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Re: Dinosaur Web Pages' Re-Opening



In a message dated 97-03-09 03:16:14 EST, tkeese1@gl.umbc.edu (T. Mike
Keesey) writes:

<< So come take a look, already! >>

Your web site is beautifully done and lots of fun to mess around in. The dino
cladogram is the most up to date I've seen, though I think you've used a
number of as-yet-unpublished taxonomic names. Of course, if your web site is
the >only< place they appear, they're still unpublished(!), but this may
mislead some viewers.

Now let me air some of my beefs with the dinosaur cladogram itself--not your
problem, since you follow conventional wisdom. Call this THE FIVE GREAT
ERRORS OF DINOSAUR CLADISTICS.

(1) Saurischia doesn't exist, or if it does, it is congruent to Dinosauria
itself. All the characters that are listed by, say, Benton in _The
Dinosauria_ that supposedly unite Theropoda and Sauropodomorpha into a clade
Saurischia are either plesiomorphies, homoplasies, or doubtful.
Sauropodomorpha and Ornithischia share a more recent common ancestor than
either group shares with Theropoda; important characters uniting the two
groups into the clade Phytodinosauria include twin sternal plates, dermal
armor, a functional fifth pedal digit (retained in a small number of
ankylosaurians), and leaf-shaped herbivorous dentition.

(2) Thyreophora doesn't exist. The dermal armor and a few superficial cranial
similarities have led cladists astray into thinking that stegosaurs and
ankylosaurs share a more recent common ancestor than either group shares with
other ornithischians. Dermal armor is plesiomorphic for all of Ornithischia
(and indeed Phytodinosauria), and the pectoral, pelvic, limb, and vertebral
anatomies of stegosaurs and ankylosaurs could scarcely differ more.
Examination of pelvic and limb anatomy indicates that Stegosauria and
Cerapoda share a more recent common ancestor within Ornithischia than either
group shares with Ankylosauria. By the way, _Emausaurus_ is not a basal
thyreophoran but a basal stegosaur remarkably similar to _Huayangosaurus_ and
sharing about a dozen cranial characters with it.

(3) Heterodontosaurids are not ornithopods (as pointed out by Albert Santa
Luca in his description of _Heterodontosaurus_). At best they are basal to
the clade Marginocephalia, united with it by the presence of a dentary fang,
premaxillary-maxillary diastema, jugal with well-developed boss (which merges
with and helps to enlarge the jugal in marginocephalians), and three fanglike
premaxillary teeth. In ornithopods (other than heterodontosaurids) the
prepubic process is enormous and invariably extends forward of the
preacetabular process of the ilium, and there is an obturator process on the
ischium. Neither of these features appears in heterodontosaurids.

(4) Segnosaurs (= therizinosaurs) are not theropods. There is not one
theropod feature of segnosaurs that doesn't also appear in at least one other
group of phytodinosaurs, except for the so-called "semilunate" carpal,
tridactyl manus, and ascending process on the astragalus. In maniraptoran
theropods, the semilunate carpal evidently formed by fusion of the proximal
carpals (radiale and intermedium), whereas in segnosaurs it formed by the
fusion of distal carpals I and II, as seen in _Alxasaurus_, whose illustrated
carpal structure differs greatly from that of theropods and cannot be derived
from theirs. The forelimb and manus of segnosaurs are highly specialized, so
the fact that it the manus is tridactyl should not be used as a character
uniting segnosaurs with theropods. The ascending process on the astragalus
differs in detail from that found in theropods and probably developed
independently. The feet, jaws, teeth, pelvis, and limbs of segnosaurs all
derive much more readily from prosauropod-like sauropodomorphs than from
theropods and, as Greg Paul once wrote but alas seems to have since recanted,
segnosaurs do indeed represent forms derived from the
sauropodomorph-ornithischian transition.

(5) _Mononykus_ is not a bird, nor is it an alvarezsaurid. It's a perfectly
good arctometatarsalian or avimimiform theropod with a highly derived,
birdlike forelimb, pelvic girdle, and skull.

This has been just a short summary. More details--fully illustrated--will
appear when I finally finish the third edition of _Mesozoic Meanderings_ #2.
(Time is at a premium for me these days.)