Chilotherium schlosseri ( Weber, 1905 )

Tissier, Jérémy, Petculescu, Alexandru & Becker, Damien, 2023, Chilotherium schlosseri (Weber, 1905) (Rhinocerotidae, Mammalia) from the late Miocene of the foreland of the Eastern Carpathians in Romania, Comptes Rendus Palevol 22 (36), pp. 729-752 : 734-744

Chilotherium schlosseri ( Weber, 1905)

( Figs 3-8; Tables 1-3 )

Aceratherium schlosseri Weber, 1905: 344 , pl. VIII, figs 1-4, pl. IX, figs 1-3.

Teleoceras ponticus Lubicz-Niezabitowski, 1912: 1 , 1913: 223, pl. XXIV, figs 1-2, pl. XXV, figs 3-5.

Aceratherium kowalevskii Pavlow, 1913: 48 , pl. 4-5.

Aceratherium wegneri Andrée, 1921: 189 , pl. I-II.

Aceratherium angustifrons Andrée, 1921: 202 , pl. III.

Chilotherium schlosseri – Ringström 1924: 85.

Aceratherium sp. – Malik & Nafiz 1933: 44, pl. VII figs 4, 5.

Aceratherium cf. kowalevskii [partim] – Nicolas 1978: 456.

Chilotherium cf. sarmaticum – ȘtiucĂ 2003: 114, fig. 2.

TYPE MATERIAL. — Lectotype, a relatively well-preserved adult cranium with associated mandible described and figured as Aceratherium schlosseri by Weber (1905: taf. VIII, figs 1-4; taf. IX, fig. 1 pars). The specimen is believed to have been destroyed during the Second World War ( Giaourtsakis 2022).

REFERED MATERIAL. — From Reghiu (Late Miocene, Khersonian, MN 10/11, RĂdulescu et al. 1995; Codrea 1996; ȘtiucĂ 2003; Geraads & Spassov 2009; Codrea et al. 2011): partial skull with maxillary tooth rows (P1-M3) and associated mandible with left p3-m3 and right p2-m3 ( ERIS-Rg/1987 001/6 ) ( Figs 3-5) .

From BacĂu (Late Miocene, latest Bessarabian, MN10 , Țibuleac 2019) : right P4-M3 ( MNS-IBB 112 ); rightM1-2 ( MNS-IBB 113 ) ; fragmented left M3 ( MNS-IBB 114 ) ; fragmented mandible with left p4-m3 and right p3-m1 and m3 ( MNS-IBB 115 ) ( Figs 6; 7).

From Pogana (Late Miocene, latest Khersonian-earliest Maeotian, MN 11/12 , Codrea et al. 2011) : left M2 ( VPM – P/355 ) ; right dp3 ( VPM – P/354 ); both illustrated by Codrea et al. (2011: figs 3.1- 3.2) ( Fig. 8).

TYPE LOCALITY AND AGE. — Samos Island, Greece; Late Miocene fluvio-lacustrine volcanoclastic deposits of the Mytilinii formation; MN 11-13 ( Weber 1905; Vlachou & Koufos 2009; Giaourtsakis 2022).

DESCRIPTION

Cranial features

The skull of Chilotherium schlosseri from Reghiu includes a partial dolichocephalic cranium associated with a betterpreserved mandible. It belongs to a young adult, with unworn M3, and very slightly worn m3. The skull has been broken into two fragments during the extraction from the host beds, and several cranial bones are lost. Moreover, the specimen is largely restored with gypsum. The rough restoration preserves together the bones but also partially covers several ones as well as the sutures between them precluding accurate observations. Consequently, only the maxillae with DP1-M3 tooth rows and incomplete nasals, frontals, parietals and jugals can be clearly identified ( Figs 3; 4). The lacrymal bones are largely covered by the gypsum during the skull restoration, and they can only be inferred. The best-preserved jugal bone is on the right side of the skull. The occipital bone does not preserve the condyles, but it keeps the external occipital crest ( Fig. 4B). No foramina have been depicted within the skull.

