Ordovician Crinoids Research Articles

Disarticulation patterns in Ordovician crinoids: Implications for the evolutionary history of connective tissue in the Crinoidea

Taphonomic information is examined to evaluate the early history of connective tissues in the Crinoidea. The pattern of stalk segmentation of Middle and Late Ordovician crinoids is consistent with the two-ligament (intercolumnal and through-going ligaments) pattern present in living isocrinid crinoids and interpreted for fossil isocrinids, holocrinids, and Lower Mississippian crinoids. A single rhombiferan was also examined; its taphonomic pattern is also indicative of this style of tissue organization. Furthermore, the taphonomy of all Middle and Late Ordovician crinoids may reflect that they lacked discretely organized muscles between arm brachials, which is consistent with the hypothesis that muscles evolved as a connective tissue between plates only once within the Crinoidea, during the Early Devonian. These data indicate that the two-ligament organization of the stalk is a primitive feature among the Crinoidea and perhaps even among stalked echinoderms. Therefore, the autotomy function of this column-tissue organization among living crinoids is an exaptation. On the other hand, discretely organized muscles as connective tissue in crinoid arms is a derived trait that first appeared during the middle Paleozoic; this adaptation proved very successful for the advanced cladid crinoids.

Whiterockian (Ordovician) crinoid fauna from the Table Head Group, western Newfoundland, Canada

Whiterockian (Ordovician) crinoid fauna from the Table Head Group, western Newfoundland, Canada

Calyx plate homologies and early evolutionary history of the Crinoidea

New, competing ideas on crinoid plate circlet homologies and the desire for a phylogenetically-based classification have led to a reexamination of the Crinoidea. The most primitive crinoids had four plate circlets, from bottom to top: lintels, infrabasals, basals, and radials. Dicyclic camerates, cladids, flexibles, and dicyclic articulates are composed of infrabasals, basals, and radials. Monocyclic camerates and monocyclic articulates are composed of basals and radials. These all follow traditional ideas on homologies. In contrast, disparids with “compound radials” are composed of lintels, infrabasals, and radials, rather than “basals,” “inferradials,” and “superradials;” and disparids without “compound radials” have lintels and infrabasals rather than “basals” and “radials.”Parsimony-based phylogenetic interpretation of Ordovician crinoids with rhombiferans as the outgroup shows that crinoids are divisible into six clades, two paraphyletic and four monophyletic, including four-circlet crinoids, cladids, disparids, camerates, flexibles, and articulates.

Functional morphologies of the columns of Upper Ordovician Xenocrinus and Dendrocrinus

Two common Upper Ordovician crinoids, Xenocrinus and Dendrocrinus, had distinctive columns that showed marked contrasts of both form and function. Xenocrinus baeri (Meek) had a tetragonal facet geometry but functioned in the normal manner for a symplectially articulated column. Column flexure was by bending, the four directions of curvature being limited by the tetragonal arrangement of crenulae. In contrast, columns of Dendrocrinus casei Meek, with a characteristic pentastellate symmetry, were able to twist, a functional adaptation not previously reported from crinoids; this was controlled by the unusual geometry of the crenularium. This suggests that some or all crinoid columns may be subject to twisting stresses, perhaps associated with changes in the current direction, that crenulae may resist passively.Functional morphology, column, crinoids, Xenocrinus, Dendrocrinus

Eoparisocrinid crinoids from the Middle Ordovician (Galena Group) of northern Iowa and southern Minnesota

Two species of eoparisocrinid crinoids from the Middle Ordovician Galena Group of northern Iowa and southern Minnesota are described, namelyEoparisocrinus crossmanin. sp. andE. grandein. sp. The post-larval development ofEoparisocrinus crossmaniis examined. Crinoid arms grow by addition of new plates at their distal tips in conjunction with calcite deposition on old plates. New branches appear where axillary plates are initiated. Consequently, the growth rates for number of brachials and length of food-gathering system compared to crown volume are much faster than if the animals were isometric. The number of food particles collected is related to the number of food-catching tube-feet, which can be estimated if the length of the arms and height of the covering plates are known. The size of the largest food item is constrained by the food groove width. Thus, food-gathering capacity is the number of food-catching tube-feet multiplied by food groove width. The food-gathering capacity increases more rapidly than if the animal grew isometrically, and the ratio of food-gathering capacity: crown volume only declines slightly over the known growth range. All Ordovician cladid crinoids examined follow nearly identical ontogenetic trajectories. The ecological niche of a stalked crinoid is related to four basic parameters: stem length, food groove width, tube-foot spacing, and branch density. Stem length limits the highest elevation above the seafloor. The column ofE. crossmanibecomes longer during ontogeny due to the formation of new columnals and height growth of old ones. Consequently, individuals gradually “move up” until the adult elevation of about 50 mm is reached. The growth rates of stem length relative to crown size are slow in the youngest and mature animals but rapid in juveniles. The food grooves become wider throughout growth so that older crinoids ate larger food particles than younger ones. The food groove width increases less rapidly than if the shape were constant, because distal plates and branches are more narrow and have more slender food grooves than proximal plates. Growth curves for food groove width versus stem length and elevation were generated forE. crossmaniand other crinoids that commonly occur in the same beds. Together, elevation and food particle size define the main dimensions of the niche. The various taxa are more or less separated by different food groove widths at most comparable elevations. This pattern minimizes ecological overlap and probably competition between the different species. The tube-foot spacing ofE. crossmaniis constant regardless of size, which suggests that it employed the same type of feeding mechanism throughout post-larval ontogeny. The arm branches of adults gradually become less densely spaced relative to the area of water filtered than in juveniles.

