Function and shape in late Paleozoic (mid-Carboniferous) ammonoids

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Abstract
Analysis of an exhaustive data base of Namurian ammonoid shell characters indicates that the morphology of the Goniatitida can be explained in terms of functional constraints, resulting particularly from hydrostatic and hydrodynamic properties. Modes of life ranging from benthic to pelagic are inferred on this basis for various goniatitid morphotypes; all morphologic features and facies associations are normally compatible with these inferences. Neutral buoyancy is shown to have been likely for all goniatitids. By contrast, the prolecanitids (Order Agoniatitida) exhibit a number of hydrostatic and morphologic anomalies; these anomalies are not explicable using the same principles and remain problematic. This is noteworthy, in that prolecanitids survived the Permian/Triassic extinctions and gave rise to the diverse ceratitic radiation in the Triassic.The applicability of these results to ammonoids outside the Namurian is assessed, and, in particular, morphologic parallels with Mesozoic ammonites are discussed.