Joseph F. Pachut

Patterns of bryozoan endemism through the Ordovician-Silurian transition

Abstract Area cladograms produced by parsimony analysis of endemicity illustrate historically developed biogeographical associations among Caradocian, Ashgillian, Llandoverian, and Wenlockian bryozoans. Areas in North America, Siberia, and Baltica were organized into three provinces and 12 biomes over a time interval of 35 million years. Six of these biomes belonged to the North American-Siberian Province and became extinct during the Ashgillian. Three biomes represent a successional series of biogeographical associations in the Late Ordovician of Baltica, and the middle biome of this succession is most closely related to that of the Wenlockian platform in North America. All four Silurian biomes are represented in Late Ordovician local areas, indicating that the associations important in the recovery radiation were already in existence prior to the extinction events. Three of these four biomes expanded their geographic extent in the wake of the Late Ordovician extinctions. Several biome extinction and replacement events took place during lowstands of sea level, suggesting that biogeographic reorganizations took place as a consequence of habitat loss in epeiric seas. Biome development largely depended on the extent of major lithotopes and their intersections with deep ocean and climatic barriers. The loss of regional habitats, associated with marine regression, was a key factor in biome extinction and reorganization, and indicates that biogeography played a significant role in the Late Ordovician mass extinctions and Silurian recovery radiations. Vicariance hypotheses are needed to account for the development of barriers subdividing ancestral areas, whereas hypotheses of congruent dispersal are required to explain the appearance of biomes in geographically disjunct areas.

The concepts of astogeny and ontogeny in stenolaemate bryozoans, and their illustration in colonies of Tabulipora carbonaria from the Lower Permian of Kansas

JOSEPH F. PACHUT, ROGER J. CUFFEY, AND ROBERT L. ANSTEY Department of Geology (Cavanaugh Hall), Indiana University-Purdue University at Indianapolis, 425 University Boulevard, Indianapolis 46202, Department of Geosciences (Deike Building), Pennsylvania State University, University Park 16802, and Department of Geological Sciences (Natural Science Building), Michigan State University, East Lansing 48224

Diversity and Distribution of Triassic Bryozoans in the Aftermath of the End-Permian Mass Extinction

Abstract Seventy-three species of stenolaemate bryozoans are documented worldwide from the Triassic. Stage-level diversity and paleogeographical analyses reveal that the recovery of bryozoans following the end-Permian mass extinction was delayed until the Middle Triassic. Early Triassic bryozoans faunas, dominated by members of the Order Trepostomida, were depauperate and geographically restricted. Bryozoan diversity increased during the Middle Triassic and diversity peaked in the Carnian (early Late Triassic). High extinction rates throughout the Late Triassic led to the extinction of all stenolaemate orders except the Cyclostomida by the end of the Triassic. Comparisons between global carbonate rock volume, outcrop surface area, and bryozoan diversity indicate that the documented diversity pattern for bryozoans may have been related, in part, to the availability of carbonate environments during the Triassic.

