Sexual dimorphism, a widespread phenomenon, has been extensively researched in extant and fossil crustaceans. However, identifying sexual dimorphism in phyllocarid fossils preserved as isolated parts is often challenging, except in cases where the specimens are exceptionally well preserved, including those with soft tissues. This study proposes a novel approach by introducing the use of geometric morphometric techniques to identify sexual dimorphism in phyllocarid fossils based on carapace morphology. It presents a comprehensive re-analysis of Soomicaris ordosensis Liu et al., 2023a, carapaces from the Upper Ordovician in North China and Tarim Plates. Elliptic Fourier analysis was applied to quantify the size and shape variation in nearly 100 specimens. The results demonstrate the presence of significant sexual dimorphism in the length and shape of the S. ordosensis carapace. The carapace shape exhibited variation between the sexes: the posterodorsal margin of one group of carapaces gradually extends backward to form a posterodorsal spine; the carapaces of the other group have a convex posterior margin and lack a posterodorsal spine. Additionally, both types manifest an overall allometric growth pattern, albeit with distinct growth coefficients. Furthermore, the observed approximately 1:1 ratio between the two forms suggests that the population of S. ordosensis may have exhibited a dioecious mating system. Geometric morphometrics are a highly effective method for elucidating the subtle variations in the carapace morphology of S. ordosensis, thereby underscoring the cryptic dimorphism characteristics of fossil animals. This finding offers the first indirect evidence for egg-brooding behavior within the extinct order Archaeostraca.

Richard Bambach was a leading figure in the “paleobiology revolution” of the late 1960s and 1970s, keeping the movement grounded with his keen geological and ecological insights. With interests ranging from the functional biology of individual organisms to the largest macroevolutionary trends in the history of life, he was especially adept at linking paleoecological and macroevolutionary patterns across spatiotemporal scales. He authored seminal publications during five different decades and was recognized with both the Moore Medal from the Society for Sedimentary Geology and the Paleontological Society Medal.

Over the past half century, paleobiologists have advanced the estimation of phylogenetic relationships among fossil taxa to explore evolutionary patterns in deep time. This study employs a breadth of phylogenetic analyses, specifically divergence time estimations and character rate evolution, within three blastozoan echinoderm clades: Diploporita, Eublastoidea, and Paracrinoidea. Utilizing reversible jump Markov chain Monte Carlo (rjMCMC) and fossilized birth–death (FBD) models, we investigated evolutionary rates through anatomical subunit partitioning. Results suggest similar rates among the three groups, although Paracrinoidea exhibits elevated rates in several anatomical subunits. The inferred trees largely agree with other recently published analyses, in that the current taxonomy of the group does not reflect true evolutionary relationships. Thus, this study adds to a growing body of literature that highlights the need to revise echinoderm taxonomy. We tested different clock models for each group and found that model choice had strong effects on resulting trees; our findings suggest linked clocks (i.e., the same clocks for all character partitions) had more support than unlinked clocks (i.e., different clocks for different character partitions). These findings indicate a need to carefully consider model choice and rates of evolution when utilizing these types of analyses.

An important question in evolutionary biology and macroecology is whether taxa show systematic trajectories in occupancy, the proportion of geographic area occupied, over macroevolutionary timescales. Past studies have used fossils to document these trajectories, showing a symmetric rise and fall. In this study, I focus on several biases in the analyses of fossil occupancy trajectories that have been unaccounted for. First, better sampling of boundary bins in a taxon’s stratigraphic range, paradoxically, results in lower mean occupancy of taxa in those bins. This is because better sampling allowed more taxa with low occupancies to be included in the mean occupancies of those bins compared with intermediate bins. Second, the possibility that taxa may have incomplete durations within boundary bins could also lower occupancies in those bins. Finally, a bias can also exist when the number of sampled sites is not constant throughout a taxon’s stratigraphic range. I use simulations to show that the first bias can be corrected by conditioning these boundary bins to be sampled in the same way as intermediate bins. To mitigate the second bias, I use higher-resolution time bins to constrain the intervals over which taxa’s occupancies are measured so that they are comparable between boundary and intermediate time bins. I also present an approach that can correct for the last bias by subsampling geographic sites, testing its impact in a simulation. Considering these factors, the occupancy trajectories of marine animal genera look to be a relatively gradual rise post-origination with a sudden decline before extinction.

