Despite decades of nearly equal numbers of men and women as students, women remain underrepresented in the ranks of professional paleontology. Scholarly publishing is a gateway to the senior ranks, and journals are the gatekeepers. We asked whether the publishing infrastructure of the Paleontological Society supports gender equity. We reviewed all papers published by the society's journal, Paleobiology, from its inception in 1975 through 2021. Over the journal's run, the number of authors per paper increased due to cultural shifts toward collaborative research and acknowledging technical and support contributions with coauthorship. These shifts opened the door to more women, particularly beginning in the early 2000s, when the first women editors held the keys to the society's journals. Despite these gains, women remain chronically underrepresented. Change that supports one underrepresented group generally supports all. Therefore, we offer four recommendations to open the publishing gate to all intersectional identities: (1) review manuscripts without author identifiers; (2) recruit more editors from diverse backgrounds; (3) democratize the review process by including more and different voices; and (4) gather data on author demographics at the time of submission and analyze and report on these data regularly to see who is and who is not passing through the publishing gateway.

Women are underrepresented in paleontology. Despite more women students, representation at senior levels remains low. To advance professionally, scientists must disseminate their research through peer-reviewed publications. We examine gendered authorship patterns in Paleobiology to ask whether the publishing infrastructure supports the Paleontological Society's gender-equity goals. We reviewed all papers published in Paleobiology from its inception in 1975 through 2021. For each paper, we recorded each author, the author's position in the author list, and the total number of authors on each paper. We coded gender based on a combination of personal communication and pronouns used in publicly available information. We compared author demographics with anonymized membership data from the Paleontological Society. Over the journal's run, the number of authors per paper increased due to cultural shifts toward collaborative work and acknowledging student contributions with coauthorship. These trends contribute to proportionally more women authors, beginning in the early 2000s. Despite these increases, women remain chronically underrepresented. In 2018, 2019, and 2021, the proportion of women authors in Paleobiology paralleled membership in the Paleontological Society. However, in 2020, Paleobiology published fewer women authors than expected based on society membership. This echoes declines in women's scholarly productivity in the first year of the COVID-19 pandemic observed across many disciplines. We offer four recommendations: (1) practice double-anonymous peer review; (2) recruit editors from diverse backgrounds who invite reviewers with diverse backgrounds; (3) democratize manuscript review by selecting reviewers from a disaggregated reviewer database; and (4) gather and analyze demographic data for both submissions and publications.

Understanding how morphological variation changes within populations over relatively short timescales in response to environmental changes and ecology (i.e., thousands of years) is a major focus of paleontology and evolutionary biology. A distinct focus is in understanding the broadscale patterns by which lineages have diversified into distinct environments over geologic time (i.e., millions of years). One major challenge has been reconciling how and whether processes acting over shorter timescales shape the patterns observed over long timescales. One way of examining morphological variation at the population level is by examining the distribution of polymorphic character states—discrete anatomical features that vary within a population. Fossil species often maintain such polymorphisms for long periods of time, with such variation even sometimes inherited by new species from their ancestors. In this article, I suggest that examining how these polymorphisms are distributed among incipient descendant lineages might help link the ecological and evolutionary processes that act at the population level (e.g., natural selection, genetic drift, competition, predation) to the paleobiological patterns that are often reconstructed across many species and over long timescales. I explore these dynamics in two lineages: Ectocion, a genus of Eocene mammals, and botryocrinids, a Mississippian cladid crinoid family. I found that new lineages typically have fewer polymorphisms than their ancestors, suggesting that ancestral variation is “sorted” into incipient lineages during speciation. This variation appears to be sorted randomly, which means that it is not possible to detect the influence of natural selection in guiding the inheritance of ancestral morphologies. I suggest that the patterns by which ancestral variation is sorted into new species may explain patterns of lineage diversification over long timescales, highlighting how population processes can extend their influence over longer timescales to shape large-scale evolutionary dynamics.

