Bradley Deline

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.

Testing the plateau: a reexamination of disparity and morphologic constraints in early Paleozoic crinoids

Abstract Studies of crinoid morphology have been pivotal in understanding the constraints on the range of morphology within a clade as well as the patterns of disparity throughout the Phanerozoic. Newly discovered and described faunas and recent study of early Paleozoic crinoid diversity provide an ideal opportunity to reanalyze Ordovician through Early Silurian crinoid disparity with more complete taxonomic coverage and finer stratigraphic resolution. Using the coarse stratigraphic binning of Foote (1999), the updated morphologic data set has a similar disparity pattern to those previously reported for the early Paleozoic. However, with the more resolved stratigraphic binning used by Peters and Ausich (2008), a significant difference exists between the original and current data sets. Both data sets have a pronounced disparity high during the late Middle Ordovician. However, the updated disparity curve has a much higher initial disparity during the Early Ordovician and a pronounced rise in disparity during the Silurian recovery. Examination of differential sampling, proportions of the crinoid orders through time, and methods of coding characters indicate these factors have little effect on the pattern of crinoid disparity. The Silurian morphospace expansion occurs primarily within disparids and coincides with the origination of the myelodactylids. These findings corroborate the rapid expansion of morphospace during the Ordovician. However, crinoid disparity did not remain static and, although less frequent than during the initial radiation, new body plans evolved following the Ordovician Extinction (e.g., the myelodactylids). These results are consistent with the hypothesis of ecology constraining the limits on morphologic disparity at the class level.

THE TAPHONOMIC SIGNATURE OF A BRINE SEEP AND THE POTENTIAL FOR BURGESS SHALE STYLE PRESERVATION

Abstract Unusually fine preservation of soft anatomy in the fossil record, often referred to as Lagerstätte deposits, has led to great advances in understanding the evolution of life. An understanding of the potential environments of deposition that might lead to exquisite preservation may help to reconstruct the effects of the taphonomic filter and thereby better interpret the completeness of fossil Lagerstätten. Seafloor brines are potential environments leading to exceptional preservation. The Shelf and Slope Experimental Taphonomy Initiative (SSETI) placed mollusc shells, decapod crustaceans, sea urchins, and wood into a Gulf of Mexico seafloor brine pool environment to study the rates and modes of skeletal and soft tissue decay. We found that skeletons, soft tissue, and wood placed directly in the sulfidic anoxic brine were essentially not degraded or discolored over nearly a decade. Where the brine mixed with overlying seawater in a brine stream, the taphonomic signature was quite different. Calcium-carbonate shell and urchin tests underwent severe dissolution, whereas terrestrial plant remains were unaltered. Farther from the brine, shell and urchin carbonate was only slightly dissolved, wood was completely consumed by xylophagus animals, and decapods were reduced to claw parts only. From these experiments, we conclude that the taphonomic signature of a brine seep can be recognized by a unique juxtaposition of preservation styles that varies across phyla. The central area of the anoxic brine would promote exquisite preservation of carbonate, soft-animal tissue, and cellulose. The central area would be ringed by a zone of near total loss of shell carbonate, but paradoxically would promote the preservation of organic tissue such as shell periostracum and ligament, wood, nuts, and cones. Where seawater salinity is normal, the taphonomic signature would return to a seafloor assemblage appropriate to the depth and depositional environment. Brine seep systems may provide a mechanism for maintaining unaltered organism remains at the sediment-water interface long enough to become buried with soft anatomy intact and undisturbed. The very important fossil deposit known as the Burgess Shale exhibits preservation styles and patterns that might be explained by presence of brine. Our experimental work in a modern brine system may shed some light on the taphonomic conditions that led to preservation known as the “Burgess Shale type.”

Comparing taxonomic and geographic scales in the morphologic disparity of Ordovician through Early Silurian Laurentian crinoids

Abstract Interpretations of morphologic radiations and macroevolutionary patterns are dependent on a priori choices of taxonomic and geographic scales of study. The results of disparity analysis at varying taxonomic (species and genus) and geographic (regional, biofacies, and community) scales are examined in a study of Ordovician though Early Silurian crinoids. Using discrete morphologic characters, we examined the disparity of 421 crinoids from 65 Laurentian biofacies. Crinoid disparity differs when analyzed at the regional and biofacies levels. Regardless of fluctuations in regional crinoid disparity, average within-biofacies disparity was static throughout the Ordovician, deviating only during the Silurian because of the proliferation of the morphologically aberrant myelodactylid crinoids. The choice of taxonomic level does not have an effect at the biofacies level. However, at the regional level, the two taxonomic scales (genus and species) can produce different results because of variation in the number of species per genus through time and the amount of morphologic variation within individual genera. Weighting disparity by abundance provides a metric combining morphology and community structure. Average weighted disparity at the community level showed patterns similar to that of the biofacies-level disparity curve, but this metric has a greater degree of variation between biofacies. Biofacies with a low ratio of weighted to unweighted disparity display the distinctive community structure (based on aerosol filtration theory) that is often reported in crinoid assemblages.

GIANTS AMONG MICROMORPHS: WERE CINCINNATIAN (ORDOVICIAN, KATIAN) SMALL SHELLY PHOSPHATIC FAUNAS DWARFED?

