MESOZOIC ECHOLOCATION

[darren.naish@port.ac.uk]

First of all, echolocation is more widespread in Mammalia than some 
of you have realised. Besides bats and toothed whales, it is also 
employed by some rodents. Passive facultative echolocation has also 
been reported in some blind humans and it has been suggested that 
some shrews may employ echolocation: to my knowledge this has not 
been proved. The cochlea of the most primitive bats known shows that 
they were echolocators, thus this is an ancient trait for bats. Only 
a few megabats echolocate, and it is still argued over whether they 
are evolving this convergently with microbats, or have inherited it 
from microbat ancestors. The isolated petrosals of some basal 
cetaceans indicate that echolocation is also an ancient trait for 
whales  - and I have spoken to biologists who insist that all aquatic 
tetrapods must emply echolocation because they simply cannot rely on 
visual acuity - and if this is true, mysticetes (which, so far as we 
know, do not echolocate) have lost echolocation. That this may have 
been the case is supported by a vestigial melon found in the heads of 
some mysticetes. Melons are not a prerequiste for echolocation, but 
are thought to be beneficial once evolved and probably aid in the the 
focusing of sound beams.

There is no correlation between echolocation and survival rates for 
KT tetrapods. Multituberculates (MTBs) survived the KT event, yet 
seem to have relied on bone-conducted hearing and certainly not 
echolocation. Primitive ungulates, similarly, have no recorded 
capacity for echolocation, nor (to my knowledge) do the marsupials 
that also survived the KT event.

As for reptiles, it has at times been wondered if marine reptiles 
like ichthyosaurs and plesiosaurs could have been echolocators. In 
water, sound waves are conducted throughout an animal's body, this 
makes directional hearing difficult. The way to solve this is to 
isolate your ear bones from the rest of the skeleton by a medium that 
is less dense than the rest of your body. Whales have done this by 
growing a thin sheet of oily bubbly fluid between their petrosals 
(ear bones) and the rest of the skull, thus they can tell with 
accuracy (as animals that hear sound waves conducted in air) from 
which direction a sound wave is coming from. Chris McGowan verified 
sometime in the 1980s that icthyosaur ear bones are not separated 
from the rest of the skull, thus directional hearing and echolocation 
are probably not possible. Mike Taylor, more recently, has verified 
that the same is true for plesiosaurs. These animals appear, 
therefore, to have relied on vision and olfaction more than hearing. 
This is perhaps verified by the fused stapes in _Thalassiodracon_, 
and the apparently absent (!) stapes in some polycotylids and 
elasmosaurs reported by Ken Carpenter.

However, some icthyosaurs, including platypterygines and at least 
some specimens of _Ichthyosaurus_, have a dished central depression 
on their frontals which Mary Wade has interpreted as space for a 
melon. If true, this could be evidence of echolocation and, maybe, 
maybe, ichthyosaurs had a different way of going about it whereby 
directional hearing was not a necessity. I don't know.

Finally, Betty Cunningham wrote..

> Sperm whales have the largest extant eyes I know of, and they use
> sonar. 

A number of toothed whales have well developed eyes and are actually 
highly visual animals (as shown by experiments in which they bump 
into the sides of their tanks when obscured by cloudy water). There 
is still debate over how big ichthyosaur eyes are: I reckon they 
filled up the whole orbit, enclosed the sclerotic ring within the 
sclera (i.e. the sclerotic ring rings the cornea), and were 
therefore huge (same goes for plesiosaur eyes), but recently it has 
been argued (but not in print!) that they were small and contained 
within the sclerotic ring. Based on extant analogues, I don't see how 
the latter can be correct, but then I don't actually know much about 
eyes (Mickey?). These confounds imply that in marine tertrapods there 
is not a direct, knowable correlation between small eyes and 
echolocation, or between big eyes and lack of echolocation.

Sperm whale eyes are big, but they are surpassed by those of big 
architeuthid squids. The Thimble Tickle (Newfoundland) squid, which 
was about 16 m long total length, is reputed to have had eyes the 
size of saucers. One often reads of architeuthid eyes the size of 
rugby balls, but I would like to know from which squid this is 
recorded (and, yes, I know that there are sightings of squids 60 m or 
more in legnth..). Among tetrapods, the biggest known eyes belonged 
to _Opthalmosaurus_, the mid Jurassic-->lower Cretaceous squid-eating 
ichthyosaur.

"Set the controls for......... medium"

DARREN NAISH
darren.naish@port.ac.uk