Abstract The Lilliput Effect, wherein assemblages decrease in mean individual body size after mass extinctions, has not been documented at a wide geographic scale in any of the Late Devonian mass extinction pulses in invertebrate taxa. Based on a dataset of 800 scolecodonts (polychaete jaw elements) from the literature, museum collections, and newly presented data from the Appalachian Basin, we find that scolecodont size distribution per temporal bin decreases across the Frasnian/Famennian Kellwasser Events from a median length of 500 μm before the Kellwasser Events to a median length of 196 μm during the Kellwasser Events. The majority of the small scolecodonts documented during the extinction interval are newly measured specimens from the Kellwasser Events of the Appalachian Basin, although this size change is not unique to the Appalachian Basin. We interpret the reduction in body size as a hypoxia-driven occurrence of the Lilliput Effect because of the susceptibility of benthic invertebrates to hypoxia and the association of this extinction event with hypoxia. While previous studies have shown that polychaete community biomass decreases in response to oxygen stress, our study provides fossil evidence of individual size reduction, plausibly due to oxygen stress.

Abstract Over the past 10 million years, coastal-marine settings along the Peruvian Margin have undergone profound geographic and oceanographic transformations, resulting in extensive changes in coastal-marine communities. While mollusk taxonomy research is slowly being integrated into ecosystem-wide analyses, which have historically centered on vertebrates, a long-term chronostratigraphically controlled analysis of molluscan diversity and compositional changes has not been undertaken for this region. We compiled a database covering 152 species, 97 genera, and 51 families of mollusk fossils from the Peruvian Margin (13–16°S) to assess long-term diversification patterns and faunal turnover from the late Miocene to the present. We identified two distinctive molluscan assemblages. The first, dating to the late Miocene (10–6 Ma), underwent a substantial shift during the Mio-Pliocene transition (6–4 Ma), culminating in a second assemblage more akin to modern counterparts. This shift resulted in an increase in diversity, with the younger assemblage (6–0 Ma) exhibiting greater genus richness than the former late Miocene assemblage. The turnover at 6–4 Ma was driven by peaks in bivalve origination (6–5 Ma) along with elevated extinction rates for gastropods (6–5 Ma) and bivalves (5–4 Ma). Ecological analyses revealed that no single ecological trait consistently changed during this interval, indicating that the turnover resulted from a broad reorganization of ecological strategies. We propose that the major molluscan turnover during the late Miocene–early Pliocene is associated with geomorphological changes related to the Andean uplift, the disappearance of semi-embayments, and a sea-level rise.

Abstract Living chondrichthyans comprise only a fraction of their historical diversity represented in the fossil record, but together they provide insights into the evolution of this ancient clade. Using a theoretical morphology approach, we sought the evolutionary drivers of mandible morphology, a key factor in the feeding ecology of the clade, across their more than 400 million year evolutionary history. Using an empirical sample of 122 extant and 95 extinct species across 35 orders, we created a theoretical morphospace that encompasses, and expands beyond, sampled variation. We sampled morphologies from this theoretical morphospace and subjected them to biomechanical analysis of speed and strength, deriving landscapes of functional performance and optimality into which we projected a phylomorphospace. We examined how the optimality landscape has been navigated by chondrichthyan evolution and how it has been occupied by taxa characterized according to habitat and trophic level. The empirical chondrichthyan morphospace occupation was dispersed from the trade-off optimality peaks. Early chondrichthyans occupy morphospace characterized by narrow, curved jaws before expanding to more robust morphologies through time. This move toward robust morphologies does not follow the most optimal trade-off morphologies, instead avoiding areas that are least optimal. Deep-water species occupy the largest morphospace area, while higher trophic level species stay closer to the trade-off optimality peaks. Our study shows that chondrichthyans, rather than being living fossils, have explored increasingly specialized jaw morphologies, likely related to shifts in ecology such as increased numbers of durophagous taxa, as opposed to a more generalist optimization of component biomechanical constraints.

