Robert J. Elias

THE WORLD'S BIGGEST TRILOBITE—ISOTELUS REX NEW SPECIES FROM THE UPPER ORDOVICIAN OF NORTHERN MANITOBA, CANADA

Abstract The largest known trilobite fossil, a virtually complete articulated dorsal shield of the asaphid Isotelus rex new species, has been recovered from Upper Ordovician (Cincinnatian, Richmondian) nearshore carbonates of the Churchill River Group in northern Manitoba. At over 700 mm in length, it is almost 70 percent longer than the largest previously documented complete trilobite, and provides the first unequivocal evidence of maximum trilobite length in excess of one-half metre. Comparisons with other fossil and extant members of the phylum suggest that in terms of maximum linear dimensions it was among the biggest arthropods ever to have lived. Sediments of the Churchill River Group were deposited in an equatorial epeiric setting and the extremely large size of I. rex n. sp. thus marks a striking example of low-latitude gigantism, in sharp contrast to the widespread phenomenon of “polar gigantism” in many modern marine benthic arthropods. Lack of extensive epibiontic colonization of the exoskeletal surface and the presence of large distinctive trace fossils in the same unit suggest that I. rex n. sp. may have been a semi-infaunal predator and scavenger that employed a shallow furrowing and probing mode of benthic feeding. The extinction of the isotelines (and virtually the entire asaphide lineage) at the end of the Ordovician cannot be related to the near contemporaneous achievement of exceptionally large adult size in some representatives. Failure to survive the terminal Ordovician extinction event was most likely a consequence of a pelagic larval life-style that proved ill-adapted to the rapid onset of global climatic cooling and loss of tropical shelf habitats.

MICROBORINGS AND GROWTH IN LATE ORDOVICIAN HALYSITIDS AND OTHER CORALS

Microborings in the Late Ordovician tabulate corals Catenipora rubra (a halysitid) and Manipora amicarum (a cateniform nonhalysitid) and in an epizoic solitary rugose coral differ from nearly all of those previously reported in Paleozoic corals. These microborings were formed within the coralla by endolithic algae and fungi located beneath living polyps. Comparable structures in the Late Ordovician tabulate Quepora ?agglomeratiformis (a halysitid) represent algal microborings, not spicules, and halysitids are corals, not sponges as suggested by Ka2mierczak (1989). Endolithic algae in cateniform tabulates relied primarily on light entering through the outer walls of the ranks rather than through the polyps; lacunae within coralla permitted appropriate levels of light to reach many corallites. The direction of boring was determined by corallum microstructure and possibly also by the distribution of organic matter within the skeleton. There is an apparent inverse correlation between boring activity and coral growth rate. The location and relative abundance of pyritized microborings within calcareous coralla can be established quantitatively and objectively from electron microprobe determinations of weight percent sulfur along appropriate traverses of the coral skeleton. The distribution of such microborings in Catenipora rubra and Manipora amicarum is comparable to algal banding in moder corals; this is the first report of such banding in the interiors of Paleozoic corals. Change in the intensity of boring within each corallum was evidently a response to variation in the linear growth rate of the coral, or to fluctuation in an environmental factor (perhaps light intensity) that could control both algal activity and growth rate in these corals. Change in the algal boring intensity and linear growth rate of the coral was generally but not always seasonal and usually but not invariably associated with change in the density of coral skeletal deposition. Cyclic bands of boring abundance maxima within fossil colonial corals provide a measure of annual linear growth comparable to the widely accepted method based on skeletal density bands. Algal bands are more sporadically developed than density bands within and among coralla, thus increasing the difficulty of interpretation. Fluctuations in the abundance of algal microborings apparently provide a detailed record of changes in the linear growth rate of colonies and of individuals within colonies. Combined analyses of microboring abundance and skeletal density will contribute significantly to our understanding of the biological and environmental factors involved in endolithic activity and coral growth. ABSTRACT- Microborings in the Late Ordovician tabulate corals Catenipora rubra (a halysitid) and Manipora amicarum (a cateniform nonhalysitid) and in an epizoic solitary rugose coral differ from nearly all of those previously reported in Paleozoic corals. These microborings were formed within the coralla by endolithic algae and fungi located beneath living polyps. Comparable structures in the Late Ordovician tabulate Quepora ?agglomeratiformis (a halysitid) represent algal microborings, not spicules, and halysitids are corals, not sponges as suggested by Ka2mierczak (1989). Endolithic algae in cateniform tabulates relied primarily on light entering through the outer walls of the ranks rather than through the polyps; lacunae within coralla permitted appropriate levels of light to reach many corallites. The direction of boring was determined by corallum microstructure and possibly also by the distribution of organic matter within the skeleton. There is an apparent inverse correlation between boring activity and coral growth rate. The location and relative abundance of pyritized microborings within calcareous coralla can be established quantitatively and objectively from electron microprobe determinations of weight percent sulfur along appropriate traverses of the coral skeleton. The distribution of such microborings in Catenipora rubra and Manipora amicarum is comparable to algal banding in moder corals; this is the first report of such banding in the interiors of Paleozoic corals. Change in the intensity of boring within each corallum was evidently a response to variation in the linear growth rate of the coral, or to fluctuation in an environmental factor (perhaps light intensity) that could control both algal activity and growth rate in these corals. Change in the algal boring intensity and linear growth rate of the coral was generally but not always seasonal and usually but not invariably associated with change in the density of coral skeletal deposition. Cyclic bands of boring abundance maxima within fossil colonial corals provide a measure of annual linear growth comparable to the widely accepted method based on skeletal density bands. Algal bands are more sporadically developed than density bands within and among coralla, thus increasing the difficulty of interpretation. Fluctuations in the abundance of algal microborings apparently provide a detailed record of changes in the linear growth rate of colonies and of individuals within colonies. Combined analyses of microboring abundance and skeletal density will contribute significantly to our understanding of the biological and environmental factors involved in endolithic activity and coral growth.

