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1. Shifts in food webs and niche stability shaped survivorship and extinction at the end-Cretaceous

   https://doi.org/10.1126/sciadv.add5040  

   García-Girón, Jorge; Chiarenza, Alfio Alessandro; Alahuhta, Janne; DeMar, David G.; Heino, Jani; Mannion, Philip D.; Williamson, Thomas E.; Wilson Mantilla, Gregory P.; Brusatte, Stephen L. (December 2022, Science Advances)

   It has long been debated why groups such as non-avian dinosaurs became extinct whereas mammals and other lineages survived the Cretaceous/Paleogene mass extinction 66 million years ago. We used Markov networks, ecological niche partitioning, and Earth System models to reconstruct North American food webs and simulate ecospace occupancy before and after the extinction event. We find a shift in latest Cretaceous dinosaur faunas, as medium-sized species counterbalanced a loss of megaherbivores, but dinosaur niches were otherwise stable and static, potentially contributing to their demise. Smaller vertebrates, including mammals, followed a consistent trajectory of increasing trophic impact and relaxation of niche limits beginning in the latest Cretaceous and continuing after the mass extinction. Mammals did not simply proliferate after the extinction event; rather, their earlier ecological diversification might have helped them survive. 

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2. A stable isotope-based investigation of mammalian paleoecology across the Cretaceous/Paleogene boundary in the Denver Basin, Colorado, U.S.A.

   Breyak, Annie; Fricke, Henry; Wainwright, Jensen; Blumenthal, Scott; Chritz, Kendra; Lyson, Tyler; Krause, David; Chester, Stephen (November 2024, Society of Vertebrate Paleontology)

   Understanding and mitigating the e ects of our ongoing biodiversity crisis requires a deep-time perspective on how ecosystems recover in the aftermath of environmental catastrophes. The mass extinction event at the Cretaceous/Paleogene (K/Pg) boundary (ca. 66 Ma) represents a natural laboratory wherein the tempo and mode of biotic recovery can be studied with high chronostratigraphic resolution. Although the morphological evolution of mammals across this event has been reconstructed from skeletal remains, the exact nature of any changes in dietary preference remains unknown. A primary goal here is to fill this gap by investigating how ecological preferences of mammals, reflected by diet, changed from the Late Cretaceous, when they shared landscapes with dinosaurs, to the earliest Paleogene, when they did not. To accomplish this, carbon and oxygen isotope ratios of fossil tooth enamel (bioapatite) were measured using laserablation mass spectrometry in order to infer animal diet and drinking water sources, which vary depending on the niche occupied by an animal. Fossil teeth were collected from two sites located within 400 meters of one another within the West Bijou Creek field area of the Denver Basin, one 9 meters (~128 ky pre-K/Pg) below the boundary (teeth from ceratopsian and hadrosaurid dinosaurs and the multituberculate mammal Mesodma, as well as gar fish scales), and the other 4 meters (~57 ky post-K/Pg) above (Mesodma teeth and gar fish scales). Carbon isotope ratios (δ13C) of Mesodma tooth enamel vary significantly across the K/Pg boundary, with Late Cretaceous teeth having lower and more variable δ13C (-10.1 to -16.4‰, n=4) and early Paleocene teeth having higher and less variable δ13C (-5.3 to 9.0 ‰, n=5), the latter being similar to values for Late Cretaceous dinosaurs. These results suggest Mesodma had very di erent dietary behaviors following the extinction event, presumably a result of the disappearance of non-avian dinosaurs as well as 57% of North American plants, both of which made new food sources and niches available to them. These results also hint at a decoupling of behavioral change from morphological change, at least in the case of Mesodma, over 10 ky timescales. Isotopic analysis of teeth from other Late Cretaceous and earliest Paleogene mammalian taxa is ongoing and will hopefully allow for more detailed interpretations of ecological change across the K/Pg extinction event in the Denver Basin. 

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3. Early Paleocene floras from the San Juan Basin (New Mexico, USA) record terrestrial ecosystem change following the Cretaceous-Paleogene Boundary

   Flynn, A.G.; Secord, R.; Geng, J.; Abbuhl, B.; Williamson, T.E.; Brusatte, S.L.; Peppe, D.J (October 2020, Geological Society of America Abstracts with Programs)

   Early Paleocene floral communities were substantially restructured as a result of the Cretaceous-Paleogene (K-Pg) mass extinction approximately 66.0 Ma. While events immediately adjacent to the K-Pg boundary have been extensively studied, comparatively little research has looked at long-term terrestrial ecosystem recovery during the early Paleocene. The San Juan Basin (SJB), located in northwestern New Mexico, preserves an exceptional, large, and well-dated early Paleocene plant record making it an ideal location to study long-term recovery of early Paleocene terrestrial ecosystems. Here we investigate early Paleocene terrestrial ecosystem change using a coupled high-resolution plant and δ13C record from the SJB. Plant macrofossils were collected from the lower Paleocene Ojo Alamo Sandstone and lower Nacimiento Formation in the SJB spanning the initial ~1.5 myr of the Paleocene. Macrofloral extinction, origination, and net diversification rates were simultaneously estimated using the Pradel capture-mark-recapture (CMR) model from 66.0 – 64.5 Ma with 100 Kyr time-steps. Two intervals of decreasing floral diversity were identified: a short interval at ~65.5 Ma and a prolonged interval from ~65.2 – 64.7 Ma. Two short intervals of rapidly increasing floral diversity were also identified: the first at ~65.3 Ma and the second at ~64.6 Ma. The onset of both intervals of decreasing floral diversity are coeval with a -1.5 to -2.5 ‰ bulk organic δ13C excursion. We also applied the Pradel CMR model to contemporaneous macrofloras from the Denver Basin (DB), Colorado and the Williston Basin (WB), North Dakota and Montana. The floral diversity patterns estimated from the DB and WB indicate intervals of increasing and decreasing floral diversity that are coeval with the same intervals identified in the SJB. This suggests a regional driver in patterns of floral diversity change during the early Paleocene in western North America, which reflects prolonged terrestrial ecosystem instability following the K-Pg mass extinction. 

