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Following Fortey and Owens (1987), the Ordovician trilobite taxon Ellipsotaphrinae is established as valid, but is argued to have full family status within the Cyclopygoidea. It encompasses the existing genera Ellipsotaphrus, Girvanopyge Gamops and Circulocrania together with two new genera proposed herein, Arisemolobes and Synaptotaphrus. Typical ellipsotaphrid genera have a totally circumscribed ‘foreglabella’, incorporating extended S1 furrows and a portion of the occipital furrow. The known range of the family is Floian to Katian. Genera are conservative in form throughout their ranges and are widespread. All occur only in deeper water sediments with palaeooceanic access. Ellipsotaphrus monophthalmus and Ellipsotaphrus infaustus are reassessed and Ellipsotaphrus zhongguoensis, from the Katian of China, is regarded as a junior synonym of the Katian Girvan species Ellipsotaphrus pumilio. Girvanopyge [ = Cremastoglottos; Nanlingia; Waldminia] is demonstrated, partly on the basis of new material from the Katian of Girvan, to be an ellipsotaphrid cyclopygoid and not to have a close affinity to the remopleuridids, as had been clained. Girvanopyge barrandei, from the Katian of the Czech Republic, is synonymised with Girvanopyge caudata from China. Gamops is revived for forms showing a relationship to both Girvanopyge and Ellipsotaphrus. It encompasses three Czech species including the Dapingian Gamops triangulatus, which probably also occurs in correlative strata in South Wales. The systematic treatment is supported by new material from the Upper Ordovician of the Girvan district, and the relevant geology of this area is described in detail. New species proposed are: Arisemolobes zhouzhiyii, Synaptotaphrus oarion and Circulocrania ? dichaulax.
Known antennae of trilobites are all flagelliform, in marked contrast to the varied first antennae of marine pancrustaceans. An exceptionally preserved specimen of the Early Ordovician (late Tremadocian) asaphid Asaphellus tataensis Vidal, 1998, from the Fezouata Shale in Morocco, exhibits both antennae in situ; they are relatively short, widen distally, and bear a series of round, dome-shaped organs along both their dorsal and ventral surfaces. These organs are vastly larger than chemo- or mechanosensory sensilla on the antennae of other arthropods, rendering their homology and function uncertain. The clavate antennae of Asaphellus reveal the furthest deviation from the inferred ancestral state of the trilobite antenna known to date. This could be an adaptation for detection of prey, because most Asaphidae have been claimed as predators and scavengers on the basis of specialized features of the calcified exoskeleton.
The tiny Ordovician trilobite Oenonella Fortey, 1980 has previously been known only from cranidia and pygidia of its Middle Ordovician type species. Two new species, Oenonella wasisnamei and O. otherfellersorum, from the Darriwilian Table Cove Formation, western Newfoundland, Canada, are known from silicified material that provides new information on the librigena, ventral morphology, and intraspecific variation within the group. The stratigraphic range of the genus is extended downward by an occurrence in the Floian of the Shallow Bay Formation, western Newfoundland. New knowledge of Oenonella confirms its likely phylogenetic relationship with Proscharyia Peng, 1990, known from the Tremadocian of South China. In addition, the overlooked Amechilus Ross, 1951, seems to belong to the same group. Both Oenonella and Proscharyia have in the past been assigned to the aulacopleuride family Scharyiidae Osmólska, 1957. New silicified material effectively rules out this possibility. The broader affinity of the group remains obscure, but together the three genera are assigned to the new family Oenonellidae.
Aglaspidid euarthropods are usually preserved as flattened carapaces. We here describe a new species and genus, Gogglops ensifer, from the Ordovician Shaanxi Province, China, which shows the best three-dimensional (3D) preservation yet discovered. We show that the cuticle was originally composed of calcium carbonate, except for the visual surfaces of the eyes which were probably originally organic but have been selectively phosphatized. A review is given of the problems pertaining to the original nature of the aglaspidid cuticle. Gogglops is probably closely related to certain poorly known aglaspidid genera from Ordovician deposits of the Siberian Platform. Phylogenetic analysis indicates that the new genus is closely related to Aglaspis itself, and securely nested within the clade Aglaspidida.
