© 2000 by The Society for Integrative and Comparative Biology
The Class Asteroidea (Echinodermata): Fossils and the Base of the Crown Group 1
1 Department of Geology, University of Illinois at Urbana-Champaign, 1301 W. Green St., Urbana, Illinois 61801
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Because of limited research, a generally accepted hypothesis has not emerged for the phylogeny of the Asteroidea. The fossil record is a potential source of needed data, although fossil asteroids are rare, and they tend to be poorly preserved.
Emphasis in the taxonomy of both recent and fossil asteroids has been on characters visible from the exterior, and paleontologists have sought to fit even the most ancient (i.e., Ordovician) specimens into taxonomic ordinal schemes devised for recent asteroids. Animal form and arrangement of body wall ossicles of Paleozoic asteroids can be similar to those of younger species, thereby suggesting close affinities, yet ambulacral arrangements indicate clear separation of Paleozoic stem groups from the crown group.
Traits taken from the ambulacral column that mark crown-group asteroids include presence of dorsal podial pores (which allowed transfer of the ampullae to the arm interior), an offset arrangement of ambulacrals on the adambulacrals, and increased complexity of the articulation structures between ambulacrals and adambulacrals. Transfer of ampullae to the arm interior provided protection and more space for ampullae within the arm, as well as space within the furrow and between the ambulacral and adambulacral ossicles for elaboration of the soft tissues that enhance arm motion.
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The primary purpose of this paper is to suggest some characters to be expected in a taxon basal to the Mesozoic diversification of crown-group asteroids. The meager fossil record of asteroids bears on efforts to identify basal traits; the second purpose is to evaluate the limitations of this fossil record.
The asteroid skeleton consists of thousands of discrete ossicles more or less tightly linked in a dermal layer at the surface of the body and enclosing a large coelom. Ossicles, although numerous, belong to one of a comparatively few series. A number of ossicular primary series form the body wall. The usually double marginal and the semi-enclosed adambulacral and ambulacral series arise proximal to the terminal at the tip of each arm. The marginal series separate the abactinals or dorsals above from the actinals or ventrals below; ossicles of these two series extend to the terminal or not in different species. Accessory ossicles are the spines, spinelets, granules, and pedicellariae that more or less cover the surface of primary ossicles of most asteroids but not those with thickened dermal tissues. Additional ossicular series can occur internally, including those of an interbrachial septum and superambulacrals. The taxonomy of asteroids traditionally has stressed features visible from the exterior, including arrangement of abactinal, actinal, and marginal series and the arrangement of accessory ossicles.
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Asteroids are highly vulnerable to taphonomic alteration. Recent asteroids are found over a broad range of habitats, from rocky intertidal settings to ocean depths. Unfortunately, many habitats are infrequently captured in the stratigraphic record. Many asteroid families are restricted in latitude, depth, or both (e.g., Blake, 1983
, 1990), thereby biasing their fossilization potential. Most asteroids are epifaunal, and wave and current action typical of shallow-water environments destroys specimens. Special rapid burial events, such as deposition beneath migrating shoals, storm-generated sedimentary layers, or turbidity flow horizons, are required. Overall body robustness is an important contributing factor to preservation; however, even the sturdiest of asteroids are vulnerable to rapid destruction. Around the time of death, extensive tissues provide an attraction to disruptive scavengers, and the comparatively small (relative to overall body size) ossicles are readily disassociated and scattered. After burial, the closed body wall provides a protective sac tending to hold the individual together, but it also generally prevents filling of the coelom with sediment, and the asteroid becomes flattened and distorted with decay and compaction. Compaction is particularly serious with constructional modes such as that of the Asteriidae, in which the coelom is large (and therefore displacement due to collapse is significant) and ossicles are small. Displacement disrupts the ossicular alignment and orientation that are important to taxonomic and phylogenetic interpretation. Accessory ossicles are seldom preserved in a manner that allows ready comparative analysis, and where they are in place they obscure the taxonomically important features of the underlying ossicles. Important internal features (e.g., of the jaw frame and ambulacral column) typically are obscured by other ossicles.
