Microstonyx major (Gervais, 1848)

Kostopoulos, D. S., Spassov, N. & Kovachev, D., 2001, Contribution to the study of Microstonyx: evidence from Bulgaria and the SE European populations, Geodiversitas 23 (3), pp. 411-437 : 414-419

publication ID	https://doi.org/ 10.5281/zenodo.5375475
persistent identifier	https://treatment.plazi.org/id/03D2217E-FFEE-981A-FCAE-FAA4FC3A7CBB
treatment provided by	Marcus
scientific name	Microstonyx major (Gervais, 1848)
status

Treatment

MATERIAL EXAMINED. — Skull specimens (K-5253, K-5254, K-5256, K-5258, K-5260, K-5262, K-5263, Pk-5265); juvenile maxillae (K-5264); palate (K-5259, K-5269); upper toothrows (entirely or partly preserved) (K-5257, K-5261, K-5266, K-5267, K- 5268, KA1, KA2, K-743); lower toothrows (entirely or partly preserved) (K-5276, K-5277, K-5278, K- 5280, K-5282, K-5283, K-5284, K-5285, K-5286, K- 5287, K-5289, K-5290, K-5292, K-5293, K-5298, E-5299); juvenile mandibular rami (K-5264, K-5294, K-5295, K-5296, K-5297).

LOCALITIES. — Kalimantsi (K), Petrelik (Pk), Ezerovo (Ez), Bulgaria.

AGE. — Late Miocene, Turolian (MN11-MN13).

PRESERVATION STATUS. — (K-5253): central part of the skull with zygomatic arches and frontals; (K-5254): part of the basioccipital and the right lateral side without teeth; (K-5256): only the palate, the zygomatic arches and a part of the frontals are preserved; (K-5258): laterally compressed, probably young female individual without teeth, except the canine roots; (K-5259): palate; (K-5260): broken right zygomatic arch and anterior part of the snout; (K-5261): palate; (K-5262): basioccipital, toothrows, left zygomatic arches and anterior part of the snout are missing; (K-5263): only the basioccipital and the left lateral side of the skull are preserved; (K-5264): part of juvenile skull (palate, maxillary bones, part of the nasal bones); (K-5269): palate; (Pk-5265): anterior part of the skull with toothrows.

DESCRIPTION

Skull

The description of the skull morphology of the Kalimantsi suid is mainly based on specimens K-5256, 5260 and 5262, while additional information are also taken from the specimens K-5253, 5254, 5258, 5259 and 5263 ( Figs 2 View FIG ; 3 View FIG ). Most of the skulls are badly or partly preserved, belonging to adult individuals. The skull specimen from Petrelik (Pk-5265) ( Fig. 4 View FIG ) is very similar to that from Kalimantsi and therefore morphological characters are presented together. Skull measurements are given in Table 1.

The occiput is triangular, moderately elevated and strongly concave, both in lateral and posterior view ( Fig. 2 View FIG ). The anterior apophysis of the basioccipital is small, while the paroccipital process is strong. The choanae opens behind M3. The palate is elongated and relatively narrow. The frontoparietal is flat, limited laterally by the well-developed temporo-parietal crests. The temporals are slightly concave. The anterior rim of the orbit is placed well behind M3. The lachrymal notch (referred also in some French bibliography as “incisure infraorbitale”) is large and wide. The zygomatic arches are strongly inflated and laterally extended. Their anterior end is placed above the limit M1/M2. The snout is elongated and narrow ( Fig. 5 View FIG ). The posterior end of the elongated nasals is placed at the point of the greatest extension of the zygomatic arches. In lateral view the nasals appear to be slightly concave. The posterior end of the narial notch is placed just above the anterior border of the canine. The maxillary depression for the elevator and depressor rostrii muscles is elongated and deep. The maxillary foramen is located above the anterior lobe of M2 or between M1/M2. The alveolar tuberosities are strongly developed with rough surfaces ( Figs 2 View FIG ; 4 View FIG ; 5 View FIG ).

Despite the general morphological homogeneity between the skull specimens, there is some developmental and size variability. Thus, the specimens K-5260 and K-5258 are relatively smaller and have less developed alveolar tuberosities than specimens K-5262, K-5269 and Pk-5265.

Mandible

Only four specimens from Kalimantsi (K-5276, 5277, 5278 and 5283) and a mandibular ramus from Ezerovo (E-5299) are available for morphological and metrical observations ( Table 2; Figs 6 View FIG ; 7 View FIG ).

The two best preserved specimens K-5276 and K-5277 ( Fig. 6 View FIG ) belong to adult individuals. They are characterized by elongated and relatively narrow snout with shallow horizontal rami. Nevertheless, the specimen K-5277 is slenderer with a more elongated symphysis (102 mm) than specimen K-5276 (87mm) ( Table 2) which appears to have a shallower horizontal ramus ( Table 2). The dark colors of preservation in specimen K-5277 may be indicate a different fossiliferous horizon than K-5276. Taking into account the similar ontogenetic age of the two individuals, the observed differences could be also attributed to sexual dimorphism. The mandible from Ezerovo ( Fig. 7 View FIG ) has a deeper horizontal ramus than that from Kalimantsi (height of the mandible behind m3: 74.7 mm in E-5299 versus 67.5 in K-5283) and similar to that of M. majo r from Dytiko ( Greece) (73.5 mm).