Lower Ordovician sections in the type Ibexian area of western Utah contain a considerably more diverse trilobite fauna than has previously been reported. Reinvestigation of these faunas, based on new field sampling, allows a reassessment of the dimeropygid genera Ischyrotoma Raymond, 1925, and Dimeropygiella Ross, 1951. These taxa have been considered synonyms, but parsimony analysis indicates each is a well supported clade, and they are best recognized as sister genera. The number of species known from Ibex has been doubled, from four to eight, and morphological information is now available for most parts of the exoskeleton. New species include Ischyrotoma juabensis (Juab Formation), I. wahwahensis (Wah Wah Formation), Dimeropygiella fillmorensis (Fillmore Formation), and D. mccormicki (Fillmore Formation). The previously named species Dimeropygiella caudanodosa, D. blanda, and D. ovata are fully revised on the basis of abundant new material. Pseudohystricurus is a paraphyletic group, with species distributed as a basal grade of the Ischyrotoma/Dimeropygiella group.
Protaspides and later growth stages are described and discussed for the Ordovician trinucleid trilobite Cryptolithus tesselatus Green and the raphiophorid Lonchodomas chaziensis Shaw. A small triangular rostral plate is described for a single protaspid instar in both Cryptolithus and Lonchodomas. The presence of this small sclerite in the ontogeny of these taxa supports the origin of the Trinucleoidea from the Ptychopariida through reduction in and then fusion or loss of the rostral plate. Earlier and later growth stages have fused ventral sclerites, with no signs of connective sutures. All members of the Trinucleoidea with known ontogenies share similar small, ovoid asaphoid protaspides, with distinct axes and varying numbers of sharp, conical to subtubular, submarginal spines on the dorsal exoskeleton and marginal spines on the hypostome. Two protaspid instars are identified in both Cryptolithus and Lonchodomas, sharing many characters that indicate homologous levels of development. A single origin for the median suture of the Asaphida is supported, with an anterior rostellum as its precursor.
Sections in the Basin Ranges provide stratigraphic standards for the Ordovician of the U.S., from which zones based on trilobites have been widely employed in regional correlation. This paper describes new trilobite faunas from a poorly known part of the succession, at the base of the Middle Ordovician (Whiterockian) in western Utah and eastern Nevada, and including the type Ibexian section near Ibex, Utah. The faunas are from the topmost Wahwah, and overlying Juab Limestone Formations, in strata equivalent to Zone L. Although well-preserved, none of the trilobites are silicified. All are typical of inshore carbonate, Bathyurid biofacies. The majority are new species congeneric with, but different from species from the overlying Kanosh Formation. The dominant bathyurid is the hitherto little-known Psephosthenaspis, after which the new Zone is named. A threefold subdivision of the Psephosthenaspis Zone is based on successive species of the genus. The lower two of these are probably equivalent to the Valhallan Stage, described from more offshore biofacies from Spitsbergen. Eighteen species are described, of which nine are formally named as new including: Goniotelina ensifer, Petigurus inexpectatus, Psephosthenaspis microspinosa, M. glabrior, Pseudoolenoides aspinosus, Ectenonotus progenitor, Kanoshia reticulata and Pseudomera arachnopyge. The type species of a remarkable new illaenid-like bathyurelline, Madaraspis magnifica gen. et sp. nov., is described.
Exuviae comprising immature growth stages allow the tracing of morphological and morphogenetical features of the reduced-eyed phacopid species Plagiolaria poothaii Kobayashi and Hamada, 1968, from the Early Devonian of Satun Province, southern Thailand. Biometric and morphometric approaches have been used to characterize the shape changes. This is the fourth phacopid genus for which ontogenetic information is known. Ontogenetic features of this species are generally similar to those of Phacops and Weyerites. However, the ankylosis of the facial sutures occurs at the end of the meraspid period, later than in Phacops, and this is probably a derived character.
