Colin D. Sumrall

Ontogeny in the fossil record: diversification of body plans and the evolution of ''aberrant'' symmetry in Paleozoic echinoderms

Abstract Echinoderms have long been characterized by the presence of ambulacra that exhibit pentaradiate symmetry and define five primary body axes. In reality, truly pentaradial ambulacral symmetry is a condition derived only once in the evolutionary history of echinoderms and is restricted to eleutherozoans, the clade that contains most living echinoderm species. In contrast, early echinoderms have a bilaterally symmetrical 2–1–2 arrangement, with three ambulacra radiating from the mouth. Branching of the two side ambulacra during ontogeny produces the five adult rays. During the Cambrian Explosion and Ordovician Radiation, some 30 clades of echinoderms evolved, many of which have aberrant ambulacral systems with one to four rays. Unfortunately, no underlying model has emerged that explains ambulacral homologies among disparate forms. Here we show that most Paleozoic echinoderms are characterized by uniquely identifiable ambulacra that develop in three distinct postlarval stages. Nearly all “aberrant” echinoderm morphologies can be explained by the paedomorphic ambulacra reduction (PAR) model through the loss of some combination of these growth stages during ontogeny. Superficially similar patterns of ambulacral reduction in distantly related clades have resulted from the parallel loss of homologous ambulacra during ontogeny. Pseudo-fivefold symmetry seen in Blastoidea and the true fivefold symmetry seen in Eleutherozoa result from great reduction and total loss, respectively, of the 2– 1–2 symmetry early in ontogeny. These ambulacral variations suggest that both developmental and ecological constraints affect the evolution of novel echinoderm body plans.

Systematics and paleoecology of Late Cambrian echinoderms from the Western United States

Although echinoderm debris is locally common, articulated specimens are rare in Late Cambrian rocks from the Great Basin and Rocky Mountains of the western United States and are mostly associated with hardgrounds. The fauna, including cornute stylophorans, trachelocrinid eocrinoids, solute homoiosteleans, and rare edrioasteroids, includes several members of the archaic Cambrian Evolutionary Fauna, which had already passed its maximum diversity for echinoderms. In addition to the low diversity, articulated specimen abundance is very low, averaging only about one-tenth that found in overlying Lower Ordovician units. The transition between the Cambrian and Paleozoic Evolutionary Faunas for echinoderms in North America apparently occurred rapidly very close to the Cambrian-Ordovician boundary, because no unequivocal examples of the Paleozoic fauna (such as crinoids, glyptocystitid rhombiferans, asteroids, or echinoids) were found in the Late Cambrian sections.New taxa include several cothurnocystid stylophorans assigned to Acuticarpus delticus, new genus and species, Acuticarpus? republicensis, new species, and Archaeocothurnus goshutensis, new genus and species; Scotiaecystis? species, a poorly preserved cornute stylophoran with lamellipores; Minervaecystis? species, a fragmentary solute homoiostelean based on several steles; Tatonkacystis codyensis, new genus and species, a well-preserved trachelocrinid eocrinoid with five unbranched arms bearing numerous brachioles; an unnamed, poorly preserved, epispire-bearing eocrinoid; an unnamed, poorly preserved, globular eocrinoid? lacking epispires; and an unnamed, heavily weathered, edrioasterid edrioasteroid. Nearly all holdfasts found in these Upper Cambrian units are single-piece blastozoan types, probably belonging to trachelocrinid and other eocrinoids. Distinctive columnals and thecal plates of several additional undescribed eocrinoids and other echinoderms were locally abundant and are also described.

Ordovician edrioasteroids from Morocco: faunal exchanges across the Rheic Ocean

A new edrioasteroid fauna from the Ordovician of Morocco, North Africa includes eight new isorophid species placed in six genera of which three are new. A phylogenetic analysis of Moroccan genera and other taxa places these new edrioasteroids into a phylogenetic framework. Pyrgocystidae is redefined to include plesiomorphic isorophids with hood plates underlying the cover plates. New pyrgocystid taxa include Streptaster nodosus sp. nov., Belochthus? chauveli sp. nov., Argodiscus espilezorum sp. nov. and Moroccopyrgus matacarros gen. et sp. nov. Isorophinid taxa include Isorophus africanus sp. nov., Isorophusella gutii sp. nov., Euryeschatia reboulorum gen. et sp. nov. and the morphological aberrant Anedriophus moroccoensis gen. et sp. nov. These taxa are attached either epibiotically on conulariids or echinosphaeritid rhombiferans, or encrust large bioclasts such as cephalopods and trilobites in a siliciclastic environment that is otherwise devoid of suitable hard substrates. The presence of close relatives of North American taxa in this Perigondwanan section indicates a high degree of communication between North America and North Africa in Ordovician time. Also described are two species of an unusual eocrinoid? Hexedriocystis gen. nov., the type species H. inexpectatus sp. nov. and H. mimus sp. nov., that is a close mimic of isorophid taxa.

