Borisodon kara

Archibald, J. David & Averianov, Alexander, 2012, Phylogenetic analysis, taxonomic revision, and dental ontogeny of the Cretaceous Zhelestidae (Mammalia: Eutheria), Zoological Journal of the Linnean Society 164 (2), pp. 361-426 : 401-403

BORISODON KARA ( NESOV, 1993)

FIGURE 26

(See Appendix 4 for synonymies, referred illustrations, and referred specimens.)

Holotype: CCMGE 106 /12455, right dentary with p5, m1-3, alveoli for p2-4, and coronoid, angular, and condyloid processes. In the original description the collection number was incorrectly indicated as CCMGE 101 /12455 ( Nesov, 1995: 123). Found in 1988.

Type locality and horizon: Near Ashchikol’ Lake , Kyzylorda Province , Kazakhstan. Drilling core from about 500 m depth. Grey siltstone of lower Turonian age ( Nesov, 1993) .

Diagnosis: Differs from Aspanlestes , Zhelestes , Parazhelestes , and Eoungulatum by tilting of coronoid process 95–105°. Differs from Aspanlestes , Zhelestes , and Eoungulatum by mandibular symphysis at p2 or more anterior. Differs from Aspanlestes , Zhelestes , and Parazhelestes by ‘coronoid’ facet present; p5 metaconid absent; p5 lingual cingulid complete. Differs from Zhelestes , Parazhelestes , and Eoungulatum by Meckelian groove present. Differs from Aspanlestes and Parazhelestes by mandibular condyle slightly above alveolar level; p5 paraconid cingulid cusp absent. Differs from Eoungulatum by masseteric fossa bordered ventrally by well-defined crest connected to condyle; m3 subequal or larger than m2.

Description: The posterior portion of the dentary horizontal ramus is about 1.5 times higher than the tooth crowns. On the labial side at the mid-height of the horizontal ramus under the p5 distal root there is a large posterior mental foramen, connected to a short groove that becomes shallower posteriorly. On the medial side of the horizontal ramus the posterior end of the mandibular symphysis is at the level between the roots of p2. There is a rather faint Meckelian groove extending between the level of the distal end of m3 and towards the point anteroventrally of the mandibular foramen.

The horizontal ramus gradually continues into the ascending ramus without constriction. The ascending ramus is about three times higher than the horizontal ramus, with a steep anterior border of the coronoid process, sloping at an angle of 75° toward the alveolar margin. The coronoid process, or its impression, was complete before its removal from the encasing matrix; thus, the outline in Figure 26 is quite accurate. It is trapezoidal in shape, with an almost straight anterior margin and a slightly concave posterior margin. The masseteric fossa is very large and deep, extending posteriorly to the condylar process, and bordered anteriorly by a very prominent coronoid crest. It is deepest at the anteroventral corner, where there is a large, slit-like labial mandibular foramen concealed laterally by the coronoid crest. Ventrally the masseteric fossa is bordered by an extensive posterior shelf continuing to the mandibular condyle. The medial side of the coronoid process is flat and slightly depressed along the anterior margin. The mandibular foramen is relatively large, oval-shaped, and faces posteroventrally. It opens above the anterior portion of the angular process, in the posterior half of the ascending ramus.

The mandibular condyle is distinctly above the alveolar level. The condyle is convex and oval-shaped in posterior and dorsal views, with the lateral end more pointed and placed more dorsally compared with the medial end ( Fig. 26B–D ).

The mandibular angle is a thin triangulate plate somewhat deflected medially. Its anteroventral margin is convex and the posterior margin is concave, forming part of the round incisura between the mandibular angle and condyle.

There are alveoli for double-rooted p2-4 preserved in the anterior portion of the dentary fragment and partially visible laterally because of the broken dentary wall. The alveoli for p4 are approximately the same size as for p5. The anterior alveolus for p2 is incomplete but it seems that this tooth was only slightly shorter than p5. The alveoli for p3 are twice as small as those of p4.

The p5 is a semimolariform tooth without a metaconid, and with a unicusped talonid with an incipient ‘basin’. It is as tall as the molars. The main cusp (protoconid) is distinctly curved, hook-like, with its apex pointed distally. Its mesial margin is a sharp vertical crest. The distal side of the cusp is wide, concave, facing linguodistally, and sloping into the talonid basin. Mesial to the base of the protoconid there is a small triangular platform at the junction of the lingual and labial cingulids and the protoconid mesial crest. This structure could be a cingulid mesial accessory cusp (paraconid). The talonid is formed by a large cusp, reaching half the height of the protoconid, and is pointed dorsodistally. Its mesiolingual side is distinctly concave, forming the incipient talonid basin. The lingual cingulid is continuous, extending from between the mesial end towards the distal point of the talonid cusp. It is elevated above the space between the roots and forms the lingual border of the talonid basin. The labial cingulid is interrupted at the space between the roots and vanishes at the posterior portion of the talonid cusp.

The lower molars are of rather similar morphology. The m2 is the largest and the m1 the smallest. The trigonid is about twice as tall as the talonid on m3 (on m1-2 the protoconid is not complete and could be taller). The metaconid seems to be only slightly lower than the protoconid. The metaconid apex is bent somewhat distally. The paraconid is smaller than the metaconid but still relatively unreduced. It is largest on m3 and smallest on m1. On m3 the paraconid is distinctly taller than the talonid of m2 whereas as in m1-2 the paraconid is confluent with the talonid surface of the preceding tooth. The mesiolingual vertical side of the paraconid forms a keel on all molars. The protocristid is almost straight and transverse. The paracristid is not straight but angled at the paracristid notch. The bases of the paraconid and metaconid are connate lingually, leaving only a slit-like opening into the trigonid basin. The trigonid basin is a small triangular area between the bases of the trigonid cusps. The talonid is distinctly wider than the trigonid on m1, slightly wider on m2, and narrower on m3. The cristid obliqua contacts the trigonid wall at the protocristid notch. The hypoconid is the largest talonid cusp and the entoconid the smallest. The hypoconulid projects distally and is closer to the entoconid but is not twinned with the latter. In m3 the hypoconulid is hook-like and separated by a greater distance from the entoconid than in m1-2. The talonid basin slopes mesiolingually towards its deepest point at the mesial end of the entocristid. The development of the precingulid increases from m1 to m2. It is a short subvertical crest whose lingual end is abutted against the hypoconulid (or talonid cusp) of the preceding tooth. A very faint postcingulid is present only in m2.

Measurements: See Appendix 3.

Figures

Figure 26. Borisodon kara, CCMGE 106/12455, right dentary with p2-4 alveoli, p5, m1-3. A, stereophotographs of p5, m1-3 in labial, occlusal, and lingual views. Photographs of dentary, B, lingual; C, posterior; D, lingual views. Line shows reconstruction of ascending ramus based on impression in surrounding matrix.