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



From: Ben Creisler
bcreisler@gmail.com


Maybe diapsid tetrapods with neural spines and chevrons on caudal
vertebrae  are restricted to downward bent tails for developmental as
well as mechanical reasons.  In addition, predatory animals that need
to breath air may need a low profile when swimming rapidly just
beneath the surface-- a strong vertical tail would break the surface,
although this event undoubtedly occurred with some ichthyosaurs. The
dorsal fin on sharks is a give-away to prey at the surface. Mosasaurs
caught pterosaurs and hesperornithids at the surface.

For discussion of a possible tail fin in plesiosaurs, see:

http://www.njan.org/files/plesiosaur%20tail%20propulsion.pdf

On Tue, Sep 10, 2013 at 5:06 PM, Dann Pigdon <dannj@alphalink.com.au> wrote:
>
> 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
> _____________________________________________________________
>