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Journal of Paleontology 91(4):767–780


and Hollingworth, 1990) and the Zechstein Reefs of Thuringia, Germany (Reich, 2007 and references therein) are both known to contain the stem group cidaroid Eotiaris keyserlingi. It is thus possible that reefal environments in the late Paleozoic may have supported diverse echinoid communities and that the assemblage described here is so diverse simply because of the reefal nature of the sediments in which it was preserved. That two species of Eotiaris, E. guadalupensis and E. keyserlingi, are both reported exclusively from reefal environments may also indicate that early crown group echinoids evolved, or at least thrived, in these environments. These Permian reefal environ- ments are also not the oldest such Paleozoic reefal settings to yield echinoid faunas. Although their framework differs from reefs of the Permian, Mississippian mud mounds from the Fort Payne Formation of Kentucky (Thompson and Ausich, 2016) and the Waulsortian mud mounds of Clitheroe, Lancashire (Hawkins, 1935; Donovan et al., 2003), andWaulsort, Belgium (Jackson, 1929), have also yielded diverse and abundant echinoid faunas and were likely favorable habitats for echinoids. It has also been recently proposed that Triassic echinoids may have had an affinity for reefal environments (Zonneveld et al., 2015) as much of the known Triassic echinoid fossil record is from reefal settings (e.g., Kier, 1977, 1984; Stanley, 1979, 1989; Smith, 1994; Zonneveld, 2001; Zonneveld et al., 2007). Many of these Triassic taxa are stem group cidaroids belonging to the Miocidaridae (e.g., Zonneveld et al., 2007) and the families Triadocidaridae and Paurocidaridae (e.g., Smith, 1994), which are likely to be descendants of miocidarids such as Eotiaris guadalupensis (Smith, 2007). Given the abundance of these stem cidaroids in reefal environments in the Permian and Triassic, it is possible that the early diversification of stem group cidaroids may have taken place in reefal environments; how- ever, more data will be necessary to further test this hypothesis.


Co-occurrence of stem group and crown group echinoids.— From a paleoecological standpoint, this fauna is important because it demonstrates a depositional environment in which archaeocidarids, proterocidarids, and miocidarids coexisted. Permian miocidarids have until now only been reported from assemblages where they are the only echinoid constituent (Kier, 1965; Reich, 2007) and that miocidarids are herein found from the same environments as archaeocidarids indicates that the most crownward stem group echinoids, the archaeocidarids, and the earliest crown group echinoids, the miocidarids, were occupying the same environments at the same time. This is particularly interesting given the survival of the miocidarids through the Permian-Triassic mass extinction (Smith and Hollingworth, 1990; Erwin, 1993, 1994), which appears to have been responsible for the extinction of the archaeocidarids. Miocidarids outside of localities in west Texas are only known from the two reefal localities described in the preceding. Archaeocidarids, however, were much more abundant and apparently more geographically widespread as they have been described from test or interambulacral plate material from localities in Texas (Kier, 1958b; Schneider, 2010; this paper), Australia (Etheridge, 1892; Webster and Jell, 1992), Kansas (Boos, 1929; Matthieu, 1949), Oklahoma (Boos, 1929), Pakistan (Waagen, 1885), Tunisia (Matthieu, 1949), Timor (Wanner, 1941), Argentina (Hlebzevitsch and Cortiñas, 2009),


Hungary (Mihály, 1980), and Bosnia (Kittl, 1904). This abundance of archaeocidarids, relative to miocidarids, makes their demise at the Permian-Triassic boundary interval even more interesting, and currently there exists no good mechanism to explain the differential survival of the miocidarids and archaeocidarids. Furthermore, there is no good understanding of the temporal distribution of Permian archaeocidarid abundance or diversity at the stage level or lower, which will be necessary to understand the dynamics of stem group echinoid richness and abundance leading up to the Permian-Triassic boundary. For example, the end-Guadelupian extinction event (Stanley and Yang, 1994), which was responsible for major extinctions in some clades (e.g., fusulinids; Stanley and Yang, 1994; Groves and Wang, 2013) and only slightly elevated extinction rates in others (Payne and Clapham, 2012; Clapham, 2015), may have played a role in extinction of the archaeocidarids; however, whether this is the case remains to be seen.


The acquisition of characters leading to crown group echinoids.—New fossil discoveries are key for establishing the sequence of character evolution associated with the transition from stem group to crown group taxa. As specimens with new morphologies are discovered, a clearer picture of the order of character changes leading from the stem group to the crown group becomes available, and the true synapomorphies defining the crown group become apparent (Donoghue, 2005). Basal crown group echinoids have previously been united by a num- ber of synapomorphies, which distinguish them from members of the echinoid stem group. Among the most conspicuous of these synapomorphies are the reduction in coronal plating to two columns of interambulacral plates and two columns of ambulacral plates and the evolution of the perignathic girdle (for a complete list of crown group echinoid synapomorphies, see Kroh and Smith, 2010). In addition, although not demon- strably present in the most basal euechinoids (for which there is little fossil evidence), the earliest, and most basal, cidaroid taxa also display crenulate tubercles and a rigid interambulacral area at and below the ambitus (Kier, 1965; Smith and Hollingworth, 1990; Thompson et al., 2015b). Before this study, neither a perignathic girdle, an interambulacral area composed of two columns of plates, and crenulate tubercles, were known to be present in the most derived stem group echinoids, which belonged to the genus Archaeocidaris. It is very unlikely, however, that the acquisition of these three characters took place all at once as evolutionary transitions marked by large numbers of character acquisitions are often incremental (e.g., Donoghue and Purnell, 2005; Mackovicky and Zanno, 2011). Although it is well known that the origination of crown group echinoids took place in the late Paleozoic (Smith and Hollingworth, 1990; Smith et al., 2006; Nowak et al., 2013; Thompson et al., 2015b), the order of character state transitions associated with, and leading up to, the origination of the crown group is not well understood. The new specimens of Archaeocidaridae indet. from west


Texas appear to shed light on the order of some of the character changes associated with the morphological transition from stem group to crown group echinoids (Fig. 4). The interambulacral plates of this indeterminate archaeocidarid are composed of pentagonal and hexagonal forms (Fig. 2.4–2.9). On one interior


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