Mode of growth and life-history strategies of a Late Ordovician halysitid coral

The upper surface of the corallum of Catenipora rubra was often at or just above the sediment-water interface during life. The vertical growth rate was barely sufficient to keep pace with background sedimentation and possible subsidence of the corallum. Therefore, the colonies were in constant danger of being covered by influxes of sediment, especially during storms. This was compensated by the ability of polyps to respond to sedimentation events and by certain aspects of colony growth. Rapid regeneration following partial mortality involved budding of uninjured polyps and rejuvenation of damaged individuals, in some cases accompanied by a type of axial increase not previously known in tabulate corals. Rapid lateral expansion was possible because small, “immature” polyps could bud and grow in a reptant manner. Interconnected ranks of the cateniform corallum served to dam shifting sediment at the periphery of the colony. Lacunae within the colony were reservoirs for material that breached peripheral ranks and for sediment that settled on the ranks and was rejected by polyps or removed by passive flow. Polyps comprising the colony were distributed over a large area of the substrate surface, thereby decreasing the probability of complete mortality during sedimentation events and increasing the probability that a sufficient number of individuals would survive to ensure optimum regeneration. The corallum, anchored in the substrate and with sediment filling the lacunae, provided a broad, stable base during high-energy events. It remains to be established how widespread these growth patterns and strategies were among other corals with cateniform colonies, a form that appeared in many unrelated stocks. Most previous workers emphasized physical strength when considering functional morphology, following a tacit assumption that the corallum rose high above the substrate and was therefore susceptible to breakage during high-energy events. An understanding of the origin of cateniform patterns and the phylogeny of these corals requires knowledge of their modes of growth and life-history strategies, which were genetically as well as environmentally controlled.

Temporal variations in tempestite thickness may be a geologic record of atmospheric CO2

Storm-bed (tempestite) thickness reflects, in part, storm intensity, which is related to the amount of atmospheric CO{sub 2}. Thus, variations in tempestite thickness through geologic time may record fluctuations of CO{sub 2}. Geologic criteria other than storm-bed data have been used to define specific intervals of time when the atmosphere was CO{sub 2} enriched (greenhouse phases) and CO{sub 2} depleted (icehouse phases). If tempestite thickness, storm intensity, and CO{sub 2} are casually linked, greenhouse phases should correspond to deposits of thick tempestites (more intense storms), and icehouse phases should be characterized by comparatively thin tempestites (less intense storms). Tempestite thickness data provide a test of the greenhouse-icehouse model, and initial results suggest general agreement with the independently derived climate (CO{sub 2}) curve for the latest Precambrian through Phanerozoic.

PALEOBIOLOGIC AND EVOLUTIONARY SIGNIFICANCE OF CORALLITE INCREASE AND ASSOCIATED FEATURES IN SAFFORDOPHYLLUM NEWCOMBAE (TABULATA, LATE ORDOVICIAN, SOUTHERN MANITOBA)

