Gandheralophus robustus, Missiaen & Gingerich, 2012

Missiaen, Pieter & Gingerich, Philip D., 2012, New early Eocene tapiromorph perissodactyls from the Ghazij Formation of Pakistan, with implications for mammalian biochronology in Asia, Acta Palaeontologica Polonica 57 (1), pp. 21-34 : 26-28

Gandheralophus robustus sp. nov.

Figs. 2G–K, 3E–I; Table 1.

Etymology: From Latin robustus , solid, referring to the larger size and more robust aspect of the dentition in comparison with G. minor .

Holotype: GSP−UM 6768, a partial jaw with left m 1–3 in place, and alveoli of the right i1 and the left i1–p4.

Type locality: Gandhera Quarry (GSP−UM locality GH−40), Balochistan Province, Pakistan. GPS coordinates of the type locality are: 30.3852° N, 69.8238° E .

Type horizon: Late early Eocene (Ypresian); upper part of the upper Ghazij Formation; Gandhera Quarry, Balochistan ( Pakistan).

Referred material.—GSP−UM 4690, left DP3−4; GSP−UM 4709, left P4−M2; GSP−UM 4711, left P4−M1; GSP−UM 4717, right m2−3; GSP−UM 4722, left P4−M3; GSP−UM 5273, left M2; GSP−UM 5274, left M1; GSP−UM 5276, left M1; GSP−UM 5278, left M2; GSP−UM 5281, left M2; GSP−UM 5283, left M3; GSP−UM 5287, left M2; GSP−UM 5289, left M1; GSP−UM 5291, right DP4; GSP−UM 5296, left M1; GSP−UM 5297, right M1; GSP−UM 5298, left M2; GSP−UM 5302, left M2; GSP−UM 5323, left M1−2; GSP−UM 5325, right M2−3; GSP−UM 5327, right M1; GSP−UM 5328, left M1−2; GSP−UM 5356, right m3; GSP−UM 5357, left m3; GSP−UM 5358, right m3; GSP−UM 5360, left p2−4; GSP−UM 5362, left m3; GSP−UM 5363, right m2; GSP−UM 5364, right m2; GSP−UM 5378, left m3; GSP−UM 5381, left m2; GSP−UM 5382, left m2; GSP−UM 5432 P4−M2; GSP−UM 5434 right M1; GSP−UM 5435, left DP4; GSP−UM 5436, left M1; GSP−UM 5437, left M2; GSP−UM 5438, right P4; GSP−UM 5441, left M2; GSP−UM 5443, left m3; GSP−UM 5446, left p3−4; GSP−UM 6255, right P4−M3; GSP−UM 6260, left M2; GSP−UM 6262, right M2; GSP−UM 6265, left M1; GSP−UM 6272, right m1; GSP−UM 6275, right m1.

Diagnosis.— G. robustus differs from G. minor in being larger in size and more robust in appearance; in having a P4 with a metaconule but without a complete metaloph; in having upper molars with an anteroposteriorly compressed parastyle cusp, a slightly stronger labial cingulum and a more anteriorly directed metaloph; in lacking p1, having a smaller p2 and having a p3 with a smaller metaconid; in having a closed trigonid on p4 and on the lower molars; and in having a relatively larger m3.

Description.—The P4 of G. robustus has a variably developed, anteroposteriorly compressed parastyle which it is clearly separated from the subequal, closely spaced paracone and metacone. The robust protocone is the largest cusp on P4, and is connected to the ectoloph by a distinct protoloph. There is no continuous metaloph on P4, but a premetaconule crista runs from the metaconule to the premetacrista.

M1 is rectangular to somewhat trapezoidal in shape and slightly wider than long. The small, anteroposteriorly compressed parastyle is positioned directly anterior to the paracone. The metacone is somewhat lower than the paracone and clearly separated from it in a more posterolingual position. The protoloph has a strongly reduced paraconule and attaches to the ectoloph between the parastyle and the paracone. The metaloph attaches to the ectoloph just anterior to the metacone. Labial and lingual cingula are variably developed, ranging from weakly developed to strongly developed and continuous.

