Just a quick post, with a very pretty picture. Here is a fossil tooth of the "extinct" Bigtooth Mako shark, aka Isurus hastalis. This shark is the ancestor of the Great White Shark, Carcharodon carcharias. (Since Isurus hastalis and Carcharodon carcharias are simply Miocene and extant end members of an anagenetic lineage, Isurus hastalis didn't exactly go extinct, since it is more or less just a morphotype as opposed to a distinct species).
Later on I'll make up a post completely on the Isurus-Carcharodon transition in the Purisima Formation; there is now a fairly good sample size of teeth from the Isurus-Carcharodon lineage from the Purisima, that demonstrate the appearance of serrations and the increase in serration size through time. Oddly enough, you can see a bit of a little depression that the tooth sits in in the rock surface; that is the remains of a hole I dug for a partial fish skull exactly one year prior (spring break 07; this was 08).
This is one of the youngest occurrences of Isurus hastalis. Needless to say, I was pretty damn happy when I found this. Its a damn pretty tooth, and I'll be donating it to the SCMNH this winter break (If I haven't already - I can't recall).