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
cornpatible 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 parallele with Mesozoic ammonites are discussed.