Re: marine reptile McMenu

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

hammeris1@bellsouth.net wrote:
> Most of the depictions of these animals seem to be in shallow Jurassic seas of 
> the time - if this was pliosaur preferred habitat, then they surely could have 
> dived to the bottom to munch on the large mollusks of the time, correct?
>
>   
I  can't think of any mechanical reason why they couldn't, and funnily 
enough Ben Kear just published this:

KEAR, B.P. & GODTHELP, H., March, 2008. Inferred vertebrate bite marks 
on an Early Cretaceous unionoid bivalve
from Lightning Ridge, New South Wales, Australia. Alcheringa 32, 65-71.

An opalized unionoid bivalve bearing potential vertebrate bite marks is 
described from Lower Cretaceous nonmarine
deposits of Lightning Ridge, New South Wales, Australia. Damage to the 
shell includes a series of regularly
spaced indentations with associated depression fracturing and some 
surface crushing. These injuries are consistent
with vertebrate feeding traces reported on other fossil molluscs 
(cephalopods) and suggest that a bivalve-eating
predator might have been present in the Lightning Ridge palaeofauna. The 
size and spacing of the apparent tooth
marks closely match the dental morphology of several sympatric 
vertebrate taxa: large osteichthyan or
chondrichthyan fish, crocodiles, and pliosauroid plesiosaurs. The 
feasibility of these and other vertebrates as
potential causes of the pathologies is discussed.

> I would think they would be opportunistic eaters.
> If they could catch it, they ate it.
>
>   
Yes, it's tempting to think of these things as just bloody big 
generalists.  Killer whales much 100,000kg whales and 0.1kg sardines, 
and lots of stuff in between - that's a prey size range covering 6 
orders of magnitude.  Sure, the upper and lower ends of that niche are 
helped by some pretty complex and famous behaviour patterns, but that's 
still a broad range.  Is this a general theme for marine apex predators?
> But what about the deeper waters when crossing from one sea to another - could 
> these leviathans hit swirling schools of larger fish like some kind of 
> monstrous seal?
>
>   

Maybe, but this isn't as easy as dolphins and sailfish make it look.  If 
you look at the open water predators that do successfully hunt baitfish, 
they tend to have some combination of: small (<1000kg) body size, and 
fast, agile swimming abilities (dolphins, seals, sharks, tuna, 
penguins?), social hunting strategies (all of these, plus sailfish), and 
anatomical oddities that allow them to hunt (sailfish, marlin).  The 
main exceptions to these are seabirds, which cheat because they fly, and 
rorquals, which cheat because they can gulp-feed.  And killer whales, of 
course, because they can do anything - but they have to work pretty hard 
to hunt baitfish, and not all pods (as far as we know) can do it.

A 10 or 20 tonne pliosaur is agile for its size, but it's nowhere the 
combination of speed and agility that dolphins, seals, tuna, and medium 
sized sharks have.  Unless you can make a case for gulp feeding (which 
is a highly specialised mode of feeding - see 
http://www.berkeley.edu/news/media/releases/2007/11/27_lungewhale.shtml 
), then perhaps some form of social hunting behaviour (or even mixed 
species?) would be necessary to take advantage of the baitball resource.

Cheers
Colin


--
Colin McHenry
Computation Biomechanics Research Group http://www.compbiomech.com/
School of Engineering (Mech Eng)
University of Newcastle
NSW 2308

t: +61 2 4921 8879