Platybelodon grangeri ( Osborn, 1929 ), 1813

Wang, Shiqi, He, Wen & Chen, Shanqin, 2013, The gomphotheriid mammal Platybelodon from the Middle Miocene of Linxia Basin, Gansu, China, Acta Palaeontologica Polonica 58 (2), pp. 221-240 : 223-232

Platybelodon grangeri ( Osborn, 1929)

Figs. 3–7 , Tables 1 and 2.

Referred material.—Laogou locality (LX200002): juvenile skulls (HMV0049, 0050, 1812?, and 1837); juvenile skull associated with mandible (HMV1813?); adolescent skulls (HMV0019, 1815?, 1828?, and 1839?); adult skulls (HMV0014–0018, 0020–0027, 1840?, and 1841?); adult skulls associated with mandibles (HMV0939 and 0940); juvenile mandibles (HMV0045–0048); adolescent mandibles (HMV0029, 0040, 0041, and 0044); adult mandibles (HMV0030–0039, 0042, 0043, 1830, 1836, and 1842?); jaw remains (HMV1800, fragmentary right maxilla with M2 and M3; HMV1801, fragmentary maxillae with M3s; HMV 1798, fragmentary right dentary with m2 and m3); isolated cheek tooth (HMV1799, right m3); incisor remains (HMV1266–1269, and 1863, left I2s; HMV1275 and 1862, left i2s; HMV1272, right i2). Laogou locality (LX200003): jaw remains (HMV1797, fragmentary maxillae with DP4s and right M1; HMV1793, fragmentary dentaries with right m1; HMV1263 and 1795, fragmentary left dentary with m2 and m3; HMV1784 and 1794, fragmentary right dentary with m2 and m3; HMV1786, fragmentary left dentary with m3; HMV1785 and 1796, fragmentary right dentary with m3); isolated cheek teeth (HMV1859, right M2; HMV1788 and 1845?, right M3s; HMV 1787, 1802?, 1803?, and 1846?, left m3s; HMV1783, 1804?, 1847?, and 1852?, right m3s); incisor remains (HMV1270, right I2; HMV1274, 1789, 1790, and 1792, left i2s).

Emended diagnosis.—The diagnosis of Platybelodon grangeri by Osborn and Granger (1931: 4) was mainly based on the mandible and the lower incisors. Here, we add some features pertaining to the cranium and the molars: the neurocranium is low and elongated, with a narrow and long dorsal table in females, and a relatively broad and short one in males. In adults, the posterior border of the external nares is located posterior to the postorbital process, while the anterior border of the orbit is situated posterior to the anterior border of M3. The alveolus of the incisor is slender, and the upper tusk is relatively weak and lacking enamel bands in most cases. The elongated and extremely flattened mandibular

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symphysis is relatively long in males, but short in females. In adults, a robust transverse ledge is developed at the narrowest part of the mandibular symphysis ( Fig. 5A, D ). The ascending ramus of the mandible is directed posteriorly, and the mandibular angle is blunt and rounded. Cementum and small enamel conules are well developed in the interloph(id)s of the upper and lower molars, whereas their cingula and cingulids are relatively weak. Pretrite trefoils are developed on at least the first two loph(id)s of all molars and the last deciduous premolars. While the pretrite trefoils are relatively symmetrical on the upper molars, advanced forms are marked by relatively weak posterior lobes (posterior pretrite accessory central conules) and the presence of posterior posttrite accessory central conules. On the lower molars, the pretrite trefoils tilt anteromedially, and are marked by strong and individualized accessory conules tending to invade the neighboring entoflexids, as well as the presence of anterior posttrite accessory central conules. The pretrite and posttrite half−lophids of the lower molars occupy alternate positions. The intermediate cheek teeth (DP4–M2, and dp4– m2, respectively) tend to have four loph(id)s, with complete fourth loph(id)s present on M2 and m 2 in the more advanced forms. By contrast, M3 and m3 are relatively narrow and possess more than four loph(id)s.

Description

Skull.—In dorsal view ( Figs. 3D 1 , 4B ), the alveolus for the incisor is slender and long. The incisive fossa between the

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premaxillae is narrow and deep, and its posterior convex ledge is located deep within the external nares. The anterior end of the nasal is blunt. The posterior border of the external nares is located between the postorbital processes in the juvenile specimens, whereas it is situated more posteriorly in adults. The latter feature is particularly well developed in males, resulting in a relatively broad and short dorsal table of the neurocranium in these individuals, as opposed to the narrow and long dorsal table found in juveniles and females. Anteriorly, the temporal lines are directed anterolaterally, thus connecting the postorbital processes. Further posteriorly they converge to form an intertemporal constriction, before extending posterolaterally to merge with the occipital crest. The zygomatic process of the squamosal is relatively weak in juveniles and females, but strong in males.

