Brian E. Bodenbender

STRATOCLADISTIC ANALYSIS OF BLASTOID PHYLOGENY

Abstract Stratocladistics combines morphological and stratigraphic data in a parsimony-based analysis of evolutionary relationships. We use stratocladistics here to provide an overview of the phylogeny of the extinct echinoderm class Blastoidea. Both cladistic and stratocladistic methods evaluate alternative phylogenies by comparing the number of ad hoc hypotheses needed to reconcile each alternative to observed data. Minimization of ad hoc hypotheses selects the phylogeny best supported by data and enables phylogenetic analyses to incorporate data from different sources. Cladistics treats ad hoc hypotheses of homoplasy, whereas stratocladistics additionally considers ad hoc hypotheses of differential preservation probability of lineages in the stratigraphic record. The blastoid phylogeny derived using stratocladistics is more resolved than hypotheses selected by cladistics. Although the morphological characters are relatively homoplasious, in this instance the stratigraphic ordering of fossils provides both structure and altered polarity for the stratocladistic hypothesis. The stratocladistic phylogeny supports previous paleontological conclusions of convergence among blastoid lineages and facilitates evaluation of specific hypotheses of character transformation that are integral to recent systematic revisions. Additionally, consideration of temporal data makes some hypotheses of ancestor-descendant relationships more parsimonious than hypotheses of derivation from a common ancestor. The ability to recognize sequential members within single lineages allows more accurate estimation of faunal diversities and more specific reconstruction of evolutionary histories. Chief among possible confounding factors in stratocladistics are instances where preservation potential shows significant geographic variation, although problems of preservation are more tractable than the difficulties homoplasy presents for cladistic analysis.

BLASTOID STRATOCLADISTICS—REPLY TO SUMRALL AND BROCHU

We applaud Sumrall and Brochus (2003) reanalysis of our data matrix and their success in finding trees shorter than those we reported. We also agree with some points in their comment, such as the importance of distinguishing between general properties of a method and operational attributes of any given application of the method. However, a number of points in their comment are misdirected. We shall try to clarify the most important areas of disagreement, keeping track of those that deal with stratocladistics in general, as opposed to our blastoid example. The distinction between precision and accuracy is not unique to phylogenetics; it is a concept native to all measurement and estimation. Yes, accuracy is hard to assess in phylogenetic studies of fossil organisms, and “we cannot know if a reduced number of optimal trees includes the true phylogeny” (Sumrall and Brochu, 2003, p. 189), but this sentence continues, “contained in a larger set.” Which set? Certainly not the cladistic solution-set, any more than that of stratocladistics. In contrast, Fox et al. (1999) compared cladistic and stratocladistic results to “true” (simulated) histories, and stratocladistics out-performed cladistics under a wide (and realistic) range of evolutionary models (Fisher et al., 2002). Given the importance of accuracy, we have always treated precision as secondary, and our discussion of the resolution of cladistic and stratocladistic results included explicit recognition of this (Bodenbender and Fisher, 2001, p. 363). Sumrall and Brochu state, “Stratocladistics is said to recover fewer optimal trees, and to have better-resolved consensus trees, than standard parsimony analysis … ” (2003, p. 189; to simplify communication, we here mostly follow their usage of cladogram = tree, although this tends to conflate cladogram-trees with “evolutionary trees”). The first claim is accurate, but the second (attributed by them to Fox et al., 1999) has never been …

