Stephanocemas, 2009

Wang, Xiaoming, Xie, Guangpu & Dong, Wei, 2009, A new species of crown-antlered deer Stephanocemas (Artiodactyla, Cervidae) from the middle Miocene of Qaidam Basin, northern Tibetan Plateau, China, and a preliminary evaluation of its phylogeny, Zoological Journal of the Linnean Society 156 (3), pp. 680-695 : 681-688

STEPHANOCEMAS PALMATUS SP. NOV. ( FIGS 3–6 )

Holotype: IVPP V15722 , nearly complete, shed left antler without pedicel ( Figs 3 , 4 ). Collected by Gary T. Takeuchi and Jack Tseng on 13 August 2006.

Etymology: palmatus, Latin , webbed, palmate; in allusion to palmate antler of the species.

Type locality: IVPP locality CD0626 (N37° 06′ 12.6″, E97° 16′ 33.4″), north limb of Barun Yawula anticline, 30 km south-west of Delingha , Haixi Mongolian Autonomous Prefecture, Qinghai Province, northern Tibetan Plateau, China ( Figs 1 , 2 ) .

Referred specimens: IVPP V15723 , palm part of antler with only one medial tine preserved ( Fig. 5 ), from IVPP locality CD0630 (N37° 06′ 24.3″, E97° 15′ 59.7″) ; IVPP V15724 , posterior half of a small juvenile antler fragment ( Fig. 6 ), from IVPP CD0630 .

Geology and age: IVPP V15722 (from IVPP CD 0626) was produced from a sequence of rusty yellow to greenish cross-bedded sandstones and fine gravels along a north–south orientated wash on the western end of the north limb of the Barun Yawula anticline ( Fig. 2 ). IVPP V15723 and V15724 ( IVPP CD 0630) are also from the same rusty yellow sandstone layers but are stratigraphically ~ 115 m above IVPP CD0626. The Stephanocemas -producing sandstone, dipping toward the north, is more than 600 m above the axis of the Barun Yawula anticline, where reddish mudstones are exposed as the stratigraphically lowest exposure. Fifty kilometres north-west of the Barun Yawula anticline is the north limb of the Keluke (Kurliq) anticline, where a sequence of ~ 5000 m of Neogene sediments is exposed. Known as the Huaitoutala section, this long sequence contains vertebrate faunas ranging from the middle Miocene to early Pliocene age ( Wang et al., 2007) and palaeomagnetically dated to 15.7–1.8 Mya ( Fang et al., 2007). Lithologically the Stephanocemas -producing strata in Barun Yawula are approximately comparable to the greenish sandstones in the Huaitoutala section, where the middle Miocene Olongbuluk Fauna is found. The main difference seems to be that the Huaitoutala section is a much thicker sequence (the Olongbuluk Fauna alone spans more than 1000 m), whereas the same section in Barun Yawula is represented by substantially thinner beds, possibly indicating a lower rate of deposition.

Faunally, the Stephanocemas -producing sandstones in Barun Yawula (localities CD 0626, 0630, 0633, 0634; see Fig. 2 ) have so far not yielded other agediagnostic taxa beside a few non- Stephanocemas cervid antler fragments, as well as fragments of turtles and teleost fishes. However, in the eastern end of the Barun Yawula anticline, ~ 6 km to the southeast of CD0626, a small assemblage of fossil mammals was collected. Of these, the most agediagnostic include an astragalus of hipparionine horse ( IVPP CD 0618), indicating a late Miocene age. Based on correlations by satellite image ( Fig. 2 ), the CD0618 locality is stratigraphically close to or slightly above the highest Stephanocemas -producing horizon ( CD 0634), although such correlations are not without difficulties, particularly in the eastern end of the Barun Yawula anticline where multiple faults have horizontal offsets of up to several hundred metres. To the best of our knowledge (see further discussion in ‘Problems with Bohlin’s Qinghai Materials’ below), Stephanocemas is restricted to the middle Miocene strata (the Olongbuluk Fauna) in the eastern Qaidam Basin, i.e. no lagomerycine deer has been collected in known late Miocene faunas ( Wang & Wang, 2001; Deng & Wang, 2004a, b; Wang et al., 2007). We thus conclude that Stephanocemas palmatus probably represents the latest appearance of the lineage in the Tibetan Plateau in the latest middle Miocene, although the possibility exists that its upper-most occurrence ( CD 0634) is earliest late Miocene.

