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

Journal of Paleontology, Nov 2004 by Bodenbender, Brian E, Hiemstra, Erik J

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 Catyocrinites 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.

INTRODUCTION

THIS STUDY reports basic patterns of skeletal crystallography in several rhombiferan, diploporan, and paracrinoid species. All are extinct echinoderms that lived during the early Paleozoic. This work is part of an ongoing exploration of the potential of skeletal crystallography both for use as a phylogenetic character in analyses of echinoderm evolutionary relationships and as a source of other paleobiological information. After documenting the skeletal crystallography of a sample of rhombiferan, diploporan, and paracrinoid species, we examine some implications of skeletal crystallography for phylogenetic and morphological interpretations of echinoderms at high taxonomic levels.

Early echinoderms exhibit a diverse array of skeletal morphologies. Among the taxa in this study, rhombiferans typically have a regular arrangement of readily homologized thecal plates, although some species have comparatively larger numbers of more variably positioned plates. In comparison to rhombiferans, diploporans generally have more skeletal plates that often vary in size, are perforated by pores, and are arranged in a less well-defined geometric pattern. A few paracrinoids, such as Platycystites Miller, 1889, resemble rhombiferans in having an identifiable arrangement of major thecal plates but with smaller, less regularly identifiable plates also included in the theca. However, most paracrinoids, including Oklahomacystis Parsley and Mintz, 1975 and Amygdalocystites Billings, 1854, resemble diploporans in having a less regular architecture featuring a variable number of thecal plates. Some paracrinoids have internally opening pores developed on inner plate surfaces, whereas others lack pores.

The morphological variation that these and other early echinoderms exhibit has led to the designation of about 20 classes for early, extinct forms whereas extant echinoderms occupy five classes. Phylogenetic relationships among the various early echinoderm groups are uncertain. The most recent general treatments in the literature are by Paul and Smith (1984) and Paul (1988), but an overall evaluation of relationships using modern phylogenetic methods has yet to be published. Given the state of our phylogenetic understanding it is not surprising that the taxonomy of early echinoderms is in disrepair, with workers relying on informal names or existing formal, nonphylogenetic groups as a means to communicate. In particular, many of the early echinoderm classes, including the Rhombifera, Diploporita, and Paracrinoidea, are informally known as cystoids [= blastozoans of Sprinkle (1973) or "cystoids sensu lato" of Paul (1988), but broader than the class Cystoidea of Kesling (1967)], a group that is thought to be monophyletic (Paul, 1988). Although some cystoid groups such as paracrinoids and blastoids are readily diagnosed and appear to be monophyletic, others such as the rhombiferans and diploporans may be polyphyletic and are in need of revision.

Paleobiological applications of echinoderm crystallographic data have been reviewed recently (Bodenbender, 1997), so here we present only highlights of previous research. Crystallographic studies of echinoids found that the orientations of c axes in skeletal elements of the test are relatively consistent within species, varying by less than 30 degrees (Raup, 1959, 1962). Crystallographic axis orientations are also generally consistent with taxonomic groupings at the family level, with some families having crystallographic axes perpendicular to plate surfaces whereas others have axes tangential to plate surfaces. In other studies, Emlet (1985, 1988) found that c axes of certain plates in the echinoid apical system yield information on larval development because they retain orientations inherited from precursor larval spicules.