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4. Small mammals in a large tree: examining the timing of the early therian radiation using a novel phylogenetic metatree; Journal of Vertebrate Paleontology, Program and Abstracts, 2023

   Grossnickle, D; Hellert, S; Ely, R; Slater, G; Lloyd, G; Angielczyk, K_D; Bormet, A; Wilson_Mantilla, G (October 2023, Society of Vertebrate Paleontology)

   Mammals achieved considerably greater ecological diversity in the Cenozoic compared to the Mesozoic. However, there remains uncertainty about the origins of this rise in diversity, such as whether it was triggered by novel ecological opportunities following the Cretaceous-Paleogene (K-Pg) mass extinction event 66 Ma. To test hypotheses on the timing of the mammalian radiation, studies commonly rely on analyses of fossil-only datasets, which often ignore the influence of phylogeny, or analyses of extant- only datasets, which struggle to recreate macroevolutionary patterns in deep time. Thus, an integrative approach is needed that incorporates paleontological data, neontological data, and phylogenetic comparative methods. To this end, we generated a time-calibrated meta-phylogeny(‘metatree’) comprising over 3000 species of trechnotherians (therians and close relatives) from the Mesozoic and Cenozoic, based on 115 published character matrices and a molecular phylogeny of extant mammals. To quantify ecomorphological patterns, we collected jaw lengths (as a proxy for size) and jaw measurements that correlate with diet for 430 extinct and extant species. We then fit a suite of evolutionary models to the data to test various hypotheses on the i) timing of the start of the therian radiation and ii) potential shift in mode of evolution. Although results are sensitive to time-calibration methods and phylogenetic uncertainty, they generally show evidence for a shift in mode of evolution prior to the K-Pg boundary. For both the jaw correlates of diet and body size analyses, results suggest a shift from a constrained (Ornstein-Uhlenbeck) or stochastic (Brownian motion) mode of evolution to an ‘early burst’ mode of evolution, with the best-fitting models being those that model this shift occurring between 76 and 66 Ma. These results suggest that the ecological diversification of therians began in the latest Cretaceous, prior to the K-Pg extinction event. However, our results are also congruent with the hypothesis that the therian radiation was a multiple-step process that involved bursts in diversification at multiple points in time, such as following both the Cretaceous Terrestrial Revolution (ca. 80 Ma) and the K-Pg extinction event. Further, we emphasize the importance of continued efforts to collect early mammal fossils and resolve relevant stratigraphic information, which will allow for more robust tests on the timing of the mammalian radiation. 

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5. Cretaceous–Paleogene plant extinction and recovery in Patagonia

   https://doi.org/10.1017/pab.2020.45  

   Stiles, Elena; Wilf, Peter; Iglesias, Ari; Gandolfo, María A.; Cúneo, N. Rubén (November 2020, Paleobiology)

   null (Ed.)

   Abstract The Cretaceous–Paleogene (K/Pg) extinction appears to have been geographically heterogeneous for some organismal groups. Southern Hemisphere K/Pg palynological records have shown lower extinction and faster recovery than in the Northern Hemisphere, but no comparable, well-constrained Southern Hemisphere macrofloras spanning this interval had been available. Here, macrofloral turnover patterns are addressed for the first time in the Southern Hemisphere, using more than 3500 dicot leaves from the latest Cretaceous (Maastrichtian) and the earliest Paleocene (Danian) of Argentine Patagonia. A maximum ca. 90% macrofloral extinction and ca. 45% drop in rarefied species richness is estimated across the K/Pg, consistent with substantial species-level extinction and previously observed extirpation of host-specialized leaf mines. However, prior palynological and taxonomic studies indicate low turnover of higher taxa and persistence of general floral composition in the same sections. High species extinction, decreased species richness, and homogeneous Danian macrofloras across time and facies resemble patterns often observed in North America, but there are several notable differences. When compared with boundary-spanning macrofloras at similar absolute paleolatitudes (ca. 50°S or 50°N) from the Williston Basin (WB) in the Dakotas, both Maastrichtian and Danian Patagonian species richnesses are higher, extending a history of elevated South American diversity into the Maastrichtian. Despite high species turnover, our analyses also reveal continuity and expansion of leaf morphospace, including an increase in lobed and toothed species unlike the Danian WB. Thus, both Patagonian and WB K/Pg macrofloras support a significant extinction event, but they may also reflect geographically heterogeneous diversity, extinction, and recovery patterns warranting future study. 

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