Biological variation fuels evolutionary change. Across longer timescales, however, polymorphisms at both the genomic and phenotypic levels often persist longer than would be expected under standard population genetic models such as positive selection or genetic drift. Explaining the maintenance of this variation within populations across long time spans via balancing selection has been a major triumph of theoretical population genetics and ecology. Although persistent polymorphisms can often be traced in fossil lineages over long periods through the rock record, paleobiology has had little to say about either the long-term maintenance of phenotypic variation or its macroevolutionary consequences. I explore the dynamics that occur when persistent polymorphisms maintained over long lineage durations are filtered into descendant lineages during periods of demographic upheaval that occur at speciation. I evaluate these patterns in two lineages: Ectocion, a genus of Eocene mammals, and botryocrinids, a Mississippian cladid crinoid family. Following origination, descendants are less variable than their ancestors. The patterns by which ancestral variation is sorted cannot be distinguished from drift. Maintained and accumulated polymorphisms in highly variable ancestral lineages such as Barycrinus rhombiferus Owen and Shumard, 1852 may fuel radiations as character states are sorted into multiple descendant lineages. Interrogating the conditions under which trans-specific polymorphism is either maintained or lost during periods of demographic and ecological upheaval can explain how population-level processes contribute to the emergent macroevolutionary dynamics that shape the history of life as preserved in the fossil record.

Coccolithophores are unicellular algae, with the cell surface covered by calcite scales (coccolith). Coccolithophore size has become progressively smaller from the late Paleogene to the present. The first size reduction (FSR) occurred ∼32 million years ago in the most successful modern lineage, the order Isochrysidales. To decipher the driving mechanism(s) behind the FSR, we conducted a morphologic analysis on the fossils of coccolithophores that lived during the late middle Eocene to early Oligocene (∼40–31 million years ago), using marine sediments from the South Atlantic Ocean. Our data show a strategic divergence between groups of large and small coccolithophores. The group of larger coccolithophores increased in size by ∼4 µm before their extinction, while the size range of the group of smaller coccolithophore contracted. In addition, the central opening on the coccolith decreased in all species from the late Eocene to early Oligocene. We infer that the increased nutrient availability during the Eocene/Oligocene climate transition may be responsible for the morphologic evolution. We hypothesize that larger species with larger central openings on their coccoliths were more dependent on mixotrophy, making them better adapted to the nutrient-poor conditions from the middle to late Eocene. On the other hand, smaller species whose central opening dwindled were better adapted to nutrient-rich conditions during the early Oligocene. Our inference supports the idea that the diversity of trophic strategies among phytoplankton is underestimated.

The first size reduction (FSR) in the Reticulofenestra-Gephyrocapsa-Emiliania (RGE) lineage (order Isochrysidales), which occurred in the early Oligocene (∼32 Ma), is of great significance for understanding the Lilliput effect that has affected coccolithophore communities from the late Eocene to this day. We conducted a morphologic analysis on the coccoliths of Reticulofenestra species that lived during the late middle Eocene to early Oligocene (∼40–31 Ma), using marine sediments from the South Atlantic Ocean. Our data show increasing size and decreasing abundance of the large species during the late Eocene, leading to their disappearance at the FSR, and a concurrent decrease in the size variability of the small- to medium-sized coccoliths whose central opening diameter had become very reduced. Although the cosmopolitan late Paleogene through Neogene size decrease in coccolithophores has been linked to the concomitant long-term decline in global pCO₂, we suggest here that the FSR was the result of environmental destabilization caused by the expansion of eutrophic environments following the late Eocene establishment of overturning circulation associated with ice buildup on Antarctica. This study also leads us to propose a hypothetical model that links coccolith morphology of species of the RGE lineage and trophic resources in the upper ocean: the small- to medium-sized, r-selected coccolithophores with smaller coccolith central openings live in nutrient-rich waters where they rely mostly on photosynthesis and little on mixotrophy, whereas the larger, K-selected species with larger coccolith central openings live in oligotrophic waters where they are more dependent on mixotrophy.

