Re: Venom in Sinornithosaurus

[Mike Habib <habib@jhmi.edu>]

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