On Dec 25, 2009, at 4:45 PM, don ohmes wrote:
The picture I am getting, especially from Pinsdorf's post, is that
of a
large group of snakes that seems to range (among species) from mere
incipient venom production (minus any delivery system) to more
highly
evolved delivery systems and more complicated toxins. In other
words, each
representing various stages of a common process, perhaps
post-diversification morphological convergences overlying a common
ancestral
ability to produce a specific type of venom. Co-existing
"transitional
species" as it were.
That is, more or less, one of the primary interpretations in the
literature.
There is a competing hypothesis (and I don't have the time to
check the
citations right now, being Xmas day and all, but will pull the refs
later)
that solenoglyphs (front-moveable fangs) come from a proteroglyph
ancestry
(front fixed fangs), but that rear-fanged origins are all separate
- that
is, that those snakes with front fangs do not have a rear-fanged
ancestry.
There are a number of different versions of venom system evolution
maps,
all of which obviously depend on the phylogenetic hypothesis
preferred.
I suppose there could be a 'basal prey' that set the 'template', as
well...
Interesting thought. If so, and if rear-fanged systems are
actually basal
(very questionable), then that 'basal prey' type would be lizards -
most
opisthoglyphous snakes are lizard feeders in the wild (with quite a
number
of frog eaters in there, as well). Note that many of these will
eat rodents
in captivity, so zoo diets cannot be taken as representative.
Or not. But they do seem rather "unfinished" when compared to the
incredible specializations of rattlesnakes.
They do, don't they? And yet, there are many independent
derivations of
rear-fanged morphology in colubrids, without much sign of a trend
towards
any sort of proteroglyphy *except* in boomslangs, which have several
shortened elements in the rostral end of the skull (included a
shortened
anterior max and premax) which brings the rear fangs forward. They
also can
have as many as three functional fangs per side (well, that's the
record
that I've seen - there might be some 8-fanged boomslangs out there
for all I
know).
Heh. the devil makes me write this: are you absolutely sure the
anticoag
focus is really there? Because I am strictly taking your word for
it... }:D
Understood; I am quite confident of the anticoagulant trend, but not
certain. It is very well reported and known for boomslangs and
twig snakes,
which are highly studied relative to other rear-fanged snakes
because they
cause human fatalities. While the general anticoag focus in less
toxic
rear-fanged taxa is less certain, it has been repeatedly noted and
reported
in the literature, including by Harry Greene, whose work on snake
ecology I
tend to hold in high regard. South American racers (some of which
do pack a
pretty good punch) are repeatedly indicated in medical journals to
produce
heavy bleeding as the primary symptom.
Cheers,
--Mike Habib