Chilotherium schlosseri (Weber, 1905) (Rhinocerotidae, Mammalia) from the late Miocene of the foreland of the Eastern Carpathians in Romania

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INTRODUCTION
In Romania, the hornless rhinoceros Chilotherium Ringström, 1924 has only been recorded so far throughout three localities in the eastern areas.Two fossil-bearing outcrops (Bacău and Pogana) are confined to the Eastern Carpathians Foreland (Moldavian and Scythian platforms, respectively), and the third one (Reghiu) within the foredeep zone (Fig. 1).In the Republic of Moldova, in the same foreland framework, the Chilotherium fossils are more common among the terrestrial vertebrates than on Romanian territory (e.g.Lungu & Rzebik-Kowalska 2011).The first report of the genus within the Romanian scientific literature is actually related to this area, Macarovici (1936) describing several isolated teeth of "Aceratherium" schlosseri Weber, 1905 from the upper Miocene sandy clay of Gura Galbenei (Cimșlia District, Republic of Moldova).
Later, Ioniță (1963) noticed a significant assemblage of fossil vertebrates originated from a Miocene succession, which is cropping out on the Reghiu River slopes, upstream of its confluence with the Milcov River (Reghiu locality, Vrancea District)."Aceratherium" schlosseri and Aceratherium incisivum Kaup, 1832 are listed among other fossil mammals (Ioniță 1963).Into an overview of mammal fauna from Reghiu, Știucă (2003) re-assigned the Chilotherium remains to C. cf.sarmaticum due to the higher level of hypsodonty than in C. schlosseri (Weber, 1905).He also highlighted a skull and postcranial bones of medium size whereas the length of upper and lower tooth rows are larger than those of "Aceratherium" kowalevskii Pavlow, 1913 (junior synonym of C. schlosseri after Antoine & Sen 2016) from Grebeniki 1 in Ukraine.Știucă (2003: fig.2) partially illustrated the Reghiu specimens (only the occlusal view of the right upper tooth row of the skull and the lingual view of the left unrestored hemimandible.However, in the text there is no reference to Ioniță 's original paper (Ioniță 1963).Rădulescu & Șova (1987) acknowledged the mammal fossils collected during the excavations for the thermal power station of Bacău city as "the most important Late Miocene bone accumulation in the Extracarpathian area of Romania" (Fig. 1).Among the numerous mammal remains, Aceratherium incisivum and Chilotherium sp. have been mentioned and summarily discussed and illustrated in this preliminary report.At least two specimens of Chilotherium (fragments of a maxillary and a mandible) were illustrated by Rădulescu & Șova (1987: pl. 2, figs 1-4).They were thought to represent an intermediate hypsodont stage between Chilotherium schlosseri (Weber, 1905) of lower Maeotian and Chilotherium sarmaticum Korotkevich, 1958 of upper Sarmatian s.l.(synonym of Khersonian).Most mammal remains from Bacău were housed in the collections of the Museum of Natural Sciences "Ion Borcea" Bacău, but for a long time this fossil fauna was thought to be lost (e.g.Codrea et al. 2011).Recently, after repeated research in the collections, these remains as well as other important fossils have been retrieved (Țibuleac 2018, 2019).Thanks to these rediscoveries, the previous Maeotian age of the faunal assemblage (Rădulescu & Șova 1987) has been re-assigned to the late Bessarabian (MN10) instead (Țibuleac 2019).
The last Chilotherium fossils so far in Romania have been collected from the Pogana Quarry (Scythian Platform, Vaslui District, Fig. 1).Codrea et al. (2011: fig. 3) described two isolated teeth of Chilotherium sp. from fluvial deposits of upper Miocene.The specimens were compared to C. sarmaticum, C. (Eochilotherium) kiliasi Geraads & Koufos, 1990, andC. kowalevskii Pavlow, 1913  The primary purpose of this paper is the taxonomical reassessment of the Chilotherium fossils found up to now in Romania involving the morphological description of the fossils and their critical comparison.Secondly, the biostratigraphical and paleogeographical significance of their occurrences is highlighted.