Abstract Saffordophyllum newcombae Flower, 1961, displays unique abilities and an unprecedented range in types of corallite increase. Cerioid growth was characteristic, but colonies on soft substrates could grow in a tollinaform manner during early astogeny. The capacity for recovery from damage and partial mortality is amazing. Rejuvenation may have been accompanied by peripheral expansion in some cases. Rapid regeneration could involve axial increase. Circular lacunae that formed during recovery became sites of rapid lateral increase or corallite decrease. Two types of axial increase occurred within coralla. Lateral increase was concentrated mainly along the basal wall and adjacent to certain circular lacunae. In typical cerioid parts of the corallum, lateral increase seldom yielded “adult” corallites, but incipient lateral offsets could be numerous. The level of colony integration was probably moderately high. There was likely soft-tissue continuity among polyps, coordination of polyp behavior, subjugation of individuals for the good of the colony, and perhaps astogenetic control. Saffordophyllum newcombae is considered to be a tabulate coral, although one type of axial increase is similar to that in a few rugose corals and the other type of axial increase as well as possible peripheral expansion resemble modes of increase in some coralline sponges. Lateral increase is considered compatible with cnidarian rather than poriferan biology. Corallite size is typical of tabulates. Saffordophyllum may not be the direct ancestor of favositid tabulates, and may not even be closely related to them; S. newcombae is very different from Paleofavosites and Favosites. The remarkable range in forms of increase discovered in S. newcombae demonstrates the critical need for detailed paleobiologic studies, if we are to understand the early evolutionary history of corals and to establish reliable criteria for distinguishing various coral groups and homeomorphs.

Life-history strategies of a species of Catenipora (Tabulata; Upper Ordovician; southern Manitoba, Canada)

Detailed study of coralla by transverse serial sections permits the determination and evaluation of life-history strategies (survival and growth characteristics) in response to different physical environments, for Catenipora foerstei Nelson, 1963 from the Selkirk Member, Red River Formation, in Manitoba. We recognize various modes of corallite increase: one type of axial increase, four types of lateral increase, and agglutinated patches of corallites in association with normal, undamaged corallites; and one type of axial increase, one type of lateral increase, and temporary agglutinated patches from the recovery processes of corallites damaged by sediment or bioclast influx. In addition, the formation of new ranks by lateral increase is the most effective method for rapid growth of a corallum or for reconstructing part of a corallum damaged by physical disturbances. Fluctuations in the tabularial area of corallites occur in cycles over vertical intervals ranging from 3.20–7.90 mm. We consider each cycle to represent annual growth. Average annual growth of the three coralla ranges from 4.20–6.27 mm. According to correlations between annual growth cycles and other growth characteristics, a high frequency of offsetting is associated with rapid vertical growth. Specifically, annual growth is relatively high in association with episodes of sediment or bioclast influx, probably generated by storms. In some coralla, however, annual growth is highest in the cycle characterized by few new corallites or by extraordinarily high rates of offsetting by normal, undamaged corallites as well as damaged corallites. This suggests that vertical growth could also be affected by factors other than storm-related disturbance.

PALEOBIOLOGIC FEATURES OF TRABECULITES MACULATUS (TABULATA, LATE ORDOVICIAN, SOUTHERN MANITOBA)

Abstract Detailed analysis of certain growth characteristics in Trabeculites maculatus contributes to an understanding of the paleobiology and phylogeny of early tabulate corals. Some coralla of T. maculatus contain peculiar, vertically oriented cylindrical lacunae (open areas) that are lenticular, or in one case circular, in cross section. The nature of these structures and their relation to adjacent corallites suggest that they were formed by the coral in response to soft-bodied biotic associates of unknown taxonomic affinity. Trabeculites maculatus is an unusual tabulate coral featuring both axial and lateral modes of corallite increase. Axial increase was common, often occurring in association with rejuvenation following injury and less commonly involving normal, undamaged corallites. Lateral increase of normal corallites was typical, but this form of increase could also be involved in the termination of lacunae and occurred in response to a divergent growth pattern around the circular lacuna. Corallite decrease was fairly common, usually taking place adjacent to lenticular lacunae but in some cases involving normal corallites not associated with lacunae. Corallite fusion was uncommon; it could be either temporary or permanent. Conspicuous relocation of corallites and restructuring of corallite arrangement generally involved mass rejuvenation and/or regeneration, usually over a large surface area of the corallum. The growth features in T. maculatus are fundamentally the same as those in the co-occurring Saffordophyllum newcombae, including types of axial increase unknown in other tabulate corals. The basic paleobiologic similarity of these species supports the interpretation that the genera they represent are closely related phylogenetically. The relationship of these taxa to other tabulates, however, remains unresolved.