Evidence that can be used to interpret the life habits of extinct organisms usually takes three forms: functional analysis, analogy with living organisms, and geological evidence. Independent quantitative tests for habit are rarely available. A theory of optimum eye design originally derived for living aquatic arthropods provides quantitative data that are used to test previous suppositions about the life habits of two Ordovician pelagic trilobites: that the telephinid trilobite Carolinites was epipelagic while the cyclopygid Pricyclopyge was mesopelagic. Optimum compound eye design theory uses measured lens diameters and interommatidial angles to determine the “eye parameter” (p), which can be used to gauge approximate optimum level of illumination for the eyes of these trilobites. The eye parameter provides an independent test for their relative paleobathymetry. Values of the eye parameter measured in the dorso-ventral direction across two eyes of Carolinites killaryensis utahensis were found to have medians of 2.13 and 3.24. Values measured in the antero-posterior direction have medians of 3.17 and 4.86 for the two eyes. Values measured in the dorso-ventral direction across two eyes of Pricyclopyge binodosa have medians of 4.23 and 4.98, while values measured in the antero-posterior direction have medians of 7.06 and 8.31. Eye parameters are higher in Pricyclopyge than in Carolinites, the difference statistically significant at p = 0.05. The eyes of Pricyclopyge are optimally designed for lower levels of illumination than are those of Carolinites. This accords with the previous interpretations of the former trilobite as mesopelagic and the latter as epipelagic.
Proetid trilobites of Lower Devonian (Pragian) age are known principally from fragmentary material from the type area in the Czech Republic and Morocco. We describe well-preserved, articulated specimens from the Ihandar Formation of the Anti-Atlas, Morocco, which include species of previously little known genera, and help to clarify their morphology. Eight new species are described: Dalejeproetus sagaouii, D. owensi, Lepidoproetus maharchianus, L. lahceni, L. splendens, Podoliproetus mirdani, P. sinespina, and Proetina ihamadii.
The progress achieved in trilobite systematics over the last 75 years is briefly reviewed. Different approaches to phylogenetics have influenced the way trilobites have been classified. Classical evolutionary taxonomy, the stratigraphical approach, and cladistics have all contributed in different ways to the current classification, which has evolved piecemeal, and is still unsatisfactory is some ways. Nonetheless, progress towards a phylogenetic classification has been made, especially as the result of information from ontogenies provided by well-preserved silificified material. Trilobites are a well-defined clade within a larger arachnomorph group. Agnostida have been excluded from Trilobita, but are perhaps best considered as specialised trilobites, at least until limbs of eodiscids are described. The outstanding problems in classification of each trilobite order are reviewed. Most are concerned with the recognition of the appropriate Cambrian sister taxa, and the discovery of the relevant ontogenies. It is very likely that post-Cambrian clades “root” deeply into the Cambrian. The coherence, or otherwise, of Proetida, Asaphida, Corynexochida and the lichid/odontopleurid groups will be resolved by such studies. The problems of paraphyly in Ptychopariida and Redlichiina may prove more obdurate. The temporal brevity of certain Cambrian family ranges may be partly a taxonomic artefact. The possibility of a late Cambrian gap in the record on some clades should be considered.
No trilobite species, and very few genera, pass from the Lower (Ibexian) into the Middle Ordovician (Whiterockian), which is a turning point in Laurentian trilobite history. Trilobites from three sections exposing the base of the Whiterock Series (basal Middle Ordovician) in Nevada are described and illustrated. They are attributable to different, and generally more open-shelf, biofacies from the Bathyurid biofacies trilobites described from the Ibex area, Utah, by Fortey and Droser (1996), but include species in common, which allow correlation into the Ibexian type section. At Little Rawhide Mountain the basal Middle Ordovician is developed in Olenid biofacies, described for the first time in western North America from rocks of this age. Correlation based on species-level similarity shows that the “spike” that has been used to define the type base of the Whiterockian (and hence the Middle Ordovician) at Whiterock Canyon is at a level younger than the base of the Whiterockian assumed in recent discussions of its international correlation. The type Whiterock base correlates with the Psephosthenaspis glabrior trilobite Subzone at Ibex, well above the major change in trilobite faunas at the base of the Psephosthenaspis Zone (P. microspinosa Subzone). In all study sections there is an abrupt change of facies after trilobite Zone J, possibly associated with regression. Four new species are described: Cloacaspis tesselata, Harpillaenus rossi, Acidiphorus? lineotuberculatus, and Benthamaspis serus.
