Thompson et al.—Permian echinoids
those described as Population A in Hoare and Sturgeon (1976). They attribute these lanterns to either Archaeocidaris or Polytaxicidaris. The foramenmagnumslopes at a gentler angle in the lanterns attributed to Archaeocidaris? jacksoni Spreng and Howe, 1963 and Archaeocidaris? remotus Spreng and Howe, 1963 than in the pyramids herein described. In addition, the indeterminate lanterns appear to have less-sloping foramen magna then those of the lanterns described by Kier (1958b) from the Wordian of west Texas. Proterocidarid lanterns are also not well enough known to confidently assign the herein described taxon to Pronechinus? sp. The lantern characteristics of particular clades of echinoids in the Paleozoic are not well known, as pointed out by Spreng and Howe (1963). This is in part because lanterns are often assigned to genera and species without the presence of any articulated test material and are often given the names of taxa that are associated with disarticulated plates and spines of preexisting genera. This results in inconsistency regarding the association of lanterns with particular genera and species, and thus the lantern characteristics attributable to higher- level taxa are not well known. Because of this, the characteristics of the lanterns described herein preclude assignment to a particular taxonomic group, and thus they are left in open nomenclature. The teeth associated with these lanterns are of interest
primarily because they do not have a serrated point and thus are not like the teeth typically associated with archaeocidarids and proterocidarids. Archaeocidaris, Polytaxicidaris, and the proterocidarids Proterocidaris de Koninck, 1882 and Fournierechinus Jackson, 1929 display serrated teeth con- structed of multiple rows of columns of primary and secondary plates. These are termed ‘compound lamellar teeth’ (Reich and Smith, 2009). That the teeth present herein do not display the serrated tip characteristic of compound lamellar teeth suggests that they are not compound lamellar in origin. The nature of echinoid tooth microstructure is best understood through scanning electron microscopy analyses; however, due to the silicified nature of the material described herein, we did not attempt to examine the microstructure of the teeth. That they do not appear to be compound lamellar, however, leaves a few hypotheses regarding the nature of the teeth. They could be from a taxon not present in the described fauna; however, this seems unlikely given the nature of the material and its preparation, which exposed the entire silicified fauna of the limestone blocks. Furthermore, these pyramids could belong to Archaeocidaridae indet. or Pronechinus? sp. The nonserrate morphology may indicate either a primitive (e.g., simple lamellar) or derived (e.g., cidaroid-type U-shaped) tooth. U-shaped teeth are known from the Permian of the Ford Formation associated with the cidaroid Eotiaris keyserlingi and had clearly evolved by the Permian (Smith and Hollingworth, 1990). The teeth present in this west Texas fauna do have the U-shaped cross section characteristic of cidaroid teeth; however, without microstructural analysis, their affinities remain unclear.
Discussion
The assemblage herein described is particularly diverse by Permian standards. That this assemblage is so diverse relative to other Permian echinoid occurrences is likely due to the interplay
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of a number of competing factors, including worker bias, paleoenvironmental setting, and taphonomy. This is the first study to methodologically identify dis-
articulated echinoid ossicles to the taxonomic rank of family in the Permian, though the approach has been used with much success in older strata (e.g., Kutscher and Reich, 2004; Reich and Smith, 2009; Thompson and Denayer, 2016). It has been demonstrated that the examination and identification of dis- articulated echinoid ossicles increases the number of taxa known from a given formation (e.g., Gordon and Donovan, 1992; Donovan, 2001; Kroh, 2007; Thompson and Denayer, 2016). In addition, it is expected that when disarticulated echi- noid ossicles are used to evaluate diversity estimates, recorded species richness will be higher than when solely articulated specimens are used (Nebelsick, 1996). Therefore, it is possible that the diversity of the assemblage herein observed is higher than that of other localities in the Permian due to the failure of previous studies to account for disarticulated ossicles. The silicified nature of fossils from the Permian of the
Guadalupe Mountains also undoubtedly plays a role in the higher reported diversity. Because the examined specimens were silicified and later dissolved out of limestone blocks, a greater number of specimens were available for study. Silicified faunas also often yield higher diversity aragonitic mollusk faunas than those that are not silicified (Cherns and Wright, 2000, 2009; Wright et al., 2003); however, it is unknown whether the same would be true of the high-magnesium calcite echinoderm faunas. Regardless, if the specimens reported herein were not silicified, and therefore could not have been easily dissolved from their interring matrix, then only specimens on bedding planes would have been visible. To our knowledge, no other silicified Permian echinoid faunas from outside of west Texas have been acquired through dissolution of CaCO3 matrix. This technique has, however, yielded exceptional preservation and increased estimates of diversity and abundance in Pennsylvanian (Kier, 1965) and Triassic (Stanley, 1989; Smith, 1994 described in Stanley, 1994) faunas. It is probable that if other silicified Permian faunas are prepared through matrix dissolution techniques, they will yield comparably diverse faunas. It is also possible that the assemblage herein described has
a relatively high diversity because of the depositional environ- ment from which it was collected. This assemblage preserves a transported reefal community (Babcock, 1977). Reefal com- munities have been shown to contain diverse regular echinoid assemblages in the Red Sea, and the sediments associated with these reefal environments also preserve disarticulated compo- nents of diverse regular echinoid assemblages (Nebelsick, 1996). A transported reefal community may also have preserved a mixed assemblage representing the wide range of microhabitats that echinoids inhabit on a reef. Although reefal environments are unlikely to preserve a full suite of articulated echinoid communities due to their high-energy setting (Nebelsick, 1996), as mentioned in the preceding, when dis- articulated material is accounted for, a more diverse assemblage is likely to be preserved. Also of importance is the fact that this is not the first Permian reef to yield fossil echinoids. The Wuchiapingian Zechstein reefs of the Ford Formation from Northern England (Hollingworth and Pettigrew, 1988; Smith
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