Systematic revision and sexual dimorphism in Choffatia (Ammonoidea: Perisphinctoidea) from the Callovian of Kutch, India

Abstract

The genus Choffatia (Siemiradzki) is an important Callovian ammonite in Kutch biostratigraphy. Several species of the genus were described by the early workers who did not recognize intraspecific variability and sexual dimorphism in terms of the modern concepts. We have described here three dimorphic species based on material reposited in the Geological Survey of India, Kolkata as well as systematically collected additional specimens with precise stratigraphic information. These species are: Choffatia recuperoi (Gemmellaro, 1873), Choffatia cobra (Waagen, 1875), and Choffatia perdagata (Waagen, 1875). The microconchs of the three species, described here for the first time, are lappeted whereas the macroconchs have a simple aperture. However, these species have similar shell morphologies and they intergrade in shell shape and sculpture; thus it is difficult to distinguish them qualitatively. Statistical analyses (both bivariate and multivariate) support that they are distinct species and there is a good correlation among the degree of involution, the degree of inflation, and the strength of ornamentation. C. cobra is the most evolute, depressed, and strongly ribbed whereas C. perdagata is the most involute, compressed, and weakly ribbed. C. recuperoi occupies the intermediate position. This interspecific diversity of Choffatia thus supports Buckman's Law of Covariation.

Introduction

Choffatia is a Middle Jurassic (Bathonian–Callovian) quasi-cosmopolitan ammonite genus recorded in several biogeographic provinces (Fig. 1), but species level assemblages seem to be characteristic and more or less different in each biogeographic domain or basin (see Spath, 1927–1933, Mangold, 1970, Cox, 1988, Riccardi et al., 1989, Sandoval et al., 1990, Zaton, 2010). Many species of Choffatia have been described from the Callovian and some of them are time-diagnostic (see Mangold, 1970, Cox, 1988). In Kutch, India, Choffatia has been studied formerly by Waagen (1873–1875) and Spath (1931) who have altogether described 18 species. The present study reveals that Choffatia in Kutch has been excessively and subjectively split into many species. Moreover, the early workers were not well aware of the prevalence of sexual dimorphism. Many previously described species appear to be microconchs.

In the present study, we have revisited the genus Choffatia based on the type specimens stored in the repository section of the Geological Survey of India, Kolkata, along with additional specimens that have been collected systematically from different sections within the mainland of Kutch. Most of the collection has been made in different field trips by the members of the Palaeontology Laboratory at the Department of Geological Sciences, Jadavpur University. The detailed systematic study reveals that Choffatia is represented by only three dimorphic species, namely, Choffatia recuperoi, Choffatia cobra, and Choffatia perdagata. Each species is distinct as revealed from statistical analyses; the microconchs are lappeted and replicate the intermediate stage of the macroconchs. Detailed comparison has been made for each species with the contemporaneous species from the other parts of the world. The present three species have almost similar stratigraphic ranges in Kutch.

The genus Choffatia shows a low specific diversity and there is a strong correlation among the shell inflation, the shell involution, and the strength of ornamentation. C. cobra, being the most evolute, depressed species, has the strongest ornamentation in contrast to C. perdagata, which is relatively involute, compressed, and weakly ornamented.