Neoceti is the taxonomic group containing the two living clades of Cetacea (Odontoceti and Mvsticeti), but excluding Archaeoceti. The two living groups are quite disparate, each distinguished by a unique combination of anatomical and ecological attributes. The Odontoceti (toothed whales, dolphins, porpoises) are echolocating macropredators, whereas Mysticeti (baleen whales) are filter feeders. Ancient cetaceans from Oligocene times (25 to 30+ Ma) also show skull structures indicative of echolocation in odontocetes and of filter feeding in mysticetes, emphasizing the early divergence of feeding habits. Apart from the feeding apparatus, however, basal odontocetes and mysticetes are much more similar to one another than are their modem descendants. Similarities include some evolutionary novelties (synapomorphies) of the skull, which are not seen in archaeocetes. Thus, odontocetes and mysticetes are regarded as sister taxa, forming a clade variably termed crown group Cetacea, or Neoceti, or Autoceta. Basal odontocetes and mysticetes also show marked similarities with archaic cetaceans (Archaeoceti), pointing clearly to an origin within the archaeocete family Basilosauridae.
I. Changing Concepts of Names
The name Cetacea was first used in a modern sense by Bris-son in 1762 for genera and species of living whales, dolphins, and porpoises. Until the mid-1800s, high-level classification was based on superficial ieatures, with no implication that patterns among living cetaceans had arisen by evolution. Formal subdivisions for the living baleen whales and toothed cetaceans were proposed by W. H. Flower for Mysticeti (1864) and Odontoceti (1867). with the implication that these were real groups (in modern terms, clades). The discovery of fossils broadened the concept of Cetacea in the early to mid-1800s. Initially, most fossils were recognized as related to living species, and modem generic names (e.g., Delphinus, Balaena) were applied to some of these. The discover)’ of the archaic Eocene whale Basilosaurus in the 1830s eventually led in 1883 to naming of a formal group of archaic cetaceans, the Archaeoceti. Thus, the concept of Cetacea was expanded to include three suborders: one extinct (Archaeoceti) and two living (Odontoceti and Mysticeti).
II. The Monophyly of Odontoceti
Odontocetes include 71-72 living species in the families Physeteridae, Kogiidae, Ziphiidae, Platanistidae, Delphinidae, Phocoenidae, Monodontidae, Iniidae, Pontoporiidae, and Lipotidae. Strictly defined, the Odontoceti comprises the most recent common ancestor of all living species, plus all the descendants of that ancestor. Such an ancestor probably lived in latest Eocene or Early Oligocene times. In practice, fossil and recent odontocetes are distinguished by osteological features, particularly in the skull. For example, above the eye, a large supraorbital process in each maxilla rises posteriorly over the frontal, usually forming a voluminous facial fossa in which open dorsal infraorbital foramina for nerves and blood vessels; in living species, this fossa forms the origin for the nasofacial muscles, which manipulate diverticula or sacs in the soft nasal passages. In turn, the diverticula probably help produce echolocation sounds. Where the rostrum passes into the facial fossa, a vertical antorbital notch forms a path for the facial nerve, which supplies the nasofrontal muscles. Anteriorly, each side of the rostrum in iront of the bony nares has a premaxillary sac fossa, premaxillary foramen, and usually premaxillary sulci; in living species, these are also implicated in sound generation in the nasal passages. Below the face, the infraorbital process is vestigial or absent, and the most posterior tooth lies far forward of the antorbital notch. In the ear region on the skull base, the parietal and squamosal roof the periotic so that this ear bone no longer contributes to the floor of the brain-case. Finally, all odontocetes that are well-preserved show evidence of a small middle sinus extending laterally from the ear toward the jaw joint in the glenoid cavity.
The only serious challenge to odontocete monophyly has come from recent molecular analyses, which placed the sperm whale, Physeter macrocephalus, closer to living mysticetes than to other odontocetes. Such a relationship would make the Odontoceti paraphyletic. Reanalyses, based both on anatomy and on molecules, confirmed that the sperm whale lies within the Odontoceti and that the group is a clade.
