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Re: Triceratops sprawl (long)



>        There's no proof that protoceratopsids as a group evolved in
>the deserts of Mongolia. All we know is that a couple branches of the=20
>marginocephalian family tree- Protoceratops and Psittacosaurus- show up=20
>there, which proves only that Protoceratops and Psittacosaurus lived in=20
>Mongolia. Their ancestors could have lived anywhere in asia, and probably=
=20
>did live all across asia, the only thing special about Mongolia is that it
>has the right conditions for these animals to show up. Their ancestors=20
>could have evolved in any number of places besides dunes.

This is the refrain for the science of paleontology, "We need more fossils t=
o
prove it."  Truth is that we will never have enough data (what would be the =
fun
otherwise).  Since _Psittacosaurus_ is also known from Europe, and since the
earliest ceratopians (Proto wasn't necessarily the first) are found in Asia,=
 the
likelyhood of their evolution to a quadrepedal form occuring there is pretty
good.  Based on the evidence we have, this seems reasonable.  As always, new
finds will force a rethinking of current views.

> If a future=20
>paleontologist found skeletons of a jackrabbit, a spadefoot toad, a=20
>tortoise and a coyote in desert sediments, could they then conclude that=20
>lagomorphs, anapsids, anurans and canids all evolved in desert=20
>conditions? Or if this future paleontologist found fossils of ground=20
>sloths in florida, and mammoths in south america, could they justifiably=20
>conclude that these were the ancestral homelands of the group?

If these are the only specimens available, sure.  However, with more digging=
,
they will discover the real ranges of these animals, as well as their place =
of
origin.  The difference here is that there are numerous specimens found in t=
he
Gobi, and from several different formations.  For now, it seems that the Gob=
i
was the place that quadrepedal ceratopians got their start.

>        Not to mention the fact that Cretaceous North America is not=20
>noted for sand dunes (I suppose Pachyrhinosaurus went dune-busting up on=20
>the North Slope?), the fact that putting the limbs out to the side is=20
>competely contradictory to the trackway evidence, and that animals like=20
>camels get along just fine with long, erect limbs.

Except that the Sprawling Forelimb Model (SFM) can and does match the trackw=
ays.
Consider this: trackway information is one piece of information.  Based on t=
his
information, any mount of the animal which seems to fit the track must be
considered as a possibility.  Since there are mounts in the Erect Forelimb M=
odel
(EFB) and SFM that match the tracks, they both must be considered equally
plausible.  To suggest that (based on this alone) one mount or another is
correct will be based more on opinion rather than on fact.

One more piece of info (from my own work).  As a part of my undergraduate
seminar, I tried to look at the limb from a different perspective.  It has b=
een
my observation that limbs designed for similar postures, tend to have simila=
r
proportions in the bone elements.  For the three posture types available (fu=
lly
erect, semi-erect, and fully sprawling), I chose animal groups (with known
forelimb postures) to represent each posture type.  I chose artiodactyls for
fully erect, crocodiles for semi-erect, and turtle for fully sprawling.  I m=
ade
measurements of the maximum length of the humerus and ulna for several diffe=
rent
animals of different sizes (attempting to attain as many species as possible=
).
After plotting the values on a scatter plot, I performed a linear regression=
 on
the data to produce a straight line (representing average values for the gro=
up).
I then superimposed the ceratopian line on top to comare the two (all of=
 this will be published in the near future).

The results of this procedure?  In the comparison of artiodactyls and=
 ceratopians, the lines were very different in slope (the junction of the=
 two formed roughly a 45 degree angle).  Certianly, if there is that much=
 difference in the slopes, then the correlation between the two is nil. =
 Therefore, EFM is inaccurate.  In the comparison of crocodiles and=
 ceratopians, the lines were still quite different (although they may be=
 close enough to allow for a secondary semi-erect posture, but this was not=
 what the limb was designed to do).  In the comparison of turtles and=
 ceratopians, there was something remarkable in that the lines were=
 virtually identical in slope (only a difference of 0.02).  If the limbs of=
 turtles and ceratopians are that similar, then the posture must have also=
 been that similar.

The point behind the dune buggy analogy is to show that both automobile
manufacturers, and the powers of evolution, both found similar solutions to =
the
problems associated with dune environments.  The similarity is there: both h=
ave
placed the wheels/feet as far apart as possible, this has the combined=
 effect of
spreading the weight over a larger area, and lowering the overall center of
gravity (producing an extremely stable morphology).  Car designers know that=
 the
more stable one builds a vehicle, the better the handling.  Since SFM is an
extremely stable morphology, certainly it would've handled just as well.  IM=
HO,
this is why this group did so well in North America.

Rob

***
Q.  What fossil fish is a blood relative?

A.  The antiarch.