Abstract
Based on material from the uppermost Tithonian La Caja Formation at Puerto Piñones, northeastern Mexico, the complete ontogenetic development (protoconch to adult) of the ammonite Salinites grossicostatum is outlined by a detailed morphometrical shell analysis. The embryonic stage, consisting of a small ellipsoid protoconch and ammonitella, ends at about 0.6 mm. Four major morphological changes are differentiated throughout ontogeny based on internal features such as reduced septal spacing and siphuncle position. Sexual dimorphism is reflected by shell size, siphuncular diameter, differences in the morphology of the apophysis, and by two distinct general trends in septal spacing. In addition, macroconchs are characterized by septal crowding at different stages, followed by the return to normal septum distances. Our analysis indicates a change in the mode of life after the neanic stage. A change in habitat preference is inferred for adult individuals. While microconchs persisted at Puerto Piñones, large mature macroconchs temporarily migrated to other areas, possibly for egg deposition. Salinites grossicostatum is endemic to the ancient Gulf of Mexico and is there restricted to outer continental shelf environments.
Highlights
Ammonite morphology and internal structures yield important information on both the systematic position of species as well as their paleobiology, but only a small number of taxa has been analyzed in detail by using a combination of external and internal sets of data (e.g. [1])
A small number (8%) of S. grossicostatum macroconchs with sizes of >53 mm diameter returned to the mating area at Puerto Piñones, possibly for a second mating event
Based on our analysis of septal spacing we propose that seasonal environmental changes are evidenced in Salinites grossicostatum
Summary
Ammonite morphology and internal structures yield important information on both the systematic position of species as well as their paleobiology (e.g. dimorphism, habitat preferences, migration during sexual maturity, responses to seasonality), but only a small number of taxa has been analyzed in detail by using a combination of external and internal sets of data (e.g. [1]). Ammonite morphology and internal structures yield important information on both the systematic position of species as well as their paleobiology (e.g. dimorphism, habitat preferences, migration during sexual maturity, responses to seasonality), but only a small number of taxa has been analyzed in detail by using a combination of external and internal sets of data An analysis of septal spacing in combination with shell diameter and ornamentation data revealed a gradual decrease in septum distances in adults reaching maturity [2]), while septal crowding was regarded as rare in premature individuals [2]. In other taxa septal crowding was attributed to adverse ecological circumstances, or injuries [2,3,4,5,6].
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