Ophiura sarsii sarsii (Echinodermata, Ophiuroidea) from the late Middle Miocene to Early Pliocene formations of North Hokkaido, Japan

Abstract

Specimens of the extant species Ophiura sarsii sarsii Lutken, collected from the late Middle Miocene to early Late Miocene Wakkanai Formation and from the middle Late Miocene to Early Pliocene Koetoi Formation in north Hokkaido (latitude ca. 45°N), are described herein. The Wakkanai specimens represent the oldest fossil record of this species. The fossil ophiuroids from both locations are found in dense aggregations suggesting that they formed high-density populations on the sea bottom of the lower shelf and the upper slope as in those of Recent ones. The maximum sizes of the fossil ophiuroids from both formations were larger than those from the Late Miocene to Middle Pleistocene formations in Honshu, north to central Japan (latitude ca. 35-39°N). This regional trend of larger body size northward resembles that of Recent Ophiura sarsii sarsii. Size structure of the ophiuroids from the Wakkanai Formation was bimodal, and the percentage of animals larger than 20 mm in disc diameter was larger in the Wakkanai Formation than in the Koetoi Formation. Such a difference in size structure may be explained by the difference in water depth.