Reef-building corals live with algae in their tissues. They form a mutually beneficial relationship called photosymbiosis, wherein algae provide corals with nutrients, and in return, the coral provides the algae with protection. Even though corals obtain a majority of their nutrients through algae, they also catch and feed on prey such as plankton. Today we can observe a relationship between the size of the coral polyp and the share of its plankton food. Polyps are not preserved in the fossil record, but the skeletal cups called corallites that make up the hard skeleton of corals are well preserved and are directly related to the size of the polyp. Corals with smaller corallites are usually more reliant on algae and thus are more efficient in photosymbiosis. In this study, we explored how the size of corallites relates to their ability to engage in photosymbiosis over the last ∼250 Myr. We used large datasets of fossil and modern corals and their corallite sizes. We applied various analytical methods to understand how photosymbiosis has evolved in reef-building corals. First, we analyzed the abundance or ecological dominance of different corallite sizes to understand ecological patterns. Second, we traced the diversity of corallite sizes over time in terms of taxonomic richness to identify evolutionary patterns. Our findings revealed interesting trends. When looking at corallite sizes by genus, we observed a slightly positive trend in corallite sizes in more recent times. However, when considering the ecological abundance of corallites, we found a significant negative trend in corallite sizes for colonial corals since the Mesozoic Era. This suggests that corals with smaller corallites became more dominant over time, thereby obtaining a competitive advantage in the shallow, tropical, nutrient-poor seas they largely occupy today.

Corallite sizes reflect a continuum in the efficacy of photosymbiosis in colonial reef corals, with smaller corallite sizes generally associated with higher autotrophy. Using a large compilation of reef-coral traits and corallite diameters as a proxy, we test here the hypothesis that photosymbiotic efficacy has increased over the evolutionary history of scleractinian corals. To gain a more comprehensive understanding of the evolutionary versus ecological patterns of corallite sizes of reef corals, we used three analytical methods: (1) occurrences-weighted within-bin analyses as a proxy for abundance or ecological dominance to depict ecological patterns; (2) unweighted range-through analyses; and (3) unweighted sampled-in-bin analyses to represent diversity in terms of taxonomic richness, enabling us to trace evolutionary patterns. By-genus, range-through analysis indicates a slightly positive trend of corallite sizes toward the Recent. However, the occurrences-weighted assessment shows a pronounced negative trend of corallite sizes in colonial corals since the Mesozoic. Random walk and directional evolution are both statistically supported to explain this long-term decrease. A driven trend is evolutionarily plausible, giving reef corals a selective advantage in the oligotrophic environments they largely occupy today.

Radiodonta is a group of early arthropods, distantly related to living insects and spiders, that provides insight into the origin of the segmented body plan shared by these animals. Radiodonts include some of the largest animals from the Cambrian Period; however, little has been known about their development due to a lack of juvenile specimens. We present an analysis of development of the radiodont Stanleycaris based on 265 exceptionally well-preserved specimens from the Cambrian Burgess Shale, ranging in size from 10 to 83 mm. We show that several aspects of the body shape of Stanleycaris changed as it grew. For example, the eyes were relatively larger in the smallest individuals, suggesting that juveniles were advanced visual predators. Additionally, segments were added sequentially at the rear of the body, a common developmental trait among arthropods. In light of the early evolutionary divergence of radiodonts from other arthropods, this finding provides direct evidence for a deep origin of this developmental mode. Finally, using a newly devised approach, we find evidence for two distinct fossil types of Stanleycaris, representing carcasses and molted exoskeletal remains, respectively. Based on comparison with fossils of other species, the general pattern of molting in Stanleycaris is likely shared among radiodont species and potentially other early arthropods. Altogether, our study demonstrates the first detailed view of the early development of a radiodont, providing key new evidence about the evolution of development at the origin of the arthropod body plan.