In dorsal view ( Fig. 4A), the skull is rather narrow, having subparallel zygomatic arches and an apparently roughly straight occipital crest. Both nasal and frontal bones are flattened, without an obvious suture between them. The posterior end of the nasal notch can be inferred above the posterior border of P 4 in the lateral view. The frontoparietal crests are distant (sensu Antoine 2002), the minimum distance between them being of 64.2 mm. In lateral view ( Fig. 3A), the postorbital processes are present on the frontal, and on the zygomatic arch, located on the jugal. The orbits are located high near the cranial roof, their anterior borders reaching the anterior border of M2. The dorsal profile of the skull is slightly concave. In ventral view, the maxillae are fragmented but the upper tooth rows are complete and well preserved. The zygomatic process of the maxilla starts at the level of M1. The anterior base of the processus zygomaticus is high in lateral view ( Fig. 3) and its transition from the maxilla follows a brutal inflection in the ventral view ( Fig. 4C).

Mandibular features

In lateral view, the corpus mandibulae is lower in the specimen from Reghiu (ERIS-Rg/1987 001/6; Fig. 5) than that from Bacau ( MNS-IBB 115; Fig. 7), with a rather constant height (H at the front of p3: 76.0 mm, of m1: 79.0 mm, of m3: 76.5 mm in Reghiu; H at the front of m1: 92.0 mm, of m3: 94.0 mm in BacĂu). The ventral border is straight in Reghiu, but more ventrally convex in BacĂu. These differences may reflect sexual dimorphism, which is known in another Chilotherium species ( C. wimani Ringström, 1924 ) and significant in the mandible ( Chen et al. 2010). The symphyseal angle is approximately of 15° in the mandible from Reghiu. The foramen mentale is below the posterior border of p2 on the specimen of BacĂu. The mandibular angle can only be partly observed because of the incompleteness of the specimens. The fragmentation of the mandibles precludes the observations on the masseteric fossa, but a very low ridge marks its basal extension ( Figs 5; 7). The condylar process, preserved on the left part of the mandible from Reghiu is very wide transversally. Underneath, the neck is wide and the mandibular notch is obtuse. The ramus is roughly vertical on the mandible from Reghiu, although its anterior border follows a very slightly anteriorly curved line ( Fig. 5A). In dorsal view, the symphysis is massive (observable on the mandible from Bacau, Fig. 7C), with a posterior margin located at the level of the p3 on both specimens. The spatium retromolare is short. The orientation of the tooth rows is not parallel to the long axis of the mandible (sensu Pandolfi 2015, character 81).

General dental features

The maxillary cheek teeth are high-crowned (sensu Antoine 2002, character 69) and without enamel foldings (sensu Antoine 2002, character 64). The enamel is wrinkled and covered with patches of cement. From the skull and the mandible of Reghiu, the premolar series are short with respect to the molar series (sensu Antoine 2002, 42 <IP /M and Ip/m> 50, character 63; Table 1 ), but p2 remains elongated ( Fig. 5B). No d1/p1 are present, as attested by the sharp ridges running anterior to p2s, whereas a DP1 is present in the upper tooth series of the skull of Reghiu ( Fig. 4C).

The symphysis of the mandible is mostly encased in gypsum or broken on the two mandibles and the presence of the central incisors (i1s) or alveoli cannot be assessed. The lower tusks (i2) are broken on ERIS-Rg/1987 001/6 and not preserved on MNS-IBB 115 ( Figs 5; 7). They are triangular-shaped in cross section with an acute posterior edge, divergent and quite strong.

Upper dentition

The occlusal morphology of the upper cheek teeth of all the specimens mainly differ according to the stage of wear, almost unworn in Reghiu to a very advanced wear in BacĂu. Several teeth are incompletely preserved ( Figs 4C; 6; 8). The dental pattern of the upper cheek teeth is rather complicated due to the presence of secondary enamel folding (= crochet, antecrochet and crista sensu Antoine 2002; see Fig. 2). In occlusal view, the lingual rims of the upper tooth rows are straight.