In anterior view ( Fig. 3C ), the transversely elliptical opening of the external nares is wide and low. The rough surface of the dorsal part of the premaxilla for the attachment of maxillo−labialis is broader in males than in females and juveniles, suggesting that the former might have possessed a stronger trunk. In ventral view ( Fig. 3D 2 ), the incisive alveolus is slender and long, and converges distally in females, whereas it diverges in males. The triangular zygomatic process of the maxilla is relatively small and does not extend far laterally. The palate is narrow, flattened, and slightly arched upward. The suture between the palatine bone and the maxilla is visible, and anteriorly terminates in a small, slit−like palatine foramen. The posterior edge of the last functional upper check tooth is located anterior to the rounded anterior border of the choanae. The latter are elliptical and have two sharp lateral edges, forming the pterygoid processes. The posterior opening of the alisphenoid canal is large, oval, and located in the posteromedial part of the alisphenoid. The sharp crest of the pterygoid process extends backwards to connect with a crest located on the anterior border of the tympanic bulla, with the confluent openings of the oval and middle lacerate foramina located in a deep groove situated dorsal to the former crest.

The tympanic bulla is triangular and extends laterally and posteriorly, with a concave and vertical posterolateral edge. There is a large, rounded opening on the medial side of the tympanic bulla, representing the canal of the internal carotid artery, while a depression located posterior to the medial side of the tympanic bulla marks the position of the posterior lacerate foramen. The stylomastoid foramen is located in a deep fossa developed posterolateral to the tympanic bulla. The glenoid fossa is relatively flattened, lacks a prominent postglenoid process, and is anterolaterally connected to the posterior end of the jugal. On the posterior part of the ventral surface of the squamosal there is a depressed region, representing the external auditory meatus. The portion of the exoccipital posterior to the squamosal is more convex ventrally than the more medial part of the exoccipital. The occipital condyles are large and triangular, and directed posterolaterally. The basioccipital is triangular posteriorly, and attaches to the cylindritransverse ledge mandibular symphysis cal basisphenoid. The vomer is sharp and extends anteriorly into the choanae.

In posterior view ( Fig. 3B ), the occipital surface is transversely elliptical and nearly perpendicular to the dorsal and ventral sides of the skull. The fossa for the ligamentum nuchae is large and rounded, and marked by a thin external sagittal crest running through its center. The occipital condyles are rounded ventrolaterally, and border a dorsoventrally compressed foramen magnum. In lateral view ( Figs. 3A, D 3 , 4B ), the neurocranium is generally flattened, though in some males it may be slightly arched. The basicranium (the part of the cranium located posterior to the pterygoid processes) and palate (the part located anterior to the pterygoid processes) are located almost in the same horizontal plane. The slender and long alveoli for the incisors have strong crests developed on their dorsal faces, and are located almost in the same plane as the palate in juveniles and females, while sloping further downwards in males. The lateral, facial part of the maxilla is

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low and elongated. The anterior border of the orbit is located in line with the middle or anterior part of M 3 in fully adult individuals. The rounded lower infraorbital foramen is relatively large and situated just in front of the zygomatic process of the maxilla, while the upper infraorbital foramen is slit−like and located on the anteromedial side of the orbit. The orbitotemporal crest extends posteroventrally from the strongly developed postorbital process, with the ethmoidal and optic foramina occurring as small openings below the central part of the crest overlapping the sutura sphenofrontalis. The lower part of the orbitotemporal crest is located dorsal to a deep groove containing the anterior lacerate and round foramina, and forms the anterior edge of the alisphenoid, with the latter overlapping the posterior part of the maxilla. The temporal fossa is fan−shaped and bordered by a short zygomatic process of the squamosal, with the external auditory meatus visible as a small opening in the ventral part of the latter. The jugal is slender.

