Kathleen A. Ritterbush

Mercury anomalies and the timing of biotic recovery following the end-Triassic mass extinction.

The end-Triassic mass extinction overlapped with the eruption of the Central Atlantic Magmatic Province (CAMP), and release of CO2 and other volcanic volatiles has been implicated in the extinction. However, the timing of marine biotic recovery versus CAMP eruptions remains uncertain. Here we use Hg concentrations and isotopes as indicators of CAMP volcanism in continental shelf sediments, the primary archive of faunal data. In Triassic–Jurassic strata, Muller Canyon, Nevada, Hg levels rise in the extinction interval, peak before the appearance of the first Jurassic ammonite, remain above background in association with a depauperate fauna, and fall to pre-extinction levels during significant pelagic and benthic faunal recovery. Hg isotopes display no significant mass independent fractionation within the extinction and depauperate intervals, consistent with a volcanic origin for the Hg. The Hg and palaeontological evidence from the same archive indicate that significant biotic recovery did not begin until CAMP eruptions ceased.

Westermann Morphospace displays ammonoid shell shape and hypothetical paleoecology

Abstract The Westermann Morphospace method displays fundamental morphotypes and hypothesized life modes of measured ammonoid fossils in a ternary diagram. It quantitatively describes shell shape, without assumption of theoretical coiling laws, in a single, easy-to-read diagram. This allows direct comparison between data sets presented in Westermann Morphospace, making it an ideal tool to communicate morphology. By linking measured shells to hypothesized life modes, the diagram estimates ecospace occupation of the water column. Application of this new method is demonstrated with Mesozoic data sets from monographs. Temporal variation, intraspecies variation, and ontogenetic variation are considered. This method can address hypothetical ecospace occupation in collections with tight stratigraphic, lithologic, and abundance control, even when taxonomy is in dispute.

Geographic ranges of genera and their constituent species: structure, evolutionary dynamics, and extinction resistance

Abstract. We explore the relationships among the geographic ranges of genera, the ranges and positions of their constituent species, and the number of species they contain, considering variation among coeval genera and changes within genera over time. Measuring range size as the maximal distance, or extent, between occurrences within a taxon, we find that the range of the most widespread species is a good predictor of the range of the genus, and that the number of species is a better predictor still. This analysis is complicated by a forced correlation: the range of a genus must be at least as large as that of each of its constituent species. We therefore focus on a second measure of range, the mean squared distance, or dispersion, of occurrences from the geographic centroid, which, by analogy to the analysis of variance, allows the total dispersion of a genus to be compared to the mean within-species dispersion and the dispersion among species centroids. We find that among-species dispersion is the principal determinant of genus dispersion. Within-species dispersion also plays a major role. The role of species richness is relatively small. Our results are not artifacts of temporal variation in the geographic breadth of sampled data. The relationship between changes in genus dispersion and changes in within- and among-species dispersion shows a symmetry, being similar in cases when the genus range is expanding and when it is contracting. We also show that genera with greater dispersion have greater extinction resistance, but that within- and among-species dispersion are not demonstrable predictors of survival once the dispersion of the genus is accounted for. Thus it is the range of the genus, rather than how it is attained, that is most relevant to its fate. Species richness is also a clear predictor of survival, beyond its effects on geographic range.

New evidence on the role of siliceous sponges in ecology and sedimentary facies development in eastern Panthalassa following the Triassic-Jurassic mass extinction

