Transitional Vertebrate Fossils
Cetaceans (whales, dolphins)
Just several years ago, there was still a large gap in the fossil record of the cetaceans. It was
thought that they arose from land-dwelling mesonychids that gradually lost their hind legs and
became aquatic. Evolutionary theory predicted that they must have gone through a stage where
they had were partially aquatic but still had hind legs, but there were no known intermediate
fossils. A flurry of recent discoveries from India & Pakistan (the shores of the ancient Tethys
Sea) has pretty much filled this gap. There are still no known species-species transitions, and the
"chain of genera" is not complete, but we now have a partial lineage, and sure enough, the new
whale fossils have legs, exactly as predicted. (for discussions see Berta, 1994; Gingerich et al.
1990; Thewissen et al. 1994; Discover magazine, Jan. 1995; Gould 1994)
or similar triisodontine arctocyonids (early Paleocene) Unspecialized
condylarths quite similar to the early oxyclaenid condylarths, but with strong canine teeth
(showing first meat-eating tendencies), blunt crushing cheek teeth, and flattened claws
instead of nails.
(mid-Paleocene) -- A transitional genus intermediate between
and the mesonychids, with molar teeth reorganizing in numerous ways to
look like premolars. Adapted more toward carnivory.
(mid-Paleocene) -- A mesonychid (rather unspecialized Paleocene meat-eating
animal) with molars more like premolars & several other tooth changes. Still had 5 toes
in the foot and a primitive plantigrade posture.
or a very similar mesonychid (early Eocene, around 55 Ma) -- A small
mesonychid with very narrow shearing molars, a distinctively shaped zygomatic arch,
and peculiar vascularized areas between the molars. Probably a running animal that could
swim by paddling its feet.
itself may be just too late to be the whale
ancestor, but probably was a close relative of the whale ancestor. Says Carroll (1988):
"The skulls of Eocene whales bear unmistakable resemblances to those of primitive
terrestrial mammals of the early Cenozoic. Early [whale] genera retain a primitive tooth
count with distinct incisors, canines, premolars,, and multirooted molar teeth. Although
the snout is elongate, the skull shape resembles that of the mesonychids, especially
(early-mid Eocene, 52 Ma) -- The oldest fossil whale known. Same skull
, still with a very terrestrial ear (tympanic membrane, no
protection from pressure changes, no good underwater sound localization), and therefore
clearly not a deep diver. Molars still have very mesonychid-like cusps, but other teeth are
like those of later whales. Nostrils still at front of head (no blowhole). Whale- like skull
crests and elongate jaws. Limbs unknown. Only about 2.5 m long. This skull was found
with terrestrial fossils and may have been amphibious, like a hippo.