Re: nocturnal eyes question

["Dino Guy Ralph" <ralph.miller@alumni.usc.edu>]

"Christopher Taylor" <ck.taylor@auckland.ac.nz> writes:

>     Not necessarily. A recent study (can't track down the exact reference
to
> it - I only glanced across it while looking for something else - someone
> help me please.... :S) looked at a close fossil relative to platyrrhines
> (New World monkeys) which has been considered nocturnal because of its
large
> orbits. CAT scans of the braincase showed, however, that the olfactory
bulbs
> in this beastie were fairly small, despite a sense of smell being usually
> well-developed in nocturnal mammals. The authors concluded that the animal
> was more likely to have been diurnal, and the large orbits were a relic
from
> nocturnal ancestors (which it definitely had).

Did these nocturnal ancestors have both large orbits and large olfactory
bulbs?  If not, then how can we conclude that the fossil species' closest
ancestors were nocturnal?  If the eyes see well enough in the dark, there
may be no need for a keen sense of smell.

Nocturnal owls have excellent night vision, but (correct me if I'm wrong)
their sense of smell is virtually nil.  I mean, they are immune to skunking,
aren't they?  On the other side of the coin, kiwis often can't see the prey
they fossick at night, but they use their senses of smell and touch to
locate their prey instead.  In each case, the animal does not require both
excellent night vision and excellent olfaction.  One or the other will
suffice.

General rules will apply in many cases, but certainly not in all cases.

Regarding visual acuity, there is another important factor that is unlikely
to fossilize: the density of visual receptor cells.  At
http://www.ornithology.com/lectures/Senses.html, it is written:

"The density of receptor cells in the retina also allows birds to see in
very dim light.  Barn owls can see an object at 2 meters with an
illumination of .00000073 foot candles.  This is equivalent to a person
seeing an object by the light of a match a mile away."

If an extinct large-eyed species -- for example, a pterosaur -- was likewise
equipped with dense retinal receptor cells, it would have excellent night
vision.  But sadly, fossils are unlikely to either validate or refute a
"dense receptor cell" hypothesis for such an animal unless there is an
osteological correlation for such a condition.

Even so, I would think that the discovery of an extinct species that has
markedly larger eyes than its close relative would support the hypothesis of
a nocturnal lifestyle for the large eyed taxon, even if the case is not
"proven" on this point alone.  Knowledge of the animal's habitat and diet
may also be helpful in arguing such inferences.

-------Ralph
-------
"Dino Guy" Ralph W. Miller III
Docent at the California Academy of Sciences
proud member of the Society of Vertebrate Paleontology