The skull from Reghiu (ERIS-Rg/1987 001/6) preserves both P1-P4 premolar rows, whereas only P4 is known in BacĂu (MNSB – 112). The P1 is triangular-shaped and the anterolingual cingulum is absent. The anterior groove of the ectoloph and a simple parastyle can be observed. The paracone fold is noticeable on both teeth. The protoloph is absent and the metaloph is transverse, with an incipient crochet. The lingual border of the paracone forms an incipient crista. The hypocone is posteriorly elongated. The metaloph is transverse on P2 and the protocone is as developed as the hypocone. The protoloph is present and weakly connected to the ectoloph. All P2-4 have a closed median valley (lingual bridge to lingual wall; semimolariform to submolariform sensu Heissig 1969) and a lingual groove between the protocone and hypocone. In occlusal view, the P2-P4 are almost quadrangular if we pass over the very long and acute parastyle (not preserved on MNS-IBB 112). The crochet is strong and anteroposteriorly elongated ( Figs 4C; 6A) and the crista is absent. The P3-P4s of ERIS-Rg/1987 001/6 develop a large metastyle and an S-shaped metaloph. The hypocone is more posterior than the metacone. The postfossette is closed by a high and narrow posterior cingulum. The protocone is constricted on P4 from BacĂu ( Fig. 6A) and this constriction should appear with wear on the specimen from Reghiu. Only the parastyle is noticeable on the ectoloph of the premolars from Reghiu (the labial border of the P4 from BacĂu is broken). The lingual cingulum is limited to an isolated bulge on the lingual border. There is a very thin labial cingulum on premolars.

The upper molars from Reghiu are almost complete, whereas the other specimens are partially broken (e.g. the right M2 of MSN-IBB 113 from BacĂu misses the mesiolingual and distolabial borders; Fig. 6B). The protoloph and the metaloph are fully continuous and connected to the ectoloph. All referred specimens lack lingual and labial cingulum and a medifossette. All M1-M2 are quadrangular with a long parastyle and metastyle except for the broken M1 MNS-IBB 112 ( Fig. 6A). A moderate mesostyle is developing on all M1-M2s except on the M1 MSN-IBB 113 ( Fig. 6B). The median valley is closed on M1 by the lingually projected antecrochet on worn teeth, but slightly open on unworn M1 from Reghiu. It is open on M2-3. The postfossette is triangular on unworn teeth and oval shaped on worn teeth. The protocone is strongly and simply constricted in all teeth of the specimen MNS-IBB 112 from BacĂu ( Fig. 6A) and those from Pogana, whereas on the Reghiu specimens the constriction is visible only on M1, but should appear with wear on M2-3. The crista is not generally present, except a very weak one on the M2 ( MNS-IBB 112: Fig. 6A) and M3 ( MNS-IBB 114: Fig. 6B) from BacĂu. The crochet is anteroposteriorly oriented and simple, with the exception of the M1 MNS-IBB -113, where it has multiple projections. On the M2s from Reghiu, the crochet is isolated from the metaloph near the unworn occlusal surface, but is connected to it with an increased wear. The M3s are subtriangular in occlusal view due to the fusion of the ectoloph and metaloph. The ectometaloph displays a smooth posterior groove. The protocone is very large and shows a trefoil-shaped constriction in worn teeth. The M3s also have well-developed antecrochet and crochet.

Lower dentition

The labial cingulum of the lower cheek teeth is missing, except a short and weak extension of the anterior one until the level of the paraconid in lower premolars. The ectolophid groove is deep, V-shaped and developed until the neck in occlusal view, and oblique in lateral view. The lingual cingulum is present in the openings of the posterior valleys of p3-4 and the anterior valleys of p2-4 ( Fig. 5).

The dp1 is absent.

The dp3 is only preserved at Pogana ( VPM – P/354) and was originally identified as a p4 ( Codrea et al. 2011). However, based on the very thin enamel, we consider it as a decidual premolar.It is much worn, especially the trigonid, which might explain its very small dimension compared with those given by Pavlow (1913). The external groove of the ectolophid is deep. The enamel has rugosities on its labial side.

