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) Eoconodon 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. Microclaenodon (mid-Paleocene) -- A transitional genus intermediate between Eoconodon and the mesonychids, with molar teeth reorganizing in numerous ways to look like premolars. Adapted more toward carnivory. Dissacus (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. Hapalodectes 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. Hapalodectes 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 Hapalodectes...." Pakicetus (early-mid Eocene, 52 Ma) -- The oldest fossil whale known. Same skull features as Hapalodectes, 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. Ambulocetus natans (early-mid Eocene, 50 Ma) -- A recently discovered early whale, with enough of the limbs and vertebrae preserved to see how the early whales moved on land and in the water. This whale had four legs! Front legs were stubby. Back legs were short but well-developed, with enormous broad feet that stuck out behind like tail flukes. Had no true tail flukes, just a long simple tail. Size of a sea lion. Still had a long snout with no blowhole. Probably walked on land like a sea lion, and swam with a seal/otter method of steering with the front feet and propelling with the hind feet. So, just as predicted, these early whales were much like modern sea lions -- they could swim, but they could also still walk on land. (Thewissen et al., 1994) Rodhocetus (mid-Eocene, 46 Ma) -- Another very recent (1993) fossil whale discovery. Had hind legs a third smaller than those of A. natans. Could probably still "waddle" a bit on land, but by now it had a powerful tail (indicated by massive tail vertebrae) and could probably stay out at sea for long periods of time. Nostrils had moved back a bit from the tip of the snout. Basilosaurus isis, Protocetes, Indocetus ramani and similar small-legged whales of the mid-late Eocene (45-42 Ma) -- After Rodhocetus came several whales that still had hind legs, but couldn't walk on them any more. For example, B. isis (42 Ma) had hind feet with 3 toes and a tiny remnant of the 2nd toe (the big toe is totally missing). The legs were small and must have been useless for locomotion, but were specialized for swinging forward into a locked straddle position -- probably an aid to copulation for this long-bodied, serpentine whale. B. isis may have been a "cousin" to modern whales, not directly ancestral. Another recent discovery is Protocetes, a slightly more advanced whale from the late Eocene. It was about 3m long (dolphin sized), and still had primitive dentition, nostrils at end of snout, and a large pelvis attached to the spine; limbs unknown. Finally Indocetus is known from only fragmentary remains, but these include a tibia. These late Eocene legged whales still had mesonychid-like teeth, and in fact, some of the whale fossils were first mis-identified as mesonychids when only the teeth were found. ( See Gingerich et al. (1990) for more info on B. isis.) Prozeuglodon (late Eocene, 40 Ma) Another recently discovered whale, found in 1989. Had almost lost the hind legs, but not quite: still carried a pair of vestigial 6- inch hind legs on its 15-foot body. Eocetus, & similar "archeocete whales" of the late Eocene These more advanced whales have lost their hind legs entirely, but retain a"primitive whale" skull and teeth, with unfused nostrils. They grew to larger body size (up to 25m by the end of the Eocene), an had an elongate, streamlined body, flippers, and a cartilaginous tail fluke. The ear was modified for hearing underwater. Note that this stage of aquatic adaptation was attained about 15 million years after the first terrestrial mesonychids. Dorudon intermedius -- a late Eocene whale probably ancestral to modern whales. In the Oligocene, whales split into two lineages: 1.Toothed whales: Agorophius (late Oligocene) -- Skull partly telescoped, but cheek teeth still rooted. Intermediate in many ways between archaeocetes and later toothed whales. Prosqualodon (late Oligocene) -- Skull fully telescoped with nostrils on top (blowhole). Cheek teeth increased in number but still have old cusps. Probably ancestral to most later toothed whales (possibly excepting the sperm whales?) Kentriodon (mid-Miocene) -- Skull telescoped, still symmetrical. Radiated in the late Miocene into the modern dolphins and small toothed whales with asymmetrical skulls. 2.Baleen (toothless) whales: Aetiocetus (late Oligocene) -- The most primitive known mysticete whale and probably the stem group of all later baleen whales. Had developed mysticete-style loose jaw hinge and air sinus, but still had all its teeth. Later, Mesocetus (mid-Miocene) lost its teeth. Modern baleen whales first appeared in the late Miocene. "인생은 예측불허. 그리하여 쭈쭈바의 의미를 찾는다" 쭈쭈바의 네딸들에서 "언제 오줌을 쌀건지는 예측불허. 그리하여 난 전봇대를 찾아헤멘다." 너의 쭈쭈바가 하늘을 날게하려면, 쭈쭈바에게 나는법을 가르쳐라. - McGill 수학과 1층 남자화장실에 쓰인 낙서.