A while back, Matt speculated on the size of the allegedly giant mamenchisaurid Hudiesaurus.  At the time, all he had to go on was Glut’s (2000) reproduction of half of Dong (1997:fig. 3), and a scalebar whose length was given incorrectly.  The comments on that article gave some more measurements, but we never got around to showing you the figures of the vertebra in question, so here it is:

Hudiesaurus sinojapanorum IVPP V. 11120 holotype "first dorsal" vertebra, composite of Dong (1997:figs. 1, 3a)

The measurements of this vertebra given in the paper are:

• Height of vertebra 76
• Length of centrum 55
• Width of centrum anteriorly 42
• Width of centrum posteriorly 39
• Height of neural spine 41
• Width of top of neural spine 27

Dong said (p. 109) that “The mounted type skeleton of Mamenchisaurus hochuanensis, which was the largest sauropod known in China at the time it was described, measures 22 m in length (Young and Chao, 1972). The centrum of the first dorsal in the type of Hudiesaurus sinojapanorum is 1.5 times longer than the type of M. hochuanensis, leading to an estimated skeleton length of 30 m.”  This is odd because 1.5 times 22 m is 33 m, not 30 m.  But it’s also odd because Young and Zhao (1972:table 1) actually gave the centrum length of D1 as 250 mm — so the alleged 55 cm long centrum of the Hudiesaurus D1 is actually 2.2 times as long, which would yield a length estimate of *gulp* 2.2 x 22 m = 48.4 m.  Which would be right up in Amphicoelias fragillimus territory.  And that is clearly nonsense for a dorsal vertebra that’s only 76 cm high.

The height:length ratio of the Hudiesaurus vertebra is 76/55 = 1.38.  That of the first dorsal of M. hochuanensis is (using Young and Zhao’s measurements) 640/250 = 2.56 — nearly twice as tall.  In M. hoch., C19 (the last cervical) has a height ratio of 660/325 = 2.03 — better, but still not very close.  C18 has 660/400 = 1.65, C17 has 630/550 = 1.15.  So based on proportions, it looks like the Hudiesaurus “dorsal” is not a dorsal but a cervical — furthermore, not even the last dorsal, but the penultimate or antepenultimate cervical.  If it is homologous with C17 of M. hoch., then its 55 cm length is exactly the same as that M. hoch.’s C17, which would suggest that Hudiesaurus was the same length (22 m); if it’s homologous with C18. then its 55 cm length is 1.38 times greater than that of M. hoch., suggesting a total length of 30 m.  It’s interesting that this last figure is the very one proposed by Dong — could it be that after writing the paper, he reconsidered the serial position of the vertebra and recalculated the total length on the basis of an assignment as the penultimate cervical, while neglecting to update the text?    It seems a bit far-fetched, but maybe it’s a possibility.

Finally: is the vertebral morphology consistent with an identity as a cervical?  I think so.  The strong opisthocoely is a point in favour, as are the lateral fossae.  (The dorsals of Mamenchisaurus lack fossae and foramina.)  On the other hand, Dong claims that the vertebra has a hyposphene, which is unknown in cervicals — but I see no hyposphene in the photo above. Dong doesn’t really say why he thinks the vertebra is a dorsal, but my guess is that it may be due to lack of a fused cervical rib, and the assumption that there was a free dorsal rib associated with it.  But Young and Zhao (1972) say of M. hoch. that “The last cervical and first dorsal vertebrae are generally distinguished by rib morphology. However, a difficulty is posed here by the last cervical’s absence of articulated ribs.”  And my own observation of casts of this specimen show that actually the last few cervical ribs are not fused.

In conclusion, it seems to me that Hudiesaurus is probably based on a posterior cervical rather than an anterior dorsal.  BUT let me clear that I have never seen the material, and everything I’ve written here is based only on what’s in the literature.  I may well have made a dumb mistake, and no-one should take my thoughts on this too seriously.  If I was certain, I’d put it in a paper instead of on a blog.

References

• Dong, Z. 1997. A gigantic sauropod (Hudiesaurus sinojapanorum gen. et sp. nov.) from the Turpan Basin, China; pp. 102-110 in Dong, Z. (ed.), Sino-Japanese Silk Road Dinosaur Expedition. China Ocean Press, Beijing.
• Glut, D.F. 2000. Dinosaurs: The Encyclopedia, Supplement 1. McFarland & Company, Inc. Jefferson, North Carolina. 442 pp.
• Young, C.C., and Zhao, X.-J. 1972. Mamenchisaurus hochuanensis sp. nov. Institute of Vertebrate Paleontology and Paleoanthropology Monographs, A, 8:1-30.