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Cladistics and biology: Long
>I was under the impression that not saying mammalian
>ancestors were reptiles was simply a matter of terminology, but that if
>you went back afr enough you could find what probably be called a reptile
>except that it was a mammal ancestor, so "technically" you couldn't CALL
That is where old, gradistic thinking got people screwed up. They pictured
a basal "reptile" something like Sphenodon, from which all other groups
evolved. Nope: Sphenodontida may be primitive within Lepidosauria, or even
Diapsida (compared to crocs, dinos, etc.), but are still not much like the
basal Reptilia or Amniota known from fossils.
If you found a living Dimetrodon, or Sphenacodon, or whatever, it turns out
that they would almost certainly NOT share all the features listed before
which are derived, reptilian features (uricotely, four-to-five color
receptors in the eye, etc.)
(Incidentally, it wasn't cladists who recognized this. Nick Hotton III,
former Curator of Fossil Reptiles and Amphibians at the Smithsonian, has
pointed this out in papers before, and Dr. Hotton is not what one would call
a rampaging cladist!)
> However, here you seem to be saying that the line that led to
>mammals came straight from amphibians (oops, excuse me. I meant basal
>tetrapods), independant of "reptiles", and that all the synapsid
>modifications for dry land, such as a skin that wouldn't dry out
>(presumably with SOME kind of scales) developed independantly from
>"reptiles" starting with those first sem-aquatic critters.
Actually, there is no strong evidence for scales in the sense of archosaurs
or turtles (and certainly not like lepidosaurs!), all previous pictures of
Dimetrodon not withstanding. Synapsids did (and still do) have vascular
skin, a primitive feature. Unlike reptiles, which have mostly avascular
skin, most vertebrates excrete various substances from glands in their
integument (muscus as a major excretion in "fish" and lissamphibians, sweat and
pheremones in mammals, someting in between in basal synapsids?)
Also, nocturnality is probably a primitive feature in tetrapods, while
diurnality is derived within Reptilia and many groups of large mammals.
What I (and Gauthier, and Hotton, and others) are saying is that reptiles
and mammals shared a common amniote ancestor which had the common features
of both (in particular, an amniotic egg) but lacked the derived features of
> What does cladistics have to say about, say, oue WAY earliest
>ancestors, the Monera (or whatever they are these days). Are we
>considered a part of this group because we are descended from Monerans?
Ugh. How many times do we have to do this? :-( I am really looking forward
to the day cladistics gets taught in high school!
Under phylogenetic taxonomy, paraphyletic taxa (such as "Protozoa") are
considered invalid. We are considered Life, the big clade which contains
"monerans" and Eukaryota.
>How about fish? I would think that unless those marine and freshwater
>critters that led to tetrapods was an example of amazing covergant
>evolution starting with those basal vertebrates, you would have to say
>we are descended from fish.
Yep. However, ichthyologists have realized the paraphyletic nature of
"Pisces" LONG ago (about the 1920s), and did not use that "class" name since
then. Instead, they have broken them up into Cyclostomes, Chondrichthyes,
and Osteichtyes (sp? on all). Only the latter is paraphyletic (if you
exclude Tetrapoda, which modern systemicisits do not do).
> Getting on to what this means for dinosaurs, I've heared people say
>that birds HAVE to be considered dinosaurs because they are descended
>from them (or something like that), as though no matter how innovative
>and unique you got, you would still be considered the same type of
>critter as your ancestors. However, it has always sounded to me
>that the whole labelling business (birds ARE dinosars, humans not from
>reptiles and fish) was more of a case of toMAEto toMAHto than something with
>real implications toward restoration of these extinct forms. The only way I
>that we couldn't be called reptiles or fish or bacteria if if those
>ancesters of ours were developed those reptilian or fishlike traits TOTALLY
>INDEPENDANT of true reptiles and fish and bacteria, and that resemblance
>to thise forms was simply an amazing example of covergent evolution
>(or independant development of life, in the case of bacteria).
> I am admittadly ignorant of cladistic terminology, so please clear
>this up for me.
Not only are you ignorant of cladistics (not your fault: as I noted, it is
still not taught to a broad audience), you suffer from the falsehoods
generated under gradistic thinking. There is no evidence that the ancestors
of mammals possesed the derived features common to turtles, lepidosaurs,
crocs, and birds (the latter four forming a clade outside of mammals).
There is no reason to speculate that dinosaurs drove mammals into the
darkness because of their superiority: nocturnality (or at least twilight
activit) is a primitive feature in tetrapods (found in primitive mammals,
lissamphibians, and the terrestrial outgroup, lungfish [coelocanths being
deep enought that "nocturnality" may not be a valid concept]).
Instead, cladistics forces us to look at the common similarities of
organisms as clues to their common ancestral features.
Hope this helps.
Thomas R. Holtz, Jr.
Dept. of Geology
University of Maryland
College Park, MD 20742