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Re: "Cursoriality", etc.
John H. seems to be supporting the common view that it is inherently so
difficult for very large animals to run that they may not be able to do so.
There are reasons to think otherwise.
In some ways is it actually easier to maintain a high speed as size
increases. Elastic similarity predicts that speed should increase with
increasing size, primarily because stride length increases. This is why a
squirrel must gallop, while a cat must only trot, while a human need just
walk, to move at about 3-4 mph. A galloping rhino needs a much shorter
ballistic suspended phase to run at the same speed (perhaps top 25 mph) as a
galloping dog. (That the speed of the trot-gallop transition increases with
size is an *advantage* of large size). Therefore giants do not have to run as
"hard" to move at a given speed as a smaller animal. Also, energy efficiency
of locomotion increases with increasing size, so giants do not have to work
as hard to run at a given speed.
It is true that among modern animals speed peaks at medium sizes, with
animals over about 3 tonnes unable to run (top elephant speed is ~12 mph,
claims of higher speeds are false). It is therefore reasonable to argue that
this is a universal truth. But is science we must not assume that this means
giants cannot run, lest we become (gasp) dogmatic. It is possible that we
happen to live at one of those times when there are no giants that run.
Recently extinct rhinos such as elasmotherium weighed perhaps 5 tonnes, yet
they appear designed for galloping like black and white rhinos. Titanotheres
also reached about 5 tonnes, and they seem to have been rhino-like in design
and have flexed elbows and knees. Even indricotheres up to 15-10 tonnes were
not built like elephants. They are more scaled up work horses with flexed
elbows and knees (noted by Granger & Gregory back in the 30s), so they may
have been able to run.
Even the argument that limb flexion must decrease with increasing size may be
false. In modern mammals the decreasing flexion of the knee is reflected in
the orientation of the joints. The limb and joint anatomy of ornithomimids
and tyrannosaurs is extremely similar regardless of size, and there is no
evidence of decreasing limb flexion with bigger bulk. This is not surprizing
in that huge T rex grew up from little hatchlings and gracile medium sized
juveniles that were probably about as swift as ornithomimids of similar size.
Most land animals do not lose speed as they grow up, there is no evidence
tyrannosaurs did so. If they did do so, then they were most peculiar. Of
course, the bird-like ankles and feet of tyrannosaurs were superbly well
suited for running. My paper on this subject is submitted, of course I
discussed this stuff in PDW.
I recently finished my part of a very extensive joint study on ceratopsian
locomotion, in which we look at just about everything including the kitchen
sink. Yes of course they had erect arms, and even the biggest examples appear
to have flexed limbs, running hindfeet, and exceptionally strong, powerfully
muscles skeletons suitable for running at speeds well above those reached by
elephants. In this sense ceratopsids appear to have been more rhino-like.
Hadrosaurs also appear to have retained flexed knees, flexible ankles, and
feet that were both bird and rhino-like to sizes of 15 tonnes or more.
Doubtful they were as slow as elephants. Hadrosaur and ceratopsid locomation
is discussed in my paper in DinoPast&PresentII.
If giant land animals were always as slow as elephants, then they should show
the adaptations for having been as slow as elephants, including the very
short immobile foot seen in nonrunning stegosaurs, sauropods and unitatheres.
Instead, a number of past giants show signs of running ability similar to
that of rhinos and big ground birds.