Spiraclavus nacoensis, a New Species of Clavate Agelacrinitid Edrioasteroid from central Arizona

Colin D. Sumrall

Abstract-- Twenty-four fragmentary specimens and numerous disarticulated plates of a new clavate agelacrinitid edrioasteroid were collected from the Middle Pennsylvanian Naco Formation in central Arizona. Spiraclavus nacoensis, n. gen. n. sp., is similar to Lepidodiscus laudoni (Bassler, 1936) and Discocystis kaskaskiensis (Hall, 1858), but differs in the direction of ambulacral curvature and in the number and arrangement of ambulacral cover plates. Numerous, well-preserved, disarticulated plates allow a full description of plate morphologies from various parts of the theca. The entire oral surface is rigidly constructed of tessellate plates. Interlocking serrate sutures between successive floor plates add strength. Plate sutures in the flexible and telescoping portions of the theca (recumbent zone and pedunculate zone, respectively) are beveled allowing considerable imbrication and potential for thecal extension. The associated fauna indicates that the unit containing the edrioasteroids was deposited in an offshore, subtidal, marine carbonate shelf with occasional clastic influxes. The specimens of Spiraclavus nacoensis were buried by clastics probably during storm events.


Thecal designs in isorophinid edrioasteroids

Colin D. Sumrall

Abstract--A study of functional morphology in the edrioasteroid suborder Isorophina reveals three major thecal designs: domal, pyrgate, and clavate. Pyrgate forms are characterized by specialized morphologic features that facilitate extension of the theca well above the substrate and contraction of the theca to a low, streamlined, domal profile. Domal forms have a low, domal thecal profile capable of little shape change. Clavate forms are characterized by modifications to the pyrgate theca to facilitate higher level suspension feeding. Pyrgate species are generalized edrioasteroids that inhabited a variety of near-shore open marine environments. Domal species are small specialized edrioasteroids that inhabited shallow-marine to intertidal environments. Clavate species inhabited offshore, open marine environments. The size reduction of the peripheral rim and the modifications to the pedunculate zone are adaptations allowing clavate edrioasteroids to compete for small attachment sites while maintaining the ability to extend the theca for higher level suspension feeding and respiration. A phylogenetic analysis of well-known isorophinid edrioasteroids to determine the relationships between the three designs indicates that domal species are a monophyletic group within the Isorophinidae, clavate species are a monophyletic group within the Agelacrinitidae, and isorophinid pyrgate species are a paraphyletic group ancestral to both domal and clavate forms. Edrioasteroidea, Echinodermata, Functional Morphology, Phylogeny


Plating and Pectinirhombs of the Ordovician Rhombiferan Plethoschisma

Colin D. Sumrall and James Sprinkle

Abstract--When first described, Plethoschisma grandis was thought to have an unusual plating arrangement compared to other pleurocystitids, although the ventral surface was largely unknown. New specimens showing more of the ventral surface indicate that Plethoschisma has very similar ventral plating to Deltacystis and that four of the original dorsal plate designations of Plethoschisma were incorrect. A new subfamily, Deltacystinae, is erected to receive the two similarly plated genera. The pectinirhombs of Plethoschisma are reinterpreted as confluent and in the B2/IL2 and IL3/L4 positions and one of the new specimens has the largest pectinirhomb and greatest number of dichopores of any known glyptocystitid. A phylogenetic analysis of the Pleurocystitidae reveals that the Deltacystinae, new subfamily, is the sister group to the Pleurocystitinae, new subfamily, and that pectinirhombs apparently evolved independently in the Pleurocystitidae and the Cheirocrinidae.


Late Paleozoic edrioasteroids (Echinodermata) from the North American Midcontinent

Colin D. Sumrall

Abstract--A new classification is herein proposed for Late Paleozoic Edrioasteroidea (Echinodermata), separating forms with the advanced clavate thecal design from those with the ancestral pyrgate thecal design, and a new Subfamily Discocystinae is erected to receive the clavate agelacrinitid edrioasteroids. Lepidodiscus Meek and Worthen, is restricted to the pyrgate type species L. squamosus (Meek and Worthen) and two unnamed species, whereas the clavate L. laudoni (Bassler), is assigned to Clavidiscus, n. gen. The clavate Discocystis priesti Strimple, and three new species, Hypsiclavus kinsleyi, n. gen., n. sp., Hypsiclavus huntsvillensis, n. gen., n. sp., and Hypsiclavus guensburgi, n. gen., n. sp. are placed in Hypsiclavus, n. gen. Bostryclavus, n. gen., is erected to receive Echinodiscus sampsoni Miller. A redescription of Discocystis kaskaskiensis (Hall) and a diagnosis of Spiraclavus Sumrall are included for completeness.


