Palaeosyops () is an extinct genus of primitive brontothere that lived in Asia and North America during the Early to Middle Eocene. Palaeosyops was among the first brontotheres to be described scientifically, and is known from a large number of fossils, particularly from the Bridger Formation in the Green River Basin in the western United States.
The largest Palaeosyops were about the size of small cattle, exceeding in weight. Palaeosyops was the largest mammal in its environment, and markedly larger in size than its ancestors and the more basal brontothere Eotitanops. Palaeosyops had a relatively robust skull, and is unique among the brontotheres for its quite extreme brachycephaly (a short and broad head). Gracile Palaeosyops skulls were once classified as the separate genus Limnohyops, but are now interpreted as showing sexually dimorphic differences. The dentition, overall anatomy, and inferred paleoenvironment (semitropical forests) of Palaeosyops all indicate that it had a diet and lifestyle similar to modern-day tapirs, though unlike tapirs Palaeosyops did not have a prehensile upper lip.
The species-level taxonomy of Palaeosyops is disputed. North American fossils show a general trend of increasing size and robusticity over the course of the Bridgerian. This has been interpreted as a succession of species, from P. fontinalis to P. paludosus to P. robustus. Different revisions have recognized additional or fewer species, and some scholars contend that the North American fossils may not be divisible into distinct species at all. In Asia, fragmentary fossils from Pakistan have been referred to the species P. dayi. Additional fragmentary fossils referred to Palaeosyops are also known from Kazakhstan and Myanmar, and from the Arctic Ellesmere Island in Canada.
Research history
North America
Palaeosyops paludosus was one of the first brontotheres to be scientifically described, and one of the first mammals described from the fossil-rich Eocene deposits of the Green River Basin in Wyoming. Palaeosyops was the first hornless brontothere to be discovered, and its relation to the brontotheres previously discovered, such as the considerably more massive Megacerops, was not realized for some time. The first Palaeosyops fossils to be described consisted of four isolated teeth found at Church Buttes in the Bridger Formation of Wyoming, found by Ferdinand Vandeveer Hayden during his geological explorations of the Great Plains and the Rocky Mountains. The teeth were described by Joseph Leidy in 1870; the name Palaeosyops, for which Leidy provided no explanation, means "ancient boar face", derived from Ancient Greek παλαíο (ancient), σύς (boar), and ὤψ (face). Leidy did not recognize Palaeosyops as a perissodactyl until 1873, when he described a skull, teeth, and limb bones and noticed their resemblance to those of Palaeotherium and Tapirus.
Over the course of the late 19th and early 20th century, a large number of fossils of primitive brontotheres from the Bridgerian of North America were discovered, and used to name a multitude of species and several genera. Much of the fossil material was found in the Bridger Formation in the Green River Basin. In 1929, Henry Fairfield Osborn recognized fourteen distinct species, divided into the three genera Palaeosyops, Limnohyops, and Eometarhinus. In addition to relatively minor dental and cranial features, the best represented genera Palaeosyops and Limnohyops were distinguished mainly by their robusticity, with the name Limnohyops used for more gracile specimens. Given that the type material of the Palaeosyops type species, P. paludosus, consists of isolated teeth, concerns have been raised that the species may be non-diagnostic, and that the genus might thus be dubious. Some neotype specimens have been suggested by different authors, but these specimens have no nomenclatural significance since the original teeth are still available. Despite the problematic type fossils, recent revisions of brontotheres have continued to use the name Palaeosyops.
In 1966, Peter Robinson synonymized Eometarhinus, used for fossils known from the Huerfano Formation, with Palaeosyops. Later revisions have maintained that Eometarhinus is a junior synonym of Palaeosyops, though it has been noted that the Eometarhinus fossils are so fragmentary that it may not be entirely possible to confidently rule it out as distinct. Limnohyops was designated a junior synonym of Palaeosyops by Bryn J. Mader in 1989, who attributed supposed generic distinctions to individual variation and sexual dimorphism. This synonymy has likewise been maintained in later revisions. In a 2000 species-level revision, Gregg F. Gunnell and Vicki L. Yarborough recognized five species of Palaeosyops: P. fontinalis from the early Bridgerian, P. laevidens and P. paludosus from the middle Bridgerian, and P. laticeps and P. robustus from the late Bridgerian. Under this model, the species are differentiated mainly by size and various dental features. In 2010, Mader published his own species-level revision, which recognized only three species: P. fontinalis from the early Bridgerian, P. paludosus from the middle Bridgerian, and P. robustus from the late Bridgerian. Mader did not consider the dental features used by Gunnell and Yarborough to be diagnostically useful, and distinguished the species mainly by their size and stratigraphic level.
