Re: stegosaur origins

[Tom Holtz <tholtz@geochange.er.usgs.gov>]

>Can anyone suggest a common ancestor for Stegosauria.  Scelidosaurus (the
>ornithischian parts of the type specimen) is currently considered an
>ornithopod.

Since Sereno's hallmark 1986 paper on ornithischian phylogeny,
Scelidosaurus has been a basal thyreophoran, and not an ornithopod.

>So I'm looking for a more primitive form or something
>fabrosaur-like from the late Triassic or early Jurassic.

Bipedal Scuttelosaurus and quadrupedal Scelidosaurus are generally
considered the outgroups to the stegosaur-ankylosaur clade.

>Also, is it possible that there existed a row of keeled plates much
>shorter than the massive spine/plates running down the back between the
>large plates.  I think that the large plates were firmly attached and did
>not have as much motion capabilities as Bakker suggests. Although the
>alternating plate arrangement is effective for thermo-regulation, I'm
>just not 100% convinced.

Primitive stegosaurians have paired spikes.  The spikes become
progressively more plate-like, starting at the neck and working their way
back (Kentrosaurus is plates to the hip, spines after the hip).

Alternating plates are known only for Stegosaurus, although other advanced
forms (Wuherosaurus, maybe) may have also had alternating plates.

                                
Thomas R. Holtz, Jr.                                   
tholtz@geochange.er.usgs.gov
Vertebrate Paleontologist in Exile                  Phone:      703-648-5280
U.S. Geological Survey                                FAX:      703-648-5420
Branch of Paleontology & Stratigraphy
MS 970 National Center
Reston, VA  22092
U.S.A.