M2 is similar to M1 but larger and more trapezoidal.

The dimensions of M3 are similar to those of M2, but M3 is distinctly more trapezoidal with a rounded posterior bor− der. No accessory structures have been observed on the posterior part of M3.

A partial DP2 preserved on specimen GSP−UM 4690 ( Fig. 2H) shows the presence of a metacone and a distinct hypocone.

DP3 is subtriangular with a well−developed parastyle, paracone and metacone which are all clearly anteroposteriorly separated. The protoloph is short and connects the well−developed protocone to the preparacrista. There is no real metaloph, but a distinct metaconule is placed between and slightly anterior to the metacone and hypocone.

P4 is fully molariform, mainly differing from M1 by its more anteriorly projecting parastyle and better developed paraconule and metaconule.

The anteriormost lower dentition of G. robustus is unknown, but the holotype specimen shows the closely appressed alveoli of a small i1, a moderately sized i2 and i3, and the root of a strong canine. The canine is followed by a diastema of 10.5 mm, followed in turn by two small, partially fused alveoli and then by four larger alveoli. This shows that p1 is absent in G. robustus , and that p2 is reduced. The symphysis was not constricted and its posterior margin is situated just before the alveoli of p2.

The p3 is subrectangular and premolariform ( Fig. 3F–G). The paracristid projects strongly anteriorly and bears a marked paraconid. The strong protoconid is placed centrally on the trigonid, while the much smaller metaconid is placed posterolingual to it. The talonid presents a cusp that is placed just labial to the midline, with a crest running anteriorly towards the back of the protoconid.

The p4 is much wider than p3, with a metaconid that is subequal in size to the protoconid. The p4 paracristid is less anteriorly projecting than on p3 and does not have a paraconid. Instead, it forms a curved crest that runs down anteriorly from the protoconid, curves rather sharply into a transverse orientation and then curves again, running up the metaconid and forming a closed trigonid basin.

The m1 is rectangular ( Figs. 3E, H) with a trigonid similar to that on p4. The transversely oriented protolophid and hypolophid are distinct, but clearly notched. The hypoconulid is strongly reduced, forming a small, low cusp pressed against the back of the hypolophid.

The m2 is very similar to m1, mainly differing from it by its larger size, more robust appearance, and shorter paracristid ( Fig. 3I).

The m3 is distinctly larger than m2. The hypoconulid lobe is somewhat variable in development, but always forms a distinct basin with a posthypocristid directed towards the center of the hypolophid and usually a small, accessory lingual cusp.

Comparison.—The molars of Gandheralophus minor and G.

http://dx.doi.org/10.4202/app.2010.0093

robustus show only few distinct morphological differences, and both species are therefore placed in the same genus Gandheralophus . The two species differ most distinctly in size ( Fig. 4 ), and in the anterior dentition. G. robustus differs from G. minor in lacking p1, in having a smaller p2 and a morphologically simplified, more premolariform p3. These features can all be considered derived features, suggesting an evolutionary reduction of the anterior premolars in G. robustus . In this light, the weaker P4 metaloph in G. robustus might also be interpreted as a derived trait, and part of the same evolutionary tendency. Additionally, the relatively larger size of m3, and especially of the hypoconulid lobe, has also been considered a derived character in perissodactyls ( Hooker 1994; Froehlich 2002). Combined, these observations suggest that G. minor is closer to the ancestral morphotype of the genus Gandheralophus , whereas G. robustus is characterized by its reduced anterior dentition, as well as its larger size and more robust teeth.