SKELETAL CRYSTALLOGRAPHY AND CRINOID CALYX ARCHITECTURE

Abstract In the first broad survey of skeletal crystallography in fossil crinoids, we examine 10 Ordovician species representing five orders and apply crystallographic data to questions of crinoid phylogeny, homology, and development. Orientations of c crystallographic axes in the large calcite crystals that form the skeletal plates of the crinoid calyx vary systematically according to the position of each plate on the calyx. Plates lower on the calyx have axes more inclined toward the stem attachment than are axes from plates higher on the calyx. Although most specimens display this general pattern, exact orientations vary widely between species with no discernible relationship to phylogeny. Furthermore, the topological pattern of variation does not correlate with the order of addition of plates to the calyx during growth. Lack of a phylogenetic signal among diverse crinoids early in the clade’s history implies that crystallographic data will be of limited use to high-level phylogenetic studies within crinoids. Neither does skeletal crystallography strongly favor any of several competing interpretations of homologies among major crinoid calyx plates. Crystallographic data are informative, however, for some minor skeletal plates. Brachial plates have c axes that roughly parallel the surface of the plate, whereas interbrachial plates have perpendicular c axes, suggesting that distinct generative processes produce these plates. Anal plates have orientations similar to interbrachials, suggesting similar developmental mechanisms. Although c axes have regular orientations relative to plate morphology within a specimen, a axes show extensive intraspecimen variability with respect to plate morphology.

A RECONNAISSANCE OF SKELETAL CRYSTALLOGRAPHY IN RHOMBIFERANS, DIPLOPORANS, AND PARACRINOIDS

Abstract We characterized the skeletal crystallography of representatives of nine rhombiferan, three diploporan, and three paracrinoid species. Crystallographic data from these groups are similar to data from previous studies of crinoids, echinoids, and blastoids in that 1) orientations of c axes are consistent within species and within higher taxonomic groups; 2) c axes typically are oriented subparallel to the medial plane of their respective plates; and 3) the inclination of axes within the medial plane varies between taxa. Rhombiferan c axes are oriented normal to plate surfaces whereas diploporan c axes are tangential to plate surfaces. Paracrinoids more closely resemble diploporans in having irregular patterns of thecal plating but their c axes are approximately perpendicular to plate surfaces as in rhombiferans. In contrast to c axes, a axes in all specimens show little regularity and cannot be distinguished from random orientations. The rhombiferan Caryocrinites ornatus displays minor differences in the inclinations of c axes depending on the location of skeletal elements on the theca. Plates at the base of the theca have slightly aborally inclined axes, whereas distal plates have axes inclined slightly adorally. This pattern matches orientations in some early crinoids, suggesting similarities between rhombiferans and crinoids in development or skeletal construction. Skeletal crystallography in various echinoderms can be compared in light of hypotheses of homology proposed in the Extraxial-Axial Theory (EAT). Skeletal elements homologized under the EAT do not correspond to any particular crystallographic axis orientation, suggesting that the homologies proposed in the EAT encompass significant underlying skeletal variation.

The Origin of Dark Sand in Eolian Deposits along the Southeastern Shore of Lake Michigan

Dune deposits on Lake Michigan’s southeastern shore contain pin stripe laminations: thin (<3 mm) laterally continuous (10 m) layers of dark sand, with vertical separations of 0.5–100 cm between laminations. On modern dune surfaces concentrations of dark sand are observed in ripple troughs and form continuous sheets that pass under ripple crests. We explore the source of the dark grains and the processes that can concentrate them in layers, using direct field observations, textural analysis, and point counts of sand grain minerals using the energy-dispersive x-ray analysis function on an electron microprobe. Large and medium size fractions of dune sand are dominated by quartz while smaller size fractions contain high proportions of dark heavy minerals (Fe-Ti oxides, garnets, and Fe-Mg silicates). Pin stripe laminations and dark surface patches are enriched in this fine-grained component, suggesting that sorting by grain size is important in their origin. The fine-grained sand fractions of glacial tills have low concentrations of dark heavy minerals. Thus, the high proportions of heavy dark minerals in finer-grained fractions of dune sands developed during postglacial transport. The association of fine dark sands with ripples suggests that they form as translatent stratification when smaller grains preferentially collect in troughs during ripple migration. Dark laminations in dunes seldom display characteristic features of grainflow (upward coarsening in grain size, troughlike cross section), indicating that this mechanism does not account for the majority of pin stripe laminations. Grainfall has been observed creating a patch of dark sand and may account for the formation of some pin stripe laminations. Pin stripe laminations indicate the orientation of past dune surfaces and can help reconstruct past dune geometries and migration histories.