ABSTRACT Paleoecological consequences of the global Triassic–Jurassic mass extinction (201.3 Ma) are poorly understood. Fossiliferous marine boundary records are rare, commonly condensed, and typically reveal facies changes previously attributed to eustacy. Sedimentology and biofacies analyses from stratigraphically expanded successions of the lowest Jurassic strata, New York Canyon, Nevada, were investigated with high-resolution paleoenvironmental observations, fossil surveys, and microfacies analysis. Following the collapse of the uppermost Triassic carbonate ramp, the lowest Jurassic Ferguson Hill Member of the Sunrise Formation records a midshelf habitat dominated by previously unrecognized siliceous sponges for approximately two million years. In addition, the earliest Jurassic strata from the Pucara Group, central Peruvian Andes, were examined and record a more greatly expanded stratigraphic succession of facies across the inner to middle shelf. Like Nevada, the lowest Jurassic Aramachay Formation is replete with intense concentrations of siliceous sponges. The revelation of widespread, ecologically dominant siliceous sponges has been overlooked despite detailed biofacies studies in both depositional systems. Sponges expanded across shallow environments with sparse benthic biocalcifier populations, and were likely aided by increased ocean silica concentrations from the weathering of the Central Atlantic Magmatic Province. Facies changes previously attributed to sea-level change are thus interpreted to result from the collapse of the carbonate factory concomitant with the mass extinction, with transition to an alternate state dominated by siliceous sponges before a return to carbonate platform development in the Sinemurian. Our study highlights the need to separate biofacies from paleoenvironmental analysis during mass extinction times when nonactualistic assemblages may dominate and deviate from expected environments (e.g., siliceous sponges as indicators of deep paleoenvironments).

Interpreting drag consequences of ammonoid shells by comparing studies in Westermann Morphospace

Differently-shaped ammonoid shells present varied hydrodynamic properties, but the relevance of shell shape to ammonoid paleoecology is unclear. To examine trends in ammonoid shell shape that relate to hydrodynamic consequences, we project ammonoid shell data into Westermann Morphospace. Operationally similar to other multivariate methods, Westermann Morphospace features a fixed frame and scaling calibrated around the most common planispiral morphotypes. First, results of hydrodynamic experiments are projected into the space, to test for associations between recognized morphotypes and measures of drag force. Discocone shell shapes produce minimal drag at small sizes and/or low speeds, while oxycone shells produce minimal drag at higher sizes and/or faster speeds. If hydrodynamic efficiency was a first-order selective pressure on shell shapes produced by ammonoids, an association is expected between larger adult shells and oxyconic geometry. To assess this, published shape data are shown in Westermann Morphospace to examine intervals of evolutionary interest. A rough analysis of Late Triassic ammonoid shell shapes shows the expected association between larger shells and oxyconic geometry, but the association is completely reversed immediately after the end-Triassic mass extinction, and the association does not return among the “recovered” middle Jurassic ammonoids. This suggests that hydrodynamics suited for high-metabolism rapid locomotion were not a first-order influence on shell shape immediately after the extinction, or that other hydrodynamic influences require assessment by different methods. Lastly, shells of a series of endemic chronospecies of Cretaceous Neogastroplites are compared, showing a shift to inclusion of more discoconic shells at smaller sizes, resulting in minimal drag for more of the ontogenetic series as a whole. Together, these results indicate that size must be considered when interpreting the hydrodynamic efficiency of ammonoid shells. Flank shape, ornament and soft tissue behaviors may be as or more important than first-order shell geometry, and quantification of their importance and influence on varied shell geometries will benefit from further experimental results.

Association between geographic range and initial survival of Mesozoic marine animal genera: circumventing the confounding effects of temporal and taxonomic heterogeneity

Abstract. We investigate the association between geographic range and survival in Mesozoic marine animal genera. Previous work using data from the Paleobiology Database (paleobiodb.org) demonstrated greater survivorship overall among Phanerozoic genera that were widespread during their stage of first appearance, but this relationship did not hold during the Mesozoic. To explore this unexpected result, we consider geographic range in conjunction with temporal variation in survival and variation in survival among higher taxa. Because average range and average survival are negatively correlated among stages, for reasons that are still unclear, and because the data are heavily influenced by cephalopods, which include many wide-ranging and short-lived genera, the effect of geographic range on survival is obscured in the aggregate data. Thus, range is not a significant predictor of survival when data are analyzed in aggregate, but it does have a significant effect when variation in average range and average survival among stages and classes is taken into account. The best-fitting models combine range with both temporal and taxonomic heterogeneity as predictive factors. Moreover, when we take stage-to-stage variation into account, geographic range is an important predictor of survival within most classes. Cephalopod genera must be more widespread than genera in other classes for geographic range to significantly increase odds of survival, and factoring in survival heterogeneity of superfamilies further increases model fit, demonstrating a nested nature in the sensitivity of range and taxonomic aggregation.