Camerate crinoids from the Middle Ordovician (Galena Group, Dunleith Formation) of northern Iowa and southern Minnesota

Five species of camerate crinoids from the Middle Ordovician Dunleith Formation (Galena Group) of northern Iowa and southern Minnesota are described:Cleiocrinus regiusBillings,Cotylacrinna sandran. gen. and n. sp.,Euptychocrinus skopaiosn. gen. and n. sp.,Abludoglyptocrinus charltoni(Kolata), andEopatelliocrinus ornatus(Billings) n. comb.Archaeocrinus desideratusBillings is assigned toCotylacrinna, andGlyptocrinus fimbriatusShumard andPtychocrinus longibrachialisBrower are placed inEuptychocrinus.The geographical affinities of the Iowa and Minnesota crinoids lie with adjacent localities in the northern midcontinent and the Appalachian province of Canada and New York. Preservation of various specimens implies thatEuptychocrinus skopaios, Abludoglyptocrinus charltoni, andEopatelliocrinus ornatusutilized parabolic arm fans.Cotylacrinna sandrais a specialized rhodocrinitid which is perhaps the largest completely known Ordovician crinoid with a stem length of over 91 cm and a total volume of about 45,740 mm3. The root morphology indicates an upright column and this animal towered above the associated echinoderms, which ranged from the substrate level to a maximum of 25 cm above the seafloor.Euptychocrinus skopaiosis marked by dwarfed morphology compared to the closely alliedE. fimbriatus, and it exhibits accelerated development of fixed brachs, number of brachials in and length of the arms, and closely spaced pinnules. The dwarfism is interpreted as a specialization for small size and adults ofE. skopaioswere only located about five or six cm above the substrate. New brachials and pinnules form at the distal arm tips ofE. skopaiosthroughout ontogeny. Consequently, the length of and the number of plates in the food-gathering system are positively allometric relative to the crown volume. Food gathering capacity equals the number of food-catching tube feet times width of the food grooves and it is also augmented more rapidly than expected for an isometric crinoid. Although distantly related to euptychocrinids, most other Dunleith camerates, namelyAbludoglyptocrinus charltoni, Eopatelliocrinus ornatus, andCotylacrinna sandra, follow the same developmental trends of the food-gathering system observed inEuptychocrinus skopaios.Comparison of the pinnulate camerates and three species of ramulate cladids and a disparid from the Dunleith reveals some striking contrasts. At equivalent crown volumes, the camerates are characterized by more numerous arm branches in the form of pinnules, more narrow food grooves, more closely spaced tube feet, and longer food-gathering systems with more plates and greater capacity. The Dunleith camerates were adapted for catching smaller food particles using more numerous and more closely spaced tube feet located on more extensively branched filtration nets than the associated cladids and disparid. The differences can be attributed to taxonomy and presumably phylogeny, that is camerates versus cladids and disparids, and/or morphology in the presence of pinnules versus ramules.

Hybocrinid and disparid crinoids from the Middle Ordovician (Galena Group, Dunleith Formation) of northern Iowa and southern Minnesota

Four hybocrinid and disparid crinoids from the Middle Ordovician Dunleith Formation (Galena Group) of northern Iowa and southern Minnesota are described: Hybocrinus conicus Billings, Ohiocrinus levorsoni n. sp., Caleidocrinus (Huxleyocrinus) gerki n. sp., and Ectenocrinus simplex (Hall). The first three taxa are rare. Ectenocrinus simplex is an abundant and protean form ranging from the Shermanian to the Maysville and from the Appalachians to the Midcontinent. One Middle Ordovician specimen from the Dunleith is a complete small adult with stem and a lichenocrinid holdfast. The column was largely upright with the crown located about 25 cm above the seafloor. The Middle Ordovician crinoids differ somewhat from the later Cincinnatian material where only young E. simplex exhibit lichenocrinid holdfasts. Older crinoids became detached and were eleutherozoic well before the column was 25 cm long. Thus, the Cincinnatian individuals lost the attachment device earlier during ontogeny than their ancestors in the Middle Ordovician. Unlike most associated crinoids, E. simplex formed a roughly conical filtration net. The arms of E. simplex are extensively branched. Ten main arms bear unbranched ramules on alternate brachials, and the arm structure converges on the pinnulate pattern. Narrow food grooves and short covering plates are present. Analogies with living crinoids indicate that small food particles were caught by small and close-spaced tube-feet. The formation of new plates and ramules at the arm tips increases the size of the food-gathering system throughout ontogeny. The food-gathering capacity comprises the number of food-catching tube-feet times the width of the food grooves, and it measures the number and size of food particles that can be caught. Both size and capacity of the food-gathering system are positively allometric compared to crown volume and the amount of tissue that must be nourished. This is mainly caused by the addition of new ramules at the arm tips, which generates an exponentially increasing plate supply rate. Examination of numerous specimens from various geographic and stratigraphic horizons with multivariate statistics shows that the species was homogeneous throughout its range aside from the differences in living habits mentioned above.

