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We propose a new proxy that employs assemblages of fossil turtle shells to estimate the timing and depth at which fossilization and lithification occur in shallowly buried terrestrial strata. This proxy, the Turtle Compaction Index (TCI), leverages the mechanical failure properties of extant turtle shells and the material properties of sediments that encase fossil turtle shells to estimate the burial depths over which turtle shells become compacted. Because turtle shells are one of the most abundant macroscopic terrestrial fossils in late Mesozoic and younger strata, the compactional attributes of a suite of turtle shells can be paired with geochronologic and stratigraphic data to constrain burial histories of continental settings--a knowledge gap unfilled by traditional burial-depth proxies, most of which are more sensitive to deeper burial depths. Pilot TCI studies of suites of shallowly buried turtle shells from the Denver and Williston basins suggest that such assemblages are sensitive indicators of the depths (~10-500 m) at which fossils and their encasing sediment become sufficiently lithified to inhibit further shell compaction, which is when taphonomic processes correspondingly wane. This work also confirms previously hypothesized shallow Cenozoic burial histories for each of these basins. TCI from mudstone-encased turtle shells can be paired with thicknesses and ages of overlying strata to create geohistorical burial curves that indicate when such post-burial processes were active. [ABSTRACT FROM AUTHOR]

The Chicxulub bolide impact has been linked to a mass extinction of plants at the Cretaceous—Paleogene boundary (KPB; ∼66 Ma), but how this extinction affected plant ecological strategies remains understudied. Previous work in the Williston Basin, North Dakota, indicates that plants pursuing strategies with a slow return-on-investment of nutrients abruptly vanished after the KPB, consistent with a hypothesis of selection against evergreen species during the globally cold and dark impact winter that followed the bolide impact. To test whether this was a widespread pattern we studied 1,303 fossil leaves from KPB-spanning sediments in the Denver Basin, Colorado. We used the relationship between petiole width and leaf mass to estimate leaf dry mass per area (LMA), a leaf functional trait negatively correlated with rate of return-on-investment. We found no evidence for a shift in this leaf-economic trait across the KPB: LMA remained consistent in both its median and overall distribution from approximately 67 to 65 Ma. However, we did find spatio-temporal patterns in LMA, where fossil localities with low LMA occurred more frequently near the western margin of the basin. These western margin localities are proximal to the Colorado Front Range of the Rocky Mountains, where an orographically driven high precipitation regime is thought to have developed during the early Paleocene. Among these western Denver Basin localities, LMA and estimated mean annual precipitation were inversely correlated, a pattern consistent with observations of both fossil and extant plants. In the Denver Basin, local environmental conditions over time appeared to play a larger role in determining viable leaf-economic strategies than any potential global signal associated with the Chicxulub bolide impact. [ABSTRACT FROM AUTHOR]

Baenidae are the most diverse clade of turtles from the latest Cretaceous and earliest Paleogene of North America. Palatobaena species have a distinctive cranial bauplan within baenid turtles and we herein describe a new species, P. knellerorum, based on a complete cranium from the lower Paleocene Denver Formation in the Denver Basin, Colorado. Palatobaena knellerorum differs from other Palatobaena species in the extreme degree of its dorsally oriented orbits, as well as the presence of a broad pentagonal midline crest formed by the supraoccipital and parietals. It can be further diagnosed from other species of Palatobaena by the following unique combination of characters: having a broadly rounded cranium, presence of upper temporal emargination that extends well anterior to the otic chamber, a broad exposure of the supraoccipital on the dorsal skull roof, a broadly rounded 'tongue groove' between the maxillae and premaxillae, and a deeply emarginated nasal region that exposes the underlying premaxillae in dorsal view. Palatobaena knellerorum most closely resembles the Maastrichtian/Danian taxon Palatobaena cohen and the Danian taxon P. bairdi. A phylogenetic analysis places P. knellerorum as sister to P. bairdi, and these taxa are sister to P. cohen. Palatobaena knellerorum overlaps in time with P. cohen, but the two are separated geographically with P. knellerorum restricted to the Denver Basin and P. cohen restricted to the Williston Basin. Cranial anatomical data and sedimentological data suggest Palatobaena spp. were bottom dwelling turtles living in shallow water environments that used a putative proboscis to prey upon freshwater-shelled invertebrates. [ABSTRACT FROM AUTHOR]

North American soft-shelled turtles, including trionychines and plastomenids, are incredibly abundant in latest Cretaceous through earliest Paleocene sediments. Here we describe a new species of plastomenid turtle, Plastomenus joycei, based on a nearly complete early Danian skeleton from the Denver Formation in the Denver Basin, Colorado. Plastomenus joycei is differentiated from all other plastomenid turtles based on the presence of large eighth costals that are much longer than wide, sinusoidal raised ridges on the carapace, flat posterior edge of the carapace, spike-like epiplastra that lack callosities, a broad entoplastron that lacks a callosity, hyoplastra with two lateral processes, presence of metaplastically ossified hyoplastral shoulders (i.e., anteriorly protruding lappets), and a broad midline contact between the hypo- and xiphiplastra. Plastomenus joycei broadly resembles the Cretaceous Gilmoremys spp. and the Eocene Plastomenus thomasii and exhibits intermediate morphology between the two, most notably in the degree of ossification of the plastron. The increase in plastral ossification, as well as a decrease in overall size and an increase in the doming of the shell, co-occurs with the Cretaceous–Paleogene mass extinction and these morphological changes may be in response to an increase in mammalian predators during the early Paleogene. Plastomenus joycei is most commonly found in riverine sandstone sediments and is hypothesized to be a riverine turtle. [ABSTRACT FROM AUTHOR]

The Cretaceous-Paleogene (K-Pg) boundary interval represents one of the most significant mass extinctions and ensuing biotic recoveries in Earth history. Earliest Paleocene fossil mammal faunas corresponding to the Puercan North American Land Mammal Age (NALMA) are thought to be highly endemic and potentially diachronous, necessitating precise chronostratigraphic controls at key fossil localities to constrain recovery dynamics in continental biotas following the K-Pg mass extinction. The Laramide synorgenic sedimentary deposits within the Denver Basin in east-central Colorado preserve one of the most continuous and fossiliferous records of the K-Pg boundary interval in North America. Poor exposure in much of the Denver Basin, however, makes it difficult to correlate between outcrops. To constrain fossil localities in coeval strata across the basin, previous studies have relied upon chronostratigraphic methods such as magnetostratigraphy. Here, we present a new highresolution magnetostratigraphy of 10 lithostratigraphic sections spanning the K-Pg boundary interval at Corral Bluffs located east of Colorado Springs in the southern part of the Denver Basin. Fossil localities from Corral Bluffs have yielded limited dinosaur remains, mammal fossils assigned to the Puercan NALMA, and numerous fossil leaf localities. Palynological analyses identifying the K-Pg boundary in three sections and two independent, but nearly identical, 206Pb/238U age estimates for the same volcanic ash, provide key temporal calibration points. Our paleomagnetic analyses have identified clear polarity reversal boundaries from chron C30n to chron C28r across the sections. It is now possible to place the fossil localities at Corral Bluffs within the broader basinwide chronostratigraphic framework and evaluate them in the context of K-Pg boundary extinction and recovery. [ABSTRACT FROM AUTHOR]