Subject: Tiger shark again
Date: Fri, 22 Nov 1996 02:44:29 -0800
From: Bill Heim
Reply-To: Sharks and Cartilaginous Fish Discussion

References:DJCripe@AOL.COM wrote: "I would be interested in any references that present the evolutionary evidence of the lineage of the tiger shark (or other sharks). What is the criteria for determining that one species of shark was the precursor to another species?.

There is very little published on shark evolution except in general terms, i.e. one of the earliest sharks was Cladeoselche in the Devonian, etc. Lineages represent what I have worked out from collecting in primarily East Coast deposits. For example: In middle Eocene deposits (Potsapansa formation) of Virginia and South Carolina (Santee limestone) you find a small tiger shark which is sometimes incorrectly called Physolgaleous secundus but in actuallity it is a smaller version of the Miocene shark Galeocerdo contortus and possibly should have an unique species name. The teeth are the same as the Miocene G. contortus except for size. The Eocene shark teeth are about 1/2 size of the Miocene ones. The shark is then again found in early and middle Oligocene deposits (River Bend and Belgrade formations) of North and South Carolina (Cooper Marl) and late Oligocene deposits in South Carolina (Chandler Bridge formation) but are slightly larger. In the middle Miocene of Maryland, Virginia (Calvert and Choptank) and North Carolina (Pungo River) they are very common and fairly large (15-20mm). In the late Miocene of Baja (Rosarito Beach formation) you find very large specimens (20mm+) but they are very rare in the Late Miocene of Florida (Hawthorne). By the early Pliocene (Yorktown formation - Virginia & North Carolina) they are completely extinct.

Thus it is possible to trace the evolutionary history of one type of shark. Unfortunately, it also becomes hard to draw delinating lines as the teeth usually show complete gradiations from one species into another. It is often arbitrary as in the example above one could argue that since the teeth are the same across the time frames except for size they should be one species. Another person could argue that since the Eocene and early Oligocene teeth are usually much smaller than the Miocene ones, then it should be a different, ancestor species. But what about Oligocene teeth which can cover both size ranges?