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I was asked by a list member to summarize the argument for rearing sauropods.
How can I refuse? This is based in part on my 1998 Modern Geology 23: 179
paper which I'm sure you are all familiar with, as well as some SVP abstracts.
There are two ways to rear. Bipedally, on the hindlegs, or tripodally, with
the tail acting as a third prop.
Elephants are fully capable of rearing without undue trouble both in
captivity and the wild even though they have no special adaptations for doing
lack of adaptations included the following -
No significant tail to act as a counterweight to the front of the body or act
as an additional support.
Elephants are front heavy, mainly because of lack of heavy tail, so forelimbs
bear most of the animal's mass (arm elements are therefore more robust than
corresponding leg bones, and forefoot is bigger than hind) making them forelimb
Arms are long, adding to front weight.
Posterior dorsals are smaller than anterior ones, a classic quadrupedal
In fact, posterior dorsals are quite small. This is because they rarely need
bear the load of the entire body while it is being held by only the hindlegs.
When elephants rear they flex the knees strongly. The pelvic muscles probably
cannot properly function when the hios are tilted up and I don't think
elephants can walk on two legs, but I'm not entirely sure because I cannot
recall what circus elephants are capable of.
All sauropods are better adapted for rearing than elephants. I mean all of
them, each and every one. Heres why -
All had large strong tails that could act as a counterweight. Even
brachiosaur tails were a heck of a lot bigger than elephant tails. This means
was always easy for a sauropod to tilt the body up, much easier than an
which can rear up despite lacking this major advantage. This was verified by
McNeil Alexander in the 80s.
Sauropod tails could be used as props. Some more than others. Large tails
with sled chevrons were the most suitable. Sled chevrons also underlie the
of ground sloths and kangaroos that use them as props. Diplodocid tails were
the best props in size and chevron morphology.
Largely because they had large tails all sauropods were hindlimb dominant in
that the legs bore most of the bodies mass, hindlimb elements being more
robust than the corresponding arm bones. This is true even in brachiosaurs, the
disparity varies with short armed diplodicids having the greatest disparity. In
fact most diplodocids (barosaurs being the exception) were so back heavy that
they should have been able to walk on the hindlegs alone with the hands just
clear of the ground, although they could not walk when fully reared up as
In some cases arms are short, further reducing front weight. Mainly
diplodocids, most sauropods had long forelimbs.
In sauropods the posterior dorsals are always much larger proportional to
mass than those of elephants, even when the sauropod is the same mass as an
elephant (plot showing this is in the Mod Geol paper). Compare a Shunosaurus
skeleton with that of an elephant. This means the sauropod dorsal column was
stronger, allowing the body to be held aloft for extended periods of time.
is considerable variation in this factor among sauropods, being least in
brachiosaurs, and taken to a fantastic extreme in apatosaurs in which the
dorsals and sacrals are colossal way beyond any basic structural needs during
normal locomotion or even rearing (so apatosaur dorso-sacrals dwarf those of
the much heavier brachiosaurs).
In many sauropods the dorsals decrease in size progressing forward from the
hips, a classic bipedal adaptation which means that the column is stressed to
regularly bear the entire load of the body anterior to the hips without the aid
of the arms. This feature is difficult to explain as other than evidence of
habitual rearing. This condition is highly variable, being most extreme in
diplodocids especially apatosaurs. Brachiosaurs are an exception, having
exceptionally large shoulder vertebrae.
A number of sauropods had retroverted pelves, in which the hip complex was
tilted posteriorly relative to the dorsal series. Jim (Dinosaur) Jensen first
noted this in Camarasaurus. Other types had it too although its sometimes
difficult to be entirely certain due to vageries of preservation. This
made sense when quadrupedal only in brachiosaurs in which the dorsals sloped
strongly upwards. Otherwise having the pelvis rotated posteriorly when
quadrupedal was a positive disadvantage because it was not optimal for walking
fours. When sauropods with retroverted hips were bipedal with the dorsal series
sloped up the pelves was still horizontal, allowing the animal to walk about.
Retroverted hipped sauropods could have used the tail as a ground clear
counterweight than as a prop. Derived therizinosaurs had a similar arrangement
of course they were fully bipedal.
