An iguanodontian sternal plate from the Upper Cretaceous Ashizawa Formation (Futaba Group) of Fukushima Prefecture, Japan

Abstract

Iguanodontia is a diverse clade of ornithopod dinosaurs known from Upper Jurassic to Upper Cretaceous deposits worldwide, including Japan. In 1999, an isolated sternal element tentatively attributed to a hadrosauroid was collected from the marine Ashizawa Formation of the Upper Cretaceous Futaba Group in Fukushima, northeastern Japan. Our description of this specimen reveals that it is a hatchet-shaped (unfused) sternal plate with an undeveloped caudomedial process, a concave lateral margin, and a relatively short (unflattened) caudolateral process. This element is assignable to an indeterminate styracosternan (Iguanodontia), likely a hadrosauroid. The size of the sternal plate suggests that it is from an individual with a body length of about 3 m. Given that all iguanodontian specimens previously recovered from Upper Cretaceous marine strata in Japan appear to belong to larger (over 3 m in body length) animals, our findings suggest that smaller individuals also inhabited coastal settings in the region.