Squamate extinctions across K/Pg boundary in Denver Basin + thecodonty and tooth evolution
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Ben Creisler
Sep 24, 2025, 2:15:58 AM (5 days ago) Sep 24
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Ben Creisler
Holger Petermann and Tyler R. Lyson (2025)
Squamate fauna from the Denver Basin shows major ecosystem disruption across K/Pg boundary
Proceedings of the Royal Society B: Biological Sciences 292(2055): 20251234
doi: https://doi.org/10.1098/rspb.2025.1234
https://royalsocietypublishing.org/doi/10.1098/rspb.2025.1234
The Cretaceous/Palaeogene (K/Pg) boundary marks the most recent and the second-most severe mass extinction in Earth’s history. Marine K/Pg boundary sections that preserve uninterrupted sedimentation are abundant, and thus the global marine response to this extinction event is well understood. Similar terrestrial sedimentary sequences that record the before and after of the extinction and that are also geochronologically well constrained are rare. We describe and compare the non-marine squamate diversity immediately before (ca 57 kyr) and after (ca 128 kyr) the K/Pg mass extinction from two geographically and chronostratigraphically well constrained localities in the Denver Basin, Colorado. The latest Cretaceous squamate record there consists of 235 isolated fossils and is extraordinarily diverse, with 27 operational taxonomic units, whereas the earliest Palaeocene record is depauperate in both richness (3 taxa) and abundance (20 isolated fossils)—a ca 91% drop in specimen abundance and a 93% extinction rate. Integrating the Denver Basin squamate fossil record with that of the Williston and Powder River basins of Montana, North and South Dakota, and Wyoming, provides a regional extinction rate of between 75 and 84% for squamates in the Western Interior of North America and suggests this group was more severely impacted across the K/Pg boundary than other small-bodied vertebrates.
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Gabriel Mestriner, Gregory F. Funston, Júlio C. A. Marsola, Sterling J. Nesbitt, Max C. Langer, David C. Evans and Aaron R. H. LeBlanc (2025)
Rethinking thecodonty: the influence of two centuries of comparative dental anatomy on our understanding of tooth evolution
Biology Letters 21(9): 20250316
doi: https://doi.org/10.1098/rsbl.2025.0316
https://royalsocietypublishing.org/doi/10.1098/rsbl.2025.0316
Free pdf:
https://royalsocietypublishing.org/doi/epdf/10.1098/rsbl.2025.0316
‘Thecodont’ refers to teeth implanted in sockets within the jaw, a condition traditionally associated with living mammals and crocodylians, which also coincidentally have teeth attached by ligaments to the socket walls (gomphosis). For over a century, the bony periodontium of many other amniotes has been described as a single tissue, ‘bone of attachment’, causing confusion over dental tissue homology. The conventional definitions of ‘thecodonty’ exclude species with fused teeth (‘ankylothecodonts’), implying a fundamental difference between mammals, crocodylians and most other vertebrates. However, the stereotypically ‘thecodont’ attachment tissues have been discovered in representatives of all major amniote clades, showing that gomphosis and ankylosis likely stem from heterochronic changes in the timing and extent of cementum and alveolar bone mineralization. This challenges (i) previous hypotheses regarding the evolution of the amniote periodontium, (ii) the ‘bone of attachment’ paradigm, and (iii) the significance of ‘thecodonty’. We suggest a new nomenclatural approach that incorporates recent histological and evolutionary research and divides thecodonty into anatomical categories to clarify their origin and evolution. We propose the terms anisothecodont and isothecodont to denote, respectively, asymmetric and symmetric implantation of teeth in their sockets. Regardless of the geometry of the connection, we propose using ankylosis and gomphosis to denote the mode of tooth attachment.
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New papers:
Holger Petermann and Tyler R. Lyson (2025)
Squamate fauna from the Denver Basin shows major ecosystem disruption across K/Pg boundary
Proceedings of the Royal Society B: Biological Sciences 292(2055): 20251234
doi: https://doi.org/10.1098/rspb.2025.1234
https://royalsocietypublishing.org/doi/10.1098/rspb.2025.1234
The Cretaceous/Palaeogene (K/Pg) boundary marks the most recent and the second-most severe mass extinction in Earth’s history. Marine K/Pg boundary sections that preserve uninterrupted sedimentation are abundant, and thus the global marine response to this extinction event is well understood. Similar terrestrial sedimentary sequences that record the before and after of the extinction and that are also geochronologically well constrained are rare. We describe and compare the non-marine squamate diversity immediately before (ca 57 kyr) and after (ca 128 kyr) the K/Pg mass extinction from two geographically and chronostratigraphically well constrained localities in the Denver Basin, Colorado. The latest Cretaceous squamate record there consists of 235 isolated fossils and is extraordinarily diverse, with 27 operational taxonomic units, whereas the earliest Palaeocene record is depauperate in both richness (3 taxa) and abundance (20 isolated fossils)—a ca 91% drop in specimen abundance and a 93% extinction rate. Integrating the Denver Basin squamate fossil record with that of the Williston and Powder River basins of Montana, North and South Dakota, and Wyoming, provides a regional extinction rate of between 75 and 84% for squamates in the Western Interior of North America and suggests this group was more severely impacted across the K/Pg boundary than other small-bodied vertebrates.
====
Free pdf:
Gabriel Mestriner, Gregory F. Funston, Júlio C. A. Marsola, Sterling J. Nesbitt, Max C. Langer, David C. Evans and Aaron R. H. LeBlanc (2025)
Rethinking thecodonty: the influence of two centuries of comparative dental anatomy on our understanding of tooth evolution
Biology Letters 21(9): 20250316
doi: https://doi.org/10.1098/rsbl.2025.0316
https://royalsocietypublishing.org/doi/10.1098/rsbl.2025.0316
Free pdf:
https://royalsocietypublishing.org/doi/epdf/10.1098/rsbl.2025.0316
‘Thecodont’ refers to teeth implanted in sockets within the jaw, a condition traditionally associated with living mammals and crocodylians, which also coincidentally have teeth attached by ligaments to the socket walls (gomphosis). For over a century, the bony periodontium of many other amniotes has been described as a single tissue, ‘bone of attachment’, causing confusion over dental tissue homology. The conventional definitions of ‘thecodonty’ exclude species with fused teeth (‘ankylothecodonts’), implying a fundamental difference between mammals, crocodylians and most other vertebrates. However, the stereotypically ‘thecodont’ attachment tissues have been discovered in representatives of all major amniote clades, showing that gomphosis and ankylosis likely stem from heterochronic changes in the timing and extent of cementum and alveolar bone mineralization. This challenges (i) previous hypotheses regarding the evolution of the amniote periodontium, (ii) the ‘bone of attachment’ paradigm, and (iii) the significance of ‘thecodonty’. We suggest a new nomenclatural approach that incorporates recent histological and evolutionary research and divides thecodonty into anatomical categories to clarify their origin and evolution. We propose the terms anisothecodont and isothecodont to denote, respectively, asymmetric and symmetric implantation of teeth in their sockets. Regardless of the geometry of the connection, we propose using ankylosis and gomphosis to denote the mode of tooth attachment.
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