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The Wild and Wacky *Carnotaurus*



               (or: "Boy, Are My Arms Tired...")

  Sometimes if ecology doesn't work off the bat, or the puniest
things seem inapplicable or lead to Lamarckian statements, or
allometry doesn't seem to explain anything ... there's always
biomechanics.

  There are two questions: 1) what are the brow horns for, 2)
and why are the arms so small?

  Both can be explainable as sexual characteristics: 1) brow
horns in extant animals are usually display devices or wrestling
devices, as in cervids, and proposed for ceratopsids
(Wellnhofer, 1993). Perhaps the style is reminiscent of bighorn
sheep: there is a broad, flat surface on the forehead with
dorsally-flat lateral horns that form a platform (conceivably
you can have two male carnotaurs pressing their foreheads
together in a sort of elephant-likeshoving match). Perhaps they
interlocked and the style was deer-like, with twisting involved;
displace the arrangement of the horns on one and you get a
cantered head, with carnotaur #1 having the left horn above his
opposite's right and the right horn below the opposite's left...

  Display features are usually in a position to be seen. Someone
brought up *Cryolophosaurus,* plus *Dilophosaurus*.... These are
front and side-view examples, whereas the best display use for
the dorsoventrally-compressed horns of carnotaurs is from the
top, and they would not be particularly visible from other
angles, especially from the side, in which the head itself is
best viewed. Seems likely then that display isn't very viable.
Not impossible, mind.

  2) Puny arms are seen in tyrannosaurine Tyrannosauroidea and
carnotaurs. They are not seemingly analogous as the manus and
forearm in the latter are much different anatomically than the
latter, and the humerus itself is mechanically different. In
tyrannosaurines, the humerus, though robust, is a form of rods
with projecting plates and an inclined head, with distinct
epiphyses to mark a wide distal end from the diaphysis (shaft).
The distal end is nearly confluent with the diaphysis proximally
to the head, with the exception of robust swellings proximally
(caudally/dorsally, the m. humeroscapularis has a strong
attachment, whereas cranially/ventrally, the deltopectoral crest
is a thick ridge whose distal/ventral end [to the shaft] is set
inward from the proximal/dorsal end [using the same criteria],
not like tyrannosaurs who lack such a robust m. hum.scap.
attachment and the deltopectoral is a large flange, not a short
ridge; similarly, the crest is longer in carnosaurs
proximodistally than in tyrannosaurines). The humeral head is
rounded and subspherical in carnotaurs than in tyrannosaurs,
suggesting greater maneuverability. So all in all, the
morphology suggests greater leverage, maneuverability, and
lesser strength, than in tyrannosaurines. Finally, the form of
the manus and epipodium is distinctly different: the manus of
tyrannosaurines (_sensu_ Horner) has a unique flexure/extensure
capacity so that when the digits are extended, the fingers
spread laterally, and contra when flexed, thus having
psuedo-opposability; the manus of carnotaurs is not completely
known, however it seems apparent that the wrist was oriented
palm-cranial or lateral, not medial or caudal, and that the hand
was extraordinarily stunted with a large fourth metacarpal [?].
Plausibility of the identification of elements aside, they are
sparely articulated, so relationships are not concrete. The
"spur"-like fourth metacarpal would normally point caudally, but
given the re-orientation of the manus, this is now laterally or
cranially.

  Both tyrannosaurines and carnotaurs have arms that cannot
touch each other, much less themselves, but they do project
beyond the chest far enough so that if the animal were to rest
its chest down on something, they could effectively contact it.
Maybe not manipulate it, but we're looking at some really strong
forelimbs here. Given the probably strong adductive ability of
the humeri, the forearms have a mechanical ability that belies
their brevity and apparent stubbiness; this, in a typcial
Holmesian fashion means that they must have done something, even
if we don't know what. If they seem undersized for prey
manipulattion, then we can suggest sexual manipulation. Some
animals (let's use cats) grip their mates during coitus; in cats
this is to [dobuly] prevent the female from bolting and to
incite aestrus; such may be the case in carnotaurines, who would
be able to use the "spur" to impale the scruff of the female's
neck or shoulder and "stimulate" her. Either spreading extension
of the digits of the hand to ice-pick--style gripping (and tiny,
curved claws) or a spur on the hand can be considered sexual
adaptations.

  I would not be adverse to criticism....

=====
Jaime A. Headden

  Aaaaaaaaaaaaaaaaaaaahhhhhhhhhhhhhhhhhhr-gen-ti-na
  Where the Wind Comes Sweeping Down the Pampas!!!!

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