1999 Volume 53 Issue 1 Pages 3-18
The authors examined the mandibular features of stegodons, Stegodon aurorae and Stegodon shinshuensis through the investigation of mandibles from the Kobiwako group of Taga Town, Shiga Prefecture, central Japan. For this examination, mandibles of six species of Stegodon and those of five species of Elephantinae were used. We observed especially the median section of mandibular symphyses and a cross-section of the mandibular bodies of these specimens. Differences in characters between stegodons and Elephantinae are: 1) the cross-sections of the bodies of Stegodon are more slender and the lateral surfaces of the bodies are flatter than those of Elephantinae; 2) a groove in the medioventral side of the body is observed in some species of Stegodon, while it is not observed in Elephantinae; 3) most of the symphyseal sections of Stegodon are more elongated antero-posteriorly than those of Elephantinae and the Stegodon's intraspecific variation in shape is higher than that of Elephantinae; 4) the posterior ends of the mandibular angles of Stegodon are flatter medio-laterally, but those of Elephantinae are round, and the mandibular angles of the Stegodon are not so expanded as those of the Elephantinae. 5) regarding the arrangement of molars in the specimens prior to M2 stages, the successor and predecessor of almost all stegodons are placed on the same plane, however, in Elephantinae, the successor is slightly inclined forward at the mesial. These characters are considered to be related to the relatively long mandibles of the Stegodon. Moreover, the authors were able to determine the specific characters of S. aurorae in the Stegodon: 1) the anterior end of the body is steeply inclined in S. aurorae; 2) the median section of the symphysis is shorter antero-posteriorly than in other Stegodon; 3) in the M1 stage specimen, the successor is arranged in the posteroventral side of the predecessor, and in the M3 stage specimen, the bending convex to lingual of the third molars is strong. These findings suggest that the mandible of the S. aurorae is closely connected to the antero-posterior shortening of the mandible as observed in the Elephantinae. The authors confirm the evolutionary trend of the mandible among the considered mandibular features of S. aurorae. It is suggested that S. aurorae has the same trend as that observed in the Elephantinae. We also believe that Stegodon zdanskyi is a separate species because of the more advanced shortening of S. shinshuensis.
