Beyond preservation: morphological analysis of Lower Jurassic serpenticone ammonoids

The Lower Jurassic family Echioceratidae comprises ammonoids crucial for upper Sinemurian biostratigraphy and is a good model for studying the serpenticone morphology present in a wide variety of groups. Their morphological characterization, however, is often hampered by taphonomic alteration and the use of parameters that are difficult to apply across varied fossil preservation in different facies/lithologies. This study proposes a morphometric approach using more readily available conch measurements, as robust proxies for Raup’s classical parameters. Based on a large dataset from various basins and collections, we show that taphonomic alteration does not significantly alter conch shape in these evolute forms, allowing the comparison of variably preserved material. The echioceratid morphospace is conservative, yet a strong allometric signal is identified: larger specimens within Paltechioceras and Echioceras are consistently more evolute than smaller ones, considered juveniles. Furthermore, we document significant covariation between ornamentation and shape: stronger ribs are associated with more depressed whorls and rursiradiate trajectories, whereas rib strength seems independent of the degree of involution. These results demonstrate that size, insofar as it is linked to ontogeny, may be the primary driver of conch shape variation within genera, while ribbing patterns, likely under stronger phylogenetic control, follow predictable covariation rules, refining morphological criteria for systematic and palaeobiological studies in ammonoids.