Subject: Re: Whale blow holes

Dr. Cathy Schaeff (schaeff@american.edu)
Thu, 09 Jan 1997 12:48:56 +0000

Dear David,

You asked: When did the blow hole first appear in the fossil record,
and what was the name of the animal it appeared in? 

Archaeocetes (elongated aquatic mammals comprising two families -
Protocetidae and Basilosauridae) evolved from the terrestrial order
Condylartha at the end of the Paleocene and then, about 50 million years
ago (Early to Mid Eocene), colonised the sea. At this time they had
nostrils located on top of their snout.  It is hard to define when the
nostrils became blow holes because soft tissue is not preserved in the
fossil, however, between 38-25 million years ago the nostils were
shifted so that they faced backwards and structures to seal them against
water were developed.  At that time there were four families
Agorophiidae, Squalodontidae, Aetiocetidae, and Cetotheriidae.

I've also read that it's generally agreed that the nearest living land
relatives  of whales are ungulates.  Can you tell me just what skeletal
features lead paleontologists to believe this? 

Cetaceans are thought to have evolved from the terrestrial order
Condylarthra which gave rise to both the Artiodactyla (ultimately the
modern ungulates) and Archaeoceti (ultimately the cetaceans).  Fossil
evidence includes info from many structures including skull formation
(including the skull's gradual elongation) and tooth structure -- teeth
of Pakicetys inachus and P. attocki, found in 1980, led to the
recognition that Ichthyolestes and Gandakasia, once considered primative
ungulates, were actually intermediate between mesonychids (primative
ungulates) and protocetid whales (most primative family of archaeoceti)
to which they are now linked.

For further reading, a good review is provided in The Natural History of
Whales and Dolphins by Peter Evans.

One last question, not at all related to whale evolution:  I understand
that when plankton counts fall below certain densities, some baleen
whales give up feeding and move off in search of greener pastures.  How
does a whale sense how much plankton is in the water?


I have been unable to unearth any real info on your last question --
I'll get back to you if I am more successful later today.


Regards,
Dr. Schaegg