Global events such as mass extinctions have lasting impacts on biodiversity: Actinopterygians (ray-finned fishes) are currently the most abundant and diverse group of living vertebrates; however, this situation only arose after previously dominant groups suffered major losses coincident with the Paleozoic Hangenberg mass extinction event. This project will examine whether: (1) the Hangenberg extinction event, associated with climatic changes, permanently restructured global vertebrate biodiversity; (2) actinopterygians underwent a large-scale radiation after an extinction bottleneck; (3) actinopterygian diversification occurred in response to vacated niches in post-extinction ecosystems, leading to the widespread evolution of novel functional morphologies. This project will assemble comprehensive vertebrate occurrence databases, reconstruct new phylogenies, and analyze the occupation/reoccupation of vertebrate morphospace.

Investigation of actinopterygian evolution is vital for interpreting current vertebrate biodiversity because these fishes are a major component of modern ecosystems. Phylogenetic, morphological, and diversity data will be placed in open repositories, providing starting points for further studies of vertebrate macroevolution. The results will be valuable for designing science curricula and other accessible programs because of paleontology's broad appeal - instilling in the general public an appreciation for broader concepts including evolution, biodiversity loss and turnover, and large-scale global change.

Project Report

Ray-finned fishes (Actinopterygii) make up more than half of all living vertebrates, represented by around 32,000 species of all forms and functions. While living actinopterygians are used as model organisms and study groups in multiple fields of biology, their early evolutionary history and the mechanisms underlying their modern dominance are not well characterized. Investigation of the context and characteristics of the initial, Devonian-Mississippian (420-323 Ma) diversification of ray-finned fishes is therefore essential for understanding the evolutionary and ecological underpinnings of the modern vertebrate fauna. We combined approaches from multiple fields, including quantitative paleobiology, phylogenetic comparative methods and community ecology with detailed re-examination of important early taxa in order to study patterns of divergence, disparity and diversity in early actinopterygians. We compiled of the first complete database for jawed vertebrate species from the later Devonian (391-359 Ma) and Mississippian (359-323 Ma) in order to investigate the patterns of global change and their relationship with two major mass extinction intervals. Analyses of this database revealed that vertebrates suffered a global mass extinction the end-Devonian Hangenberg event (359 Ma), equivalent in the magnitude to the end-Permian and end-Cretaceous extinctions. This devastated a previously stable fauna of armored placoderms and lobe-finned fishes (Sarcopterygii) that dominated the previous "Age of Fishes." Surviving groups were also impacted, reduced to just a handful of lineages, and ecosystems collapsed. "Romer’s Gap," a well-known interval from which few tetrapod fossils are known, was found to coincide with a global lull in vertebrate diversity following the mass extinction. The length of this recovery period, marked by homogenous ecosystems, lasted some 10-20 million years, in line with estimates for other, better-studied mass extinction events. In the aftermath of the Hangenberg event, actinopterygians, chondrichthyans (sharks and allies) and tetrapods, all rare in the Devonian, underwent pronounced diversification, giving rise to major extinct and extant groups. These three divisions of vertebrate life dominated all subsequent ecosystems from the Mississippian to now. Thus, modern vertebrate biodiversity was founded in the aftermath of the Hangenberg extinction. The patterns of vertebrate diversification during the post-Hangenberg recovery were revealed through comparison with the records for other animals and examination of morphological and ecological traits. While several vertebrate lineages survived the extinction, nearly half of these were "dead clades walking" which persisting without proliferating for millions of years, a class equivalent to modern "living fossils". The rise of new predators among successful but previously rare vertebrate groups changed post-extinction ecosystems, impacting the diversity of connected groups in their food webs. Comparison of the records for shell-crushing vertebrates and armored crinoid echinoderms (sea lilies), the latter unaffected by the Hangenberg extinction, showed that mass extinction can have on-going ecological impacts. Some crinoids thrived after their predators were largely eliminated at the end-Devonian, but went into decline when new predators with new kinds of teeth appeared among surviving fishes, such as actinopterygians and chondrichthyans. Thus, a large-scale predator-prey (Lotka-Volterra) cycle occurred on a geological timescale as a result of selective extinction, making crinoids lagging victims of the Hangenberg event millions of years after the fact. This illustrates how mass extinctions can have long-term, cascading and often unpredictable consequences beyond the initial losses. Contrary to expectations, post-Hangenberg radiations of modern forms, such as actinopterygians, largely failed to replace lost diversity or recover previous ecosystems. Instead, these radiations produced a wealth of novel body morphologies (e.g., angelfish and eels) and ecological strategies (e.g., crushing predators). This occurred as a two-stage "head first" process. First, new feeding niches were invaded, linked to significant changes in the jaws and other aspects of the skull. Later, body shapes diversified dramatically, associated with changes in habitat and swimming mode. This is was contrary to previous models of adaptive radation, which favored either a single early burst of diversification, ongoing change, or an early stage involving body form. All this change among actinopterygians was in response to new ecological opportunities afforded in the aftermath of the Hangenberg extinction. Many of these new actinopterygian groups dispersed worldwide, comprising major components of vertebrate ecosystems for at least 100 million years. The Hangenberg event therefore set the stage for the diversification ofactinopterygians: all Mississippian and modern aquatic faunas are dominated by ray-finned fishes. While it was once thought that divergent body plans arose only rarely among non-teleosts, our re-investigation of relevant species showed that such morphologies and traits appeared in multiple lineages. The functional, ecological and morphological diversity of early ray-finned fishes might approximate the same in Mesozoic and later vertebrate biotas dominated by teleosts. Thus, the pre-requisites of modern actinopterygian dominance were achieved in the aftermath of the end-Devonian mass extinction, a major contingent event in subsequent vertebrate evolution.

Agency
National Science Foundation (NSF)
Institute
Division of Environmental Biology (DEB)
Type
Standard Grant (Standard)
Application #
1011002
Program Officer
Maureen Kearney
Project Start
Project End
Budget Start
2010-06-01
Budget End
2012-05-31
Support Year
Fiscal Year
2010
Total Cost
$14,979
Indirect Cost
Name
University of Chicago
Department
Type
DUNS #
City
Chicago
State
IL
Country
United States
Zip Code
60637