A rare occurrence of Lamellaptychus (ammonoid jaw) from the Tethyan Himalaya and updated Indian aptychus record

Abstract

The jaw apparatuses of ammonoids are rarely discovered in the fossil record. Here, we describe the first and sole record of the aptychus type lower jaw of ammonoid Lamellaptychus lamellosus (Parkinson, 1811) from the Tethyan Himalaya. The aptychus was collected from the Demul section, representing the succession of Upper member of the Spiti Shale Formation. It is discovered from a micaceous silty clay horizon with bivalves (Inoceramus), serpulids, gastropods, ?brachiopods, crinoid ossicles, belemnites and ammonites. The age-diagnostic ammonites, ?Streblites (Oppeliidae) and cf. Aulacosphinctes sp. (Himalayitidae), suggest a Late Tithonian (Ponti–Microcanthum zones) age for the aptychus. The para-species L. lamellosus is recorded for the first time and attributed to oppeliid, though in-situ material is required for a conclusive result. Given the latest advances in the aptychus study, the previous Indian records are re-evaluated. The results suggest that L. kachhensis (Trauth, 1938) from Kachchh Basin is a para-subspecies of L. sparsilamellosus (Gümbel, 1861) rather than a valid para-species of Lamellaptychus genus. In addition, a closer observation of placenticeratid ammonite jaws from the Cretaceous of Narmada Basin shows that bivalves were misidentified as aptychi.