OPPORTUNISTIC ECOMORPHS OR ICARUS POPULATIONS? INTERPRETING ASSEMBLAGES OF DIMINUTIVE BRACHIOPODS ASSOCIATED WITH A DEVONIAN TURNOVER EVENT

Marine benthos in the northern Appalachian Basin of central New York undergo regional extinction and turnover (60-80% of endemic fauna) during the Late Givetian. Rapid turnover (10-100 kyr) is associated with transgression and expansion of dysoxic facies. The brachiopod Allanella tullius, present in nearshore facies prior to the Hamilton-Tully turnover, does not go extinct but instead undergoes a facies shift to finer-grained dysoxic shales. Associated with this facies shift is a reduction in the size of individual Allanella specimens. Specimens preserved in the dark gray to black dysoxic shales of the Moscow Fm. are generally smaller than their siltstone counterparts, with shell length ranging from 1.8 mm to 15.7 mm and a mean shell length of 4.2 mm. In siltstones, shell length ranges from 1.9 mm to 17.5 mm with a mean specimen length of 7.9 mm. Further, measurements of specimen maximum size in siltstone assemblages are underestimated, as large specimens are typically fragmentary.

Diminutive assemblages of Allanella have long been recognized and explanations for their formation have included paedomorphosis and juvenile mortality, among others. Determining the cause of these assemblages has been stymied by preservational differences between facies, and the inability of linear measurements to morphologically differentiate specimens from different populations and at different ontogenetic stages. Simple linear measurements of shell width to length fail to distinguish any allometric trends among specimens within nearshore facies that would be usable in interpreting the diminutive fauna. What previously has been interpreted as an example of ecophenotypic variation (i.e., adaptation to dysoxic soft substrates), may be indistinguishable morphologically from an “Icarus population”: an assemblage of juveniles killed while dispersing into stressful fringe environments. Eigenshape analysis is now being employed to look for morphological variation missed by traditional linear morphometric techniques.