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

Abstract

Vertebrates have experienced diverse eras that shaped their evolutionary trajectory, from the development of jaws to the transition from water to land. However, the patterns of morphological evolution in deep time that underlie macroevolutionary changes such as these remain not well understood. In this dissertation, I investigate the early success and subsequent decline of lobe-finned fishes, a now mostly extinct assemblage of bony fishes at the base of the vertebrate tree of life, by considering vignettes at varying time scales to assess the tempo of their evolutionary history. First, I use two independent types of trait data to assess if, across a span of hundreds of millions of years of evolution, three fish lineages identified as iconic “living fossils” share common patterns as implied by this title. I find that there are marked differences in patterns recovered among and within groups and traits, implying a variety of unique evolutionary trajectories at the largest timescales and urging the reassessment of the use of qualitative descriptors like “living fossils” without forming the proper quantitative foundation. Then, I adjust to a smaller scale by describing a ‘phaneropleurid’ lungfish from Late Devonian fluvial deposits in the Canadian Arctic. The articulated fossil represents a new species that possesses characters that intimate a transitional nature, such as skeletal adaptations for buccal pumping as well as the elongation of the base of the second dorsal fin. I show how this structurally intermediate lungfish can elucidate the conditions punctuating the lungfish transition from Devonian, generalized lobe-finned anatomy, to a more modern, post-Devonian one. Finally, I step back to examine the Devonian as a period of changing macroevolutionary patterns in lobe-finned fishes using their jaws as a robust, ecologically informative system to interrogate whether this early diversification bears the hallmarks of an adaptive radiation. I detect patterns fitting the hypothesized expectations of high initial evolutionary rates followed by a 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 paleontological data can provide insights into the intricacies of macroevolutionary patterns at varying scales, including during periods of accelerating or decelerating phenotypic change.