Question of the month

Armour plate

What's this bone that we found on the beach?

Answer

An isolated pleural bone from a marine turtleAn isolated pleural bone from a marine turtle

Skeleton of a marine turtle, viewed from above. One of the pleural bones is indicated in blueSkeleton of a marine turtle, viewed from above. One of the pleural bones is indicated in blue.

The specimen is a 'pleural bone', which is one of the bones that makes up a turtle's shell. All turtles have a unique set of bones that form the hard structure of their shells, and in most species this is also covered by large scales (an exception is the Leatherback Turtle). The midline of the shell is made up of neural bones. The flanks are composed of pleural bones. The rim of the shell is made of peripheral bones, with a single proneural bone (also called a nuchal bone) immediately above the turtle’s neck, and a pygal bone above the tail. These various bones fuse to adjoining bones at zig-zagging suture lines, similar to the sutures that join the bones in our skulls. The internal skeleton of the turtle is fused to the bones in the shell; pleural bones are fused to ribs, and in many cases a rib shaft is clearly visible projecting from the pleural bone. However, in the case of the specimen in the photo, the extremity of the rib has snapped and is missing. This suggests that the specimen is from a Green Turtle (Chelonia mydas), as this species has a long rib shaft that is prone to breaking.

Seven species of marine turtles are found worldwide, and six of these are found in Queensland coastal waters. Marine turtles are famously long-lived, and will not breed until they are more than 30-50 years old. Marine turtles are threatened by plastic pollution, entanglement in fishing tackle and shark nets, and boat strikes. This battery of threats, coupled with their ‘slow-lane’ life histories, means that populations of marine turtles have declined drastically around the world.

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