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Re: Shuvosaurus, 3rd impressions



David Peters (davidrpeters@earthlink.net) wrote:

<A quick look at Carroll 1988 showed me a dorsal view of the skull of
Dromiceomimus, an ostrich mimic dino. Here the lacrimal l sneaks in, like a
finger from the posterior!>

  Unfortunately for Carroll (1988), it only illustrates one dinosaurian skull
in dorsal view, and that is that of *"Ornithomimus" edmontonicus* which Dave
mentioned. However, skulls of earlier dinosaurs are known, such as
*Herrerasaurus*, *Coelophysis*, *Thecodontosaurus*, and *Eoraptor*, which shows
the presence of an (at times) small lachrymal component in dorsal view. This
exposure is expanded in theropods to supplant at times the dorsal aspect of the
prefrontal, which in some cases can be rendered to a splint removes from the
lateral margin of the orbit, or simply ... obliterated due to fusion with
lachrymal, making any division or comparison between the two of them
impossible. In many ornithischians, the prefrontal is the same as supraorbital
1, and often reverses the condition in dinosaurs and the lachrymal is exposed
only laterally due to expansion of the prefrontal. *Lesothosaurus*, for example
(Knoll, 2002, JVP 22(2):238-242), lacks a dorsally exposed lachrymal but this
region also shows erosion and the area even in Sereno's reconstruction in 1991
is not unambiguous; it is impossible to tell in thyreophores for the most part
to to cranial modification, and the same is true in pachycephalosaurus. And
while the lachyrmal is visible in dorsal view in many ornithischians, it is not
dorsally exposed on the skull roof, but this is referencing the figures in
Weishampel, Dodson and Osmólska (1990).

  The skull of *Herrerasaurus*, as an example of basal saurischian morphology,
shows a small portion of the lachrymal exposed dorsally rostral to the
prefrontal (Sereno & Novas, 1993, JVP 13(3):451-476), and they possess a
slightly interlocking articulation (scarf join). This alone should help define
theropods/saurischians in their natural condition. *Plateosaurus* and
*Thecodontosaurus* both preserve a rostral splint of a lachyrmal dorsally
exposed (Galton, 1990, in Weishampel et al.(eds.), 320-344; Yates, 2003, JSP
1(1):1-42).

  I hope this helps,
  Cheers,

Jaime A. Headden

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

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