Three echinoid assemblages with the earliest cidaroid (Echinodermata: Echinoidea) fossil record from the Middle Miocene of Taiwan

Cidaroids (Echinoidea: Cidaroida) are the basalmost taxa among extant echinoids. However, the relative scarcity of intact cidaroid tests in the Cenozoic fossil record poses challenges for understanding their biogeographic patterns and evolutionary trends. This study reports the 200-meter thick Middle Miocene Nangang Formation, including three echinoid assemblages, namely “Cidaroida assemblage”, “Astriclypeoidea assemblage”, and “Spatangoida assemblage”. The potential paleoenvironmental implications and comparisons to widespread Miocene echinoid faunas of the circum-Mediterranean area are discussed. Notably, an articulated cidaroid fossil with intact spines recovered from the Cidaroida assemblage. The specimen exhibits shallow and transverse oval areoles along with distinct, rod-like, spinose primary spines, suggesting that it belongs to the genus Prionocidaris. In summary, the Prionocidaris fossil stands as the earliest occurrence of this order in Taiwan, and the associated echinoid assemblages offer a valuable record in the underexplored Western Pacific Region.