Chilotherium schlosseri (Weber, 1905)

Figs 9, 10, 11

Neotype.

A well-preserved skull (GPIH 3015) with an associated mandible (GPIH 3015 a), designated by Kampouridis et al. (2023 b).

Type locality.

Upper Miocene deposits of Samos Island (Greece); exact locality unknown.

Material.

An almost complete juvenile skull preserving D 1 – D 4 and M 1 on both sides (NHMW -GEO-2009 z 0088 / 0001); a partial juvenile skull preserving the maxillary with D 1 – D 4 on both sides (SMNS 47913); a partial juvenile skull preserving only the maxillary, which bears D 2 – D 4 on both sides (SMNS 47914); an isolated D 1 (MGL 106691); two isolated D 2 s (GMM 570 and MGL 107103); a partial mandible preserving the right i 2, p 2, p 3, d 4, m 1, and m 2 and the left i 2, p 3, d 4, m 1, and m 2 (NHMW -GEO-1911/0005/0033); a partial mandible preserving the right d 1 – d 4, erupting m 1, the left d 2 – d 4, and erupting m 1 (SMF M 6814); a partial mandible preserving the right p 2, d 3 – d 4, erupting m 1, the left p 2, d 3 – d 4, and erupting m 1 (SMF M 6815); and a partial juvenile mandible preserving d 2 – m 1 on both sides, while the i 2 s are erupting (GMM 593).

Description.

One almost complete (NHMW -GEO-2009 z 0088 / 0001) and one partial juvenile skull (SMNS 47913), along with some partial maxillae of C. schlosseri, exist in the studied material from the Upper Miocene of Samos in Greece (Fig. 9). The premaxillary bones are broken off in all specimens. In dorsal view, the suture between the nasals and the frontals is a relatively straight mediolateral line at the level of the anterior edge of the orbit. The suture between the two frontals is a straight line, when present, and the bones seem quite porous in comparison to the more massive nasals. In the youngest partial skull (SMNS 47913), no frontal-parietal crests are visible. In the ontogenetically older NHMW -GEO-2009 z 0088 / 0001, the frontal-parietal crests are well formed and already widely separated (minimum distance = 64 mm). The frontal-parietal suture forms a 50 ° angle to the frontal suture. Posteriorly, the frontal suture merges with the suture between the parietals.

In lateral view, the suture between the nasals and the maxillary is relatively straight, starting from the nasal notch and ending at the lacrimal. The position of the nasal notch slightly varies in the three available C. schlosseri specimens. In the youngest specimen (SMNS 47914), the nasal notch reaches the level of the contact between D 2 and D 3, whereas, in the slightly more mature SMNS 47913, it reaches the level of mid-D 3. In the ontogenetically oldest specimen (NHMW -GEO-2009 z 0088 / 0001), the nasal notch reaches the level of the contact with D 3 and D 4. In lateral view, the nasal and lacrimal share a short suture, thus separating the maxillary from the frontal. Immediately behind the orbit, the beginning of the postorbital process on the frontal is visible in SMNS 47914. The maxillary and lacrimal are positioned below the nasals and frontals. The maxillary exhibits several infraorbital foramina, which can vary significantly, even in the same individual, from three to up to four foramina of variable size. Posteriorly, the maxillae extend into pockets, which house the unerupted molars. The lacrimal is relatively small, and it connects dorsally to the frontal and nasal, anteriorly to the maxillary, and ventrally to the jugal. The jugal shares a wide suture with the maxillary. The zygomatic arches are not preserved on any of the specimens. In lateral view, one of the most prominent parts is the orbital cavity, which is placed very high dorsally and has a thin dorsal margin. In ventral view, the maxillary constitutes the largest portion of the palatine. Posteriorly, the maxilla is sutured to the palatine. The palatine forms the anterior part of the choana, and two symmetrical foramina are placed near the maxillary-palatinal suture, at the posterior portion of D 4.

The upper dental material of C. schlosseri can be studied based on a number of complete or partial deciduous tooth rows and some isolated teeth (Fig. 10). The D 1 s in SMNS 47913 are not fully erupted, as they are placed well below the occlusal level of the other teeth (Fig. 9 B 3, B 4, 9 C). Thus, D 1 seems to finalise its eruption and start being worn after D 4. The root of the tooth in this specimen is not yet formed, also demonstrating the early developmental stage of D 1 in SMNS 47913. A continuous lingual cingulum is present, which continues on the posterior side of the tooth, becoming stronger and extremely high. The most prominent enamel fold is probably the metaloph, separating a large postfossette. Another weaker enamel fold in the middle of the tooth probably corresponds to the protoloph. The protocone and metacone seem to be connected to some degree lingually. The parastyle and metastyle are relatively well developed in comparison to the generally reduced D 1.

Concerning the other deciduous premolars, D 2 to D 4 are hypsodont and highly molarised. In D 2, the protocone is slightly smaller than the hypocone and bears a posterior and very weak anterior constriction, forming a short antecrochet. A small anterior constriction is present in the hypocone. An enamel pillar is present at the entrance of the median valley in all specimens. A very strong, continuous lingual cingulum is also present in all specimens except SMF M 6805 and NHMW -GEO-2009 z 0088 / 0001. The protoloph and metaloph are relatively thin, and a closed medifossette is always present. In all specimens that are adequately worn, a secondary fold splits off the crista and connects to the protoloph, creating a prominent fossette anterior to the medifossette. Very strong anterior and posterior cingula are present, forming large pre- and postfossettes. Both cingula continue slightly onto the ectoloph, even forming an extremely weak buccal cingulum in SMNS 47913. The parastyle and metastyle are very large. Weak paracone and metacone folds are present. All three specimens bear some enamel plications in the median valley. They are most prominent in the medifossette of the left D 2 of SMNS 47913, where five small plications are visible.