Diagnosis: Stephanocemas palmatus is easily distinguished from species of Paradicrocerus ( Paradicrocerus flerovi , Paradicrocerus elegantulus , and Paradicrocerus brevistephanos ) in its derived characters such as large size, loss of dorsal ridges between anterior and posterior tines, enlarged central (palm) portion of antler, and horizontally orientated side branches (tines). Stephanocemas palmatus differs from all previously known species of Stephanocemas ( Stephanocemas aralensis , Stephanocemas chinghaiensis , Stephanocemas rucha , Stephanocemas thomsoni ) in the following derived characters: large size, expanded central (palm) part of antler, more horizontally orientated tines, and multiple (three or more) tines being plate-like.

Description

The unusual arrangement of antlers in Stephanocemas confounded Colbert (1936), who was unable to find a suitable method to describe its peculiar patterns. Although he agreed in principle with a nomenclatural scheme by Pocock (1933), who envisioned a series of dichotomous branching patterns in the posterior beams as seen in living cervids (see also Colbert, 1940), in practice Colbert found such a system difficult to apply to Stephanocemas . Instead, Colbert devised a rather arbitrary system of antler nomenclature of anterior (brow tine), posterior (beam), median, and external tines. Because the branching patterns in Stephanocemas are apparently derived independently from those in living cervids, Colbert’s system is adopted in this description to avoid implications of homology with living cervids.

The orientation of antlers is not always easily ascertained without associated cranial elements or large ontogenetic series. Our determination for the holotype of S. palmatus was based on comparisons with the best-preserved series for P. elegantulus and S. thomsoni . In general, the widest tine is the posterior one, and its opposite tine is, by definition, the anterior tine. The lateral and medial tines are generally shorter than the anterior and posterior tines. The lateral tines tend to be longer and more upwardly curved relative to their medial counterparts. Given the above observation, we regard IVPP V15722 as a left antler.

IVPP V15722 is a large, full adult individual. A juvenile partial antler from a higher stratigraphical horizon (see ‘Geology and age’), IVPP V15724 , offers some sense of ontogenetic variations. Even in a very young individual, such as IVPP V15724 , the palmate nature of the antler is readily apparent ( Fig. 6 ), and distinctly different from juveniles of S. thomsoni ( Colbert, 1936: fig. 5). The entirely palmate IVPP V15724 has four branches and the palm portion is folded along the anteroposterior axis.

The palm portion of IVPP V15722 is mediolaterally expanded with a relatively high width/length ratio. Toward the medial-central aspect of the palm, there is a distinct dorsal spur, which is broken off at the base. Only a rounded scar is left for this spur. Other than this spur, the dorsal surface is flat and smooth without any trace of the dorsal ridge seen in Paradicrocerus (see Phylogeny). On the ventral side, an indication of a burr (coronet) is marked by small bony spurs at the bases of posterior and lateral tines. The burr is oval-shaped with its long axis orientated anteroposteriorly, which obviously corresponds to an oval cross-sectioned pedicel (stem). This burr-like structure, a concave shedding scar, and rough surface surrounding the burr clearly indicate a deciduous antler.

The most characteristic aspect of the tines in IVPP V15722 is their spread-out appearance with their nearly horizontal orientation and their flattened cross-section. In lateral view, anterior and posterior tines spread out from the palm, initially horizontally toward the periphery and then gently curving upward toward their tips ( Figs 3 , 4 ). In anterior view, the medial tines (only the posterior one is preserved) are essentially horizontal whereas the lateral tines curve upward slightly, creating an asymmetrical appearance in terms of the upward curvatures of medial and lateral tines.

Of the preserved tines, the posterior, posterior lateral, and anterior lateral tines are bifurcated, although, judging from the condition of the anterior lateral tine, the bifurcated tines do not extend far from the base of the split. The anterior tine is relatively short and wide in dorsal view. Its cross-section is basically dorsoventrally flat with the dorsal surface slightly convex. The medial corner of the tip is broken, but judging from the remaining parts, the anterior tine may be slightly bifurcated.

