Amynodon is an extinct genus of amynodont that lived in North America during the Middle Eocene, from the late Bridgerian to the late Uintan land mammal age. Several species of Amynodon have been named but only the type species, A. advenus, is generally considered valid today. Amynodon fossils have also been reported from Asia (China, Kazakhstan, Mongolia, and Japan) and Europe (Hungary) but these fossils are no longer considered to belong to the genus.
Amynodon is the type genus of the family Amynodontidae. It was a medium-sized member of the family, estimated to have weighed . Amynodon is the earliest known amynodont in North America, and was probably descended from Asian ancestors that migrated across Beringia. It was a characteristic animal of the Uintan land mammal age, sometimes treated as an index taxon. Amynodon was a relatively primitive and unspecialized, with teeth intermediate between those of more primtitive rhinocerotoids and more derived amynodonts. The canines of Amynodon took the form of semi-erect tusks, which were further developed in more advanced amynodonts.
Amynodon was historically interpreted as a semiaquatic animal, but reinterpretations in the 20th century argued that Amynodon does not preserve any obvious semiaquatic adaptations; such adaptations did evolve in amynodonts, but may have been restricted to the more derived metamynodontines, such as Metamynodon. Amynodon was instead interpreted as a terrestrial browser. The broad range of Amynodon across the modern-day United States further support to this idea, since this indicates that Amynodon was accustomed to the dominant environment type of the Uintan, savannas with open-canopied forests. Recent isotope analyses of Amynodon teeth have lent renewed support to the idea that it might have been semiaquatic.
Research history
Description of A. advenus
Amynodon is the best represented amynodont in the fossil record. The first fossils of Amynodon were briefly reported by Othniel Charles Marsh in 1875 from the Uinta Basin (Uinta Formation) in Utah. Marsh recorded several fragmentary fossils, most notably an isolated upper P3 (third upper molar; YPM 11763) and a corresponding lower molar. Marsh believed that the fossils came from a true rhinoceros and named the new species Diceratherium advenum based on the material. The species was mainly based on the upper molar, for which Marsh provided measurements.
In 1877, Marsh named the new genus Amynodon based on a nearly complete skull (YPM 11453), also from the Uinta Formation in Utah. The type species was A. advenus, "which was provisionally referred to the genus Diceratherium when first described." Marsh believed the animal to be the earliest rhinoceros yet discovered, with a skull intermediate between rhinoceroses and tapirs but with molars entirely like those of rhinoceroses. Marsh's description was wrong on several anatomical details since the skull had not been removed from its matrix (surrounding rock) or properly put together before it was described.
Marsh did not designate a type specimen, a problem compounded by the use of different fossils to diagnose Diceratherium advenum in 1875 (YPM 11763) and Amynodon advenus in 1877 (YPM 11453). In 1890, Henry Fairfield Osborn interpreted YPM 11453 as the type specimen of A. advenus. In 1921, Edward Leffingwell Troxell instead interpreted the upper molar YPM 11763 as the type specimen. Later authors generally follow Troxell's assessment. In 1955, Jean Hough considered YPM 11453 to be a paratype specimen.
Description of Orthocynodon
In 1882, William Berryman Scott and Osborn described the new genus and species Orthocynodon antiquus, based on an associated skull and lower jaw (PU 10047), and another partial skull, found in the Washakie Formation in Wyoming. Scott and Osborn distinguished the fossils from Amynodon due to several features of the teeth; Orthocynodon was described to have more erect canines, a small postero-internal (internal, towards the back) cusp on the upper P2 and P3 (second and third premolar in the upper jaw), and generally more molariform (molar-like) premolars than Amynodon. Orthocynodon was also Bridgerian in age, and thus predated Marsh's fossils. Scott and Osborn presented it as the new oldest known rhinoceros. In 1883, Scott and Osborn described O. antiquus in more detail and established the new family Amynodontidae to contain Orthocynodon and Amynodon.
YPM 11453 was eventually prepared out of its matrix, and Marsh allowed Scott and Osborn to examine the skull and an associated lower jaw. Scott and Osborn found several features which they had believed made Orthocynodon distinct, such as erect canines. They designated Orthocynodon as a probable synonym of Amynodon in 1887 and in 1890, Osborn treated O. antiquus as a species of Amynodon (A. antiquus). The only remaining features believed to distinguish A. antiquus as a species were in the premolars; according to Osborn, A. antiquus retains the lower P1 (the first premolar), a tooth lost in A. advenus, and has a more molariform P4.
Additional fossils and species
In 1890, Osborn described a third partial Amynodon skull (PU 10309), from the Uinta Formation, referred to a new species, A. intermedius. Osborn distinguished A. intermedius from A. advenus by its larger size, the retention of the upper P1, and the procumbent position of the canines. Osborn believed that the three described Amynodon species formed an evolutionary sequence, with A. antiquus evolving into A. advenus, which then evolved into A. intermedius. In 1895, Osborn reported the discovery of a relatively complete Amynodon skeleton in the Uinta Formation, which he referred to A. intermedius. Osborn appears to never have figured these fossils.
