[CINC] Marine Mammal Evolution

[whalebear at cs.com]

In response to  a recent question regarding marine mammal evolution, I sent the following reply.  If there are any questions, pleas don't hesitate to ask....

Regards to all,

Chuck Rennie


MARINE MAMMAL PALEONTOLOGY

 

Marine mammals were not contemporary with thedinosaurs.  The great adaptive radiationof dinosaurs began approximately 200 million years ago (late Triassic) and wasover by the end of the Cretaceous (65 million years ago).

 Marine mammals first appear in the fossil recordapproximately 50 million years ago. Ancestral groups were either raoellids (small, deer-like animals) oranimals related closely to mesonychids (furred mammals ranging in size frommodern dogs to modern bears), both of which were even-toed ungulates (whichinclude cattle, sheep, goats, giraffes, deer, and hippopotamuses,).  Extant Cetaceans are most closely relatedamong living species to hippopotamuses (determined by molecular biology).  Initial evolution of Cetaceans appears tohave been in the Tethys Sea area (covering what is today Pakistan, parts of India,and Indonesia, and extendingfrom Africa to the Pacific).  Initial species were fresh-water orshallow-water marine, had four limbs, and had limited capability for underwaterhearing.  The past 30 years has seen anexplosion of descriptions of fossil Archaeocetes; there are now excellentdescriptions of species intermediate between terrestrial vertebrates and modernCetaceans.  The earliest species that iswell-described is in the genus Pakicetus(49.5-49 million years before present). By approximately 28 million years ago, the Archaeocetes disappear fromthe fossil record.

 Mysticetes (baleen whales) enter the fossil record with theappearance of Llanocetus 35 millionyears ago.  This species, like otherarchaic Mysticetes, had teeth, although the wide spacing of the teeth may be anindication that some type of “proto-baleen” was also present.  Among the modern families, the oldest fossilin the family Balaenidae (right whales) dates to 28 million years ago, and theoldest fossil in the family Balaenopteridae (Blue, Fin, Sei, Bryde’s, Omura’s,and Minke whales) dates to approximately 25 million years ago.  The oldest fossil in the family Eschrichtidae(gray whales) dates to only 0.5 million years ago, although some analyses claimgreat antiquity for this family and place it closest to some of the archaicfamilies.

 The oldest of the Odontocetes (toothed whales) dates to 32million years ago.  Among current families,the Pyseteridae (sperm whales) have the oldest pedigree, extending back to 23million years before present (although the Kogiidae, or pygmy and dwarf spermwhales, extend back only 6 million years in the fossil record).  Ziphiids (beaked whales) extend back 13-15million years, Platanistids (river dolphins) to 16 million years.  Archaic dolphins are present in the fossilrecord as early as 23 million years ago, although modern delphinids don’tappear until 11 million years before present. Early phocoenids (porpoises) appear 12-16 million years before present,and appear to have evolved in the northern Pacific.  Monodontids (belugas and narwhales) don’tenter the fossil record until approximately 2 million years before present.
 
Pinnipeds are of more recent origin than Cetaceans.  Although great debate existed in the past asto whether they were of monophyletic (one ancestor) or biphyletic (withotariids, or eared pinnipeds, derived from ursids (bears), and phocids, orearless pinnipeds, derived from mustelids (skunks, weasels, and otters)), mostauthors today consider them to be monophyletic. The earliest pinnipeds appear in the fossil record 27-25 million yearsago.  Phocids appear approximately 25million years ago, Otariids approximately 11 million years ago, and Odobenidae(walruses) 16-14 million years ago.

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