Geologically rapid climate change is anticipated to increase extinction risk nonuniformly across the Earth's surface. Tropical species may be more vulnerable than temperate species to current climate warming because of high tropical climate velocities and reduced seawater oxygen levels. To test whether rapid warming indeed preferentially increased the extinction risk of tropical fossil taxa, we combine a robust statistical assessment of latitudinal extinction selectivity (LES) with the dominant views on climate change occurring at ancient extinction crises. Using a global data set of marine fossil occurrences, we assess extinction rates for tropical and temperate genera, applying log ratios to assess effect size and Akaike weights for model support. Among the classical “big five” mass extinction episodes, the end-Permian mass extinction exhibits temperate preference of extinctions, whereas the Late Devonian and end-Triassic selectively hit tropical genera. Simple links between the inferred direction of climate change and LES are idiosyncratic, both during crisis and background intervals. More complex models, including sampling patterns and changes in the latitudinal distribution of continental shelf area, show tropical LES to be generally associated with raised tropical heat and temperate LES with global cold temperatures. With implications for the future, our paper demonstrates the consistency of high tropical temperatures, habitat loss, and the capacity of both to interact in generating geographic patterns in extinctions.

Reduction of body size is a common response of organisms to environmental stress. Studying the early Toarcian succession in the Lusitanian Basin of Portugal, we tested whether the shell size of benthic marine communities of bivalves and brachiopods changed at and before the global, warming–related Toarcian oceanic anoxic event (T-OAE). Statistical analyses of shell size over time show that the mean shell size of communities decreased significantly before the T-OAE. This trend is distinct in brachiopods and is caused by larger-sized species becoming less abundant over time, whereas it is not significant in bivalves, suggesting a decoupled response to environmental stress. Reductions in shell size precede the decline in standardized sample-level species richness associated with the early Toarcian extinction event. Such decreases in the shell size of marine invertebrates, well before the onset of biodiversity change, suggest that reductions in body size more generally may be a precursor of a subsequent loss of species and turnover at the community level caused by climate change. Sedimentological evidence is against hypoxia as a driver of extinction and the preceding size decrease in the brachiopod fauna in the studied succession, although low oxygen levels are widely held responsible for elevated early Toarcian extinction rates globally. Reduction of mean shell size in brachiopods but stasis in bivalves is difficult to explain with ocean acidification, because experimental work shows that brachiopods can be resilient to lowered pH, albeit long-term metabolic costs and potential evolutionary adaptations are unknown. Rising early Toarcian temperatures in the Lusitanian Basin seem to be a plausible factor in both diversity decline associated with the T-OAE and the preceding reductions in mean shell size, because thermal tolerances in modern bivalves are among the highest within marine invertebrates.

The coastal environment of the Changjiang delta has been influenced by recent anthropogenic activities such as dam construction and increased sewage and fertilizer inputs. Previous work examined the compositional shift of marine plankton to assess ecological impacts of these activities on marine ecosystems in the Changjiang discharge area. Here we used benthic marine ostracodes collected in the Changjiang estuary and the adjacent East China Sea in the 1980s and the 2010s, respectively, to investigate temporal changes of the benthic community and controlling factors for the benthic fauna. Our results revealed more shoreward distribution of some well-known offshore ostracode species in the 2010s compared with the 1980s and a relatively more important role for environmental processes (e.g., bottom-water temperature, bottom-water salinity, and eutrophic conditions of surface water) than spatial processes (e.g., the flow of ocean currents) in structuring ostracode compositions. The temporal changes in the ostracode community are likely attributable to the combined effects of reduced fresh water and sediment discharge and eutrophic conditions of the Changjiang due to the many dams constructed along the Changjiang and population expansion in the Changjiang Basin. Results of redundancy analysis and variation partitioning suggest that ocean currents facilitated environmental filtering of ostracode species such that they could disperse to preferred environmental conditions. These findings highlight the potential uses of marine microfossils to better understand ecological impacts on benthic ecosystems in vulnerable Asian mega-deltas and provide insights into the integration of metacommunity concepts in disentangling dynamics of marine benthic communities.

Earliest Paleocene megafloras from North America are hypothesized to be low diversity and dominated by long-lived cosmopolitan species following the Cretaceous/Paleogene (K/Pg) mass extinction. However, megafloras used to develop this hypothesis are from the Northern Great Plains (NGP) of North America, and relatively little is known about floras from southern basins. Here, we present a quantitative analysis of an earliest Paleocene megaflora (<350 kyr after K/Pg boundary) from the Ojo Alamo Sandstone in the San Juan Basin (SJB), New Mexico. The megaflora, comprising 53 morphotypes, was dominated by angiosperms, with accessory taxa composed of pteridophytes, lycophytes, and conifers. Diversity analyses indicate a species-rich, highly uneven, and laterally heterogeneous flora. Paleoclimate estimates using multivariate and univariate methods indicate warm temperatures and relatively high precipitation consistent with a modern tropical seasonal forest.