III. The Monophyly of Mysticeti
Mysticetes include 13-14 living species in the families Bal-aenidae, Neobalaenidae, Balaenopteridae, and Eschrichtiidae. Mysticete monophyly has never been in serious doubt (cf. Physeter and odontocete monophyly). Strictly, the Mysticeti comprises the most recent common ancestor of all living species, plus all the descendants of that ancestor. Plausible ancestors of latest Eocene and Early Oligocene age are known. Unlike living species, fossil mysticetes are not recognized by the presence of baleen; although this probably occurred in most extinct species, it preserves rarely. Rather, osteological features, particularly in the skull, distinguish the Mysticeti. The rostrum is relatively large, with thin edges and a flat broad lower surface. The main bones in the rostrum (vomer, premaxilla, and maxilla) are sutured loosely with each other and, posteriorly, with the cranium. The lacrimal is also loosely sutured, between the frontal and a prominent transverse preorbital ridge 011 the maxilla. Loose sutures between the feeding apparatus and cranium account for the common loss of the rostrum in fossil mysticetes; perhaps such sutures function in skull kinesis during filter feeding. Ventrally, the maxilla is usually toothless, forming an origin for baleen, but in archaic forms the maxilla may have teeth that lie well forward of the orbit (cf. ar-chaeocetes). Posteriorly, the maxilla extends toward the orbit, forming a prominent infraorbital plate below the frontal. Finally, the halves of the mandible are joined by ligaments at a short symphysis. Other putative diagnostic features of the skull are seen in most, but not all, mysticetes, as noted later for archaic forms.
Figure 1 Changing concepts of the Cetacea. (A) Preevolntionary classification as used by Jardine and others, early to mid-1800s. Species are clustered on the basis of sometimes superficial features. Genealogical relationships are not particularly implied, (B) Widely held concept of cetacean diphyly, as used by Slijper in the 1960s, and supported by many other cetologists (based on Slijper). (C) Widely cited but now abandoned concept of relationships between living Odon-toceti and Mysticeti showing the sperm whale, Physeter macrocephalus, as more closely related to living mysticetes than to other odontocetes (based on Milinkovitch). (D) Current concept of Cetacea showing crown group Cetacea (Neoceti) with two sister taxa, Odontoceti and Mysticeti, and a cluster of progressively more stemward archaeocete groups (based on Uhen and others). Grade taxa are shown with gray infill. (E) Summary of relationship between later archaeocetes, the Basilosauridae, and Odontoceti + Mysticeti.
Figure 2 Morphological similarities and differences among later archaeocetes and basal odontocetes and mysticetes. (A) Archaeocete. Lateral, dorsal, and ventral views of the archaeocete skull show the dorudon-tine Zygorhiza kochii (Basilosauridae: Dorudontinae: Priabonian, latest Eocene), based on Kellogg (1936). Dorsal view of the archaeocete mandible shows the dorudontine Saghacetus osiris (Basilosauridae: Dorudontinae: Priabonian, latest Eocene), based on Stromer (1908). (B) Archaic mi/sticete. Lateral, dorsal, and ventral views of skull and dorsal view of mandible show Chonecetus geodertorum (Mysticeti: Aetiocetidae; Chattian, Late Oligocene), based on Barnes et al. (1995), with the addition of some features. Figures of teeth are not available. (C) Basal platanistoid odontocete. Lateral, dorsal, and ventral views of skull and dorsal view of mandible show Waipatia maerewhenua (Odontoceti, Waipatiidae; Chattian, Late Oligocene), based on Fordyce (1994),with modifications, including the addition of some features. Some teeth are in situ on the original fossil, but for simplicity are not shown in other than the lateral view.
IV. The Monophyly of Odontoceti + Mysticeti
In the later 1800s and indeed until the 1960s, the known archaeocetes and fossil mysticetes and odontocetes seemed rather divergent from one another. Slijper, Kellogg, and other influential cetologists doubted a close relationship between odontocetes and the other two groups and were uncertain about mvs-ticete origins among the archaeocetes. Thus, the two living groups of cetaceans were regarded as diphyletic, of different ancestiy. They were sometimes classified as distinct orders.
From the 1970s to 1990s, several major advances overturned the notions of diphyly, and ultimately changed cetacean nomenclature. The fossil record of Eocene archaeocetes and of early (especially Oligocene) odontocetes and mysticetes expanded markedly, helping to bridge the structural and strati-graphic “gap” among the three groups. It became clear that evolution is not always slow and gradual and that major structural change can occur in short geological intervals. Developments in the deep sea drilling project led to much improved geological correlation, helping to date and clarify evolutionary sequences. Molecular and biochemical approaches to phy-logeny indicated close relationship between odontocetes and mvsticetes. The rise of cladistics (phylogenetic svstematics) also clarified many concepts of relationship and nomenclature.
