A NEW SPECIES OF ALCA (AVES, ALCIDAE): AND A NOVEL APPROACH FOR ASSESSING DIVERSITY AMONG TAXA KNOWN FROM ISOLATED SPECIMENS

Alcidae is a clade of pelagic wing-propelled diving birds with a fossil record including auks, auklets, puffins, guillemots, murres and murrelets. Knowledge of extinct alcid diversity has potential to clarify our presently poor understanding of alcid paleobiogeography and may help answer questions such as whether alcids originated in the Pacific or Atlantic, and whether radiations and extinctions of alcids were driven by paleoclimatic events such as changes in oceanic circulation patterns. Correlations between alcid evolutionary history and paleoclimatic events also holds potential to inform our interpretations of present-day shifts in avian population ranges due to the current global warming trend.

Despite the many thousands of Pliocene fossils referred to the Atlantic alcid taxon Alca (auks), species diversity among this clade has remained unresolved due to the paucity of associated specimens, and the fragmentary preservation of the vast majority of specimens (~97%). In addition to the identification of diagnostic morphological features, measurements of multiple variables from >3000 Alca fossils were categorized by hierarchical cluster analysis, and resulted in the recognition of five "species clusters". Statistical support for these clusters was assessed by means of discriminant function analysis. The reliability of this method was tested using the exact same measurements taken from 13 extant species of alcids, and was found to be extremely robust with regard to the accurate recovery of distinct species groups. Clustered groups of fossils were then re-evaluated to identify diagnostic morphological features for use in phylogenetic analysis.

The holotype specimens of all currently recognized Alca species were recovered in separate, statistically supported clusters. Both morphometric and morphological assessment of all material referred to extinct Atlantic alcids support Alca grandis, Alca stewarti, Alca ausonia, and Alca torda as valid Pliocene species. Identification of a fifth species of Alca increases our knowledge of Pliocene alcid diversity, and demonstrates the potential of this method for assessing species diversity in other taxa known from fragmentary and isolated remains.