GEOLOGICAL FRAMEWORK
The Romanian occurrences of the genus Chilotherium are confined to the Eastern Carpathians Foreland (Bacău and Pogana) and its foredeep (Reghiu).The Eastern Carpathians Foreland (ECF) includes the major pre-alpine platforms of north-eastern Romania (Fig. 1).The most widespread is the Moldavian Platform, which represents the south-westernmost part of the East-European/Russian Platform prolonged under the Carpathian Orogenic Belt.The Scythian and Covurlui platforms are developing towards the south and are separated by the major crustal faults Bistrița and Trotuș, respectively.The latter is an Alpine basin, having as a basement the prolongation of North Dobrudja Orogen also known as North Dobrudja promontory west of the Danube (Ionesi 1994).The ECF also covers a Moesian Platform sector located northeast of the Intramoesian Fault (Dobrogean sector or Eastern Moesia) with heterogeneous pre-alpine basements of Central and Southern Dobrogea (e.g. Săndulescu & Visarion 1988;Visarion et al. 1988;and references therein).
Starting from the Neogene, the Eastern Carpathians Foreland evolved roughly similarly to the large framework of the Galiczian Gulf (Popov et al. 2004).The different depositional systems are related to the main tectonic events and resets of the land-sea report.Generally, after the middle Bessarabian, when the Eastern Carpathians uplifted, the Paratethys Sea retreated southeastward favouring the development of a river network on the emerged land.Consequently, a large prograding fluvial-deltaic depositional system developed in the widespread areas of Romania, the Republic of Moldova, and Ukraine (Matoshko et al. 2016; and references therein) coeval with the brackish sea retreat.
The Miocene successions have not been wholly and accurately formalized due to various reasons.From the literature, there are two main attempts.Ionesi et al. (2005) proposed several lithostratigraphic units, but only for the middle and upper Sarmatian, using geochronological boundaries to delineate each one.Following the "Balta Stage" sensu Barbot de Marny (1869), Matoshko et al. (2016) basically argued the Balta Formation (upper Bessarabian -Pontian = upper Tortonian -Messinian stages of the geological time scale after Raffi et al. 2020) on a unique depositional system, and not on the lithology, which largely changes within its framework.

Reghiu
According to Ioniță (1963) and Rădulescu et al. (1995), the most important fossils of Chilotherium have been sampled from the surroundings of the Reghiu locality (Vrancea District).The outcrop (Reghiu Stream slopes) is confined to the Focşani Sub-basin/Depression of the ECF foredeep.During the Bessarabian, the last orogenic nappe (Pericarpathian/ Molasse Nappe) emerged and thrust over the Miocene successions of the foreland marking the major shortening event of Eastern Carpathians.Along with the orogen-foreland tectonic contact, a syn-and post-orogenic foredeep develops striking variations of width and thickness.Among its framework, Bacău, Adjud, and Focșani areas started to evolve as Badenian -Sarmatian s.l.depocenters along to the basement faults of the ECF (Tărăpoancă et al. 2003;Krézsek & Olariu 2021;and references therein).

BacĂu
The second outcrop is located in the southern part of Bacău city (Fig. 1).The age of the Chilotherium host-beds is not certainly assessed, because the succession was open only during the excavation for a thermal power plant and cannot be seen nowadays.All remains of various large vertebrates (Rădulescu & Șova 1987)   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 P4 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 < I P/M and I p/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 triangularshaped 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).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).
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.
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 squa-mosal, 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