The well-preserved asteroid skeleton had to remain in chemical equilibrium with its environment for millions of years, yet surficial detail and ossicular meshwork typically are delicate and taphonomically vulnerable. Ossicles can be obscured by infilling or replacement with foreign mineral material, or they can recrystallize and fuse with each other or with enclosing lime mud, becoming impossible to extract from the rock. Many of the best-preserved specimens were buried in fine mudstones or muddy sandstones, although ossicles are leached from many such settings, leaving molds whose fidelity depends in part on grain size of enclosing sediments, and in part on sediment compaction about the ossicles; compaction must be intimate and yet not disruptive. Isolated ossicles are found in sediments only through diligent effort such as that of generations of European paleontologists searching the Cretaceous chalks and related rocks; ossicles likely are more widely distributed in the fossil record but remain largely unrecognized (Howe, 1942).
The fossil record of asteroids is limited; however, absence of any recent family from Triassic or older rocks and presence of divergent surviving families in Lower and Middle Jurassic rocks indicates a period of rapid evolution. A survey of monographs (e.g. Fisher, 1919; Downey, 1973; Walenkamp, 1976, 1979) suggests few species and specimens are typical of many recent settings. European Cretaceous chalk deposits have been relatively well sampled, and based on his own and earlier studies, Gale (1986) decided that absence of asteroid beds resulted from an initial low density, although he does not elaborate on his inference. In addition, Gale considered effects of bioturbation and current action to be important. Parallel arguments to those of Gale might be typical of fossil occurrences.
The Paleozoic record, which includes no species closely related to surviving representatives, is difficult to interpret. The Lower Devonian Hunsrück Slate of Germany contains a truly unique diversity of stelleroids (both asteroids and ophiuroids) (Lehmann, 1957) preserved during a relatively brief time span of a few million years (Bartels et al., 1998) in a restricted geographic area. The fauna accumulated in a geologically and chemically complex and incompletely understood active plate tectonic setting (Bartels et al., 1998). Many of the stelleroids are relatively complete, but flattened and pyritized, although preservation of some specimens is exquisite. For geological reasons, the depositional setting of the Hunsrück is unique; the fauna is not the product of a very brief, extraordinary evolutionary flowering. The Hunsrück fauna therefore is uniquely instructive on the state of stelleroid evolution in mid-Paleozoic times.
In the Hunsrück, Kutscher (1976) recognized 50 stelleroid species distributed among 33 genera and 13 families; assigned species are approximately equally divided between ophiuroids and asteroids. Many of the asteroid species are relatively large, even by recent standards: Lehmann illustrated specimens in a size range between 50 mm and 125 mm, and both multiarmed species and those with five arms are included. Taphonomic folding of the body wall of some specimens suggests bulkiness, and thus Paleozoic asteroids were capable of constructing and supporting considerable tissue mass. A broad range of skeletal robustness is known, ranging from those of comparatively delicate construction (e.g., Palasteriscus devonicus) to armored species (e.g., Palaeostella solida). Individual ossicles can be thickened, as in P. solida, or delicate, as in P. devonicus, and they can bear prominent spines, as in Hystrigaster horridus. Asteroids tend to be feeding generalists, and therefore it is not possible to link form and function in a simple manner, yet morphology and function are strongly related among recent asteroids (e.g., Blake 1990), and Hunsrück Slate diversity therefore surely demonstrates varied habits and ecologic partitioning comparable to that of the most complex recent stelleroid fauna. The Hunsrück indicates that stelleroids long have been major contributors to marine ecosystems.
In summary, the fossil record is misleading in terms of taxonomic diversity whereas the general scarcity of asteroids from the record reflects both taphonomic constraints and probable original abundances.
