Hidden Patterns of Ray-Finned Fish Evolution: Bridging Paleontology and Neuroanatomy

Abstract

The fossil record of ray-finned fishes dates back to more than 400 million years and the clade is remarkably diverse in extant settings. However, there is still little understanding of the early evolution of the clade. The apparent lack of morphological disparity in Paleozoic forms has been challenging for a precise placement of early fossils within the ray-finned fish tree, both with respect to each other and to living groups. In this dissertation I expand on the knowledge of the evolution of ray-finned fishes based on poorly understood aspects of their morphological diversity, phylogeny and geographic distribution. Focus is given to describing important aspects of endocranial and soft tissue anatomy that can directly link fossil and living taxa in an ecomorphological and phylogenetic framework. I first describe important new Paleozoic ray-finned fish taxa from poorly sampled high-latitude localities in the southern hemisphere in chapters 2 and 3. In these chapters I present in detail the anatomy of new taxa and discuss the implications of the observed morphologies for our understanding of early ray-finned fish evolution. Additionally, I discuss the importance of poorly explored axes of morphological diversity of stem ray-finned fishes, with emphasis on endocranial anatomy of Permo-Carboniferous taxa. By joining description of taxa from poorly known high-latitude localities with the description and comparison of hidden aspects of endocranial anatomy across early ray finned fishes, chapters 2 and 3 provide a starting point for future biogeographic and phylogenetic work on Paleozoic actinopteygians. In Chapters 4 and 5 I provide a detailed account of soft tissue preservation in Paleozoic ray-finned fish fossils, with special emphasis on aspects of their neuroanatomy. This represents the first description of three-dimensional soft-tissue fossilization in Paleozoic ray-finned fishes and the oldest evidence of morphological disparity in vertebrate fossil brains. The results of these two chapters summarize important morphological variation in fossil brains and other associated soft tissues, providing a tentative reconstruction of the early evolution of the ray-finned fish brain. In Chapter 6 I expand on the knowledge of morphological diversity of living ray-finned fish brains and endocasts through the application of diffusible iodine contrast enhanced micro-tomography (dice-CT) of more than 70 living ray-finned fish taxa. Through a new landmark scheme, volumetric comparisons and general description of morphological variation in living ray-finned fish brains I expand on the knowledge of ecomorphological diversity of the ray-finned fish brain, as well as providing a new morphological map for living ray-finned fish endocasts which can be directly compared to extinct counterparts. With the results presented in this dissertation I expect to provide a new framework for future research on fossil and living ray-finned fishes where the description of endocranial and soft tissue anatomy of both fossil and living taxa will play a major role in drawing more robust assessments on the phylogenetic relationship of early ray-finned fishes as well as more accurate reconstructions of neuroanatomical evolution through time and space.