Relationships between internal and external morphology in Paleofavosites (Tabulata); the utility of growth and growth form

During growth of colonial corals, the basic organization of skeletal elements was determined by inherent factors, but arrangement of corallites within a colony could be affected if environmental change induced a modified growth form. Comparisons of internal and external characters during colony development indicate how environmental and genetic factors determined growth form. The results of these comparisons have implications for understanding of colony integration, functional morphology, and systematics. This study is based on serially sectioned coralla of the cerioid tabulate Paleofavosites subelongus, from the uppermost Ordovician to lowermost Silurian of the east-central United States. Colony growth from resulted from changes in maximum growth angle of marginal corallites, and in the shape of the growth surface. These features were coordinated with corallite characters and were dependent on variation in corallite growth. At the same time that a colony became broader by expanding its maximum growth angle and developing a taller growth surface, its corallites became larger, more new corallites were initiated, and recently initiated corallites expanded more rapidly. When a colony9s maximum growth angle was reduced and the growth surface became flatter, corallites also became smaller, fewer corallites were initiated, and those corallites that were recently initiated expanded slowly. Genetic constraint of growth is illustrated by consistent patterns of initial colony growth, and by relationships among characters of internal and external morphology. Frequent small-scale variations in growth angle and growth surface height: width during astogeny indicate fluctuating environmental factors. Sedimentation and subsidence of the colony were probably the major environmental controls on form.

Corallite Increase and Mural Pores in Lichenaria (Tabulata, Ordovician)

Abstract Lichenaria may be a representative of the most primitive stock of tabulate corals. The degree of paleobiologic complexity discovered in L. globularis and L. grandis is therefore surprising. Six types of corallite increase are recognized. All are lateral, which is the predominant mode in tabulates. Most types, however, are unique or are comparable to those in few other Ordovician taxa. Only Type 1 (L. globularis), yielding a single offset with a simple basal mural pore, is typical of tabulates. In Type 2 (L. globularis), one parent produces two offsets simultaneously, whereas in Type 3 (L. globularis), two offsets arise from separate parents at nearly the same time and join via a connective mural pore. Types 4 (L. globularis, L. grandis), 5 (L. grandis), and 6 (L. globularis, L. grandis), respectively, involve one, two, and two to four corallites in addition to the parent, which join via a connective mural pore at the site of offsetting. Several features of L. globularis and L. grandis point to unexpectedly high levels of colony integration. Continuously fused common walls lacking back-to-back epithecae suggest soft tissue continuity among polyps above the corallum. Connective mural pores indicate temporary fusion of polyps. Coordinated behavior of polyps is suggested by the development of conjoined offsets from two parents during Type 3 increase, and by fusion during Types 4 to 6 increase. Attempts at certain types of increase sometimes failed to yield offsets, suggesting expendability of incipient buds, perhaps reflecting subjugation of individuals for the good of the colony. In light of this study, genera that have previously been included in Lichenariidae and Lichenariida require reassessment and their phylogenetic relationships should be reconsidered. Unfortunately, this is hindered because fundamental characters such as corallite increase and wall structure remain inadequately known in most early tabulates.

MORPHOMETRICS OF MANIPORA (TABULATA; UPPER ORDOVICIAN; SOUTHERN MANITOBA, CANADA)

Abstract Multivariate morphometric analysis was applied for differentiation of closely related species and evaluation of intra- and interspecific variation in Manipora from the Selkirk Member, Red River Formation, in southern Manitoba. Seven morphological characters were quantified in transverse thin sections of 46 coralla and statistically tested for selecting effective characters in discriminating species. Cluster analysis was performed on a raw data matrix coordinated with 46 coralla by three selected characters. Two major clusters on the resulting dendrogram were regarded as morphospecies, following comparative examination of the coralla using serial sections. Cluster analyses were also conducted on principal component score matrices obtained from the raw data set coordinated with 46 coralla by all seven characters, and from an experimental data set including the 46 coralla plus two replicates of each and six of the characters. The results agree closely with the first cluster analysis, but discrimination of morphospecies was slightly degraded. The validity of two morphospecies recognized in the first cluster analysis was verified by discriminant analyses, descriptive statistics, and bivariate plots. The results show that tabularium area is the most meaningful character for distinguishing these morphospecies; ranges of variations of the other six characters overlap between morphospecies. Another cluster analysis like the first was performed, but with the addition of 11 type specimens and reference coralla of Manipora species from the Upper Ordovician of southern and northern Manitoba and Texas. Based on this analysis, together with comparative examination of thin sections, the two morphospecies are identified as valid species: M. amicarum Sinclair, 1955 and M. manitoba (Sokolov, 1955). Manipora magna Flower, 1961 is considered to be a synonym of M. amicarum, while M. trapezoidalis Flower, 1961 and M. garsonensis Caramanica, 1992 are considered to be synonyms of M. manitoba, and the hypotypes of M. amicarum of Nelson (1963) are assigned to M. manitoba.