CLADISTIC AND PHENETIC RECOGNITION OF SPECIES IN THE ORDOVICIAN BRYOZOAN GENUS PERONOPORA

Abstract We compare cladistic, phenetic, stratophenetic, and typological methods of recognizing species within a single well-sampled, long-lived Ordovician bryozoan genus, Peronopora Nicholson, 1881. A consensus of 11 methods recognizes 14 species within Peronopora, each of which has both cladistic and phenetic support, and support by at least four different methods. Comparison of methods was based on: 1) the number of species recognized; 2) the degree of splitting of consensus groups; 3) consistency in determining first and last appearance datums, and stratigraphic ranges; 4) consistency in the number of specimens assigned to species; 5) stability in recognizing geographic distributions; and 6) correlations with character heritability. The five methods most closely approaching consensus were: 1) Ward clustering on Euclidean distance; 2) K-means splitting; 3) Ward clustering on correlation coefficients; 4) cladistic parsimony; and 5) cladistic parsimony plus iterative discriminant analysis. The three methods farthest from consensus were: 1) stratophenetics; 2) average linkage on Euclidean distance; and 3) conventional typology. Cladistic parsimony is the only method that can recognize all 14 consensus clusters. We argue that the overall advantages of parsimony analysis outweigh the merits of the various phenetic approaches in recognizing paleontological species. We also argue that given sufficient allochrony in sampling, cladistic structure is detectable both within and among species. Recognizing species based on fixed character states would require at least 72 species in our material. The crown groups of Pachut and Anstey (2002) are herein recognized as monophyletic species in the sense of Mishler and Theriot (2000), one of which we designate as a new species, P. browni. Six of our eight nonmonophyletic stem groups are recognized in this paper as metaspecies in the sense of Donoghue (1985) and Gauthier et al. (1988). Two of our stem groups are phenetically indistinguishable from phyletically contiguous crown groups, and we attribute the failure of parsimony analysis to recognize the monophyly of these groups to homoplasy.

PHYLOGENY, SYSTEMATICS, AND BIOSTRATIGRAPHY OF THE ORDOVICIAN BRYOZOAN GENUS PERONOPORA

Abstract Specimens of Peronopora Nicholson, 1881, are abundant in Upper Ordovician rocks of the North American Midcontinent. Based on the positions of units in the Composite Conodont Standard Section, we have sampled 211 specimens over a stratigraphic interval of 9.1 million years. The average duration of sample spacing is 61,664 years but is commonly as small as 32,800 yr. Thirty-four morphometric characters were measured in each specimen and were converted into multistate characters; character-state breaks were established based upon each characters ability to discriminate between phenetic groupings. Each character was subsequently weighted based on the number of derived states, degree of independence from other attributes, and estimated heritability. Cladistic analysis of these data indicate that there are eight species in Peronopora each consisting of an optimally defined crown group and a basal stem group (or paraclade). Character states shared by stem and crown groups define species but, within species, stem and crown groups also differ in some character states. The species are, in ascending order from the base of the tree, Peronopora decipiens (Rominger, 1886), P. compressa (Ulrich, 1979), P. pauca Utgaard and Perry, 1964, P. milleri Nickles, 1905, P. horowitzi new species, P. vera Ulrich, 1888, P. sparsa Brown and Daly, 1985, and finally P. dubia (Cumings and Galloway, 1913). Diagnostic keys permit the unique assignment of each specimen to a species and the separation of members of stem groups from those of crown groups. Thirty-one characters are required to discriminate between all 211 specimens. This contrasts with previous studies of Peronopora where eight or fewer characteristics were used. Of the ten characters most useful in discrimination, only three had been used in the conventional species literature. This accounts, largely, for only 29.8 percent (51 of 171) of previously identified specimens being classified as members of the same species in this analysis. Discriminant function analysis of original measurements, using species identity as the grouping criterion, produces statistically significant separation of species. It appears that stratigraphic position had an explicit and undue effect on previous concepts of species many of which could not be recognized independently of stratigraphic position. All species of Peronopora appear, or are inferred to have appeared, within the Lexington Limestone between the base of the Grier Member and the top of the Millersburg Member. The cladogram indicates that species evolved in a sequential order, but their first appearance datums have been stratigraphically punctuated. Three species have ranges terminating in the Early to Middle Maysvillian, one in the Middle Richmondian, and four in the Late Richmondian. The latter four (or five) of these species died out in the extinction associated with the unconformity at the top of the Richmondian.

