Ecomorphological change in lobe-finned fishes (Sarcopterygii): disparity and rates

Abstract

Patterns of vertebrate cranial/post-cranial trait diversification are not well-understood.

Two ecological hypotheses, the ‘head-first’ model and the ‘stages’ model, predict the ordered

decoupling of cranial/post-cranial ecomorphological diversification. Rate (tempo) analyses and

macroevolutionary model-fitting (mode) analyses are often used to test whether cranial traits or

post-cranial traits diversify earlier in vertebrates. Here we reconstructed the tempo and mode of

cranial/post-cranial trait evolution in lobe-finned fishes (Sarcopterygii), a group containing

coelacanths, lungfishes, and tetrapodomorph fishes, to test for differences in the timing of

vertebrate cranial/post-cranial trait diversification. We collected full-skeletal geometric

morphometric coordinate and PC-reduced data for 57 species of aquatic and semi-aquatic lobe- fins since the Paleozoic. We performed rate, raw disparity through time, phylogenetic signal and

model-fitting (multivariate BM, EB and OU) analyses using both coordinate data and PC- reduced data but did not model-fit the coordinate data due to computational constraints. The BM

(best fit model) rate parameter is highest for post-cranial traits but cranial trait disparity is

highest throughout the majority of lobe-fin evolution, supporting the ‘head-first’ model, with

peaks in the Middle to Upper Devonian and Jurassic. We also find that PCA reduction increases

trait variance, creating bias in favor of early post-cranial trait diversification. We conclude that

cranial ‘head-first’ ecomorphological diversification drove the early Paleozoic radiation of

aquatic and semi-aquatic lobe-fins, but more research may be needed to understand conflicting

patterns in the Mesozoic when taxonomic diversity was substantially lower among “living

fossils”.

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Ecomorphological change in lobe-finned fishes (Sarcopterygii): disparity and rates

By: Bryan H. Juarez1

and Lauren C. Sallan2

1Department of Ecology and Evolutionary Biology, University of Michigan, Ann Arbor;

bryanhjuarez@gmail.com

Earth and Environmental Science & Evolution Cluster, University of Pennsylvania;

lsallan@sas.upenn.edu

Keywords: Lobe fin, geometric morphometrics, PCA, Brownian Motion, Early Burst