Kailidiscus, A New Plesiomorphic Edrioasteroid from the Basal Middle Cambrian Kaili Biota of Guizhou Province, China

Abstract A new genus and species of edrioasteroid grade echinoderm, Kailidiscus chinensis, is described from the Kaili Biota of the basal lower Middle Cambrian Kaili Formation from Guizhou Province, China. This echinoderm has a strong resemblance to isorophid edrioasteroids, bearing a well-developed peripheral rim, long curved ambulacra, and slightly imbricate interambulacral plating at maturity. However, the presence of pore-bearing, double biserial floor plates, tiered cover plates, lack of radially positioned oral frame plates, and unincorporated hydropore/gonopore are unknown in isorophids. Many of these features bear strong resemblance to eocrinoids and helicoplacoids, attesting to the plesiomorphic nature of this taxon. Despite the obvious anatomical differences, specimens of this species preserve a complete ontogeny that resembles that of isorophids. Juveniles show a discoidal theca with straight ambulacra that transitions to an inflated theca with strongly curved ambulacra with maturity.

Oral region homologies in paleozoic crinoids and other plesiomorphic pentaradial echinoderms.

The phylogenetic relationships between major groups of plesiomorphic pentaradial echinoderms, the Paleozoic crinoids, blastozoans, and edrioasteroids, are poorly understood because of a lack of widely recognized homologies. Here, we present newly recognized oral region homologies, based on the Universal Elemental Homology model for skeletal plates, in a wide range of fossil taxa. The oral region of echinoderms is mainly composed of the axial, or ambulacral, skeleton, which apparently evolved more slowly than the extraxial skeleton that forms the majority of the body. Recent phylogenetic hypotheses have focused on characters of the extraxial skeleton, which may have evolved too rapidly to preserve obvious homologies across all these groups. The axial skeleton conserved homologous suites of characters shared between various edrioasteroids and specific blastozoans, and between other blastozoans and crinoids. Although individual plates can be inferred as homologous, no directly overlapping suites of characters are shared between edrioasteroids and crinoids. Six different systems of mouth (peristome) plate organization (Peristomial Border Systems) are defined. These include four different systems based on the arrangement of the interradially-positioned oral plates and their peristomial cover plates, where PBS A1 occurs only in plesiomorphic edrioasteroids, PBS A2 occurs in plesiomorphic edrioasteroids and blastozoans, and PBS A3 and PBS A4 occur in blastozoans and crinoids. The other two systems have radially-positioned uniserial oral frame plates in construction of the mouth frame. PBS B1 has both orals and uniserial oral frame plates and occurs in edrioasterid and possibly edrioblastoid edrioasteroids, whereas PBS B2 has exclusively uniserial oral frame plates and is found in isorophid edrioasteroids and imbricate and gogiid blastozoans. These different types of mouth frame construction offer potential synapomorphies to aid in parsimony-based phylogenetics for exploring branching order among stem groups on the echinoderm tree of life.

Universal Elemental Homology in Glyptocystitoids, Hemicosmitoids, Coronoids and Blastoids: Steps Toward Echinoderm Phylogenetic Reconstruction in Derived Blastozoa

Abstract Universal elemental homology (UEH) is used to establish homology of thecal plates and elements of the ambulacral system among clades of stemmed echinoderms by placing these structures into a testable hypothesis of homology. Here UEH is used to explore hypotheses of homology in blastoids, coronoids, Lysocystites, hemicosmitoids, and glyptocystitoids. This new approach to analyze homology is particularly powerful in understanding the nature of the thecal plates of blastoids and how they relate to other taxa in a common nomenclatural lexicon. In blastoids, deltoids are interpreted as oral plates that are homologues to oral plates of glyptocystitoids and hemicosmitoids whereas side plates are interpreted to be ambulacral floor plates. Thecal plates are homologous among blastoids, coronoids and Lysocystites but these morphologies cannot be reconciled with plate circlets of glyptocystitoids and hemicosmitoids. A phylogenetic analysis of these taxa presents the origin of blastoids as sister taxon of coronoids within a testable series of homologies.

