Phylogeny and classification of the Asteroidea (Echinodermata)

Post-Palaeozoic asteroids share a large number of derived characters of the ambulacral column and the mouth frame, and constitute the crown group of the monophyletic group Asteroidea. This crown group is here called the Neoasteroidea (new subclass). The stem species of the crown group lived in the Permian or early Triassic and so the evolution of the asteroids parallels that of the echinoids. Character distribution within the Neoasteroidea, especially morphology of the skeleton, digestive system, larvae and tube feet, allows subdivision into four orders (Paxillosida, Notomyotida, Valvatida, Forcipulatida). The latter three orders possess the synapomorphy of suckered tube feet and are united as the Surculifera (new superorder); the Paxillosida are their primitive sister group. Palaeozoic asteroids represent the stem group of the class, and may be divided into plesions according to the order of appearance of synapomorphies with the crown group. Classification of Palaeozoic asteroids requires much further study. The appearance of new characters within the crown group asteroids, such as suckered tube feet, implies that these were absent in the stem group. The range of life-habits possible in Palaeozoic asteroids can thus be partly deduced from evidence provided by living asteroids. Palaeozoic asteroids are deduced to have lacked suckered tube feet and were presumably unable to evert the stomach; hence they were precluded from life on hard substrates and extraoral feeding on epifaunal organisms. It is suggested that they lived on soft substrates by deposit feeding, scavenging and predation on small benthos.     

Predatory asteroids and the decline of the articulate brachiopods

Various causes, such as increased predation pressure, the lack of planktotrophic larvae, a 'resetting' of diversity, increased competition from benthic molluscs and the decline of the Palaeozoic fauna, have been suggested to explain the failure of the brachiopods to reradiate following the Permo-Triassic mass extinction. Increased predation pressure has hitherto appeared improbable, because typical predators of brachiopods, such as teleostean fish, brachyuran crabs and predatory gastropods, did not undergo major radiation until the late Mesozoic and early Cenozoic. However, new evidence strongly suggests that one important group of predators of shelly benthic organisms, the asteroids, underwent a major radiation at the beginning of the Mesozoic. Although asteroids appeared in the early Ordovician, they remained a minor element of the marine benthos during the Palaeozoic acme of the brachiopods. However, these early asteroids lacked four important requirements for active predation on a bivalved epifauna: muscular arms (evolved in the early Carboniferous); suckered tube feet, a flexible mouth frame and an eversible stomach (all evolved in the early Triassic). Thus radiation of the Subclass Neoasteroidea coincided with both their improved feeding capability and the decline of the articulates. The asteroids were the only group of predators of brachiopods that underwent a major adaptive radiation in the earliest Mesozoic. The asteroids may therefore have contributed to inhibiting a Mesozoic reradiation of the brachiopods. Epifaunal species lacking a muscular pedicle may have been particularly vulnerable. Unlike bivalve molluscs, modern brachiopods show only a limited range of adaptations to discourage asteroid predation. □ Asteroidea, Brachiopoda, evolution, predation, functional morphology.     

Comparative histological and immunohistochemical study of sea star tube feet (Echinodermata, Asteroidea)

Adhesion in sea stars is the function of specialized structures, the tube feet or podia, which are the external appendages of the water-vascular system. Adhesive secretions allow asteroid tube feet to perform multiple functions. Indeed, according to the sea star species considered, the tube feet may be involved in locomotion, fixation, or burrowing. Different tube foot shapes usually correspond to this variety of function. In this study, we investigated the variability of the morphology of sea star tube feet as well as the variability of the composition of their adhesive secretions. This second aspect was addressed by a comparative immunohistochemical study using antibodies raised against the adhesive material of the forcipulatid Asterias rubens. The tube feet from 14 sea star species representing five orders and 10 families of the Class Asteroidea were examined. The histological study revealed three main tube foot morphotypes, i.e., knob-ending, simple disc-ending, and reinforced disc-ending. Analysis of the results suggests that tube foot morphology is influenced by species habitat, but within limits imposed by the evolutionary lineage. In immunohistochemistry, on the other hand, the results were very homogeneous. In every species investigated there was a very strong immunolabeling of the adhesive cells, independently of the taxon considered, of the tube foot morphotype or function, or of the species habitat. This indicates that the adhesives in all the species considered are closely related, probably sharing many identical molecules or, at least, many identical epitopes on their constituents.     

