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Re: did mosasaurs echolocate?



  Stephan Pickering brings up an interesting perspective on mosasaurs
using cetaceans as a jump-off point (or comparison), and brings to voice
some other taxa which can be used, in conversation with Davic Marjanovic.
Thus, I would like add some certain mechanical constraints to
mammalian-style echolocation, if inferred for reptiles, and perhaps answer
the question with some gross, soft anatomy.

  Known warm-blooded echolocators include oilbirds (*Steatornis* sp.),
most microchiropteran bats, and cetaceans. All taxa use a fluid medium in
which to transmit a pulse of energy which, when recieved in the ear,
translates as sound after bouncing of an object of high density (close to
meeting solid state). Oilbirds, as in some bats (and the cave swiftlet),
echolocate in caves and darkness, and use a simple set of vocally induced
frequencies (sounding as clicks) to navigate; as in bats, the ear receives
the signals via minute vibrations of the vibrissae in the cochlea (a bat
can deafen itself if it hears its own clicks, resulting in a specialized
stapedial muscle that detaches the bone from the cochlear wall when it
transmits, but reasserts the bone in time to receive the muted return
signal), and the ear bone(s) are/is slender, not thick. This is the first
constraint on echolocation: tests show that a thick stapes is incapable of
vibrating at the rates needed for high-frequency reception, meaning
echolocation is impossible, and such stapes are known in all mosasauroids
as well as lizards and most dinosaurs where the stapes is known. Oilbird
and swiftlets use low frequency sounds, however, and it is quite effective
for the limited use they put it through. Typically, however, both birds
hunt by vision rather than hearing.

  Cetaceans are more specialized in that odontocetes (not mysticetes) use
either the spermaceti organ (*Kogia* and *Physeter*) or the melon
(Delphinioidea, possibly paraphyletic, including monodontids, extinct
odobenocetopsids, extinct and living porpoises and dolphins, and orca and
blackfish), which is connected to the pharynx and transmits sound at a
frequency audible to the human ear, unlike most bat sounds (both are
high-frequency, though, but dolphins have been known to use low, medium,
and high frequency sounds in combination -- call it "language", some say);
this structure is unique to cetaceans and involves an osteological marker,
including a distinct depressed rostrum and an assymetrical premaxillary /
frontal system which is concave on its dorsal surface and "cups" the organ
of transmission, as Dave Marjanovic detailed.

  However, cetacean hearing involves the lower jaw and a unique,
mechanical array of teeth that are utilized much as an antenna array such
as the Big Ear. The ear bones of cetaceans are similarly delicate, and
study of fossil cetacean ear ossicles has resulted in the finding of
stapes that could and could not receive echolocation. This gives us our
second constraint: transmitting organ. In cetaceans, bats, and birds, the
pharynx is a highly adapted structure which is capable at first of
producing high-frequency sounds, and cetaceans and bats can modulate them;
birds cannot, having shifted this ability to the syrinx. Reptiles have
nowhere near the known pharyngeal adaptation to either produce
high-frequency sounds, or hear them, with some exceptions, the best being
gekkonids. For a more detailed discussion on reptile hearing, Melissa
Kaplan* has done work on this subject, and a short synthesis can be found
at:

  http://www.anapsid.org/reptilehearing.html

  In short, the cranial anatomy of squamates is specialized to
low-frequency sound, and the likelihood of mosasaurs having echolocation
is very, very slim given their large stapes, loose skulls, and
craniofacial anatomy. More like, as in extant sea snakes and marine
iguanas, who regularly use their tongues when underwater when they
navigate, their sense of taste was of some utility, and in monitors and
monitor-like mosasauroids the cranial arrangement is almost identical,
resulting in a very simple simile to be drawn. This is not exact; it is
possible, but not likely given comparisons of echolocating animals and
their gross anatomy with comparison to extant reptile hearing, that
mosasauroids have acheived another form of underwater sensing, aside from
(but including) vision; the presence of overlapping concurrent fields of
vision is high in the taxa, but not as in binocularly-capable animals,
this leaving the two chemosenses, which lepidosaurian reptiles have
prefected on the tongue, nasal organs, and the Jacobsen's organs; crocs,
similarly, have developed organs similar to the selachian ampullae of
Lorenzini in their skin, but this is akin to the sense of touch, and does
not help the croc hunt prey (but does to help navigate in some cases).
Cetaceans, despite their mean body size, have very large eyes with acute
vision, and it is a problem [for me, anyway] to refer to their eyesight
being poor because of the ratio of the size of the eye to its gross
weight.

* Melissa Kaplan, who wrote _Iguanas for Dummies_ (HungryMinds, Inc.,
2000), is a veterinarian who specializes in herpetiles, mostly iguanas,
and whose work has come through some extraordinary means. Her website,
http://www.anapsid.org, goes into some detail on the subject of her work,
including many things reptilian.

  Cheers,

=====
Jaime A. Headden

  Little steps are often the hardest to take.  We are too used to making leaps 
in the face of adversity, that a simple skip is so hard to do.  We should all 
learn to walk soft, walk small, see the world around us rather than zoom by it.

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