Rich Mooi

Are homalozoans echinoderms? An answer from the extraxial-axial theory

Abstract Homalozoans include four classes of non-pentamerous Paleozoic echinoderms: Homostelea (cinctans), Ctenocystoidea (ctenoid-bearing homalozoans), Homoiostelea (solutes), and Stylophora (cornutes and mitrates). Their atypical morphologies have historically made it difficult to relate them to other classes. Therefore, their systematic positions have been represented by two hypotheses (H): as stem taxa to echinoderms (H1) or as stem taxa to chordates (H2). These conclusions rest on previous inability to recognize synapomorphies with more crownward echinoderms, resulting in a forcing of the homalozoans down the phylogenetic tree that is more artifactual than evolutionary. The Extraxial-Axial Theory (EAT) identifies body-wall homologies, common ontogenetic patterns, and major events in bodyplan evolution. Therefore, the EAT can identify synapomorphies among even the most disparate of echinoderms. Application of the EAT undermines both H1 and H2 and strongly suggests that the bizarre asymmetry of homalozoans is a derived characteristic, and not indicative of plesiomorphic morphology for either chordates or echinoderms. Each of the four homalozoan clades and their major features are reexamined using the EAT. New findings are presented concerning homologies of thecal body wall, but we focus on stems, arms, and brachioles, which are recognized as very distinct products of independent evolutionary events. The results support a new interpretation (H3) of homalozoans as a polyphyletic assemblage that can be parsed out into other, clearly echinoderm clades. The Homoiostelea and Homostelea share the blastozoan synapomorphy of a brachiole. The enigmatic Ctenocystoidea also seem to have brachioles. The Stylophora have an arm as in crinoids. H3 is also more congruent with the known fossil record. Although they are stratigraphically early echinoderms, homalozoans are not indicative of the plesiomorphic morphology of the phylum.

Paedomorphosis, Aristotle's lantern, and the origin of the sand dollars (Echinodermata; Clypeasteroida)

-Convincing hypotheses of the origin of major invertebrate groups are difficult to make in the absence of phylogenetic analyses. In spite of this, several scenarios exist for the origin of the unusual echinoid order Clypeasteroida. I expand upon the most probable of these models by performing a phylogenetic analysis on three clypeasteroid suborders, the enigmatic fossil genus Togocyamus, and the extinct Oligopygoida. This analysis shows that the oligopygoids are the sister group of the Clypeasteroida plus Togocyamus. The latter is here considered a plesion (extinct sister group) to the crown group Clypeasteroida. Within that order, the suborder Clypeasterina is the sister group to the Laganina plus Scutellina. A new classification of all these taxa is presented. The phylogeny is based on 47 characters and incorporates data on external appendages, Aristotles lantern anatomy, and test structure of irregular echinoids, as well as new information on the morphology of Togocyamus. The earliest clypeasteroids had a lantern similar to that of adult oligopygoids, which in turn inherited their lantern from a cassiduloid-like ancestor that retained the lantern into adulthood. This lantern is absent in adult cassiduloids. Subsequent changes, including modification of the lantern into a crushing mill, extreme flattening of the test, and proliferation of food-gathering tube feet have allowed clypeasteroids to become epifaunal inhabitants of environments characterized by fine, shifting substrates, a habitat previously inaccessible to most other irregular echinoids. Rich Mooi. NHB-163, Department of Invertebrate Zoology, Smithsonian Institution, Washington, D.C. 20560 Accepted: October 27, 1989

Urchins in the meadow: paleobiological and evolutionary implications of cidaroid predation on crinoids

Abstract Deep-sea submersible observations made in the Bahamas revealed interactions between the stalked crinoid Endoxocrinus parrae and the cidaroid sea urchin Calocidaris micans. The in situ observations include occurrence of cidaroids within “meadows” of sea lilies, close proximity of cidaroids to several upended isocrinids, a cidaroid perched over the distal end of the stalk of an upended isocrinid, and disarticulated crinoid cirri and columnals directly underneath a specimen of C. micans. Guts of two C. micans collected from the crinoid meadow contain up to 70% crinoid material. Two of three large museum specimens of another cidaroid species, Histocidaris nuttingi, contain 14–99% crinoid material. A comparison of cidaroid gut contents with local sediment revealed significant differences: sediment-derived material consists of single crinoid ossicles often abraded and lacking soft tissue, whereas crinoid columnals, cirrals, brachials, and pinnulars found in the cidaroids are often articulated, linked by soft tissue, and unabraded. Furthermore, articulated, multi-element fragments often show a mode of fracture characteristic of fresh crinoid material. Taken together, these data suggest that cidaroids prey on live isocrinids. We argue that isocrinid stalk-shedding, whose purpose has remained a puzzle, and the recently documented rapid crawling of isocrinids are used in escaping benthic predators: isocrinids sacrifice and shed the distal stalk portion when attacked by cidaroids and crawl away, reducing the chance of a subsequent encounter. If such predation occurred throughout the Mesozoic and Cenozoic (possibly since the mid-Paleozoic), several evolutionary trends among crinoids might represent strategies to escape predation by slow-moving benthic predators.