Hettangian Asteriidae (Echinodermata: Asteroidea) from southern Germany: taxonomy,phylogeny and life habits

Germanasterias amplipapularia andHystrixasterias hettangiurnus are new genera and species of Asteriidae (Asteroidea: Forcipulatida) described from the Hettangian (lowest Jurassic) of southern Germany. They are among the oldest known relatively complete asteroids assignable to surviving families, yet they are remarkably similar to certain living species. Modern asteriids are efficient predators of sessile and slow-moving stoutly-armored mullusks, however much asteriid morphology is suggested to have originated with suspension-feeding habits. The lineage leading to the modern Forcipulatacea might have separated from that of the remainder of the modern asteroids as early as the Carboniferous.     

Molecular phylogeny of the Valvatacea (Asteroidea: Echinodermata)

The Valvatacea is one the most ecologically important, taxonomically diverse, and widespread groups of post‐Palaeozoic (i.e. modern) Asteroidea. Classification within the group has been historically problematic. We present a comprehensively sampled, three‐gene (12S, 16S, early‐stage histone H3) molecular phylogenetic analysis of the Valvatacea. We include five of the six families within the Paxillosida, the monotypic Notomyotida, and 13 of the 16 families of the living Valvatida. The Solasteridae is removed from the Velatida (Spinulosacea) and joins the Ganeriidae and the Leilasteridae as members of the clade containing the Asterinidae. The Poraniidae is supported as the sister group to the large cluster of Valvatacea. Asteropseids and poraniids are phylogenetically distant, contrary to morphological evidence. Several goniasterid‐like ophidiasterids, such as Fromia and Neoferdina are supported as derived goniasterids rather than as Ophidiasteridae. The Benthopectinidae (Notomyotida) are supported as members of the Paxillosida as are two members of the Pseudarchasterinae that have traditionally been considered members of the Goniasteridae. Our data suggest that Antarctic valvataceans may be derived from sister taxa in adjacent regions. © 2011 The Linnean Society of London, Zoological Journal of the Linnean Society, 2011, 161 , 769–788.     

A new species of Xyloplax (Echinodermata: Asteroidea: Concentricycloidea) from the northeast Pacific: comparative morphology and a reassessment of phylogeny

Abstract. Xyloplax janetae n. sp. is described from the northeast Pacific Ocean. This is the third species recognized for the monogeneric Concentricycloidea. Skeletal structures are elaborated by scanning electron microscopy and compared with those of Xyloplax medusiformis from New Zealand and Xyloplax turnerae from the Bahamas. Critical-point-dried specimens show fibrous connective tissue emerging from openings on the abactinal surface, on the abactinal spine bases, and at broken cross sections of the adambulacral spines. Knob-like structures emerging through the stereom openings of the abactinal spine base are also observed. Tube feet have round, swollen knobs and show few striations relative to other asteroids. Cosmopolitan bathyal–abyssal echinoderm species tend to exhibit few morphological differences over broad geographic ranges; this generalization is true of Xyloplax . The Concentricycloidea is proposed as an infraclass within the Asteroidea, as the sister branch to the Neoasteroidea (the crown-group asteroids) within the subclass Ambuloasteroidea. The hypothesis presented is compatible with recent phylogenetic data supporting affinities between Xyloplax and the Asteroidea. Characters plesiomorphic in basal neoasteroids are consistent with a sister-group relationship to concentricycloids. Actinal plate presence, an important synapomorphy for the Neoasteroidea, is absent from concentricycloids. The substantial morphological departure of Xyloplax is considered to be associated with the post-Paleozoic diversification of crown-group asteroids rather than from modification of an established and conservative morphology.     

Phylogenetic Signal Dissection Identifies the Root of Starfishes

Relationships within the class Asteroidea have remained controversial for almost 100 years and, despite many attempts to resolve this problem using molecular data, no consensus has yet emerged. Using two nuclear genes and a taxon sampling covering the major asteroid clades we show that non-phylogenetic signal created by three factors - Long Branch Attraction, compositional heterogeneity and the use of poorly fitting models of evolution – have confounded accurate estimation of phylogenetic relationships. To overcome the effect of this non-phylogenetic signal we analyse the data using non-homogeneous models, site stripping and the creation of subpartitions aimed to reduce or amplify the systematic error, and calculate Bayes Factor support for a selection of previously suggested topological arrangements of asteroid orders. We show that most of the previous alternative hypotheses are not supported in the most reliable data partitions, including the previously suggested placement of either Forcipulatida or Paxillosida as sister group to the other major branches. The best-supported solution places Velatida as the sister group to other asteroids, and the implications of this finding for the morphological evolution of asteroids are presented.     

