HOW DOES MORPHOLOGICAL VARIABILITY IN FOSSIL AND RECENT TEREBRATULIDE BRACHIOPOD SPECIES COMPARE?

When evaluating taxonomic diversity through time, patterns of morphological variability in named Recent and fossil species are assumed to be comparable. However, this assumption is rarely tested, and some paleontologists consider fossil brachiopod species to be "over-split," encompassing less variability than Recent brachiopod species. If true, this could lead to a consistent overestimate of true brachiopod species-level diversity over time. We test this assumption by comparing morphological variation in fossil and Recent species of terebratulide brachiopods, a clade whose modern and fossil species are externally similar.

Shell length, width, and height were measured in 773 individuals from 17 fossil and five Recent terebratulide species. To account for size differences, measurements were log transformed and the geometric mean, a proxy for size, was regressed from the data. Principal component analysis was performed on the size-regressed residuals for each species; morphological variability was estimated as the sum of the range of PC1 and PC2 scores for each species. Variability estimates for the fossil species were bootstrapped 10,000 times to derive a frequency distribution of morphological variability in fossil terebratulide species. Variability estimates of five Recent terebratulide species were compared to this distribution. Shell variability in four out of the five modern terebratulide species was statistically significantly less than the amount of variability in the fossil species. When combined, the five Recent terebratulide species were significantly less variable than the distribution of fossil terebratulide species (unequal variance t-test).

Significant differences in variability between fossil and Recent species may exist. Using modern terebratulide species to calibrate variation, fossil terebratulide species may be under-split. Geometric morphometrics will allow additional investigation into the morphological "equivalence" of fossil and Recent species.