Embryonic shell structure of Early–Middle Jurassic belemnites, and its significance for belemnite expansion and diversification in the Jurassic

Abstract

N. 2014: Embryonic shell structure of Early-Middle Jurassic belemnites, and its significance for belemnite expansion and diversification in the Jurassic. Lethaia, Vol. 47, 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 SinemurianBajocian 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'. □ Belemnites, embryonic shell, expansion, inorganic-organic shell matter, Jurassic.

Ontogenetic trajectories of septal spacing in Early Jurassic belemnites from Germany and France, and their palaeobiological implications

“…Measured protoconch sizes (two measurements for a single specimen: PL1 and PL2, approximately L2 and L3 in Doguzhaeva et al . ) are shown in Figure . The protoconch sizes (PL1) of Passaloteuthis laevigata are 0.287–0.392 and 0.291–0.493 mm from Buttenheim and Lixhausen, respectively.…”

“…; Ward and Bandel ; Doguzhaeva et al . , , ; Košťák & Wiese ; Larson ; Doguzhaeva ; Fuchs ). Based on these detailed observations, the embryonic shells of belemnites are thought to have a protoconch with no or a few chambers (Hanai ; Bandel et al .…”

“…Protoconch sizes were measured on the polished sections, using two methods: (1) as a distance between the top of the protoconch and the conjunction between the protoconch and phragmocone (PL1; approximately equal to L2 in Doguzhaeva et al . ); and (2) as a distance between the top of the protoconch and a point of a maximum curvature of concave aperture of the protoconch (PL2; approximately equal to L3 in Doguzhaeva et al . ).…”

“…); and (2) as a distance between the top of the protoconch and a point of a maximum curvature of concave aperture of the protoconch (PL2; approximately equal to L3 in Doguzhaeva et al . ). The shell microstructure of the protoconch was also examined using an SEM (JEOL, JSM‐6510LA) to see if there is a pattern that corresponds to a certain change of septal spacing.…”

Diversity and morphological evolution of Jurassic belemnites from South Germany

“…Nevertheless, some eurytopic taxa likely migrated over several hundreds of kilometers and different depths during their lifetime (Christensen, 1997;Zakharov et al, 2006;Alsen and Mutterlose, 2009;Sørensen et al, 2015). According to Doguzhaeva et al (2013), their colonization of deeper depths in the water column could rest on modifications of embryonic shell structures, preventing hatchings from implosion in deeper waters. By this extraordinary profusion in diverse ecological niches at global scale,…”