Mandible.—In dorsal view ( Figs. 5A, B 1 , D), the body of the mandible is well developed and connected by a troughshaped symphysis, with the latter being concave dorsally and convex ventrally, and terminating in a straight anterior edge. A robust transverse ledge is developed at the narrowest portion of the symphysis in adolescents and adults, while being more poorly developed in juveniles. The ascending ramus is thin and the mandibular foramen large and triangular. In immature individuals, deciduous molars are present anterior to the mandibular foramen. In lateral view ( Figs. 5B 2 , C), the body of the mandible is high and steeply descends from the anterior border of the postcanine tooth row to form the extremely flattened and elongated mandibular symphysis. Laterally, the symphysis is bordered by a well−developed crest. A large, rounded mental foramen is located close to the anterior edge of the postcanine tooth row, with a second, slit−like foramen located anterior to the first one near the narrowest part of the mandibular symphysis. The ascending ramus tilts backwards, and terminates in a blunt and rounded angular process. The coronoid process is rounded and directed dorsally, while the mandibular condyle has the shape of a transversely oriented, cylindrical bar.

Incisors.—Only specimen HMV1813 preserves DI2 ( Fig. 4 ), which is long and slender, and covered by a layer of enamel. The distal end is blunt, and the tooth is rounded or oval in cross section. By contrast, I2 ( Fig. 3A, C, D ) ranges in development from slender to strongly columniform, and develops either a sharp or blunt distal end after wear. While short in females (exposed length generally <300 mm), the tooth is long in males (exposed length generally> 300 mm), although in life it did not protrude beyond the tip of the lower tusk. Though nearly straight, the teeth point slightly ventrally and diverge laterally, allowing left and right fragments to be distinguished. There is no enamel band on the upper incisors from the Linxia Basin; however, at least one I2 from the Tunggur area (AMNH26567) preserves vestiges of enamel at the tip.

Both HMV0047 and HMV1813 preserve di2 ( Fig. 5B 1 , B 2 ), which is flattened and thin, with an upwardly concave cross section. The anterolateral corner of the distal end of this tooth is relatively rounded, while its lateral edge is serrated. Similarly, i2 ( Fig. 5A, C, D ) is flattened, shovel−shaped, and upwardly concave in cross section. The tooth is furthermore slightly outwardly twisted, with its medial side being perpen−

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dicular to both its ventral and dorsal surfaces, and a little thicker than the rounded lateral side. In cross section, i2 is composed of multiple layers of dentinal tubules, which are enclosed in a layer of dentine. In some specimens, the tubules are strong but the number of layers is relatively low (6–7 layers), whereas in others the tubules are thin but arranged in a greater number of layers (~10 layers).

Cheek teeth.—DP2 ( Fig. 6A ) is preserved in HMV0050 and HMV1813. The tooth is oval and composed of two lophs, with the second loph being stronger. The first pretrite main cusp (protocone) is smaller and more posteriorly positioned than the posttrite one (paracone). The second pretrite (hypocone) and posttrite (metacone) main cusps are almost the same size. The anterior cingulum is more developed than the posterior one. DP3 ( Fig. 6A ) is moderately to strongly worn in all specimens in which it is preserved (HMV0049, HMV0050 and HMV1813). The tooth has a rectangular outline in occlusal view and is composed of two lophs, with the second loph being stronger. The wear patterns of the pretrite half−lophs are shaped like a trefoil, while the posttrite half−lophs are marked by elliptical wear surfaces oriented perpendicular to the long axis of the crown. The structure of the second loph is similar to that of the first one. Small enamel conules are developed in the interloph, and the anterior and posterior cingula are equally developed. DP4 ( Fig. 6B ) is rectangular and composed of three lophs oriented perpendicular to the long axis of the tooth, plus a posterior cingulum. The first pretrite trefoil is well developed, with a serrated anterior accessory central conule linked to the anterior cingulum, a simple posterior accessory central conule, and a weak mesoconelet. The first posttrite half−loph comprises a mesoconelet and a posterior accessory central conule. The second pretrite trefoil is symmetrical, with equal development of the anterior and posterior accessory central conules and a weak mesoconelet. The second posttrite half−loph is similar to the first one. The third pretrite half−loph consists of a mesoconelet and an anterior accessory central conule, with the former often shift− ed anteriorly to fuse with the latter. The third posttrite half−loph only has an anteriorly located mesoconelet. The posterior cingulum is composed of a row of enamel conules. Small enamel conules are also well developed in the interlophs, which are covered by a thin layer of cementum. The cingulum is developed along the anterior border of the tooth and the lingual side of the first interloph.

