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The embryonic conch structure as a supposed imperative factor on the hatchling dispersal and geographical expansion of belemnites: an example of Callovian (Middle Jurassic) pachybelemnopseins from Aragón (NE Spain)

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Scanning electron microscopy, energy dispersive spectrometry and taphonomic analysis are applied for studying of about 250 shells of the pachybelemnopsein belemnites Hibolithes and Pachybelemnopsis. They are abundantly exposed on solid carbonate bedding surfaces of the middle-late Callovian deposits in the vicinity of the village Ricla, Iberian Range, Aragón, NE Spain. The evidences of their autochthonous burial are as follows: (i) high occurrence of very small, small, and medium-size specimens; the maximum diameter of the rostrum in each category is 2-3 mm, 3-6 mm, 6-10 mm, respectively, whereas large specimens are rare; (ii) small and medium-size shells are dispersed or grouped in small accumulations comprising 3-10 specimens lacking a common orientation or size selection; large shells are disperse; (iii) the apical part of the phragmocones commonly retains fragile embryonic and early post-hatching parts; (iv) the mechanical abrasion is minor; (v) a large number of shells are "hollow belemnites" lacking the diagenetic filling of the protoconch and the apical chambers of the phragmocones. The above set of characters illuminates a high mortality of juvenile and immature pachybelemnopsein belemnites as well as their fast in situ burial. This suggests that the adults did not live constantly in a shallow-water environment. The studied belemnites support a viewpoint on the adaptation of the embryonic conch of the belemnites for a nekto-pelagic lifestyle of the hatchlings. This may provide their effective dispersal and growing expansion from the late Early Jurassic onwards.
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Keywords: BELEMNITES; CALLOVIAN; EMBRYONIC SHELL STRUCTURE; HABITAT; LIFESTYLE OF HATCHLINGS; MIDDLE JURASSIC; NE SPAIN

Document Type: Research Article

Publication date: March 1, 2017

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