In 2008, Matthew C. Mihlbachler doubted multi-species classification schemes for Palaeosyops, since size differences and dental features are not reliable diagnostic traits in brontotheres. Although Mihlbachler noted that there appeared to be a general trend of increasing size and robusticity over the course of the Bridgerian, he opinioned that Palaeosyops may not be divisible into several distinct species under a phylogenetic species concept due to overlap in features between specimens. In 2023, Oscar Sanisidro, Mihlbachler, and Juan L. Cantalapiedra suggested that any observed anatomical differences between Palaeosyops species could be explainable as individual variation within a single species. Supplementary Materials
In 2015, fragmentary brontothere fossils from the Bridgerian of the Margaret Formation of the Eureka Sound Group, on Ellesmere Island in northern Canada, were referred to Palaeosyops sp. by Jaelyn Eberle and David Eberth. These fossils included jaw fragments, isolated teeth, and an incomplete astragalus.
Asia In addition to the large number of Palaeosyops fossils known from North America, a number of fragmentary fossils from Asia have also been referred to the genus. In 1958, and Therese zu Oettingen-Spielberg described fragmentary brontothere fossils, including maxillae and tooth fragments, from the Kuldana Formation of Pakistan. Dehm and Oettingen-Spielberg identified the fossils as those of a primitive brontothere, and noted similarities to Palaeosyops, but referred the fossils to a new species of the related Eotitanops, E. dayi, based mostly on slight differences in the length-width ratio of the third molar. In 2011, Pieter Missiaen, Gunnell, and Philip D. Gingerich reclassified E. dayi as Palaeosyops dayi. Missiaen, Gunnell, and Gingerich noted that P. dayi appeared intermediate in size between Eotitanops and other species of Palaeosyops, that dental features previously used to place it in Eotitanops were no longer considered diagnostically useful, and that the teeth resembled those of Palaeosyops more than those of Eotitanops.
In 1977, Leo Gabunia described a right first or second molar from the Aksyr River in the Zaysan Basin, Kazakhstan, and referred the tooth to Palaeosyops sp. In 2008, Mihlbachler agreed that the tooth came from a primitive brontothere, similar to Palaeosyops or Eotitanops, but that it did not have enough character data to properly identify. Gabunia's tooth is from an animal smaller than any North American species of Palaeosyops, but Mihlbachler noted that it could plausibly represent an Asian species of the genus or a similar brontothere. In 2000, Patricia A. Holroyd and Russell Ciochon described the new primitive brontothere genus and species Bunobrontops savagei from the Pondaung Formation of Myanmar. In addition to the fossils referred to B. savagei, Holroyd and Ciochon also referred two right molar fragments (AMNH 32523a and AMNH 32523b) to Bunobrontops sp. In 2008, Mihlbachler instead referred the two molar fragments to cf. Palaeosyops sp., noting that they were more similar to Palaeosyops molars in their proportions and wear patterns than to those of B. savagei.
Description Palaeosyops was a medium-sized brontothere, and was a considerably larger animal than the more primitive brontotheres that preceded it (Eotitanops). The largest Palaeosyops grew to be 1.5 to 2 metres (4.9–6.6 ft) long. Osborn estimated P. leidyi (a synonym of P. robustus) to have stood tall at the shoulder. In 2023, Oscar Sanisidro, Mihlbachler, and Juan L. Cantalapiedra estimated the body mass of Palaeosyops at . Supplemental Material (Data S1, Sheet 6)
As a more massive and robust animal than its predecessors, the skeleton of Palaeosyops was more graviportal (adapted to be more weight-bearing), foreshadowing the more massive brontotheres that would evolve after its time. Palaeosyops is thought to have had a lumbering gait, indicated by its spreading feet.
Skull and dentition
Palaeosyops had a robust skull, and is noted for its relative extreme brachycephaly (short and broad head) compared to other brontotheres. The strongly brachycephalic skull, and robust and sharply curved zygomatic arches, are synapomorphies that distinguish Palaeosyops from all other brontotheres. The brachycephaly of Palaeosyops also distinguishes it from more primitive perissodactyls, whereas the more primitive Eotitanops is noted for greater cranial and dental similarities to primitive equids and palaeotheres. Additional synapomorphies that distinguish Palaeosyops from other brontotheres include its sharply curved and distally tapering nasals, and the presence of a dome or low convexity in the region of the frontoparietal border.
Palaeosyops had the dental formula . The canines of Palaeosyops were large, and more prominent than in other brontotheres. The other incisors were also large and caniniform (canine-like). There was either a very small upper diastema (gap between teeth; in this case between the canine and premolars), or no upper diastema at all, and a moderately sized lower diastema. The premolars were not molarized (not molar-like), the molars had large paraconules, and the third molar sometimes had a hypocone or pseudohypocone.
The skull of Palaeosyops had a thin sagittal crest, and large areas of attachment for the masseter and temporal muscles. In combination with the robust zygomatic arches, and a deep and heavy lower jaw, these features sugggest great vertical movements of the jaw, and that Palaeosyops used great power when crushing food. The large canines may have been used for rooting around on the forest floor. Palaeosyops is believed to have had a diet similar to modern-day tapirs; it was likely well adapted to feed on tubers, roots, and bulbs from the ground, as well as for browsing on vegetation such as leaves or twigs. Unlike tapirs, the nasals of Palaeosyops were not retracted, and there is no evidence that it had a trunk or prehensile upper lip.
Classification