When compared with contemporaneous floras from the Denver Basin (DB) of Colorado and the Williston Basin (WB) of North Dakota, the SJB flora had significantly higher species richness but lower evenness. Paleoclimate estimates from the SJB were 7–14°C warmer than the estimates for the DB and WB, indicating a shift from a temperate forest in the NGP to a tropical forest in the SJB. These results demonstrate the presence of a latitudinal floral diversity and paleoclimatic gradient during the earliest Paleocene in western North America. We hypothesize that the warm, wet conditions in the earliest Paleocene SJB drove rapid rates of speciation following the K/Pg boundary, resulting in a diverse and heterogeneous flora.

Deep-sea benthic ostracod assemblages covering the last 2 Myr were investigated in Integrated Ocean Drilling Program Site U1426 (at 903 m water depth) in the southern Sea of Japan. Results show that (1) orbital-scale faunal variability has been influenced by eustatic sea-level fluctuations and oxygen variability and (2) secular-scale faunal transitions are likely associated with the mid-Brunhes event (MBE, ~0.43 Ma) and the onset of the Tsushima Warm Current (TWC, ~1.7 Ma). Krithe, Robertsonites, and Acanthocythereis are the three most abundant genera throughout the core, accounting for 78.5% of total specimens. Multiple-regression tree analysis indicated that the TWC, the MBE, and oxygen content are the significant controlling factors of ostracod dominance. Changes in assemblages exhibit decline and recovery patterns corresponding to orbital-scale cyclicity of sea-level changes. In the Sea of Japan marginal ocean setting, this cyclicity shows a close relationship with bottom-water oxygen variability since the onset of the TWC influx. The MBE amplified the influence of the TWC and oxygen variability to the deep-sea ecosystem through larger sea-level fluctuations. Acanthocythereis dunelmensis, a circumpolar species, dominates before the TWC onset. After the TWC onset and during the mid-Pleistocene transition (MPT, ~1.2–0.7 Ma) Krithe spp., known for their low-oxygen tolerance, substantially increase under moderate oxygen depletion. At the end of the MPT, Krithe dominance diminishes and is replaced by Robertsonites hanaii and Propontocypris spp. after the MBE. The post-MBE assemblage, characterized by R. hanaii, suggests a slightly warmer environment under the development of the TWC. In addition, the post-MBE high-amplitude climate system may have caused the increased abundance of active-swimming Propontocypris spp. due to their superior migration ability. Benthic ecosystems in marginal seas are sensitive and vulnerable to both short- and long-term climatic changes, and the MBE is suggested to be a global biotic event affecting benthic ecosystems substantially.

The work of Arthur J. Boucot (vale, 1924–2017) is foundational to Devonian biogeography. Cited more than 1000 times and mentioned in excess of 105,000 times, his classification of the Old World, Eastern Americas, and Malvinokaffric realms has become the framework for Devonian biogeographic and faunal studies. This study is the first to test and quantify the biotic area classification using the distributional data of trilobite, brachiopod, and fish taxa across the Lower, Middle, and Late Devonian. The biotic similarity analysis of 676 genera (ca. 30,000 occurrences) of late Silurian (Ludlow–Pridoli) and Devonian taxa represents the largest quantification of distributional data undertaken in Devonian biogeography. The temporal area approach (TAAp), applied here for the first time, failed to recover in the area phenogram many of the realms and regions used by Devonian biogeographers, excepting the Malvinokaffric realm, across the entirety of the Devonian. The Old World and Western Gondwana realms are found to be artificial and in need of revision. Without natural areas, bioregionalizations (area classifications) become unstable and unusable, resulting in authors creating further artificial regions and provinces. This study offers a better solution, namely, to identify artificial areas in existing area classifications using TAAp and biotic similarity analysis and revise them. Area classifications are modeled on taxonomic process (Wilson and Brown 1953; Starrett 1958) and the movement toward revision is necessary to break the cycle of reinvention of realms and regions within bioregionalization.

Approximately 50,000–11,000 years ago many species around the world became extinct or were extirpated at a continental scale. The causes of the late Pleistocene extinctions have been extensively debated and continue to be poorly understood. Several extinction models have been proposed, including two nutritionally based extinction models: the coevolutionary disequilibrium and mosaic-nutrient models. These models draw upon the individualistic response of plant species to climate change to present a plausible scenario in which nutritional stress is considered one of the primary causes for the late Pleistocene extinctions.