The posterior tine (beam) is the widest of all the tines and maintains its width throughout its length, and slightly widens toward the distal end. The crosssection is very flat, with a convex ventral surface and a slightly concave dorsal surface. The tine is undoubtedly bifurcated, even though its lateral tip is broken off at the base of the bifurcation (see reconstruction in Fig. 10 ).

The anterior lateral tine is slightly longer than the anterior tine. This flattened tine has shallow grooves on both dorsal and ventral surfaces throughout its length. This is also the only tine that twists almost 90° – its bifurcated tips are almost vertically aligned, with the effect of a maximum width when viewed laterally. This tine also preserves the best tips (both are slightly restored with epoxy resin).

The middle lateral tine is broken midway along the length. As in the anterior lateral tine, it is dorsoventrally compressed. Its cross-section shows shallow grooves on dorsal and ventral sides, indicating bifurcated tips.

The posterior lateral tine is longer than the posterior tine. Its dorsoventrally compressed surfaces have shallow grooves that lead to slightly bifurcated tips, which are not as distinct as those on the anterior lateral tine. The grooves taper off toward the anterior aspect of the tine.

The posterior medial tine is also longer than the posterior beam. Its cross-section is somewhat triangular at its base, gradually becoming flattened toward the tip, and its tip is not bifurcated. The middle and anterior medial tines are broken off at the base. Both have a rounded cross-section, indicating a single tip, as in the posterior medial tine. The base of the middle medial tine is elevated to form a prominent hump above the dorsal surface of the palm.

Measurements (mm): maximum anteroposterior diameter of palm: 77.7; maximum mediolateral diameter of palm: 53.6; maximum ¥ minimum diameters at base of burr: 35.0 ¥ 20.0; length of anterior tine: 39.3; maximum ¥ minimum diameters at base of anterior tine: 19.4 ¥ 9.9; length of posterior tine: 48.3; maximum ¥ minimum diameters at base of posterior tine: 28.2 ¥ 12.8; length of posterior medial tine: 54.7; maximum ¥ minimum diameters at base of posterior medial tine: 16.6 ¥ 12.6; maximum ¥ minimum diameters at base of middle medial tine: 11.2 ¥ 10.7; maximum ¥ minimum diameters at base of anterior medial tine: 14.9 ¥ 11.7; length of anterior lateral tine: 52.7; maximum ¥ minimum diameters at base of anterior lateral tine: 17.7 ¥ 10.9; maximum ¥ minimum diameters at base of middle lateral tine: 20.3 ¥ 12.8; length of posterior lateral tine: 58.0; and maximum ¥ minimum diameters at base of posterior lateral tine: 18.3 ¥ 10.9.

COMPARISON AND DISCUSSION

In this study, we restricted our concept of Stephanocemas to those Asiatic species that fall within the advanced clade of the Paradicrocerus – Stephanocemas lineage (see Phylogeny). Primitive species formerly included in Stephanocemas , such as S. elegantulus ( Roger, 1898) and S. brevistephanos Baschanov & Nurumov, 1955 , are here placed in the genus Paradicrocerus . In such a phylogenetic framework, S. palmatus is easily distinguishable from species of Paradicrocerus in its lack of dorsal ridges between anterior and posterior tines, expanded palm portion of the antler, and laterally spread tines.

Within the genus Stephanocemas , the type species, S. thomsoni Colbert, 1936 , is still the best documented species with a large array of individual antlers representing a nearly complete ontogenetic series. Such a series demonstrates a large amount of intraspecific variation in size, number, position, and orientation of individual branches (tines). Such a range of variations is probably typical for other species of the Paradicrocerus – Stephanocemas lineage as well because one of its basal species, P. elegantulus ( Roger, 1898) , shows similarly large variations (see Stehlin, 1937). The large ontogenetic series of S. thomsoni from the Tunggur Formation also allows a certain amount of confidence that S. palmatus falls outside the variations of S. thomsoni . Despite the substantial amount of flattening in the posterior tine in S. thomsoni , the rest of the tines remain essentially rounded in cross-section, with the exception of the anterior median tine in the holotype ( AMNH 26782). These rod-like tines with a single tip seem to be a primitive condition in contrast to the mostly platelike tines with branched tips in S. palmatus . Furthermore, the palm portion of the antlers in S. palmatus is also mediolaterally widened relative to those in S. thomsoni . If such a comparison is correct, then S. palmatus represents the most derived member of the genus and helps to envision a morphological framework for other lesser known species.