The British Ordovician crinoid fauna

The British Ordovician crinoid fauna

Late Ordovician extinction of North American and British crinoids

After reaching a diversity peak in the Caradocian, North American Ordovician crinoids underwent a gradual decline to a nadir in the early Ashgillian (Maysvillian). This interval, recording extinction of the Cleiocrinidae, Merocrinidae, Ottawacrinidae, Hybocystitidae, and several lineages of camerate crinoids, was apparently caused by major environmental shifts in seas of eastern North America resulting from a westward-prograding wedge of terrigenous clastics derived from the Taconic Highlands, possibly coupled with a marine transgression in the Maysvillian that allowed colder water slope biofacies to invade the craton. Crinoids suffered a major episode of extinction in the late Ashgillian (late Richmondian/Rawtheyan). This event, preceding the end of the Ordovician by at least one stage or 2 to 4 million years, resulted in extinction of 12 families of crinoids including the Xenocrinidae, Tanaocrinidae, Reteocrinidae, Archaeocrinidae, Anthracocrinidae, Cincinnaticrinidae, Iocrinidae, Anomalocrinidae, Carabocrinidae, Cupulocrinidae, Porocrinidae, and Hybocrinidae. Glacio-eustatic lowering of sea level may have triggered this crisis by partially draining the North American craton, resulting in changes in oceanic circulation, salinity, and temperature. Latest Ordovician (Hirnantian) carbonates of the North American mid-continent region contain pelmatozoan assemblages from which Silurian crinoids radiated. These taxa were largely unaffected by a minor extinction event at the Ordovician/Silurian boundary.

Ramseyocrinus (Crinoidea) from the Arenig of Morocco

The discovery of the arms of the disparid inadunate crinoid Ramseyocrinus Bates from the Arenig (Lower Ordovician) of Morocco extends the geographic range of this genus, previously only known from south Wales, United Kingdom, and the Montagne Noire, France. It is probable that the Moroccan specimen represents a new species. This is the first Lower Ordovician crinoid to be described from Africa.

Pycnocrinus altilis, a new Late Ordovician channel-dwelling crinoid from southern Ontario

Pycnocrinus altilis n. sp. is described from the Upper Ordovician Georgian Bay Formation of Ontario. Closely related to P. dyeri (Meek), 1872, the cup of P. altilis is characterized by stellate ornamentation of plates comprising ridges bounded by deep subtriangular pits. Pycnocrinus altilis inhabited submarine channels in the prodeltaic regime of the Queenston deltaic complex and is inferred to have been anchored to the substrate by a distally coiled column.

The crinoids of the Al Rose Formation (Early Ordovician, Inyo County, California, U.S.A.)

Upon re-examination Proexenocrinus inyoensis Strimple & McGinnis, the oldest camerate crinoid known, is interpreted as a diplobathrid camerate assigned to the family Rhodocrinitidae rather than a monobathrid assigned to the Tanaocrinidae, as suggested by previous workers. The slab containing the holotype of P. inyoensis also has two undescribed crinoid specimens. These new Early Ordovician crinoids are described and assigned to Inyocrinus strimplei gen. et sp. nov. These specimens are from the Al Rose Formation of west-central California, U.S.A. (upper Ibexian; Arenigian).

Ontogeny ofHybocrinus punctatus (Miller and Gurley), an ordovician crinoid

Relative growth of the Ordovician crinoid Hybocrinus punctatus (Miller and Gurley)was studied using regression, correlation, and R-mode factor analysis. Systematic growth gradients of both initial intercepts and slopes existed throughout the calyx. Development of a single plate was best correlated with growth of the directly adjacent plates and was less well correlated with that of more distant plates. A total of 86 percent of the R-mode correlation matrix variance is associated with overall size increase of all dimensions and size-correlated shape changes. Most of the remaining variance is linked with divergent growth of small groups of specimens with respect to various shape parameters.

Review: Fossil Homalozoa and Crinozoa

Abstract Volume S of this series comprises an introduction to echinoderms and systematic descriptions of fossil Homalozoa and Crinozoa. Following the standard editorial preface, the introductory section contains two papers of importance to all students of the Echinodermata, whether dealing with fossil or recent material. The first article, by Ubaghs, is an essay on the general characters of the phyllum, and although it is only just over 50 pages in length it is quite remarkable in scope and detail. Several planes of bilateral symmetry are recognised—to the dismay of those only. conversant with the Lovenian echinoid orientation—and those of Loven and Carpenter are correlated differently to the interpretation previously given in this series by Fell and Moore (1966). Ubaghs regards all homologies based on the various planes of reference as not proven and also regards Fell's analogy betwen somasteroid virgulia and crinoid pinnulate arms as rather superficial. He notes that associated Lower Ordovician crinoids...