Gobiconodontidae Chow and Rich, 1984

Cuenca-Bescós, Gloria & Canudo, José I., 2003, A new gobiconodontid mammal from the Early Cretaceous of Spain and its palaeogeographic implications, Acta Palaeontologica Polonica 48 (4), pp. 575-582 : 576-577

Family Gobiconodontidae Chow and Rich, 1984

(= Gobiconodontinae Chow and Rich, 1984)

The systematic position of Gobiconodontidae has been a subject of discussion in several papers. Traditionally authors included it in the order Triconodonta Osborn, 1888 ( Trofimov 1978; Jenkins and Crompton 1979; Chow and Rich 1984; Jenkins and Schaff 1988; Maschenko and Lopatin 1998). A recent review of Kielan−Jaworowska et al. (in press) included Gobiconodontidae in the orderEutriconodonta. Some features of the Gobiconodontidae dentition are apomorphies for this group, i.e., the raised position of the cingulum and the outline of the crown of the upper molars ( Fig. 2 ). In gobiconodontids, the three main cusps of the lower molariforms are aligned mesio−distally, as in other eutriconodontans (Kielan−Jaworowska et al. in press), whereas in the uppers, the main cusps form an obtuse angle.

Gobiconodontidae indet.

Figs. 2–4 .

Description.—One left upper molariform, M3 or M4 (MPZ 2002/73) of small size. It has a robust cusp A, characterised by probably autapomorphic lateralbulges on each mesialand distal flanges of that cusp. It has a discontinuous cingulum raised at the lingual side. The tooth is 1.67 mm long and 1.13 mm wide ( Table 1). It is almost complete, only the mesiolingual corner is broken, and a small portion of the anterolingual part of cusp B is missing. The tooth has two roots, the anterior one being broken. The posterior root is complete and compressed anteroposteriorly. In oclusal view the crown is “pillow−shaped”, more or less rectangular, and somewhat constricted in the middle. The mesial half is transversely narrower than the distal. The labial side is more indented than the lingual ( Figs. 2–4 ). The tooth crown is fanshaped in labial and lingual views. The central cusp A is the highest point of the crown ( Fig. 2 ).

The three main cusps: the anterior B, medial A, and posterior C form a wide obtuse angle. Cusp A is more lingual with respect to cusps B and C. On the distal side the distal cingulum has a broad wear facet that may have had a cuspule D ( Fig. 2A ) with a lingual slope. Cusp A is the highest and the broadest with a small wear surface on its tip ( Fig. 4A, B ), and it has two mesial and distal bulges aligned anteroposteriorly with its tip, which gives it a massive appearance ( Fig. 2A, B, D ). Both protuberances are less visible but still noticeable in oclusal view, specially the distal one ( Fig. 3 ). Cusp B is heavily damaged. Cusp C is about a third lower than the height of the main cusp A and has a distolingual wear facet. The cusps are separated by shallow incisions visible in both lingual and labial views. The notches have V−shape and they are less than 25% as deep as the cusps.

The cingulum widens on the lingual side, where ithas two small cingular cusps. The proximal cingular cusp forms the mesiolingual corner of the base of cusp A, and the other is distal and at the bottom between cusps A and C, slightly higher and taller than the proximal ( Figs. 2A, B, C , 4B ).

A thin enamel layer, as observed in the broken part of cusp B ( Fig. 4C, D ), covers the crown. The effects of acid etching allowed us to study the internal structure of the enamel and dentine. The enamel is prismatic, with the prisms more or less perpendicular to the oclusal surface and the enamel−dentine junction arranged in a similar pattern described as the radial type ( Koenigswald and Clemens 1992). The enamel is thin compared with the massive dentine. The radial structure of the dentine can be seen in Fig. 4C . The pulp cavity is visible in the anterior part ( Fig. 2E ), one can see the thickness of the dentine compared to the cavity, indicating that the molar belongs to an adult individual rather than to a juvenile.

Figures

Fig. 2. Gobiconodontidae indet. SEM micrographs of the upper molariform M3 or M4 MPZ 2002/73 (Vallipón, Teruel, Spain, upper Barremian) in lingual (A), occlusal (B), posterior (C), labial (D), and anterior (E) views. White line in A shows the distal cingulum and the direction of tooth wear made possibly by cusp a of the corresponding lower molariform. In B, mesial is up, the arrows on the left show the lingual cingulum, the arrow at the bottom the distal cingulum. In E the arrow shows the pulp cavity.

Fig. 3. Gobiconodontidae indet. Stereo SEM micrographs of the occlusal view of the upper molariform M3 or M4, MPZ 2002/73 (Vallipón, Teruel, Spain, upper Barremian). The mesial side is up.

Fig. 4. Gobiconodontidae indet. SEM micrographs of the distal cingulum and the enamel at the occlusal view of the broken cusp B of the upper molariform M3, 4 MPZ 2002/73 (Vallipón, Teruel, Spain, upper Barremian) in occlusal view, mesial side is down (A), detail of distal cingular cuspule (B), dentine layers at the anterior broken part of the tooth and enamel cover detailed in D (C), a detail of the enamel ultrastructure showing the radial type of the enamel prisms (D).