The Odontoceti and Mysticeti are now widely regarded as forming a clade or monophyletic group, equivalent to the Cetacea in the sense of Brisson and, indeed, of many modern systematics. Strictly, this group is crown group Cetacea; it includes all descendants, living and extinct, of the most recent common ancestor of Odontoceti + Mysticeti.
Odontocetes and mysticetes do share bony features not seen in archaeocetes, supporting their sister group relationship. In both, the posterior portion of the maxilla is at least slightly concave, rather than smoothly convex, and carries one or more dorsal infraorbital foramina that open dorsally rather than anteriorly. On the rostrum, an open mesorostral groove extends far anteriorly so that the premaxillae have little or 110 contact in the midline. The posterior-most teeth in odontocetes and toothed mysticetes lie anterior to the antorbital notch. Most (not all) basal species are polydont, with more than the usual mammalian number of cheek teeth, and a tooth succession is unknown. Compared with basilosaurid archaeocetes, the zygomatic process of the squamosal is more robust and anteriorly produced, with a more delicate jugal. Finally odontocetes and mysticetes are amastoid, with the posterior (mastoid) process of the periotic not exposed laterally on the skull wall.
Odontocetes and mysticetes are often identified as having a “telescoped” skull in which bone positions have moved dramatically relative to familiar mammalian landmarks such as the nose and eye. However, “telescoping” is a wide-ranging term applied to at least four different functional shifts involving both the facial region and the braincase. It should not be cited to support the monophyly of odontocetes and mysticetes.
V. Primitive Features in Basal Odontocetes and Mysticetes
Some early fossil odontocetes and mysticetes have features similar or even identical to those seen in some basilosaurid archaeocetes. Such fossils include, among odontocetes, species of Xenorophus, Mirocetus, and Archaeodelphis, whereas mysticetes include species of Aetiocetidae, Mammalodon, Llanoce-tus, and some unnamed taxa. (Perhaps some of these are stem rather than crown group members; e.g., Archaeodelphis possi-blv represents a stem odontocete.) The most obvious basilosaurid-like features of archaic odontocetes and mysticetes are the prominent intertemporal constriction, formed by elongate parietals dorsally 011 the braincase, and heterodont teeth. In all, the anterior teeth have single roots and simple crowns clearly distinct from two- or three-rooted cheek teeth with complex denticulate crowns. Multiple denticles on the crown are an evolutionary novelty linking basilosaurids, odontocetes, and mvsticetes. Further, the posterior mandibular cheek teeth in archaic odontocetes have a distinctive anterior vertical groove, as in basilosaurids. (The loss of the last upper molar, M3, in basilosaurids, has been used to dismiss a basilosaurid origin for odontocetes and mysticetes. However, a widely variable tooth complement in the latter and the likelihood that polydonty involved an increase in the number of mid- to posterior cheek teeth would allow a basilosaurid origin.)
Other parts of the feeding apparatus are revealing. In basilosaurids and basal odontocetes and mysticetes, the mandible has a large mandibular fossa (reduced in more crown-ward mysticetes), and the temporal muscle has a distinct vertical origin 011 the frontal. (This origin changes dramatically in most odontocetes, becoming overridden by facial bones and, in mysticetes, migrating over the orbit.)
In all groups, the complex of foramina in the orbit are not tightly clustered (as in living species) but are scattered antero-posteriorly. Ventrally, in the skull base, an enlarged subspheri-cal pterygoid sinus fossa is formed by alisphenoid and pterygoid. (Such a fossa is absent in Protocetidae and more basal ar-chaeocetes and becomes more elaborate in more crowned Odontoceti and Mysticeti.) Common features in the ear region include a lack of fusion between periotic and tympanic bulla (fusion occurs in later mysticetes), laterally compressed processes on the periotic (becoming more inflated in many later odontocetes), and a rather low squat tympanic bulla (becoming smoothly rounded in later mysticetes, but more elevated and delicate in later odontocetes). Other basilosaurid features include persistent postparietal foramina in the braincase and prominent exoccipital condyles.
VI. The Paraphyly of Archaeocetes
Because odontocetes and mysticetes (= crown group Cetacea) arose from among basilosaurid archaeocetes, the suborder Archaeoceti is paraphyletic. Archaeocetes form an artificial cluster of cetaceans that lack the features of Odontoceti or Mysticeti. (Many cladists would not recognize archaeocetes as a formal group and would use the term “stem group Cetacea” as an alternative to Archaeoceti.) Note that this expands the concept of Cetacea beyond the crown group, and indeed expands it beyond the concept used by Brisson.
