like structures called denticles, which even contain a pulp cavity. In whale sharks, up to 3,000 of these denticles also cover their eyeballs, providing protection in
“ Shark teeth are adapted to each species’ specific diet and function – even the filter-feeders!”
the absence of eye lids- it’ s enough to make your eyes water!
Migrating back to the mouth, an elasmobranch’ s jaws contain a fine set of teeth, and far more than 36! Tooth anatomy consists of a crown and root with a varied composition. The teeth of mackerel sharks( great whites and makos or the extinct Megalodon) lack a pulp cavity and contain only osteodentine, a spongy material that resembles bone. While most other shark species have teeth comprised of an orthodentinefilled crown and an osteodentine-filled root. All shark teeth are covered in a hard enameloid( an enamellike tissue that differs in structure and chemistry from true enamel) and contain high concentrations of fluoroapatite. They are naturally programmed to fight tooth decay!
The morphology( shape) and arrangement of elasmobranch teeth is unique to each of the 1,226 species known today. Whether sharp and pointy, needle-like or triangular, small, flat, or plate like. Each is specially adapted to a species’ specific diet and function. Even the filter feeders( basking
PHOTOS: SIMON ROGERSON
The needle-sharp teeth of lemon sharks have evolved for hunting fish, while the coxcomb-shaped teeth of the tiger shark [ above ] are suited to a variety of prey, from turtles to dolphins sharks and whale sharks) have residual teeth that they likely use for mating.
Unlike us, elasmobranchs are renowned for their conveyor belt tooth production line. The teeth, which are loosely held in the connective tissue of the jaw, will continuously fall out and regenerate over weeks. The replacement process can even be a matter of days in some species, thanks to well-regulated stem cell production.
It’ s these high tooth losses accumulating in ocean sediments which forms a fossil record, providing great insight into the evolutionary history of the elasmobranchs and changes in environmental conditions over geological time. The colour of fossilised teeth varies due to the sediment type and the chemical composition of the mineralised tooth, which can also help determine the species.
Analysing a fossil’ s geochemistry by measuring the isotope composition provides information on an animal’ s habitat use and the ocean chemistry and sea temperature at the time. While the stratigraphical arrangement of unique fossilised teeth unlocks the taxonomic diversity of living and extinct lifeforms through time.
Who has not wanted to find a rare Megalodon fossil while on a dive or at the beach! If you didn’ t, I bet you do now. �
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