Re: infuriating Bakker

["Colin McHenry" <cmchenry@westserv.net.au>]

Scott wrote
>Actually, I was refering to the functional analysis of muscle insertation
movement within a phylogenetic context.  Not enough of those, although I'd
like to see people other than just Bob tackle the subject.  I won't disagree
with any of your points on tooth shape from a functional point of view.  I
would like to caution that adaptation does not always reach an optimum.

Absolutely - and as Steven Jay used to love reminding is, adaptation
probably rarely reaches an optimum as opposed to a sufficiency.  Problem is,
this explanation is a bit like citing sexual selection to account for e.g.
sails on spine backed dinosaurs - it sould be a theory of last resort,
because it is immensley difficult to test.  And yet, like sexual selection,
it is probably a very immortant process in determining the pattern of
biological diversity.

>Historical contingency plays a large role in constraining how closeely
animals parallel one another.  For example, there's the possibility that the
teeth were already exapted for use as large "serrations" in a continuous
cutting surface, in which case the anterior-posterior enlargement of the
teeth would have reduced the "serration" count of the tooth row, and would
be less effective without at least a corresponding shift in behavior.

This is a really quick read of the situation - 'cos I know next to nothing
about theropods, so I don't want to be held to it - but if didn't know you
were talking about dinosaurs I'd guess you were talking about the situation
in sharks.  Although shark teeth have sometimes been cited as analogues for
theropod teeth, the operation of the shark jaw is so different to that of
tetrapods that I have up till now believed (reluctanty) that we would be
denied the chance to
to use feeding behaviour in sharks as an analogue for reptiles.  But the way
Robert T. is describing the allosaurs reminds me strongly of large
carcharhinids, like bull sharks and bronze whalers, that shake their prey
and let the row of fine, serrated teeth work like a saw to cut the prey to
bits.

Taken to the extreme you get the situation in the  tiger shark, where the
tips of the teeth are'bent' over to give a shape like this (hope this comes
out, curse this html block):

   C
   ^
  /   \___  B
 /
/
A

Where the line A-B is the line of the gums - tilt your head 45 degrees to
the left and you get the shape of the tooth.  Oh, and this is an upper jaw
tooth, so it's up-side down, but you get the idea.  The 'leading edge' (line
A-C) is strongly serrated, so as the jaw closes and the tooth is pushed into
the prey the tooth acts like a saw.  And that's just during the bite - when
the tiger shark has a good hold it can then shake the prey as well.  The
tiger shark jaw and tooth apparatus allows it to shred even the hardest
prey -  I've seen 3 metre tiger sharks bite through the shell of adult green
turtles (1.5 metre carapace length) in a few shakes.  It's amazing.  Ever
dissected a big green turtle? - it takes ages to get through the shell, even
with bone saws.

Perhaps the reason that we don't see the tiger shark tooth type in allosaurs
is that you don't need it if you're only trying to inflict soft tissue
wounds - in that case the bull shark tooth type is probably good enough.  Oe
can speculate would you might have seen had Bakker's allosaurs ever come
across ankylosaurs, though!  Lots of other carcharinids are intermediate,
though, between the 'bull shark type' and the 'tiger shark type', and it
would be interesting to see a proper mechanical analysis of this variation
to see how well it correlates with feeding ecology.  There are lots of works
on using varitions in shrak tooth shape for taxonomic purposes, of course,
but I've come across very little in the way of comparative analyses of the
mechanics of shark teeth.  Anyone know of anything?

Where the analogy between allosaurs and sharks seems to break down, though,
is where Nick correctly points out;
>he
cites the mediolateral expansion of the teeth in allosaurids as making
them stronger to better resist being drawn backwards. This is true, but
expanding them anteroposteriorly- as in sabertoothed mammals- would make
them far stronger for the same amount of material.
In terms of dentition, sabertooths tend to have exceptionally long
upper teeth which are mediolaterally compressed. In other words the
teeth are... saber like. _Ceratosaurus_ seems to fit this bill pretty
well, _Allosaurus_ doesn't

Looking at the teeth of those carcharhiniformes that seem to be able to take
large prey, the base of the tooth is wide - look at the 'posterior' surface
of the tiger shark tooth (line C-B), where the bottom of the tooth is
expanded greatly.  One can imagine this having the result of buttressing the
tooth again the side-to-side forces (i.e. from left to right as you are
looking at it on your screen).

