The most highly aquatically adapted group of marine mammals are the mammalian order Cetacea, which are commonly called whales, dolphins, and porpoises. Living cetaceans are a diverse group of mammals whose specialized anatomy and behaviors mask their origin from terrestrial mammals. Ranging from under 5 feet to over 90 feet in length, cetaceans are ecologically diverse; they live in rivers and oceans and in tropical and polar latitudes. About 78 species live today, classified in 13 or 14 families, and in the suborders Mysticeti (baleen whales) and Odontoceti (toothed whales, dolphins, and porpoises). Hundreds of fossil species are known, going back to the Middle Eocene, about 55 million years (Ma) ago, including members classified in an extinct third suborder, the Archaeoceti.
I. Suborder Archaeoceti: Archaic Cetaceans
The oldest Cetacea are the toothed, extinct suborder Archaeoceti, a paraphyletic group. Archaeocetes are clearly related to living Cetacea; they show typical cetacean features such as an elongate upper jaw with bony nostrils set back from the tip, a broad shelf of bone above the eye, lack the incisive foramina, incisors aligned anteroposteriorly, a dense outer ear bone (tympanic bulla), and an enlarged mandibular canal on the inner side of the lower jaw; later archaeocetes have an expanded basicranial air sinus, like living cetaceans. Archaeocetes lack the “telescoped” skull, in which bones of the snout and cranium slide over one another, as in the later odontocetes and mysticetes.
Sinuous eel-like bodies have been depicted by authors and artists, especially for the later archaeocetes, but this would be contradictory to the vertical tail movements and caudal flukes typical of other cetaceans. A quite whale-like body shape can be reconstructed around the skeletal outlines of even the longest of the basilosaurine archaeocetes. Stomach contents of Basilosmtms cetoides suggest that it was an ambush predator on bony fishes and small sharks in shallow water. There is fossil evidence that most, if not all, archaeocetes had external hindlimbs. Early archaeocetes have well-developed pelves, sacral vertebrae, and hindlimb bones. The smaller yet still-functional hind limbs of Basilosaurus isis may have aided locomotion in shallow waters. Archaeoceti also have a primitively mobile elbow joint, with a long humerus and less streamlined foreflipper than in extant Cetacea. The more primitive forelimbs might indicate that they were semiamphibious-like pinnipeds.
Most older archaeocete fossils are from India and Pakistan, in sediments deposited around the ancient Tethys Seaway, which extended between India and Africa to the south and Eurasia to the north. Since 1980, new fossils have greatly expanded our knowledge of protocetids and other early archaeocetes.
A. Family Pakicetidae: Amphibious Early Cetaceans
Nalacetus is mainly known from isolated teeth, which show that it is the most primitive cetacean. Pakiceius was a small archaeocete with eyes on top of its head. Oxygen isotope ratios in its tooth enamel indicate that it drank only fresh water.
B. Family Ambulocetidae: Walking Whales
Ambulocetiis had feet that were much longer than its hands and a long tail that probably was without flukes.
C. Family Remingtonocetidae: Gavial-Convergent Cetaceans
Remingtonocetus and Artdrewsiphius had a long and narrow skull, very tiny eyes, and a long symphysis (junction) between the two lower jaws.
D. Family Protocetidae: First Pelagic Cetaceans
Indocetus had large molars that have a complex, primitive morphology. Rodhocctus was approximately 3 m long and had nostrils near the front of the snout. It retained a large pelvis and small hind legs. Protocetiis had a skull that lacks the complex tooth crowns and expanded basicranial air sinuses of later archaeocetes. It had a small pelvis and flexible posterior part of the trunk.
E. Family Basilosauridae: Zeuglodonts
BasUosaums was a large whale (approximately 20 m). It had cheek teeth with many small denticles (saw-tooth like) and had enlarged air sinuses around the ears. Basibsaums also had large and elongate vertebral centra and short hindlimbs.
Sometimes included in the basilosaurids, but sometimes treated as a separate family are the Donidontinae, including Donidon, Saghaceftis, and Zygorhiza.
