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Re: Prognathodon (Mosasauridae) had shark-like hypocercal tail fin



I may have partially answered my own question. According to research at Harvard:
http://www.news.harvard.edu/gazette/2004/09.16/08-shark.html

"While a symmetrical fish tail leaves a one-part wake behind, the shark 
experiments clearly show a 
two-part wake. The larger upper lobe of a shark's tail cuts the oncoming water 
slightly before the 
smaller lower lobe. This creates a wake within a wake, giving the shark both 
thrust and lift, both 
forward and upward motion."

Perhaps this implies that air-breathing marine reptiles were too buoyant for 
their own good, and 
that natural selection favoured a tail with a dominant lower lobe to help 
compensate for that.

On Wed, Sep 11th, 2013 at 8:53 AM, Dann Pigdon <dannj@alphalink.com.au> wrote:

> 
> It's interesting that several reptile lineages with shark-like tails 
> (ichthyosaurs, mosasaurs,
> ocean 
> crocs) all seemed to have kinked the caudal bones downward to reinforce the 
> lower lobe of the
> fin, 
> whereas sharks with asymmetrical tail lobes seem to all continue the caudals 
> into the upper
> lobe.
> 
> Was this just coincidence (an evolutionary toss of the coin), or could it 
> have something to do
> with 
> different buoyancy issues (lungs verses liver)? Does an asymmetrical tail 
> subtly change the pitch
> of 
> the animal while it swims, with a dominant upper lobe compensating in one 
> direction and a 
> dominant lower lobe compensating in another?
> 
> On Wed, Sep 11th, 2013 at 1:27 AM, Ben Creisler <bcreisler@gmail.com> wrote:
> 
> > From: Ben Creisler
> > bcreisler@gmail.com
> > 
> > 
> > A new paper in Nature Communications:
> > 
> > Johan Lindgren, Hani F. Kaddumi & Michael J. Polcyn (2013)
> > Soft tissue preservation in a fossil marine lizard with a bilobed tail fin.
> > Nature Communications 4, Article number: 2423
> > doi:10.1038/ncomms3423
> > http://www.nature.com/ncomms/2013/130910/ncomms3423/full/ncomms3423.html

-- 
_____________________________________________________________

Dann Pigdon
Spatial Data Analyst               Australian Dinosaurs
Melbourne, Australia               http://home.alphalink.com.au/~dannj
_____________________________________________________________