Radiodonta is a clade of stem euarthropods of central importance to our understanding of the evolution of this phylum. Radiodonts include some of the largest early Paleozoic animals; however, little is known about their ontogeny. We present an analysis of molting patterns and ontogeny in the radiodont Stanleycaris based on 265 exceptionally preserved specimens from the mid-Cambrian (Wuliuan) Burgess Shale. Ranging in size from 10 to 83 mm, they constitute the most extensive radiodont ontogenetic series known. Using a novel morphospace approach, we show that putative carcasses and exuviae can be quantitatively distinguished by the particular suites of structures preserved and their modes of preservation. We propose that Stanleycaris, and probably other radiodonts, molted via a suture near the anterior of the trunk. Similar anterior molting strategies, with a suture located at the head–trunk boundary, are shared with some Cambrian euarthropods and are potentially ancestral. Allometric analyses suggest that as Stanleycaris body size increased, the head sclerite and neck became relatively broader, while the trunk and flaps became slightly longer. The eyes developed precociously, indicating an important role of visual processing in juveniles. Finally, we find evidence for an initial anamorphic developmental phase, where segment number increased at least from 11 or 12 up to 17, followed by an epimorphic phase, in which growth continued without segment addition. This is consistent with the hypothesis that finite postembryonic segment addition (hemianamorphosis) is ancestral for arthropods and refines the timing of the origin of this important developmental mode.

The Mississippi River delivers tremendous amounts of freshwater and nutrients to the northern Gulf of Mexico, which results in the explosive growth of phytoplankton populations that are typically nutrient limited. Decomposition of phytoplankton blooms by aerobic bacteria can deplete oxygen concentrations in coastal systems, leading to the establishment of oxygen-limited “dead zones.” Changes in the availability of food and dissolved oxygen, as well as changes in temperature, can have wide-reaching effects on coastal food webs. Here, we investigate how primary productivity, dissolved oxygen, and sea-surface temperature affect the sizes of molluscan predators and prey in the northern Gulf of Mexico using collections of shells preserved in seafloor sediment on the continental shelf. We find that the size of bivalves, and the frequency of predatory drilling by snails, are most affected by dissolved oxygen: prey size increases and drilling predation decreases with decreasing concentrations of dissolved oxygen. Sea-surface temperature is positively associated with the size of both molluscan predators and prey. In contrast, net primary productivity has little direct association with size, and the predator-to-prey size ratio also does not vary consistently with environmental conditions in the northern gulf. Larger bivalves in areas with lower oxygen could reflect reduced pressure from predators and, consequently, greater life spans. Larger predator and prey sizes in warmer waters may reflect more optimal conditions for growth. The shells of recently deceased bivalves, and the associated traces of drilling snails on those shells, can be used to investigate long-standing hypotheses about the roles of environmental variation in body-size evolution through geologic time. Furthermore, future studies comparing these historical data with data from present-day communities may help us understand how coastal food webs are changing in response to various human activities.

The Mississippi River watershed drains 40% of the continental United States, and the tremendous primary productivity in the adjacent north-central Gulf of Mexico has created one of the most extensive dead zones on Earth. In contrast, smaller watersheds deliver fewer nutrients to the northeastern gulf, and consequently, productivity is limited and hypoxia is uncommon. How has variation in primary productivity, oxygen availability, and sea-surface temperature affected coastal food webs? Here, we investigate environmental controls on the size of molluscan predators and prey in the northern Gulf of Mexico using Holocene death assemblages. Linear mixed models indicate that bivalve size and the frequency of drilling predation are affected by dissolved oxygen concentrations; drilling frequency declines with declining oxygen, whereas bivalve size increases. In contrast, sea-surface temperature is positively associated with the size of molluscan predators and prey. Net primary productivity contributes relatively little to predator or prey size, and predator-to-prey size ratios do not vary consistently with environmental conditions across the northern gulf. Larger bivalves in areas of oxygen limitation may be due to decreased predation pressure and, consequently, greater prey longevity. The larger size of bivalves and predatory gastropods in warmer waters may reflect enhanced growth under these conditions, provided dissolved oxygen concentrations exceed a minimum threshold. Holocene death assemblages can be used to test long-standing hypotheses regarding environmental controls on predator–prey body-size distributions through geologic time and provide baselines for assessing the ongoing effects of anthropogenic eutrophication and warming on coastal food webs.