Arrays in rays: terminal addition in echinoderms and its correlation with gene expression

Summary The echinoderms are deuterostomes that superimpose radial symmetry upon bilateral larval morphology. Consequently, they are not the first animals that come to mind when the concepts of segmentation and terminal addition are being discussed. However, it has long been recognized that echinoderms have serial elements along their radii formed in accordance with the ocular plate rule (OPR). The OPR is a special case of terminal growth, forming elements of the ambulacra that define the rays in echinoderms. New elements are added at the terminus of the ray, which may or may not be marked by a calcified element called the terminal plate (the “ocular” of sea urchins). The OPR operates in every echinoderm, from the occasionally bizarre fossils of the Cambrian to the most familiar extant taxa. Using the OPR and other criteria of recognition, echinoderm body wall can be divided into two main regions: extraxial components are associated with the somatocoels, axial components (formed in accordance with the OPR) with the hydrocoel. We compare patterns of development in axial regions of echinoderms with those found in the anterior–posterior axes of the earliest echinoderms as well as other invertebrates. Although axial and extraxial skeletons appear to be composed of the same biomineral matrix, the genes involved in patterning these two skeletal components are likely distinct. During development of the axial skeleton, for instance, the genes engrailed and orthodenticle are expressed in spatial and temporal patterns consistent with the OPR. Other genes such as distal‐less seem to demarcate early ontogenetic boundaries between the axial rudiment and the extraxial larval body. There is a complex and pervasive reorganization of gene expression domains to produce the highly divergent morphologies seen in the Echinodermata. We integrate morphological and genetic information, particularly with respect to the origins of radial symmetry in the rudiment, and the concomitant development of the rays.

Catalogus Fossilium Austriae. Band 2. Echinoidea Neogenica : Book Review

Catalogus Fossilium Austriae. Band 2. Echinoidea Neogenica. Andreas Kroh. 2005. Verlag der Osterreichischen Akademie der Wissenschaften, Vienna, 210 p., 82 pls., pb. ISBN 3-7001-3491-6 (printed edition), ISBN 3-7001-3560-2 (online edition). Many types of sea urchins fossilize well, with preservation of minute details. Fossil echinoids are therefore of great scientific value. From them, much can be learned about past environmental conditions, biogeography, and biodiversity. However, echinoids are also objects of great beauty, holding fascination at both macroscopic and microscopic inspection. In spite of the rather dry, ancient-sounding title, this book captures much of this scientific and aesthetic fascination. It is also an object lesson in how to do taxonomic monography in a time when this activity is regarded by many bandwagon-jumping peers as something to be left in their dust. A monograph of this kind should put that attitude where it belongs. Long after trendy works in current vogue have had their electronic and paper corpora fully recycled, solid books like the Catalogus will continue to be consulted for …

PHYLOGENETIC SYSTEMATICS OF TERTIARY MONOPHORASTERID SAND DOLLARS (CLYPEASTEROIDA: ECHINOIDEA) FROM SOUTH AMERICA

Abstract Sand dollars in the Monophorasteridae Lahille, 1896, form an important part of the South American Cenozoic echinoid fauna. Re-examination of type and other material adds significantly to our knowledge of the morphology and taxonomy of the family, and shows that besides Monophoraster darwini (Desor, 1847), M. duboisi (Cotteau, 1884), Amplaster coloniensis Martínez, 1984, and A. alatus Rossi de Garcia and Levy, 1989, there is a new species, A. ellipticus. We also show that Karlaster Marchesini Santos, 1958, is not a monophorasterid as once thought. A phylogenetic analysis of 24 characters assessed from all species of Monophoraster Lambert and Thiéry, 1921, and Amplaster Martínez, 1984, along with genera of the Mellitidae Stefanini, 1912, produced a single most parsimonious tree. The analysis demonstrates monophyly of mellitids and monophorasterids, and that Iheringiella Berg, 1898, should be excluded from the latter. Although both Monophoraster Lambert and Thiéry, 1921, and Amplaster Martínez, 1984, retain many features of an ancestor in common with the Mellitidae Stefanini, 1912, they also exhibit bizarre morphologies quite different from those of mellitids. The study has also resulted in a clearer picture of the biogeography and biostratigraphy of the Monophorasteridae, and their great significance in the evolution of lunulate sand dollars in the Americas.

