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1. Skull of a new periptychid mammal from the lower Paleocene Denver Formation of Colorado (Corral Bluffs, El Paso County)

   https://doi.org/10.1007/s10914-024-09716-5  

   Weaver, Lucas_N; Crowell, Jordan_W; Chester, Stephen_G_B; Lyson, Tyler_R (April 2024, Journal of Mammalian Evolution)

   Abstract 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 erectMilitocodon lydaegen. et sp. nov. The dentition ofM. lydaeexhibits synapomorphies that diagnose the Conacodontinae, but it is plesiomorphic relative toOxyacodon, resembling putatively basal periptychids likeMimatutaandMaioranain several dental traits. As such, we interpretM. lydaeas a basal conacodontine. Its skull anatomy does not reveal clear periptychid synapomorphies and instead resembles that of arctocyonids and other primitive eutherians.M. lydaefalls 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 partialM. lydaeskull represent an important step toward unraveling the complex evolutionary history of periptychid mammals. 

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2. THE PHYLOGENY OF THE EARLY PALAEOCENE ORDER TAENIODONTA USING NEW SPECIMENS FROM NEW MEXICO, USA

   Kynigopoulou, Z.; Shelley, S.L.; Williamson, T. E.; Wible, J. R.; Brusatte, S. L. (June 2022, European Association of Vertebrate Paleontology Abstract Volume)

   After the Cretaceous-Palaeogene (K-Pg) mass extinction mammals thrived in the Cenozoic. However, the phylogenetic affinities of early Palaeogene ‘archaic’ mammals that lived immediately after the extinction remain unresolved. Taeniodonta is a group of puzzling ‘archaic’ mammals that appeared in the early Palaeocene of North America. They are arranged into two subgroups; the Conoryctidae and Stylinodontidae and are characterised by their extreme degree of dental wear, indicating an abrasive diet, which led to hypsodonty in the most derived species. Due, in part, to their worn teeth and their rarity in the fossil record, the position of taeniondonts in the mammalian phylogenetic tree remains unresolved. New fossils from San Juan basin, New Mexico, USA, including unworn teeth of four genera and postcranial elements of an early taeniodont, Conoryctes, shed light on their dental and postcranial anatomy. Both in the forelimb and hind limp of Conoryctes, there are anatomical adaptations towards fossoriality. Using these specimens, we scored taeniodonts and other Palaeogene mammals into a phylogenetic data matrix (620 characters, 135 taxa). We then conducted a phylogenetic analysis using parsimony. Our results show that Taeniodonta is a monophyletic group within Eutheria. We also found that Onychodectes is basal to the two subgroups previously proposed. Based on the new postcranial fossils and revised phylogeny, we concluded that digging behaviours were likely ancestral for taeniodonts. Therefore, a more fossorial mode of life may have been beneficial for their surviving and thriving in the wake of the K-Pg extinction. 

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3. Basicranial evidence suggests picrodontid mammals are not stem primates

   https://doi.org/10.1098/rsbl.2023.0335  

   Crowell, Jordan W.; Wible, John R.; Chester, Stephen G. (January 2024, Biology Letters)

   The Picrodontidae from the middle Palaeocene of North America are enigmatic placental mammals that were allied with various mammalian groups but are generally now considered to have close affinities to paromomyid and palaechthonid plesiadapiforms based on proposed dental synapomorphies. The picrodontid fossil record consists entirely of dental and gnathic remains except for one partial cranium of Zanycteris paleocenus (AMNH 17180). Here, we use µCT technology to unveil previously undocumented morphology in AMNH 17180, describe and compare the basicranial morphology of a picrodontid for the first time, and incorporate these new data into cladistic analyses. The basicranial morphology of Z. paleocenus is distinct from plesiadapiforms and shares similarities with the Palaeogene Apatemyidae and Nyctitheriidae. Results of cladistic analyses incorporating these novel data suggest picrodontids are not stem primates nor euarchontan mammals and that the proposed dental synapomorphies uniting picrodontids with plesiadapiforms and, by extension, primates evolved independently. Results highlight the need to scrutinize proposed synapomorphies of highly autapomorphic taxa with limited fossil records. 

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4. Basicranial evidence suggests picrodontid mammals are not stem primates.

   Crowell, Jordan W; Wible, John R; Chester, Stephen G (January 2024, Biology letters)

   The Picrodontidae from the middle Palaeocene of North America are enigmatic placental mammals that were allied with various mammalian groups but are generally now considered to have close affinities to paromomyid and palaechthonid plesiadapiforms based on proposed dental synapomorphies. The picrodontid fossil record consists entirely of dental and gnathic remains except for one partial cranium of Zanycteris paleocenus (AMNH 17180). Here, we use μCT technology to unveil previously undocumented morphology in AMNH 17180, describe and compare the basicranial morphology of a picrodontid for the first time, and incorporate these new data into cladistic analyses. The basicranial morphology of Z. paleocenus is distinct from plesiadapiforms and shares similarities with the Palaeogene Apatemyidae and Nyctitheriidae. Results of cladistic analyses incorporating these novel data suggest picrodontids are not stem primates nor euarchontan mammals and that the proposed dental synapomorphies uniting picrodontids with plesiadapiforms and, by extension, primates evolved independently. Results highlight the need to scrutinize proposed synapomorphies of highly autapomorphic taxa with limited fossil records. 

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5. 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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