II. Suborder Mysticeti: Baleen Whales
Mysticetes are small to very large filter-feeding whales with a loose mandibular symphysis and a type of jaw articulation that allows the mandible to open widely for bulk feeding. All mysticetes also have a smooth ventral surface of the maxilla (the palate), which in the baleen-bearing species has grooves that hold the blood vessels that nourish the baleen plates, an infraorbital plate of the maxilla ventral to the eye, and a laterally projecting antorbital process of the maxilla. Mysticeti are a natural clade that originated from the Archaeoceti. The oldest mysticete is Late Eocene in age and the group survives today, having been represented throughout most of its history by at least five or six contemporaneous family-level groups.
Just a few years ago, it would have seemed paradoxical to need to differentiate between baleen-bearing mysticetes and tooth-bearing mysticetes, the latter seemingly a misnomer. However, this need was brought about largely by the discovery of the Aetiocetidae and other Late Eocene and Oligocene whales that are transitional between the toothed Archaeoceti and the later baleen-bearing mysticetes. There is as much recognized diversity now at the family level among both named, and known but undescribed, fossil tooth-bearing mysticetes as there is among the fossil and living baleen-bearing mysticetes.
The primitive, extinct tooth-bearing mysticetes had het-erodont dentitions, differentiated into incisors, canines, premolars, and molars, and these animals are classified in the families Llanocetidae, Kekenodontidae, Mammalodontidae, and Aetiocetidae, and in possibly two other undescribed families. Some of these families had skull characters that were very much like those of archaeocetes, thus confirming the origin of baleen whales from them.
More highly evolved toothed mysticetes, such as the Aetiocetidae, had skulls and jaws that were similar to the later baleen-bearing mysticetes, e.g., the cetotheriids. so they represent a transitional morphology. In many ways, the “intermediate” structure of aetiocetids helps us understand how baleen-bearing mysticetes might have evolved from archaeocetes. A possibly unanswerable question is which toothed mysticete taxon was the first to have rudimentary baleen plates between its teeth. It is most parsimonious to assume that baleen originated only once in cetacean evolution.
General evolutionary trends among the later Mysticeti include the acquisition of large body size, shortening of die intertemporal region as anterior and posterior parts of the skull “telescope” over and under each other, reduction of the coronoid process and mandibular foramen, lateral bowing of the mandibles, and shortening of the neck. Mysticetes are virtually cosmopolitan in dieir distribution, being found both in the past and now from polar to equatorial latitudes, with fossils known from all ocean basins. The great increase in size among baleen-bearing mysticete whales was a phenomenon of Miocene and later time, with the largest known species being among the living species.
A. Family Llanocetidae: Archaic-Toothed Mysticetes
Llanocetus had a large triangular braincase and its brain was shaped much as in later mysticetes. It had a deep mandible with widely spaced, large, multicusped teeth.
B. Family Kekenodontidae: Toothed Mysticetes
Kekenodon had long-rooted, multicusped cheek teeth, with large accessory cusps.
C. Family Mammalodontidae: Southern Ocean-Toothed Mysticetes
Mammalodon was a toothed mysticete with a strangely abbreviated and blunt rostrum. The posterior end of its mandible was deeper and more heavily built than that of the more highly derived Aetiocetidae. It had more than the primitive eutherian number of teeth per jaw (11). The teeth crowded each other, were implanted somewhat obliquely in the mandible, and had small accessory cusps.
D. Family Aetiocetidae: Small-Toothed Mysticetes
Aetiocetidae had heterodont denititions that were differentiated as incisors, canines, premolars, and molars, with small cuspules on comparatively small molar and premolar crowns. Chonecetus had a small braincase and relatively narrow rostrum. Moratvanocetus had large molars, a wide and dorsoven-trally flattened braincase, large tympanic bullae, and small, anteroposteriorly compressed cervical vertebrae. Aetiocetus was of relatively large size, with wide flat rostrum, anteroposteriorly foreshortened intertemporal region, and large zygomatic process of the squamosal. Some Aetiocetinae had increased numbers of teeth (as do embryonic modern baleen whales) and smaller teeth, possibly indicating the transition to incipient baleen plates.