Conodonts were marine vertebrates with tooth-like structures that are commonly preserved in the fossil record. The Late Triassic conodont species Mockina ex gr. carinata and Mockina ex gr. englandi were exceptionally prevalent among marine animals from the middle Norian through to the Rhaetian stages of geologic time. Leading into the complete extinction of conodonts near the Triassic/Jurassic boundary, a significant change in conodont forms occurred across the Norian/Rhaetian boundary (NRB), with the common Rhaetian varieties from Europe exhibiting thinner shapes. This trend is not as evident in North America, where these varieties are very rare, but analyses on M. ex gr. carinata and M. ex gr. englandi specimens from across western Canadian demonstrate that comparable shifts in form occurred in North America across the NRB, confirming the global extent of these trends. The specimens of M. ex gr. carinata display a sequential width reduction from the middle Norian to late Norian to Rhaetian, whereas the specimens of M. ex gr. englandi display a width reduction only from the late Norian to Rhaetian. Specimens of both species that have a length to mid-breadth ratio greater than 3:1 are restricted to the Rhaetian. Specimens from the Kennecott Point section on Haida Gwaii, British Columbia, demonstrate that this shift in form occurred somewhat later than other events and changes associated with the NRB. The global trend of width reduction in many conodonts may reflect a change in diet away from hard food sources, perhaps suggesting some degree of pressure on the biological precipitation of minerals beginning around the NRB. This interpretation would support CO₂ release as the causal mechanism of the environmental disturbance at the NRB and identify the NRB as a significant turning point for Late Triassic ecosystems, marking the beginning of a protracted, multiphase end-Triassic mass extinction.

The Late Triassic conodont species Mockina ex gr. carinata and Mockina ex gr. englandi were exceptionally prevalent among the marine fauna of the Panthalassan realm from the middle Norian through to the Rhaetian. Leading into the complete extinction of conodonts near the Triassic/Jurassic boundary, a significant turnover event occurred in conodont fauna across the Norian/Rhaetian boundary (NRB), with the pectiniform elements of common Rhaetian genera from Tethys exhibiting minimal or absent platforms. This intergeneric trend of platform reduction is not as evident in Panthalassa, where these genera are very rare, but morphometric analyses of M. ex gr. carinata and M. ex gr. englandi specimens from across the Canadian Cordillera demonstrate that comparable shifts in morphology occurred intraspecifically in Panthalassa across the NRB, confirming the global extent of these trends. Pectiniform elements of M. ex gr. carinata display a sequential reduction of platform width from the middle Norian to late Norian to Rhaetian, whereas pectiniform elements of M. ex gr. englandi display a reduction of platform width only from the late Norian to Rhaetian. Specimens of both species that have a mid-platform length to breadth ratio greater than 3:1 are restricted to the Rhaetian. Specimens from the Kennecott Point section on Haida Gwaii, British Columbia, demonstrate that this morphological shift occurred somewhat later than other biostratigraphic proxies for the NRB. The global trend of platform width reduction in many conodont pectiniform elements may reflect a change in primary diet away from hard food sources, perhaps suggesting some degree of carbonate biomineralization suppression beginning around the NRB. This interpretation would support CO₂ outgassing as the causal mechanism of the environmental disturbance at the NRB and identify the NRB as a significant turning point for Late Triassic ecosystems, marking the beginning of a protracted, multiphase end-Triassic mass extinction.

The unique body plan of frogs has been largely conserved from at least 200 Myr, and its evolution from a more generalized tetrapod condition is still poorly understood, in part due to the scarce early fossil record of the group. The origin of the frog body plan has been classically explained as an adaptation to jumping, but recent studies incorporating new data in a phylogenetic context have challenged the popular jumping hypothesis. Here we revisit and test this hypothesis from a paleobiological perspective by integrating limb data from a wide range of extant and fossil frogs. We first explored the evolution of limb proportions from the Jurassic to the Paleogene to understand when the present limb diversity originated and whether, and to what extent, limb proportions have been conserved over the last 200 Myr. We then inferred the locomotor capabilities of extinct species, and from these inferences, we studied the frog locomotor diversity over geological time and reconstructed the ancestral state. The evolution of limb proportions is characterized by an early diversification that was underway in the Jurassic, followed by a repeated evolution of a limited range of limb morphologies that were already explored by the Early Cretaceous. In agreement with this early limb diversity, the Jurassic species were also locomotory diverse, and their inferred locomotor modes do not support the jumping hypothesis. We propose that the patterns found herein of repeated convergent evolution of both limb proportions and locomotor capabilities over geological time hamper any attempt to confidently infer the ancestral locomotion mode, and it therefore might be time to start focusing on other hypotheses on the origin of the frog body plan that are not related to locomotion.

