Abstract

Considerable refinement of the surficial geology and biostratigraphy Bermuda has resulted in the proper ordering of the phylogenetic sequence of Poecilozonites, and thus offers an opportunity to examine evolutionary pathways within this taxon. Paedomorphism, the retention of juvenile ontogenetic characteristics into adulthood, is a recurrent morphological manifestation in fossil land snail shells of the subgenus P. (Poecilozonites) on this isolated oceanic island. The paleontology of this endemic taxon has been examined over the past century and was a key example of “punctuated equilibria” (PE) in the late 1960s and early 1970s. In a previous study, we documented the biostratigraphy and geochronology from the known fossil record of P. (Poecilozonites) representing at least the past 500 kyr. Here we focus specifically on paedomorphic forms that appeared in shells at the onset of the last interglaciation, marine isotopes stages (MIS) 6/5e and again at the beginning of the Holocene (MIS 2/1). There are many possible mechanisms to explain the occurrence of paedomorphism including PE, but of importance to this discussion is that neither the size nor the fossil record of Poecilozonites show three independent lineages as proposed by Gould (1969). Gould's several named paedomorphic forms supposedly branching from P. bermudensis over the past 300 kyr, occur only during the last interglacial (sensu lato) MIS 5, and the Holocene (MIS 1). Both punctuation and stasis characterize the morphological changes of this taxon over the past 140 ka, but these changes are reversible, and no speciation is evident.

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