HERITABILITY AND INTRASPECIFIC HETEROCHRONY IN ORDOVICIAN BRYOZOANS FROM ENVIRONMENTS DIFFERING IN DIVERSITY

Environmental conditions affect both the character and variability of developmental patterns (=astogeny) within each of four species of Ordovician bryozoans. Regressions between pairs of stereological measurements for populations from both high- and low-diversity habitats differ significantly in 79 percent of all comparisons. Deviations from a rigid pattern of development, measured as dispersion from regression, were greater in species populations from low diversity settings in 77 percent of comparisons. Therefore, both developmental patterns and their variability differ intraspecifically along a diversity gradient in representatives of four bryozoan families. Additionally, dispersion values were larger in younger rather than older colonies in two species irrespective of diversity level, thus suggesting an age-related reduction in variation. Changes in developmental trajectories indicate that colonies from low-diversity settings are generally paedomorphic relative to conspecific populations from high-diversity habitats. The heritability of these characters and developmental patterns, estimated using variance partitioning techniques, is greater in high-diversity associations. These findings suggest that character state modifi- cations dependent upon astogeny, or a consequence of astogenetic modifications, are more heritable in high-diversity settings. However, if the environment can cause facultative heterochrony, the possibility of fixing such patterns in subsequent generations is increased, although the mechanism for accomplishing this is presently unknown.

Morphological integration and covariance during astogeny of an Ordovician trepostome bryozoan from communities of different diversities

Correlations exist between diversity and colonial growth pattern and variability in Paleozoic stenolaemate bryozoans. Habitats occupied by diverse communities contained colonies whose growth trajectories were different and were more highly constrained than those of low-diversity habitats. Because previous studies have not examined variability during colony growth, it was unclear whether observed variabilities were constant during growth (astogeny) or whether variability changed with age. Astogenetic trajectories and growth variability are here reexamined in populations of colonies of Heterotrypa ulrichi from the same high-diversity and low-diversity communities examined by Pachut (1989). Growth trajectories of populations differ in both intermonticular and monticular regions of colonies for fundamental stereological characters. Total zooidal surface areas generally decrease during astogeny whereas zooidal densities increase. Surface areas per zooid also decrease with increasing age, but are consistently larger in monticular zooecia and in populations from low-diversity communities irrespective of the colony region sampled. These patterns of declining values are caused by increasing wall thicknesses, larger and more abundant mesozooecia and acanthostyles, and the development of maculae during colony growth. Larger values in low-diversity settings may have provided reproductive and feeding benefits necessary for survival. Covariances reflect levels of morphological integration and are larger in populations from high-diversity communities in both intermonticular and monticular regions. Values differ more during early stages of growth (1–3) than later in astogeny (stages 4–5), and are consistently higher in intermonitulcar than in monticular zooecia. Lower levels of integration in monticules may be caused by spatial adjustments as the number of monticules increases across the expanding colony surface during growth. In general, stabilizing selection may have been weaker and less effective in establishing character covariance patterns in populations from low-diversity communities, perhaps because of greater environmental stress. These findings suggest that evaluations of astogenetic patterns and age-related changes in levels of morphological integration within a paleoenvironmental context are important in measuring species richness, in determining the direction of character evolution, and in assessing tempo and mode of evolutionary changes.

Fourier packing ordinate: A univariate size-independent measurement of the polygonal packing variation in Paleozoic bryozoans

The Fourier packing ordinate provides a highly sensitive, univariate, and size-independent measurement of the polygonal packing variation in Paleozoic bryozoan colonies. It is potentially useful in quantifying a wide variety of natural packing arrays or polygonal networks, and is preferable to counting the sides of polygons, the frequency of triple or quadruple junctions, the nearest-neighbor statistic, measurements of surface area per unit cell, or individual harmonic amplitudes. Fourier shape analysis provides exact measurements of the levels of two- to six-fold rotational symmetry in all natural packing gradients. In bryozoans these symmetries are intercorrelated because as each order of symmetry is increasing, the previous order is decreasing. Shapes in these packing arrays are normally hybrids of two or more orders of rotational symmetry. The levels of rotational symmetry involved in these packing gradients are significantly correlated with a single principal component, the Fourier packing ordinate, which is independent of both size and cell boundary phenomena. Spatial analysis of the Fourier packing ordinate within an Ordovician bryozoan colony reveals both variation in packing caused by subcolony budding, as well as large scale trends which vary from the colony center to the free-growing margin.