WHEN CLOCKS (AND COMMUNITIES) COLLIDE: ESTIMATING DIVERGENCE TIME FROM MOLECULES AND THE FOSSIL RECORD

time was the domain of paleontology.Origination time could be assessed only through referenceto first appearance data in the rock record. This changed almostfrom the beginnings of modern molecular biology, when it wasrealized that molecules could be used to calculate divergencetimes between living species. Early studies relied on immunolog-ical distance information, and the underlying rationale was sim-ple: because evolution involves changes to the genetic code, andbecause these changes accumulate over time, we should expectthe number of accumulated changes (the molecular distance) be-tween living taxa to increase as their time of divergence becomesolder (Zuckerkandl and Pauling, 1962). By inferring a rate ofevolution of the genetic code, we can place absolute time esti-mates on divergence points.Unfortunately, temporal estimates from molecules did not al-ways coincide with first appearance data from the fossil record.This first became apparent with the divergence between humansand living great apes. Based on presumed relationships betweenfossil and living primates at the time, paleontologists put the hu-man-ape split at roughly 20 million years (Simons, 1961; Pilbeam,1968). Protein clock studies, and later sequence-based analyses,suggested a much more recent divergence within the past 10 mil-lion years (Sarich and Wilson, 1967; Hasegawa et al., 1985).The outcome of this particular conflict colored the nature ofthe debate for many years. New fossil discoveries eventually putthe human-chimp split much closer to the molecular estimate(Fleagle, 1998). This became the standard textbook example ofthe molecule-fossil rift, and it seemed to lend credence to claimsthat molecular approaches were somehow more reliable thanthose restricted to paleontology. Throughout the 1980s and up tothe present, it was not uncommon to see published claims thatmolecular data were inherently superior to morphology and fossilsin phylogeny reconstruction and divergence time estimation (e.g.,Sibley and Ahlquist, 1987; Goodman, 1989; Hedges and Sibley,1994; Givnish and Sytsma, 1997; Douady et al., 2002)—eventhough molecules and morphology usually support very similarresults.Several other temporal conflicts have come to the surface overthe past 10 years. The most prominent involve the origin of livingmammalian and avian ‘‘orders.’’ Most first appear as fossils inthe latest Cretaceous or Paleogene, but are predicted to have muchlonger roots in the Mesozoic from molecular evidence (Cooperand Penny, 1997; Bromham et al., 1999a; Foote et al., 1999; vanTuinen et al., 2000; Archibald and Duetschman, 2001; Springeret al., 2003). Similarly, the origin of metazoan phyla, most ofwhich first appear during the ‘‘Cambrian Explosion’’ 550 ma butwhich are estimated to have originated as much as 1.5 billionyears ago on the basis of mitochondrial data (Wray et al., 1996;Bromham, 2003; Budd, 2003). Other conflicts have received lessattention but remain no less perplexing, such as the origin ofangiosperms (Doyle, 1998; Wikstro¨m et al., 2001) and gavialoidcrocodylians (Brochu, 1997; Harshman et al., 2003). These haveremained robust to improved data sets and techniques—the morewe look at fossils, molecules, or algorithms, the stronger the dis-parity seems to grow.What causes these conflicts? We believe part of the problemcontinues to be a lack of interaction between proponents of fossil-based and molecule-based dating approaches. This is unfortunate,because the centralization of these conflicts and a lack of dialoguemask the basic fact that molecules and fossils often support thesame answer, and in some cases, the answers are different becausethe questions are different (even if we fail to recognize the dif-ference). And both approaches must ultimately refer to the fossilrecord.We organized a symposium at the Sixth North American Pa-leontological Convention entitled ‘‘When Clocks Collide: Cali-brating Lineage Divergences from Fossils and Molecules.’’ Itbrought experts together from a wide range of disciplines, spe-cialists on a large array of taxonomic problems, and proponentsof a diversity of methods to discuss these issues and try to reacha level of understanding of the sources of the conflict and addresssome possible solutions. Attendees of that symposium were treat-ed to some remarkable talks and post-talk exchanges. Not sur-prisingly (to us), very little blood was drawn—the fields of pa-leontology and molecular systematics are beginning to integrate,as they should.Ours was not the only such event in recent years (e.g., Hadly,2003). The number of papers and funded research programs in-volving the collaboration between molecular biologists and mor-phologists is growing. This is remarkable considering the lack ofdialogue in the years since Colin Patterson (1987) edited Mole-cules and Morphology in Evolution: Conflict or Compromise? Weare seeing the synthesis our field has needed for so long.We here expand on our point that miscommunication is as im-portant as real data conflict in these debates. It is written from apaleontologist’s perspective, and is perhaps slanted against mis-interpretations of the paleontological literature by neontologists.Paleontologists may feel beleaguered when revised fossil infor-mation overturns long-held beliefs and conforms to a molecularestimate, but the unease is misplaced; in many cases, fossils andmolecules have never been in conflict. The conflict is a matter ofinterpretation, and as long as competing hypotheses are not com-pletely understood, the magnitude of the ‘‘conflict’’ will be ex-aggerated. As seen in the papers in this issue, we still have a longway to go to resolve some of the inconsistencies between sourcesof information—but an integrative approach holds the greatestpromise in achieving the one goal of understanding the historyof life as thoroughly as possible.