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).
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   Comparing different dental morphologies at different stages of wear has always been problematic in mammals, including Rhinocerotidae.It is particularly difficult in those taxa that have very complex teeth, with numerous features, that sometimes totally disappear with wear, or radically change their morphology.A notable example is the connection between the protocone and hypocone that sometimes only appears with wear, but that are clearly separated when the tooth is unworn (see, e.g., Antoine 2002: fig.103).High crowned teeth are even more problematic, as they can vary through more different stages of wear than low-crowned teeth.This is especially true in Chilotherium, which has both high-crowned cheek teeth and a complex dental morphology, with numerous features varying with wear such as the crochet or antecrochet.Although modern methods such as CT-scan can be used in such cases (as recently done by Kampouridis et al. 2022a), these methods are not always available.
Therefore, we illustrate here and describe the morphology of three different cheek teeth of Chilotherium schlosseri (P4, M1 and M2), through four different stages of wear (from early wear stage to advanced), including the specimens from Romania (Fig. 10).The main differences are reported in Tables 5-7.
These results led to assign the referred specimens from Romania to a single species, Chilotherium schlosseri, and to confirm C. kowalevskii as a junior synonym of C. schlosseri, as suggested by Antoine & Sen (2016) and apparently supported 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.

DISCUSSION
The revision of the dental remains of Romanian Chilotherium highlights very close affinities of the dental specimens with advanced wear of Pogana (MN10, Codrea et al. 2011) andBacău (MN10, Codrea 1996) with the type-material of Chilotherium schlosseri from Samos in Greece.As for the associated skull and mandible of a young adult of Reghiu, identified as Chilotherium schlosseri by Ioniță (1963) andC. cf. sarmaticum by Știucă (2003), they are similar to the unworn dental specimens assigned to Chilotherium kowalevskii from the type-locality Grebeniki 1 in Ukraine.
By observing the evolution of the dental patterns according to the stages of dental wear of the material-type of these two species and of the studied material, some characters, such as a concave profile of the posterior part of the ectoloph and a mesostyle on M1-2 or a very oblique orientation of the hypolophid on lower molars, can be modified, fade or even vanish with dental wear.by the phylogenetic analysis of Pandolfi (2015).In contrast, the poorly known C. sarmaticum, although similar to the referred material, is kept as a valid species on the base of the description of Korotkevich (1958Korotkevich ( , 1970) ) and as suggested by Geraads & Spassov (2009).These results are proposed herein until a taxonomic revision of Chilotherium species and their definite need for phylogenetic analysis.
Among the Chilotherium species, the stratigraphical range of C. schlosseri is the most important and its geographical distribution, from Eastern Europe (Greece, Ukraine, Republic of Moldova, Bulgaria) to Turkey and Central Asia (Kyrgyzstan, Tajikistan, Uzbekhistan), makes it one of the most common species of the genus.Chilotherium schlosseri, as most of the genus representatives, is considered as a grazer (high-crowned cheek teeth and short limbs compared to other Aceratheriini, according to Deng 2002) inhabiting steppe environment in arid or subarid habitat (Liang & Deng 2005).In Eastern Europe, C. schlosseri is commonly associated with Aceratherium incisivum, a browser form from forested habitats (Becker 2003).The latter is not recorded in Central Asia and China, whereas Chilotherium species are absent in Western Europe (Heissig 1999).This is probably the result of climatic changes that divided the European continent in two really distinct environments from the Late Miocene onward.Indeed, Western Europe environments were still dominated by closed and semi-humid tropical forests whereas Eastern Europe had more open and drier forest landscapes due to a more continental climate (Vislobokova & Sotnikova 2001).Thus, Eastern Europe can be regarded as a transition area between the more closed and wooded environments of Western Europe to the more open ones in Asia.
fig. 9. -Box plots (min, lower quartile, middle quartile, upper quartile, max) of coefficients of variation of upper and lower cheek teeth measurements (length and width) of Chilotherium schlosseri(Weber, 1905) compared to those of current Rhinocerotidea, based on data from Table4 and Appendix 3.

appenDix 2 .
-Bivariate plots of the length and width of cheek teeth of Chilotherium schlosseri(Weber, 1905), based on the measurements (mm) from Appendix 1. A, upper premolars; B, upper molars; C, lower premolars; D, lower molars.appenDix 3. -Coefficient of variation (CV) of cheek teeth measurements (length and width) of current Rhinocerotidae (after data from Guérin 1980).

table 2 .
(Weber, 1905) schlosseri(Weber, 1905).Measurements (mm) of the upper cheek teeth from Romania and Samos (type locality, taken from Weber 1905).Measurements in brackets are indicative and are presented as left/right.