[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]
Re: thanks and a note about _Alxasaurus_
<Opisthopubic pelvis in segnosaurs has nothing to do with opisthopubic
pelves in theropods; it's derived from an intermediate condition
between the propubic pelvis in prosauropods and opisthopubic pelvis in
ornithischians. Once it is understood that segnosaurs are >not<
theropods but are derived from lineage linking prosauropods with
ornithischians, the mystery surrounding their opisthopubic pelves (and
prosauropod-like feet) evaporates. If you imagine that segnosaur
opisthopubic condition is homologous with, say, opisthopubic condition
in velociraptorines and birds, you must then also explain why
segnosaurs share so few other features with these taxa.>
Okay, I am operating on the assumption that segnosaurs are
theropods. Indeed, very strong characters are similar between the
segnosaurs, and the group Phytodinosauria: pes, jaw, pelvis, profile
of skull in *Erlikosaurus*, quadrate articulation, and some others I
The pes, however, is hardly unique to prosauropods, as the toes and
claws and even the distal articular surfaces of the metatarsals are
all distinctly autapomorphic and exist in no other dinosaurian taxa.
In fact, primitive prosauropods do not exhibit this type of pes, only
the melanorosaurids and plateosaurids. This would suggest that the pes
of segnosaurs, were they phytodinosaurs, are convergent to
prosauropods, or that segnosaurs are advanced prosauropods. Anterior
(flexor) expression of the proximal end metatarsal V in both groups is
again blown in that primitive prosauropods and *Alxasaurus* (which
preserves this bone) both possess proximally _obscured_ mt V's.
The pelvis hardly asserts a relationship with ornithschians, becuase
the the obturator prong in the segnosaurs occludes the pubis, which it
does not in any other group except birds. Okay, okay, this is hardly
creditable evidence for relationship, and I'm not advocating such
relationship, even though that clade came out that way on the last
post I made. The form of the ilium is nearly identical to
caenagnathids, *Caudipteryx*, and *Rinchenia* except for the lateral
deflection of the anterior blade in which case it resembles some
sauropods, pachyderms, and giant ground sloths. An apparent feeding
adaptation, if you ask me.
The mandible is similar in form to platerosaurs, but not in
structure, in which case it is closer to ornithomimosaurs, such as the
articular region. The teeth are similar to those of troodontids and
hypsilophodontians, but lack of strong lateral compression makes it
more similar to the former than the latter.
The manus is tridactyl in *Therizinosaurus*, unlike any
phytodinosaur, but the "semilunate" in *Therizinosaurus* and
*Alxasaurus* are composed of apparently two elements, one lateral the
other medial. They function identically to the form of maniraptorans,
and the ulna and radius perform the same rotations. The form of the
humerus in both anterior and lateral aspects are identical in both
oviraptorosaurs and segnosaurs, but arguably also in *Plateosaurus*
(structure of the deltopectoral crest differ, more similar in the
first two than either to the latter).
*Segnosaurus* and *Nanshiungosaurus* have six sacral vertebrae, only
advanced genusaurians (Thyreophora + Cerapoda) reach this number.
Overall, segnosaurs possess more characters relating to the
theropods (and, in fact, the Maniraptoriformes and Maniraptora) than
to phytodinosaurs. While I will not argue with an analysis that places
segnosaurs near phytodinosaurs, as in Paul, 1986, I do note that
convergence does occur; it only goes so far before it is more than
likely the animal _is_ what it resembles.
Ichtyosaurs are still lizards.
- Often, it is the man who is brought
down the path to the end who does
not see his own steps. -
Jaime A. Headden
Qilong, the website, at:
DO YOU YAHOO!?
Get your free @yahoo.com address at http://mail.yahoo.com