Early Jurassic belemnites are of particular interest to the study of the evolution of skeletal

morphology in Lower Carboniferous to the uppermost Cretaceous belemnoids,

because they signal the beginning of a global Jurassic

–Cretaceous expansion and diversification

of belemnitids. We investigated potentially relevant, to this evolutionary pattern,

shell features of Sinemurian

–Bajocian Nannobelus, Parapassaloteuthis, Holcobelus

and

Pachybelemnopsis from the Paris Basin. Our analysis of morphological, ultrastructural

and chemical traits of the earliest ontogenetic stages of the shell suggests that

modified embryonic shell structure of Early

–Middle Jurassic 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 Sinemurian

–

Bajocian 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

Pliensbachian

Passaloteuthis. The inorganic–organic primordial rostrum tightly coating

the protoconch and phragmocone supposedly enhanced protection, without

increase in shell weight, of the Early Jurassic belemnites against explosion in deepwater

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’.