The distributions of trilobite species were controlled by a combination of habitat preferences and paleogeographic constraints, which tend to limit their extent. The Lower Ordovician trilobite Carolinites genacinaca Ross, 1951, however, had a remarkable range unequaled among polymerid trilobites; it has been recognized on all Ordovician paleocontinents. Its distribution has been explained by an epipelagic mode of life, based on evidence from functional morphology, analogy with modern pelagic crustaceans, and geological occurrence. In such a case, morphological identity throughout its range might be anticipated, if all occurrences can be postulated to be members of a single pandemic population. Rotational superimposition has been used to compare variation within samples drawn from Alberta, Spitsbergen, and Australia with a benchmark population from the western United States. All are morphometrically similar. By any criterion, specimens identical to the benchmark population are found within the Alberta, Spitsbergen and Australia samples, which represent the extremes of the species' geographic range. A lone cranidium from France, previously referred to Carolinites vizcainoi, may be a juvenile of C. genacinaca or C. tasmaniensis; its differences are consistent with ontogeny. A small number of specimens from Siberia and central China show differences in cranidial proportions from the Utah specimens that may be the result of preservational factors and/or photographic technique, or may represent genuine morphological disparity; this could be clarified if more specimens were to become available. This study suggests that C. genacinaca was ubiquitous in the epipelagic environment in a belt that encircled the planet between paleolatitudes of approximately 30°N and 30°S.
Despite substantial advances in plate tectonic modeling in the last three decades, the postulated position of terranes in the Paleozoic has seldom been validated by faunal data. Fewer studies still have attempted a quantitative approach to distance based on explicit data sets. As a test case, we examine the position of Avalonia in the Ordovician (Arenig, Llanvirn, early Caradoc, and Ashgill) to mid-Silurian (Wenlock) with respect to Laurentia, Baltica, and West Gondwana. Using synoptic lists of 623 trilobite genera and 622 brachiopod genera for these four plates, summarized as Venn diagrams, we have devised proportional indices of mean endemism (ME, normalized by individual plate faunas to eliminate area biogeographic effects) and complementarity (C) for objective paleobiogeographic comparisons. These can discriminate the relative position of Avalonia by assessing the optimal arrangement of inter-centroid distances (measured as great circles) between relevant pairs of continental masses. The proportional indices are used to estimate the “goodness-of-fit” of the faunal data to two widely used dynamic plate tectonic models for these time slices, those of Smith and Rush (1998) and Ross and Scotese (1997). Our faunal data are more consistent with the latter model, which we use to suggest relationships between faunal indices for the five time slices and new rescaled inter-centroid distances between all six plate pairs. We have examined linear and exponential models in relation to continental separation for these indices. For our generic data, the linear model fits distinctly better overall. The fits of indices generated by using independent trilobite and brachiopod lists are mostly similar to each other at each time slice and for a given plate, reflecting a common biogeographic signal; however, the indices vary across the time slices. Combining groups into the same matrix in a “total evidence” analysis performs better still as a measure of distance for mean endemism in the “Scotese” plate model. Four-plate mean endemism performs much better than complementarity as an indicator of pairwise distance for either plate model in the test case.