Abstract Our ability to measure evolution by natural selection in the fossil record is limited by the near impossibility of estimating the fecundity and thus relative fitness of most fossil organisms. Neocheilostome bryozoans are an important exception, because they have calcified larval brood chambers known as ovicells that provide an approximate estimate of the colony’s sexual fecundity. This clade has a rich fossil record dating back ~100 million years, providing potential opportunities to observe changes in relative fitness and natural selection through many past intervals of environmental change. However, neocheilostome fossil specimens are often highly fragmented, and fragments are not necessarily randomized subsets of a colony. To make use of the majority of the neocheilostome fossil record, we need to test the effect colony organization has on our methods of inferring colony fecundity from fragmented specimens. In this study, we measure colony fecundity in a population of Recent neocheilostome bryozoan specimens of the species Parasmittina eccentrica Winston & Jackson, 2021 and quantify the nonrandom spatial arrangement of ovicells due to colony organization. We then simulate fragmenting these specimens and test the statistical robustness of standard methods one might use to reconstruct fecundity from fossil specimens. We find that ovicells are clustered and concentrated at mid-distances from the ancestrula (the oldest part of the colony). As a result, estimates of a colony’s fecundity from a single fragment have higher variance than would be expected if ovicells were randomly distributed. When estimating average population fecundity, observed variance among fossil fragments is a better estimator of sample variance than methods that assume spatial independence (such as a binomial distribution), especially for fragment sizes of 8 mm or less. While there is much to be learned about neocheilostome ovicell arrangement across taxa and environments, we can robustly estimate fecundity from small fossil fragments even in extinct neocheilostome species.

Abstract Mammalian species richness is commonly highest at mid- to high elevations, but the accumulation of sediment that might bury and preserve skeletal remains generally occurs at lower elevations, leading to concerns that fossil assemblages are biased toward low-elevation taxa. Here, I use extant mammals as an analogue to test the basin-scale spatial overlap between species ranges and sediment sinks where burial and fossilization would be possible. Sediment sinks are estimated within five topographically complex regions in western North America by identifying areas with both a low slope and a high contributing area of runoff and are compared with point occurrences of mammals compiled from the Global Biodiversity Information Facility (GBIF). I find that, among the test areas, 82–96% of all species have occurrences that overlap with a sediment sink, despite common offsets in the elevations of maximum sink area and maximum species richness: summed across all test areas, 83% of species and 87% of total sediment sink area are found in the lowest 1000 m of the test areas. Although many other factors can act against the fossilization of terrestrial mammals, these results indicate that the spatial distribution of mammal species with respect to sediment sinks should not in itself impose a major bias at the basin scale.

Abstract The evolution of the mandible in mammalian carnivores is influenced by ecological demands that have changed over their phylogenetic history. We combined geometric morphometrics and biomechanical analysis (including beam analysis and finite element analysis, or FEA) to assess the interaction between form and function as the mandible has adapted independently to carnivorous diets in therian clades including Metatheria, Mesonychia, “Creodonta,” and Carnivoramorpha. Our goal was to determine the relative contributions of mechanical advantage, mandibular force, and mandibular resistance to bending and torsion, to the evolution of mandibular shape in these groups, as well as whether they produce differential rates of shape evolution in the horizontal and ascending rami, which respectively are the tooth-bearing and muscle-loading parts of the structure. We found that the ascending ramus has higher rates of evolution than the horizontal ramus, making it the more rapidly evolvable portion of the mandible. Statistical evaluation supports this interpretation, as mechanical advantage and resistance to force explain more of the variance in shape than do the beam mechanic estimates that are heavily influenced by the mandibular body. Regression analysis shows that the evolution of specialized carnivory was associated with stronger mandibles in which mandibular shape changed by shortening and thickening of the mandible, increasing the areas of muscle attachment, and increasing the carnassial blade length. Principal component analysis of mandibular shape shows that different clades in Theria have been able to fill out similar specialized carnivorous niches with similar functional metrics despite having different mandibular morphologies.

Abstract Linear state space models provide a useful framework for investigating phenotypic evolution in fossil lineages for a wide variety of models representing Brownian motion, Ornstein-Uhlenbeck processes, and the potential influence of environmental covariates. A state space framework also provides access to residuals for the predicted and observed values at each time point as well as improved numerical stability. We illustrate the value of the state space approach by reanalyzing a classic dataset of trait evolution in the diatom lineage Stephanodiscus yellowstonensis. A series of increasingly complex models were fit to these data, including a novel modification of an Ornstein-Uhlenbeck model in which a trait tracks an exogenous covariate. These model results suggest that the number of spines on the periphery of the diatom is best explained by adaptation to changing solar insolation over time.

