Pathologic Gigantism in Middle Carboniferous Cephalopods, Southern Midcontinent, United States

Abstract

Giant genera and species occur in virtually every phylum of the Kingdom Animalia. For the most part, their existence has been viewed as an illustration of Cope’s Rule: the tendency toward phylogenetic size increase among groups of organisms. Giant Mesozoic ammonites are frequently cited examples of this trend, and the giants are typically treated as discrete taxonomic entities. In contrast, pathologic gigantism is a rare condition that results in abnormal growth of an individual beyond the normal size limits of its species, and does not reflect Cope’s Rule. That condition is little known among invertebrates, although pathologic gigantism has been reported in prosobranch and pulmonate gastropods, where it is associated with infestation by larval trematodes that caused parasitic castration.

In Middle Carboniferous strata of the southern midcontinent, United States, cephalopod occurrences are dominated by ammonoids in unusual abundance. These assemblages represent single horizons and localities where most individuals of a species are of strikingly similar size, apparently mature, and seem to reflect mass mortality, possibly related to reproduction (semelparity). Associated with these assemblages are rare, conspecific individuals thought to be pathologic giants that became abnormally large because they failed to achieve sexual maturity that would have caused their growth to cease.