Comparative Morphology of Tube Feet Among the Asteroidea: Phylogenetic Implications

Tube-foot morphology has been included among a variety of taxonomiccriteria for the Asteroidea over the past twenty-five years.Other than a few families belonging to the order Paxillosida,which are thought to have pointed, non-suckered tube feet thatare used for digging and burial in soft sediments, the presumptionhas been that asteroids have flat-tipped, suckered tube feet.This has become an accepted model despite the fact that thecomparative morphology of asteroid tube feet has not been considered.In the present study we examine tube-foot morphology of 45 speciesof Asteroidea representing 19 families. Our analysis confirmsthat members of the Luidiidae and Astropectinidae (order Paxillosida)lack suckers on the tips of their pointed tube feet. We demonstratethat there is considerable variation in tube-foot morphologyamong members of the Asteroidea including an entirely new typeof flat-tipped, non-suckered tube foot in species belongingto the order Valvatida. The external morphology of tube feetin species belonging to the order Velatida could not be distinguishedfrom "typical" flat-tipped, suckered tube feet; nonetheless,histological sections revealed a distinctive internal morphology.Finally, we report the first observations of the tube-foot morphologyof representatives of deep-sea asteroids belonging to the ordersNotomyotida and Brisingida, a group that also lacks the typicalflat-tipped, suckered tube-foot morphology. The results of ourstudy demonstrate that the current tube-foot morphology modelneeds to be reconsidered, as there is considerably greater variationthan was previously believed to be the case. Moreover, we concludethat while tube-foot morphologies show consistent similaritieswithin orders, tube-foot morphology is less appropriate as ataxonomic character below this level.     

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.     

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.     

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.     

The origin of multiplacophorans – convergent evolution in Aculiferan molluscs

Abstract: Multiplacophorans are Palaeozoic (Silurian to Permian) stem group polyplacophorans with 17 shell plates in a particular arrangement of single terminal plates separated by three columns of plates forming five transverse rows. Their distinctive morphology has prompted disparate interpretations of their relationship to polyplacophorans. Some features are strikingly similar to crown group polyplacophorans and even to some living families. Here we describe two Devonian forms, Protobalanus spinicoronatus sp. nov., a hercolepadid from northeast Ohio, USA, and Hannestheronia australis gen. et sp. nov., a strobilepid from South Africa. Using the results from a Bayesian relaxed molecular clock to test competing scenarios of the relationship of multiplacophorans to crown group polyplacophorans, we demonstrate that multiplacophorans are stem group polyplacophorans in which certain characters of the crown group evolved convergently.     

Tracing the evolution of the holothurian body plan through stem‐group fossils

The fossil echinoderm Palaeocucumaria, from the early Devonian Hunsrück Slate of southwestern Germany, has been studied using both traditional techniques and X‐ray microtomography, and its anatomy clarified. Phylogenetic analysis shows that it is a stem‐group holothurian with a combination of characters that help understand how the modern (crown‐group) holothurian body plan developed. Echinoids and holothurians have evolved along different paths, by differential growth of the larval‐ and rudment‐derived body regions. Palaeocucumaria shows that late stem‐group holothurians had a water vascular organization with a single external madreporite and calcified stone canal leading to the aboral end of the peripharyngeal coelom, and five primary radial water vessels that gave rise to tentacle‐like tube‐feet. This fossil data, in combination with a molecular phylogeny based on 18 s‐like rRNA gene sequence data, is used to order evolutionary steps in the making of the crown‐group holothurian body plan. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 109 , 670–681.     

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.     

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.      

Cladistic Analysis of A Problematic Ammonite Group: the Hamitidae (Cretaceous, Albian–turonian) and Proposals for New Cladistic Terms

The Hamitidae are a family of mid–Cretaceous heteromorph ammonites including lineages leading to four other families. Problems are outlined in trying to describe the phylogeny of completely extinct groups such as these heteromorph ammonites using the existing cladistic terminology, which is largely concerned with extant taxa and their ancestors. To solve these problems, two new terms are proposed: †crown groups and †stem groups, which are equivalent to crown and stem groups in terms of the evolutionary history of a clade, but are not defined on the basis of extant taxa. Instead they are defined by the topology of the phylogenetic tree, the †crown group being a clade defined by synapomorphies but which gave rise to no descendants. A †stem group is a branch of a phylogenetic tree which comprises the immediate sister groups of a given †crown group but is not itself a clade. Examples of these terms are described here with reference to the phylogeny of the Hamitidae and their descendants. The Hamitidae are paraphyletic and form †stem groups to a number of †crown groups, namely the Anisoceratidae, Baculitidae, Scaphitidae, and Turrilitidae. The definitions of the genera and subgenera are refined with respect to the type species and the clades within which they occur, and four new genera are described: Eohamites , Helicohamites , Sziveshamites , and Planohamites .     

