Embryonic shell structure of E arly– M iddle J urassic belemnites, and its significance for belemnite expansion and diversification in the Jurassic

Early J urassic belemnites are of particular interest to the study of the evolution of skeletal morphology in L ower C arboniferous to the uppermost C retaceous belemnoids, because they signal the beginning of a global J urassic– C retaceous expansion and diversification of belemnitids. We investigated potentially relevant, to this evolutionary pattern, shell features of S inemurian– B ajocian N annobelus , P arapassaloteuthis , H olcobelus and P achybelemnopsis from the P aris B asin. Our analysis of morphological, ultrastructural and chemical traits of the earliest ontogenetic stages of the shell suggests that modified embryonic shell structure of E arly– M iddle J urassic belemnites was a factor in their expansion and colonization of the pelagic zone and resulted in remarkable diversification of belemnites. Innovative traits of the embryonic shell of S inemurian– B ajocian belemnites include: (1) an inorganic–organic primordial rostrum encapsulating the protoconch and the phragmocone, its non‐biomineralized component, possibly chitin, is herein detected for the first time; (2) an organic rich closing membrane which was under formation. It was yet perforated and possessed a foramen; and (3) an organic rich pro‐ostracum earlier documented in an embryonic shell of P liensbachian Passaloteuthis . The inorganic–organic primordial rostrum tightly coating the protoconch and phragmocone supposedly enhanced protection, without increase in shell weight, of the E arly J urassic belemnites against explosion in deep‐water environment. This may have increased the depth and temperature ranges of hatching eggs, accelerated the adaptation of hatchlings to a nektonic mode of life and promoted increasing diversity of belemnoids. This study supports the hypothesis that belemnite hatchlings were ‘a miniature of the adults’.