Hyotherium lacaillei, Orliac & Antoine & Duranthon, 2006

Orliac, Maeva J., Antoine, Pierre-Olivier & Duranthon, Francis, 2006, The Suoidea (Mammalia, Artiodactyla), exclusive of Listriodontinae, from the early Miocene of Béon 1 (Montréal-du-Gers, SW France, MN 4), Geodiversitas 28 (4), pp. 685-718 : 688-698

publication ID	https://doi.org/ 10.5281/zenodo.4651010
persistent identifier	https://treatment.plazi.org/id/03C487F9-9F6C-FFBC-FF75-50BFFE3BFBEE
treatment provided by	Felipe
scientific name	Hyotherium lacaillei
status	sp. nov.

Treatment

Hyotherium lacaillei n. sp. ( Figs 2 View FIG ; 3 View FIG ; 5 View FIG )

Hyotherium soemmeringi – Crouzel et al. 1988: 86. — Antoine & Duranthon 1997: 211. — Antoine et al. 1997: table 1. — Duranthon et al. 1999: 86.

HOLOTYPE. — MHNT Béon 2003 H5-4, left hemimandible with alveolus of canine and p1 and p2-m3, associated with a left premaxilla with I1 and a portion of maxilla with P1-P2 ( MHNT Béon 91 G4 41), a fragment of P3, and associated maxilla fragment with P4-M1 ( MHNT Béon 91 G4 111).

ETYMOLOGY. — In memory of the late Michel Lacaille (1946-2005) who initiated the field work for teenagers at Béon 1, and took part in all the summer excavations.

TYPE LOCALITY. — Béon 1, Montréal-du-Gers, Gers, France.

MATERIAL EXAMINED. — I1 (l), MHNT Béon 92 G3 111; I2 (l), MHNT Béon AR SN 2; I3 (r), MHNT Béon AR SN 3; Cm (r), MHNT Béon AR SN 1; i1 (l), MHNT Béon 92 G3 1170; i2 (r), MHNT Béon 93 G4 468, MHNT Béon 99 G4 408; cm (r), MHNT Béon 92 G4 268; p1 (r, l), MHNT Béon G4 537; m2 (l), MHNT Béon AR SN 6 associated with m3, MHNT Béon AR SN 4; postcranial elements: humerus (l), MHNT Béon SN 4520; astragalus (l), MHNT Béon SN 181; astragalus (l), MHNT Béon 92 F3 738 (juvenile); calcaneum (l), MHNT Béon SN 4535; calcaneum (r), MHNT Béon SN 4533; navicular (l), MHNT Béon F3 810; cuboid (l), MHNT Béon 92 F3 739; Mt III (l), MHNT Béon F3 794, MHNT Béon G3 629.

DIAGNOSIS. — Hyotherium with complete and continuous row of upper and lower cheek teeth; I1 with a distal cusplet; P1 located lingually to the canine alveolus; upper premolars globular in their anterior part; M1 without buccal cingulum.

DIFFERENTIAL DIAGNOSIS. — Hyotherium with no firstsecond premolar diastema on both lower and upper dentition, in contrast to other species of the genus; upper premolars more globular in their anterior part than in H. soemmeringi and H. shanwangense , P1 located in an anterior position, lingually to the canine, as in H. meisneri ; lower premolars more slender than those of H. meisneri and H. major , but more globular than those of H. soemmeringi and H. shanwangense ; posterior part of the lower premolars weakly developed, less than in H. soemmeringi and H. shanwangense .

COMPARATIVE DESCRIPTION

Upper dentition ( Figs 2 View FIG : 1; 3: 5-8)

A portion of premaxilla with the I1 and the I2-I3 alveoli, a portion of maxilla with P1-P2, a broken P3, and another portion of maxilla with P4-M1, are referred to the same individual and associated with the hemimandible MHNT Béon 2003 H5-4. The I1 exhibits a high bicuspid crown of typical hyotheriine morphology. Another I1 (MHNT Béon 92 G3 111, Fig. 3 View FIG : 5) is similarly robust with a large labio-lingual diameter. There is a major apical wear facet and a reduced posterior contact with I2. The curvature of the crown in lateral view follows the general curvature of the root, so that the tooth is strongly bent downwards. The overall morphology of I1 is similar to that of H. soemmeringi with two apical cusps, whereas the morphology of that of H. major (Pomel, 1847) (MNHN SG 3618) and H. meisneri (Meyer, 1829) ( Van der Made 1994: figs 4, 5) is simpler with only one apical cusp. On MHNT Béon 91 G4 41, the upper incisor alveoli row is rectilinear and the two premaxillae form a sharp angle (c. 60°). The precanine diastema is reduced, which could be a sexually dimorphic difference. The anterior part of the snout is narrow, in agreement with the morphology of the associated mandibular symphysis.