P3 ( Fig. 6B ), preserved in HMV1837 and HMV1812, is weak and oval in occlusal view. Two nearly isometric cusps form a middle loph. Weak anterior and strong posterior cingula are present and composed of a series of small conules, with the outermost two conules being the largest in both cingula. By contrast, P4 ( Fig. 6C ), preserved in HMV1839, HMV1815, and HMV1828, is rectangular in occlusal view and composed of two lophs. The anterior loph is a little stronger than the posterior one, and on each loph there are two main cusps. Rudimentary pretrite trefoils are present, while the posttrite half−lophs are simple. The posterior cingulum is stronger than the anterior one.

The structure of M1 ( Fig. 6B, C ) is similar to that of DP4, but the tooth is larger and has a better developed posterior cingulum and cementum in the interlophs, with the latter also bearing small, well−developed enamel conules. M2 ( Fig. 6C ) resembles DP4 and M1, but is larger, hypsodont (here defined as having an unworn crown height of more than 50 mm, almost the same as its width), and has a better developed posterior cingulum and thicker cementum in the interlophs than M1. By contrast, the development of small enamel conules in the interlophs is somewhat diminished. M3 ( Fig. 6D, E ) is rectangular in occlusal view and marked by a wide anterior portion. It is hypsodont and consists of 4–5.5 lophs. The first two lophs are similar to those of M2, with well−developed pretrite trefoils, posttrite half−lophs comprising mesoconelets and posterior accessory central conules, and the first anterior pretrite accessory central conule being linked to the anterior cingulum. On the third, fourth, and fifth lophs, only anterior pretrite accessory central conules are developed, and tend to diminish in size from the anterior to the more posterior lophs, or are absent altogether. The posterior posttrite half−lophs are simple, bearing only anteriorly shifted mesoconelets, but no posterior accessory central conules. The posterior cingulum is generally composed of two large cusps, of which the pretrite is the larger. There is always an anterior cingulum, and the cementum in the interlophs is strongly developed. Some small conules are present in the interlophs, but they are more weakly developed than on the more anterior teeth. The above observations are based on the specimens from Zengjia. By contrast, the posterior pretrite accessory central conules on the anterior lophs are somewhat reduced, and the cementum in the interlophs is even more developed in the specimens from Laogou.

Unlike in specimens from Tunggur (Osborn and Ganger 1931), dp2 is not preserved in any of the Linxia specimens. Only HMV0050 has an alveolus for a single root in front of dp3, implying that dp2 is only weakly developed in this group. By contrast, dp3 ( Fig. 7A ) is preserved in four specimens ( HMV0047 , HMV0048 , HMV1813 , and HMV0045 ), and is long, triangular, and composed of two lophids. Double trefoils are developed on the first lophid, with the posterior pretrite accessory central conule being distinct from the main cusp and connected to the latter via an enamel crest. The second lophid is wider than the first and the second pretrite trefoil is developed, while the posttrite half−lophid is rather simple. The anterior cingulid is relatively weak, whereas the posterior one is stronger. Compared to the corresponding upper tooth, dp4 ( Fig. 7A ) is narrower and longer. It is rectangular and composed of three lophids plus a posterior cingulid, with the lophids, especially the anterior two, being tilted both anteriorly and lingually. The first pretrite is trefoliate, with the anterior pretrite accessory central conule being linked to the anterior cingulid, while the posterior pretrite accessory central conule is individualized and has a tendency to invade the neighboring entoflexid. The mesoconelet is weak. The first posttrite half−lophid has a mesoconelet and weakly outlined anterior and posterior accessory central conules. The second pretrite half−lophid is also trifoliate, comprising a strong anterior and posterior pretrite accessory central conule and a weak mesoconelet. The second posttrite half−lophid has a mesoconelet and an anterior accessory central conule. The third pretrite half−lophid also has a mesoconelet and an anterior pretrite accessory central conule, with the former often shifting anteriorly to fuse with the latter. By contrast, the third posttrite half−lophid only has an anteriorly located mesoconelet. The posterior cingulid is relatively strong, with two main cusps and other smaller conules. Small enamel conules are well developed in the interlophids, and are covered by a thin layer of cementum. The cingulid is developed along the anterior border of the tooth and the buccal side of the first interlophid .

