Palaeosyops robustus ( Marsh 1872 )

Mader, Bryn J., 2010, A species-level revision of the North American brontotheres Eotitanops and Palaeosyops (Mammalia, Perissodactyla), Zootaxa 2339, pp. 1-43 : 39-40

Species Palaeosyops robustus ( Marsh 1872)

= P. laticeps Marsh 1872 = P. diaconus Cope 1873 b = P. leidyi Osborn 1908

= P. grangeri Osborn 1908 = P. copei Osborn 1908

Holotype: YPM 11122, a fragmentary skull.

Referred specimens: ACM 1794, AMNH 1544 (type of P. leidyi ), AMNH 1554, AMNH 1558, AMNH 1580, AMNH 2361, AMNH 5105, AMNH 5106 (type of P. diaconus ), AMNH 11678, AMNH 11683, AMNH 11708 (type of P. copei ), AMNH 11710, AMNH 12185, AMNH 12189 (type of P. grangeri ), AMNH 12190, UCM 19489, USNM 6704, USNM 12694, USNM 13454, USNM 13464, USNM 13465, USNM 13466, USNM 16660, USNM 16661, USNM 16690, USNM 26120, USNM 26138, YPM 11000 (tentatively included, type of P. laticeps ), YPM 11150, YPM-PU 10009.

Diagnosis: Species of Palaeosyops distinguished from P. fontinalis and P. paludosus by its larger size (length P2 to M3 is approximately 150 to 165 mm).

Discussion: As indicated in the Discussion section for the genus Palaeosyops , I recognize only a single valid species of Palaeosyops from the Twin Buttes Member of the Bridger Formation (Bridger C and D). Because " Limnohyus " robustus Marsh (1872) is the first species of Palaeosyops that is based on a type reliably known to have been collected from this level I have referred all specimens of Palaeosyops from the Twin Buttes Member of the Bridger Formation to this taxon.

In the same paper, however, Marsh (1872) also described another new species of Palaeosyops , which he named P. laticeps . This specimen was recorded from "Marsh's Fork", approximately fifteen miles (24 km) from Fort Bridger. Although there are several creeks located ten to fifteen miles (16–24 km) southwest of Fort Bridger, there is no creek known as "Marsh's Fork". It is not certain, therefore, which stratigraphic horizon this specimen is from, but if it was collected in the vicinity of Cottonwood Creek or Black's Fork, then it is probably from Bridger B (Blacks Fork Member). However, if the distance is accurately recorded as fifteen miles (24 km), then the locality would almost certainly be in the Twin Buttes Member. The Sage Creek White Layer, which is the boundary between Bridger B (Blacks Fork Member) and Bridger C (Twin Buttes Member) is found in deposits located approximately eleven to twelve miles (18–19 km) from Fort Bridger. Because of the uncertainty of the stratigraphic level I only tentatively regard P. laticeps as a synonym of P. robustus .

In accordance with the Principle of First Reviser (Article 24.2.1, International Code of Zoological Nomenclature, Ride et al. 1999) I select the name Palaeosyops robustus as having priority over the name P.

laticeps , as this will best serve nomenclatural stability (Recommendation 24A), because the stratigraphic level from which the type of P. laticeps was collected is uncertain and P. laticeps could thus be a junior synonym of P. paludosus from Bridger B. It should be noted that Marsh discussed Palaeosyops laticeps before he discussed " Limnohyus " robustus in his 1872 paper, but the International Code does not recognize the concept of “page priority” ( Nemésio 2007).

Specimens of Palaeosyops from the Adobe Town Member of the Washakie Formation, which is chronologically equivalent to the Twin Buttes Member of the Bridger Formation, are also referred to P. robustus . Because of the small sample size of Palaeosyops from the Washakie Formation (n=4) statistical analysis of the sample is not practical at this time.

In the same year that Marsh described " Limnohyus " robustus, Leidy (1872a) described the type of P. humilis , which Osborn (1929) alleged to be from Bridger C. As indicated in the discussion for the species P. paludosus , however, this specimen is probably from Bridger B and is thus referred to P. paludosus .

In 1873 Cope described parts of two maxillae with some of the cheek dentition preserved (AMNH 5106), which he identified as a new species of Palaeosyops , P. diaconus ( Cope 1873b) . This specimen is from Henry's Fork and is thus from the Twin Buttes Member of the Bridger Formation and represents P. robustus .

In 1908, Osborn described three new species of Palaeosyops , all of which I refer to P. robustus . Palaeosyops leidyi was based on a well-preserved skull (AMNH 1544) from Henry's Fork; P. grangeri was based on a palate from Twin Buttes with the grinding teeth and parts of the lower jaw and skull preserved (AMNH 12189); and P. copei was based on a series of upper grinding teeth (AMNH 11708) from Henry's Fork at Lone Tree.

In addition to Palaeosyops robustus, Gunnell and Yarborough (2000) recognized a second valid species of Palaeosyops from Bridger C: P. laticeps (here regarded as a synonym of P. robustus ). Gunnell and Yarborough distinguished P. laticeps from P. robustus by the former’s smaller size (especially in upper premolar dimensions) and relatively distinct hypocones on M3. However, specimens that Gunnell and Yarborough assign to the two species overlap in size for many variables (based on their published data). Furthermore, as indicated above, the distal part of the brontothere tooth row is highly variable and the size and morphology of the M3 hypocone generally makes a poor diagnostic character. Thus, the weight of evidence suggests that there is a single species of Palaeosyops in the Twin Buttes Member of the Bridger Formation and its stratigraphic equivalents.