Ontogeny of Upper Cretaceous (Turonian-Santonian) scaphitid ammonites from the Western Interior of North America : systematics, developmental patterns, and life history. Bulletin of the AMNH ; v. 185, article 2
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Aconeceras trautscholdi ammonitellae mass occurring in the Aptian of Symbirsk, central Russia represent consecutive calcification stages of the primary organic shell wall. Already after the formation of the organic shell with proseptum, the first whorl and umbilical walls of the initial chamber were calcified, then the remaining part of the initial chamber, and finally the nacreous primary constriction was formed and the proseptum was calcified. The original mineral participating in calcification was aragonite, which formed primary prismatic layers. The ammonite embryonic shell was thus formed similarly to the archaeogastropod larval shell. This explains the microstructural distinction of the ammonitella and proseptum walls with respect to the rest of the ammonite shell.
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Previous views on the type of embryogeny in ammonites are discussed by the present writer who also points out some morphological characters of the initial chamber, along with the first whorl, which are indicative of a non-larval type of embryogeny. The swelling of shell wall on the nepionic constriction is analyzed in detail and the manner of forming this swelling explained, together with the hypothesis on an adaptative character of the swelling. The diagrams of a relative density of septa in the genus Quenstedtoceras are analyzed and used as a basis for showing certain similarities in the postembryonal development of ammonites and the development of the Recent Decapoda. The occurrence of the morphological hiatus lack of one whorl characteristic for the sexual dimorphism of type A is explained, on the basis of the Quenstedtoceras, by the difference of one whorl in the ending of the only sexual cycle in males and the first sexual cycle in females.
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An embryonic ammonoid assemblage was discovered in a carbonate concretion recovered from a dysoxic, relatively offshore marine shale of Virgilian (Upper Pennsylvanian) age in Kansas, USA. The assemblage consists primarily of two species of the Goniatitina, Aristocerassp. and Vidrioceras sp., whose initial chambers (protoconchs) differ in size and shape. Microscopic observations of serial thin sections of specimens at different growth stages reveal the sequence of embryonic shell development starting with the formation of the initial chamber and ending with the synchronous secretion of a prismatic proseptum and nacreous swelliig (primary varix) at the aperture. The mode of occurrence of the embryonic shells of the two species in the concretion suggests that these ammonoids produced numerous small offspring, a reproductive strategy similar to that in many extant coleoids.
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α08cαPんπθ8μLθγ侃Zμ8(JIMBo)and Scαρんπθ8μαημ8(YABE)occur abundantly in the Turonian of Hokkaido and south Sakhalin． The two species are nearly satisfied with an indispensable condition of the fossil population concept by reason of the characteristic in the mode of occurrence． Biometric and hydrostatic examinations on 42 samples for the two species from Naibuchi， Saku， Obira and Oyubari areas， have been made to obtain the quantitative data on their functional evolution． The analyses of individual relative growth of radius length to total rotation angle， septal and ventral wall thickness to radius length and variation of phragmocone size of adult shells indicate that the two species evolved in parallel in the direction of a buoyancy excess． Approximate calculation of the buoyancy of several we11-preserved adult shells suggests that the two species changed with time in the mode of life of adult stage from benthic to planktic． The suture complexity of the two species increases with time in inverse proportion to the growth of septal and ventral wall thickness． Their inhabitable depth range may be shallower than that of modem 2Vαμ碗玩8， because of their smaller value of the relative siphuncular strength than in Nα励批8． Judging from the coexistent and sympatric mode of occurrences， similarity of ontogeny unti180r 9πstage and parallel histo㎡cal change of certain she11 characters， it may be possible that the relationship between the two nominal species is a sexual dimorphism．
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