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RE: rate of speciation
Thomas R. Holtz, Jr. wrote:
1) Nearly every popular account of shark history to the contrary, there has
been a HUGE amount of change in shark anatomy during their tenure in the
seas. There are very few sharks cruising the seas today that closely
resemble the sharks of the Paleozoic, or Triassic, or much of the Jurassic.
Indeed, the sharks most familiar to people (tigers, makos, great whites,
their ilk; and the rays and skates) are evolutionarily very recent (more
recent than mammals, for instance).
I'll second that! The shark-ray crown-group (Neoselachii) are relative
newcomers and first appear for certain in the Jurassic - although there are
tantalizing (but probably misleading) hints of certain neoselachian groups
in the Triassic (e.g., as gill-rakers that have an uncanny resemblence to
those of the modern basking shark; but this may be due to convergence.)
Unfortunately, many non-neoselachian elasmobranch groups are called "sharks"
in the popular literature because they show a vague and overall resemblence
to modern sharks.
Of living sharks, cow sharks and Port
Jackson sharks are found in the Early and Middle Jurassic; skates, rays,
open-sea predatory sharks show up in the Late Jurassic or Cretaceous.
The fossil record of sharks is both superb and lousy. Superb, because their
teeth are a dominant component of the marine fossil record; lousy, because
for the vast majority of shark species all we have is their teeth. Thus, it
is sometimes difficult to pin-point when exactly an individual lineage
arose, since the teeth are not always good indicator of the phylogenetic
affinities of their owners.
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