The low−crowned molars of Gandheralophus , together with its distinct protolophs and metalophs on the upper molars, upper molars with short, straight ectolophs and very small paraconules, and distinct hypolophids on the lower molars are typical of a group of basal tapiroids often placed in the probably paraphyletic family Isectolophidae ( Radinsky 1963; Schoch 1989; Gingerich 1991; Ting 1993; Froehlich 1999; Lucas et al. 2003; Maas et al. 2001; Holbrook et al. 2004). Gandheralophus can be characterized as moderately lophodont, having relatively rectangular upper molars without a strongly projecting parastyle and with very weak upper molar conules, lacking a diastema between p1 and p2, having a p3–4 with a well−developed, molariform trigonid, and having lower molars with weak m1–2 hypoconulids and lacking a metastylid.

Its smaller size, the shape of the upper molars, the absence of a diastema between p1 and p2, the more molariform p3–4 and the absence of a metastylid clearly differentiate Gandheralophus from Cardiolophus and Homogalax from the early Eocene of North America ( Gingerich 1991) and from similar taxa such as Homogalax wutuensis and Chowliia laoshanensis ( Tong and Wang 2006) from the early Eocene of East Asia. The absence of a p1–2 diastema and of a metastylid are shared with Isectolophus , known from North America and Asia ( Radinsky 1963; Lucas et al. 2003). However, Ganderalophus differs from Isectolophus by its less advanced lophodonty, upper molars with a small paraconule and more oblique cross−lophs, by lower molars with a more lingually closed talonid basin, and a labiolingually wider m3 hypoconulid lobe.

The more rectangular, less trapezoidal shape of the upper molars and the absence of a metastylid or twinned metaconid make Gandheralophus morphologically closer to Orientolophus and Karagalax . The poorly known Orientolophus was described from the earliest Eocene Lingcha Formation in South China ( Ting 1993). Orientolophus is generally considered to be the most primitive isectolophid known ( Froehlich 1999; Hooker and Dashzeveg 2004), and is more primitive than Gandheralophus in terms of a weaker parastyle and a weaker metaloph and hypolophid, as well as stronger upper molar conules and m1–2 hypoconulids. Karagalax was described by Maas et al. (2001) from the Barbora Banda locality in Pakistan, and is probably early middle Eocene in age ( Gingerich 2003). Karagalax shares the absence of a diastema between p1 and p2 and a similar degree of lophodonty with Gandheralophus , but differs from the latter by a slightly more projecting upper molar parastyle, a transversely narrower lower dentition, and a strongly projecting paracristid on p3–4.

These observations may suggest that Gandheralophus and Karagalax formed a separate, South Asian isectolophid lineage that was possibly derived from a taxon close to Orientolophus from the earliest Eocene of South China. This southern lineage would then differ from other isectolophids from East and Central Asia and from North America by a smaller, less projecting parastyle, a more transverse metaloph and a smaller hypoconulid. This southern lineage would additionally differ from Cardiolophus − and Homogalax −like forms by the absence of a p1–2 diastema and lower molars with a more transverse hypolophid and lacking a metastylid, as well as possibly a stronger P3 postprotocrista. Finally, it would differ from Isectolophus by its less advanced lophodonty and a lingually less open lower molar talonid. The possibility of this southern isectolophid lineage is biogeographically significant, but it may require a more exhaustive phylogenetic analysis beyond the scope of this paper, as well as a better morphological knowledge of Orientolophus to be confirmed.

Figures

Fig. 4. Histograms showing the distribution of m1 (A) and m3 (B) size in Gandheralophus gen. nov. specimens of the two Pakistan species described here: G. minor and G. robustus (solid bars). The holotype of each species falls in the modal class. Comparison to a mixed male and female sample of the North American reference species, Hyracotherium tapirinum (Cope, 1875) (open bars; after Gingerich 1981) shows that the variability of Gandheralophus specimens is too great to represent a single species. Empirically, mammalian species have natural−log (length X width) ranges of 0.4 units (mean ± 2 standard deviations; Gingerich 1981). Each species shown here fits within a 0.4−unit range. Distributions of size of G. minor and G. robustus are strongly bimodal. Taken together they would have a range of 0.55 units (2.90 to 3.45) for m1 and of 0.80 units (3.45 to 4.25) for m3. Thus G. minor and G. robustus represent separate species.