Abstract Punctuated equilibria argue for intervals of long-term net stasis and comparatively abrupt change in the morphology of individual species lineages resulting from the process of allopatric speciation as recorded in the stratigraphic and fossil record. The concept of coordinated stasis extends punctuated equilibria to posit that not only individual species, but groups of coexisting lineages within a basin, display concurrent morphological and ecological stability over the same extended intervals of geologic time (105 to 106 yr). These blocks of stability termed ecological–evolutionary subunits (EESUs) are separated by shorter-lived (on the order of 103 to 104 yr) episodes of change characterized by varying combinations of speciation, extinction, immigration, and emigration. The result is a pattern of evolutionary and ecological stasis and change that is coincident and highly punctuational. Here, we assess the connections among environment, evolution, and ecology by documenting patterns of stability, geographic extent, and synchronous turnover during medium-scale bioevents in the Middle Devonian of the eastern United States, and we briefly compare these with patterns of EESUs across the Late Ordovician mass extinction (LOME) based on ongoing work. We quantify the geographic extent and stability of faunas originally documented in the Appalachian Basin and identify their likely places of origin and refugia during turnovers. Faunas are geographically widespread during times of stability and border comparably stable faunas in adjacent provinces. During geologically brief intervals, assemblages display near-synchronous shifts involving local extirpation/extinction and coordinated migration of biogeographic boundaries over very long distances. Allopatric speciation in small, locally isolated populations along the edges of basins during brief windows of dramatically altered environmental conditions is more consistent with the geological record, emphasizes the role of environment and biogeography in driving evolutionary change, and confirms the prevalence of punctuated equilibria.

Abstract Ammonoid cephalopods are excellent model systems for evolutionary biomechanics due to their volatile evolutionary dynamics and remarkable fossil record. During the Mesozoic marine revolution, natural selection increasingly favored ammonoid shells with specific ranges of ornamentation patterns (projections that influence surface roughness). While this evolutionary pattern has been attributed to enemy-driven evolution (i.e., escalation), many morphologies lack clear defensive roles. Using a combination of 3D modeling, physical experiments, and computer simulations, we investigate these patterns from a hydromechanical perspective. We model theoretical morphologies along a continuum of increasing ornamentation coarseness. Neutrally buoyant, 3D-printed models, weighted to match the mass distribution of their virtual counterparts, demonstrate that coarser patterns progressively attenuate rocking motions. Flow visualization experiments reveal these coarser patterns produce higher energy dissipation rates in the disturbed fluid. Computational fluid dynamics simulations were performed to characterize the hydrodynamic costs of ornamentation patterns over the majority of biologically relevant swimming speeds and shell sizes for planispiral ammonoids. Only the coarsest categories incur substantial increases in hydrodynamic drag. However, ornamentation patterns with intermediate coarseness effectively avoid this physical trade-off, experiencing dynamic stabilization without considerably reducing swimming efficiency. These trade-off-defying morphologies were progressively favored during the Mesozoic, becoming more abundant than others by the end of this era. Ultimately, these experiments highlight important hydromechanical selective pressures involved in ammonoid evolutionary trends and some fundamental constraints on aquatic locomotion more broadly.

Abstract Paleo-ecological niche modeling (paleoENM) estimates the niches and distributions of extinct species using fossil paleo-coordinates and local environmental data. While general circulation models (GCMs) have been used to estimate climate conditions in deep time, primarily for terrestrial vertebrates, variations in paleo-elevation models used in GCM construction can influence paleoENM outcomes. This study (1) examines the impact of the Cretaceous–Paleogene (K-Pg) mass extinction on the niche dimensions of the marine invertebrate group Turritellinae (Cerithoidea: Turritellidae) and (2) compares two paleo-elevation models’ effects on GCM-based species’ distribution predictions. Fossil occurrence data from the Maastrichtian and Danian periods were collected from the Paleobiology Database (PBDB), museum collections, and published literature. Environmental data were extracted from HadCM3L GCM simulations using Scotese- and Getech-based paleogeographic and pCO2 boundary conditions. We estimated the niche dimensions of turritellines using maximum entropy (MaxEnt) and performed ordination analysis using kernel density estimation. MaxEnt model metrics showed that the Getech-based GCM outperformed the Scotese-based GCM. Geographic projections revealed minor differences in suitable habitat between the Maastrichtian and Danian in the Getech-based GCM, but overinflated predictions in the Scotese-based GCM. Niche overlap between the Maastrichtian and Danian was high, with both GCMs supporting niche similarity and equivalency. Our results suggest that differences in elevation model boundary conditions affected predicted distribution and niche patterns. This study offers a novel approach to understanding ecological persistence in invertebrates after mass extinction events, examines the robustness of GCM boundary conditions in paleoENM studies, and provides a framework for future paleoecological research on fossil invertebrates.