The p2 can be observed only on the right tooth row of Reghiu. The paralophid is short, anteriorly curved without constriction, and the paraconid is developed. The protolophid is oblique, forming an acute angle with the lingual border towards the anterior part. The metaconid is very large and not constricted. It is bigger than the protoconid and entoconid, but roughly same-sized as the hypoconid. The talonid is also larger than the trigonid. The posterior valley is much bigger than the anterior one, both are open.

Both p3s of the specimen from Reghiu are preserved, but only one is preserved on the specimen from BacĂu, and very worn. The hypolophid is bigger than the protolophid, which is also bigger than the paralophid. The labial branch of the paralophid is similarly oriented as on p2. The metaconid is simple and large. Both valleys are open and V-shaped in lingual view, the posterior being very large. In occlusal view, the trigonid is narrow and almost V-shaped whereas the talonid is large and rounded.

The p4s are preserved on the mandibles from Reghiu and BacĂu. The lingual branch of the paralophid is long, reaching the same level as the protolophid and hypolophid. The posterior valley is bigger that the anterior valley, and are U-shaped. The metaconid is rounded. The trigonid is narrow and almost V-shaped and the talonid is rounded on p4.

The m1s from Reghiu and BacĂu are similar to p4s, but due to their different stage of wear the metaconid is larger and the anterior valley is smaller. The m2s also have the similar pattern as p4s except for the square-shaped trigonid in occlusal view. The m3s are all preserved. The metaconid and entoconid are round and simple. The protolophid and hypolophid are very oblique in occlusal view and become more transverse with wear, as noticeable on the mandible from Bacau ( Fig. 7C). On the mandible from Bacau, the left m3 is much more worn than the right one, indicating a possible dental pathology of this individual.

REMARKS

The studied material can be undoubtedly assigned to Aceratheriini by the following set of characters: presence of wrinkled enamel on the cheek teeth, the developed crochet and the usual absence of a crista on upper molars, the constricted protocone and of a weak paracone fold on M1-2, as well as by the lingual bridge between the hypocone and protocone on P4 ( Antoine et al. 2010; Lu 2013; Becker et al. 2013; Pandolfi 2015).

According toPandolfi (2015), the combination of additional morphological features, such as high-crowned cheek teeth with the presence of cement patches, a lingual cingulum usually present on lower premolars as well as short metaloph on M1-2, allow for assigning them to an informal cluster ( Pandolfi 2015), including “chilothere” sensu lato ( Antoine & Sen 2016; i.e., Persiatherium Pandolfi, 2016 , Acerorhinus Kretzoi, 1942 , Shansirhinus Kretzoi, 1942 , Chilotherium Ringström, 1924 ) as well as Subchilotherium intermedium ( Lydekker, 1884) and Plesiaceratherium gracile Young, 1937 .

Subchilotherium intermedium differs by the general absence of antecrochet (only present on M1) and protocone constriction, by the separated protocone and a hypocone on P4, by a strong paracone fold, a weak constriction of the protocone and a short metaloph on upper molars, an antecrochet and a hypocone separated on M1, an hypocone isolated on M2, and the presence of lingual and labial cingulum on lower cheek teeth ( Lydekker 1884; Colbert 1935; Pandolfi 2015).

Plesiaceratherium gracile differs by an arched lingual rim of the upper tooth row, the absence of cement on cheek teeth, the presence of a continuous lingual cingulum on upper premolars and of usual medifossette on P2-4, as well as a protocone constriction and an antecrochet missing on P4 ( Young 1937; Yan & Heissig 1986; Pandolfi 2015; Becker & Tissier 2020).

The referred dental specimens share the following features with the “chilothere” sensu lato (following Antoine & Sen 2016): presence of a protocone constriction on P3-4, strong antecrochet on upper molars, deep ectolophid groove reaching the neck on lower cheek teeth and absence of labial cingulum on the lower cheek teeth.

However, they differ from Persiatherium rodleri Pandolfi, 2016 by the absence of lingual cingulum on M1-2, and a stronger antecrochet on M1 ( Pandolfi 2015).

Shansirhinus ringstroemi Kretzoi, 1942 can be excluded by the presence of elaborate enamel plications on cheek teeth, a well-developed medifossette on P3-4, strong lingual cingulum on P2-4, antecrochet and hypocone separated on P4 and M1, and a trigonid angular and U-shaped on lower cheek teeth ( Deng 2005; Pandolfi 2015).

