Middle Carboniferous cephalopod assemblages across northern Arkansas are dominated
  by a small number of ammonoid species that occur in unusual abundances.  Individuals of
  a particular species exhibit strikingly similar size, yet both small and large species are rep-
  resented.  Assemblages occur in either high energy deposits of conglomeratic sandstone or
  oolite, or low energy concretionary, dark shales, but in neither case do they seem to have
  undergone post-mortem sorting.  Ammonitellas and protoconchs are notably absent in the
  matrix preserving the ammonoids in both settings.  These assemblages may represent sexu-
  ally mature adults that experienced mass mortality following reproduction (semelparity) in
  a manner similar to that found in some modern squids.