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Abstract Isolated pan-chelydrid turtle shell fragments are common in Late Cretaceous and early Paleocene sediments across western North America, but more complete and associated specimens are rare, obfuscating our understanding of the group’s early evolution. Here we describe a new genus and species,Tavachelydra stevensoni, of stem-chelydrid turtle from the early Paleocene of the Denver Formation (Danian, Puercan I and II) of Colorado based on complete shells, associated pelvic material, and referred cranial material. Our phylogenetic analysis placesT. stevensonias the immediate sister to crown chelydrids based on, among others, a purely ligamentous attachment of the plastron and carapace. The costiform process of the nuchal, an important character complex in chelydroid turtles, shows variation in either ending in peripheral II or III. TheT. stevensonimaterial was mostly found in laminated fine-grained deposits, suggesting this taxon inhabited ponded-water environments. The referred cranial material shows broad triturating surfaces indicating a durophagous diet, further underscoring durophagy as an important feeding strategy during the early Paleocene. 

Understanding the role of humans as ‘ecosystem engineers’ requires a deep-time perspective rooted in evolutionary history and the fossil record. However, no con-ceptual framework exists for studying the rise of ecosystem engineering in deep time, requiring us to consider effects that fall outside the scope of traditional defini-tions. Here, we present a new framework applicable to both modern and ancient engineering-type effects. We propose a new term – ‘Earth system engineering’ – to describe biological processes that alter the structure and function of planetary spheres, and which combines core tenets of ecosystem engineering, niche construction, and legacy effects. We illustrate this framework using the fossil record, and show how it can be applied across the tree of life, and throughout Earth history. 

Alongside the Chicxulub meteorite impact, Deccan volcanism is considered a primary trigger for the Cretaceous–Paleogene (K–Pg) mass extinction. Models suggest that volcanic outgassing of carbon and sulfur—potent environmental stressors—drove global temperature change, but the relative timing, duration, and magnitude of such change remains uncertain. Here, we use the organic paleothermometer MBT′_5meand the carbon-isotope composition of two K–Pg-spanning lignites from the western Unites States, to test models of volcanogenic air temperature change in the ~100 kyr before the mass extinction. Our records show long-term warming of ~3°C, probably driven by Deccan CO₂emissions, and reveal a transient (<10 kyr) ~5°C cooling event, coinciding with the peak of the Poladpur “pulse” of Deccan eruption ~30 kyr before the K–Pg boundary. This cooling was likely caused by the aerosolization of volcanogenic sulfur. Temperatures returned to pre-event values before the mass extinction, suggesting that, from the terrestrial perspective, volcanogenic climate change was not the primary cause of K–Pg extinction. 

Mass extinctions are major influences on both the phylogenetic structure of the modern biota and our ability to reconstruct broad-based patterns of evolutionary history. The most recent mass extinction is also the most famous—that which implicates a bolide impact in defining the Cretaceous/Palaeogene boundary (K/Pg). Although the biotic effects of this event receive intensive scrutiny, certain ecologically important and diverse groups remain woefully understudied. One such group is the freshwater ray-finned fishes (Actinopterygii). These fish represent 25% of modern vertebrate diversity, yet the isolated and fragmentary nature of their K/Pg fossil record limits our understanding of their diversity dynamics across this event. Here, we address this problem using diversification analysis of molecular-based phylogenies alongside a morphotype analysis of fossils recovered from a unique site in the Denver Basin of western North America that provides unprecedented K/Pg resolution. Our results reveal previously unrecognized signals of post-K/Pg diversification in freshwater clades and suggest that the change was driven by localized and sporadic patterns of extinction. Supported inferences regarding the effects of the K/Pg event on freshwater fish also inform our expectations of how freshwater faunas might recover from the current biodiversity crisis. 