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Background
Taxonomists working with recent asteroids traditionally have stressed exterior characters, although some papers treated aspects of internal anatomy as well (e.g., Agassiz, 1877
; Viguier, 1879; Fisher 1911; Jangoux, 1982). Paleontologists have sought to fit fossils of all ages into taxonomic concepts developed for recent asteroids. Schuchert (1915), for example, took the terms "Cryptozonia" (for hidden or small marginal ossicles) and "Phanerozonia" (for those with marginals obvious from the exterior) from Sladen (1889), and Spencer and Wright (1966) applied E. Perrier's ordinal terms to Paleozoic fossils. Unfortunately, exterior characters of asteroids of all ages can be very similar (e.g., Fig. 1A, B), and it is argued here that ambulacral structure provides the better guide to broad taxonomic affinities, although ambulacral differentiation is subtle and added research is needed.
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Since the work of Spencer (1951)
, a common ancestry for asteroids and ophiuroids has been thought to lie in a poorly known group of fossil stelleroids, the Somasteroidea. Because the stelleroid classes (or subclasses) have been closely aligned, assignment of individual species or specimens might be expected to be equivocal, but this has not been generally true; most uncertainty probably has resulted from taphonomic loss.
Asteroid apomorphies
Apomorphies of the Asteroidea (Blake 1998) that are most readily recognized among fossils are ambulacral (Fig. 1C, D): 1) The ambulacral ossicles are erect, arranged as a series of A-frames along the ambulacrum. 2) The ambulacrals articulate laterally and ventrally with the adambulacrals throughout the length of the arm, although in many Paleozoic asteroids they abut only the dorsal adradial corner of comparatively large adambulacrals. 3) The adambulacral series define the margin of the ambulacral furrow. 4) Some differentiation of articulation facets and tissue support pads is present. These articular structures are better developed in post-Paleozoic asteroids, and development in Paleozoic representatives requires further documentation. Mouth frames potentially are important sources of information, but they are seldom well exposed.
All Jurassic and younger asteroids either can be assigned to a recent family, or to an extinct family close to a survivor, whereas all Paleozoic fossils are distinct at the ordinal level. Three apomorphies (or apomorphy complexes) serve to demarcate the crown group (Fig. 1C, D): 1) presence of dorsal podial pores between subsequent ambulacrals. Dorsal podial pores are first known from the Devonian. 2) offset positioning of ambulacral and adambulacral ossicles, first known from the Carboniferous; and 3) differentiation of complex abradial ambulacral-adambulacral articulation structures. Character 3 is correlated with character 2, but character states for 3 are poorly understood because of lack of adequate fossils.
The three apomorphy complexes are shared by all known post-Paleozoic asteroids, and at present they are known from a single Paleozoic species, Calliasterella americana Kesling and Strimple, 1966 (Fig. 2A, B). Interpretation of character sequence is based on stratigraphic succession over a long geologic interval. Changes plausibly are subject to homoplasy, and therefore presence in a fossil taxon does not prove close affinities with modern asteroids. Additional defining traits can be expected to emerge with future research.
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Dorsal podial pores
Schöndorf (1907
, Schöndorf1909) emphasized the significance of podial pores and arrangement of ambulacrals and adambulacrals whereas other paleontologists have argued that podial pores are widely scattered taxonomically and therefore they have not stressed the character complex in their interpretations, see below. Dorsal podial pores (Figs. 1C, D; 2B, H) are approximately vertically oriented passageways developed between subsequent ambulacral ossicles in a mid-ossicular position. The tube foot descends from the interior ampulla through the podial pore to the exterior.
Configuration is crucial to the recognition of dorsal podial pores. Linkage is short and direct in that the ampulla is directly above the podium of which it is a part. Dorsal pores are large and smoothly contoured passageways and there are no angular ossicular ridges to interfere with podial movement. Dorsal podial pores occur in all known post-Paleozoic asteroids, and they have been recognized in species as old as Devonian (Haude, 1995; Webster et al., 1999).
