''Herpetotherium'' 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 nom ...

metatheria Metatheria is a mammalian clade that includes all mammals more closely related to marsupials than to placentals. First proposed by Thomas Henry Huxley in 1880, it is a more inclusive group than the marsupials; it contains all marsupials as wel ...

n mammal, belonging to the possibly paraphyletic family

Herpetotheriidae Herpetotheriidae is an extinct family of metatherians, closely related to marsupials. Species of this family are generally reconstructed as terrestrial, and are considered morphologically similar to modern opossums. Fossils of herpetotheriids com ...

. Native to North America from the

Eocene The Eocene ( ) Epoch is a geological epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period in the modern Cenozoic Era. The name ''Eocene'' comes from the Ancient Greek (''ēṓs'', " ...

to Early

Miocene The Miocene ( ) is the first epoch (geology), 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 mea ...

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

s have been found in

California California is a state in the Western United States, located along the Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the most populous U.S. state and the 3rd largest by area. It is also the m ...

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and

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, and

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. The oldest species, ''H. knighti'', is dated to around 50.3 mya, and the most recent, an unnamed species, may be as recent as 15.97 mya. A morphological analysis of marsupials and basal metatherians conducted in 2007 found ''Herpetotherium'' to be the sister group to extant marsupials. It is the youngest known metatherian from North America until the migration of the

Virginia opossum The Virginia opossum (''Didelphis virginiana''), also known as the North American opossum, is the only opossum living north of Mexico, its range extending south into Central America. It is the northernmost marsupial in the world. In the United S ...

from South America within the last 2 million years. Some authors have regarded the species assigned to ''Herpetotherium'' (for instance, ''H. comstocki'', ''H. marsupium'', and ''H. merriami'') as belonging to African and European genus '' Peratherium'' Aymard, 1850 on the basis of Simpson's observation that "the upper molars ... agree closely with '' Peratherium'', less closely with other didelphid genera" and on Cope himself recognizing "there is no valid distinction between ''Herpetotherium'' and '' Peratherium''" (Simpson, 1928, p. 6) Yet other authors have continued to maintain generic separation for ''Herpetotherium'' and continue to assign new North American species of herpetotheriid marsupials to this genus.

Species

*''H. fugax'' (syn. ''Didelphys pygmaea'', ''H. scalare'', ''H. tricuspis'', '' Peratherium fugax'') ( type) *''H. huntii'' *''H. knighti'' (syn. ''Centracodon delicatus'', ''Entomacodon minutus'', '' Peratherium morrisi'') *''H. marsupium'' Troxell, 1923, p. 508 (syn. '' Peratherium marsupium'' according to Simpson, 1928Simpson, George Gaylord
"American Eocene Didelphids."
''American Museum Novitates'', no. 37, pp. 1-7, 1928.) *''H. merriami'' (Stock and Furlong, 1922, originally named within '' Peratherium'' as Stock agreed with Simpson that ''Herpetotherium'' Cope 1873 was synonymous with ''Peratherium'') *''H. tabrumi'' Korth, 2018 *''H. valens'' (syn. '' Peratherium donahoei'') *''H. youngi'' (syn. '' Peratherium spindleri'')

Anatomy and morphology

''Herpetotherium'' has long been recognized as having a primitive morphology compared to other marsupials. For instance, Osborn (1907, p. 111) thought it represented an ancestral stock to both "later carnivorous and herbivorous types of marsupials", and thought it was evidence that polyprotodont opossum-like mammals must have given rise to other marsupials (Osborn, 1910, p. 154). Troxell (1923) strongly fixated on mandibular characters of ''Herpetotherium'' (though speaking specifically of ''H. marsupium'', yet these are generally true of ''Herpetotherium'') that he felt were especially comparable to ''

Didelphis ''Didelphis'' is a genus of New World marsupials. The six species in the genus ''Didelphis'', commonly known as Large American opossums, are members of the ''opossum'' order, Didelphimorphia. The genus ''Didelphis'' is composed of cat-sized om ...

'', such as "(1) the strong canine, (2) weak anterior premolar, (3) dominance of the

protocone A cusp is a pointed, projecting, or elevated feature. In animals, it is usually used to refer to raised points on the crowns of teeth. The concept is also used with regard to the leaflets of the four heart valves. The mitral valve, which has two ...

in the premolars, (4) fourth tooth of the series molariform, (5) position of

mental foramen The mental foramen is one of two foramina (openings) located on the anterior surface of the mandible. It is part of the mandibular canal. It transmits the terminal branches of the inferior alveolar nerve and the mental vessels. Structure The ...

under this tooth, (6) identical form of the molars, (7) groove on the jaw suggesting an inflected angle," yet he pointed out several differences, such as "(1) absence of

diastema A diastema (plural diastemata, from Greek διάστημα, space) is a space or gap between two teeth. Many species of mammals have diastemata as a normal feature, most commonly between the incisors and molars. More colloquially, the condition ...

ta around the anterior premolar, (2) trend of the mental foramen forward instead of backward, (3) the inflected angle, seeming to begin beneath the molars" (p. 510).

References

External links

3D Digital skull of ''Herpetotherium fugax''.
{{Taxonbar, from=Q601759 Paleogene mammals of North America Prehistoric metatherians Fossil taxa described in 1873 Prehistoric mammal genera