Some Eocene Cerithioids (Gastropoda, Mollusca) from Kutch, Western India, and Their Bearing on Palaeobiogeography of the Indian Subcontinent

The cerithioids are a diverse group of gastropods found globally as fossil and living animals during the Cenozoic Era. Their systematics is riddled with problems stemming from large morphological variability, homoplasy, and wide geographical distribution. Six cerithioid species are described here from the lower Middle Eocene of Kutch, Gujarat, western India. All but two are new species. They are Palmerella kutchensis, Tenagodus? sowerbyi, Potamides archiaci, and Cerithium harudiensis. The rest are new records from Kutch and were known only from Pakistan. A palaeobiogeographical review based on major published records of benthic molluscs from Pakistan reveals strong endemism. The cerithioid faunas from different parts of Pakistan and this report from Kutch also show strong endemism and often quite localized development. It is argued here that unhindered faunal mixing was not possible in this province since these newly evolved basins were restricted in nature in their Lower Cenozoic history. 1. Introduction Cerithioids are an extremely diverse and less understood group of gastropods. It includes the family Turritellidae with one of the highest specific diversity among gastropods since the Late Cretaceous period. Their taxonomic prodigality does not reflect on morphological disparity. The simple turreted coiling characterizes all cerithioids. Cerithioid species are often defined based on very small and minute differences in characters. Again, many species, especially geographically widespread ones, have extremely variable morphological features resulting in large overlapping in several attributes with one or another species (e.g., see Houbrick [1] and Reid et al. [2]). Morphological convergence is also common in higher taxonomic categories, such as families and genera [3]. The superfamily Cerithioidea has been variously interpreted in recent times. In an effort to make sense of this poorly understood group different researchers have placed varied importance to different sets of characters, often without a critical appreciation of their evolutionary significances. The superfamily does not include the family Turritellidae in Bandel’s [3] classification scheme based primarily on protoconch characters. Protoconch characters, generally appearing to remain fairly constant in genera and families, have been given high weightage. However, classification schemes based on other presumably stable characters, such as radular features, do not always match with those based on protoconch characters. For example, Houbrick [4] placed Campaniloidea close to Cerithioidea