Podial basins
Podial basins are closed cup-like or shelf-like depressions shared by sequential ambulacrals or adjacent ambulacrals and adambulacrals. They are bordered either by the contoured surface of ambulacrals and adambulacrals or by narrow skeletal ridges that define the basin. No cylindrical passageway to the interior is present and therefore any ampullae were external. No direct fossil evidence of ampullae exists; however, podial basins are capacious, which strongly suggests their presence. Podial basins (Fig. 1G, H) are known only from Paleozoic species.
Other pore-like features
A number of authors have argued that podial passageways to arm interiors were present in various lower and middle Paleozoic species (Spencer 1919, p. 184; Schuchert, 1915; Kesling, 1962; Branstrator, 1975; Haude, 1995), in effect suggesting that presence or absence of pores is a comparatively minor taxonomic and phylogenetic issue. Of these, only the example of Haude (1995) represents dorsal podial pores as defined here; the remainder are of a number of possible origins including 1) products of taphonomic alteration; 2) skeletal gaps; 3) possible gaps for soft-tissues other than podia; and 4) tiny, lateral supposed openings.
1. Taphonomic changes can significantly alter stelleroid appearance. Ossicular separation can result from displacement at the time of burial or from subsequent dissolution. Sediment and ossicles commonly are similar in color, which is itself potentially misleading, and color similarities have contributed to the overzealous laboratory preparation that has corrupted many specimens. Kesling (1962) quoted Schuchert (1915) on the presence of pores in Hudsonaster matutinus, but based on restudy of the type specimen, these are podial basins that are difficult to see because of color and other aspects of preservation. The reconstruction of Branstrator (1972, his Text-fig. 2) shows podial basins and small, irregular, lateral openings; the latter appear to be taphonomic, although they could have been skeletal gaps (below).
2. Skeletal arrangement at the juncture of ambulacral and adambulacral ossicles is complex, and skeletal gaps (Fig. 1G) superficially suggestive of lateral podial pores are present in a number of taxa. Skeletal gaps fail the criterion of form because they are more or less angular, rather than rounded, and they lack smoothly rounded, funnel-like passageways to the interior of the arm. The pattern in Platanaster (Fig. 1G) is important because the margins of the podial basins are raised, rounded, and nearly closed, thus defining limits to the ampullae. Further, the skeletal gaps of Platanaster are filled with small ossicles that might either have been displaced from the dorsal surface, or, more likely (based on uniformly small size and similar ossicular shape), they represent remains of a flexible tissue and ossicular covering present in life. Similar skeletal configurations are known in a number of recent taxa, e.g., the reticulated dorsal surface of some Asteriidae. The purported pores of Branstrator (1975, his Pl. 1, Fig. 4, Pl. 2, Fig. 6) are skeletal gaps.
3. In the literature, any skeletal separations at the ambulacral-adambulacral junctions have been treated as related to the water vascular system. In post-Paleozoic asteroids, however, skeletal discontinuities are present for articular tissues between the abradial articular flanges of the ambulacrals and the dorsal surface of the adambulacrals (Fig. 1D). Small notching of ambulacrals in the type specimen of Hudsonaster narrawayi (Fig. 2I, J) suggests presence of comparable tissues, although these were less complexly differentiated than those of post-Paleozoic taxa. Articular tissue gaps are angular and small (articular tissues need not span significant distances).
4. Arrangement of ossicles and soft tissues was complex, and specimens with supposed very small pores to the arm interior have been reported near the ambulacral-adambulacral juncture (e.g., Spencer 1916, Pl. 4, Fig. 3). The complex configuration is unlike that of true podial pores. Any connection between the interior and the furrow tube feet through such an opening would have been twisted, extended, and narrow. Further, the putative link at ossicular boundaries is at an area of active flexure, which seemingly must interfere with soft tissues. If openings were present, they are very different in nature from dorsal podial pores and if present should be recognized as a separate character.
