Living Cetacea are fully aquatic and share a hydrodynamically streamlined body form with forelimbs modified into flippers and loss of external hind limbs, while locomotion is powered by a heavily-muscled tail bearing a broad terminal horizontal fluke. Communication with other whales and sensory perception in an aquatic medium are largely sound-based, for which cetaceans have characteristically-dense tympanic bullae; isolated, highly modified periotic or petrosal bones; and large mandibular canals with thin lateral acoustic fenestrae. Feeding is accomplished by straining krill and small fish in the baleen whales (Mysticeti) or by catching larger fish, squid, and other animals and swallowing these whole or in large pieces in the toothed whales (Odontoceti). These divergent specialized adaptations to life in water, and the living suborders that have them, Mysticeti and Odontoceti, can be traced backward in the fossil record from the present to the Oligocene epoch (Fordyce and Barnes 1994).
Late Eocene Archaeoceti of the family Basilosauridae (especially Dorudontinae; Uhen 1996) resemble later mysticetes and odontocetes in having a hydrodynamically streamlined body form with forelimbs modified into flippers and locomotion powered by a heavily-muscled fluked tail, while retaining reduced but functional hind limbs (Gingerich et al. 1990). Basilosaurids have dense tympanic bullae, pterygoid sinuses, partially isolated periotics, and large mandibular canals with lateral acoustic fenestrae, which would have enabled them to hear directionally in water. Feeding was different in that cheek teeth retain complex morphology and functional occlusion, and heavy wear shows that food was chewed before swallowing. Thus, late Eocene basilosaurid archaeocetes have many, but not all, characteristics of later whales. Early mysticetes and odontocetes are difficult to distinguish from basilosaurids, and all are marine and fully aquatic. Temporal, morphological, and environmental/geographical continuity between late Eocene basilosaurids and following Oligocene mysticetes and odontocetes indicates basilosaurids are closely related to modern whales, and their derived aquatic characteristics affirm inclusion in Cetacea.
Middle Eocene Archaeoceti of the family Protocetidae resemble basilosaurid archaeocetes, mysticetes, and odontocetes in having a hydrodynamically streamlined body form and locomotion powered by a heavily-muscled tail, while retaining large functional hind limbs (Gingerich et al. 1994). The form of the forelimbs is as yet poorly documented, and the presence of a fluke is only a possibility. Protocetids have dense tympanic bullae and large mandibular canals with lateral acoustic fenestrae, but isolation of the periotic was limited, making directional hearing questionable. Like basilosaurids, protocetids had cheek teeth different in detail but retaining complex morphology and functional occlusion, and here too heavy wear shows that food was chewed before swallowing. Protocetids were probably good swimmers and all are found in marine strata. Thus, middle Eocene protocetid archaeocetes have many, but not all, characteristics of basilosaurid archaeocetes and some characteristics of later whales. Here again, it is the relative continuity in time, form, and place between middle Eocene protocetids and late Eocene basilosaurids that indicates protocetid archaeocetes are closely related to basilosaurids, and their derived aquatic characteristics affirm inclusion in Cetacea.
Early Eocene Archaeoceti of the family Pakicetidae are poorly known postcranially. Pakicetus has a dense tympanic bulla with a characteristically cetacean sigmoid process (Gingerich and Russell 1981), but the periotic was firmly integrated in the basicranium, making directional hearing questionable (Gingerich et al. 1983). Pakicetus had no enlargement of the mandibular canal and the incus was intermediate in morphology between those of modern artiodactyls and cetaceans, suggesting not only that it could not hear directionally but it could not hear well in water (Thewissen and Hussain 1993). Pakicetus had cheek teeth retaining complex morphology and functional occlusion, with larger protocones but otherwise the same general pattern of cusps and crests as later protocetids and basilosaurids. Early Eocene pakicetid archaeocetes are found in river and estuarine deposits in association with land mammals, but these deposits are peripheral to the Tethys Sea and pakicetid-bearing deposits are overlain by protocetid-bearing marine strata. Pakicetids have some, but not all, characteristics of later protocetid and basilosaurid archaeocetes and they have but few characteristics of later whales. Relative continuity in time, form, and place indicates early Eocene pakicetids are closely related to middle Eocene protocetids, and the derived aquatic characteristics of pakicetids affirm inclusion in Cetacea (Gingerich et al. 1983, Thewissen and Hussain 1993, Thewissen 1994). Pakicetus, the remingtonocetid Remingtonocetus (Kumar and Sahni 1986; Gingerich et al. 1995a), and ambulocetid Ambulocetus (Thewissen et al. 1994), all discovered in recent years, have such important primitive and nonaquatic characteristics that all have forced us to expand our concept of Cetacea.
The Paleocene mammals most similar to pakicetids and later protocetids are Condylartha or Mesonychia of the family Mesonychidae (Van Valen 1966, 1969, 1978). Mesonychidae did fit comfortably in the archaic order Condylarthra until Van Valen suggested that dental similarities to later Protocetus were important. This possible connection to later whales now overshadows their clear connection to earlier Paleocene condylarths but the original temporal, morphological, and geographical resemblance has not changed. Protocetid teeth and mesonychid teeth have similarly-unusual proportions, and a protocetid of uncertain generic attribution, one pakicetid (Ichthyolestes), and a possibly Ambulocetus-like genus (Gandakasia) were originally described as mesonychids by Pilgrim (1940) and by Dehm and Oettingen-Speilberg (1958). These were included in Mesonychidae by Szalay and Gould (1966), although they are now known to be primitive archaeocetes rather than mesonychids.
Some Mesonychidae like Asian late Paleocene Sinonyx (Zhou et al. 1995) and North American early Eocene Pachyaena (Zhou et al. 1992; O'Leary and Rose 1995) are known from virtually complete skeletons showing them to be hoofed cursorial mammals with no aquatic specializations and no distinctively cetacean characteristics (they have ossified auditory bullae, but lack in particular the dense enlarged bullae with sigmoid processes seen in Pakicetus). Thus we distinguish Cetacea from non-Cetacea at a gap between Mesonychidae and Pakicetidae. This gap is not a rigid boundary, but one subject to revision in light of new discoveries (postcranial remains of pakicetids will be critical here): the connection may become weaker if similarities in form that we see now are in the future overshadowed by similarity to some other group, or the connection may become stronger if new similarities are discovered that reinforce it (similarity in any particular case, like continuity, is always relative to that in competing cases).
What makes a primitive archaeocete like Pakicetus (Gingerich and Russell 1981) or Ambulocetus (Thewissen et al. 1994) a whale, when mesonychids like Sinonyx and Pachyaena are not whales? Berta (1994) asked: "What is a whale?". Gish (1994) asked: "When is a whale a whale?" And a third question might be: "Where is a whale a whale?" It is not possible to answer one question without the others, and the answer to all three is that a primitive early fossil whale is a whale when continuity in temporal, morphological, and geographical range connects it to living whales—ideally through a closely-connected series of temporal, morphological, and geographical intermediates—and its form shows one or more of the specializations of whales. The former reflects the "shared" and the latter the "derived" components of synapomorphy diagnosing Cetacea. In our view as paleontologists, whales became whales when they first showed evidence of the evolutionary transition in grade from terrestrial to aquatic life characteristic of living cetaceans.