More reptile stuff

["Thomas R. Holtz, Jr." <th81@umail.umd.edu>]

Dwight Stewart writes:
>       Basically, we are taught that mammals are "superior" to reptiles.
>Usually birds are placed
>       in "2nd place", with reptiles and amphibians ranked only above
>insects, IF that.

I REALLY hope people aren't still being taught that, especially in schools.
Value judgements of "superiority" and "inferiority" hearken back to the
pre-Darwinian idea of a Scale (Ladder) of Nature.

>The facts simply don't bare that out; it's a kind of
>prejudice (IMHO!).  Reptiles are better suited for certain niches.

Damn straight!!  Depending on how you define "superiority", even amphibians
can be considered superior to mammals (at least in terms of species numbers:
some studies, mentioned in a recent issue of _Herpetological Review_,
suggest that amphbians, like squamates and birds, might outnumber mammals).

>The fact that dinosaurs
>       don't fit the model of modern reptiles, doesn't mean that dinosaurs
>WEREN'T reptiles.

Therein lies the interesting sociological thing.  Here we have lots of
people on the net who want to stay as current as possible with regards to
new fossils, new paleobiological interpretations, etc., but still clinging
to old-fashioned ideas of taxonomy.  Taxa (groups of organisms) aren't
"models": they are historical entities, groups of common descent.

Reptiles aren't "creepy crawlies" anymore (even if that is the derivation of
the name).  They represent a particular branch of the tree of life.

You can check numerous sites (including the dinosaur listserve archive or
the U Cal Berkeley Museum of Paleontology (http://www.ucmp1.berkeley.edu))
for more info.

>          Correct me if I'm wrong (really asking for it here!), but aren't
>both birds and mammals evolved
>       from reptiles? 

You're wrong, at least under modern vertebrate systematics.  To be terribly
unfashionable and quote myself:
"The category Reptilia is now considered a node-based taxon: the most recent
common ancestor of turtles, lepidosaurs (lizards [including snakes] and the
tuatara), and archosaurs (crocodiles and birds and their extinct relatives).
Thus Aves (the birds) is part of the larger monophyletic Reptilia.  Mammals,
however, are not part of this clade, because our ancestors diverged from the
common ancestor of all reptiles (as now defined) before the
turtle-lepidosaur-archosaur divergence.  Thus under cladistics, the
ancestors of mammals by definition were not reptiles (i.e., were not part of
the clade Reptilia), while birds' ancestors and birds themselves are true
reptiles (i.e., members of the clade Reptilia)."  p. 105 in Farlow &
Brett-Surman's The Complete Dinosaur.

Following Peter Bucholz's response (which contained much of the same info I
provided), Dwight asks:
>          I see what you mean.  But, at some more distant point, mammals &
>reptiles would share a common ancestor.

Yes.  Together, mammals and their ancestors (Synapsida) and reptiles and
their ancestors (Sauropsida) comprise the group Amniota.  (None of these
terms are new to cladistics: they are many, many decades old).

>       Which brings up a point about so-called "mammal-like" reptiles like
>Dimetrodon.  Is that term still applicable?

Not really, since under modern systematics, they aren't reptiles.  Some
prefer the term "protomammals", but this has its own teleological problems
(that is, these animals weren't part of a "mammal factory": they were their
own diverse clade, of which only the line containing mammals survives).
Basal synapsids, or "pelycosaurs", might do the trick.

>       If so, were these animals an "evolutionary dead-end"?

In as far as we are here, the basal synapsids per se aren't an evolutionary
dead-end.  Of course, the huge numbers of individual species within
Synapsida were evolutionary dead ends (i.e., haven't left living
descendants), but that's true of every single group of animal.

>I frankly wonder if non-avian dinosaurs had not become extinct, if we would
even be > here to discuss this?  :-)

Greg Paul, in his review of Dougal Dixon's _The New Dinosaurs_, pointed out
that as primate ancestors were already around by the latest Cretaceous, and
as Mesozoic dinosaurs never seemed to compete directly with these arboreal
herbivores/omnivores, there doesn't seem to be any evidence for competative
exclusion of Primates by Dinosauria.  He suggested that human-like forms
might well have been possible in a world dominated by dinosaurs, although
large herd mammals, big carnivorous mammals, and the like might not have
appeared (since dinos were already doing those things).

For an interesting update to this, the evidence of latest Cretaceous parrots
does suggest that some Mesozoic dinosaurs were arboreal
herbivores/frugivores.  However, parrots and arboreal primates do coexist in
South America, Africa, and tropical Asia.  (And, of course, there still
remains the question of whether competative exclusion is a real phenomenon
or not, but that's a question for another time).

Hope this helps.

Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:tholtz@geol.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661