Semester

Summer

Date of Graduation

2022

Document Type

Thesis

Degree Type

MS

College

Eberly College of Arts and Sciences

Department

Geology and Geography

Committee Chair

James Lamsdell

Committee Co-Chair

Kathleen Benison

Committee Member

Kathleen Benison

Committee Member

Sarah Sheffield

Abstract

Horseshoe crabs, a group of aquatic chelicerate arthropods of the class Xiphosura, are strongly linked with the concept of “living fossils” – a term colloquially used to refer to clades that display a consistently low rate of morphological evolution through time. The concept of living fossils has been hotly debated, as it is considered to simplify or obscure millennia of evolutionary change. Recent methodological and computational advances in the paleobiological sciences have allowed for the investigation of these claims. Xiphosura are a model taxon for this type of investigative study because they exhibit a complex evolutionary history, despite their reputation as “living fossils”. Xiphosura first appear in the fossil record during the Ordovician and are represented today by four extant species. During these 480 million years of evolution, Xiphosura have displayed a remarkably stable body plan, most notably in Limulidae, the sole surviving clade from the Mesozoic onwards; however, there are also aberrant forms associated with the exploration of nonmarine niches. The presence of these forms in the fossil record indicates a more complex evolutionary history than is usually attributed to Xiphosura, including events of both rapid evolutionary change and stasis. A comprehensive study of evolutionary rate in horseshoe crabs through their fossil record has not yet been conducted. Thus, their status as a clade with consistently low rate of evolution has not been evaluated. An investigation of evolutionary rates in Xiphosura using discrete character data reveals heterogeneous rates of evolution; specifically, high rates of evolution cluster within Belinurina and Austrolimulidae – two clades that display heterochronic morphological trends and affinities for non-marine environments. Additionally, these high evolutionary rates occur after the transition to nonmarine habitats, indicating a driving environmental pull behind shifts to heterochronic morphologies. Conversely, the clades that show no concerted heterochronic trends – Limulidae and Paleolimulidae – produced varied high and low rates, with a tendency towards lower rates of discrete character evolution. Xiphosura also show variable rates of evolution through time, with higher rates corresponding to mass extinction events and radiations. Average evolutionary rate in Xiphosura has also decreased through time, though there is still variability present. Overall, horseshoe crabs show variable and dynamic evolutionary patterns through time, therefore indicating that they are not “living fossils.”

Included in

Paleontology Commons

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