In D 3, the protocone and hypocone are of comparable size. The protocone bears strong anterior and posterior constrictions, forming an antecrochet. The hypocone is weakly constricted anteriorly. An almost continuous lingual cingulum is present in many specimens, although it is very weak in NHMW -GEO-2009 z 0088 / 0001 and absent in SMF M 6805. At the entrance of the median valley, a small enamel pillar is present in all specimens. A well-developed crochet is always present. A small crista is present in all specimens, varying in size and shape. Strong anterior and posterior cingula are present, forming a large postfossette. The parastyle and metastyle are well developed. The paracone fold is strong, the mesostyle is extremely weak, and the metacone fold is faintly visible.

D 4 has a similar morphology to D 3, although the protocone is somewhat larger than the hypocone. The protocone bears strong anterior and posterior constrictions, forming a prominent antecrochet. The hypocone bears only an anterior constriction. A slight, discontinuous lingual cingulum is present in some D 4 specimens. A large crochet is always present, but the crista is lacking in all specimens, although a very weak enamel bump might be present instead. A strong anterior cingulum is present, and a strong posterior cingulum forms a wide postfossette. The parastyle is well developed and the metastyle large. The paracone fold is well developed, a slight mesostyle is present, and a weak metacone fold is faintly visible. All D 4 s exhibit thin horizontal grooves at the base of the crown on the lingual and buccal sides of the enamel, which can be interpreted as hypoplasias (Mead 1999; Böhmer and Rössner 2018; Hullot and Antoine 2020).

Branching furrows are visible on the teeth of three specimens (MGL 107103, SMNS 47913, and SMNS 47914). The traces occur as a branched network of shallow, bleached furrows, covering large parts of the teeth of SMNS 47913 and SMNS 47914 (Fig. 10 C, D) and almost the whole ectoloph of MGL 107103 (Fig. 10 B). The concentration of these furrows varies significantly. Some areas are only sporadically covered with a few furrows, while in other areas, the furrows cover the surface of the enamel so densely that a continuous, bleached area is produced. These branching furrows represent root etching, which is commonly observed in extant and fossil bones and usually takes the form of irregular furrows, indicating that the bones were altered by plant growth (Behrensmeyer 1978; Andrews and Cook 1985; Fisher 1995; Montalvo 2002; Bader et al. 2009). Humic acids – produced by the plant roots to extract nutrients from the substrate – cause shallow depressions and bleaching of the bone (Behrensmeyer 1978; Morlan 1980), which can result in the pattern seen on the enamel of the studied teeth.

Four juvenile C. schlosseri mandibles from Samos in Greece (Fig. 11) were studied. One of them, GMM 593, was published by Andree (1921, pl. 2, figs 4, 5) and originally referred to as Aceratherium wegneri?, a junior synonym of C. schlosseri (Kampouridis et al. 2022 b; Svorligkou et al. 2025). Three additional mandibles are preserved in the studied collections (NHMW -GEO-1911/0005/0033, SMF M 6814, and SMF M 6815). Only one of these specimens, SMF M 6814, preserves the left d 1. This tooth is extremely small and has a single, rounded tip that is approximately 6 mm in diameter and 10 mm in height. The d 2 is preserved in only two specimens (GMM 593 and SMF M 6814) but is heavily worn in both, with the trigonid and talonid widely connected. The paralophid is constricted and anteriorly oriented. The metalophid projects lingually and slightly distally. The hypolophid projects lingually and is slightly wider than the metalophid. Both anterior and posterior valleys remain open until completely worn. A shallow ectolophid groove exists buccally. A slightly discontinuous buccal cingulum is visible, and it is probably connected to the anterior and posterior cingula, which are not preserved due to wear. The preserved d 3 s are quite worn down in all specimens, and a wide connection between the trigonid and talonid has been established. The paralophid is less developed than the metalophid and hypolophid. A weak constriction in the metalophid is visible in GMM 593, though only faintly, due to heavy wear. Both the anterior and posterior valleys remain open until completely worn in GMM 593; while in SMF M 6814, the anterior valley is already completely worn off, a small remnant of the posterior valley is still present. In SMF M 6815, both the anterior and posterior valleys are already worn off. The ectolophid groove is relatively shallow but deeper than in d 2. A discontinuous cingulid is visible in buccal view, and a very small discontinuous cingulid is also visible lingually at the entrance of the anterior valley. The d 4 is moderately to heavily worn in the studied specimens. A narrow connection between the trigonid and talonid is established in the ontogenetically youngest specimens, GMM 593 and SMF M 6814. In NHMW -GEO-1911/0005/0033 and SMF M 6815, the connection between the trigonid and talonid is somewhat wider. The paralophid is relatively short, whereas the metalophid and hypolophid are similarly well developed. In GMM 593, the paralophid of the right d 4 exhibits a small distal projection, which is not visible on any other d 4. The hypolophid exhibits an anterior constriction in GMM 593 and SMF M 6814. The anterior valley is quite small, whereas the posterior one was probably larger. The anterior valley probably would close shortly before being completely worn due to a small cingulid being present at its entrance in most d 4 s. The ectolophid groove is deep, and a small enamel pillar is located at its base in all four specimens. High anterior and posterior cingulids are present; they continue faintly on the buccal side and less so on the lingual side.