Palaeobiological and morphological aspects of Jurassic Onychites (cephalopod hooks) and new records from the New Zealand Jurassic

Abstract

Study of Jurassic belemnite specimens with well-preserved soft parts, including reasonably intact arm crowns, has indicated that the belemnite animal probably had 10 arms. Each arm was covered with double rows of small hooks (0.3–5.0 mm in length), grouped as micro-Onychites. Large hooks (5.0–40.0 mm in length), grouped as mega-Onychites, are also present and were probably positioned either within the arm crown or on the tips of two modified arms (hectocotyli). In modern large squids the large hooks are developed as a function of maturity and their presence only in males suggests that their principal role is in reproduction rather than predation. It is likely that the mega-Onychites fulfilled a similar role in belemnites. The mega-Onychites O. macnaughti Marwick (Kimmeridgian–Tithonian of New Zealand) has some characters in common with O. contractus Quenstedt (Pliensbachian, Swäbian Alb, southern Germany). The micro-Onychites Falcuncus tainui n. sp., also from the Kimmeridgian–Tithonian of New Zealand, is comparable to taxa described from the Jurassic of Poland and northern Italy.