Bizarre beach 'comb'

August 2009

Bizarre beach 'comb'

We found this bizarre-looking object poking out of the sand in Redland Bay during a recent early morning beachcomb. Is it a tooth from a gigantic marine creature or a weathered tree branch?

Answer

The mysterious object is part of the underside or plastron of a sea turtle skeleton. The underside of sea turtles is protected by the plastron, a bony scaffold to which the shell is attached. This particular bone is called the hypoplastron. This bone is one of four similar ones, fused in pairs, which together form the plastron.

Interestingly, the plastron incorporates special bones called belly ribs, or gastralia. Belly ribs are found in the skeletons of many reptiles, including dinosaurs. In other reptiles, belly ribs reinforce the underside of the animal, acting as a girdle to hold in the guts. In turtles they have been fused and expanded to help form its armour.

The species of turtle can be determined from just this one bone. This bone is from a Green Sea Turtle Chelonia mydas which would have been approximately 30-50 years old when it died.

Turtle underbelly bones Four bony plates are fused together to form the 'plastron' or underbelly Green Sea Turtle A nesting Green Sea Turtle (Chelonia mydas)

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