E. Family Cetotheriidae: Primitive Baleen-Bearing Mysticetes
Cetotheriidae were fiat-snouted, edentulous, baleen-bearing mysticetes. Their palates had characteristic grooves marking the courses of blood vessels that nourished the baleen plates, and fossil baleen has even been found with some specimens. The dentaries were elongate and relatively slender and had nutrient foramina along the dorsal border marking the previous locations of dental alveoli. Cetotheres had relatively small heads compared to their body size, similar in this regard to Recent rorquals of the family Balaenopteridae.
Herpetocetus and Nannocetus are characterized by their dentary-squaniosal articulation: the zygomatic process of the squamosal is elongate anteroposteriorly and deepened dorsoventrally, and the angular part of the dentary projects posteriorly ventral to the postglenoid process and the ear region.
F. Family Balaenopteridae: Rorquals and Relatives
Balaenopterids have a deep transverse sulcus between the rostral portion of the maxilla and the supraorbital process of the frontal. They also have an elevated cranial vertex (relative to the supraorbital process of the frontal), a tapered and somewhat twisted (in derived taxa) horizontal ramus of the dentary, and a large, spheroid mandibular condyle. They also lack any transverse exposure of the parietals at the apex of the skull posterior to the nasal bones.
G. Family Eschrichtiidae: Gray Whales
Eschrichtius robustus is characterized by a relatively narrow and dorsally arched rostrum with large nares that are located at the apex of the curve, two tuberosities 011 the braincase near the nuchal crest, and a massive mandible with a low coronoid process and a large rounded mandibular condyle.
H. Family Balaenidae: Right Whales, Bowheads, and Relatives
Morenocetus, a fossil, probably had an upper jaw that was narrow and arched. Living balaenids have extremely long baleen plates, similar to those of the extant Balaena mi/sticetus.
I. Family Neobalaenidae: Pygmy Right Whales
Extant Caperea marginata is very small, has expanded ribs, an extremely large occipital shield of the braincase, a short and tapered rostrum, and tapered and twisted mandible.
III. Suborder Odontoceti: Toothed Cetaceans
Odontocetes are echolocating toothed cetaceans that have extensions of the basicranial sinuses, around their ears, into other parts of the skull. The large pterygoid sinus extends anteriorly around the sides of the nasal passages and, in derived taxa, in front of them. The Odontoceti are known as far back as the Eocene-Oligocene boundary, approximately 35 Ma.
Odontoceti are commonly called “toothed whales,” but this is a bit of a misnomer because all of the Archaeoceti, some of the primitive fossil Mysticeti, and all fetal baleen-bearing mysticetes are toothed, whereas some Odontoceti had undergone dramatic tooth loss. Therefore, “presence of teeth” is not diagnostic of the Odontoceti. The earliest odontocetes have a primitive dental formula, differentiated into incisors, canines, premolars, and molars. Later odontocetes have all teeth alike, homodonty, and some taxa have additional teeth (polydonty) whereas others have reduced numbers of teeth.
Similarly, Odontoceti are commonly thought of as having asymmetrical crania, but this is not true for all Odontoceti, especially many of the fossil species, and, again, is not diagnostic of the group. Cranial asymmetry, a derived character state, is related to the development of complex echolocation abilities involving the production of both high- and low-frequency sound. Different odontocete species have either symmetrical or asymmetrical crania, and asymmetry is a derived character state that apparently has been independently acquired within different lineages at different times. In most odontocetes having asymmetrical crania, the skew is to the left side, but the skulls of members of the extinct kentriodontid delphinoid subfamily Pithanodelphinae are skewed to the right side.