The unique body plan of frogs (Lissamphibia: Anura) has been largely conserved from at least 200 Myr, and its evolution from a more generalized tetrapod condition is still poorly understood, in part due to the scarce early fossil record of Salientia, the anuran total-group. The origin of the anuran Bauplan has been classically explained as an adaptation to jumping, but recent studies incorporating new data in a phylogenetic context have challenged the popular jumping hypothesis. Here we revisit and test this hypothesis from a paleobiological perspective by integrating limb data from a wide range of extant and fossil frogs. We first explored the evolution of limb proportions from the Jurassic to the Paleogene to understand when the present limb diversity originated and whether, and to what extent, limb proportions have been conserved over the last 200 Myr. We then inferred the locomotor capabilities of extinct species by phylogenetic flexible discriminant analysis, and from these inferences, we studied the locomotor diversity of frogs over geological time and reconstructed the ancestral state for frog-like salientians. The evolution of limb proportions is characterized by an early diversification that was underway in the Jurassic, followed by a repeated convergence over a limited area of the morphospace that was already explored by the Early Cretaceous. In agreement with this early limb diversity, the Jurassic stem species were also locomotory diverse, and their inferred locomotor modes do not support the jumping hypothesis. We propose that the patterns found herein of repeated convergent evolution of both limb proportions and locomotor capabilities over geological time hamper any attempt to confidently infer the ancestral locomotion mode and, it therefore might be time to start focusing on other hypotheses on the origin of the anuran Bauplan that are not related to locomotion.

Oviraptorosaurs, small, feathered dinosaurs from the Cretaceous, have left an extensive fossil record of egg clutches, including numerous nests preserving adults on top of their eggs. Despite the volume and quality of these finds, the bizarre nest arrangement makes them difficult to interpret. Oviraptorosaur-style nests consist of up to three layers of eggs, organized in concentric rings within the interior walls of a volcano-shaped mound. Such nests are unknown among modern animals, and so this study used actualistic experimentation to investigate the thermodynamics of the clutch. Experiments used 36 infertile emu eggs, which are close in mass to those of medium-sized oviraptorosaurs, arranged in a sand nest as interpreted from the fossil record. A surrogate dinosaur was constructed (warmed by an interior water bath) to represent an adult attending its nest. Energy from the surrogate was monitored as it flowed throughout the nest. Resulting clutch temperatures are significantly above ambient conditions and fall in a range between modern bird and crocodile incubation temperatures. These experiments seem to support the notion that an attending adult oviraptorosaur would have the capacity to raise clutch temperatures above ambient using its body temperature, perhaps representative of crucial innovations on the path from crocodilian-type nesting to modern avian reproductive practices.

Numerous, high-quality reproduction-related oviraptorosaur fossils have been described. However, oviraptorosaur-style nests are unknown among extant animals, and their curious construction makes nesting behavior difficult to interpret. Experiments were undertaken to better understand oviraptorosaur nesting strategies. A surrogate was constructed and placed atop mock-oviraptorosaur nests built from sand and 36 infertile emu eggs (as Macroolithus approximations) arranged according to the most current nest reconstructions. Thermometers, placed within each egg and throughout the experimental area, recorded energy flow from the surrogate dinosaur into the nesting microenvironment. One experiment examined a basic open nest warmed from above; the second, a fully buried clutch warmed from above; and the third, a nest open like the first but with heating elements (representing hindlimbs) extending down into the nest. It was found that egg temperatures in each scenario surpassed ambient temperatures without requiring excessive energy input. Final clutch temperatures were below most avian values, closer to crocodilian incubation, but are likely conservative, considering experimental parameters. These results may support the idea that an oviraptorosaur could use adult-generated energy to warm a clutch above ambient conditions. Additionally, egg tiers would be warmer and more uniform in temperature if heated by elements within the nest, such as hindlimbs, instead of solely from above. Results from the second experiment indicate that an endothermic adult could possibly warm a clutch fully buried beneath itself despite a barrier. Although not likely a behavior exhibited by oviraptorosaurs, such results suggest an important evolutionary step bridging guarded subterranean eggs and contact-incubated subaerial eggs.