Colbert (1936) described a second species of Stephanocemas , S. triacuminatus , from the Tunggur Formation. However, following a suggestion by Stehlin (1937), Colbert (1940) soon recognized it as a species of Lagomeryx , a practice that has been followed since then (e.g. Young, 1964), as well as in this study.

Beliajeva (1949) figured and described a partial antler of Stephanocemas sp. near Sarybulak River from the Zaysan Basin in eastern Kazakhstan. The Zaysan form is distinguished by its relatively small palm portion, rounded tine cross-sections, and more vertically orientated tines, characters that easily differ from those in S. palmatus .

Young (1964) named a new species, Stephanocemas chinghaiensis , based on an antler from Ledu (= Nien-Pai) County, Qinghai Province, described by Bohlin (1937) (see additional comments in ‘Problems with Bohlin’s Qinghai Materials’ below). Based on published figures ( Bohlin, 1937: text figs 42–44), the Ledu antler is distinct from S. palmatus in its smaller size, smaller palm portion, fewer medial and lateral tines (one each), more rounded cross-sections of the tines, and slightly more upright orientation of the tines.

Beliajeva (1974) erected a new species, Stephanocemas aralensis , from the middle Miocene of western Kazakhstan. The holotype ( PIN 1551-67) is a nearly complete left antler collected near Bes-Tyube, north of the Aral Sea. Based on the illustrated holotype and referred materials ( Beliajeva, 1974: figs 2–4), S. aralensis is easily distinguished from S. palmatus in its smaller size, much more elongated burr (i.e. shedding scar; corresponding to a more mediolaterally compressed pedicel), less mediolaterally expanded palm portion, more upwardly (vertically) orientated peripheral tines, more rounded cross-sections for most peripheral tines, fewer lateral (one to two) and medial (two to three) tines, and more proximally branched posterior tine (more so in the holotype and a referred antler, PIN 1568-42, than in PIN 1551-68). With the possible exception of the elongated burrs, most of the characters in S. aralensis are primitive, and are probably far removed from those of S. palmatus .

More recently, Ginsburg & Ukkakimapan (1983) described a new species, Stephanocemas rucha , from lignitic deposits of the Li Basin in northern Thailand, which represents the southern-most occurrence of the genus. Initially thought to be late middle to early late Miocene in age mostly based on the presence of proboscideans and Stephanocemas ( Ginsburg & Ukkakimapan, 1983; Ginsburg, 1984), the age of the Li Fauna was revised downward to early Miocene (equivalent to the MN4 of the European mammal zones), based on long-distance comparison of mammals from Europe ( Mein & Ginsburg, 1997). However, more recent studies of magnetostratigraphy ( Benammi et al., 2002) and fossil rodents ( Chaimanee et al., 2007) from the nearby Mae Moh Basin, placed the Li Basin strata (considered equivalent in age to that of the Mae Moh Basin) back in the late middle Miocene age, around 13–12 Mya. Regardless of its age, the Thai species is the southernmost occurrence of the genus and is zoogeographically important. Unfortunately, the holotype of S. rucha is poorly preserved. Additional antler fragments were listed, but these were not figured nor described ( Mein & Ginsburg, 1997). Our present comparison is thus based on the holotype (T Li 67) only. The palm portion of S. rucha is smaller and less laterally spread than that of S. palmatus . Judging from the anterior tine, the only fully preserved one on the holotype, and the bases of the two lateral tines, the radiating tines are all orientated rather vertically, i.e. forming an angle greater than 45° with the horizontal plane. In this condition, the Thai species is thus somewhat primitive among advanced species of Stephanocemas (see Fig. 9 ). If our reconstructed outline of the posterior tine ( Fig. 10 ) is correct, an admittedly speculative reconstruction based on published figures, then S. rucha shares with S. thomsoni and S. palmatus a flattened posterior tine.

