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Re: Venom in Sinornithosaurus



Venomous snakes replace fangs in a back-to-front manner, with new fangs rotating into position behind an old fang, and developing a connection to the venom duct, before the older fang drops out. In some cases, there is an "overlap" period in which both the old fangs and replacement fangs are functional, producing functionally four- fanged animals (albeit only transiently).

--Mike H.


Michael Habib
Assistant Professor of Biology
Chatham University
Woodland Road, Pittsburgh PA  15232
Buhl Hall, Room 226A
mhabib@chatham.edu
(443) 280-0181



On Dec 26, 2009, at 10:35 AM, Lee Hall wrote:

So how do you handle the issue of tooth replacement when you have venom glands?

Lee Hall
Paleontology Undergraduate
Museum of the Rockies
Montana State University
Bozeman, MT
lhall@montana.edu
http://sites.google.com/site/leehallpaleo/Home




On Fri, Dec 25, 2009 at 9:04 PM, Mike Habib <habib@jhmi.edu> wrote:
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