<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.     

A new paleozoic Symmoriiformes (Chondrichthyes) from the late Carboniferous of Kansas (USA) and cladistic analysis of early chondrichthyans

Background

The relationships of cartilaginous fishes are discussed in the light of well preserved three-dimensional Paleozoic specimens. There is no consensus to date on the interrelationship of Paleozoic chondrichthyans, although three main phylogenetic hypotheses exist in the current literature: 1. the Paleozoic shark-like chondrichthyans, such as the Symmoriiformes, are grouped along with the modern sharks (neoselachians) into a clade which is sister group of holocephalans; 2. the Symmoriiformes are related to holocephalans, whereas the other Paleozoic shark-like chondrichthyans are related to neoselachians; 3. many Paleozoic shark-like chondrichthyans, such as the Symmoriiformes, are stem chondrichthyans, whereas stem and crown holocephalans are sister group to the stem and crown neoselachians in a crown-chondrichthyan clade. This third hypothesis was proposed recently, based mainly on dental characters.

Methodology/Principal Findings

On the basis of two well preserved chondrichthyan neurocrania from the Late Carboniferous of Kansas, USA, we describe here a new species of Symmoriiformes, Kawichthys moodiei gen. et sp. nov., which was investigated by means of computerized X-ray synchrotron microtomography. We present a new phylogenetic analysis based on neurocranial characters, which supports the third hypothesis and corroborates the hypothesis that crown-group chondrichthyans (Holocephali+Neoselachii) form a tightly-knit group within the chondrichthyan total group, by providing additional, non dental characters.

Conclusions/Significance

Our results highlight the importance of new well preserved Paleozoic fossils and new techniques of observation, and suggest that a new look at the synapomorphies of the crown-group chondrichthyans would be worthwhile in terms of understanding the adaptive significance of phylogenetically important characters.     

A New Extinct Genus of Cavioidea (Rodentia,Hystricognathi) from the Miocene of Patagonia (Argentina) and the Evolution of Cavioid Mandibular Morphology

The family Caviidae is represented in modern faunas by cavies and maras, whereas the family Hydrochoeridae is represented by capybaras. The evolutionary origin of these families has been related to a diversity of plesiomorphic fossil forms (recorded from the late Oligocene up to the middle Miocene) traditionally grouped in the family “Eocardiidae”. These fossil forms were included, together with Caviidae and Hydrochoeridae, within the Cavioidea s.s. (sensu stricto), because they share high crowned cheek teeth, double-hearted occlusal surface, short lower incisors, and moderate hystricognathy. Within Cavioidea s.s., caviids and hydrochoerids were interpreted as forming its crown group, because they have unique craniomandibular and dental features. In this contribution, a new taxon of Cavioidea s.s. from the middle Miocene of central Patagonia, Argentina, is described, and its phylogenetic position is determined on the basis of a morphological cladistic analysis in which “eocardiids” were included. The study permits the understanding of the sequence of appearance of characters that originated the highly divergent morphology of crown-group cavioids. The analysis of the sequence of appearance of the characters that traditionally diagnosed the crown group indicates that these changes did not occur at the same time. On the contrary, many of these features seem to have appeared at different nodes of the evolutionary history of Cavioidea s.s. The remarkably derived morphology of modern cavioids is the result of a stepwise appearance of a mosaic of evolutionary innovations that originated gradually along the history of Cavioidea during the late-middle Miocene.     

