COMPLEXITY IN INTERPRETING BODY SIZE EVOLUTION IN LONG-LOOP BRACHIOPODS (TEREBRATELLIDINA)

The distribution of body size within a group provides insights into numerous dimensions of evolution, such as the evolutionary control of development, and the functional implications of size. Nearly 50% of named extant genera of Terebratellidina (Brachiopoda) are observed to be less than 1 cm in valve length (presumably their adult size), a higher proportion of small taxa than for all Phanerozoic brachiopods.

Why might this be? For many years, hypotheses of heterochrony have been proposed to explain the evolution of terebratulide brachiopods: small size in extant taxa indicates paedomorphosis from larger ancestors. But, does the abundance of small terebratellidines really provide evidence of repeated paedomorphic size reduction, driven by some common selective impetus? Hypotheses of heterochrony require data on size and shape, ontogenetic age, and phylogenetic relationships. A comprehensive phylogenetic analysis has been lacking, and has hindered our ability to fully understand the complexity of body size evolution in the most diverse group of extant brachiopods.

Using a matrix of 84 characters of shell morphology for 25 of the 55 extant long-loop terebratellidine genera, we inferred phylogenies under both maximum parsimony and Bayesian approaches, with several extant short-loop terebratulidines as our outgroups. Our analyses suggest that small body size is the ancestral condition and not secondarily derived numerous times, as predicted from hypotheses of convergent paedomorphosis.

However, the closely related short-loop terebratulidines exhibit morphological characteristics as adults, irrespective of size, that are typical for juvenile and some small adult long-loop taxa. This may bias our results against detecting paedomorphosis. Testing these results with genomic data, as well as adding extinct terebratulide morphology and stratigraphic information to these extant-only analyses will provide important genetic and deep-time comparisons. Different methods for determining character polarity – outgroup comparison, relative stratigraphic position, ontogenetic transformations – can yield different phylogenetic patterns. Our results indicate the importance of using all available and relevant data and methods in the testing of evolutionary hypotheses about body size.