[MM] Gobiconodon hoburensis
DESCRIBER Trofimov, 1978
TIME Cretaceous Early
FOSSILSITE Hovoor Beds, Gobi Dessert, Mongolia
FALL UNDER Gobiconodon borissiaki
Gobiconodon borissiaki (Trofimov, 1978) > Gobiconodon hoburensis (Trofimov, 1978 )[ >> Guchinodon hoburensis (Trofimov, 1978 )] > Gobiconodon hopsoni (Rougier, Novacek, McKenna and Wible, 2001) > Gobiconodon ostromi (Jenkins and Schaff, 1988) > Gobiconodon zofiae (Li, Wang, Hu & Meng, 2003) > Gobiconodon sp. (1) (Rougier, Novacek, McKenna & Wible, 2001) > Gobiconodon sp. (2) (Tang et al., 2001) > Gobiconodon sp. (3) (Cuenca-Bescós & Canudo, 2003 )
In Gobiconodon hoburensis the contact between succeeding teeth is formed by cusp d fitting between a mesiolingual cusp e and a mesiolabial cusp f. A cusp f, however, seems to be very small and rapidly eroded or missing altogether. Cusp d is supported labially by the anterior slope of b. The remaining cusps of the crown are missing. The m5 is the best preserved molariform of this jaw fragment. The contact between m4 and m5 is different from that between m3 and m4. The distal cusp d of the m4 does not fit in an embrasure between cusp e and the anterior slope of b, or possibly cusp f. A mesiobuccal cusp or extension of the crown is absent, and cusp d abuts directly labial to cusp e. Cusp b is away from the front of the tooth and a cusp f is absent; cusp e, however, is prominent. In short, m4 does not interlock with m5, but is arranged en echelon. In all other species of Gobiconodon, cusp d of the m4 interlocks with cusp e and either the slope of b or cusp f (Kielan-Jaworowska and Dashzeveg, 1998: fig. 7).
All the remaining major cusps of the m5 are damaged and worn, but enough remains to show that they were essentially aligned anteroposteriorly. On the lingual side of the m5, a moderate cingulum runs from cusp e to cusp d. The roots of the molariforms do not show the thickening at the level of the alveolus present in G. borissiaki and the fragmentary lower jaw referred to G. hopsoni. The roots of the m5 are directed less distally than those of the m5 of the Asiatic species of Gobiconodon (Kielan-Jaworowska and Dashzeveg, 1998; personal obs.) and are more similar to those of G. ostromi (Jenkins and Schaff, 1988).
The dentary is robust, with thick cortical bony walls that are thicker than those of Gobiconodon borissiaki. The masseteric fossa is the most conspicuous feature on the labial surface of the dentary; it is moderately deep and limited ventrally by a thick and blunt masseteric ridge. This fossa extends anteriorly, but stays behind the level of the m5. The anterior limit of the fossa was formed by a mostly missing coronoid ridge. In the anterior part of the masseteric fossa, a small foramen opens into the substance of the dentary.
This foramen occupies a position similar to the masseteric foramen, or labial mandibular foramen, of other Mesozoic mammals (Dashzeveg and Kielan-Jaworowska, 1984; Rougier, 1993; Cifelli et al., 1998). On the labial aspect of the body of the jaw, between the lower molariforms, conspicuous depressions corresponding to the position of the upper molariforms can be identified. Similar depressions are present in Gobiconodon ostromi, but not in the remaining Asiatic species, and these may simply be related to the larger size of G. ostromi and G. hopsoni.
On the lingual aspect of the dentary, a very subtle sulcus runs mesiodistally along the ventral third of the dentary toward the broken edge of the mandibular foramen. This sulcus is the impression of the remnant of Meckel’s cartilage. It is less pronounced than in other species of Gobiconodon. The mandibular foramen is very posteriorly placed and marks the anterior edge of the pterygoid fossa. The mesialmost edge of this fossa extends mesiodorsally from the foramen in the broken base of the coronoid process. An incomplete oval depression lies ventral to the mandibular foramen, a muscle scar likely produced by the medial pterygoid. This depression is known in all species of Gobiconodon, but not in other Mesozoic mammals, and is a good diagnostic feature for this group.