neural spine bifurcation in sauropod dinosaurs of the - PalArch

Wedel & Taylor, Neural Spine Bifurcation in Sauropods PalArch’s Journal of Vertebrate Palaeontology, 10(1) (2013)
Figure 9. Degree of neural spine bifurcation of presacral vertebrae in well-preserved Morrison Formation sauropod specimens
representing several taxonomic groups. In all taxa with deep bifurcations, these are concentrated around the cervico-dorsal
transition. ‘No data’ markers may mean that the vertebrae are not preserved (e.g., posterior dorsals of Suuwassea emilieae
ANS 21122), that the degree of bifurcation cannot be assessed (e.g., anterior cervicals of Diplodocus carnegii CM 84/94), or
that the serial positions of the vertebrae are uncertain so they contribute no information on serial changes in bifurcation
(e.g., the four cervical vertebrae known for Barosaurus lentus YPM 429). The Camarasaurus specimens are roughly in
ontogenetic order: C. lentus CM 11338 is a juvenile, C. grandis YPM 1905 and GMNH-PV 101/WPL 1995, and C. supremus
AMNH 5761 are adults, and C. lewisi BYU 9047 is geriatric. See text for sources of data.
The presacral vertebral formulae are assumed
to be as follows: 15 cervicals and 10 dorsals
in Apatosaurus and Diplodocus, 16 cervicals
and 9 dorsals in Barosaurus, and 12 cervicals
and 12 dorsals in Camarasaurus. In each group,
only the informative specimens are listed; for
example, the cervical vertebrae of Barosaurus
lentus YPM 429 are of uncertain serial position,
so this specimen is not included in the descriptions
of cervical vertebrae.
A note on preservation: when material is
broken or incomplete, it is easier to detect deep
bifurcations than shallow ones. The neural
spine tips are usually narrow, fragile, and easily
broken or lost. If a vertebra is missing the
top half of its spine but the bottom half is not
split, it is impossible to say whether the com-
plete vertebra was bifid or not. But if the spine
is deeply bifurcated, even a small piece of bone
from the base of the trough or one of the metapophyses
is enough to confirm that it was bifid.
Cervical Vertebrae
Apatosaurus louisae CM 3018 (Gilmore, 1936:
195 and plate 24) – Impossible to assess in C2-
C5, at least shallowly bifid in C6, deeply bifid in
C7-C15. According to Gilmore (1936: 195): “Unfortunately
the type of A. louisae lacks most of
the spine tops, only those of cervicals eight, ten
and twelve being complete; thus the point of
change from single to bifid spines cannot be determined
in this specimen.” However, Gilmore
(1936: plate 24) shows the base of the cleft preserved
in C6 and C7, indicating that those ver-
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