In this study, we tested predictions of the coevolutionary disequilibrium and mosaic-nutrient extinction models through the study of dental wear and enamel hypoplasia of Equus and Bison from various North American localities. The analysis of the dental wear (microwear and mesowear) of the samples yielded results that are consistent with predictions established for the coevolutionary disequilibrium model, but not for the mosaic-nutrient model. These ungulate species show statistically different dental wear patterns (suggesting dietary resource partitioning) during preglacial and full-glacial time intervals, but not during the postglacial in accordance with predictions of the coevolutionary disequilibrium model. In addition to changes in diet, these ungulates, specifically the equid species, show increased levels of enamel hypoplasia during the postglacial, indicating higher levels of systemic stress, a result that is consistent with the models tested and with other climate-based extinction models. The extent to which the increase in systemic stress was detrimental to equid populations remains to be further investigated, but suggests that environmental changes during the late Pleistocene significantly impacted North American equids.

This paper is aimed at constraining the phylogenetic frame of the acquisition of endothermy by Archosauromorpha. We analyzed the bone histology of Azendohsaurus laaroussii. Stylopodial and zeugopodial bones show three tissue types: (1) avascular lamellar zonal bone formed at low growth rates; (2) a scaffold of parallel-fibered bone containing either small primary osteons or simple vascular canals; and (3) fibrolamellar bone formed at high growth rates. We used quantitative histology to infer the thermometabolic regime of this taxon. We define endothermy as the presence of any mechanism of nonshivering thermogenesis that increases both body temperature and resting metabolic rate. Thus, estimating the resting metabolic rate of an extinct organism may be a good proxy to infer its thermometabolic regime (endothermy vs. ectothermy). High resting metabolic rates have been shown to be primitive for the clade Prolacerta–Archosauriformes. Therefore, we inferred the resting metabolic rates of A. laaroussii, a sister group of this clade, and of 14 extinct related taxa, using phylogenetic eigenvector maps. All the inferences obtained are included in the range of variation of resting metabolic rates measured in mammals and birds, so we can reasonably assume that all these taxa (including Azendohsaurus) were endotherms. A parsimony optimization of the presence of endothermy on a phylogenetic tree of tetrapods shows that this derived character state was acquired by the last common ancestor of the clade Azendohsaurus–Archosauriformes and that there is a reversion in Crocodylia.

The evolution of novel morphologies can signify expansion of a clade into new niches. This can be studied in the fossil record by investigating the morphospace occupancy of organisms, with small morphospaces signifying low morphological disparity and more diffuse morphospaces suggesting a broader range of morphologies adapted to different environments. Morphological disparity of many taxa (arthropods, crinoids, etc.) from the Cambrian to modern intervals have been studied in this manner. However, no study has investigated this in archaeocyaths, which, as reef builders, can have a disproportionate effect on early Cambrian biodiversity relative to their frequency. Here, we collect morphological data on archaeocyathan sponges, mostly from Laurentia. More than 600 museum specimens and 400 field samples were measured for traditional morphometric characters and discrete gross morphological characteristics. We find that archaeocyaths have an average cup/individual (body) diameter of 10.6 mm. This is significantly smaller than a selected group of modern demosponges and lithistid sponges that measure 94.1 mm and 66.8 mm in diameter, respectively, and each has a larger size variance. Archaeocyathan gross morphologies are also highly constrained to a few simple morphologies (three to six categories), while modern demosponges and lithistids are more diverse (nine categories each). These data indicate that Laurentian archaeocyaths were restricted in their morphological disparity, potentially due to limitations imposed by having a robust calcareous skeleton while still maintaining a large intervallum cavity space to facilitate passive entrainment. The fact that these Cambrian reef builders were restricted in their morphological complexity may have had a strong influence on the biodiversity of early Phanerozoic ecosystems. Furthermore, a clade limited to only a few specific morphologies is at an increased risk of extinction.

