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?