Earl Douglass described a poorly preserved jaw (CM 734) in 1903, from Sage Creek in Montana. Douglass referred the jaw to Metamynodon but Horace Elmer Wood identified it as Amynodon, possibly a new species, in 1934. According to Wood, CM 734 is more advanced than A. antiquus and similar in size to A. intermedius, but distinguished by having one less pair of upper incisors. In 1910, Walter W. Granger reported a new Amynodon skull (AMNH 14601) from Beaver Divide in Wyoming. Walter initially referred the skull to A. antiquus but later changed his mind to A. advenus. Elmer S. Riggs reported new Amynodon fossils from the Uinta Basin in 1912 but did not describe them. The fossils found by Riggs included a skull (FMNH 12184) and a lower jaw (FMNH 12191), which were later referred to A. advenus. In 1919, reported Amynodon fossils from the Uinta Basin, from several individuals. Peterson referred the fossils to both A. advenus and A. intermedius, almost entirely based on size.
In 1921, Troxell suggested that YPM 11453 was distinct from YPM 11763 on the species level. All of the features that Troxell considered to be distinguishing are in the upper P3 since YPM 11763 preserves nothing else. These features included that the P3 of YPM 11453 is smaller, that both its posterior and anterior cingula are stronger, that the postsinus (gap behind the posterior cingulum) is deeper, and that the medisinus (gap between the two cingula) is much narrower. Since Troxell interpreted YPM 11763 as the type specimen of A. advenus, he moved YPM 11453 to a new species, A. erectus. Troxell distinguished A. erectus from A. antiquus by A. erectus having lost P1 in both jaws and its smaller size, though noted that both species had similar erect canines; the retention of P1 in A. antiquus was considered so significant by Troxell that it might justify Orthocynodon as a subgenus. A. erectus was distinguished from A. intermedius by A. intermediuss larger size and more procumbent canines.The description of the new genus Amynodontopsis by Chester Stock in 1933 was the first record of amynodonts from California. Stock proposed that A. intermedius could belong to Amynodontopsis because A. intermedius shared some features with the new species, notably the depth and backward extent of the fossae (grooves in front of the eye orbits). In 1939, Stock described the new species Amynodon reedi, also from California. The type specimen of A. reedi is a fragmentary left maxilla, LACM (CIT) 314/2529, which preserves the upper P4–M3. Stock stated that the fossil was from "Poway conglomerate" in San Diego, California; modern stratigraphic revisions of the region place this in the Friars Formation. Stock compared the maxilla to a small Amynodon fossil found in the Uinta Formation, AMNH 1936A; some later authors have misinterpreted this as A. reedi also being present in the Uinta Formation. A. reedi was distinguished from the other species by Stock through its small size, and by P4 being narrow, with a metaloph at a right angle to the tooth's external border. Stock also reported an Amynodon tooth, possibly A. advenus, from the Sespe Formation in California. David J. Golz and Jason A. Lillegraven reported additional Californian Amynodon fossils in 1977, from the Santiago Formation and referred these to A. intermedius.
Revisions In 1981, John Andrew Wilson and Judith Schiebout described a large number of Amynodon fossils from Uintan-age strata at Pruett Tuff in the Buck Hill Group of Brewster County, Texas. Wilson and Schiebout referred the fossils to A. advenus. As the largest published sample of Amynodon from a single population, the assemblage allowed Wilson and Schiebout to study the degree of individual variation in a single Amynodon species. High variability was observed in several features: overall body size varied considerably, as did canine position (erect/procumbent/oblique), strength of the cingula, and the depth of the fossae. Some of the previously described Amynodon material, attributed to A. antiquus, A. erectus, A. advenus, and A. intermedius, was examined for comparison and was determined to show less variability than the single-population sample from Texas, despite being from different sites and slightly different stratigraphic levels (early/late Uintan). Wilson and Schiebout thus suggested that A. antiquus, A. erectus, and A. intermedius were all synonyms of A. advenus.
William P. Wall revised Amynodon in 1982. Wall recognized only A. advenus and A. reedi as valid, and agreed with Wilson and Schiebout's assessment that A. antiquus, A. erectus, and A. intermedius are junior synonyms of A. advenus. Wall pointed out that Wilson and Schiebout had made some mathematical errors in their analysis; contrary to their calculations, the Texan Amynodon sample shows less variability than the total sample. The species other than A. advenus were reassessed by Wall individually, which resulted in the designation of all except A. reedi as synonyms:
The distinguishing features of A. antiquus, the retention of P1 and the molariform P4, are not actually present in the type specimen. The type jaw is broken towards the back, and Scott and Osborn misinterpreted P2–M3 as P1–M2. This means that P1 is not actually retained, and that the supposed molariform premolar is just a molar. Troxell's features to distinguish A. erectus from A. advenus were assessed as "quite minor", and the A. erectus fossils as "well within the range" of A. advenus.