Archaeocyaths are the most abundant sponges from the Cambrian period, having formed the first animal reef communities more than 500 million years ago. The Archaeocyatha are index fossils for correlating rocks of similar ages globally because of their abundance, extensive geographic distribution, detailed anatomy, and well-established taxonomy. Their ecological significance remains incompletely explored, yet they are known to have strongly competitively interacted, unlike modern sponges. This study examines the feeding ecology of the fossil remains of Siberian archaeocyath assemblages. As suspension feeders, archaeocyaths filtered plankton from the water column through pores in their outer wall. Here we outline a new method to estimate the limit on the upper size of plankton that could be consumed by an archaeocyath during life. The archaeocyaths examined were predominantly feeding on nanoplankton and microplankton such as phytoplankton and protozooplankton. Size-frequency distributions of pore sizes from six different Siberian archaeocyath assemblages, ranging from Tommotian to Botoman in age, reveal significantly different upper limits to the prey consumed at each locality. Some of the assemblages contain specimens that could have fed on larger organisms extending into the mesoplankton, including micro-invertebrates as a possible food resource. These results show that during the establishment of the first animal reef systems, prey partitioning was established as a way of reducing competition. This method has applicability for understanding the construction and the functioning of the first reef systems, as well as helping to understand modern reef systems and their development though time and space.

Deriving ecological and evolutionary descriptions of, and implications from, faunal assemblage patterns is commonly addressed by observation and a variety of exploratory techniques (scaling and clustering), along with qualitative evaluations of species occurrences and relative abundances. We argue that interpretations of faunal patterns, especially those documented by the fossil record, should be based upon the composition and structure of entire communities to provide strong conclusions and replicable results.

As an example, we use benthic foraminiferal data at high resolution (1–2 cm, corresponding to 300–1400 yr) over a section corresponding to about 20 kyr across the beginning of the Paleocene–Eocene thermal maximum (PETM). The PETM was an episode of rapid global warming about 55.5 Ma, associated with ocean acidification and lowered open oceanic productivity and deoxygenation and marked by severe turnover in benthic foraminiferal assemblages. Here we provide a stand-alone approach applicable to any dynamic faunal system, perturbation detection analysis (PDA), to recognize and identify community disruption evidenced as either positive growth or negative decline, and we use this methodical approach to obtain new information on foraminiferal communities before, during, and after the initiation of the PETM.

We conclude that the late Paleocene benthic foraminiferal community (FCOM1) was in a growth stage of positive increasing diversity, suggestive of favorable environmental conditions. This stage continued through the initial changes at the onset of the PETM, when disruption through environmental stress led to this community's termination. A second community (FCOM2) formed with declining diversity and high variability, showing a lack of adaptation to changing conditions. Knowledge of total assemblage status under both adverse and advantageous conditions is necessary, but not recognized by methods that rely upon analysis of single samples only: individual samples cannot be used to recognize disruptive changes in a community's structure, but these are easily identified using PDA.

Accurately recognizing analogues between fossil and modern ecosystems allows paleoecologists to more fully interpret fossil assemblages and modern ecologists to leverage the fossil record to address long-term ecological and environmental changes. However, this becomes increasingly difficult as taxonomic turnover increases the dissimilarity between ecosystems. Here we use a guild-based approach to compare the ecological similarity of Cretaceous cold-seep assemblages preserved in the Pierre Shale surrounding the Black Hills and modern cold-seep assemblages from five previously recognized biofacies. We modify modern assemblage data to include only those taxa with fossilizable hard parts greater than 5 mm in length to make these modern data sets more comparable to potential fossil analogues. We find that while the Black Hills assemblages are more similar in ecological guild composition to the modern thyasirid biofacies, subsets share similarities in ecological structure to the lucinid and mussel-bed biofacies. The fossil seep assemblages are also more similar to one another than are modern assemblages belonging to the same biofacies, despite greater geographic and temporal dissimilarity among the fossil samples. Furthermore, guild-level ordination analyses show a secondary faunal gradient that reflects community succession in the hard substrate–dominated modern assemblages and reveals a parallel faunal gradient in the soft sediment–dominated Cretaceous assemblages, consistent with a gradient in the influence of seep fluids on the faunas. Thus, while the Black Hills assemblages are quite homogeneous in their composition, they capture ecological variation similar to successional patterns in modern seep systems.

Bone healing is an important survival mechanism, allowing vertebrates to recover from injury and disease. Here we describe newly recognized paleopathologies in the hindlimbs of the early tetrapods Crassigyrinus scoticus and Eoherpeton watsoni from the early Carboniferous of Cowdenbeath, Scotland. These pathologies are among the oldest known instances of bone healing in tetrapod limb bones in the fossil record (about 325 Ma). X-ray microtomographic imaging of the internal bone structure of these lesions shows that they are characterized by a mass of trabecular bone separated from the shaft's trabeculae by a layer of cortical bone. We frame these paleopathologies in an evolutionary context, including additional data on bone healing and its pathways across extinct and extant sarcopterygians. These data allowed us to synthesize information on cell-mediated repair of bone and other mineralized tissues in all vertebrates, to reconstruct the evolutionary history of skeletal tissue repair mechanisms. We conclude that bone healing is ancestral for sarcopterygians. Furthermore, other mineralized tissues (aspidin and dentine) were also capable of healing and remodeling early in vertebrate evolution, suggesting that these repair mechanisms are synapomorphies of vertebrate mineralized tissues. The evidence for remodeling and healing in all of these tissues appears concurrently, so in addition to healing, these early vertebrates had the capacity to restore structure and strength by remodeling their skeletons. Healing appears to be an inherent property of these mineralized tissues, and its linkage to their remodeling capacity has previously been underappreciated.

