Assessing competition during the transition from jawless to jawed vertebrates with morphometrics, ecomorphology, and comparative biomechanics

Scott, Bradley Raymond

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Description

Title

Assessing competition during the transition from jawless to jawed vertebrates with morphometrics, ecomorphology, and comparative biomechanics

Author(s)

Scott, Bradley Raymond

Issue Date

2023-04-25

Director of Research (if dissertation) or Advisor (if thesis)

Anderson, Philip S. L.

Doctoral Committee Chair(s)

Anderson, Philip S. L.

Committee Member(s)

• Fuller, Becky C.
• Cheng, Chi-Hing C.
• Wilson, Mark V. H.

Department of Study

Evolution Ecology Behavior

Discipline

Biology

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• agnathan
• gnathostome
• comparative biomechanics
• ecomorphology
• paleontology
• early vertebrate evolution

Abstract

The transition from jawless to jawed vertebrates (agnathan/gnathostome transition) is one of the most significant events in early vertebrate evolution and an excellent example of a faunal shift associated with an evolutionary novelty (jaws) but not a mass extinction. During the Silurian and Devonian periods (444–359 million years ago), jawless vertebrates (agnathans) gradually reduced in diversity and abundance while jawed vertebrates (gnathostomes) increased in diversity and abundance. Competition has been the most commonly proposed cause of the agnathan/gnathostome transition. According to the competition hypothesis, agnathans were competitively excluded by gnathostomes, but so far, competition and competitive exclusion have not been supported or refuted in extinct early vertebrates. Direct behaviors cannot be observed in fossil taxa, so proxies of competition must be used. Competition is stronger between taxa that are morphologically similar. To test whether competition from gnathostomes drove nearly all agnathans to extinction, ecological similarity was approximated from similarity in body forms of early vertebrates. Then both ecological and functional analyses of the similarities and differences in body form among agnathan and gnathostome groups were used to support or refute competition among early vertebrates. Morphological similarity was assessed using Principal Coordinate Analysis (PCOa) of morphometric measurements of 120 early vertebrate taxa, representing nearly all major early vertebrate groups (Chapter 2). Similarity of habitat use, an important ecological parameter for competition in modern fishes, was estimated for early agnathans and gnathostomes using PCOa of only habitat-relevant characters (Chapter 4). Morphometric analysis of extant shark species (Selachiformes) was used to determine which characters are most relevant to habitat use (Chapter 3). Sharks share key features of buoyancy and fin control with early vertebrate taxa, making them a suitable modern analogue. Linear discriminant analysis and ANOVAs showed that the position, size, and shape of the fins of sharks are most important for differentiating habitat use, especially when compared to the shape of the head, body, or tail. Overall morphological comparisons of agnathans and gnathostomes are supported by habitat-specific analysis: fork-tailed thelodonts (agnathans) are different from all other early vertebrate groups, resembling modern laterally compressed reef fishes, while jawless osteostracans and heterostracans resemble the jawed placoderms (Gnathostomata, specifically antiarchs and phyllolepids, respectively) and non-fork-tailed thelodonts have a generalist morphology similar to that of many gnathostomes. For morphological comparisons to have ecological impact, there must be functional similarities. The final study compared function of body forms of early vertebrate groups by simulating hydrodynamic performance using computational fluid dynamics (Chapter 5). Competition between heterostracans and phyllolepids and between osteostracans and antiarchs was not supported based on differences in lift-to-drag ratios and benthic performance. Overall, competition between agnathans and gnathostomes is not supported, and it is considered to be very unlikely that gnathostomes outcompeted agnathans. Other ecological changes (predation, environmental change) are potential alternative causes for the agnathan/gnathostome transition and warrant further investigation.

Graduation Semester

2023-05

Type of Resource

Thesis

Handle URL

https://hdl.handle.net/2142/120555

Copyright and License Information

Copyright 2023 Bradley Scott. Chapter 2 Published in Paleobiology DOI: https://doi.org/10.1017/pab.2022.32. with the following authorship: Bradley Scott and Philip S. L. Anderson. Copyright © The Author(s), 2022.

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