It was thought that
Therizinosaurus reached 12 m -- is this outdated?
Well, since Therizinosaurus is only known
from forelimbs, ribs and pedal elements, it's length must be estimated using
other taxa. Using Alxasaurus as a guide, since it is the only segnosaur
known from a nearly complete skeleton, gives a length of 7.7 meters.
This is based on comparing humeral lengths (760 vs. 375 mm) of Therizinosaurus
(GI 100/15) with the larger individual of Alxasaurus (IVPP V 88402), which was
estimated to be 3.8 meters long by Russell and Dong. Comparing different
elements (ulna, metacarpal II) will give different lengths, but they all fall
around 8 meters (7.8 and 9.8 in this case). Of course,
Therizinosaurus could have had different proportions than Alxasaurus, but this
is currently unknown. In fact, the hypothetical reconstructions I've
seen give it a much shorter tail which would make it less than seven meters
long.
Hmmm... if it exists. Sereno puts it closer to
Ornithomimosauria all the time, and there are still people (like my 0.02 â
worth) going around who think segnosaurs are more probably
prosauropods...
Well, the studies of Holtz (1996, 1999, 2000),
Frankfurt and Chiappe (1999), Makovicky and Sues (1998), Sues (1997), Xu et
al. (1999) and my own all support an oviraptorosaur-segnosaur
group. Only Sereno (1997, 2000) and Dong and Currie (1994) have
performed analyses that dispute this. Sereno places them as the sister
group to ornithomimosaurs and alvarezsaurids (which I also disagree with),
while Dong and Currie place them as sister group to troodontids, then to
oviraptorosaurs. Other studies, such as those of Britt (using only
vertebral characters) and Elzanowski (using only skull characters), place
them in a group with troodontids, ornithomimids and oviraptorosaurs, but
aren't specific regarding relationships within that group. It has
become increasingly clear that segnosaurs are theropods, with the discovery
of Alxasaurus and especially Beipiaosaurus. I'm fairly certain even
George and Greg agree now.
As the extent of pneumatisation is heavily
influenced by ontogenetic age and the fossil record of (of? for? what's
correct?) segnosaurs isn't too good, couldn't we have found only subadults
of Therizinosauroidea?
Dong and Currie (1994) consider the large
specimen of Alxasaurus adult, based on the firmly closed sutures between the
neural arches and centra of dorsal and caudal vertebrae, and the fused
sacral centra and ribs. As for Nanshiungosaurus brevispinus (the only
other taxon that matters in this regard, as it has dorsal centra preserved),
I'm not sure if Dong has an opinion (as the paper's in Chinese), but the
pelvic elements are fused which may suggest the holotype is
adult.
Mickey
Mortimer