sauropod rearing

[GSP1954@aol.com]

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 
so. The 
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 
dominant. 

Arms are long, adding to front weight.

Posterior dorsals are smaller than anterior ones, a classic quadrupedal 
adaptation. 

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 
entirely 
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 
that is 
was always easy for a sauropod to tilt the body up, much easier than an 
elephant 
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 
tails 
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 
explained below. 

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 
far 
stronger, allowing the body to be held aloft for extended periods of time. 
There 
is considerable variation in this factor among sauropods, being least in 
brachiosaurs, and taken to a fantastic extreme in apatosaurs in which the 
posterior 
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 
arrangement 
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 
on all 
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 
and 
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 
widely. 

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. 
When 
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 
short 
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 
position. 

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 
is 
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 
accurately 
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 
well 
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 
is an 
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 
this has 
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. 
If 
nonfloral grit wear is present then it is possible that sauropod feeding 
height depended on season and climatic trends. During the rainy season ground 
cover 
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 
droughts 
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. 

G Paul