Of course, the important teeth in the shark jaw for the killing bite are
towards the front of the jaw, and that jaw is 'square fronted' - i.e. the
main orientation of the jawline, and of the teeth, is medio - lateral,
rather than anterior-posterior as it is in the theropods.  In tiger sharks
the head is especially wide and square fronted (seeing a square snouted
shark coming at you is one of the quickest ways of picking a tiger from a
whaler), increasing the width of the jaw and hence the amount of jaw that is
aligned medio-laterally, and hence the amount of big front teeth that can be
brought into play in the side-to-side head shake.

But if the shark is using its jaw like a saw, and is inflicting lateral head
shakes on its prey to do so - while the Bakkerian allosaur is pulling its
head down and back to 'saw' away at Brontosaurus - then the net result on
the tooth shape should be the same.  I would expect the tooth of a Bakkerian
allosaur to be much wider (er, anterior-posteriorly...I'm getting
disorientated now!) than he figures.  Yes, more like the ceratosaur.

Anyway, Scott then said:

>One the other hand, reducing tooth size could maximize the effectiveness of
the tooth row for slicing as a single surface, while widening the teeth
would be the only remaining avenue to increase overall strength.  The tooth
row would be maximally fit as a functional complex, though individual parts
would not be maximally fit when atomized.


That's precisely what you see in white sharks - bigger teeth, with (shock)
_spaces_ in between them (very unusal in macropredatory sharks, and one of
the ways in which you can make a positive id of a white shark bite), so far
fewer teeth for your shark (not really good value for money for tooth
hunters?!) From memory, the white shark tooth is also thicker (anterior
posteriorly, which is equivalent to medio-laterally in the dinosaur!) than
the carcharhiniform teeth - just as the allosaur tooth is thicker than the
ceratosaur tooth in Bakker's figure 8.

Now white sharks are one of the few sharks that will bite (live) prey bigger
than themselves....Although they usually go for marine mammals (seals and
dolphins) smaller than themselves, they do follow whale breeding migrations,
presumably to try their luck on the year-old calves that are still with
their mothers.  In Morton Bay (near Brisbane), where the humpbacks come very
close to the coast on their way up to the breeding grounds in the coral sea,
there is a big increase in large white shark catches on the shark lines when
the whales are in the area (in fact one of the largest whites taken off
Eastern Australia was caught 200 metres off the one of the most popular
swimming beaches in the bay!).  Yes, the sharks are probably interested in
the new born whales (which they can chase when the whales come back down
from the coral sea), but they also seem interested in the year old calves.
Of course, they just have to fight over them with the orcas and the false
killers, producing the predator-phile's ultimate fantasy - white shark vs.
killer whale!  Who'd be a humpback calf?

Now where was I going with this?  Oh yes, the point is that year-old
humpbacks are not particularly small marine mammals, and may even be as
large or larger than the shark.

Alright, I'm starting to waffle. I just can't help myself when someone
mentions sharks....Okay, I mentioned the sharks.  Sorry. Back  to the point.

>    I should point out that this would be hard to test without a more
complete fossil record, and is not intended as positive evidence for Bob's
hypothesis.


Well, this is where I disagree with Scott.  I think it's eminantly testable
if you make some (very thorough, carefully chosen) comparisons, and augment
them with some FEA simulations, etc.


So what do we reckon, all you theropod types?  Might this be of any use?

By the way, I really liked Bakker's paper.  I love the left-field stuff.
Better to be wrong than dull, as S.J. used to say. ( Someone once said of my
grandfather that, of ten ideas he'd put forward seven would be impossible -
but of the other three, one would be a winner.  What do think, Darren?  Runs
in the family?)

Enough

Col

"They're chickens, you dolt!  Apart from you they're the most stupid animals
on the planet. They don't plot, they don't scheme, and they are not
organised."

Colin McHenry
56 Gaskill St
CANOWINDRA,  NSW 2804,  Australia
Ph: +61 2 6344 1009
Mobile phone: 0428 131 858
email: cmchenry@westserv.net.au