PHYLOGENETIC NOMENCLATURE AND PALEONTOLOGY

That TAXONOMY should be phylogenetic—i.e., that named taxa should be restricted to monophyletic groups—is widely accepted throughout systematic biology. A recently proposed phylogenetic nomenclatural system (de Queiroz and Gauthier, 1990, 1992, 1994; Cantino and de Queiroz, 2000) goes further by specifying that all supraspecific taxon names be explicitly defined on the basis of common ancestry. This phylogenetic nomenclatural system would eventually replace the Linnean system currently in use. In this note, we discuss why this is a good thing for paleontology and suggest ways in which we, as a community, can make the transition from typological to evolutionary systems while minimizing confusion. The Linnean system appears to have served us well for more than two centuries, and its abandonment makes many uneasy. But phylogenetic nomenclature follows from a view all of us share—that taxa are the dynamic products of descent with modification rather than static classes of objects sharing some arbitrary set of physical membership criteria (Griffiths, 1974; Ghiselin, 1984; de Queiroz, 1988; de Queiroz and Gauthier, 1990). Because the hierarchical pattern generated by evolution creates natural groups that we discover (or whose presence we hypothesize), taxa should be viewed as historically-bound entities. An evolutionary group exists because of common ancestry. The only natural boundaries for these groups are speciation and extinction. This is why monophyletic groups are individuals, but paraphyletic assemblages above the species level are not; to create a paraphyletic assemblage, we are forced to create an artificial upper bound on the basis of characters we deem significant for group exclusion. Modern applications of Linnean taxonomy acknowledge the individuality of species and recognize the fact that higher taxa are the products of evolution, but as long as taxa are based on their physical characters and not common ancestry, they continue to be viewed as …

THE BIOLOGICAL IMPLICATIONS OF AN EDRIOASTEROID ATTACHED TO A PLEUROCYSTITID RHOMBIFERAN

Abstract The attachment of a small edrioasteroid to the nonperiproctal surface of a small specimen of the pleurocystitid rhombiferan Amecystis has implications for the life posture of pleurocystitids and indicates that Amecystis was epifaunal. Articulation of the preserved portions of both thecae indicates that the edrioasteroid and Amecystis were alive at or very near the time of burial and that the edrioasteroid used the Amecystis theca as an attachment surface. Other examples of edrioasteroids attaching to live organisms are known suggesting that an edrioasteroid attached to a live rhombiferan is not highly unusual. The position of the edrioasteroid on the rhombiferan suggests that Amecystis was oriented with the anal side down in life and that this pleurocystitid was fully epifaunal.

Ponticulocarpus, a new cornute-grade stylophoran from the Middle Cambrian Spence Shale of Utah

Four specimens of a new cornute-grade stylophoran Ponticulocarpus robisoni new genus and species are described from the Spence Shale of northern Utah. Ponticulocarpus has wide and thin spinal and glossal processes interpreted as a mechanism for snowshoeing in a soft substrate. The presence of a posterior bar across the inferior posterior lobe, and two small bridges connecting the left adoral to M92 on the superior face and connecting M1 to M3 on the inferior face are unique in cornute-grade stylophorans. The posterior bar may be homologous with the posterior marginals M95 and M6 of other cornute-grade stylophorans, and an opening of the most posterior portion of the theca may have resulted in the development of the digital and glossal processes from marginals.