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Taxonomy, Phylogeny and Paleobiogeography of the Cassiduloid Echinoids

  • Author(s): Alves Souto, Camilla
  • Advisor(s): Marshall, Charles R.
  • et al.
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Cassiduloids are rare and poorly known irregular echinoids, which include the sand dollars and heart urchins, that typically live buried in the sediment, where they feed on small organic particles. Cassiduloids evolved during the Marine Mesozoic Revolution, but despite their rich fossil record, species richness (diversity) is very low. The goal of this thesis is to improve our taxonomic knowledge of the group, propose hypotheses of relationship among its representatives and analyze their patterns of geographic distribution through time, thereby contributing to our understanding of their evolutionary history.

In the first chapter , I used synchrotron radiation-based micro-computed tomography (SRµCT) images of type specimens to describe a new Cassidulus species and a new cassiduloid genus that could not have been discovered with traditional techniques. I also designate a neotype for the type species of the genus Cassidulus, Cassidulus caribaearum, provide remarks on the taxonomic history of each taxon, a diagnostic table of all living cassidulid species, and extend the known geographic and bathymetric range of two species. Besides rendering novel morphological data, the SRµCT images provided significant insights in the evolution of bourrelets of these cassiduloid echinoids. However, determining how the bourrelets have evolved, as well as the evolution of all the cassiduloid traits, requires a phylogenetic framework of the group.

Therefore, in the second chapter, I reconstructed the phylogeny of the cassidulids using morphological characters and inferred their patterns of geographical distribution through time. Because morphological and geographic histories are erased by extinctions, unraveling phylogenetic relationships and biogeographic patterns based on only the living species can be challenging, especially for groups that have experienced extensive extinction such as the cassiduloids. Studies have shown that fossil taxa generally improve phylogenetic resolution because of their unique morphological information that have often been modified in Recent species. Thus, inclusion of fossils can be critical to addressing evolutionary questions. Surprisingly for marine invertebrates, there are relatively few studies that have included fossils in their phylogenetic and biogeographic analyses. I performed a cladistic analysis of 45 cassidulids based on 98 characters, which resulted in 24 most-parsimonious trees. The monophyly of the family Cassidulidae was not supported because the genera Eurhodia and Glossaster were placed within the family Faujasiidae. Analyses to determine the sensitivity of the resulting clades to missing data did not result in significantly different topologies or resolution of the tree, but the coding of partial uncertainties changed the relationship within some subclades (particularly within genera). The taxonomic implications of these results and the evolution of some morphological features are discussed. The evolutionary history of the cassiduloids has been dominated by high levels of homoplasy and a dearth of unique novel traits. The cassiduloids (as defined in this study) most likely originated in the Early Cretaceous (oldest records from the Aptian), and no conspicuous novelties were added during their evolution. Biogeographically, a time-stratified DEC model with range constraints indicates that the cassidulids had a south Tethyan and northwest Atlantic origin probably dating back to the Late Cretaceous (Campanian–Maastrichtian) or Paleocene. Most cassiduloids are endemic to small regions and their evolution has mostly been influenced by dispersal rather than vicariant events. Speciation occurred mainly within the northwest Atlantic during the Late Paleocene to the early Eocene. Despite their high diversity during the Paleogene, cassidulids and faujasiids have only seven extant species, and three of them are relicts of lineages that date back to the Eocene. Future studies of the biology of these poorly known species, some of which brood their young, will yield further insights into the evolutionary history of this group.

While the first two chapters focused on cassiduloids, the third chapter is a broader macroevolutionary study of the echinoids. Specifically, I performed an analysis of how their genus richness (diversity) has changed since their appearance in the fossil record and estimated the turnover rates (origination and extinction) throughout their evolutionary history. The ability to document macroevolutionary trends has been accelerated by the development of free-access online databases. However, despite their undeniable benefit to research, these databases are not free from error and their data need to be checked on a regular basis. Therefore, with the goal of analyzing the echinoid’s diversity dynamics since their appearance in the fossil record, I assess the quality of the echinoid entries in the Paleobiology Database (PBDB), correct errors and improve the dataset by including missing information. Assessments of data quality included cross-referencing classifications and checking stratigraphic ranges and synonymies against the literature. The entries in the PBDB were derived from 1,100 references and include ~9,500 occurrences representing 445 genera, about 65% of all valid echinoid genera. Fifty percent of the occurrences were from Europe and the USA, and 41% percent were from the Cretaceous. Genus classifications were mostly outdated and some species were misclassified. Diversity curves were then generated using three different methods to account for preservation and sampling biases. Overall, the echinoid data from the PDBD reflected major trends known for the evolution of the class (e.g. increased diversification during the Marine Mesozoic Revolution) and recovered extinction events that have affected all marine biota (e.g. the end-Cretaceous mass extinction). However, spatial, temporal and taxonomic biases exist, so we need to be mindful of these when analyzing the data. The fossil record of the echinoids dates back to the Middle Ordovician. Although affected by mass extinctions, the group’s diversity has been steadily increasing since the origin of the irregular echinoids in the Mesozoic. In addition to the echinoid diversity curve and turnover rates, contrasting and similar diversity trajectories for closely related major echinoid clades are also presented.

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This item is under embargo until March 29, 2023.