Cetaceans descended from terrestrial mammals around 50 million years ago (Thewissen et al., 2001; see also review in Uhen, 2007, this issue). The cetacean sister group is one of the artiodactyls (e.g., Luckett and Hong, 1998; Milinkovitch et al., 1998; Gatesy and O'Leary, 2001; Gingerich et al., 2001; O'Leary, 2001; Thewissen et al., 2001; Geisler and Uhen, 2003), and among the extant artiodactyls, hippopotamids are the most likely sister group of cetaceans (Nikaido et al., 1999; see also Fisher et al., 2007, this issue). The order Cetacea can be characterized by their ears (Berta, 1994). The transition from land to water involved a great number of changes in individual organ systems (see Berta et al., 2006). These different changes happened in less than 10 million years (Thewissen and Williams, 2002). The sound transmission mechanism underwent pervasive anatomical changes that are well understood functionally (Nummela et al., 2004a).
Early whale evolution is well documented in the fossil record (see reviews by Fordyce and de Muizon, 2001; Thewissen and Williams, 2002). The Eocene cetacean families Pakicetidae, Ambulocetidae, Remingtonocetidae, paraphyletic Protocetidae, Basilosauridae and paraphyletic Dorudontidae represent respective higher branches on the phylogenetic tree of Cetacea (O'Leary, 2001: Geisler and Sanders, 2003; Geisler and Uhen, 2003).
Regarding pakicetids, Gingerich and Russell (1981) and Gingerich et al. (1983) provided descriptions of the braincase and the auditory region of Pakicetus inachus, and analyzed pakicetid hearing mechanisms in air and water. Their interpretation was that Pakicetus was already specialized in aquatic hearing. Oelschläger (1986a, b, 1987, 1990) expanded this work and made further functional inferences. Thewissen and Hussain (1993) described a Pakicetus incus, the oldest middle ear ossicle known for cetaceans, and showed that the Pakicetus middle ear is land mammal-like and resembles the modern artiodactyl incus in relative length of its crura. Luo (1998) and Luo and Gingerich (1999) studied the pakicetid ear structures, and added further details to the functional analyses of the transformation of the cetacean basicranial structures during the evolution of whales. Nummela et al. (2004a, 2006) provided detailed descriptions of the largest collection of cranial material of all three pakicetid genera; the corresponding postcranial material is described by Madar (2007). Regarding ambulocetids, Thewissen et al. (1994, 1996) described this new family while describing the genus Ambulocetus. Much of the ear material of Ambulocetus is poorly preserved.
Kumar and Sahni (1986) described the ear region of Remingtonocetus, within their new family Remingtonocetidae, making suggestions of the hearing mechanism of this species. Nummela et al. (2004a) expanded the study of remingtonocetid ear and hearing, with new cranial material, tympanic bullae, and all three auditory ossicles, and found the remingtonocetids to be the first cetacean family with a whale hearing mechanism for underwater hearing. The sound path from water to the inner ear passed through the lower jaw and mandibular fat pad to the tympanic plate, a lateral wall of the tympanic bone, to the ossicular chain, and the oval window. This mechanism was fully functional, although not yet very sophisticated.
The ear morphology of protocetids has been described for Georgiacetus (Hulbert et al., 1998), Rodhocetus (Gingerich et al., 1994), Gaviacetus, Takracetus, and Dalanistes (Gingerich et al., 1995). Luo (1998), and Luo and Gingerich (1999) described ear morphology of Gaviacetus and Indocetus and discussed their function and hearing abilities. Nummela et al. (2004a) described the hearing function of protocetids, including Indocetus tympanic and malleus. Geisler et al. (2005) described the ear structures and the mandible for Carolinacetus. Regarding basilosaurids and dorudontids, Kellogg (1936) described their auditory morphology in detail, but he did not address functional aspects of the fossil whale ear.
The most explicit study of functional character transformations in early whales was by Lancaster (1990), building on earlier work of Fleischer (1978). At the time when Lancaster wrote his study, early cetacean ears with functional interpretations were only known for basilosaurids and dorudontids, and these Eocene whales were included in the hypothetical transformation series he proposed for the whale ear. Lancaster determined that the late Eocene whale ear was a highly derived hearing mechanism capable of sound transmission underwater. Uhen (1998, 2004), Luo (1998), Luo and Gingerich (1999), and Nummela et al. (2004a) added functional interpretations based on the morphology of the lower jaw and the ear region, which indicate that these animals heard well in water, and had already relatively good directional hearing, but did not yet echolocate.