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Re: Concavenator corcovatus, a new humped carcharodontosaurid from Las Hoyas



Jonas,

Actually, I'd say that caution should be placed on the idea that
thermoregulatory concerns were behind the evolution of elongated
neural spines, or any other such structures in dinosaurs, for that
matter...  Off the top of my head, I know of no extant animals that
employ such structures primarily for regulating body temperature.
Instead, the general "rule" is that these weird, expensive structures
(some of which actually seem to put the animal at a disadvantage) are
primarily grown and employed for display purposes of one sort or
another.  Thermoregulatory functions, if any, are what's secondary.

As for your question concerning phylogenetically widely spaced animals
evolving similar structures... what about the Artiodactyla?

And speaking of horns, a study by Hoefs on the thermoregulatory
potential of Ovis horn cores concluded that horn core size varies with
ambient temperature; that being, for sheep living in cold climates,
heat conservation is important, while for those living in hot
environments, enhanced heat dissipation would be advantageous, so
their horns, having secondary thermoregulating properties, should
differ depending on the environment.  Using measurements from a few
hundred horns from many species of sheep, an  index of heat-exchange
capacity was created  by dividing the combined surfaces of two horn
cores by the mass of the animal.  What was discovered was that the
thinhorn sheep of subarctic and arctic have the smallest horn cores
(smaller vascularized plexas), while some desert-dwelling types have
cores that are of more than twice  the subarctic/arctic (larger
vascularized plexas), while other are of intermediate sizes.   It was
also the author's position that the evolutionary trend to vary core
size in response to ambient temperature is independent of a parallel
trend to increase horn size for the benefit of enhancing reproductive
success. (Interestingly enough to mention as a broad brush to apply to
antelope and deer horns, American pronghorn do not use their horns for
thermoregulation. They instead use external heat exchange with the
environment via varying blood supplied to the skin, and internal heat
exchange between the carotid artery rete and cavernous venous sinus
blood to regulate body temperature.)

What's funny is that if you take an admittedly huge leap and apply
such conclusions to the sails and plates of various dinosaurs
(vascularity focus), the larger the sail or the plates, meant that the
animal was evolving them to cool down!  They were heat radiators, not
heat gainers.

However with that said, I still don't buy the idea that dinosaur
sails, plates and such were primarily for thermoregulation.  For
example, I’d assume it is relatively safe to say that Concavenator is
considered (today, anyway) to be a precursor to what we eventually see
as a configuration of elongated neural spines down the entire length
of the animal in Acrocanthosaurus.  Oddly enough, the progression of
this elongation of spines seems to have been far from even, apparently
(and this is an educated guess) beginning around the hips and
proceeding along the remaining length of the spine in the more derived
forms to come.  I find this scenario slightly daffy if
thermoregulation was the concern.  I base this conclusion off of the
fairly well-known (in terms of fossil record) Dimetrodon and its ilk.
Granted, thermoregulation as the function of the “sails” in the
Sphenacodontidae is far from proven (some find it to be nothing but a
current “best guess” that has been grossly over-popularized as being
fact), but, at least in the lesser derived Sphenacodontids, ALL of the
neural spines show an elongation trend that eventually reached its
zenith in the “sail” seen in Dimetrodon.  Such a sequence of events,
if based on thermoregulation, makes more sense in regards to the
steady increase in surface area available for heating/cooling. i.e.
selective advantages at work.  In this respect, the "fin" on
Concavenator just doesn't fit the bill.

And speaking of Dimetrodon... During the Permian, the locations of
Sphenacodontid fossils (Texas/New Mexico mostly, some mid US, and
central Europe) was wet and tropical, and continued toward a hot, arid
state as time marched forward (http://www.scotese.com/epermcli.htm).
Again, taking a leap based on real experimentation... If the Ovis horn
study applies, and the sails on Sphenacodontids were used for
thermoregulation, then Sphenacodontids were evolving them in response
to a warming world.

In addition, Tomkins et al came to the conclusion that for at least
two  well represented taxa, Pteranodon and Dimetrodon,
thermoregulation was not the driving factor behind the evolution of
the head gear in the pterosaur or the sail in the Pelycosaur. In the
study, it was concluded that for Pteranodon, the crests of larger
animals were disproportionately large for their body size, which
supported what the authors expected if head gear had everything to do
with sexual selection.  Similarly, the larger species of Dimetrodon
had increasingly larger sails compared to their body size, which was
also interpreted as sexual selection.