Some isolated anterior teeth are preserved: I2 (MHNT Béon AR SN 2, Fig. 3 View FIG : 6) is globular, and bears a clear separate posterior cusp. Its structure is simple, with a rounded lingual cingulum. I3 (MHNT Béon AR SN 3, Fig. 3 View FIG : 7) also presents a simple asymmetrical and short crown, without any cingulum. A right upper canine (MHNT Béon AR SN 1, Fig. 3 View FIG : 8) is known; it exhibits all typical hyotheriine characters: an anterior and a posterior crest of enamel and an anterior groove. The ventral enamel band is present but limited to a small inflection in the lingual anticline. The tooth is globular and of large size and most likely belongs to a male individual.

The upper premolars are present in the holotype ( Fig. 2 View FIG : 1); they are robust and globular: their anterior part is inflated and their transverse diameter is higher than in H. soemmeringi (see measurements in Table 1). The two first premolars are closely juxtaposed and there is no postcanine diastema, the canine emerging laterally to the P1. This morphology differs from that of H. soemmeringi from Sandelzhausen (MN5, Germany; Schmidt-Kittler 1971), which presents a clear postcanine diastema and in which P1 lies posterior to the canine alveoli. Lingually, the P2 bears a small enamel bud in front of the precrista; the posterior part of the tooth presents only a small incipient lingual basin. The lingual basin in H. soemmeringi is more developed. The P3 attributed to the same individual is too fragmentary to be described. The P4 bears two well separated buccal cusps. There is no lingual cingulum. The lingual roots of the P4 are coalescent, separated by a shallow groove; there are two buccal roots. The M1 is a square tooth with four well separated roots, without buccal cingulum.

Mandible ( Fig. 2 View FIG : 2, 3)

The anterior part of the mandible (MHNT Béon 2003 H5-4) is broken in front of the lower canine alveolus; the mandibular angle and the ascending ramus of the mandible are intact, except from the coronoid process, which is broken at its base. The symphysis appears to be narrow (c. 20 mm), but this dimension is certainly underestimated because of slight distortion. The diameter of the lower canine alveolus is wide (DMD = 14 mm; DLL = 8 mm), but its depth is reduced indicating that the tooth was not ever-growing and that the specimen was a female. The main mandibular foramen opens at the level of p2, while a smaller one lies below p4. The height of the horizontal ramus is similar to that of the female specimen of H. major housed in the MNHN. The ventral edge of the horizontal ramus is thick. The vascular incisura is present but reduced; its anterior edge bears a small tuberosity on the lingual side. The posterior edge of the angle of the mandible is thin and forms a small process extending beyond the posterior border of the ascending ramus. The masseteric fossa is deep. The lateral profile of the articular condyle is slender and sub-cylindrical; in dorsal view it is slender and slightly inclined medially.

Lower dentition ( Fig. 3 View FIG : 1-4)

One i1 and two i2s of H. lacaillei n. sp. are known; these lower incisors present a typically hyotheriine morphology with mesio-distally compressed roots but broad labio-lingually, endocristids and no lingual synclinid ( Fig. 3 View FIG : 2). The lower canine MHNT Béon 92 G4 268 has a scrofic section (i.e. the endofacet is much more developed than the postfacet). The ectofacet is slightly convex. Enamel bands are visible on the endo- and postfacet. A pair of p1s (MHNT Béon G4 537, Fig. 3 View FIG : 1) displays a short and weakly 2a 2b 3a 3b developed posterior part, which confers a globular aspect to the tooth. In the mandible of H. soemmeringi from Sandelzhausen, the posterior part of p1 is more elongated (Schmidt-Kittler 1971).

The hemimandible MHNT Béon 2003 H5-4 displays a complete cheek tooth series ( Fig. 2 View FIG : 3a). All the teeth are preserved except p1; the corresponding alveolus indicates that p1 had two coalescent roots. There is no diastema, neither between c1 and p1 nor between p1 and p2. The size of the diastemata can vary individually and with sex and age ( Hellmund 1991; Van der Made 1991), but members of Hyotherium , including H. soemmeringi , exhibit diastemata anterior and posterior to p1, even reduced ones. The lower cheek tooth enamel is thick and subtly wrinkled. The premolars are slender, which is characteristic of H. soemmeringi , but the teeth are still slightly inflated at the level of the central cusps ( Fig. 4A View FIG ) and not constricted as observed in H. soemmeringi from Germany (Sandelzhausen, MN5; Georgensmünd, MN6; Meyer 1834; Schmidt-Kittler 1971) or France (Baigneauxen-Beauce, MN5). The antero-posterior lengths of the lower premolars increase from p1 to p4 and the size of the p2 is reduced with respect to p3-p4 ( Fig. 4A, B View FIG ). The central cusps of the premolars lie in a subcentral position so that the tooth appears subsymmetrical in lateral view. On p4, the metaconid