The Periptychidae, an extinct group of archaic ungulates (‘condylarths’), were the most speciose eutherian mammals in the earliest Paleocene of North America, epitomizing mammalian ascendency after the Cretaceous–Paleogene (K–Pg) mass extinction. Although periptychids are mostly known from fragmentary gnathic remains, the Corral Bluffs area within the Denver Basin, Colorado, has yielded numerous exceptionally well-preserved mammalian fossils, including periptychids, from the earliest Paleocene. Here we describe a partial cranium and articulated dentaries plus an additional unassociated dentary fragment of a small-bodied (~273–455 g) periptychid from ca. 610 thousand years after the K–Pg mass extinction (Puercan 2 North American Land Mammal ‘age’) at Corral Bluffs. Based on these new fossils we erect Militocodon lydae gen. et sp. nov. The dentition of M. lydae exhibits synapomorphies that diagnose the Conacodontinae, but it is plesiomorphic relative to Oxyacodon, resembling putatively basal periptychids like Mimatuta and Maiorana in several dental traits. As such, we interpret M. lydae as a basal conacodontine. Its skull anatomy does not reveal clear periptychid synapomorphies and instead resembles that of arctocyonids and other primitive eutherians. M. lydae falls along a dental morphocline from basal periptychids to derived conacodontines, which we hypothesize reflects a progressive, novel modification of the hypocone to enhance orthal shearing and crushing rather than grinding mastication. The discovery and thorough descriptions and comparisons of the partial M. lydae skull represent an important step toward unraveling the complex evolutionary history of periptychid mammals. 

Abstract Saxochelys gilbertiis a baenid turtle from the Late Cretaceous Hell Creek Formation of the United States of America known from cranial, shell, and other postcranial material. Baenid turtles are taxonomically diverse and common fossil elements within Late Cretaceous through Eocene faunas. Detailed anatomical knowledge is critical to understanding the systematics and morphological evolution of the group. This is particularly important as baenids represent an important group of continental vertebrates that survived the mass extinction event associated with the Cretaceous/Paleogene boundary. High-resolution micro-computed tomography scanning of the holotype skull reveals additional anatomical details for the already well-knownSaxochelys gilberti. This includes the revision of some anatomical statements from the original description, but also detailed knowledge on internal anatomical features of the braincase and the description of a well-preserved axis (cervical vertebra 2). Our new detailed description and previous work on the shell and postcrania makeSaxochelysone of the best-described, nearly complete baenid turtles, which are often only known from either isolated shell or cranial material. A revised phylogenetic analysis confirms the position ofSaxochelys gilbertias a derived baenid (Eubaeninae) more closely related toBaena arenosathan toEubaena cephalica. 

Denazinemys nodosais a Late Cretaceous representative of the North American turtle clade Baenidae diagnosed, among others, by a shell surface texture consisting of raised welts. We provide a detailed description of a partial skeleton from the late Campanian Kaiparowits Formation of Utah, USA, including bone-by-bone analysis of its cranium based on images obtained using micro-computed tomography. A revised phylogenetic analysis confirms placement ofDenazinemys nodosaclose toEubaena cephalicaandBoremysspp. within the clade Eubaeninae. Comparison with a second skull from the Kaiparowits Formation previously assigned toDenazinemys nodosaquestions its referral to this taxon. An assortment of specimens from the Early to Late Campanian of Mexico and the USA had previously been referred toDenazinemys nodosabased on shell surface texture alone, even though this characteristic is known to occur in other baenids. Our review of all available material concludes thatDenazinemys nodosais currently only known from the Late Campanian of New Mexico and Utah. 

The origin of turtles and their uniquely shelled body plan is one of the longest standing problems in vertebrate biology. The unfulfilled need for a hypothesis that both explains the derived nature of turtle anatomy and resolves their unclear phylogenetic position among reptiles largely reflects the absence of a transitional fossil record. Recent discoveries have dramatically improved this situation, providing an integrated, time-calibrated model of the morphological, developmental, and ecological transformations responsible for the modern turtle body plan. This evolutionary trajectory was initiated in the Permian (>260 million years ago) when a turtle ancestor with a diapsid skull evolved a novel mechanism for lung ventilation. This key innovation permitted the torso to become apomorphically stiff, most likely as an adaption for digging and a fossorial ecology. The construction of the modern turtle body plan then proceeded over the next 100 million years following a largely stepwise model of osteological innovation.