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Re: Silesaurus



Dave Peters (davidrpeters@earthlink.net) wrote:

<The latest SVP cover story is about Silesaurus, an _early_ ornithischian.>

  Actually, Dzik offers extensive data to show that *Silesaurus* is not even a
dinosaur, as it appears to lack some adaptations that even *Pisanosaurus,*
*Herrerasaurus,* or *Saturnalia* possess. This includes the proximally-placed
deltopectoral crest and no cervical epiphyses; additionally, other features
appear to separate them, as *Silesaurus* possesses a large and proximally
positioned fourth trochanter, associated with splayed femora and elevated
femoral heads, lacks a "hooked" fourth trochanter separating it from
ornithischians, lacking a greater trochanter distinguished from, as well as a
rectangular or rounded femoral head (its triangular instead), the distal femora
lacks epiphyseal extensions of the tibial and fibular condyles, a shorter tibia
than femur, and absence of a distinct olecranon on the ulna. These are not all
important per se, but of the three options for relationships Dzik offered, being
a basal ornithischian is only one of these, and as it would appear that his
option 3 is congruent with the fossils, as Holtz notes onlist, *Silesaurus* is
no silly ornithischian (pardon the pun). Absence of a predentary and presence of
a region likely bearing a cornified "hook" or "beak" is possibly analogous, and
as there appear to be a lack of ornithischian features present in *Pisanosaurus*
and *Lesothosaurus,* versus basal sauropodomorphans or theropods including
*Eoraptor,* it seems this form is a rather non-dinosaurian (using the
Saurischia+Ornithischia definition) taxon where *Eoraptor* is considered a basal
saurischian/theropod.

<Could Saltopus be a sister taxon?>

  Based on age perhaps? In *Silesaurus,* the radius is nearly the length of the
tibia, whereas in  *Saltopus,* the radisu is less than 50% this. The ilium is
only around half the femoral length, though in *Saltopus* it is almost the same
length, over 80%. The femur is strongly bowed in the Scottish taxon, but but
less recurved in the Polish one, as in the tibia/fibula. Trunk vertebrae are
less elongated in *Silesaurus.*

  Finally, though Dzik states that *Silesaurus* has three sacrals (pg. 571) but
describes the sacrum as consisting of four (pg. 565); this is simialr to the
count apparently available in *Saltopus,* wich also bears a bowed
olecranon-bearing ulna. The forelimb anatomy of *Silesaurus* resembles the
reductionist anatomy seen in *Lewisuchus* and *Marasuchus,* which were likely
faculative quadrupeds that could run bipedally. These functional aspects appear
to be absent, as do features of the proximal femur and tarsus of *Saltopus,*
that would lead one to suggest relationships. Mickey Mortimer did a good job of
summing up his perceptions of *Saltopus* using the published plates
(http://www.cmnh.org/dinoarch/2000Sep/msg00051.html), which I used as well.

  Dzik, J. 2003. A beaked herbivorous archosaur with dinosaur affinities from
    the early Late Triassic of Poland. _Journal of Vertebrate Paleontology_
    23(3):556-574.

  Cheers,

  Jaime A. Headden

  Little steps are often the hardest to take. We are too used to making leaps in
the face of adversity, that a simple skip is so hard to do.  We should all learn
to walk soft, walk small, see the world around us rather than zoom by it.

  "Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)




David Peters
St. Louis