The type of cranial bone telescoping of odontocetes differs from that of the mysticetes; the maxilla extends posteriorly over die supraorbital process of the frontal and there is no laterally projecting antorbital process of the maxilla. The pterygoid sinus fossa in Odontoceti extends from the ear region anteriorly around the nasal passage to invade the posterior part of the palatine bone. In the earliest Odontoceti, the nasal bones primitively were elongate, flat, and overhung the posterior part of the nar-ial opening, as in the derived Mysticeti. In later Odontoceti, the nasal bones are reduced, being knob-like and retracted posterior to the nasal opening. All Odontoceti have a premaxillary foramen, a branch of the infraorbital foramen system, located in each premaxilla anterior to the narial opening. Around or anterior to the narial opening is a widened and flattened part of the premaxilla where the premaxillary sac, a diverticulum of the dorsal nasal passage, lies. This flat part of the premaxilla is called the premaxillary sac fossa or the spiracular plate.
Odontocetes have the ability to echolocate and to selectively take individual prey items. This separates them from the Mysticeti, which cannot echolocate and are bulk feeders. During echolocation, high-frequency sound, in the form of clicks, is produced by the movement of air, recycled within the diverticula, sacs, and valves of the nasal passages. The sound is focused by the melon on the face, which is an acoustic lens, and projected into the environment. Sound is reflected off of objects and animals in the water and is returned to the odontocete. The external acoustic meatus is closed; sound is instead transmitted to the ears via the side of the face, through the thin posterior part of the dentary, through a fat body, and thence to the ear region. Directional hearing is possible because the ear bones are isolated in fat bodies, and the sound arrives at each ear at a different time. Even the earliest odontocetes had spiracular plates and enlarged peribullary sinuses, and this implies that they could actively echolocate, at least to some extent.
Throughout their history, odontocetes have been more diverse taxonomically than mysticetes, with several different families existing simultaneously. Different families of odontocetes are diagnosed particularly on details of sutures around the bony nostrils and the base of the rostrum, the basicranial sinuses, and the ear region. Trends among odontocetes include the evolution of paedomorphism (adults showing a spectrum of juvenile-like features), repeated convergent evolution of very long rostra, increasing facial asymmetry, development of more complex basicranial sinuses, and shortening of the neck.
A. Family Agorophiidae: Primitive Toothed Whales
Agorophius was a medium-sized odontocete with heterodont dentition, wide mesorostral groove, elongate nasal bones, very large temporal fossae bordered by large cranial crests for the attachment of very massive jaw musculature, and a relatively elongate and narrow intertemporal constriction.
B. Family Physeteridae: Giant Sperm Whales
In physeterids the periotic is distinctive, having a large posterior process, a round internal acoustic meatus, and a part of the outer whorl of the bulla (called the “accessory ossicle”) that is attached to the anterior process of the periotic. The teeth of all physeterids are homodont. Ferecetotherium had both upper and lower functional teeth. Diaphorocetus had a skull with the distinctive physeterid type of supracranial basin.
Scaldicetus had a relatively long, slender rostrum, large zygomatic arches, large tympanic bullae, prominent occipital condyles, and a forward-sloping occipital shield that is deeply emarginated laterally by large temporal fossae. It appears to have had 12 teeth on each side of the mandible. The living sperm whale, Physeter nwcrocephalus, lacks upper teeth.
C. Family Kogiidae: Pygmy and Dwarf Sperm Whales
Kogiids and physeterids share highly asymmetrical crania, supracranial basin, derived lachrimal-jugal structure, and a large mastoid process. Kogiids differ from physeterids by having lesser development of the melon and thus a less concave facial skeleton. In addition, the kogiid skull has a blunt and squared zygomatic process of the squamosal and an anteropos-teriorly aligned bony crest or eminence in the middle of the facial basin of the skull.
Extant Kogia have skulls that are largely composed of spongy and inflated bone, with very short and tapered rostra, no upper teeth, an elevated cup or basin on the sagittal crest in the supracranial basin, and the orbit placed posteriorly within the temporal fossa.