All sauropods had to be able to rear. At least the boys did. To have sex. So
what the fuss about? Rear to have sex. Rear to eat. Not much difference when
you think about it.
Brachiosaurs were better adapted for rearing than elephants, and probably did
so more often than the latter Jurassic Park style (but I will never forgive
JP for making the brachiosaurs lumpy, Gumby leg cereal box toys rather than the
well proportioned, elegant beasts they were). But is makes sense that these
long armed forms had the least adaptations among sauropods for standing on the
hindlegs, and probably did it the least among the group.
Among most other sauropods the adaptations for rearing were markedly stronger
and it was probably a more regular habit, with many specialized for it. But,
being a diverse group, the adaptations for and mode of rearing differed
Those sauropods with retroverted pelves probably stood bipedally, and were
able to walk slowly as the fed, giving them a feeding mobility advantage. When
rearing with a functionally oriented pelvis, the legs should have remained
straight at the knees in the normal working posture. This includes camarasaurs
which were specialized for this feeding mode, a number of derived Chinese
examples probably as well, and brachiosaurs when they did rear up. When walking
bipedally the tail was held clear of the ground, to act as a counterweight.
standing still the tail could be used as a third support.
Sauropods with unretroverted pelvis were unable to walk when rearing because
the hip muscles were screwed up. Presumably like elephants the knees were
strongly flexed, and the tail was used as a prop to ease the leg load. The
armed, massive hipped, big tailed diplodocids with well developed sled chevrons
were specialized for this feeding mode; with the somewhat longer armed,
smaller tailed barosaurs being least so among the group, and least needing to
because the neck was so long and could reach so high on all fours. As noted in
earlier posts the diplodocids ability to ventro-extend the neck appears to have
been an adaptation for extending feeding range without having to shift
There is not really much need for biomechanical studies on this, because they
will not tell us much. It is obvious that if tailless, front heavy elephants
with weak posterior dorsals can rear heavy tailed sauropods with the center of
mass near the hips and stout posterior dorsals could do so more easily,
usually a lot more easily. We know sauropods had to rear up to have sex so it
not a question of basic ability in the first place. Sauropods of all sizes were
massively constructed and the notion they would have collapsed just becuase
they stood on two legs only is silly. The enormous size of the dorso-sacrals
compared to elephants confirms that. As I have often stated, detailed
biomechanical studies are not really feasible because it is not possible to
determine the mass distribution with air-sacs whose dimensions - at least those
not contained within bones - are not recorded in the fossils.
What those who oppose regularly rearing to feed - and perhaps fight predators
and each other - sauropods have never explained is why sauropods could not
rear to feed on a regular basis, and why so many of the land whales were so
adapted for rearing if they did not do so regularly. The burden is actually
on those who disagree with rearing sauropods.
As for those who say it is not possible to falsify the argument that
sauropods reared, the same is true of the argument that they could not rear. It
anatomical question based on the predominance of the morphological evidence.
Sauropods look like they should have been trophically diverse. Of course
brachiosaurs probably fed low rarely at best, and camarasaurs were too short
necked to readily feed at ground level. Short armed diplodocids look like they
could and should have readily feed low. But I have yet to see evidence of
extensive grit wear that should be present on ground cover grazer teeth. If
been published let me know. If not then work needs to be done. Why it has not
been is obscure, looks like an obvious project for a grad student. Fiorillo did
not find grit wear on diplodocid teeth, so maybe they did not feed near
ground level. I repeat that Kent and others who advocate grazing sauropods have
failed to address this serious problem.
Unless extensive grit wear compatible with grazing is found on sauropod teeth
it will have to be presumed they almost always fed high. If grit wear is
found then it will need to be shown that it was from dirt and not phytolithes.
nonfloral grit wear is present then it is possible that sauropod feeding
height depended on season and climatic trends. During the rainy season ground
would be inviting. When dry the higher the better. Because they have thick
cuticles and can produc deep tap roots many conifers are adapted for dry
conditions, just take a tour out west. During the dry season and especially
sauropods able to reach the crowns of conifers either on all fours with long
necks or rearing would have had a strong selective advantage over shorter
herbivores, both in nutrition and water.