The odontocete families usually included in Delphinoidea J, are Kentriodontidae, Odobenocetopsidae, Albireonidae, f Delphinidae, Phocoenidae, and Monodontidae. Among these, only the last three have Recent (=living) representatives and are discussed in this article. Odobenocetopsidae and Al-bireonidae are small families, consisting of a few known fossil species. The fossil kentriodontids are diverse and are the basal delphinoid family.
I. Family Monodontidae: Belugas, Narwhals, and Relatives
The family Monodontidae includes two living Arctic species: the living beluga (Delphmapterus leucas), in the subfamily Del-phinapterinae, and the tusked narwhal (Monodon monocerus), in the subfamily Monodontinae. The skulls of these two cetaceans are similar, with the tusk of the narwhal differentiating them superficially. The fossil record of Monodontidae extends back to the Late Miocene (circa 11 or 12 Mya). Mon-odontids appear most closely related to the porpoises of the family Phocoenidae and to the dolphins of the family Delphinidae, but detailed study of their interrelationships has yet to be made.
Fossil representatives of Delphinapterinae, animals having a relatively narrow and slightly downturned snout like that of the modern beluga, Delphinapterus, have been collected from deposits bordering the western North Atlantic and from California and Japan.
Fossils of latest Miocene and Pliocene members of the extinct, aberrant, broad-headed monodontid Denebola are known from coastal sites in central and southern California and various localities in Baja California. The skull of this genus is broad and the snout is wide, flat, and blunt, causing it to mimic the skull of the pilot whales (family Delphinidae, genus Globicephala). Phylogenetic analysis is needed that would elucidate the relationships of this group to the other monodontids. We do not yet understand the evolutionary origin of the bizarre spiral-tusked narwhal of the Arctic because no fossils have been found.
The fossil record of the Monodontidae demonstrates that the group was much more numerous, diverse, and widespread in the past than it is now and that it was characteristic of the temperate and subtropical latitudes of the North Atlantic and the North Pacific oceans. No monodontid has been found in the Southern Hemisphere. The fossil distribution is seemingly-paradoxical to the present strictly Arctic distribution, and the living monodontid species are relics. Past members of the family may have occupied the ecologic niches of several species of modem dolphins. The sole surviving species have relict distributions in a possibly marginal habitat.
II. Family Delphinidae: Dolphins
The modern, mostly pelagic dolphins of the family Delphinidae are the most diverse living family of Cetacea and are nearly cosmopolitan in the world’s oceans. The family includes a taxonomically and ecologically diverse range of small to large species with long or short rostra, and narrow or broad rostra. Rostral shape and numbers and size of teeth are related to diet, with the various species ranging from generalists, to predators, to squid eaters. Common names reflect the animal s sizes: some of the large “dolphins” are called pilot whales (Globicephala spp.) and killer whales (Orcinus orca), and smaller species are called dolphins, such as Delphinus spp. All Delphinidae are unified by skull osteology, especially details of the left-skewed asymmetrical cranial vertex and narial region, the basicranium, and the periotic.
Although the delphinids are the most diverse group of living cetaceans, they are surprisingly rare in the fossil record. The oldest documented delphinid is Late Miocene (about 10 to 12 million years ago) in age from southern California, and delphinids are not very common in subsequent latest Miocene and Pliocene deposits. All pre-Late Miocene supposed fossil delphinids are now known to be members of other odontocete families.
The present abundance of delphinids is apparently the result of explosive evolution that probably occurred in the later part of the Pliocene, resulting in die replacement of the earlier more diverse kentriodontids, phocoenids, and monodontids. The rather abrupt appearance of Delphinidae, in relative abundance, in Pliocene time is a notable phenomenon in cetacean evolution.
Morphology of the earliest fossil delphinid indicates that the family probably arose from within the primitive, extinct family Kentriodontidae, but no definitive study has been made that would prove this.
III. Family Phocoenidae: Porpoises
The true porpoises of the family Phocoenidae have a fossil record dating back to the Late Miocene, or about 11 or 12 million years ago. All pre-Pliocene fossil records of this family from the Atlantic realm are suspect. Phocoenids have a raised eminence on each premaxilla anterior to the nasal openings, 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 (paedomorphic characters) and spatulate teeth of living phocoenids only appear in the more recent members of the group, known from the Late Pliocene and more recently. Late Miocene phocoenids have longer snouts, more prominent cranial crests, and conical tooth crowns, all of which are primitive characters, which the early phocoenids share with kentriodontids or delphinids. Phocoenids were much more diverse in the past than now, and in rocks dating from 3 to 12 million years ago around the North Pacific margin their fossils are more diverse and more abundant than those of delphinids.
The oldest apparent phocoenid is represented by a single periotic of Middle Miocene age from Baja California Sur, Mexico. In morphology it closely matches the holotypic periotic of the most primitive named phocoenid, Piscolithax tedfordi, also from Baja California, which is considerably younger (latest Miocene, circa 9 Mya). However, P. tedfordi, while being the most primitive named phocoenid is for its time a relict taxon, because die somewhat more derived Salumiphocaena stocktoni from southern California is about 10-12 million years old. Piscolithax was a large phocoenid, approximately the size of extant Tursiops truncatus, and had relatively large and wide pectoral flippers.
Piscolithax and other relatively primitive genera of Phocoenidae, some with unusually long rostra, have been discovered in Late Miocene and Pliocene deposits in coastal Peru. There is no published fossil record that would elucidate the origins of the living genera Neophocaena, Phocoena, and Phocoenoides.