Gould's Wonderful Life (1989) was a landmark in the investigation of the Cambrian radiation. Gould argued that a number of experimental body plans (“problematica”) had evolved only to become extinct, and that the Cambrian was a time of special fecundity in animal design. He focused attention on the meaning and significance of morphological disparity versus diversity, and provoked attempts to quantify disparity as an evolutionary metric. He used the Burgess Shale as a springboard to emphasize the important role of contingency in evolution, an idea that he reiterated for the next 13 years. These ideas set the agenda for much subsequent research. Since 1989 cladistic analyses have accommodated most of the problematic Cambrian taxa as stem groups of living taxa. Morphological disparity has been shown to be similar in Cambrian times as now. Konservat-Lagerstätten other than the Burgess Shale have yielded important new discoveries, particularly of arthropods and chordates, which have extended the range of recognized major clades still further back in time. The objective definition of a phylum remains controversial and may be impossible: it can be defined in terms of crown or total group, but the former reveals little about the Cambrian radiation. Divergence times of the major groups remain to be resolved, although molecular and fossil dates are coming closer. Although “superphyla” may have diverged deep in the Proterozoic, “explosive” evolution of these clades near the base of the Cambrian remains a possibility. The fossil record remains a critical source of data on the early evolution of multicellular organisms.
We have produced computer simulations of multiramous graptoloids with the intention of defining the rules governing branching strategies and colony form. Close matches between such simulations and real graptolites show that complex rhabdosomes may be produced by the permutation of relatively simple sets of rules. Those designs found in nature produce an efficient and regular distribution of zooids through the area included by an essentially planar rhabdosome. Strikingly geometrical arrays of stipes, such as the Goniograptus and yin/yang patterns, closely approach paradigmatic harvesting arrays. For dichotomously branching anisograptids the evolutionary trend in reduction of “primary stipes” can be explained by the production of larger spreading colonies. Multiramous graptoloids fed during vertical movement through the water column. Changes in a single branching decision may produce considerable changes in rhabdosome morphology, but these are not necessarily of high taxonomic importance; this is proved by a specimen which is a morphological combination of two “genera.” Although primarily under genetic control, certain modifications to colony form were probably the result of inhibitory interaction between adjacent stipes.
The geometry of the simpler types of graptolite stipes can be quantified by assuming them to consist of stacked cylinders representing the thecal tubes. It can be shown that specification of any three characters, for example, thecal inclination, thecal spacing, and stipe width, effectively defines the others (thecal overlap, thecal length and width, thecal “density”); hence the parameters commonly used in the definition of species are not independent. Variation within and between species can be represented by three-character plots. The effects of altering the apertural angle from 90° on such characters as thecal spacing can also be represented geometrically. If thecae are considered as cones rather than cylinders, it follows that as stipe width increases, thecal curvature will describe a sine curve if the thecae are to remain in contact; this observation is matched on real graptolites. Thecal curvature beyond 90° to the dorsal wall is generally impossible without thecae detaching from their neighbors to produce thecal isolation. Growth of thecae along a stipe can be represented graphically. Computer fits to growth curves for five dichograptids show that they can be closely described by an exponential decrease in growth increments along the stipe, and that this model is more accurate than two possible alternatives.
The longevity of genera living at the edge of the early Ordovician North American plate is related to their position on the former shelf, that is preserved in richly fossiliferous sections of Spitsbergen. Genera which endure for a long period (SO Myr or more) tend to be confined to shallow-water sites associated with algal mounds at the edge of the carbonate platform (illaenid-cheirurid community type) or deep-water sites with low oxygen concentration, and probably beneath the thermocline (olenid community type). The latter includes “conservative” forms that have survived from the Cambrian. The long ranges of the olenid community type are attributed to the adaptation of only a few genera to a highly specific, stressed environment. The endurance of the illaenid-cheirurid genera is due to the persistence of the tropical shelf-edge habitat, with high predictability of resources, to which the Ordovician forms became adapted early on. The nileid community type, which occurred between the platform edge and olenid-bearing environment, was the site of rapid generic turnover and may have been the source for subsequent recruitment into other environments. The inner shelf also had rapid generic replacement, dominated by a single family, the Bathyuridae. Rapidity of evolution in this site may be due to the spatial heterogeneity of the epeiric environment, as in the Devonian phacopids studied by Eldredge (1974).