The species assigned to Acerorhinus differ by the presence of lingual cingulum on P2-4, a weak constriction of the protocone on M1, a posterior groove on the ectometaloph of M3, and of an external groove vanishing before the neck on lower cheek teeth ( Deng 2005; Pandolfi 2015).

Among the “chilotheres”, the Reghiu specimens can basically be identified as belonging to Chilotherium based on the well-marked processus postorbitalis of the frontal and squamosal, and a massive mandibular symphysis bearing large tusk-like i2s (Geraads & Spassov 2009). More broadly, the referred dental remains can be assigned to Chilotherium by having a protocone constriction and a strong antecrochet on P4, usually a strong protocone constriction and an absence of lingual cingulum on upper molars, and an external groove developed until the neck on lower cheek teeth ( Pandolfi 2015; Antoine & Sen 2016). From their dimensions ( Tables 2 ; 3 ), the referred dental specimens roughly match most of those of Chilotherium species, except C. primigenius Deng, 2006a that are clearly smaller and C. anderssoni that are slightly larger ( Weber 1905; Ringström 1924; Ji et al. 1980; Deng 2001, 2006a; Antoine & Sen 2016; Sun et al. 2018).

Following mainly Deng (2006a), Sun et al. (2018) and Kampouridis et al. (2022b), ten species of Chilotherium can be considered with confidence as valid: Chilotherium samium ( Weber, 1905) , Chilotherium schlosseri , and Chilotherium kiliasi ( Geraads & Koufos, 1990) from Europe, Chilotherium anderssoni , Chilotherium habereri , Chilotherium wimani , Chilotherium xizangensis , Chilotherium primigenius , and Chilotherium licenti from China, and Chilotherium persiae from Iran.

Regarding C. schlosseri , many authors have suggested a series of European Chilotherium species more recently described as junior synonyms (e.g. Heissig 1975; Giaourtsakis 2003, 2009; Deng 2006b; Antoine & Sen 2016). Unfortunately, the type material of C. schlosseri was lost during the Second World War (e.g. Giaourtsakis 2003). Likewise, the holotype of Chilotherium angustifrons ( Andrée, 1921) is also considered lost ( Meiburg & Siegfried 1970; Giaourtsakis 2009). However, following the recent overview on European Chilotherium of Kampouridis et al. (2022b), Chilotherium wegneri ( Andrée, 1921) and Chilotherium angustifrons ( Andrée, 1921) are treated as junior synonyms of C. schlosseri . Likewise, Chilotherium kowalevskii , described in the Late Miocene locality of Grebeniki 1 in Ukraine, is also considered as a junior synonym of C. schlosseri as suggested by Antoine & Sen (2016) and this study (see below). Finally, we also consider Teleoceras ponticus Lubicz-Niezabitowski, 1912 from Odessa ( Ukraine) as a junior synonym of C. schlosseri because of their very similar morphology and dimensions (Appendices 1; 2), as mentioned by Giaourtsakis (2022).

In contrast, according to Geraads & Spassov (2009) and despite the lack of well-known specimens and the absence of recent literature, the species Chilotherium sarmaticum from Berislav in Ukraine is also regarded as a valid species.

According to Deng (2006a: figs 1, 2), the very worn dental remains of the primitive species C. primigenius from Zhongmajia (Hezheng County, Gansu Province, China) differ by a sagittally oriented antecrochet on upper molars and an almost transverse hypolophid on m3. The i2s are also much more upraised and nearly vertical ( Deng 2006a: fig. 2.3).

Chilotherium wimani is also precluded by a strong and continuous lingual cingulum as well as usually the presence of a medifossette on upper premolars ( Ringström 1924: pl. VII, figs 2, 3).

Chilotherium xizangensis and the more derived C. licenti points to specific differences by low crowned cheek teeth and a poorly developed parastyle on upper molars for the former ( Ji et al. 1980; Deng 2001), as well as the presence of a crochet and a well-developed crista linked together to form a medifossette on P2–M2 for the latter ( Sun et al. 2018).

