TRENDS, STASIS, OR DIFFUSION: FUNCTIONAL AND PHYLOGENETIC EVOLUTION OF GRAPTOLOID THECAE

Graptoloid thecae, the tubes which housed individual graptoloid zooids, show a striking diversity of forms in the Silurian, particularly among the monograptids. The functional or ecological role of thecal morphologies has been relatively underexplored. Only two viable functional hypotheses have been suggested for differences in thecal shape: (1) feeding adaptations and (2) defense against small pelagic predators. Alternatively, thecal form may have no overall ecological role.

I tested the functional importance of various thecal forms by testing for the presence of biased directional change between ancestors and descendents. If the Silurian increase in morphological disparity of graptoloid thecae is related to a significant shift in predatory stress, there should be an observable directional bias in ancestor-descendent change for traits related to defense. I measured ten morphological traits of potential functional importance from graptoloid species throughout the mid-Ordovician and Silurian and combined this trait data with a large supertree of graptoloid species built from morphology-based cladograms and traditional classifications. The supertree was scaled to time using temporal data from a global database of graptoloid occurrences (Sadler and Cooper, unpub.). A partial analysis of late Ordovician and early Silurian graptoloid thecae shows weak evidence for the presence of directional biases in defensive traits, suggesting that the Silurian morphological diversification of thecae was not the result of a sudden increase in predation intensity.

I further assessed the evolutionary dynamic of these potential functional traits by comparing the fit of alternative models of trait evolution to the partial dataset. I tested models of constrained evolution, directional trends, early burst, and random diffusion. Ornstein-Uhlenbeck models were the best supported, based on AIC, indicating that graptoloid thecae were restricted to a static range of functional variation through the late Ordovician and early Silurian. Thus, thecal functional diversity was either limited due to intrinsic constraints on morphology or experienced consistent stabilizing selection through the Hirnantian mass extinction.