The hadrosaurids were a successful group of herbivorous dinosaurs. During the Late Cretaceous, 100 to 66 million years ago, hadrosaurids had high diversity, rapid speciation rates, and wide geographic distribution. Most hadrosaurids were large bodied and had similar postcranial skeletons. However, they show important innovations in the skull, including disparate crests that functioned as socio-sexual display structures, and a complex feeding apparatus, with specialized jaws bearing dental batteries. Little is known about the macroevolutionary processes that produced these evolutionary novelties. Here we provide novel perspectives using evolutionary rate and disparity analyses. Our results show that hadrosaurid cranial evolution was complex and dynamic, but their postcranial skeleton and body size were conservative. High cranial disparity was achieved through multiple bursts of phenotypic innovation. We highlight contrasting evolutionary trends within hadrosaurids between the disparate facial skeleton and crests, which both showed multiple high-rate shifts, and the feeding apparatus, which had low variance and high rates on a single phylogenetic branch leading to the diverse Saurolophidae. We reveal that rapid evolutionary rates were important for producing the high disparity of exaggerated crests and present novel evidence that the hadrosaurid diversification was linked to both a key adaptive innovation in the feeding apparatus and multiple bursts of innovation in socio-sexual displays.

Vertebrate microfossil assemblages in a stratigraphic sequence often yield similar assortments of taxa but at different relative abundances, potentially indicative of marginal paleocommunity changes driven by paleoenvironmental change over time. For example, stratigraphically younger assemblages in the Dinosaur Park Formation (DPF) yield proportionally more aquatic taxa, consistent with marine transgression. However, individual deposits may have received specimens from multiple source paleocommunities over time, limiting our ability to confidently identify ecologically significant, paleocommunity differences through direct assemblage comparisons. We adapted a three-source, two-tracer Bayesian mixing model to quantify proportional contributions from different source habitats to DPF microfossil assemblages. Prior information about the compositions of separate, relatively unmixed terrestrial, freshwater, and marine assemblages from the Belly River Group allowed us to define expected taxon percent abundances for the end-member habitats likely contributing specimens to the mixed deposits. We compared the mixed assemblage and end-member distributions using 21 different combinations of vertebrate taxa. Chondrichthyan, dinosaur, and amphibian occurrence patterns ultimately allowed us to parse the contributions from the potential sources to 14 of the 15 mixed assemblages. The results confirmed a significant decline in terrestrial contributions at younger DPF sites, driven primarily by increased freshwater specimen inputs—not incursions from the adjacent marine paleocommunity. A rising base level likely increased lateral channel migration and the prevalence of freshwater habitats on the landscape, factors that contributed to increased paleocommunity mixing at younger channel deposit sites. Bayesian methods can account for source-mixing bias, which may be common in assemblages associated with major paleoenvironmental changes.

Researchers often interpret the presence of tortoises in Pleistocene assemblages as evidence of an interglacial age, based on an assumption that these fossils indicate thermic climates, as modern giant tortoises require. Since the Paleocene, tortoises have been common components of terrestrial fossil assemblages and have repeatedly evolved species of giant size. Whereas extant giant tortoises are found only on islands off the coasts of South America and Africa, at least two species persisted in North America until the terminal Pleistocene. These tortoises, Hesperotestudo crassiscutata and Gopherus “hexagonatus,” both of which reached carapace lengths of >1 m, were distributed across the southern United States. This study provides new metrics to derive quantitative weight estimates from measurements of the tortoise shell. The linear measurements of 69 anatomical features of the shells of 108 live tortoises indicate that the regression between straight carapace length and weight is most significant, with a maximum r2 > 0.99. This regression is useful for tortoises that weigh between 1.8 and 339 kg. This mass estimate, coupled with a heat dissipation rate derived from thermoregulation modeling, provides estimates of how long tortoises can maintain a viable body temperature at low ambient temperatures. Depending on size, a tortoise can survive a maximum of 2.3 to 33 hours of freezing temperatures, which corresponds to a mean annual temperature ≥22°C and a mean winter low temperature ≥7.5°C. This analysis infers warmer temperatures at Pleistocene sites with fossil tortoise occurrences than previous qualitative estimates.