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Diapsids from synapsids? (was Archosaurian turtles?)



<Date: Sun, 30 May 1999 03:00:00 -0700 (PDT)
>From: Pharris Nicholas J <pharrinj@plu.edu>
>To: Dinosaur Mailing List <dinosaur@usc.edu>
>Subject: Re: ARCHOSAURIAN TURTLES?
>Message-ID: <Pine.OSF.4.10.9905300254280.2677-100000@sage.plu.edu>


>Larry Febo wrote:

>> Hmmm....I remember backing down from my theory that diapsids may have
>> developed from the synapsid condition (somewhat embarrassingly I might
add)
>> when I realized that what I was saying implied that turtles (as anapsid
>> representatives) should therefore be more closely related to humans than
>> birds! Now,...if turtles are actually lepidosaurian.........I may still
be
>> on the right track about a one time only development of the
"warm-blooded"
>> condition in terrestrial vertebrates.

>OK, so you're advocating relationships something like this for the extant
>tetrapods:

>----------------------amphibians
> \--------------------lizards & snakes
>  \          \--------turtles
>   \
>    \-----------------crocodiles
>     \----------------birds
>             \--------mammals

>???

>No offense, but it looks pretty unlikely to me...

>-Nick Pharris


I`m not sure what you`re indicating....(sorry, I can`t follow the shorthand
you are using).I`ll post the following excerpt of what I`ve written to
various other paleontologists of my opinion in order to reiterate my
original position on the subject:

      Here`s an excerpt from an earlier letter to various paleontologists...
To start, I firmly believe that endothermy developed only once in the
vertebrate line. It`s a very complex metabolic process, requires a lot of
input of energy and resources, and most likely required a long time to fully
develop. Once established in a given line of reptiles, it would prevent the
re-development of such a condition in other species, due to direct
competition. I think that its inception occurred sometime in the early
Pennsylvanian, before the synapsid-diapsid split. I`m thinking that perhaps
the diapsids evolved directly from the synapsid condition. Although there is
apparently no fossil evidence for this, I believe this is due merely to a
lack of fossils, and not the lack of the possibility.I know this is a poor
argument >for< my theory, but, I don`t think we should claim that both
evolved separately from the anapsid condition (which surely seems
ectothermic). Such a claim would lead us to believe that both diapsids and
synapsids have evolved endothermy separately. This conclusion I think to be
a misconception based upon the limited fossil evidence at hand. In fact, if
Petrolacosaurus was not discovered in its single location of upper
Pennsylvanian strata, wouldn`t the diapsid condition then be "postponed"
even further until the later Permian? Endothermy, as I see it, was developed
primarily as a means of incubating the young. The advantages of raising the
young in a colder environment (or micro-environ) as protection from
cold-blooded predators would be enormous. A warm-blooded metabolism could
impart many secondary advantages as well, allowing its possessor to
outperform an ectotherm on many levels. This endothermic condition, having
first evolved in the synapsid group, would have presented an environmentally
challenging block to an independent re-development of such a warm-blooded
condition at any later date. For a separate diapsid type to develop, as a
possible offshoot of synapsids, I believe they had to take to the trees, to
find a new and separate niche, and an even safer environ in which to raise
their young. This branching off of the diapsid line, of course, is not a
re-development of the endothermic condition f "from scratch". That condition
would already have been in place, at least to some degree, and from there
further modified to suit the specific new environment at hand. As for the
fact that many diapsids today are considered "cold-blooded", I can only
attribute this to their being secondarily so. It`s easy for evolution to
"backslide" if the energy input needed to maintain a certain state is rather
high. These forms probably found it easier to adapt to an environ that
offered only an occasional meal by slipping back into a cold-blooded
metabolism. They may have "outclassed" certain other ectothermic reptiles
and amphibians due to their previous modifications under an endothermic
condition (being,..on a sliding scale, somewhat more "warm-blooded" than
their competitors). As possible evidence of this secondary ectothermy I cite
the four chambered crocodilian heart, and even the partial separation of the
ventricle in lizards and snakes. (Why a partial separation at all, unless a
holdover from earlier times of a complete ventricular separation?). It`s in
the trees that I believe the diapsid line fully developed. First as
climbers, then gliders, then fully capable flying forms. It`s here that the
lighter skull and hollow bones developed, as well as a much enhanced version
of endothermy . Periodically, however, some of these arboreal forms came
back down to ground level, and evolved into larger land forms. Perhaps it
was the higher metabolic rate necessary for the development of true flying
forms that enabled these larger descendants to outclass the existing
mammal-like reptiles in the Triassic, perhaps in many cases leading to their
extinction.

So,...that`s it. Basically I can`t see endothermy developing twice. It seems
more parsimonious that it developed once (perhaps even started in the
anapsid condition), and was passed along as diapsids that developed from the
synapsid condition. Even the mathematics of the situation bears me out.
Dosen`t it seem more likely that the natural progression would be from
anapsid (no holes in back skull)... to synapsid (one pair)...and then to
diapsid (two pair)??