Lakes can serve as cradles of biodiversity by facilitating adaptive radiation linked to distinct environmental conditions and ecological opportunities. On San Salvador Island, the Bahamas, karstification during Holocene transgressive flooding created anchialine lakes that have supported rapid evolutionary diversification, most notably among endemic pupfishes with specialized feeding strategies. To assess whether similar phenotypic divergence occurs in other members of these lake communities, we investigated morphological variation in shells of the chemosymbiotic infaunal bivalve Ctena sp. (Lucinidae). Live and dead Ctena specimens were sampled from muddy sediment filling karst pockets along the shorelines of three anchialine lakes, a habitat type from which chemosymbiotic bivalves have not previously been reported. Using two-dimensional geometric landmarks of shell interiors and valve margins, we found significant differences in shell shape, size, and allometric relationships among living populations from each lake. Death assemblages preserved similar among-lake patterns, suggesting that morphological divergence predates the window of time averaging represented by death assemblages. Living and dead-shell assemblages within each lake also differed significantly in shell shape, and temporal shape-change vectors from the dead to living included both shared and distinct trends among lakes.Ongoing genomic work suggests that lake-dwelling Ctena on San Salvador represent a single endemic species. Thus, spatiotemporal patterns among lake populations likely reflect ecophenotypic responses to distinct sedimentological and geochemical conditions that influence their chemosymbiotic lifestyle. These results demonstrate the potential of combined life and death assemblage analyses to detect patterns of phenotypic divergence over geologically instantaneous timescales.
Abstract By linking form and function within a particular ecological context, ecomorphology provides a method for reconstructing unknown aspects of the ecology of fossil species. To test how differences in morphology can separate among ecological categories, canonical variates analysis (CVA) is typically employed. However, whether CVA is the most appropriate method for ecomorphology is questionable, as its underlying assumptions, particularly regarding sample size, are often unmet in high-dimensional biological datasets. Moreover, CVA cannot account for the influence of phylogeny. Therefore, alternative statistical methods, which can appropriately handle limited sample sizes and phylogenetic covariance are needed to improve ecomorphological inferences. To achieve this, we employ Bayesian ordinal regression models that discriminate among habitat categories using 3D landmark data of extant African Bovidae astragali, both with and without a phylogenetic parameter. Without a phylogenetic parameter, classification success rates of each model either meet or exceed those of CVAs using the same variables. Including a phylogenetic grouping variable generates nearly perfect predictions of habitat category when species identity and phylogenetic attributions are known, but poor predictions when unknown. Although consideration of similarity due to phylogenetic covariation is critical in evolutionary studies, our results indicate that separating the effect of phylogeny from morphological variables can be detrimental to group membership prediction for taxonomically indeterminate specimens. Therefore, whether to include a phylogenetic effect is contingent upon the research question and the study system. Our work indicates that a Bayesian modeling framework is a robust tool for investigating ecomorphology, with promising future applications for paleoecological studies.
Studies on paleophysiology and thermoregulation are central topics in fossil vertebrate paleobiology because they allow inferences about extinct organismal function beyond anatomical description. Pterosaur thermophysiology remains actively debated. Evidence such as relatively rapid growth and elevated metabolic rate estimates has been used to argue for endothermic-like physiologies consistent with the energetic demands of powered flight, whereas reproductive traits, including burial incubation and precocial development, have led some authors to propose ectothermic strategies more akin to those of extant non-avian diapsids. Here, we provide quantitative estimates of resting metabolic rate (RMR) for the tapejarid pterosaur Caiuajara dobruskii using phylogenetic eigenvector maps and osteocyte lacunar density as proxy. Analysis of six histological sections from three appendicular elements (femur, tibia, humerus) at different ontogenetic stages (adult and juvenile) yielded mass-independent RMR estimates of 0.24-0.76 ml O2 h-1 g-0.86, substantially higher than those of lepidosaurs but lower than typical mammalian and avian values. The retrodicted mass-independent maximum metabolic rate (MMR) of 5.68 ml O2 h-1 g-0.87 falls between values for highly active reptiles and birds, suggesting sufficient aerobic capacity to sustain powered flight without fully matching the maximal performance of small endotherms. Together, these results suggest active-flighted pterosaurs fall among a metabolically intermediate continuum, limiting straightforward actualistic comparisons for inferring their thermoregulatory strategy.
Diversification of benthic mollusks in the Mesozoic and Cenozoic was a key component in the evolution of the modern marine fauna, but the effects of preservational biases on the apparent magnitude of diversification are not fully quantified. Specifically, the taxonomic radiation coincides with increased frequency of unlithified sedimentary deposits and increased quality of aragonitic fossil preservation, trends that could magnify the appearance of diversification by improving fossil recovery. Given that both lithification and aragonite dissolution are expected to preferentially hinder the recovery of small fossils, we explore the relationship between preservational quality and body size in Cretaceous and Paleogene gastropods. We measured shell size for more than 7000 species occurrences illustrated in more than 200 published sources; the literature-based approach allowed broad geographic and taxonomic sampling, although quality of preservation could not be assessed in as much detail as in field- and specimen-based studies. Three aspects of preservational quality were assessed for each measured specimen: overall shell completeness, surface pristineness, and features related to lithification and shell dissolution. We find that shells <9-14 mm are underrepresented in the paleontological literature as preservational quality declines. An improvement in either the lithification or pristineness metric was associated with a decrease in mean shell size. Average preservational quality in sampled specimens increased through time, notably between the Cretaceous and Paleogene. These results support the contention that taphonomic biases affect observed biodiversity trends in gastropods by obscuring small taxa, but also suggest that the magnitude of this effect may be quantifiable in future studies.
The evolution of the muscular stomach was a key innovation in the avian body plan, enabling high metabolic rates by functionally replacing oral processing. However, its deep-time origins within Archosauria have remained poorly understood, primarily due to the extreme rarity of fossilized stomachs. Here, we demonstrate that gastrolith shape is a robust proxy for digestive function and, by extension, stomach muscularity and diet. By assembling a comprehensive dataset from 104 individuals across 46 extant archosaur species, we establish a strong correlation between gastrolith shape, stomach muscularity, and diet. Application of this validated method to the fossil record reveals divergent digestive strategies among major dinosaur clades. Herbivorous ornithischians retained angular gastroliths, consistent with limited gastric abrasion and compensation through oral processing. Sauropods likewise retained angular gastroliths, consistent with limited gastric abrasion and digestion relying more on long retention times and fermentation. Conversely, the muscular stomach, indicated by rounded gastroliths, evolved early within Theropoda, appearing at least by the base of Maniraptoriformes. This innovation was likely a crucial prerequisite for repeated transitions to herbivory in maniraptoriform theropods with reduced dentition, shifting mechanical processing from the jaws to the stomach and reducing reliance on heavy jaw adductor musculature.