The latest Miocene fossil pygmy sperm whale. Praekogia, differed from the Recent species of Kogia by having dense rather than spongy (=oil-filled) cranial bone, cranial crests that are not so elevated, and the orbit situated anterior to the end of the zygomatic process of the squamosal (the typical cetacean condition), not posteriorly within the temporal fossa.
Scaphokogia had an elongate and narrow skull, a long, parallel-sided, thick, dense snout, and nearly circular supracra-nial basin.
D. Family Ziphiidae: Beaked Whales
Ziphiidae have reduced the number and size of teeth. Male modern ziphiids generally retain one or two prominent teeth in the lower jaw, but primitive species showed rows of small dolphin-like teeth.
E. Family Squalodontidae: Shark Toothed Dolphins
Squalodontidae were medium-sized to large, dolphin-like cetaceans with long rostra and a relatively primitive, heterodont dentition composed of incisiform anterior teeth and premolars and molars commonly widi triangular crowns and two roots. Their skulls were fully telescoped, the narial openings being located between the eyes, and the premaxillae and maxillae extending posteriorly to reach die cranial vertex. In Patriocetus and Kelloggia, die skull had a narrow intertemporal region. In Sqnalodon and related genera, the skull had a more disk-like facial region.
F. Family Eurhinodelphinidae
Eurhinodelphinidae (also called Rhabdosteidae) were medium to large sized and have fully telescoped skulls with extremely long, narrow rostra bearing many small, homodont teeth with single roots and conical crowns. The vertebrae are elongate and the neck is flexible. The lower jaw was shorter than the upper jaw. The front part of the upper jaw that overhung carried no teeth.
Argyrocetus and Rhabdosteus had narrow facial regions, but Eurhinodelphis had a wide, rounded facial region. Macrodel-phinus attained the size of a living killer whale.
G. Family Waipatiidae
Waipatia was a dolphin-like cetacean with heterodont teeth and a relatively wide rostrum. The cranial vertex was slightly asymmetrical.
H. Family Squalodelphinidae
Squalodelphinids were medium-sized cetaceans with stout rostra bearing homodont denititions composed of conical-crowned teeth that somewhat resembled those of modern dolphins, but which had somewhat rugose enamel caps. The cranial vertex was asymmetrical and there was a moderate-sized elevated crest aligned anteroposteriorly over each orbit.
I. Family Dalpiaziniidae
Dalpiazinia had a nasal region and cranial vertex that were symmetrical, the rostrum was relatively short, and the dentition was homodont.
J. Family Acrodelphinidae
Acrodelphinids (also called Eoplatanistidae) were medium-sized cetaceans with relatively large vertebrae and pectoral limbs, and on the skull there was a relatively low anteroposteriorly aligned crest on top of the supraorbital process of the frontal. The pterygoid bone was continuous with the alisphe-noid bone to form a thin, vertically oriented sheet of bone within the back of the orbit (similar to platanistids, a so-called “reduplicated pterygoid). The lower jaw was the same length as the rostrum and had an elongate groove on each side, mirroring a similar groove on each side of the rostrum. Such grooves are also present in some Pontoporiidae.
K. Family Platanistidae: Indian River Dolphin (Platanista gangetica) and Relatives
Platanistidae have a generalized vertebral column, but a highly modified foreflipper. The skull has tiny eyes with reduplicated pterygoids. The narial region and cranial vertex show strong left-skew asymmetry. The jugal is thick, and the zygomatic process of the squamosal is dorsoventrally expanded and medially excavated. The maxillary crests are large and form a parabolic surface on the facial region, “accessory ossicles” surround bulla and periotic, and the rostrum is long and transversely compressed with elongate anterior teeth and short back teeth.
L. Family Iniidae: Amazon River Dolphin and Relatives
Inia geoffrensis has a delphinid-like basicranium and ear region, but its combination of elevated maxillary crests, premaxillary eminences, asymmetrically left-skewed narial region, elevated cranial vertex, and elongate zygomatic processes of the squamosals differentiate it from other families of Odontoceti.