Chilotherium xizangensis also displays a reduced lingual cingulum on upper molars, a pronounced crista on M3, a medifossette on P2-4, and a narrow mandibular symphysis ( Ji et al. 1980: pl. IV).

The type species Chilotherium anderssoni and C. habereri differ by a protoloph and metaloph separated on P4 and an antecrochet not connected to the hypocone on M1( Ringström 1924: pl. III, figs 3, 4, pl. IV, fig. 3). The small-sized Chilotherium samium differs by the absence of antecrochet and a protocone and hypocone separated on P4 ( Weber 1905: pl. IX, fig. 5). Finally, Chilotherium kiliasi differs by having pronounced crista on upper premolars, separated protocone and hypocone on P2, a poorly-developed antecrochet on upper molars, and a labial cingulum on lower premolars ( Geraads & Koufos 1990: pl. 3, figs 4, 5).

The Iranian C. persiae is distinguished by the presence of a thin anterior transverse branch (protoloph) on D1, separated protocone and metacone on P2, the unusual presence of crista and medifossette on upper cheek teeth, the quadrangular shape of the M 3 in occlusal view, labial cingulum on lower premolars, an isolated and a constricted paralophid on p2 ( Deng 2006a; see specimens MNHN-27802,-27803 and AMNH.F.MAR3053, 3859, 3073D, 38223827, 3860 and 3892). According to the descriptions and illustrations of Korotkevich (1958: figs 1, 2; 1970: figs 9-12), C. sarmaticum differs by having a more developed lingual cingulum and a crista on upper premolars, a labial cingulum on P2, a medifossette on P3-4, a weak antecrochet on upper molars, a labial cingulum on lower molars as well as a convex ventral border and an anteroposteriorly weakly increasing height of the corpus mandibulae.

Taking into account the evolution of the occlusal morphology regarding the state of tooth wear, the studied chilotheres remains of the Romanian Eastern Carpathians Foreland share the closest affinities with Chilotherium schlosseri from the type-locality of Samos in Greece and its junior synonym Chilotherium kowalevskii from the type-locality Grebeniki 1 in Ukraine ( Weber 1905: pl. IX, figs 1-3; Pavlow 1913: pls IV-V; Krokos 1917: pls I-II; Pandolfi 2015; Antoine & Sen 2016). In particular, they share dental features such as the absence of labial cingulum, the presence of strong crochet and antecrochet on upper cheek teeth, a lingually-projected antecrochet joining the metaloph on worn upper cheek teeth, a weak mesostyle and a concave posterior profile of the ectoloph on unworn M1-2, a reduced lingual cingulum on lower premolars, missing on lower molars, semimolariform to submolariform upper premolars, a medifossette usually absent on upper premolars, and a curved paralophid without constriction on p2. Regarding the mandible’s morphology, they additionally share a straight ventral border of the corpus mandibulae ( Figs 5A, C; 7A, B).

Regarding the cheek teeth dimensions, the coefficients of variation are rather high for the specimens referred to C. schlosseri . However, C. schlosseri actually displays very close coefficients of variation to extant Rhinocerotidae (Table 4; Fig. 9). The apparent high variation in cheek teeth measurements of C. schlosseri can be considered as normal for the family Rhinocerotidae and does not disqualify the assignation of the referred specimens from Romania to this species and the suggested synonymies.

Tables

TABLE 1. — Upper and lower premolars/molars indices (IP/M = 100xLP3-4/LM1-3),Ip/m = 100xLp3-4/Lm1-3) of Chilotherium schlosseri (Weber,1905) from Reghiu (ERIS-Rg/1987 001/6) and Bacău (MNS-IBB 115).

	LP3-4	LM1-2	IP/M	Lp3-4	Lm1-3	Ip/m
ERIS-Rg/1987 001/6 (left)	66.3	146.4	45.29	57.9	124.8	46.39
ERIS-Rg/1987 001/6 (right)	67.8	136.5	49.7	61.8	129.0	47.91
MNS-IBB 115 (right)	–	–	–	64.5	129.9	49.66