is weak, whereas the posterior accessory cuspid is well developed. The metaconid differentiation occurs posteriorly to the protoconid. The premolars lack a lingual cingulum. As in all Hyotheriinae , m1 is only slightly smaller than m2; the transverse diameter of the anterior lobe of m3 is clearly greater than the second lobe, as in H. soemmeringi . The lower molars lack an endoentofossid, which is correlated with the convergence of the two posterior cusps. An isolated m2 (MHNT Béon AR SN 6) and one m3 (MHNT Béon AR SN 4) belong to the same individual ( Fig. 3 View FIG : 4a). The teeth present globular cusps with a reduced groove pattern and a well developed talonid, particularly on m3. This morphology is similar to that of H. soemmeringi .

Postcranial skeleton

Forelimb. Humerus ( Fig. 5 View FIG : 4): the adult left humerus MHNT Béon SN 4520 is much smaller than available specimens of Eurolistriodon sp. (Orliac in press), the distal width of MHNT Béon SN 4520 being c. 60% that of specimens of Eurolistriodon sp. The former is here attributed to H. lacaillei n. sp. The proximal end is missing. The diaphysis, preserved from the level of the deltoid tuberosity, is badly crushed medio-laterally; the distal end is intact. The morphology of this bone closely resembles that of recent Sus scrofa Linnaeus, 1758 being slenderer than in Eurolistriodon . The median groove is shallow and the distal trochlea is slender in distal view. The distal extension of the medial epicondyle is somewhat more developed than in Sus scrofa .

Hindlimb. Astragalus ( Fig. 5 View FIG : 1): two mediumsized suoid astragali (smaller than those of Eurolistriodon sp.) are attributed to Hyotherium lacaillei n. sp. Their inclined proximal trochlea and their convex sustentacular facet clearly indicate their suoid affinities. Even though their morphological features are identical, the astragalus MHNT Béon

1a 1b 1c 1d

4a 4b SN 181 is much larger than MHNT Béon 92 F3 738 ( Table 2); this could be due to individual age, the small one belonging probably to a juvenile or subadult individual. The bone is slender and high, more than in extant suids. However, the astragalus is more robust than in Taucanamo sansaniense from Sansan (specimens MNHN Sa 4632-38, 4640-44, 7802, 9319, 10749). The sustentacular facet presents a strong convexity; its maximum width occurs at the mid-level of the facet; the facet is slender at its distal end and the medial border bears a clear crest, as in all extant Suinae . The cuboid facet represents a small portion of the distal trochlea. The “P index” ([cuboid-facet length/total distal trochlea length] × 100) defined by Dehm (1934) equals 39.5 for the large specimen and 42.2 for the small one; these values are higher than in Aureliachoerus aurelianensis (33.0-36.5 range measured on the three specimens of A. aurelianensis from Artenay, France, MN4; MNHN Ar 2789, 2790, 2791) and close to that of Sus scrofa (P = 40; Leinders 1976). In lateral view, the articular surface corresponding to the coracoid process of the calcaneum is wide and concavo-convex, whereas it is slender and concave in Palaeochoerus typus Pomel, 1847 (specimens from Saint-Gérandle-Puy; MNHN SG 133390, SG 9862).

Calcaneum ( Fig. 5 View FIG : 5): two calcanea are preserved (MHNT Béon SN 4594; MHNT Béon SN 4533). The global morphology is close to that of Sus scrofa : the sustentaculum tali is short with a posterior side sloping ventro-laterally. The posterior border of the sustentaculum tali is, however, slightly inflated, more than in Aureliachoerus aurelianensis (MNHN Ar 2792, 2794, and 2795; Artenay, France, MN4). The calcaneum presents a slight constriction anterior to the tuber calcanei. In dorsal view, the tuber calcanei is higher than wide and bears a deep sulcus.

