Re: feathers, hair and Compsognathus

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

At 09:07 AM 10/24/96 +0930, Jonathan Wagner wrote:

>>         The origional Class Reptilia (Linneaus) was paraphyletic, as it
>> includes early synapsids but not the mammals, and early Sauropsids, but not
>> birds.  As it stands now, Reptilia has been redefined so that it is
>> monophyletic, but excludes the "mammal-like reptiles" and includes the
>> birds.  For some perspective, see the Tree of Life homepages (sorry, URL not
>> handy, e-mail off-list me if you can't get it).
>
>      Does this mean that mammal like reptiles are NOT descended from
>animals with reptilian traits that got these reptilian traits from a
>same common ancestor that reptiles got thier reptilian traits from?
>Exactly why are synapsids excluded?

Because of a long-time association of "reptilian" = "any amniote unlucky
enough not to have evolved into a bird or mammal", I understand your
confusion.  However, this is not the definition of "Reptilia" which is used
by modern systemicists.

Under the system of phylogenetic taxonomy, taxa are defined by common
ancestry (which is recognized by the shared presence of derived features).
For example, Reptilia = all descendants of the most recent common ancestor
of turtles, lepidosaurs (tuataras and squamates), and archosaurs
(crocodilians and birds).  Given most of the recent work on amniote
phylogeny, mammals and their ancestors (together, the Synapsida) are well
outside this group.

It turns out that Reptilia is united by a large number of soft- and
hard-tissue features which are lacking in mammals (and other synapsids, so
far as we can tell), lissamphibians, lungfish, etc.  The chemistry of the
reptilian integument differs from those of other vertebrates, reptiles have
skins without lots of glands (slime glands in "fish" and lissamphbians,
sweat glands and pheremone glands and such in mammals), reptile eyes have
more types of color receptors than those of other vertebrates, reptiles are
capable of uricotely (excreting wastes in a whitish powdery mess rather
than, well..., you know...), and many others.

It is unlikely that the extinct synapsids (Edaphosaurus, Dimetrodon,
Moschops, Lystrosaurus, and the rest of the gang) possessed any of these
features.  The skins of these critters were probably not the wonderfully
scaled aglandular skins of lizards or turtles or birds, but were likely
loaded with various sorts of glands.  Basal synapsids, in fact, LACKED
reptilian traits.  Instead, they shared the traits common to all primitive
amniotes (laid eggs, had keratinous claws, etc.) and all primitive tetrapods
(were four-limbed, were ectothermic, etc.)  However, none of these traits
are "reptilian".

Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:th81@umail.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661

"There are some who call me...  Tim."
-- Tim the Enchanter, "Monty Python and the Holy Grail"
---------- subtitle --[Monty Python ik den Holy Grailen]

"Tim?!?  They called me TIM?!?!"
-- Tom the Paleontologist, on seeing "The Ultimate Guide to T. rex" :-)