Genus Parapatoceras SPATH from Kachchh and the likely ancestor of Epistrenoceras BENTZ (Ammonoidea, Middle Jurassic)

摘要

The age of the heteromorph ammonite genus Parapatoceras SPATH in Kachchh (western India) is now lowered to the late Middle Bathonian Arkelli Zone (= Bremeri Zone) and globally to Early Bathonian (Zigzag Zone). The upper limit of the genus, has also been recently revised to Late Callovian (Collotiformis Subzone, Athleta Zone). The late Middle Bathonian Parapatoceras distans (BAUGIER & SAUZE) from Jumara (Kachchh), as recorded here, is the best ancestral candidate for the mid-Late Bathonian Epistrenoceras BENTZ; both have now been recorded from the same section at Jumara. A tentative Strenoceras-Spiroceras-Parapatoceras-Epistrenoceras lineage is plausible with a suggestion that possibly both Parapatoceras and Epistrenoceras came for the same ancestor, the Spiroceras, and that in turn from the Bajocian Strenoceras. Analysis of coiling variations of Parapatoceras SPATH suggests that differentiation based on coiling is not a defining parameter for the introduction of a subspecies. Six global heteromorph events are proposed that are either influenced by sea level highstands or transgressive phases.