Ontogeny and origin of the brooding system in Antarctic urechinid sea urchins (Echinodermata, Holasteroida)

SummaryEchinoids usually broadcast gametes, and do not generally engage in a high degree of parental care. However, when they do, juveniles are typically maintained among the spines, or in shallow, external depressions in the test itself. The brooding Antarctic holasteroids Urechinus mortenseni and Plexechinus nordenskjoldi are bizarre exceptions: females develop an elaborate brooding system in which a small number of direct developing young are protected. Ontogeny of post-natal brooding urechinids is marked by profound divergence in the growth trajectories of male and female apical systems. In females, this leads to dramatic departures from the patterns found in all other echinoids. Otherwise, coronal skeleton allometry of males and females is almost identical. Juveniles in brood pouches grow larger than the diameter of the apical aperture through which they must pass to reach the external environment. The apical plates, from which the brooding system is suspended, “hinge” downward to enlarge the aperture, allowing the young to emerge from the female. A possible origin for the brooding system suggests derivation by centripetal plate addition from the ocular plates in the coronal skeleton. We develop a contrasting model for the origin of the brooding system that relies on a proposed homology between genital and periproctal elements of the apical system of echinoids and the more highly developed dorsal skeleton of other echinoderm classes.

A NEW SPECIES OF LEODIA (CLYPEASTEROIDA: ECHINOIDEA) FROM THE NEOGENE OF VENEZUELA AND ITS IMPORTANCE IN THE PHYLOGENY OF MELLITID SAND DOLLARS

Abstract Leodia divinata new species is described from Early Pliocene deposits in Venezuela. It is only the second known species of Leodia, which was otherwise represented by a single extant and Late Pleistocene species, L. sexiesperforata. We also provide an overview of the characters that distinguish Leodia from other mellitid genera and that distinguish L. divinata from L. sexiesperforata. The evolutionary importance of L. divinata is discussed, along with its contribution to our knowledge of the phylogeny of the Mellitidae. It is now possible to partially fill a major and previously vexing gap in the stratigraphic record of the mellitids, and place in a phylogenetic context the strong preference of modern Leodia for biogenic, calcareous sand bottoms.

A NEW SPECIES OF ABERTELLA (ECHINOIDEA: SCUTELLINA) FROM THE GRAN BAJO DEL GUALICHO FORMATION (LATE EARLY MIOCENE–EARLY MIDDLE MIOCENE), RÍO NEGRO PROVINCE, ARGENTINA

The genus Abertella Durham, 1953 initially was described to include one of several problematic species of Miocene sand dollars originally placed in Scutella Lamarck, 1816. Durham (1953) named Scutella aberti Conrad, 1842 as the type of Abertella , and later (1955) tried to resolve issues concerning familial relationships of North American scutellines by placing the genus in a monogeneric family, the Abertellidae. Durham (1953, p. 351) separated Abertella from other members in his assemblage of taxa related to Scutella because the former has: 1) an immediately submarginal periproct between the second pair of post-basicoronal plates; 2) moderately closed petaloids about three-quarters the length of the corresponding aboral ambulacrum (although he states this ratio as being two-thirds in the description itself); 3) widely disjunct oral interambulacra; 4) ambulacral basicoronals that are larger than interambulacral basicoronals (misstated, and corrected to say just the reverse in Durham [1955]); and 5) a well-developed notch in the posterior margin of the test. Abertella aberti (Conrad, 1842) is known from the coastal Miocene of the eastern United States, notably Maryland, North Carolina, and Florida (Durham, 1953; Cooke, 1959; McKinney, 1985). In his 1953 paper, Durham also named to Abertella the following species: S. floridana Cooke, 1942 from Florida; S. cazonesensis Kew in Dickerson and Kew, 1917 from California; and S. habanensis Sanchez Roig, 1949 from Cuba. After creating the family Abertellidae, Durham (1955) felt that Echinarachnius sebastiana Jackson, 1922 (from Puerto Rico) might also be referable to Abertella as he conceived it. Durham (1957) also named Abertella palmeri from Guatemala and A. kewi from Chiapas, Mexico, whereas Cooke (1959) eventually synonymized his own S. floridana with A. aberti . The most recent species to be placed in the genus was A. complanata Brito, 1981, found in Miocene strata of northeastern …

Crinoid Ancestry Without Blastozoans

At present, a debate in the paleontologic literature focuses on whether or not the immediate ancestry of the Crinoidea lies in an unidentified member of the Blastozoa, which includes eocrinoids and an assemblage known variously as the “cystoids”. Those proposing to derive crinoids from within the blastozoans have recently argued for homologies in the construction of the oral region of certain derived taxa from both groups. An opposing viewpoint, outlined here, finds evidence that aside from plesiomorphies, proposed similarities are superficial and homoplastic. We suggest these superficialities represent convergent adaptive strategies. Earliest crinoids express ambulacral traits unlike any blastozoan but that are expressed in the only other pentaradial echinoderms with a known record early enough to be considered in the context of crinoid origins, edrioasteroids and edrioasteroid-like stem echinoderms.