Giganticlavus, a new genus of Pennsylvanian edrioasteroids from North America

Colin D. Sumrall and Arthur L. Bowsher

Abstract--Two new species of Late Paleozoic edrioasteroids Giganticlavus bennisoni, new genus, new species, and G. gelasinus, new genus, new species, are described from the Early Pennsylvanian of Oklahoma and New Mexico. Giganticlavus differs from other clavate edrioasteroids by its large size, extreme ambulacral length and ambital overlap, extreme thickness of interambulacral and recumbent zone plates, and the tessellate nature of the recumbent zone plating. The extreme length of ambulacra and thickness of the plating are interpreted as adaptations for increased size and rigidity.


Paleontology and Taxonomy of Late Cambrian Echinoderms from the Western United States

Colin D. Sumrall, James Sprinkle, and Thomas E. Guensburg

Abstract--Although echinoderm debris is locally common, articulated specimens are rare in Late Cambrian rocks from the Great Basin and Rocky Mountains of the western United States, represented by cornute stylophorans, trachelocrinid eocrinoids, solute homoiosteleans, and rare edrioasteroids. The fauna includes primarily components from the archaic Cambrian Evolutionary Fauna which had already passed its maximum diversity for echinoderms. In addition to the low diversity, articulated specimen abundance is very low, averaging only about one-tenth that found in overlying Lower Ordovician units. The transition between the Cambrian and Paleozoic Evolutionary Faunas for echinoderms in North America apparently occurred rapidly, very close to the Cambro-Ordovician boundary because no unequivocal examples of the Paleozoic fauna (such as crinoids, glyptocystitid rhombiferans, asteroids, and echinoids) were found. New taxa include Tatonkacystis codyensis, n. gen. and sp., a well preserved trachelocrinid eocrinoid with five unbranched arms bearing numerous brachioles; an undescribed, poorly preserved, epispire-bearing eocrinoid?; several cothurnocystitid stylophorans including Acutucarpos deltas, n. gen. and sp., Acutucarpos republicia, n. gen. and sp., Archaeocothurnus goshutensis, n. sp.; Scotiaecystis sp., a poorly preserved cornute stylophoran with lamellipores and a fragmentary solute homoiostelean previously assigned to Minervaecystis; and a heavily weathered edrioasterid edrioasteroid. Nearly all holdfasts found in these Upper Cambrian units are single-piece blastozoan types, probably belonging to trachelocrinid and other eocrinoids. Disarticulated thecal plates and columnals of several additional undescribed eocrinoids were locally abundant and a possible Late Cambrian crinoid is noted based on a single occurrence of disarticulated plates.


The role of fossils in the phylogenetic reconstruction of Echinodermata

Colin D. Sumrall, Department of Geology, Tulane University, New Orleans, LA 70118

Abstract--Morphological data can be used effectively in phylogenetic analyses to determine relationships among echinoderm clades. These data in the form of characters are simply hypotheses that any observed morphological state among taxa results from a single character state transformation and is therefore, homologous. All such character states must be scored as potentially homologous unless the hypothesis of homology can be rejected by the tests of similarity, conjunction (a priori), or character congruence (a posteriori). Fossils are not always more incomplete than extant forms and incompleteness originates from non-preservation and long phylogenetic branches. The greatest strength of fossil data lies in its ability to effectively shorten long phylogenetic branches by occurring on the tree nearer to the nodes than extant terminal taxa and thus circumventing positively misleading results encountered in parsimony analysis under long branch conditions.


Phylogenetic analysis of living Echinodermata based on primitive fossil taxa

Colin D. Sumrall and James Sprinkle

Abstract--We performed a phylogenetic analysis of fossil taxa to revise the systematics of the Phylum Echinodermata, including the five living classes. Shared descent forms the basis for redefining larger taxonomic groupings in this new classification, rather than the traditional method of defining groups on the presence of key morphological features. Forty-eight genera and species were selected for the analysis with at least one representative from each major echinoden-n group. We chose preservationally complete fossil taxa to minimize missing data, and early members of fossil groups to preserve primitive states and to avoid long branch-length problems that confound parsimony analysis. A set of 146 morphological characters was scored for the 48 taxa, and the resulting matrix was analyzed with PAUP 3.Os using multiple heuristic searches to detemiine relationships. We divided the data set into a core analysis, including the 37 best preserved and most completely understood taxa, plus 1 1 incomplete taxa that were fragmentary and/or poorly understood. We chose three Early Cambrian taxa as outgroups based on their early appearance and on their retention of character sets that represent presumed primitive states. Multiple runs using different outgroup combinations yielded the same results. We performed experiments on the core analysis by individually adding the incomplete taxa, by deleting identified clades, and by deleting sets of characters. Three slightly different hypotheses of branch order were nearly always recovered in the set of most parsimonious trees. We prefer one hypothesis over the other two because its placement of labile taxa was not based on ecologically correlated sets of characters. Two of the hypotheses support the same arrangement for living echinoderms. The preferred hypothesis has the following arrangement: (Crinoidea (Echinoidea, Holothuroidea), (Asteroidea, Ophiuroidea)). However, the alternate hypothesis: (Crinoidea (Holothuroidea (Echinoidea (Asteroidea, Ophiuroidea)))) is only one step longer, not a significant difference.