Metamynodon is an extinct genus of amynodont that lived in North America from the Middle Eocene to the Middle Oligocene. Metamynodon was a large, robust, and semiaquatic animal that displayed a suite of adaptations towards an amphibious lifestyle. Many of these features showcase convergent evolution with the modern hippopotamus, though Metamynodon is most closely related to rhinoceroses.
Fossils of Metamynodon were first discovered in the late 20th century. The type species of Metamynodon, M. planifrons, is known from near-complete fossils and is thus relatively well understood anatomically. Two additional species based on more fragmentary remains are often recognized, M. chadronensis and M. mckinneyi, though their taxonomic validity has been questioned since the 1980s. M. planifrons was the largest species, about the same size as large modern rhinoceroses. The largest M. planifrons measured up to long and weighed about . The skull of Metamynodon was broad and flat. Dentally, it was characterized by large and tusk-like canines, and high-crowned cheek teeth. The body was stout and barrel-shaped, and the legs were relatively short.
Metamynodon likely inhabited river systems across much of North America. It was a grazer, possibly adapted to grind down abrasive vegetation. Metamynodon is recorded from the late Uintan to the Whitneyan land mammal age, and was most common in the Orellan, which corresponds to the Early Oligocene. Although dwarfed by brontotheres in the Eocene, Metamynodon was the largest mammal in North America during the Early Oligocene.
Research history
Discovery of M. planifrons
Metamynodon planifrons was described by William Berryman Scott and Henry Fairfield Osborn in 1887, based on a skull and jaws found in the White River Formation. The fossils were among a large number of mammal fossils found by Samuel Garman in Nebraska and Dakota. The original Metamynodon fossils were found by Garman in 1880–1881 in the Big Badlands of South Dakota. Garman's fossils were entrusted to Scott and Osborn by Louis Agassiz for preparation and description.
The type specimen of M. planifrons is the well-preserved skull MCZ 9157. The teeth of MCZ 9157 are badly damaged, and the skull completely lacks the canines and incisors. Scott and Osborn also described two fossil jaw rami: a left ramus tentatively associated with MCZ 9157 (and thus also numbered MCZ 9157) and the right ramus MCZ 11931, which belongs to a different individual and is considered a paratype specimen. Scott and Osborn's original description was unclear in which lower jaw specimens were used for the diagnosis, and their description of the dentition was almost entirely conjectural since the teeth are damaged. A slightly amended description of the fossils was published in 1960 by Horace Elmer Wood. All of the original M. planifrons fossils are kept at the Museum of Comparative Zoology at Harvard University.
Scott and Osborn identified Metamynodon as a "highly modified successor" of Amynodon, which had been described a decade earlier. The name Metamynodon means "after Amynodon". Amynodon had for the previous decade been considered a predecessor of modern rhinoceroses, and perhaps the earliest known true rhinoceros (i.e. the family Rhinocerotidae), an idea challenged by the discovery of Metamynodon. In 1890. Osborn opinioned that Metamynodon was "certainly far from the rhinoceros line" and yet "undoubtedly a successor" to Amynodon. Osborn deduced that amynodonts represented a parallel lineage of "pseudo-rhinoceroses" that were not ancestral to any modern forms.
In 1892, an expedition from the American Museum of Natural History found additional M. planifrons fossils in the White River Formation. The most significant new fossils included a relatively well-preserved skeleton, which included the jaw but only fragments of the skull (AMNH 546), as well as the skull and jaw of another individual (AMNH 545). Further fossils found in 1892 included another skull (AMNH 547), a forelimb (AMNH 553), and several complete and fragmentary lower jaws (AMNH 549–553 and AMNH 696). Another search in 1894 discovered additional postcranial fossils, including a left scapula (AMNH 1092), a complete right manus (front foot, AMNH 1095), and a complete left pes (hind foot, AMNH 1100). Phalanges, caudal vertebrae, and ribs were also found. The exceptional fossils left only parts of the vertebral column unknown. The skeleton AMNH 546 and the other fossils collected in 1892 were described by Osborn and Jacob Lawson Wortman in 1894, the first description of the postcranial anatomy of any amynodont. The other fossils were described by Osborn and Wortman in 1895.
Further finds and other species
In 1921, Edward Leffingwell Troxell described the new species Metamynodon rex, based on the skull YPM 10274, found in the White River Formation in the Pine Ridge Indian Reservation, South Dakota. YPM 10274 had originally been purchased from C. H. Little of South Dakota by Othniel Charles Marsh in 1889, but was not prepared out of its matrix and studied until the 20th century. Although other Metamynodon fossils, including new skulls, had been discovered in the intervening years, Troxell's description of YPM 10274 in 1921 marked the first new Metamynodon fossil to be described since Osborn and Wortman's work in the 1890s. Troxell distinguished M. rex from M. planifrons by several minor dental features, and slight differences in the zygomatic arch.In 1942, Miklós Kretzoi named the new genus and species Cadurcopsis dakotana, based on a skull in the Hungarian National Museum. The C. dakotana skull is from the South Dakota badlands, and was prior to 1942 identified as a M. planifrons skull. Kretzoi distinguished Cadurcopsis from Metamynodon by several features, including a shorter skull, the height of the facial portion of the skull, the slender and more narrow cheekbones, the lack of a diastema (gap) between the canines and premolars, and some dental features. In 1960, Horace Elmer Wood designated both M. rex and C. dakotana as junior synonyms of M. planifrons. Wood's assessments of synonymy have been maintained by later authors.
