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On the Origin and Evolution of True Crabs: Insights from Tropical America
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- Author / Creator
- Luque, Javier
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A full understanding of the evolution of novel forms requires inference about their origins through the study of variation in extant taxa and clues from the fossil record. However, the origins of morphological diversity in many groups are obscured by the scarcity of transitional fossils or reliable early occurrences of crown groups near the base of major branches. This is the case for true crabs, or Brachyura, a group whose evolutionary history and internal phylogenetic relationships remain unresolved. Although molecular and morphological phylogenetics bring powerful tools to the study of relatedness at the genotypic and phenotypic levels, the fossil record provides a unique glimpse into the origins of such relatedness by revealing a past morphological diversity otherwise inaccessible. Furthermore, fossils are pivotal for understanding the evolution of key traits, and provide geographic and chronologic data critical to the calibration of nodes of interest. Unfortunately, in spite of the overall good crab fossil record in Late Cretaceous and Cenozoic deposits worldwide, records of early brachyurans are still poorly known, and a strong collection bias towards modern high latitudes limits our understanding of the origins of the group’s origin and early morphological variation.
Here I examine the fossil record of true crabs from the tropical Americas based on newly discovered crustacean-rich assemblages from the Early and ‘mid’ Cretaceous and re-examination of museum specimens, with emphasis on some brachyuran higher taxa (e.g., Raninoida, Eubrachyura, and a new chimaeric lineage described herein). I test hypotheses about the relationships among the main fossil brachyuran lineages, and investigate the distribution of visual systems in ‘intermediate’ brachyuran groups. Analyses of morphological data from fossil and extant taxa support the view that podotreme brachyurans (crabs with sexual openings at the base of the legs) form a paraphyletic grade, and that some derived podotreme groups might be closer to Eubrachyura or ‘higher’ true crabs than to less inclusive podotremes. New Cretaceous fossils push the envelope for what a ‘crab’ is, and challenge conventional views of crab evolution. My findings show that the evolution of shovel-like and paddle-like pereiopods, and decarcinization, or loss of a typical ‘crab-like’ form, have occurred independently at least five times since the Early Cretaceous or before. Similarly, my findings reveal that the loss of the plesiomorphic ‘mirror’ eyes — seen among most decapod crustaceans including the earliest brachyurans — has occurred in several podotremes and in eubrachyurans since at least the Early Cretaceous. In addition, the distribution of eye types among brachyuran crabs supports a paraphyletic podotreme grade, as suggested by recent phylogenetic works. The versatility of the crab body form, and the interplay between development and ecological invasion, may have acted as drivers of morphological innovation. These findings provide novel insights about the time, place, and phylogenetic pathways of early diversification in crabs, with the aim of contributing to the vast ongoing inter–institutional effort to resolve the Decapoda Tree of Life. -
- Graduation date
- Fall 2018
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- Type of Item
- Thesis
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- Degree
- Doctor of Philosophy
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- License
- Permission is hereby granted to the University of Alberta Libraries to reproduce single copies of this thesis and to lend or sell such copies for private, scholarly or scientific research purposes only. Where the thesis is converted to, or otherwise made available in digital form, the University of Alberta will advise potential users of the thesis of these terms. The author reserves all other publication and other rights in association with the copyright in the thesis and, except as herein before provided, neither the thesis nor any substantial portion thereof may be printed or otherwise reproduced in any material form whatsoever without the author's prior written permission.