Join us for Rafael Rivero’s upcoming dissertation defense!

Title: From Living Fossils to Adaptive Radiations: Examining Phenotypic Evolution in Lobe-Finned Fishes

Abstract:
Vertebrates have experienced diverse eras pivotal in shaping their evolutionary trajectory, with some groups surviving while myriad go extinct. However, the patterns of morphological evolution that underlie their history in deep time remain not well understood. In this dissertation, I use phenotypic data from lobe-finned fishes, a group of mostly extinct and structurally intermediate bony fishes that hold a significant phylogenetic position at the base of the vertebrate tree of life, to analyze the tempo of their early success and subsequent decline. First, I use two independent types of trait data to assess if three fish lineages identified as iconic “living fossils” share common patterns in rates of change through their evolutionary history as implied by their title. I find that there are marked differences in patterns recovered among and within groups and traits, implying unique evolutionary trajectories across time and urging the reassessment of the use of qualitative descriptors without forming the proper quantitative foundation. Then, I describe a ‘phaneropleurid’ lungfish from late Devonian fluvial deposits in the Canadian Arctic. The articulated fossil represents a new species that possesses skeletal adaptations along its dorsal fin and skull that intimate its transitional nature. I show how this structurally intermediate lungfish can offer insights into the conditions punctuating the transition to a more modern, post-Devonian anatomy. Finally, I shift focus to sarcopterygian macroevolutionary patterns across the Devonian, using jaws as a robust, ecologically informative model system to interrogate whether this early diversification bears the hallmarks of a putative adaptive radiation. I detect the hypothesized patterns of high initial evolutionary rates followed by deceleration, with clades within the group showing more diffusive patterns and more extensive or restricted exploration of shape space early in their evolutionary history. This dissertation offers new perspectives on hundreds of millions of years of evolution in sarcopterygian and “living fossil” lineages, and further underscores how morphological data can provide insights into the intricacies of macroevolutionary patterns during periods of rapid phenotypic change.