Wood named the new species Metamynodon chadronensis in 1937. M. chadronensis was based on two associated rami of the lower jaw (AMNH 11866), with preserved cheek teeth, found in the Chadron Formation in South Dakota. The fossil was collected by fossil collecter H. F. Wells in 1903, and originally referred to Metamynodon sp. by William Diller Matthew in 1909. Wood described M. chadronensis as a primitive species of Metamynodon, transitional between earlier amynodonts such as Amynodon and Paramynodon and the more derived M. planifrons, both in its dental features and in its overall body size (being close to that of Paramynodon). Referral to Metamynodon was noted to be provisional, since a skull of M. chadronensis would be needed for comparison.
In 1981, John Andrew Wilson and Judith Schiebout named the new species Metamynodon mckinneyi, based on a lower jaw and some additional jaw fragments found in late Uintan strata the Buck Hill Group in Brewster County, Texas. The species was named after the B. P. McKinney family of the Agua Fria Ranch in Brewster County, who had been helpful to the paleontologists. Wilson and Schiebout referred the fragmentary fossils to Metamynodon based on dental similarities, such as the relative size of the molars and premolars and the loss of one pair of premolars. Wilson and Schiebout suggested that M. mckinneyi could represent an intermediate form between Amynodon and M. chadronensis, though noted that better fossils were required to determine this.
M. chadronensis and M. mckinneyi are based on relatively fragmentary fossils. Both are typically treated as valid species of Metamynodon, though their validity has also been called into question. Both were proposed to be transitional forms between Amynodon and more derived Metamynodon, but several amynodont genera are more derived than Amynodon but less derived than M. planifrons (e.g. Megalamynodon, Sharamynodon, Sellamynodon). In 1989, William P. Wall considered the assignment of both species to Metamynodon was "open to question", especially in the case of the more primitive M. mckinneyi. Wall suggested detailed comparisons to the closely related Megalamynodon to determine the validity of M. chadronensis and M. mckinneyi. In 2006, Spencer G. Lucas proposed that M. mckinneyi could belong to Zaisanamynodon. In 2023, Veine-Tonizzo et al. considered revisions to be necessary for both M. chadronensis and M. mckinneyi to assess whether they are valid species, as their diagnostic characters are "not specific enough to distinguish them from the type species".
Purported finds in Asia
Historically, a number of Asian rhinocerotoid fossils have been referred to Metamynodon. In 1916, Guy Ellcock Pilgrim and George de P. Cotter described the new species Metamynodon birmanicus from the Pondaung Formation in Myanmar. In 1925, Pilgrim named another species from Myanmar, Metamynodon cotteri. Recognizing differences between the Burmese fossils and Metamynodon fossils from North America, William Diller Matthew referred M. birmanicus and M. cotteri to the new genus Paramynodon in 1929. P. cotteri has since also been recognized as a synonym of P. birmanicus.
In 1922, Clive Forster-Cooper described the large species Metamynodon bugtiensis based on fragmentary fossils (including a palate, with twelve teeth preserved) from Dera Bugti in Pakistan. Since 1922, the M. bugtiensis fossils have been variously interpreted as remains of a giant amynodont or as remains of Paraceratherium. Modern reexamination of the fossils revealed that the dentition was misinterpreted by Forster-Cooper, and that M. bugtiensis was an aberrant form of Paraceratherium bugtiense.
Upper molars from the Early Oligocene Caijiachong Formation in Yunnan, China, were assigned to Metamynodon sp. by Xu Yu-Xuan in 1961. Xu's molars are considered unlikely to belong to Metamynodon and probably belong to the same taxon as the dubious Gigantamynodon giganteus, known from the same site.
Description
Size
M. planifrons was a massive animal, about the same size as large modern rhinoceroses. M. planifrons was long. The relatively complete M. planifrons skeleton AMNH 546 measures long, in the smaller range of known specimens, and stood nearly high, and was about broad at the chest. The body mass of M. planifrons has been estimated at . Supplemental material. Supplemental Material (Data S1, Sheet 8)
M. planifrons was larger than M. chadronensis and M. mckinneyi. The size of the other two species is difficult to estimate on account of their fragmentary fossils; recent estimates put the body mass of M. chadronensis at and that of M. mckinneyi at .
Skull and jaw
The skull of Metamynodon was large, broad, and flat. Among the amynodonts, Metamynodon had one of the most brachycephalic skulls (skull wide relative to its length); the skull of the larger Zaisanamynodon was even more brachycephalic. The zygomatic arches of Metamynodon were expanded and massive, and it had the widest muzzle of all known amynodonts. Metamynodon had a powerful sagittal crest (a ridge on the top of the skull, for jaw muscle attachment).