Bones are living tissues and require oxygen from the blood circulation to grow, to replace damaged bone with new bone, and to carry out other functions. The amount of blood flowing into bones is related to the energy costs of these functions. It is difficult to measure blood flow rate into tissues, even in living animals, so it was thought impossible for it to be measured in extinct ones. However, blood enters the shaft of a leg bone through a hole, called a foramen, and the size of the foramen is related in turn to the size of its artery and the rate of blood flow. We can now use this surprisingly simple “foramen technique” to evaluate bone blood flow rates from fossil bones. In this study, we measured blood flow rate to the tibia (a lower hind-limb bone) of the dinosaur Maiasaura that lived more than 74 million years ago. They grew astonishingly fast, reaching 200–400 kg in their second year and reproductive adulthood in their third year and exceeding 3 metric tons before their teens. The fast growth of these hadrosaurs is associated with relatively high blood flow rates to the bones of juveniles compared with adults. Blood flow to 1 g of bone is 15 times higher in a 2 kg hatchling than in a full-grown adult, and that in a 1-year-old is 4 times higher. These differences reveal the enormous cost of building bones in the growth stages compared with the costs of maintaining them later in life.

Fossil bones were once living tissues that demanded internal blood perfusion in proportion to their metabolic requirements. Metabolic rates were primarily associated with bone growth (modeling) in the juvenile stages and with alteration and repair of existing bone affected by weight bearing and locomotion (remodeling) in later stages. This study estimates blood flow rates to the tibia shafts of the Late Cretaceous hadrosaurid Maiasaura peeblesorum, based on the size of the primary nutrient foramina in fossil bones. Foramen size quantitatively reflects arterial size and hence blood flow rate. The results showed that the bone metabolic intensity of juveniles (ca. 1 year old) was greater than fourfold higher than that of 6- to 11-year-old adults. This difference is much greater than expected from standard metabolic scaling and is interpreted as a shift from the high metabolic demands for primary bone modeling in the rapidly growing juveniles to a lower metabolic demand of adults to remodel their bones for repair of microfractures accumulated during locomotion and weight bearing. Large nutrient foramina of adults indicate a high level of cursorial locomotion characteristic of tachymetabolic endotherms. The practical value of these results is that juvenile and adult stages should be treated separately in interspecific analyses of bone perfusion in relation to body mass.

The body size of fossil organisms has been an important area of research for paleobiologists for well over a century, because body size can tell us much about widespread trends in the evolution of major groups of living things. However, paleobiologists often study body size by focusing only on size-related information collected from fossils. Without information about a fossil organism's biology and geologic history beyond its size, we cannot understand what is driving body-size change over evolutionary time in a meaningful way.

Luckily, the ways evolutionary biologists already think about growth and development (ontogeny) and evolutionary relationships among taxa (phylogeny) can help us resolve this issue. In particular, by looking at how a species' body size, age, and other observable traits (phenotype) change over its growth and development, we can track how a species' body-size changes over the course of its time on Earth. Furthermore, we can compare these patterns between closely related species, and identify the sources of body-size change in deep time.

To show how these ideas are a practical solution to problems in the fossil record, we applied them to a common pattern called the “Lilliput effect.” Named after the island of Lilliput and its tiny inhabitants in Gulliver's Travels, this pattern describes a sharp decrease in organism body size during extinctions in Earth history. Despite the Lilliput effect being very common, we understand little about how it occurs. Along with providing a stronger definition for the Lilliput effect, we use our framework to note some likely processes for the Lilliput effect (such as changes to development), and some famous cases where we could easily test these ideas.

Body size has a long history of study in paleobiology and underlies many important phenomena in macroevolution. Body-size patterns in the fossil record are often examined by utilizing size data alone, which hinders our ability to describe the biological meaning behind size change on macroevolutionary timescales. Without data reflecting the biological and geologic factors that drive size change, we cannot assess its mechanistic underpinnings.