No specimens of p3 were found. There is no evidence that this tooth existed in platybelodonts from the Linxia Basin, and p3 has not been reported in Platybelodon grangeri from elsewhere. Only specimen HMV0044 preserves a partially erupted p4, the posterior part of which is still partially obscured by m1 ( Fig. 7B ). While the structure of this tooth can thus not be clearly observed, it seems to resemble specimens from Tunggur showing a double−lophed p4 with rudimentary pretrite trefoils, which is similar to P4 except for being narrower and longer. The structure of m1 ( Fig. 7B ) is similar to that of dp4, except for a larger and more developed posterior cingulid and cementum in the interlophids. Small enamel conules are well developed in the interlophids. Similarly, m2 ( Fig. 7C, D ) also resembles dp4 and m1, but is larger and more hypsodont than the latter, while the small conules in the interlophids tend to be reduced. The posterior cingulid is strong and better developed in the specimens from Laogou than those from Zengjia, and consequently a complete fourth lophid mainly tends to be present in the former. Finally, m3 ( Fig. 7E, F ) is long anteroposteriorly, hypsodont, and has 4.5–6.5 lophids. Generally, the pretrite half−lophids are marked by well−developed trefoils on the first three lophids, in which the mesoconelets shift anteriorly to merge with the an−

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terior accessory central conules, while the strong, serrated and individualized posterior accessory central conules tend to invade the neighboring entoflexid. The corresponding posttrite half−lophids have mesoconelets and anterior accessory central conules. On the posterior lophids, the mesoconelets and anterior accessory central conules of the pretrite and posttrite half−lophids can be either separated or fused, but tend to diminish in size or may even be entirely absent. The pretrite and posttrite sides of the same lophid show alternate positions. The posterior cingulid is composed of one or two large cusps, and the cingulid is generally also developed on the anterior border of the tooth. The cementum in the interlophs is strongly developed, especially in specimens from Laogou. Compared to the more anterior teeth, the development of small conules in the interlophids is weak. Specimens from Laogou are generally narrower than those from Zengjia.

Stratigraphic and geographic range.—Haramagai Formation, Xinjiang; Hujialiang Formation, Gansu; Zhongning area (formation not yet established), Ningxia; and Tunggur Formation, Inner Mongolia, all from the Middle Miocene of northern China ( Osborn and Granger 1931, 1932; Tobien 1973; Chen

1978, 1988; Tobien et al. 1986; Ye and Jia 1986; Guan 1988, 1996; Ye et al. 1989; Wang and Qiu 2002).

Figures

Fig. 3. Skull of the gomphotheriid mammal Platybelodon grangeri (Osborn, 1929) from locality LX200002 of the Linxia Basin, Middle Miocene. A. Adult male (HMV0940), lateral view (horizontally reversed). B. Adolescent (HMV0021), distal view. C. Adult male (HMV0024), anterior view. D. Adult female (HMV0023), dorsal (D1) and ventral (D2), and lateral (D3) views.

Fig. 4. Juvenile skull of the gomphotheriid mammal Platybelodon grangeri (Osborn, 1929) (HMV1813) from locality LX200002 of the Linxia Basin, Middle Miocene; lateral (A) and dorsal (B) views.

Fig. 5. Mandible of the gomphotheriid mammal Platybelodon grangeri (Osborn, 1929) from locality LX200002 of the Linxia Basin, Middle Miocene. A. Adult male (HMV0031), dorsal view. B. Juvenile (HMV1813), dorsal (B1) and lateral (B2) views. C. Adult male (HMV0940), lateral view (horizontally reversed). D. Adult female (HMV0042), dorsal view.

Fig. 6. Upper cheek teeth of the gomphotheriid mammal Platybelodon grangeri (Osborn, 1929) from localities LX200002 (A–C, E) and LX200003 (D, F), the Linxia Basin, Middle Miocene, occlusal view. A. Left DP2 and DP3 (HMV0050), horizontally reversed. B. Right P3, DP4, and M1 (HMV1812). C. Right P4, M1, and M2 (HMV1828). D. Right M3 (HMV0014). E. Right M3 (HMV1788).

Fig. 7. Lower cheek teeth of the gomphotheriid mammal Platybelodon grangeri (Osborn, 1929) fromf localities LX200002 (A–C, E) and LX200003 (D, F), the Linxia Basin, Middle Miocene, occlusal view. A. Left dp3 and dp4 (HMV1813). B. Right p4 (not fully erupted) and m1 (HMV0044). C. Right m2 (HMV1798). D. Right m2 (HMV1784). E. Right m3 (HMV1799). F. Left m3 (HMV1787), horizontally reversed.

Fig. 1. Map showing fossil platybelodont localities in the Linxia Basin, Gansu, western China.

Fig. 2. Gomphotheriid dental nomenclature (left M2 of Protanancus chijiensis), from Tassy (1983: fig. 4, slightly revised).