M. Family Pontoporiidae: Franciscana and Relatives
Pontist.es, Pliopontos, and the living Pontoporia blainvillei have symmetrical crania and long rostra. Brachydelphis was extremely short snouted, but its cranium was otherwise similar in construction to those of the Pontoporiinae and was symmetrical around the narial region. Parapontoporia (also classified as a lipotid) was extremely long snouted and had 77 to 88 teeth on each side of the slender rostrum and mandible. Fossil species are known from 8 to 3 Ma ago in the North Pacific.
N. Family Lipotidae: Yangtze River Dolphin
Lipotes vexillifer shares important derived characters of the cranial vertex and facial region with Parapontoporia. Its post-cranial morphology is relatively primitive, but its skull has several unique derived characters, including crests on the supraorbital process of the maxilla over the orbit, a stout rostrum, a left-skewed highly asymmetrical nasal region, and a highly elevated asymmetrical cranial vertex.
O. Family Kentriodontidae: Primitive Delphinoid Dolphins
Kentriodontids were small to medium-sized fossil dolphins. The cranial vertices of all kentriodontids, with the exception of Pithanodelphis and Atocetus, were symmetrical, the skulls were fully telescoped, and their dentitions were more or less homodont. Kamphohphus was relatively large, with a large skull that bore prominent cranial crests, and large teeth with vestiges of heterodonty—the posterior teeth had accessory cups. Ken-triodon was similar in body form to the small living dolphins. Pithanodelphis and Atocetus displayed cranial asymmetry: die cranial vertex was skewed to the right side. This skew was in the opposite direction of that present in all other odontocetes with an asymmetrical skull. These two genera were also characterized by having exceptionally large and elevated nasal bones. Lophocetus and Haclrodelphis had large and elevated nasal bones, which were constricted medially between the posterior ends of the maxillae.
P. Family Albireonidae
Albireo was a Tursiops-sized dolphin that had a large, massive skull with an up-turned rostrum containing many large, conical-crowned teeth, a large brain, and an extensive basicranial air sinus system. Its flippers resembled those of Platanista gangetica and the stout body form that of Lagenorhynchus spp. It had numerous thoracic and lumbar vertebrae and these bear long processes.
Q. Family Odobenocetopsidae: Walrus-like Whales
Odobenocetops had asymmetrical, posteroventrally directed tusks, the right much longer than the other, similar to those of a narwhal, and a blunt snout. There were no other upper teeth. The facial region retained the odontocete nasal morphology, but lacked the bony indications of nasal sacs and air sinuses. The optic nerve tracts were exceptionally large for a cetacean.
R. Family Monodontidae: Belugas, Narwhals, and Relatives
Delpninapterids have a relatively narrow and slightly down-turned snout like modern beluga (Delphinapterus leucas). Del-phinapterus and Monodon monoceros have similar skulls, but the latter bears a large tusk, Denebola was a strange fossil mon-odontid of temperate latitudes that had a broad skull and a snout that was wide, flat, and blunt.
S. Family Delphinidae: True Dolphins
Delphinidae are characterized by skull osteology, especially details of the left-skewed asymmetrical cranial vertex and narial region, the basicranium, and the periotic. Delphinids range from small to large species with long or short rostra, and narrow or broad rostra, and are the most diverse living family of Cetacea.
T. Family Phocoenidae: Porpoises
Phocoenids have a raised eminence on each premaxilla anterior to the narial opening, a lobe of the pterygoid air sinus extending dorsal to the orbit between the frontal and maxillary bones, and anteriorly retracted posterior premaxillary terminations. The short rostra, rounded crania, and spatulate teeth of living phocoenids are paedomorphic characters that only appear in the more recent members of the group, known from the Late Pliocene and more recently. Late Miocene phocoenids had longer snouts, more prominent cranial crests, and conical tooth crowns. Piscolithax was a large phocoenid, approximately the size of extant Tursiops truncatus, and had relatively large pectoral flippers.