A starfish with three-dimensionally preserved soft parts from the Silurian of England

Palaeozoic asteroids represent a stem-group to the monophyletic post-Palaeozoic Neoasteroidea, but many aspects of their anatomy are poorly known. Using serial grinding and computer reconstruction, we describe fully articulated Silurian (ca 425 Myr) specimens from the Herefordshire Lagerst?tte, preserved in three dimensions complete with soft tissues. The material belongs to a species of Bdellacoma, a genus previously assigned to the ophiuroids, but has characters that suggest an asteroid affinity. These include a pyloric system in the gut, and the presence of large bivalved pedicellariae, the latter originally described under the name Bursulella from isolated valves. Ampullae are external and occur within podial basins; the radial canal is also external. Podia are elongate and lack terminal suckers. The peristome is large relative to the mouth. Aspects of the morphology are comparable to that of the extant Paxillosida, supporting phylogenetic schemes that place this order at the base of the asteroid crown group.     

Predation by the Ordovician asteroid Promopalaeaster on a pelecypod

Blake, D.B. & Guensburg, T.E. 1994 10 15: Predation by the Ordovician asteroid Promopalaeaster on a pelecypod. An Ordovician Prornopalaeaster (Echinodermata: Asteroidea) wrapped about a Cuneamya? (Mol-lusca: Pelecypoda) in the extraoral feeding posture characteristic of Jurassic to modern members of the Asteriidae documents an early origin for this behavioral complex. Modern asteriids are convergent on Promopalaeastet; there is no direct phylogenetic linkage between the two. This fossil occurrence, combined with the success of modem Asteriidae, demonstrates that biological evolution and geological change need not outmode complex life habits. The fossil supports the notion of asteroids as Paleozoic-type predators, and its existence suggests that asteroids were not significant contributors to changing faunal structures in shelf seas during the Phanerozoic. Asteroidea, Pelecypoda, functional morphology, evolutionary ecology.     

Pale biological implications of some Upper Ordovician juvenile asteroids (Echinodermata)

Blake, D. B. 1990 10 15: Pale biological implications of some Upper Ordovician juvenile asteroids (Echinodermata). Lethaia , Vol. 23, pp. 347–357. Oslo. ISSN 0024–1164.
Two patterns have been recognized in the early dorsal skeletal development of modern asteroids. The skeleton of well-preserved juveniles of the Late Ordovician species Promopalaeaster finei is similar in fundamental ossiclar arrangement to one of these patterns, suggesting continuity of developmental sequence. Similarities include presence of large terminal ossicles and double rows of marginal ossicles. Ancient and modern juveniles differ in that in P. finei , podial pores leading to the interior of the arms are lacking, thus providing an ontogenetic argument that such pores are phylogenetically derived within the class. An unpaired interbrachial marginal, or axillary, is present distal to the oral frame; positioning supports earlier suggestions that the axillary is the homologue of the odontophore ossicle of modern asteroids. Comparisons between P. finei and another Ordovician species, Macroporaster matutinus , suggest the need for greater mouth frame flexibility contributed to the evolution of the modern odontophore. Axillary development is linked to the common absence of actinal ossicles from Paleozoic species. ▭ Asteroidea, Echinodermata, functional morphology, ontogeny, phylogeny, Ordovician, Paleozoic .     

Is the adhesive material secreted by sea urchin tube feet species‐specific?