I've hunt deer, and know a fair amount about elk and moose behavior.
Bigger horns wows the ladies and intimidates rivals. In other words,
what Tomkins et al are saying makes perfect sense.

Even if my above babble is senseless, if the position is that the
evolutionary road to elongating neural spines was primarily driven by
environmental factors; i.e. thermoregulation, then this, by extension,
would imply that Acrocanthosaurus’ elongated neural spines formed a
structure that was primarily utilized for thermoregulatory purposes.
If so, then why?  What was so special about Acrocanthosaurus’
environment that required it to evolve such a structure? And further
to the point… sticking with theropods… why did Spinosaurs and SOME
Carcharodontosaurids evolve their elongated spines, while other
theropod “groups”... even those living at the same time and in the
same locations, never mind at a later time in similar environments...
didn’t???  Why didn't Allosaurus do the same before it?  Why not
tyrannosaurs after it?  I would think that if an environmental factor
was involved, then the elongation of neural spines to form a
thermo-regulating structure would have been much more common.

So, looking back at the two aforementioned articles, thermo or no
thermo function, I see the trend in increasing spine length in at
least
one "lineage" of Carcharodontosaurids and some hadrosaurs, the
increase in plate size in stegosaurids, and the increase in sail size
in Spinosaurids, to have everything to do with these particular
animals' strategy for increasing reproductive success (amongst other
social-oriented functions).  That seems to be the fundamental point
for evolving goofy structures in the animal kingdom today, so I'd have
to say it is the sturdiest conclusion to apply toward dinosaurs and
the like as well.

Kris

Hoefs, Manfred. The Thermoregulatory potential of Ovis horn cores.
Can. J. Zool. 78(8): 1419–1426 (2000)  doi:10.1139/cjz-78-8-1419
Mitchell, Jeanette, G. Mitchell and A. Lust. Thermoregulatory anatomy
of pronghorn (Antilocapra americana). European Journal of Wildlife
Research. Volume 55, Number 1, 23-31, DOI: 10.1007/s10344-008-0210-y
Tomkins, J., LeBas, N., Witton, M., Martill, D., & Humphries, S.
Positive Allometry and the Prehistory of Sexual Selection The American
Naturalist (2010) DOI: 10.1086/653001





On Mon, Sep 13, 2010 at 11:57 PM, Jonas Weselake-George <ee555@ncf.ca> wrote:
> On Mon, 13 Sep 2010 22:07:13 -0400
> K Kripchak <saurierlagen1978@gmail.com> wrote:
>> So... based just on analogy's sake (as Darren mentioned on TetZoo),
>> I'm pretty convinced Concavenator (and other sailed and plated
>> dinosaurs for that matter) were using their goofy, obtrusive, awkward,
>> and metabolically expensive structures for what said structures are
>> usually evolved for... impressing the ladies, doing a good DeNiro Taxi
>> Driver impression, and rabble rousing.
>>
>> Kris
>
> On Thu, 09 Sep 2010 13:34:00 +0200
> Heinrich Mallison <heinrich.mallison@googlemail.com> wrote:
>> In fact, you drew a general conclusion (which, I'll give you that, you
>> qualified), despite there not being any strong indication that a sail
>> was present.
>>
> On Thu, Sep 9, 2010 at 5:32 AM,  <GSP1954@aol.com> wrote:
>> >>The arrangement tends to favor that
>> >>dinosaur sails were for display
>> >>rather than supporting fat deposits
>> >>in at least some cases.
>>
>> (incidentally, I fully agree that the fat deposit hypothesis is
>> unlikely.)
>
> I'd tend to recommend caution regarding any move to assign display as
> anything more than a secondary function. The existence of convergent
> evolution in four geographically and phylogenetically widely spaced
> taxa - all within a twenty million year period of each other - seems to
> imply *some* type of climatic adaptation - either to temperature
> extremes or seasonality.
>
> If anyone can come up with another mechanism that could trigger such
> adaptations, largely unique adaptations compared to species in other
> eras, I'd love to hear it.
>
> Similarly, if anyone knows more about possible climatic stressors it
> would be very interesting. It would be interesting to assess the
> likelihood of short-term climatic instability or temperature peaks
> being most important vs. a long term general trend in temperature.
>
> S!
>
> -Jonas Weselake-George
>
>