[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: Climate vs. Display (was Concavenator corcovatus, a new humped...)




Kris,

Thank you for your in-depth reply.

I am quickly and carefully reconsidering my doubts. As usual it seems
that caution all-round is advisable.

I'm replying with my thoughts, which I hope will be of some use in
thinking through the positions.

The first epiphany is a simple one: 

If elongated neural spines (or caudal chevrons in some groups) are
interpreted as an unusual sail or whithers like structure - then their
appearance in Iguanodontids, basal Ornithopods (the tail of
Tenontosaurus), Spinosaurids and Charcharodontosaurids, within a twenty
million year time-span, appears significant and unique compared to other
time periods.

However, if these features are interpreted as being some form of
display - then one considers that almost all dinosaur groups, in all
time periods, show some form of ornamentation and these structures
become an unremarkable coincidence.

Different answers may be a result of different ways of categorising the
data.

Regarding the analogies you made, these comparisons are quite
informative and very attractive to discuss, I don't find either to be
an analogy which is compelling without at least some caveats:

The Artyodactyla analogy (a trend in exaptation from a common
body plan) works well for the trend within theropods or
within ornithopods, but it becomes less likely when both (more distantly
related) groups show similar tendencies. However, both do represent at
least partially bipedal dinosaurs, so it doesn't seem impossible.

Regarding any comparison with the development of Sphenacodontids - one
must take into account the unique requirements of bipedal locomotion,
which may well favour an initial elongation near the hips.


Turning back to the question itself:

I've generally assumed that such structures were the simplest way to
add volume and surface area without increasing lateral dimensions
(important for an animal in mixed terrain). 

In this case they could act as an adaptation to temperature extremes
via surface area (perhaps coupled with an emphasis on water retention -
assuming some mechanism like gular fluttering is avoided) or a way to
vary mass seasonally within a body plan that doesn't favour generalised
fat deposits.

Your observation regarding the lack of similar adaptations in other
theropods is astute and I think that both the thermoregulatory and
seasonal food storage hypothesis rest decisively upon this point:

Acrocanthosaurus and Spinosaurids represent some of the largest
theropods. If their activities involved prolonged exposure heat
dissipation may have been a very real problem for animals of such large
mass (eg. a Spinosaurid that is ambush hunting fish). In this case
behaviours may lead to considerable differences in micro-habitat and
niche segregation (with niches conceived of temporally as well as
spatially) within the same formation. This could explain why such
structures are not totally universal.

The only two animals which seem to pose obvious problems in this area
are Carcharodontosaurus and Lurdusaurus for Ouranosaurus (while making
an exception for Sauropods which we can assume already had
solid adaptations to heat dissipation and/or food storage).

Most other predators and herbivores in these ecosystems are much
smaller animals (and the expansion of Thyreophora, along with
the prevalence of potentially burrowing Hypsilophodontids supports the
idea of an unusually  harsh environment).

In comparison, the Late Jurassic and Late Cretaceous have very different
faunas from the ones we are considering and it may be the case that the
Barremian-Albian, with its extinctions and unusual diversifications,
may represent a unique climatic regime.

The increased frequence of ocean anoxic events towards the end of the
Early Cretaceous and the marked periods of global warming both seem to
suggest climatic instability - although these may tend to post-date the
earliest species showing a trend toward sails (I've done relatively
little climatic research - so I may be off on the extent of earlier
disruptions).

Challenges for the heat dissipation or whithers hypotheses:
- Show the existence of unique climatic conditions or habitats (possibly
  including climatic variability on the millennial or greater
  timescales)
- Possibly some explanation for the one or two taxa of similar mass
  which lack this feature and are found in the same deposits.

Challenges for the display hypothesis:
- An explanation why these display structures are favoured in the Early
  Cretaceous, but other display structures predominate in the mature
  Late Jurassic and Late Cretaceous ecosystems (as well as in other
  Early Cretaceous ecosystems).
- Possibly demonstrate that bipedalism doesn't favour the development
  of neural spines over the hips and/or doesn't favour an increase in
  height over longitudinal/lateral dimensions.

Better estimates on the heat dissipation capacity of "sails" and
assessment of the amount of calorie storage provided by a hump (vs.
volumes in the rump and ribcage) would help a great deal of course.

One interesting possibility is that climatic conditions favoured large
size and early maturation or produced changes in population densities
or some acting through some other mechanism favoured the development of
these display structures.

Thank you for stimulating this outburst of thought,

-Jonas Weselake-George


On Wed, 15 Sep 2010 18:50:12 -0400
K Kripchak <saurierlagen1978@gmail.com> wrote:

> 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, Iâd 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 âsailsâ in the
> Sphenacodontidae is far from proven (some find it to be nothing but a
> current âbest guessâ 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 âsailâ 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 Acrocanthosaurusâ elongated neural spines formed a
> structure that was primarily utilized for thermoregulatory purposes.
> If so, then why?  What was so special about Acrocanthosaurusâ
> environment that required it to evolve such a structure? And further
> to the pointâ sticking with theropodsâ why did Spinosaurs and SOME
> Carcharodontosaurids evolve their elongated spines, while other
> theropod âgroupsâ... even those living at the same time and in the
> same locations, never mind at a later time in similar environments...
> didnât???  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): 1419â1426 (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, Â<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
> >
> >