From: pmb2@ellis.uchicago.edu (Peter Bell)
Subject: Re: vision in snakes, evolution of snakes
Keywords: This article printed on 100% recycled electrons
Reply-To: pmb2@midway.uchicago.edu
Organization: University of Chicago
Date: Sat, 9 Jan 1993 17:16:20 GMT

At the start of this thread, the question was more about discriminating light
colored prey from dark-colored prey, rather than about color vision per se...
However, in the reference given below, it is stated that few snakes have a 
fovea, which is an indicator of having evolved a region of great visual acuity.
the reference also states that all snakes lack oil droplets, which I think
is in contrast to what has been stated for Thamnophis--don't they have some 
sort of unusual oil droplet?

In article <1993Jan8.175159.16472@progress.com> gerard@progress.COM (Gerard Bras) writes:
>kyle@Csli.Stanford.EDU (Kyle Wohlmut) writes:
>
>>The Emerald Tree Boa is at least one example of multiple stenopeic
>>pupils.  Well, actually, most of them have two.  From observation I
>>would conclude that most colubrids have fixed pupils.  Which is rather
>>confusing considering that the colubrids evolved much later!  Could
>>their eyes de-evolve? (Although they probably prefer the term,
>>'Evolutionarily challenged')
well, it need not be that the eyes de-evolved in colubrids;  it is possible 
that the most recent common ancestor of boas and colubrids had fixed pupils;
after the boas diverged from that lineage, some groups evolved stenopeic 
pupils while the lineage giving rise to the colubridae did  not.  Remember that
I don't think it has been suggested that the colubrids arose from *within* the
Boidae, and that both lineages have been present on the planet and subject to
selection for equal lengths of time...  It is certainly possible, though, that
the common ancestor had substantially superior pupillary mobility to what 
most modern colubrids had--secondary reduction are known to occur in many 
characters...

>I think so, I've heard that the snakes evolved from subterranean lizards which
>had become legless and nearly sightless.  Upon resurfacing they re-evolved vision
>in some unique ways.
>
>Again, this is just from reading.  Anyone who knows for sure is invited to educate me.
>
>cheers,
>gerard

Hi, gerard--

No one knows for sure.  The fossil record for snakes is not particularly 
good.  Most of the speculations on snake evolution come from living forms;  in
the recent _Vertebrate Paleontology and Evolution_ (Robert Carroll, Freeman:
1988), there is an interesting section on the fossil history and possible
evolution of lizards and snakes on pp 224-238.  As far as I can tell from 
it, there are very few fossils available from the transitions in which snakes 
originated; in terms of remains convincing enough to use in figure (ie, showing
the major structures)  there is a good snake skull, and there is a very in-
teresting complete skeleton of an aquatic varanid with no pectoral and reduced
pelvic girdles.  There are also a number of distinctively snakelike vetebrae 
which have been located.  The index indicates about 80 known extinct taxa,
which are sometimes based only on vertebrae, and most of them date to after
the modern groups of snakes were all well-established.....

With that in mind, be aware that most of what is said about the origin of 
snakes amounts to a just-so story.  I thought Carroll's was well-enough told
to paraphrase a bit of it, though....

He starts by directing attention to the skulls of amphisbaenians, the truly 
burrowing lizards, which like snakes have reduced limbs.  Other than that, 
the amphisbaenians do not resemble snakes much at all.  In particular, teh 
skull is highly modified to use as a burrowing tool-- many of the bones are
heavy and fused, and most of the cranial mobilility known for snakes and
lizards is completely lost.  In addition, the eye rudiments are deeply 
buried within flesh, and the eye sockets are greatly reduced.  The fossil 
record for this group (due in part to teh heavy skull!) is pretty good, and
indicates that they arose from within the lizards, to which they are closer
than they are to snakes.

About the evolution of snakes from a burrowing ancestor, he says:

"Despite these arguments, there are several problems in accepting a burrowing
 stage in the origin of snakes.  The specializations that occurred in the
 skulls of burrowing lizards and amphisbaenids are very different than those
 that characterize early snakes such as Dinilysia.The bones of the skull are
tightly knit into a solid structure , in contrast with the extremely mobile
skull of most snakes.  Modern burrowing snakes, like burrowing lizards, have 
upper jaws that are strongly attached to the skull and tend to reduce the 
length of the lower jaws.  these genera appear to have specialized their jaw
structure from a more mobile antecedent condition, but the skull is otherwise
quite distinct from that of burrowing lizards.  The earliest known genera that
have been identified as snakes or intermediates between lizards and snakes
are not burrowers but were aquatic and have a highly mobile skull.  

  He goes on to point out that several of the characteristics of snakes that
have been attributed to burrowing may have a different basis. Teh loss of the
eardrum and middle ear cavity are seen in nonburrowing lizards and may be a 
consequence of teh articulation of the jaw in snakes.  Changes in eye structure
in snakes, he argues, parallel those seen in eyes during the origin of mammals
and are there attributed to nocturnal habits rather than burrowing.  Other 
changes, like the spectacle, might be attributed to life on the ground and 
movment in thick vegetation.

Most persuasive to Carroll, and to me, are the fact that the earliest snake
skull and one of the better "transition" fossils show a lot of aquatic 
characters, and an affinity to the enourmous varanid lizard family, rather than
teh extensive modification for digging seen in amphisbaenian fossils.

I know, though, that Carroll is not the only interpreter of these fossils--
hence the xpost to sci.bio, where if anyone knows the primary literature and
would like to weigh in with their two cents I'd like to hear it!

Peter
bell@minerva.cis.yale.edu