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1. Shifts in trophic architecture and ecospace stability determining survivorship and extinction at the end-Cretaceous

   Chiarenza, Alfio Alessandro; García-Girón, Jorge; Alahuhta, Janne; DeMar, D. G.; Jani, Heino; Mannion, P. D.; Williamson, T. E.; Wilson Mantilla, G. P.; Brusatte, S. L. (November 2022, Journal of Vertebrate Paleontology)

   An explanation for why some species, such as non-avian dinosaurs, became extinct, whereas others, including mammals, survived the Cretaceous/Paleogene (K/Pg) mass extinction, 66 million years ago (Ma) is still debated. What were the mechanisms behind community restructuring and the emergence of new ecological opportunities after the K/Pg event, selectively driving extinction and survivorship patterns? Using Markov networks, ecological niche partitioning and Earth System models, we reconstructed disruptions in continental food web dynamics, simulating long-term trajectories in ecospace occupancy through the latest Cretaceous (83.6–66.0 Ma) and early Paleogene (66.0–61.6 Ma). This method uses partial correlation networks to represent how different trophic groups interact in a food web and builds on empirical spatial co-variations to explore dependencies between trophic groups. Our analyses are based on a spatiotemporally and taxonomically standardized dataset, comprising more than 1,600 fossil occurrences representing more than 470 genera of fish, salamanders, frogs, albanerpetontids, lizards, snakes, champsosaurs, turtles, crocodylians, dinosaurs (including birds), and mammals across the best sampled region for this interval, the Western Interior of North America. We explicitly tested whether: 1) shifts in food web architecture underwent major restructuring before and after the K/Pg transition, including whether some trophic guilds were more prone to these shifts than others; and 2) any of these changes were associated with fluctuations in the realized niche space, helping to explain survivorship and extinction patterns at the boundary. We find a shift in latest Cretaceous dinosaur faunas, as medium-sized species counterbalanced a loss of large herbivores, but that dinosaur niches were otherwise resilient and static until the K/Pg boundary. Smaller terrestrial vertebrates, including mammals, followed a consistent trajectory of increasing trophic impact and relaxation of ecological niche limits that began in the Cretaceous and continued after the extinction. Patterns of mammalian ecological radiation and niche restructuring indicate that these taxa did not simply proliferate after the extinction; rather, their earlier ecological diversification might have helped them survive the K/Pg event, whereas the static niche of dinosaurs might have contributed to their demise. 

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2. Quantitative assessment of tarsal morphology illuminates locomotor behaviour in Palaeocene mammals following the end-Cretaceous mass extinction

   https://doi.org/10.1098/rspb.2021.0393  

   Shelley, Sarah L.; Brusatte, Stephen L.; Williamson, Thomas E. (May 2021, Proceedings of the Royal Society B: Biological Sciences)

   null (Ed.)

   Mammals exhibit vast ecological diversity, including a panoply of locomotor behaviours. The foundations of this diversity were established in the Mesozoic, but it was only after the end-Cretaceous mass extinction that mammals began to increase in body size, diversify into many new species and establish the extant orders. Little is known about the palaeobiology of the mammals that diversified immediately after the extinction during the Palaeocene, which are often perceived as ‘archaic’ precursors to extant orders. Here, we investigate the locomotor ecology of Palaeocene mammals using multivariate and disparity analyses. We show that tarsal measurements can be used to infer locomotor mode in extant mammals, and then demonstrate that Palaeocene mammals occupy distinctive regions of tarsal morphospace relative to Cretaceous and extant therian mammals, that is distinguished by their morphological robustness. We find that many Palaeocene species exhibit tarsal morphologies most comparable with morphologies of extant ground-dwelling mammals. Disparity analyses indicate that Palaeocene mammals attained similar morphospace diversity to the extant sample. Our results show that mammals underwent a post-extinction adaptive radiation in tarsal morphology relating to locomotor behaviour by combining a basic eutherian bauplan with anatomical specializations to attain considerable ecomorphological diversity. 

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3. 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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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. Testing the phylogeny of Periptychidae and "archaic" Paleocene mammals uner different optimality criteria

   Puschel, H. P.; Shelley, S. L.; Williamson, T. E.; Wible, J. R.; Brusatte, S. L. (October 2019, Journal of Vertebrate Paleontology, Abstracts and Program)

   It is now well established that the end-Cretaceous mass extinction had enormous repercussions for mammalian evolution. Following the extinction, during the Paleocene, mammals started to radiate, occupying new and diverse ecological niches. However, the phylogenetic relationships between the socalled “archaic” mammals of this time, and their position within Placentalia, remain contentious. The Periptychidae are a clade of distinctive “archaic” ungulates, composed of ~17 genera of small to large bodied, highly bunodont, terrestrial herbivores that were among the first placental mammals to appear after the end-Cretaceous mass extinction. Although the Periptychidae has been historically considered a distinctive “condylarth” subgroup, their higherlevel relationships have been rarely tested. Here, we present an inclusive cladistic analysis to determine and test the phylogenetic affinities of Periptychidae and other key Paleocene groups within Placentalia under different cladistic optimality criteria. We scored 140 taxa for 503 dental, cranial and postcranial characters, incorporating new morphological and taxonomic data. The data were then subject to parsimony and Bayesian tree of morphological evolution, running 5000000 generations with samples every 200 generations and discarding 25% of the samples as burn-in. Stationarity was achieved and a 50 percent majority rule consensus tree from the sampled trees was obtained. The parsimony analysis recovered 48 most parsimonious trees. The two consensus trees derived from the different analyses are largely congruent and recover a monophyletic Periptychidae, although the parsimony consensus tree is better resolved. These results are consistent with simulation studies showing that parsimony tends to be more precise (more nodes reconstructed) than Bayesian analyses, although less accurate. The main topological differences between the results relate to the position of poorly known Puercan (earliest Paleocene) species. Our results affirm the monophyly of Periptychidae and its nesting within a group of “condylarths” positioned at the base of Laurasiatheria and closely related to Artiodactyla. Within Periptychidae we found support for the three major subfamilial divisions in both analyses. These results highlight the importance of using different optimality criteria when resolving a phylogeny and provide a new insight into how placental mammals were evolving after the end-Cretaceous extinction. Grant Information: CONICYT PFCHA/DOCTORADO BECAS CHILE/2018, European Research Council Starting Grant (ERC StG 2017, 756226, PalM), National Science Foundation (NSF EAR 1654952, DEB 1654949) 

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