Journal of Paleontology, 91(4), 2017, p. 767–780 Copyright © 2017, The Paleontological Society 0022-3360/17/0088-0906 doi: 10.1017/jpa.2016.158


A diverse assemblage of Permian echinoids (Echinodermata, Echinoidea) and implications for character evolution in early crown group echinoids


Jeffrey R. Thompson, Elizabeth Petsios, and David J. Bottjer


Department of Earth Sciences, Zumberge Hall of Science, University of Southern California, 3651 Trousdale Parkway, Los Angeles, California 90090-0740, USA ⟨thompsjr@usc.edu⟩, ⟨petsios@usc.edu⟩, ⟨dbottjer@usc.edu⟩


Abstract.—The Permian is regarded as one of the most crucial intervals during echinoid evolution because crown group echinoids are first widely known from the Permian. New faunas provide important information regarding the diversity of echinoids during this significant interval as well as the morphological characterization of the earliest crown group and latest stem group echinoids. A new fauna from the Capitanian Lamar Member of the Bell Canyon Formation in the Guadalupe Mountains of West Texas comprises at least three new taxa, including Eotiaris guadalupensis Thompson n. sp. an indeterminate archaeocidarid, and Pronechinus? sp. All specimens represented are silicified and known from disarticulated or semiarticulated interambulacral and ambulacral plates and spines. This assemblage is one of the most diverse echinoid assemblages known from the Permian and, as such, informs the paleoecological setting in which the earliest crown group echinoids lived. This new fauna indicates that crown group echinoids occupied the same environments as stem group echinoids of the Archaeocidaridae and Proterocidaridae. Furthermore, the echinoids described herein begin to elucidate the order of character transitions that likely took place between stem group and crown group echinoids. At least one of the morphological innovations once thought to be characteristic of early crown group echinoids, crenulate tubercles, was in fact widespread in a number of stem group taxa from the Permian as well. Crenulate tubercles are reported from two taxa, and putative cidaroid style U-shaped teeth are present in the fauna. The presence of crenulate tubercles in the archaeocidarid indicates that crenulate tubercles were present in stem group echinoids, and thus the evolution of this character likely preceded the evolution of many of the synapomorphies that define the echinoid crown group.


Introduction


Echinoids are members of the phylum Echinodermata and are important and common constituents of modern ecosystems (e.g., Kier and Grant, 1965; de Beer, 1990; Gagnon and Gilkinson, 1994; Nebelsick, 1996; Linse et al., 2008). Although they encompass a wide morphological diversity in the post- Paleozoic (Hopkins and Smith, 2015), echinoids are first known from the Ordovician (Smith and Savill, 2001; Reich and Smith, 2009) and following a maximum Paleozoic generic richness in the Carboniferous (Kier, 1965; Smith, 1984) underwent a severe bottleneck at the Permo-Triassic mass extinction (Erwin, 1993, 1994; Twitchett and Oji, 2005). Apart from disarticulated spines, echinoids in Paleozoic strata are relatively rare. Prior to the Permian, most echinoids had flexible tests, with many clades displaying imbricate plating presumably lacking the stereomic interlocking present in post-Paleozoic clades (Smith, 1980a). Because of this nonrigid test plating, Paleozoic echinoids dis- articulated rapidly following their death, and thus articulated echinoid material from the Paleozoic is often limited to Lager- stätte (e.g., Schneider et al., 2005). Given their propensity to disarticulate (e.g., Thompson and Ausich, 2016; Thompson and Denayer, 2016), echinoid diversity in the Paleozoic is almost certainly underestimated, and thus all new taxa are important. Furthermore, the late Paleozoic has been demonstrated to be the


interval in which the first crown group echinoids are known from the fossil record (Smith et al., 2006; Nowak et al., 2013; Thompson et al., 2015b). As such, new faunas are important as they allow for a greater understanding of the morphological innovations present in these earliest crown group and latest stem group echinoids and the environments and communities in which the evolution of the first crown group echinoids took place. Examination of material in the collections of the National


Museum of Natural History in Washington D.C. revealed new specimens representing several taxa of at least three families, the Miocidaridae, Archaeocidaridae, and Proterocidaridae, from the Guadalupian Bell Canyon Formation of West Texas. The fauna herein described is the first well-preserved assemblage of Permian echinoids comprising taxa of multiple families and represents one of the most diverse assemblages of echinoids


known thus far from the Permian. The assemblage contains the first known occurrence of the Proterocidaridae in the Permian of North America and increases the number of stem cidaroid taxa known from the Permian to three. The discovery of this fauna also indicates that echinoids were likely more geographically widespread in the Permian than previously thought. Among these new specimens is the recently described, and herein vali- dated, Eotiaris guadalupensis Thompson n. sp. representing the earliest known cidaroid and crown group echinoid known in the


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