Existing frameworks for studying ontogeny and phylogeny can remedy this problem, par ticularly the classic age–size–“shape” space originally developed for studies of heterochrony When evaluated based on metrics for age, size, and phenotype in populations, propose mechanisms for size change can be outlined theoretically and tested empirically in the record Using this framework, we can compare ontogenetic trajectories within and between specie and determine how changes in size emerge. Here, we outline ontogenetic mechanisms for evo lutionary size change, such as heterochrony, as well as how geologic factors can drive appar ent non-biolo ical size chan e (e ta honomic size sortin)

To demonstrate the utility of this framework in actual paleobiological problems, we apply it to the Lilliput effect, a compelling and widely documented pattern of size decrease during extinction events. However, little is known about the mechanisms underlying this pattern. We provide a brief history of the Lilliput effect and refine its definition in a framework that can be mechanistically tested. Processes that likely produce Lilliput effects include allometric and sequence repatterning (including heterochrony) and evolutionary size-selective sorting. We describe these mechanisms and highlight relevant examples of the Lilliput effect for which feasible empirical tests are possible.

Ediacaran-age sedimentary rocks (635–538.8 million years ago) contain the oldest animal fos sils that are visible to the naked eye. Several explanations have been suggested for the origins o animals in the Ediacaran, their disappearance at the end of the Ediacaran, and the followin Cambrian explosion of animals (538.8–485.4 million years ago). For this study, we examine Ediacaran–Cambrian evolutionary patterns and how fossils (data from the Paleobiolog Database) are related to the amount of sedimentary rock (data from Macrostrat) from th same time. Amounts of Cambrian rock increase to more than five times the amount o rock in the Ediacaran. The number of fossils increases in an equally dramatic manner from the Ediacaran to the Cambrian, and there are strong positive correlations between the amoun of rock and the number of fossils. It is well known that in the Cambrian, sea level rose, leadin to the flooding of the North American continent. This relative rise in sea level would hav increased the amount of rock deposited on the continent. Cambrian flooding of the continen would have also provided a wider variety of shallow-marine environments for Cambrian ani mals to expand into, providing at least a partial explanation for the dramatic increase in th number and physical diversity of Cambrian fossils. A smaller flooding event during th Ediacaran may have enabled early fossil animals to develop evolutionary traits for shallow marine environments that allowed them to rapidly evolve during the larger flooding in th Cambrian. The results of this study demonstrate that relative sea-level rise and associated con tinental-scale flooding known to influence the amount of rock may have played a role in shap ing evolutionary patterns of Earth's earliest animals.

Strata of the Ediacaran Period (635–538.8 Ma) yield the oldest known fossils of complex, macroscopic organisms in the geologic record. These “Ediacaran-type” macrofossils (known as the Ediacaran biota) first appear in mid-Ediacaran strata, experience an apparent decline through the terminal Ediacaran, and directly precede the Cambrian (538.8–485.4 Ma) radiation of animals. Existing hypotheses for the origin and demise of the Ediacaran biota include: changing oceanic redox states, biotic replacement by succeeding Cambrian-type fauna, and mass extinction driven by environmental change. Few studies frame trends in Ediacaran and Cambrian macroevolution from the perspective of the sedimentary rock record, despite well-documented Phanerozoic covariation of macroevolutionary patterns and sedimentary rock quantity. Here we present a quantitative analysis of North American Ediacaran–Cambrian rock and fossil records from Macrostrat and the Paleobiology Database. Marine sedimentary rock quantity increases nearly monotonically and by more than a factor of five from the latest Ediacaran to the late Cambrian. Ediacaran–Cambrian fossil quantities exhibit a comparable trajectory and have strong (r_s > 0.8) positive correlations with marine sedimentary area and volume flux at multiple temporal resolutions. Even so, Ediacaran fossil quantities are dramatically reduced in comparison to the Cambrian when normalized by the quantity of preserved marine rock. Although aspects of these results are consistent with the expectations of a simple fossil preservation–induced sampling bias, together they suggest that transgression–regression and a large expansion of marine shelf environments coincided with the diversification of animals during a dramatic transition that is starkly evident in both the sedimentary rock and fossil records.