early evolution of the Tyrannosauridae in Asia, The

Journal of Paleontology, Nov 1999 by Manabe, M

ABSTRACT-An isolated premaxillary tooth of a tyrannosaurid from the Lower Cretaceous section of the Tetorj Group, Central Honshu. Japan, complements Siamotyrannus, which is based upon an incomplete posteranium for existence of tyrannosaurids in the Early Cretaceous of Asia. The occurrence of a tyrannosaurid tooth in the Japanese early Early Cretaceous further supports the possibility that tyrannosaurids originated during the Early Cretaceous in Asia and migrated to North America when the two continents were connected via a land bridge during the early Late Cretaceous. Thickening of the premaxillary teeth might have predated the increase in body size in tyrannosaurid evolution,

INTRODUCTION

TYRANNOSAURIDS WERE the dominant carnivorous dinosaurs

in Asiamerica during the Late Cretaceous (Molnar et al., 1990). Although they are among the most famous of dinosaurs, until recently their evolution was far from well understood. They were traditionally classified within the infraorder Carnosauria with other large theropods such as allosaurids and megalosaurids (Molnar et al., 1990). A cladistic analysis, however, has demonstrated that tyrannosaurids are derived members of the Coelurosauria, a group of otherwise small theropods, and Camosauria (as traditionally envisioned) is polyphyletic (Holtz, 1994). Until recently tyrannosaurids were believed to appear suddenly in the fossil record in the Late Cretaceous, i.e., Cenomanian (ca. 97 Ma), and flourished during rest of the Late Cretaceous (Benton, 1993). However, Siamotyrannus isanensis, an incomplete skeleton of a tyrannosaurid, was reported from the Early Cretaceous (pre-Aptian, ca. 124 Ma) Sao Khua Formation of Thailand, and the fossil record of the family was extended to the Early Cretaceous (Buffetaut et al., 1996). Buffetaut et al. (1996) suggested that the early evolution of tyrannosaurids may have taken place in Asia.

An isolated premaxillary tooth (IBEF VP 001) pertaining to the Tyrannosauridae is reported herein from the early Early Cretaceous Jobu Formation, Itoshiro Subgroup, Tetori Group, Japan. It is a small premaxillary tooth with a well-developed D-shaped cross-section having carinae running along the posterolingual edges, a character only found in the members of the Tyrannosauridae (Currie, personal commun., Manabe, personal observation). Premaxillary teeth with a D-shaped cross-section have also been reported in dromaeosaurids (e.g., Molnar, 1978). Premaxillary teeth in dromaeosaurids, however, are asymmetric in cross-section (e.g., Ostrom, 1969; Manabe, personal observation).

SYSTEMATIC PA LEONTOLOGY

Superorder DINOSAURIA Owen, 1842

Order SAURISCHIA Seeley, 1887

Suborder THEROPODA Marsh, 1881

Family TYRANNOSAURIDAE Osborn, 1905

Genus and species indeterminate Material-IBEF VP 001, collection of the Izumi Village Board of Education, Fukui Prefecture, Japan. An isolated premaxillary tooth.

Occurrence.-The premaxillary tooth was collected in Hayashi-dani, Izumi Village, Fukui Prefecture (approximately 35 deg 53'N, 136 deg 46'), in 1996 by Mr. Masatoshi Okura of Konan City, Aichi Prefecture of Japan. The specimen was subsequently skillfully prepared by Mr. Okura. It is from the Jobu Formation of the non-marine Itoshiro Subgroup of the Tetori Group (Yamada et al., 1989). The Jobu Formation is believed to be contemporary with the Okurodani Formation of Gifu Prefecture which is dated between 120 to 140 Ma based upon fission track dating (Gifu-Ken Dinosaur Research Committee, 1993).

Description.-This small premaxillary tooth is 11 mrn in tooth crown height, 4.5 mm in anteroposterior basal length, and 3.8 turn in tooth basal width. The tooth has a well-developed symmetrical D-shaped cross-section and serrated carinae running along the posterolingual edges. There is occlusal wear for 2 mm, on the distal end of its posterior surface. The posterior surface is flat and has no central ridge. There is a weathered surface running for 3 mrn from the distal tip of the crown on its anterior surface. One carina is more strongly defined than the other. A shallow groove runs immediately mesial to this carina. There are four denticles per I mm on the carinae. and these are significantly larger than in similarly sized tyrannosaurid teeth from the Judith River Formation (six denticles per I mm, for example, in Royal Tyrrell Museum of Palaeontology 89.65.9).

DISCUSSION

IBEF VP001 complements Siamotyrannus, which is based upon an incomplete postcranium, and demonstrates the existence of tyrannosaurids during the Early Cretaceous in Asia. The occurrence of a tyrannosaurid tooth in the Japanese early Early Cretaceous further supports the possibility that tyrannosaurids originated during the Early Cretaceous time in Asia and migrated to North America when the two continents were joined via a land connection in the early Late Cretaceous (Buffetaut et al., 1996; Kirkland 1996).

This suggestion is consistent with the earliest previous fossil record of the family from Asia: Alectrosaurus olseni from the Iren Dabasu Formation (?Cenomanian) of Mongolia (Gilmore, 1933; Mader and Bradley, 1989). Tyrannosaurids are placed as the sister group to ornithomimids and troodontids (Holtz, 1994). The diversity of ornithomimids and troodontids is highest in Asia (Barsbold and Osmolska, 1990; Osmolska and Barsbold, 1990). In North America, the earliest tyrannosaurids (teeth from the Cenomanian Cedar Mountain Formation of Utah) appear simultaneously with other Asian dinosaurs unknown earlier in North America (troodontids, ankylosaurids, ceratopsians, basal hadrosaurids; Russell, 1993; Eaton et al., 1997).