RECOGNITION OF THE ASTEROID (ECHINODERMATA) CROWN GROUP: IMPLICATIONS OF THE VENTRAL SKELETON

Journal of Paleontology, Mar 2004 by Blake, Daniel B, Hotchkiss, Frederick H C

ABSTRACT-All known post-Paleozoic asteroids belong to the crown group, and no crown-group asteroid is known from lhe Paleo/.oic. A scanty fossil record provides limited data on morphology of both Paleozoic stem-group sister lineages and on the Triassic crown group diversification. Timing of events is weakly constrained. Interpretations based on this meager record are tentative. Within limitations of the record, recent work suggests that skeletal arrangement of the ventral surface offers apomorphies of crowngroup diversification. Enlarged disk size is common in the crown group. Large disks are constructed in part by addition of many ventral and ventral-lateral so-called "actinal" ossicles. Actinals in the crown group are comparatively uniform in size, shape, and arrangement within each species. Actinal alignment is of one of two patterns, parallel to adambulacrals or parallel to marginals. Actinals in the crown group are tentatively considered to be an apomorphy of the crown group, although the incomplete fossil record leads to uncertainty.

Enlarged disks are found in some Paleozoic (stem-group) asteroids. Axillary ossicles, marginal series extending onto ventral interbrachia, and enlarged disk adambulacrals are modes of disk size increase known only from Paleozoic asteroids. Actinal ossicles are found in a few stem-group species but arrangements are unlike those of the crown group.

Certain Carboniferous and Permian genera share aspects of ambulacral column construction with the crown group, but lack actinal apomorphies. Actinal arrangement is available for two of the three known Triassic genera. In certain ways, patterns are intermediate between stem-group and jurassic and younger crown-group asteroids.

INTRODUCTION

THE PHYLOGENY and taxonomy of the Asteroidea (Echinodermata) led Gregory (1899, p. 341) to begin his paper with a note of frustration: "The classification of the Palaeozoic starfishes has long been in chaos." Since 1889, systematists have become more aware of differences between stem-group (currently known only from Paleozoic rocks) and crown-group (currently known only from post-Paleozoic rocks) asteroids, but Gregory might find little else has changed since he wrote his classification.

Difficulty in understanding asteroid history results in large part from a poor fossil record. Specimens are rare at all but a very few horizons. Asteroids are epifaunal, with a skeleton of comparatively small, unfused ossicles imbedded in a surface dermal layer. With death, unless quickly buried, the body rapidly disassociates and the ossicles are scattered and lost. If buried still more or less complete, the body generally collapses and relationships among ossicles can be hidden by the juxtaposition of ossicles. Alternatively, abutted ossicular faces tend to recrystallize and fuse, destroying detail.

Perhaps in part because of the limited fossil record, earlier generations of paleontologists aligned Paleozoic species with postPaleozoic ordinal groups. In contrast, based largely on the structure of the ambulacra, Blake (1987) and Gale (1987) independently argued that post-Paleozoic asteroids are clearly separable, indicating major phylogenelic changes near the end of the Paleozoic or during the earliest Mesozoic (a scanty fossil record does not identify precise timing). In a series of publications, Mooi and David (e.g., 2000) stressed the significance of the ambulacra in the interpretation of echinoderm phylogeny, thus providing philosophical basis for the interpretations of Blake (1987) and Gale (1987).

Blake, Tintori, and Hagdorn (2000) and Blake and Hagdorn (2003) restudied the scanty Triassic record in light of that of the late Paleozoic. These authors argued that essential characters of the ambulacra! column of post-Paleozoic, crown-group asteroids had emerged by the Carboniferous in now-extinct lineages, although refinements render even the single crown-group ambulacral ossicle recognizable as such. Many families of extant asteroids are known from the early and middle jurassic, indicating significant diversification by that time.

Blake and Hagdorn (2003) argued that presence and expression of actinal ossicles provide apomorphies of the crown group (within the resolution of the fossil record). This paper broadens and exemplifies the phylogenetic interpretations of Blake and Hagdorn (2003) but it retains the published cladogram of that paper, which is not duplicated here.

TERMINOLOGY

Dorsal and ventral are used for the upper and lower surfaces of the complete animal. Adraclial is toward the arm axis and abradial is away from this axis. Proximal is toward the mid-disk dorsal-ventral axis and distal is away from this axis. Length is approximately parallel to the arm axis, width approximately perpendicular to length in a horizontal plane, and height (low, high) is perpendicular to length in a vertical plane. Ossicular series can either arc gradually (as in Ogmaster Martens, 1865 (Fig. 1.1) or more or less abruptly (as in Neopalaeaster Schuchert, 1914, Fig. 1.6) around the inlerbrachial disk, and orientation of equivalent ossicular surfaces changes more or less gradually depending on the shape of the arc. As a result, consistent usage of orientation descriptors (e.g., "length") is not possible. For uniformity, all ossicles are described as if oriented in a distal arm position.