INFERRING EVOLUTIONARY ORDER AND DURATIONS USING BOTH STRATIGRAPHY AND CLADISTICS IN A FOSSIL LINEAGE (BRYOZOA: PERONOPORA)

Abstract In the Ordovician bryozoan genus, Peronopora, stratigraphic occurrences and cladistic branching order are significantly correlated, indicating sequential development of both patterns in geological time. Five species have stratigraphic first appearances in the exact order predicted by cladistics, but eleven species require downward-range extensions to match cladistic order. Reduced major-axis regression-based corrections and ghost range extensions represent two alternative modifications of first appearance data, with the latter more strongly supported by stratigraphic congruence indices. Tests of the robustness of observed first appearances, based on the density of sampled horizons and magnitudes of stratigraphic gaps, support the probabilistic appearance of descendant species stratigraphically above their putative ancestors in eight of fifteen ancestor-descendant pairs. A 24 m sampling gap occurs below the base of the Brannon Member of the Lexington Limestone, a unit marking the first appearances, or extended ranges, of nine species of Peronopora. A test for the presence of a uniform distribution of occurrence probabilities indicates that seven of the nine species could have originated during the time interval represented by the gap. The early branching rate within Peronopora, during the time span encompassing all first appearances of species, is 1.05 cladogram nodes per meter of strata. The rate of clade production within that interval is 0.73 (baseline) clades per meter of strata. Extrapolating downward using both rates indicates that the median position of the root of the generic clade is approximately 1.08 myr earlier than sampled. The estimated speciation rate in Peronopora is 5.73 species per myr, while the average time between speciation events is 186 kyr. Intraspecific clades, possibly including cryptic species, formed at a rate of 19.35 clades per myr, with an average waiting time per clade of 48.107 kyr. These metrics indicate very short origin times for species within the genus compared to their total stratigraphic ranges, a pattern consistent with punctuated speciation.

Depth-related associations of cryptic-habitat bryozoans from the leeward fringing reef of Bonaire, Netherlands Antilles

The distribution of modern reefdwelling bryozoans provides a guide for recognizing and interpreting bryozoan zonation in ancient reefs. Toward that end, cluster analysis and gradient analysis were used to examine depth zonation of bryozoan species occupying cryptic habitats on the leeward fringing reef of Bonaire. Both analytical techniques produced clusters of sample depths representing shallow(1-9 m), intermediate(12-33 m), and deepwater (40-61 m) habitats. Similarly, both grouped species into 4 depthrelated associations, including shallow, intermediate, intermediate-todeep, and deep water associations from cluster analysis, and shallow, shallow-to-intermediate, intermediate-to-deep, and deep water associations from gradient analysis. Results were corroborated by canonical discriminant function analyses. In both analyses, interpretable associations of species were recognizable in spite of intergradational distributions down the reef front; most displayed subtle differences in abundances across a range of depths with only a few species restricted to narrow ranges of depths. Gradient analysis produced the more interpretable results because, unlike cluster analysis, a single ordering of species along the depth-gradient resulted. In contrast, terminal branches or subclusters, determined by cluster analysis, may be rotated at branch bifurcations into a variety of species arrangements, with no objective means of determining which permutation most accurately portrays interspecific relationships along the environmental gradient. Additionally, the ordination of species by gradient analysis produced species associations which contained a higher percentage of the total number of colonies sampled across member species than was the case for cluster-defined species groupings. Because taphonomic processes may strengthen the expression of gradients, similar species associations in fossil reefs have a high probability of being recognized, especially through the use of gradient analytical techniques, and may provide a framework for the establishment of paleobathymetry.