Sea urchin adoral tube feet are highly specialized organs that have evolved to provide efficient attachment to the substratum. They consist of a disk and a stem that together form a functional unit. Tube foot disk tenacity (adhesive force per unit area) and stem mechanical properties (e.g., stiffness) vary between species but are apparently not correlated with sea urchin taxa or habitats. Moreover, ultrastructural studies of sea urchin disk epidermis pointed out differences in the internal organization of the adhesive secretory granules among species. This prompted us to look for interspecific variability in the composition of echinoid adhesive secretions, which could explain the observed variability in adhesive granule ultrastructure and disk tenacity. Antisera raised against the footprint material of Sphaerechinus granularis (S. granularis) were first used to locate the origin of adhesive footprint constituents in tube feet by taking advantage of the polyclonal character of the generated antibodies. Immunohistochemical assays showed that the antibodies specifically labeled the adhesive secretory cells of the disk epidermis in the tube feet of S. granularis. The antibodies were then used on tube foot histological sections from seven other sea urchin species to shed some light on the variability of their adhesive substances by looking for antibody cross‐reactivity. Surprisingly, no labeling was observed in any of the species tested. These results indicate that unlike the adhesive secretions of asteroids, those of echinoids do not share common epitopes on their constituents and thus would be “species‐specific.” In sea urchins, variations in the composition of adhesive secretions could therefore explain interspecific differences in disk tenacity and in adhesive granule ultrastructure. J. Morphol., 2011. © 2011 Wiley Periodicals, Inc.     

A comparative study of the life style of two Jurassic irregular echinoids

The irregular echinoids Plesiechinus ornatus (Buckman) (Pygasteroida) and Galeropygus agariciformis (Forbes) (Cassiduloida) occur together in beds of the murchisonoe Zone, Bajocian, outcropping in the Cheltenham region of Gloucestershire. These species were largely restricted to different lithofacies within the carbonate shelf environment. Both adopted a hidden mode of life but achieved this by different techniques. Plesiechinus had fairly short spines and strongly muscular podia over the whole corona and was able to cover itself with coarse substrate particles. The oral tubercles are bilaterally symmetrical and are radially arranged. The oral spines are thought to have pulled sediment out from beneath the test, excavating a small depression for it. Galeropygus bore a dense covering of very small spines and its tube feet were differentiated into aboral respiratory podia and oral suckered podia. It had a preferred anterior direction of locomotion and is thought to have buried itself completely by excavating and ploughing into the substrate as it moved forward. Plesiechinus fed using only its lantern and postulated peristomial tube feet, whereas Galeropygus was a continugus sediment swallower and used its phyllode tube feet and peristomal lip spines in transferring particles towards the mouth.     

Identification of asteroid genera with species capable of larval cloning

Asexual reproduction in larvae, larval cloning, is a recently recognized component of the complex life histories of asteroids. We compare DNA sequences of mitochondrial tRNA genes (Ala, Leu, Asn, Pro, and Gln) from larvae in the process of cloning collected in the field with sequences from adults of known species in order to identify asteroid taxa capable of cloning. Neighbor-joining analysis identified four distinct groups of larvae, each having no, or very little, sequence divergence (p distances ranging from 0.00000 to 0.02589); thus, we conclude that each larval group most likely represents a single species. These field-collected larvae cannot be identified to species with certainty, but the close assemblage of known taxa with the four larval groups indicates generic or familial identity. We can assign two of the larval groups discerned here to the genera Luidia and Oreaster and another two to the family Ophidiasteridae. This study is the first to identify field-collected cloning asteroid larvae, and provides evidence that larval cloning is phylogenetically widespread within the Asteroidea. Additionally, we note that cloning occurs regularly and in multiple ways within species that are capable of cloning, emphasizing the need for further investigation of the role of larval cloning in the ecology and evolution of asteroids.     

Preservation of tube feet in an ophiuroid (Echinodermata) from the Lower Devonian Hunsrück Slate of Germany and a redescription of<Emphasis Type="Italic">Bundenbachia beneckei</Emphasis> and<Emphasis Type="Italic">Palaeophiomyxa grandis</Emphasis>

The preservation of non-mineralized tissues in the fossil record is extremely rare. The Lower Devonian Hunsrück Slate of Germany has long been known for the preservation of non-mineralized tissues in pyrite but whether or not these remnants represent true soft tissues has been questioned. This is especially true for struetures visible only on radiographs that are too delicate for excavation by traditional methods. Here we report the discovery of well-preserved pyritized tube feet in six fully prepared specimens of the protasterid brittle starBundenbachia beneckei from the Hunsrück Slate. This discovery represents the first report of fossilized ophiuroid tube feet in the fossil record. The successful excavation of the delicate tube feet was made possible by improved airbrasive techniques developed by German fossil collectors. The relatively large size of the fossil tube feet inBunden-bachia beneckei is consistent with earlier inferences on size based on the presence of large podial basins. Protasterid ophiuroids lack the specialized arm musculature and articulations that provide increased flexibility and strength to the arms of modern ophiuroids with typically reduced tube feet. How-ever, tube foot form and perhaps function inBundenbachia might have been similar to those of living asteroids in which large tube feet are used primarily for locomotion and food-manipulation thus compensating for a lack of specialized arm musculature and articulation. Hence, feeding and life mode of protasterid ophiuroids was not necessarily limited to sedentary, infaunal microphagy as traditionally suggested. Two Hunsrück protasterid ophiuroids,Bundenbachia benecki andPalaeophiomyxa grandis are redescribed and compared.      

