Shedloads of Awesome, part 3: Dicraeosaurus
December 21, 2008
In view of all the awesome that is the Humboldt Museum’s gigantic brachiosaur mount, it’s too easy to overlook another nearly-complete Tendaguru sauropod, mounted in the very same hall, that is also worthy of respect and, yes, awe. Ladies and gentlemen, I give you: Dicraeosaurus hansemanni!
Dicraeosaurus is a member of Dicraeosauridae, the family that, together with Diplodocidae makes up the whip-tailed clade Flagellicaudata; which in turn, with rebbachisaurids and a few bits and pieces, makes up the great neosauropod clade Diplodocoidea. Dicraeosaurus was first named and briefly described by Janensch (1914:83); typically, Janensch went on to make full and detailed descriptions of its osteology, and also to describe the mounted skeleton (Janensch 1936).
It’s not really apparent from the photo above, but Dicraeosaurus is really small — like, embarrassingly small. Especially when it’s standing next to Brachiosaurus brancai. Gunga et al. (1999) estimated its mass at 12810 kg, but since that was the same paper that estimated B. b. at 74420 kg, based on a similarly grotesque baloon model, we can probably assume an accurate mass would be about one third of that, or 4000-5000 kg. Smaller than a big elephant. (I don’t know of any other published mass estimates for Dicraeosaurus; if I’ve missed any, please shout.) This is typical for dicraeosaurs: the South American Amargasaurus and Brachytrachelopan are even smaller.
Dicraeosaurs are interesting for several reasons. One is the dwarfism, and attendant shortening of the neck (which is taken to the extreme by Brachytrachelopan: reconstructed, that baby looks more like an ornithopod). But maybe most interesting is the peculiar construction of the vertebrae, which have very tall neural spines:
Check this out. The spines of C2-4 slope backwards; that of C5 is upright; from C6 onwards, they slope forwards. Very strange. Oh, and this is real: the verts are in good shape, and definitely not distorted.
One thing that Matt and I have been working on recently is the mechanics of sauropod necks, and particularly the attachment points of the epaxial ligaments and muscles. Among diplodocids and other sauropods with bifid neural spines, you occasionally find a nice clear ligament attachment knob in the floor of the trough between the metapophyses (i.e. the paired neural-spine halves) — but the Humboldt Dicraeosaurus mount is the first specimen I’ve ever seen that has such a knob at the base of every single cervical’s metapophyseal trough. See for yourself:
Unfortunately, as I was taking this last photo, and others like it, I came in a bit too close to the neck and touched one of the left cervical ribs (around C5). Aaaand off it came, to shatter on the hard flooring. It was a horrible moment … especially as I did it right in front of the curator, noted dicraeosaur lover Daniela Schwarz-Wings. With his usual impeccable tact, Matt took the opportunity of snapping a photo of me showing her the pieces, and trying to show how two of them fit together. Two more fragments lie on the floor at her feet:
Happily, the museum’s crack conservation unit swung into action immediately — I mean, literally within an hour — and I believe that rib is now back in place and as good as new. Frightening.
The tall, bifid neural spines of dicraeosaurs continue into the dorsal sequence, resulting in a “tall back” that carries through the sacrum and into the anterior part of the tail — as the posterolateral view below sort of shows. Just as the dicraeosaurid neck-shortening trend is taken to its extreme by Brachytrachelopan, so the elongation of neural spines reaches its apotheosis in Amargasaurus, which we must remember to show you one of these days.
Update (22 December)
David Hone, of Archosaur Musings fame, has sent me this photograph of the Dicraeosaurus mount in the process of being put together by the good people at RCI.
Janensch, Werner. 1914. Ubersicht uber der Wirbeltierfauna der Tendaguru-Schichten nebst einer kurzen Charakterisierung der neu aufgefuhrten Arten von Sauropoden. Archiv fur Biontologie, Berlin, III, 1 (1), pp. 81-110.
Janensch, W. 1936. Ein aufgestelltes Skelett von Dicraeosaurus hansemanni. Palaeontographica (suppl. 7), 299–308.