Cuboid ( Fig. 5 View FIG : 2): the overall shape of the cuboid is close to that of Listriodontinae with a robust body, thicker than that of A. aurelianensis (specimen from Artenay, MNHN Ar 5655). The articular surface for the astragalus is more developed than in the latter taxon, this character being directly correlated to the medio-lateral width of the cuboid-facet of the astragalus described above. The scissure for the tendon of the fibularis longus muscle is short and rounded as in A. aurelianensis . On the medial surface of the bone, the contact for the ectocuneiform is short, triangular and anteriorly oriented as is the case in extant Suinae ; in contrast the ectocuneiform facet is oriented medially in A. aurelianensis .

Navicular ( Fig. 5 View FIG : 3): the navicular MHNT Béon 92 F3 810 is associated with the cuboid described above. The extension of the posterior process of H. lacaillei n. sp. is similar to that of Eurolistriodon ; it is, however, more slender and its proximal development does not reach the level of the proximal surface of the bone contrary to what occurs in Listriodontinae . The lateral part of the posterior process bears a large sulcus, absent in Aureliachoerus and in Eurolistriodon . Like in Listriodontini Gervais, 1859, the contact for the mesocuneiform is reduced in comparison to extant suids ( Van der Made 1996b; Orliac in press).

Mt III ( Figs 5 View FIG : 6): two fragmentary left Mt IIIs of H. lacaillei n. sp. are known from Béon 1; MHNT Béon G3 629 is almost complete, only lacking the posterior part of the proximal surface; MHNT Béon F3 794 consists only of a proximal extremity. The bone is slender and elongated ( Table 2); its distal articular surface bears a well developed median crest. On the proximal surface, the mesocuneiform facet is small, as is the mesocuneiform facet of the navicular.

DISCUSSION

The larger hyotheriine specimens from Béon 1 were initially attributed to H. soemmeringi (Antoine & Duranthon 1997) . Many dimensions and mor- phological features are indeed similar to those of H. soemmeringi : the lower premolars are slender with a p4 devoid of a clear metaconid; the I1 bears a well developed distal cusp and P4 is globular. However, other characters differ from those of H. soemmeringi : the lack of diastemata; the anterior position of the P1, located lingually to Cm; premolars more globular, especially in their anterior part; the lack of buccal cingulum in upper molars (while it occurs in all other European Hyotherium species). The plausible morphology of the anterior part of the snout of H. lacaillei n. sp. and a comparison with H. soemmeringi from Sandelzhausen are proposed in Figure 6 View FIG . It is noteworthy that diastemata are also lacking in some specimens of H. soemmeringi : a specimen from Eibiswald (Styria, Austria; basal MN5, Steininger et al. 1990) and figured by Peters (1868); this specimen also presents inflated lower premolars and may document the new species. Another specimen from Buchenthal figured by Stehlin (1899 -1900: table 1, fig. 3) also lacks diastema, but the anterior two premolars are incompletely erupted suggesting a juvenile individual, in which case a short diastema could develop with maturity.

Hyotherium from Béon 1 differs from H. meisneri and H. major by its elongated and slender lower premolars, by the lack of a clear metaconid on the p4, and the lack of diastema. The anterior position of the P1 of H. lacaillei n. sp., lingual to the CM, also occurs in H. meisneri (Orliac pers. obs. on unpublished material from Grépiac, France) and H. major ( Hellmund 1991: pl. 2) but the latter two species lack of a clear distal cusp on the I1 and present a stronger reduction of the two anterior premolars than H. lacaillei n. sp. from Béon 1.

Hyotherium shanwangense Liu, Fortelius & Pickford, 2002 , from the late early Miocene of Shanwang (Shandong, China), is also closely related to H. soemmeringi and it presents a morphology close to the specimen from Béon 1 with reduced diastemata, reduced buccal cingula on the upper molars, and globular premolars ( Liu et al. 2002). The size of the material from Shanwang is also congruent with that of H. lacaillei n. sp., but the p3 and p4 of H. shanwangense are wider, the latter being characterised by an inflated p4 bearing a posterobuccal bulge as in Tetraconodontinae ( Liu et al. 2002) . H. lacaillei n. sp. presents a very short premolar row with closely juxtaposed teeth, and the P1-P2 are much more inflated in their anterior part (see measurements in Table 1) than is the case in H. shanwangense .

Van der Made (1998) considers two subspecies of H. soemmeringi : H. s. soemmeringi and H. s. wylensis (Meyer, 1866), the latter subspecies being “morphologically similar, but smaller” than the former. H. s. wylensis includes material from Sandelzhausen and Baigneaux-en-Beauce; this material differs from the material from Béon 1 by more slender lower and upper premolars, with more developed posterior part of the teeth and by the morphology of the anterior part of the snout ( Fig. 6 View FIG ).