Eotitanops () is an extinct genus of primitive brontothere that lived in Asia and North America during the Early Eocene. Eotitanops is generally considered to be the most basal (primitive) brontothere and may represent the earliest member of the group.
Two species of Eotitanops are currently recognized from North America, the type and larger species E. borealis, known from the Colorado, Wyoming, and Baja California, and the smaller species E. gregoryi (also known under the junior synonym E. minimus) from Colorado and Wyoming. Eotitanops fossils from Pakistan, contemporary with the North American species, have been assigned to a third species, E. pakistanensis. Fragmentary fossils of primitive brontotheres found on the Arctic Ellesmere Island in Canada may also belong to Eotitanops.
Eotitanops lived during the warm Early Eocene Climatic Optimum, and appears to have inhabited humid and densely forested habitats. Eotitanops was a small animal, weighing less than and standing up to tall at the shoulder. Other than features of its dentition, which place Eotitanops with the brontotheres, it would probably have been difficult to tell Eotitanops apart from primitive representatives of other perissodactyl lineages.
Research history
Discovery and taxonomy
The type species of Eotitanops, E. borealis, was originally described by Edward Drinker Cope in 1880 as a species of the related Palaeosyops, P. borealis. Cope based the new species on a maxilla fragment (AMNH 4892), found in the Lost Cabin Formation of the Wind River Basin in Wyoming. The species name borealis was chosen in reference to Boreas, the Ancient Greek god of the north wind, which in turn alluded to the Wind River. In 1881, Cope named another species of primitive brontothere from the Lost Cabin Formation, Lambdotherium brownianum, based on the lower jaw AMNH 4885.
In 1897, Henry Fairfield Osborn determined P. borealis to be generically distinct from Palaeosyops, and transferred the species to the genus Telmatotherium, as T. boreale. In 1907, Osborn moved P. borealis to the new genus Eotitanops but did not provide a diagnosis for the genus. Osborn treated Eotitanops in slightly more detail in 1908, provided a formal diagnosis, and noted that he distinguished the genus from Palaeosyops mainly by its elongated skull; Palaeosyops is otherwise known for its brachycephalic (short and broad) skull. Osborn also referred Cope's Lambdotherium brownianum to Eotitanops, as E. brownianus. The name Eotitanops was derived from Ancient Greek ἠώς (dawn), Τιτάν (titan), and ὤψ (face), and was intended to evoke its presumed status as the first of the brontotheres.
In a 1913 revision based on fossils in the collections of the American Museum of Natural History, Osborn recognized two "phyla" (lineages) within Eotitanops, one composed smaller and more light-limbed forms, and one composed of larger forms. Osborn named three new species of Eotitanops, all from the Lost Cabin Formation: E. gregoryi, based on a lower jaw and maxillary fragment (AMNH 14889), E. princeps, based on a jaw, partial limb fossils, and vertebrae (AMNH 296), and E. major, based a metatarsal and a tibia fragment (AMNH 14894). E. gregoryi was noted for its small size, whereas E. borealis, E. princeps, and E. major were larger, and E. brownianus was intermediate in size. Osborn named E. gregoryi in honor of his colleague William King Gregory, who assisted Osborn in his research on brontotheres. Osborn named a sixth species of Eotitanops in 1919, E. minimus, based on isolated teeth from the Huerfano Formation in Colorado. Osborn distinguished E. minimus as even smaller than E. gregoryi but noted that the teeth of the two species were otherwise highly similar.
The best preserved skull of Eotitanops is the relatively poorly preserved AMNH 14887, found by Walter W. Granger in 1909 on Dry Muddy Creek in Wyoming. In 1929, Osborn designated AMNH 14887 as the neotype (replacement type specimen) of E. borealis. Since the original type specimen of E. borealis is still available, Osborn's designation of a neotype has no significance, and AMNH 4892 remains the type specimen.
In 1958, and Therese zu Oettingen-Spielberg described fragmentary brontothere fossils from the Kuldana Formation of Pakistan. Dehm and Oettingen-Spielberg noted that the material was similar to Palaeosyops, but referred it to a new species of Eotitanops, E. dayi, based mostly on slight differences in the length-width ratio of the third molar. In 1991, Michael J. Novacek, Ismael Ferrusquía-Villafranca, John J. Flynn, André R. Wyss, and Mark A. Norell referred an isolated right second molar from the Las Tetas de Cabra Formation of Baja California, Mexico to Eotitanops cf. princeps.
Revisions and further discoveries
In 2000, Gregg F. Gunnell and Vicki L. Yarborough published a new revision of the North American Eotitanops material, and the fossils of some other primitive brontothere genera. Gunnell and Yarborough distinguished Eotitanops as a genus from Palaeosyops by its smaller size, and several dental features, and recognized only two valid species of Eotitanops in North America: the larger E. borealis and the smaller E. minimus. E. brownianus, E. gregoryi, E. princeps, and E. major were all designated as synonyms of E. borealis, though some E. gregoryi material was also referred to E. minimus. According to Gunnell and Yarborough, E. borealis is distinguished from E. minimus by its larger size, and by the more elongated and better developed hypoconulid (a minor cusp) on the third molar.
