KIIMETRA MIOCENICA, A NEW GENUS AND SPECIES OF THE FAMILY CALOMETRIDAE (ECHINODERMATA: CRINOIDEA) FROM THE MIDDLE MIOCENE OF SOUTHWESTERN JAPAN

Journal of Paleontology, Mar 2007 by Shibata, Tomoko F, Oji, Tatsuo

INTRODUCTION

EXTANT COMATULID (stalkless) crinoids consist of approximately 530 species, classified in 143 genera and 18 families (Clark, 1931, 1941, 1947, 1950; Clark and Clark, 1967; Messing and White, 2001). Recent comatulids account for almost 85% of all the extant crinoid species (comatulids plus stalked crinoids), and their taxonomy is currently being extensively revised (e.g., Hoggett and Rowe, 1986; Rowe et al., 1986; Messing and White, 2001). In stark contrast to such a high extant diversity, records of fossil comatulids are far less common. This is partly due to their low fossilization potential. From the Cretaceous, only 18 genera and nine families of comatulids have been documented (Rasmussen, 1978).

The oldest record of a comatulid dates back to the late Triassic (Paracomatula Hess, 1951; Hagdorn and Campbell, 1993; Simms et al., 1993). Up to the late Cretaceous, the most diverse taxa were notocrinids and solanocrinids (Rasmussen, 1961; Jagt, 1999). The comatulids probably diversified during the Cenozoic (compare Jagt et al., 2002).

Cenozoic records of comatulids are thus very important for understanding comatulid diversification. In contrast to stalked crinoids, which occur only at depths below 100 m in modern seas, comatulids are the only crinoid group which flourishes in shallowwater settings. In order to understand the timing and pattern of comatulid survival and diversification, data on Cenozoic comatulids are crucial. Until now, most reports of fossil comatulids chiefly described centrodorsals, because other skeletal parts are easily disarticulated and lost after death (Meyer and Meyer, 1986).

In the current paper, we describe a very well-preserved new comatulid species from the Miocene of southwestern Japan, the first report of a fossil member of the Calometridae. It is named Kiimetra miocenica n. gen. and sp. The sample studied contains over 100 specimens on a single bedding plane. They are almost all articulated, and preserve delicate structures of arms, pinnules, and cirri because of rapid and virtually in situ burial. Skeletal terminology is based on Moore et al. (1978).

MATERIAL AND GEOLOGICAL SETTING

The comatulid specimens were collected from a bedding plane of large sandstone slabs (approximately 2.5 m � 1.5 m) exposed near the northern tip of the Torinosu Peninsula on the southern coast of Tanabe Bay (Fig. 1 ). The comatulid bed occurs in the middle part (the M2 Member) of the Shirahama Formation (Tanabe Group). The Tanabe Group is assigned to the late Early to early Middle Miocene (ca. 16 Ma) age (Tsuchi, 1981). According to the Tanabe Research Group (1984), the uppermost part of the Asso Formation, which is the lowermost unit of the Tanabe Group and which is overlain by the Shirahama Formation, correlates with the planktonic foraminifer stage N8 of Blow (1979). According to Hisatomi (1998), this part of the Shirahama Formation was deposited on a shallow upper shelf and represents a high-energy paleoenvironment.

A few large slabs containing fossil comatulids were extracted from the exposure and transported to the National Science Museum, Tokyo. By combining these slabs, we were able to reconstruct their original arrangement (Fig. 2).

At this locality and in its environs, a total section of 2.8 m of the M2 Member may be observed (Fig. 3) and consists of alternating beds of fine-grained and medium-grained sandstone of varying thicknesses. The fine-grained sandstone is usually muddy, associated in places with bluish gray mudstone, and contains carbonaceous matter. The beds are occasionally cross-stratified. The medium-grained sandstone is well consolidated and sticks out beyond the fine-grained sandstone. It is poorly sorted, and sometimes conglomeratic. The conglomeratic layer, less than 1 m thick, consists of rounded pebbles of quartzite with smaller amounts of sandstone and shale.

All the comatulid specimens are contained in a single bed in the middle part of a 40 cm thick sandstone bed. This layer roughly coincides with the boundary between the lower conglomeratic part and the upper pebble-free, medium-grained sandstone layer within this sandstone bed. The specimens stand out well from the bedding plane. Disarticulated shells of large oysters [Crassostrea gravitesta (Yokoyama, 1926), 5-25 cm in length] and trace fossils (Skolithos isp. Haldeman, 1840) are associated with the comatulids. Aside from pieces of wood, the bed yielded no other fossils.

MODE OF OCCURRENCE

The individuals are scattered on a bedding plane of mediumgrained sandstone with no apparent preferred orientation (Fig. 4), which suggests that, at least in the last stage of their burial, there was no strong current (e.g., local turbulence accompanying a storm).

The comatulid bed forms part of a conglomeratic sandstone with rounded pebbles and cobbles. The bed is predominantly conglomeratic in the lower half and becomes less conglomeratic towards the top. This suggests that the deposition of this conglomeratic sandstone reflects the energy level of currents, and the comatulid individuals settled during the last stage of a single depositional event.