Abstract Small fossils are preserved as phosphatic (carbonate fluorapatite) micro-steinkerns (~ 0.5 mm diameter) in Upper Ordovician beds of the Cincinnati area. Mollusks are common, along with bryozoan zooecia, echinoderm ossicles, and other taxa. Similar occurrences of Ordovician micromorphic mollusks have been interpreted as ecologically dwarfed and adapted to oxygen-starved conditions, an interpretation with implications for ocean anoxia. An alternative explanation for small phosphatic steinkerns is taphonomic. Stable carbonate fluorapatite selectively filled small voids, thus preserving small fossils, including larval/young mollusks. Reworking concentrated small phosphatic steinkerns from multiple generations while larger, unfilled calcareous shells were destroyed, resulting in small fossils progressively replacing larger fossils. With thin sections and insoluble residues, we document evidence that many of these steinkerns are incomplete (“teilsteinkerns”) recording small parts of larger, normal-sized animals, or juveniles, along with smaller species. This finding suggests that these fossil assemblages are taphonomically, not ecologically, size-limited. Based on the ecology of modern oxygen minimum zones in which shelled mollusks are rare, the presence of abundant shelled organisms actually argues against severe oxygen stress. Our results also imply that the process by which the “small shelly fossils” of the Cambrian were preserved continued into the Ordovician.

Character selection and the quantification of morphological disparity

Abstract. A priori choices in the detail and breadth of a study are important in addressing scientific hypotheses. In particular, choices in the number and type of characters can greatly influence the results in studies of morphological diversity. A new character suite was constructed to examine trends in the disparity of early Paleozoic crinoids. Character-based rarefaction analysis indicated that a small subset of these characters (~20%of the complete data set) could be used to capture most of the properties of the entire data set in analyses of crinoids as a whole, noncamerate crinoids, and to a lesser extent camerate crinoids. This pattern may be the result of the covariance between characters and the characterization of rare morphologies that are not represented in the primary axes in morphospace. Shifting emphasis on different body regions (oral system, calyx, periproct system, and pelma) also influenced estimates of relative disparity between subclasses of crinoids. Given these results, morphological studies should include a pilot analysis to better examine the amount and type of data needed to address specific scientific hypotheses.

The role of preservation on the quantification of morphology and patterns of disparity within Paleozoic echinoderms

Abstract. The loss of information resulting from taphonomic degradation could represent a significant bias in the study of morphological diversity. This potential bias is even more concerning given the uneven effect of taphonomy across taxonomic groups, depositional facies, and stratigraphic successions and in response to secular changes through the Phanerozoic. The effect of taphonomic degradation is examined using character-based morphological data sets describing disparity in Paleozoic crinoids and blastozoans. Characters were sequentially excluded from the analyses following progressive taphonomic loss to determine how morphologic metrics, such as the relative distribution of taxa in morphospace and partial disparity, changed with increasing taphonomic alteration. Blastozoans showed very little change in these metrics with decreasing preservational quality, which is a result of characters that create distance in morphospace being recognizable in isolated plates. The opposite result is present in crinoids as the characters that are important in structuring the morphospace require intact modules (i.e., the calyx) to accurately assess. Temporal and stratigraphic trends produced encouraging results in that patterns could be largely recovered even with exaggerated taphonomic biases. However, certain parts of a stratigraphic sequence should be avoided and morphological outliers could potentially play a larger role through time, though both of these biases can be easily identified and avoided. The methods presented in this study provide a way to assess potential taphonomic biases in character-based studies of morphological diversity.

Evolution of metazoan morphological disparity

Significance We attempt to quantify animal “bodyplans” and their variation within Metazoa. Our results challenge the view that maximum variation was achieved early in animal evolutionary history by nonuniformitarian mechanisms. Rather, they are compatible with the view that the capacity for fundamental innovation is not limited to the early evolutionary history of clades. We perform quantitative tests of the principal hypotheses of the molecular mechanisms underpinning the establishment of animal bodyplans and corroborate the hypothesis that animal evolution has been permitted or driven by gene regulatory evolution. The animal kingdom exhibits a great diversity of organismal form (i.e., disparity). Whether the extremes of disparity were achieved early in animal evolutionary history or clades continually explore the limits of possible morphospace is subject to continuing debate. Here we show, through analysis of the disparity of the animal kingdom, that, even though many clades exhibit maximal initial disparity, arthropods, chordates, annelids, echinoderms, and mollusks have continued to explore and expand the limits of morphospace throughout the Phanerozoic, expanding dramatically the envelope of disparity occupied in the Cambrian. The “clumpiness” of morphospace occupation by living clades is a consequence of the extinction of phylogenetic intermediates, indicating that the original distribution of morphologies was more homogeneous. The morphological distances between phyla mirror differences in complexity, body size, and species-level diversity across the animal kingdom. Causal hypotheses of morphologic expansion include time since origination, increases in genome size, protein repertoire, gene family expansion, and gene regulation. We find a strong correlation between increasing morphological disparity, genome size, and microRNA repertoire, but no correlation to protein domain diversity. Our results are compatible with the view that the evolution of gene regulation has been influential in shaping metazoan disparity whereas the invasion of terrestrial ecospace appears to represent an additional gestalt, underpinning the post-Cambrian expansion of metazoan disparity.