Morphological channeling by structural constraint: convergence in styles of dwarfing and gigantism in Cerion, with a description of two new fossil species and a report on the discovery of the largest Cerion

Abstract

A survey of all Cerion taxa and geographic variants reveals that a distinctive shape—a narrowly spired “smokestack” shell more than three times higher than wide—occurs only in dwarfs and giants and never in the much more common populations of normal size. The smokestack shape evolved once in giants (in the newly described species C. excelsior from Great Inagua and Mayaguana—the largest of all Cerion), but at least seven and probably eight times independently in dwarfs. A study of complex allometric patterns in Cerion's ontogeny, and of covariance sets in growth, indicates that smokestacks can easily evolve in dwarfs and giants by the common route of relative increase in whorl number during an allometric phase that adds height but no width to the shell. (Giants simply add more whorls to a shell with whorls of normal size; dwarfs grow a normal number of whorls in a shell with small whorls and reduced maximum width.) This pathway is not open to Cerion of normal size, and the restriction of smokestacks to both extremes of the size range records a channel set by Cerion's invariant allometries and the geometry of spiral growth in general, not an immediate adaptation conferring special advantages via the elevated spire itself.