Discussion of pores
Schöndorf (1909) and Blake and Guensburg (1988) recognized lateral podial pores in certain Devonian genera (Fig. 1E, F) extending from the furrow above the adambulacrals. The enlarged, rounded configuration is similar to that of dorsal pores whereas the abradial ambulacral-adambulacral offset (see below) and well-defined articulation structures found in recent forms are lacking. The phylogenetic relationship between lateral and dorsal pores is unknown, although direct derivation of the latter from the former requires only minor displacement. Selection in favor of direct linkage and provision of added space for adambulacral articular structures and tissues might have driven a positional shift.
Possible precursors to lateral podial pores are known. In certain Paleozoic genera (e.g., Promopalaeaster [Fig. 1H] and Hudsonaster [Fig. 2I]), deep reentrants extend abradially between the adambulacrals and ambulacrals from the podial basins. Loss of the abradial edge of the ambulacral (and ossicular reshaping) would provide the opening of the lateral pore condition.
Dorsal podial pores might have originated more than once and in different patterns. For example, pores could have originated at different times in ontogeny, or scattered in different patterns through the arm, or in only some members of populations, and so on. A Devonian specimen assigned to Promopalaeaster(?) sp. by Haude (1995) apparently has true dorsal podial pores that are large and clearly configured. Pores do not occur between every ossicular pair of this specimen thus suggesting a gradational pattern of pore derivation.
True dorsal podial pores first occur in the Upper Devonian in the single poorly preserved specimen of Plediaster inceptus (Webster et al., 1999). They also occur in the Carboniferous species Neopalaeaster enigmaticus Kesling, (Fig. 2G, H) and Calliasterella americana, (Fig. 2A, B), and in the Permian species Permaster grandis and Monaster carnarvonensis Kesling, 1969. Articulation structures (facets and muscle pads) of Calliasterella are suggestive of those of post-Paleozoic species whereas corresponding features of the other taxa are comparatively simple.
Offset placement of ambulacrals and adambulacrals
In nearly all Paleozoic asteroids, each ambulacral abuts a single adambulacral (Figs. 1E–H, 2H), whereas in post-Paleozoic asteroids (Fig. 1D) and Calliasterella americana (Fig. 2B), the two are offset, with soft tissues extending from the ambulacral to both adjacent adambulacrals. (In some younger taxa [e.g., Luidia], the main body of the ambulacral is aligned with that of the adambulacral in a manner suggestive of pairing, therefore shape and articular arrangement of fossils must be evaluated with care.) Unlike C. americana, ambulacrals and adambulacrals are paired in other described Paleozoic asteroids with dorsal podial pores, including Devonaster and Neopalaeaster.
Articulation of the ambulacral column ossicles
The skeleton of the asteroid is constructed of unfused ossicles that are both held in position and articulated by more or less extensive soft tissues. Soft-tissue pads (i.e., typically rimmed depressions) are present in Paleozoic asteroids; these tissues, combined with the probable presence of catch-connective tissues (Motokawi, 1984, 1985) would have permitted complex movement in taxa ranging from blocky Hudsonaster (Figs. 1B, 2I, J) to the more delicate Calliasterella (Fig. 2A, B). Morphology and movement of the ambulacral column of asteroids has received comparatively little study.
For descriptive purposes, articulation structures between ambulacrals and adambulacrals can be separated into four components: 1, structures between successive adambulacrals; 2, structures between ambulacrals and adambulacrals; 3, structures between longitudinally successive ambulacrals; and 4, cross-furrow structures between ambulacrals.