What makes an ophiuroid? A morphological study of the problematic Ordovician stelleroid Stenaster and the palaeobiology of the earliest asteroids and ophiuroids

Extant asteroids and ophiuroids [EchinodermataJ are distinguished by differences in arm support, water vascular system structures and in details of arm and jaw structure. However, some lower Palaeozoic taxa show combinations of both asteroid-like and ophiuroid-like characters and their morphology and functional biology is poorly understood. This paper redescribes one such taxon, the middle-upper Ordovician stellate echinoderm Stenaster and clarifies its phylogenetic status. Characters in common with extant and Ordovician ophiuroids, include arm support due primarily to ambulacral ossicles, presence of extensive longitudinal arm musculature, a mobile jaw and an internalised radial water vessel with internalised podial pores. In addition, Stenaster lacks several characters which are conventionally considered to be asteroid-like, for example an axillary, madreporitc, marginal ossicles and a true ambulacral groove. However, in overall shape Stenaster is remarkably asteroid-like, showing short, broad-based arms shared podial basins and a small disc. A cladistic analysis of early asteroids, ophiuroids and somasteroid taxa consistently places Stenaster within the ophiuroids and suggests secondary convergence to asteroids. In functional terms, Stenaster is interpreted as an ophiuroid which has secondarily adopted a semi-infaunal, deposit-feeding mode of life, analogous to that of some extant paxillosid asteroids.     

A Matter of Taste? Quality of Life in Day-to-Day Living with ALS and a Feeding Tube

Although people often refer to quality of life and there is a respectable research tradition to establish it, the meaning of the term is unclear. In this article we qualitatively study an intervention of which the quantitative effects are documented as indecisive. We do this in order to learn more about what the meaning of the term quality of life means when it is studied in daily life. With the help of these findings we reflect on the intricacies of objectifying and measuring quality of life using quantitative research designs. Our case is the feeding tube for patients suffering from ALS, a severe motor neuron disease that rapidly and progressively incapacitates patients. We studied how these patients, who lived in the Netherlands, anticipated and lived with a feeding tube in the course of their physical deterioration. Our analysis shows that the quality of life related to the feeding tube has to be understood as a process rather than as an outcome. The feeding tube becomes a different thing as patients move through the various phases of their illness, due to changes in their condition, living circumstances, and concerns and values. There are very different appreciations of the way the feeding tube changes the body’s appearance and feel. Some patients refuse it because they feel it disfigures their body, whereas others are indifferent to its appearance. Our conclusion is that these differences are difficult to grasp with a quantitative study designs because ‘matters of taste’ and values are not distributed in a population in the same ways as physiological responses to medication. Effect studies assume physiological responses to be more or less the same for everyone, with only gradual differences. Our analysis of quality in daily life, however, shows that what a treatment comes to be and how it is valued shows shows generalities for subgroups rather than populations.     

Conodont anatomy, chordate phylogeny and vertebrate classification

Interpretations of conodont anatomy and affinity continue to generate controversy. Fossilized soft-tissue evidence indicates that conodonts possessed eyes, extrinsic eye muscles, a notochord, myomeres, a differentiated tail with fin radiais, possible otic capsules and possible branchial structures. Indirect evidence suggests a differentiated brain and cartilaginous head skeleton. The multi-component phosphatic tissue complexes of the conodont feeding apparatus cannot be compared to the amorphous apatite of extant agnathan otoliths. By limiting cladistic analysis to a restricted selection of these characters the hypothesis that conodonts are a sister group of the clade comprising extant hagfish, lampreys and gnathostomes can be supported. However, exhaustive analysis of a more complete character-set strongly supports the hypothesis that conodonts are more derived than hagfish. From a taxonomic perspective, these two hypotheses have no effect on how conodonts should be classified. Whether they are a stem group (the former hypothesis) or part of the crown group (the latter), conodonts are clearly part of the total group Vertebrata (=Craniata).     

New early crown-group asteroids (Echinodermata; Triassic of Germany)

Migmaster angularis n. gen. n. sp. (Trichasteropsiidae, Forcipulatacea, Asteroidea) is described from the German Triassic Lower Muschelkalk of Anisian (Pelsonian) age, and new specimens ofTrichasteropsis bielertorum broaden understanding of this species. Phylogenetic events linking the stem groups, known only from Paleozoic rocks, with the crown group, known only from post-Paleozoic rocks, are problematic; the new fossils partially constrain phylogenetic changes associated with emergence of crown-group asteroids.