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Continuing from
Andreas Christian & Wolf-Dieter Heinrich wolf-dieter.heinrich@rz.hu-berlin.de:
The neck posture of Brachiosaurus brancai, Mitteilungen
aus dem Museum für Naturkunde zu Berlin, Geowissenschaftenliche Reihe
1, 73 -- 80 (19 November 1998)
There's an impressive table of "Estimates of the cross-sectional
areas A and calculations of the compressive forces F acting on the
intervertebral discs at different colations along the neck of B[...]
for three different hypothetical postures. x: distance from the occipital
condyle." At x = 82 cm F is highest for the vertical and lowest for the mounted
posture, at x = 1.42 m it's highest for the horizontal and lowest for the
mounted posture, and at all other x the vertical posture is best and the
horizontal one worst. This effect increases strongly with x. "The accuracy
of these data depends on the precision of the estimates of segment masses
provided by Gunga et al. (1995). A systematic error in these data would not
affect our conclusions [...]. The pattern of compressive forces is very
different from the pattern of [...] [A] in both horizontal and mounted neck
posture. [...] Hoewever, even in this posture considerable deviations between
the curves of cross-sectional areas and compressive forces, respectively, occur
at the base of the neck. This might indicate that the middle fraction of the
neck was slightly inclined backwards [!] giving the neck a [very slightly]
S-like shape [...] as suggested by Paul (1988). Such a change of the neck
posture would barely affect the pattern of compressive forces in the cranial two
thirds of the neck but with the neck's center of mass being clocated more or
less directly above its base, bending moments and compressive forces would be
considerably reduced in this region." This fits the short, thin tail that was
probably unable to counterbalance the neck.
"The neural spines of the cervical vertebrae 9
to 13 are not preserved (see above) and might have been somewhat higher than
reconstructed. Furthermore, it cannot be excluded that some ligaments were
located above the spinal processes at the base of the neck where it was curved
backwards. Therefore, the lever arms of the epaxial muscles might be
underestimated at the base of the neck, starting at a[n x] [...] of about 6 m
[...]. Longer lever arms would lead to lower compressive forces in this
region making fit better the curves.
Discussion
[...] conclusively proves[...] that the neck of B[...]
was habitually kept in a nearly vertical posture [...] The reconstructions of
Bakker (1987) and Paul (1988) fit well to this result [...]. The conspicious
[sic] cervical ribs are not strong enough for transmitting high forces onto the
vertebral centra. Due to the short distance between the cervical ribs and the
intervertebral joints, the resulting torques about the intervertebral joints
cannot have been sufficiently high for supporting a more horizontal neck posture
as suggested by Frey & Martin (1997) [and the Oryctos paper]. However,
tensile forces transmitted by the cervical ribs might have been effective in
stabilizing the neck when it was occasionally slightly bent backward or in
braking backward movements of the neck."
This ironically fits the fact that the Oryctos paper says
that "engineering masts [...] must have at least three, symmetrically
disposed, members of similar sense (all tensile or all compressional" -- the
dorsal muscles, ligaments etc. plus the 2 sets of cervical ribs makes 3 tensile
members rather than 1 tensile and 2 compressional ones. BTW, wouldn't 1 medially
placed ventral compressional brace be more effective than 2 ventrolateral
ones?
"It should be pointed out[...] that the method used in this study
does not rely on absolute data on the mass distribution in the neck. Only marked
relative differences between the masses of different neck segments would
considerably change the calculated pattern of forces along the neck. Therefore,
the same results would have been obtained with an overall heavier or lighter
neck."
"It is very unlikely that the middle part of
the neck of B[...] frequently formed angles of more than about 15° to
20° with the vertical in any direction. B[...] might have been able to
hold the neck in a very inclined or even in a fully horizontal posture [assuming
the animal ever drunk], but regardless of how flexible the neck was (see e.g.
Paul (1988) vs. Martin (1987)) this could have been done only occasionally" or
"would be reflected the distribution of" A.
"The neck posture must have been nearly
vertical during most of the feeding time, [... t]herefore, B[...] could
not exploit a large volume of feeding space as it was suggested for the use of a
long neck by Martin (1987)"
Then they calculate said volume at about 31 m³, and think 20 -- 30
m³ or less are more probable for regular feeding. "The usual variation in
feeding height probably was just about 2 m." Intraspecific (as well as
interspecific) competition must have been very low.
"The results presented here cannot be applied to other sauropods
without specific examination. Body proportions differ considerably among
sauropods (McIntosh 1990, 1997), important morophological features of
B[...], like the relative forelimb length, are not typical of sauropods
(Dodson 1990), and the neck posture might have been very different in other
forms (see e.g. Borsuk-Bialynicka 1977) [don't ask me why they cite
Opisthocoelicaudia for neck
affairs...]."
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