In 2006, fragmentary brontothere fossils from the Margaret Formation of the Eureka Sound Group, on Ellesmere Island in northern Canada, were identified as cf. Eotitanops by Jaelyn Eberle. Eberle noted that the fossils were most similar to Eotitanops, and fell within the size range of E. borealis, but that they had some derived features in the upper premolars that appeared closer to Palaeosyops. The Ellesmere Island brontothere was thus suggested to possibly represent a transitional form between the two genera. In 2008, Matthew C. Mihlbachler doubted that the Ellesmere Island fossils belonged to Eotitanops, since they have a metacone (another cusp) on the second premolar; all more derived brontotheres have a metacone on the second premolar, whereas Eotitanops otherwise lacks it. Mihlbachler also designated E. dayi as a nomen dubium, doubting its classification in Eotitanops. In 2015, Eberle and David A. Eberth maintained that the original Ellesmere Island fossils were cf. Eotitanops, and also described additional brontothere fossils from the island, which they referred to Palaeosyops.
In 2010, Bryn J. Mader published a new species-level revision of Eotitanops and Palaeosyops from the United States. Mader's assessment of Eotitanops was broadly consistent with the previous revision by Gunnell and Yarborough; Mader recognized two species, distinguished mainly by size, and used the name E. borealis for the larger species. Mader used E. gregoryi for the smaller species, disagreeing with Gunnell and Yarborough's designation of E. gregoryi as a junior synonym of E. borealis. Per Mader, E. gregoryi is distinguished from E. borealis by its smaller size and the presence of an entoconid (another cusp) on the fourth premolar. The presence entoconid instead unites E. gregoryi with E. minimus, which Mader thus designated as a junior synonym of E. gregoryi.
In 2011, Peter Missiaen, Gunnell, and Philip D. Gingerich described a new species of Eotitanops from the Ghazij Formation of Balochistan, Pakistan, E. pakistanensis, and reassessed E. dayi as a species of Palaeosyops. E. pakistanensis was based on a left maxilla (GSP-UM 6558) and some isolated teeth, and was easily identified as a very primitive brontothere. Missiaen, Gunnell, and Gingerich identified E. pakistanensis as a species of Eotitanops based on the dental features of the genus as defined by Gunnell and Yarborough in 2000. E. pakistanensis was distinguished from the other two species by its third molar being subequal in size to the second molar; in E. borealis, the third molar is slightly larger than the second molar, and in "E. minimus" the third molar is smaller than the second molar. Missiaen, Gunnell, and Gingerich cited Mader's revision in the paper but continued to use the name E. minimus for the smaller species, without comment, as have some other researchers.
Description
Eotitanops was a small and primitive brontothere. The earliest perissodactyls of different lineages were probably almost indistinguishable from each other. It would thus probably have been difficult to tell the difference between Eotitanops and other primitive (non-brontothere) perissodactyls, such as Hyracotherium. Eotitanops is classified as a brontothere because it had the characteristic dental features that set the group apart from other perissodactyl lineages.
Eotitanops fossils are generally fragmentary, complicating size estimates. Eotitanops has variously been described as rabbit-sized and sheep-sized. Osborn estimated E. gregoryi to have stood tall at the shoulder, and E. princeps (=E. borealis) to have stood tall at the shoulder. Osborn provided no size estimate for the largest fossils, which he assigned to E. major, but noted that they indicated an animal nearly the size of the modern South American tapir. Modern estimates vary in placing Eotitanops between and tall at the shoulder. As noted by Osborn, E. gregoryi was smaller than E. borealis. E. pakistanensis was similar in size to E. gregoryi. In 2023, Oscar Sanisidro, Mihlbachler, and Juan L. Cantalapiedra estimated the body mass of E. borealis at , that of E. pakistanensis at , and that of the cf. Eotitanops of Ellesmere Island at . Supplemental Material (Data S1, Sheet 6)
Skull
Unlike later and more derived (advanced) brontotheres, Eotitanops is believed to have had a relatively long face, similar to other primitive perissodactyls. In more derived brontotheres, the eyes were positioned further forward on the skull. The relatively long face probably distinguishes Eotitanops from all other brontotheres. Reconstructions of Eotitanops with a long face are based on AMNH 14887, which has been heavily restored with plaster. In 2008, Mihlbachler stated that a complete skull of Eotitanops had since been identified, UCMP 132049, which showed that the preorbital (before the eyes) portion of the skull of Eotitanops was shorter than the postorbital (behind the eyes) portion, as in other brontotheres. Mihlbachler did not figure UCMP 132049, and Mader concluded in 2010 that the facial region of AMNH 14887 was probably much longer than in other brontothere genera, even accounting for possible distortion due to its restoration.
Other than the potential long face, there are no identified synapomorphic traits that distinguish Eotitanops from all other brontothere genera, but all other brontothere genera have synapomorphic traits that distinguish them from Eotitanops. Other features of the skull of Eotitanops include that it was relatively long and narrow, and that the molar series was relatively short when compared to the skull's total length.
Dentition Eotitanops had the dental formula . The upper canine was relatively well developed. There was a long diastema (gap between teeth) between the canines and first pair of premolars, and between the first and second pair of premolars, in both the upper and lower jaw.