Upper toothrow

The morphology and dimensions of the upper incisors and canines is unknown; they are usually broken or in an advanced stage of wear. A single I2 from the specimen Pk-5265 has DAP = 22.5 mm. According to their alveoli, the canines seem to be small relatively to the skull size ( Fig. 5 View FIG ). The length of the upper toothrow (P2-M3) is 148.3 mm in K-5257 and 151 mm in Pk-5265. The length of the molar row ranges between 89.6-93.6 mm in Kalimantsi (n = 5; K-5261: 89.6 mm, K-5256: 91 mm, K-5257: 92.2 mm, K-5267: 93.6 mm), while it appears slightly larger in Petrelik (n = 1; Pk-5265: 97 mm). Tooth measurements are given in Tables 3 and 4.

P 1. Looking at the Bulgarian material of Microstonyx , three different situations can be observed:

– P1 small, rudimentary, single cuspid and single rooted, separated from P2: case K-5259 ( Fig. 5 View FIG ); the obviously vestigial P1 is present only in the left side of the palate, while it is missing from the right one. The specimen belongs to an adult individual (M 2 in the first stage of wear). The distance between P1-P2 is 11 mm, while the distance between P1-C is 35.2 mm;

– P1 well-developed, double rooted and separated from P2: case Pk-5265 ( Fig. 4 View FIG ); P1 is present in both toothrows and has a simple elongated form. The distance P1-P2 is 14.5 mm and the distance P1-C is 39.4 mm;

– P1 well-developed, double rooted and close to P2: case K-5268; only the roots are preserved, indicating a normally developed P1. The specimen K-5268 is darker colored than the rest of the specimens, allowing for a possibly different stratigraphical origin.

P2. The three available specimens (K-5257, 5259 and 5266) show a simple P2 with weak anterior and strong postero-lingual cingulum.

P3. In the unworn specimen K-5268, the para- and metastyles and the antero-lingual cingulum are strong. In its upper part, the main cusp is divided into three tubercles. The internal talon is strong. In the worn specimens K-5259, 5260, 5266, 5269 and Pk-5265 the occlusal outline of

Kostopoulos D. S., Spassov N. & Kovachev D.

the tooth is triangular with strong postero-labial style (metastyle) and internal talon.

P4. P4 is clearly molarized, subsquarish and wider than long (K-5256, 5259, 5267, 5260, 5266, 5261, 5268, 5269, Pk-5265). The protocone is well-developed, while the paracone and the metacone are divided in the labial wall by a shallow furrow. The anterior and postero-lingual cingula are strong.

M1/2. The anterior cingulum is moderately to strongly developed. The posterior cingulum is strong, forming a rudimentary talon, more developed in M2 than in M1. Based on the studied specimens, the lingual and labial accessory cusps are more or less well-developed but low.

M3. Similar to M2 with a strong anterior cingulum. There are two morphotypes of the talon:

– the first one has a main cusp in the posterolingual position (K-5260, 5256, 5253, 5259, 5267, 5257, Pk-5265, KA1);

– the second one is less frequent (K-743, KA2) with two posterior cusps and double “hypoconule” ( Fig. 8A View FIG ).

Lower toothrow

The incisors present a crest on the lingual surface, due to the lateral compression of the teeth. The maximal measured length (DAP) is 33.3 mm for i2 (K-5284) and 23.3 mm for i3 (worn). The canines are small and slightly curved posteriorly ( Fig. 6 View FIG ). The length of the Kalimantsi lower toothrow (p2-m3) ranges between 142.8-161 mm ( Table 2) with a length of the molar row between 86 and 101.5 mm (n = 7; data as in Table 2 and

K-5280: 87 mm, K-5292: 90.8 mm, K-5285: 92.4 mm). The respective values for the mandible from Ezerovo are Lp2-m3: 153.6 mm and Lm1-m3: 101.5 mm. Tooth measurements are given in Tables 4 and 5.

p 1. It is absent from the two best preserved specimens K-5276 and K-5277.

p2/3. They are very similar morphologically, although the third premolar is larger. The anterior stylid (parastylid) is well developed, while the metastylid is larger and more complicated.

p4. It is even larger than p2/3 and more complicated. The cingulum is very weak and the anterior stylid moderately developed. The labial (protoconid) and the lingual (metaconid) cusps are strong, separated only in the first stage of wear. The talonid is well-developed with a main cuspid, separated from the trigonid by a weak furrow.

m1/2. The first two molars have a weak anterior and a strong posterior cingulum, which forms a rudimentary talonid. The accessory cuspids are small, both lingually and labially.

m3. It is similar to m1/2. Two types of talonid can be distinguished:

– with rounded or quadrangular posterior border (K-5276, 5277, 5285, 5280). The endoconid is slightly stronger than the hypoconulid, limited posteriorly by a small cuspid. The posterior accessory cuspid can be single or double;

– with elongated, rhomboid posterior border (K-5298, 5286) ( Fig. 8B View FIG ). The posterior cuspid is strong. The endoconulid is formed of a number of smaller accessory cuspids.