Dental Plates
After shark teeth, the most common fossils on Brownie's Beach are fragments of ray dental plates. Instead of teeth, the jaws of rays are covered with bony plates, which are used to crush their prey. What you find on the beach are the individual "bricks" that were once fused into slabs of "pavement" that wrapped around the jaws.The rays at B.B. can be divided in to three kinds. The two most common types are called "Myliobatis" (Eagle Rays) and "Aetobatis" (Bonnet Rays). However, especially the "Myliobatis" plates include many different species, and only the lower (upper?) dental plates from Aetobatis are easily recognized as such. The uppers are bundled with the other Myliobatis. Both the Myliobatis genus and Aetobatis genus belong to the Myliobatidae family and the Myliobatiformes order. For more information check out Jim Bourdon's pages at Lee Creek on Guitarfishes, Skates and Rays, and their teeth and dentitions.
For now, we'll keep it simple. Dental plates from the Myliobatis are elongated and rectangular, with grooves on one or two sides (Fig.). Aetobatis' dental plates are V-shaped, with grooves on the tapering edges on the inside of the V. (Fig.). The side with the grooves is the root. The smooth side is the actual "tooth" used to crush prey.
Fragments of Myliobatis (left) and Aetobatis (right) dental plates. (both pictures same scale. Each picture about 12cm across). |
the third type of plates belongs to a species of Devil ray, probably Plinthicus. these dental plates are very flattened, and resemble broken edges of black shells. Actually, my wife was collecting these for a long time, but each time I sorted through our daily finds I discarded them as pieces of shell. Until I saw a clear illustration of a devil ray dental plate, and realized what I had been chucking in the trash can... since then we keep them. Of all ray plates at B.B., only 1 or 2 per cent are Devil ray, the rest is about 60% Myliobatis, 40% Aetobatis.
 Fragments of Devil ray dental plates (left, on 20% larger scale than pictures above) and two close-ups showing how thin these are as compared to Myliobatis and Aetobatis. |
Barbs
Rays have one or more very typical double saw-like "spikes" in, on or under their tails, and you can find fragments of these ray-barbs.
 Five fragments of ray barbs, measuring 16mm, 22mm, 18mm, 12mm and 40mm. |
Skin Scutes
Along their back and tail, some species of ray have bony plates under their skin, with very typical shiny scales. It took me forever to figure out what these were:
The three ray skin scutes measuring (fltr): 14mm, 26mm, 25mm. The thorn on the right scute stands 8mm high. Both others are relatively flat. |