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Allometric growth between different parts of the shell often hampers the identification of mollusk shells, particularly in such cases where preadult shell growth varies strongly. Especially in gastropods, the terminal aperture is often less variable and yields morphological information essential for species determination (e.g. Vermeij 1993; Urdy et al. 2010a, b). In fossil mollusk shells, the adult aperture (peristome)is often missing, partially due to an early death, and partially due to destructive processes, which occurred post mortem (taphonomy). Therefore, the entire shell ontogeny is known only from a small fraction of all ammonoid taxa (e.g., Landman et al. 2012). Nevertheless, knowledge of the adult shell of ammonoids is very important since it can yield morphological information essential for systematics and for the reconstruction of various aspects of their paleobiology.

Cloudina, an important Ediacaran index fossil, is considered as one of the earliest biomineralizing organisms. Its biological affinities have not been fully resolved and phylogenetic links with both annelids and cnidarians have traditionally been suggested. Differences in tube morphology, ultrastructure and biomineralization suggest that Cloudina is not closely related to any recent skeletal annelid (e.g., serpulids, sabellids and cirratulids) and their skeletons are not homologous. The way of asexual reproduction in Cloudina resembles more that of cnidarians. The presence of a closed tube origin (base) in Cloudina is also compatible with the hypothesis of an animal of cnidarian grade.Key Words: Cloudina; Annelida; Serpulidae; Cnidaria; Ediacaran; biomineralization. Résumé : Incohérences dans les affinités supposées de l'organisme primitif biominéraliséCloudina (Ediacaran) avec les annélides : preuves relevant de la structure et du développe-ment ontogénétique.-Cloudina, un important fossile édiacarien, est considéré comme l'un des premiers organismes biomineralisés. Ses affinités biologiques n'ont pas encore été entièrement élucidées : des relations phylogénétiques avec les annélides et les cnidaires sont habituellement envisagées. Des différences dans la morphologie, l'ultrastructure et la biominéralisation du tube suggèrent que Cloudina ne puisse être étroitement apparentée à aucun squelette d'annélides actuelles, qu'il s'agisse de serpulidés, de sabellidés ou de cirratulidés ; leurs squelettes ne sont pas homologues. Le mode de reproduction asexuée de Cloudina ressemble davantage à celui des cnidaires. De plus, la configuration fermée de la partie initiale (proximale) du tube chez Cloudina est compatible avec l'hypothèse d'un animal de la catégorie des cnidaires.

Bored and encrusted carbonate concretions, termed hiatus concretions, coming from the Middle Jurassic (Upper Bajocian and Bathonian) siliciclastics of the Polish Jura, south-central Poland, have been subjected to detailed paleoecological investigation for the first time. The concretions possess variable morphology and bear distinct traces of bioerosion and encrustation as a result of exhumation on the sea floor during intervals of low sedimentation and/or erosion. The borings are dominated by Gastrochaenolites and Entobia. Epilithozoans, represented by at least 26 taxa, are dominated by sabellid/serpulid worm tubes and bryozoans, while sponges and corals are minor. No relationship between the concretion size and the number of encrusters has been found, suggesting that concretion size was not the primary factor controlling diversity. Stable isotope analyses and the presence of crustacean scratch marks and Rhizocorallium traces on many of the hiatus concretions indicate that they formed just below the sediment-water interface, within the sulfate reduction zone. Moreover, crustacean activities may have been a prelude to their origin, as shapes of many concretions closely resemble thalassinoidean burrow systems. It is also possible that crustacean activity around the concretions promoted their exhumation by loosening the surrounding soft sediment. The presence of borings and encrusters on different concretion surfaces, as well as truncated borings and a number of abraded epilithozoans, indicate that after the concretions were exhumed they were repeatedly overturned and moved on the sea floor, probably due to episodic storm-related bottom currents in shallow subtidal environment.