Towards a Better Understanding of the Origins of Microlens Arrays in Mesozoic Ophiuroids and Asteroids

Echinoderms are characterized by a calcite endoskeleton with a unique microstructure, which is optimized for multiple functions. For instance, some light-sensitive ophiuroids (Ophiuroidea) and asteroids (Asteroidea) possess skeletal plates with multi-lens arrays that are thought to act as photosensory organs. The origins of these lens-like microstructures have long been unclear. It was recently proposed that the complex photosensory systems in certain modern ophiuroids and asteroids could be traced back to at least the Late Cretaceous (ca. 79 Ma). Here, we document similar structures in ophiuroids and asteroids from the Early Cretaceous of Poland (ca. 136 Ma) that are approximately 57 million years older than the oldest asterozoans with lens-like microstructures described thus far. We use scanning electron microscopy, synchrotron tomography, and electron backscatter diffraction combined with focused ion beam microscopy to describe the morphology and crystallography of these structures in exceptional detail. The results indicate that, similar to Recent light-sensitive ophiuroids, putative microlenses in Cretaceous ophiuroids and asteroids exhibit a shape and crystal orientation that would have minimized spherical aberration and birefringence. We suggest that these lens-like microstructures evolved by secondary deposition of calcite on pre-existing porous tubercles that were already present in ancestral Jurassic forms.     

Mitogenomes Reveal Alternative Initiation Codons and Lineage-Specific Gene Order Conservation in Echinoderms

The mitochondrial genetic code is much more varied than the standard genetic code. The invertebrate mitochondrial code, for instance, comprises six initiation codons, including five alternative start codons. However, only two initiation codons are known in the echinoderm and flatworm mitochondrial code, the canonical ATG and alternative GTG. Here, we analyzed 23 Asteroidea mitogenomes, including ten newly sequenced species and unambiguously identified at least two other start codons, ATT and ATC, both of which also initiate translation of mitochondrial genes in other invertebrates. These findings underscore the diversity of the genetic code and expand upon the suite of initiation codons among echinoderms to avoid erroneous annotations. Our analyses have also uncovered the remarkable conservation of gene order among asteroids, echinoids, and holothuroids, with only an interchange between two gene positions in asteroids over ∼500 Ma of echinoderm evolution.     

<Emphasis Type="Italic">Compsaster formosus</Emphasis><Emphasis Type="SmallCaps">Worthen &; Miller</Emphasis> (Asteroidea; Echinodermata): A Carboniferous homeomorph of the post-Paleozoic Asteriidae

All adequately known post-Paleozoic asteroids are either assignable to surviving families or closely related families whereas no Paleozoic species assignable to a surviving order has been recognized. The Mississippian speciesCompsaster formosus is similar enough to various Recent taxa in overall form as well as in the form and arrangement of body wall ossicles to raise the issue of affinities: IsC. formosus nested within a phylogenetic branch hitherto known only from post-Paleozoic strata or is it only homeomorphic?The nature of the ambulacral system is critical to interpretation of echinoderms, and post-Paleozoic asteroids share three fundamental ambulacral characters or character suites: dorsal podial pores, staggered positioning of ambulacrals and adambulacrals, and complex articular structures between these two ossicular types.Calliasterella americana, a Carboniferous asteroid, shares the three ambulacral features, although it is distinctive from post-Paleozoic asteroids in other ways.Compsaster formosus exhibits at least two of the three ambulacral characters, although presence of staggering has not been finally established. LikeC. americana, C. formosus differs from post-Paleozoic species in details of ambulacral anatomy as well as aspects of ventral body surface ossicular arrangement. Although approaching crown-group organization,C. formosus nevertheless represents a branch basal to the crown group.Because asteroids are generalists, understanding of life habits ofCompsaster is sketchy in spite of morphological similarities between it and younger genera.Compsaster probably was epifaunal and its overall form is strongly reminiscent of that of Recent predatory asteriids but it is also similar to small-particle feeding echinasterids.