Among Paleozoic fossils, structures between successive adambulacrals (1 above) and cross-furrow structures (4 above) appear most similar to those of post-Paleozoic asteroids. Interadambulacral contact facets lie beneath the ambulacral and overlie tissue depressions for the longitudinal muscles that lower the arm. Cross-furrow articular features in Paleozoic asteroids tend to be subdued (Fig. 2I, J), although in Promopalaeaster (Fig. 1H), well-developed ridges and grooves are present.
In taxa lacking dorsal podial pores, articular structures between successive ambulacrals (3 above) can extend the full ossicular width. In Promopalaeaster (Fig. 2E, F), the medial and abradial portions of the lateral faces of subsequent ambulacrals form a cylinder-and-groove arrangement that would allow rotation between ambulacrals along transverse ossicular axes (i.e., up and down motion of the arm). Adradially, both lateral faces of the ambulacral are recessed, suggesting a tissue pad equivalent to that of recent asteroids linked successive ambulacrals.
In post-Paleozoic species and Calliasterella americana, complex articular structures link ambulacrals and adambulacrals (2 above); distinctive wing-like articular flanges occur on the abradial extremities of the ambulacrals (Figs. 1D; 2B, D). Corresponding structures of typical Paleozoic species are subtle.
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Interpretation of function in ancient asteroids is hampered by near absence of parallel research on recent asteroids (but see Eylers 1976). Brief, largely descriptive interpretations of fossils are common in the literature; perhaps most comprehensive was that of Spencer (1914–1940
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Transfer of ampullae to the arm interior would appear to offer a number of advantages to the asteroid. Vaulting potential of the ambulacra is limited, and therefore furrow space is limited; displacement provides interior space for ampullae and it also frees space for larger tube feet and their operation. Transfer of ampullae to the dorsal arm interior partially protects the water vascular system from small predators (e.g., Röttger et al., 1972) while reducing the tissue mass in the furrow, thereby allowing larger tube feet and more readily allowing furrow closure and protection (Blake, 1983). Internal positioning of ampullae provides additional surfaces of attachment and support for the tube feet.
Lateral podial pores also provide extra space in the furrow and protection, but internal space between ambulacrals and lateral arm walls is limited in species with narrow arms, and lateral passageways occupy space otherwise available for development of ambulacral-adambulacral articular structures.
Origin of dorsal pores from a strictly external positioning might begin with formation of expanded lateral reentrants above the ambulacrals followed by opening of the wall to form lateral pores, then shifting of lateral pores (Fig. 1E, F) to the more direct dorsal pore position (Fig. 1C, 2D). Once the dorsal position was attained, ambulacral-adambulacral offset and differentiation of ambulacral-adambulacral articulation structures became possible. Shift of the podial complex restricted positioning of the longitudinal ambulacral-ambulacral articular structures to the adradial end of the ossicle, a pattern that would seem to enhance partial arm rotation about the longitudinal axes of the arm.
The general uniformity among post-Paleozoic asteroids of offset of ambulacrals and adambulacrals and their complicated articular arrangements suggest monophyletic derivation of the complex, but articulation appears to be a functionally integrated whole logically subject to polyphyletic derivation.
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ACKNOWLEDGMENTS |
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I am indebted to J. Thompson and C. Ahern, U.S. National Museum (USNM), Dr. P. H. von Bitter, Royal Ontario Museum, Dr. D. J. Fisher, University of Michigan Museum of Paleontology (UMMP), H. I. Cook, British Geological Survey (GSM), and Dr. R. Norby, Illinois State Geological Survey (ISGS) for kindly making specimens and casts available to me, and to authorities of the different museums for permission to visit museums and work with collections; and to Dr. R. Haude for making rubber pulls of Promopalaeaster? sp. available. Unnumbered specimens are in the author's private collection. I am indebted to an anonymous reviewer for many useful suggestions.
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FOOTNOTES |
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1 From the Symposium Evolution of Starfishes: Morphology, Molecules, Development, and Paleobiology presented at the Annual Meeting of the Society for Integrative and Comparative Biology, 6–10 January 1999, at Denver, Colorado.
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