Orthoceratoid cephalopods had straight or slightly curved shells that often contained enigmatic calcareous structures in their chambers. These cameral deposits have been interpreted as counterweights, allowing these cephalopods to assume postures other than a default, downward-facing orientation. These animals must have balanced the proportions of their soft body, cameral deposits, and air-filled chambers to maintain a condition near neutral buoyancy. Lower body chamber ratios (BCRs) allow more mass to be dedicated to cameral deposits, increasing their influence over the total mass distribution. Using 43 computer reconstructions, we calculated the proportion of chamber contents that satisfy a neutrally buoyant condition across different BCRs. Furthermore, we explored the limits of cameral deposit distributions inside the shell to better understand their influence over orientation, stability, and maneuverability. Cephalopods with 40% BCR cannot accommodate any deposits and assume stable, downward-facing orientations. Cephalopods with 30% BCR allow some cameral deposits, which negligibly reduce stability. A slight reduction in BCR to 25% can considerably improve maneuverability, allowing these cephalopods to assume a wider range of postures while swimming. While our results are most relevant to some subset of orthocone cephalopods (Pseudorthoceratida), we also highlight similar constraints faced by broader orthocone groups. Swimming capabilities are extremely sensitive to BCR, which likely constrains the life habits and ecology of these animals. Our results add context to (1) the physical constraints of orthocone cephalopods, (2) their functional complexity in Paleozoic ecosystems, and (3) how these early swimmers navigated physical trade-offs between stability and maneuverability.

Bones preserved in fluvial sediments make up the majority of the terrestrial vertebrate fossil record, and unsteady flows (overbank floods, levee breaches, debris flows, etc.) are often invoked as agents of bone transport and burial. Experiments exploring transport of mammal bones under steady-state flow led to the development of Voorhies Groups, which are used as indicators of winnowing and transport at fossil sites. Some studies have raised concerns about the use of transport groups beyond the scope of the original experiments, especially regarding untested taxa and flow conditions. Here we investigate transport of hadrosauroid dinosaur bone models and modern sheep bones in experimental sheet floods. We find that evolving flow dynamics in unsteady flows can influence bone mobility behaviors. Factors such as bedforms and interactions with other bones caused shorter transport distances than might be expected in some elements, which would be heightened in real flooding situations where trapping mechanisms are common. Our hadrosauroid bones sorted into two statistically significant groups and one overlapping intermediate group based on transport distance. However, those groups could not be identified among sheep bones. Distributions of transport distances in both taxa do not fully match predictions based on Voorhies Groups. Our results indicate that Voorhies Groups do not quantitatively apply to all potential fluvial settings and taxa, and we thus advise caution in interpretations of fossil site taphonomic history based on Voorhies Groups. Further exploration of variables underlying bone transport and burial may allow for more broadly comparative examinations of fluvial biostratinomy.

The extinction of clades outside mass extinction events remains an understudied aspect of evolutionary dynamics. This study examines the Dactylioceratidae, an ammonite family that disappeared during the Early Jurassic, outside a recognized mass extinction event. By using high-resolution taxonomic (species-level) and temporal (subchronozone) data, we assess its evolutionary trajectory, from diversification to extinction. Our analysis reveals that Dactylioceratidae experienced an initial expansion in diversity and geographic range, followed by increased specialization. Morphological disparity and diversity peaked before a sharp decline, suggesting a possible link between ecological specialization and extinction risk. This pattern aligns with hypotheses proposing that overspecialization limits adaptability, leading to extinction under background conditions. In contrast to mass extinctions driven by sudden catastrophic events, background extinctions may be influenced by gradual ecological changes and evolutionary constraints. By comparing the case of Dactylioceratidae with broader ammonoid trends, this study provides insights into long-term extinction mechanisms. These findings are relevant for understanding both past and present biodiversity crises, shedding light on how species’ evolutionary strategies impact their survival over time.

The patterns by which ancestral species give rise to descendants offer critical insights into the processes governing evolutionary and ecological change through time. One such pattern, predicted by both theoretical models and empirical studies, is the persistence of long-lived ancestral species that give rise to multiple descendants. While models such as the birth–death process long employed by paleobiologists predict the occurrence of such “super-progenitors,” the extent to which they should appear in fossil clades remains unknown. To address this, I apply a birth–death-sampling model to four marine clades to evaluate the expected prevalence of super-progenitors and the distribution of sampled descendants. I also explore through analytical and simulation-based predictions how variation in preservation, turnover, and net diversification rate influences these expectations. The model predicts that super-progenitors should be common across nearly all of the clades examined, provided that sampling completeness exceeds approximately 50% at the taxon level. Although the threshold excludes some poorly sampled terrestrial groups, my findings suggest that super-progenitors should be expected across a broad array of clades. Continued integration of super-progenitors into phylogenetic inference and models of diversification may thus contribute to a more complete understanding of macroevolutionary pattern and process.

Continuous morphological traits play a crucial role in phylogenetic inference, yet they are often discretized due to limited software support and challenges of handling missing data efficiently. We present a new implementation of the Brownian motion model for continuous trait evolution in the Bayesian phylogenetics software MrBayes. Our approach efficiently accommodates any proportion of missing data and supports evolutionary rate variation across characters and data partitions. It is compatible with both non-clock and relaxed clock models. We validate the implementation through simulations and apply it to empirical datasets of pterosaurs and ancient humans, demonstrating that continuous traits can improve phylogenetic resolution. This development expands the methodological tool kit for morphological and total-evidence phylogenetics and is applicable across diverse taxonomic groups.