Re: Thagomizers and Proto-Thag

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

>How many thagomizeriferous stegosaurians were still around in the
>Maastrichtian?  Weishampel et al., edd., _The Dinosauria_, in chapter
>21 by Peter M. Galton on the Stegosauria has a table (21.1 on p. 450)
>of Stegosauria.  The only stegosaur of Late Cretaceous date is
>Dravidosaurus blanfordi from Tamil Nadu state, India, assigned to
>the Coniacian (88.5 to 87.5 million years ago).  On p. 452 Galton
>describes this as a small stegosaur from the Upper Cretaceous.
>("...the tail spike has a uniquely expanded middle region.")
>Elsewhere in the same volume, David B. Weishampel's chapter on
>Dinosaurian Distribution (ch. 3), on page 130, lists both the
>Dravidosaurus blanfordi and from elsewhere in Tamil Nadu state a
>Stegosaurid indet. of Maastrichtian date.

There are some serious doubts as to Dravidosaurus being a true
stegosaurian.  The illustrations of it published so far are not of a
particularly good quality, and do not show unequivocal stegosaurian
characters.  Some of Indian material discovered earlier and described by
Matley and von Huene was called "stegosaurian", but they were using the
term as it was typically used in the early part of this century, when
Stegosauria meant all armored dinosaurs (what we now call Thyreophora).  In
fact, they compare most of the Indian stegosaur fossils to North American
"stegosaurians" like Ankylosaurus, Euoplocephalus, and Panoplosaurus.

Also, there are questions regarding the number and position of tail spikes
in most stegosaurs.  Marsh's & Gilmore's versions of Stegosaurus have
strongly influenced perceptions about other stegosaurians, resulting in
(for example) four upraised spines shown in Tuojiangosaurus.  We now know
that, in Stegosaurus stenops itself, the four tail spines are oriented
horizontally and backward, not upraised.  It may be that more complete
skeletons of the Asian genus will show four (or probably more) horizontal
and backward pointing tail spikes.

>Were stegosaurians pretty rare in the last 20 million years or so
>of the Cretaceous (with uniquely expanded thagomizers or otherwise)?
>It's a long way from Tamil Nadu to Montana.  Was Thag's forebear
>pretty much safe from death by tail spike?

Stegosaurians are *extremely* rare in the Late Cretaceous.  There are
serious questions about Dravidosaurus being a stegosaurian, rather than an
ankylosaurian (for example).  The oldest unequivocal stegosaurians are
species of Wuherosaurus from the mid-Cretaceous (Aptian-Albian) of China.

So, Thag's forebears were safe from stegosaurian tail spikes.  But watch
out for ankylosaurid tail clubs and nodosaurid lateral spines..

                                
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.