''Squalodon'' is an extinct

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial n ...

whale Whales are a widely distributed and diverse group of fully aquatic placental marine mammals. As an informal and colloquial grouping, they correspond to large members of the infraorder Cetacea, i.e. all cetaceans apart from dolphins and ...

s of the

Oligocene The Oligocene ( ) is a geologic epoch of the Paleogene Period and extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that define the epoch are well identified but ...

and

Miocene The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and means "less recent" ...

epochs, belonging to the family Squalodontidae. Named by Jean-Pierre Sylvestre de Grateloup in 1840, it was originally believed to be an iguanodontid

dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...

but has since been reclassified. The name ''Squalodon'' comes from '' Squalus'', a genus of shark. As a result, its name means "shark tooth". Its closest modern relative is the

South Asian river dolphin South Asian river dolphins are toothed whales in the genus ''Platanista'', which inhabit fresh water habitats in the northern Indian subcontinent. They were historically considered to be one species (''P. gangetica'') with the Ganges river dol ...

(with its two subspecies the Ganges river dolphin and Indus river dolphin).

Description

Species of ''Squalodon'' are

odontocete The toothed whales (also called odontocetes, systematic name Odontoceti) are a parvorder of cetaceans that includes dolphins, porpoises, and all other whales possessing teeth, such as the beaked whales and sperm whales. Seventy-three species of ...

s that lived during the late Oligocene into the middle Miocene, about 28 to 15 million years ago. The genus ''Squalodon'' belongs to the order Odontoceti, the toothed whales. They are named after the shark ''Squalus'' because their cheek teeth look like the teeth of a squalus shark. The largest species, ''Squalodon whitmorei'', reached up to 5.5 meters in length. The unique-looking squalodontids were likely distributed throughout the world in warm waters during the Oligocene and Miocene. Squalodontidae became extinct in the middle of the Miocene, leaving no descendants. Hypotheses of why this family lead to extinction have to deal with competition of other groups of dolphins as well as climate change.

Ancestral and modern features

These whales are characterized by both ancestral and modern features. Their teeth are the most evident ancestral feature. At this time in history other toothed whales were evolving simple conical teeth while Squalodontidae retained their primitive dentition that their ancestors (the archaeocetes) had developed.Marine Mammal Biology: An Evolutionary Approach By A. Rus Hoelzel. Published 2002 Blackwell Publishing. Today living odontocetes have little variation in their teeth. Squalodontids' teeth are much more complex: they are widely spaced apart; their cheek teeth are triangular and serrated for grasping and cutting. Due to the efficiency of their primitive dentition squalodontids could have a diverse variety of prey. Another ancestral quality of the Squalodontidae is their necks. Squalodontid necks are more compressed than their ancestors, the Archaeoceti. Compared to toothed whales at that time, the squalodontids were likely more mobile. Paleontologists also believe that the dorsal fins were reduced but larger than that of the ancestors. Shark toothed whales also possess many modern features. Their crania were well compressed, their rostrums were telescoped outward, and their skulls show proof of the origin of echolocation.

Fossil record and classification

Fossils of this genus are identified mainly by the teeth but several different species have been named based on skull characteristics and size (the biggest being S. whitmorei). Most of the fossil record consists of teeth. These odontocete fossils have been discovered in Europe, eastern North America, New Zealand, and Argentina. Because isolated teeth are insufficient for species identification, most specimens lacking the skull can only be identified to genus. The fossils of squalodontids indicate that this species is more closely related to endangered species of dolphins and not to most of the living dolphins today.C. Muizon. 1984. Les vertebres fossiles de la Formation Pisco (Perou) II: Les Odontocetes (Cetacea, Mammalia) du Pliocene inferieur de Sud-Sacaco. Institut Francais d'Etudes Andines Editions Recherche sur les Civilizations Memoire 50:1–188 Squalodon bariensis skull

The systematic placement of ''Squalodon'' within Odontoceti was long unclear. For a long time, it was thought to be close of the ancestry of modern dolphins and porpoise. Many of the fresh-water dolphins are differentiated phylogenetically very well, while the argument of some of the species has been going on for more than a century. The taxon is characterized during the Oligocene and Miocene in which heterodont teeth are standard amongst the family. Some modern features of the scapula, however, contradict with current phylogenetic relationships. Squalodontids were believed to be the last common ancestor of the odontocetes until 1984. Muizon came to the conclusion that rather than to any of the living species this family is closer related to the endangered species. Therefore, the ancestry of today's dolphins has little to do with the squalodontids.

Species

As the type genus of Squalodontidae, ''Squalodon'' has become a repository for various squalodontids or even taxa that were once thought to belong to Squalodontidae. However, there has been no revision of ''Squalodon''.

Species currently recognized as valid

* ''Squalodon grateloupii'' Meyer, 1843 (type species) * ''Squalodon antverpiensis'' van Beneden, 1861 * ''Squalodon bariensis'' (Jourdan 1861) * ''Squalodon barbarus'' Mchedlidze and Aslanova 1968 * ''Squalodon calvertensis'' Kellogg 1923 * ''Squalodon whitmorei'' Dooley 2005 * ''Squalodon catulli'' Molin 1859

Questionably or originally assigned to ''Squalodon''

* ''Arionus servatus'' Meyer, 1841 = ''Squalodon meyeri'' Brandt, 1873 * ''Pachyodon mirabilis'' Meyer, 1838 * ''Rhytisodon tuberculatus'' Costa, 1852 * ''Smilocamptus burgueti'' Gervais, 1859 * ''Phocodon melitensis'' (Blainville, 1840) = ''Phoca melitensis'' Blainville, 1840 = ''Phocodon scillae'' Agassiz, 1841 * ''"Squalodon" kelloggi'' Rothausen, 1968 * ''Squalodon bellunensis'' Dal Piaz, 1901 * ''Squalodon peregrinus'' Dal Piaz, 1971 * ''Squalodon imperator'' Cigala-Fulgosi & Pilleri, 1985 * ''Squalodon gambierensis''Glaessner 1955

References

External links

Squalodon Facts and Information: Fossilguy.com

Tree of Life

The Free DictionaryFamily †Squalodontidae – Hierarchy – The Taxonomicon
{{Taxonbar, from=Q774918 Miocene mammals of South America Miocene mammals of North America Miocene mammals of Europe Miocene mammals of Oceania Oligocene mammals of South America Oligocene mammals of North America Oligocene mammals of Europe Oligocene mammals of Oceania Prehistoric toothed whales Oligocene cetaceans Miocene cetaceans Miocene genus extinctions Prehistoric cetacean genera Fossil taxa described in 1840 Rupelian genus first appearances Fossil cetaceans misidentified as reptiles