Ye (1989) referred to Stephanocemas aff. thomsoni 20 partial antlers, as well as dental materials, from the lower part of the Halamagai Formation (middle Miocene), Junggar Basin, Xinjiang Province. The relatively large sample size, representing both adult and juvenile individuals, reveals a substantial amount of variations. Nonetheless, the following features in the Junggar materials are consistently different from those in S. palmatus : smaller size, smaller palm part of the antler, more rounded crosssections in tines, and slightly more upright tines, although the two taxa seem to share a mediolaterally expanded palm.

Abdrachmanova (in Tleuberdina, Bayshashov & Abdrachmanova, 1993) described a new species, Stephanocemas actauensis , from the Aktau Mountain area in the Ily Basin of eastern Kazakhstan. The upper Aktau Mountain strata that produced S. actauensis were estimated to be early Miocene in age ( Tleuberdina, 1994; Lucas & Bendukidze, 1997). The holotype is a small partial antler with five or six tines. Assuming the Aktau antler is an adult, it differs from S. palmatus by the following primitive characters: small size, five to six tines with rounded crosssections, and more upward orientation of the tines.

Most recently, Li et al. (1998) referred to S. thomsoni four antlers, two jaws, and several teeth from the Hongliugou Formation in Tongxin, Ningxia Autonomous Region, an area that was previously known to contain some fragmentary materials of the genus ( Chen, 1978). As typified by the Dingjiaergou local fauna, the middle part of the Tongxin strata is middle Miocene (Tunggurian) in age ( Qiu, Wu & Qiu, 1999). Judging from a figured antler ( BPV 1212) ( Li et al., 1998: figs 1, 2) and from the authors’ description, the Tongxin materials seem closest to S. palmatus from Qaidam. BPV 1212, presumably a mature individual, has reached the stage of palm expansion of the latter. Its peripheral tines seem also substantially flattened. Furthermore, examination of another specimen, IVPP V5586 (a relatively young individual), shows a similarly flattened and mediolaterally expanded palm portion of the antler, as also noted by Chen (1978). We are thus inclined to treat the Tongxin materials as conspecific to S. palmatus . If the Tongxin species is correctly referred, it would significantly extend the geological range of the species to the early middle Miocene.

Figures

Figure 1. Map of Qaidam Basin showing surrounding mountains and major vertebrate fossil localities.

Figure 2. Satellite image of the Barun Yawula anticline, with key fossil localities and their relative stratigraphical positions indicated. The east–west trending fold is asymmetrical with the south limb dipping more steeply than the north limb. A prominent resistant bed (a dark–light band combination, indicated by black dashed lines) within the rusty green sandstones layers helps to trace stratigraphical relationships between localities in eastern and western ends of the anticline, although multiple faults (with offsets ranging from 50 to 500 m), particularly those in the eastern end, complicate correlations. White lines are the measured section.

Figure 3. Stereophoto of dorsal view of IVPP V15722, left antler without pedicel, holotype of Stephanocemas palmatus sp. nov. from Qaidam Basin, northern Tibetan Plateau. Top is posterior and bottom is anterior.

Figure 4. IVPP V15722, left antler without pedicel, holotype of Stephanocemas palmatus sp. nov. from Qaidam Basin, northern Tibetan Plateau. A, medial, and B, ventral views. Left is posterior and right is anterior.

Figure 5. IVPP V15723, referred specimen of Stephanocemas palmatus sp. nov. A, dorsal, and B, ventral views of palm portion of antler.

Figure 6. IVPP V15724, referred specimen of Stephanocemas palmatus sp. nov. A, dorsal, B, ventral, and C, medial views of posterior palm portion of a juvenile antler.

Figure 9. Strict consensus of four shortest trees (tree length = 12) of the Paradicrocerus–Stephanocemas clade found by the branch and bound option of the PAUP program on a ten taxa ¥ nine characters data matrix (Table 1).

Figure 10. Species ranges, phyletic relationship, and zoogeographical positions of the Paradicrocerus–Stephanocemas clade. Most of the species ranges are approximate. Phyletic relationship is based on one of the shortest trees in our cladistic analysis, and some indeterminate taxa not included in the cladogram are inserted here based on our estimates of their relationships. The antlers are scaled to their approximate relative size, and dashed lines are mostly our own reconstructions of missing tines.