3. Paleocene Biochronology: The Puercan Through Clarkforkian Land Mammal Ages

Changes in tooth shape have played a major role in vertebrate evolution with modification of dentition allowing an organism to adapt to new feeding strategies. The current view is that molar teeth evolved from simple conical teeth, similar to canines, by progressive addition of extra "cones" to form progressively complex multicuspid crowns. Mammalian incisors, however, are neither conical nor multicuspid, and their evolution is unclear. We show that hypomorphic mutation of a cell surface receptor, Lrp4, which modulates multiple signaling pathways, produces incisors with grooved enamel surfaces that exhibit the same molecular characteristics as the tips of molar cusps. Mice with a null mutation of Lrp4 develop extra cusps on molars and have incisors that exhibit clear molar-like cusp and root morphologies. Molecular analysis identifies misregulation of Shh and Bmp signaling in the mutant incisors and suggests an uncoupling of the processes of tooth shape determination and morphogenesis. Incisors thus possess a developmentally suppressed, cuspid crown-like morphogenesis program similar to that in molars that is revealed by loss of Lrp4 activity. Several mammalian species naturally possess multicuspid incisors, suggesting that mammals have the capacity to form multicuspid teeth regardless of location in the oral jaw. Localized loss of enamel may thus have been an intermediary step in the evolution of cusps, both of which use Lrp4-mediated signaling.V ertebrates exhibit remarkable diversity in their dentitions, which is a feature of the importance of tooth shape in adaptation to new feeding strategies in evolution. Even quite closely related species of mammals can have different shapes of teeth and thus tooth development provides an excellent model for molecularly based evolutionary developmental biological studies (evo/devo). These tooth evolutional changes took place by the activation or inactivation of gene function, and thus evolutionary lost structures or gene activation/inactivation during evolution are occasionally retained as vestigial structures or latent gene activation/inactivation at embryonic stages.The current view is that all mammalian teeth evolved from simple ancestral teeth with a conical shape not dissimilar to mammalian canines (1). Mammalian heterodont dentitions contain a variety of tooth shapes and most evo/devo studies have focused solely on the molar dentition, with cuspal morphology being used as the main comparative feature between specimens (1). A cusp is a pointed or rounded projection of the tooth that is composed of both enamel and dentin, and the general consensus is that multicuspid teeth (molariform) evolved from conical teeth by progressive addition of extra "cones" (1). Incisors however are a uniquely mammalian tooth type that are neither conical nor multicuspid and their evolutional process is not understood.Among mammalian teeth, murine dentition has been used as a powerful tool for evo/devo studies because of the relative ease of gene manipulation. A major defining feature of R...

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“…4 R-U). Furthermore, several extinct mammalian species also had multicuspid incisors (28)(29)(30)(31)(32)(33).…”

Section: Resultsmentioning

confidence: 99%

A role for suppressed incisor cuspal morphogenesis in the evolution of mammalian heterodont dentition

Ohazama

Blackburn

Porntaveetus

et al. 2009

Proc. Natl. Acad. Sci. U.S.A.

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“…We then (1) assess humeral morphotype richness through the latest Cretaceous and earliest Paleogene of our study area; (2) quantitatively and qualitatively constrain taxonomic affinities of humeral morphotypes on the basis of size and abundance; (3) infer body size changes across and following the K-Pg boundary; and (4) constrain morphofunctional affinities of select, well-preserved latest Cretaceous and earliest Paleogene distal humeri on the basis of a geometric morphometric analysis of a broad sample of extant, smallbodied mammals of diverse locomotor function. 4 the boundary between the Lancian and Puercan North American Land Mammal "ages" (NALMAs; Cifelli et al, 2004;Lofgren et al, 2004;Sprain et al, 2015; but see Fox, 1989 andKelly, 2014). Our specimens are from the Hell Creek and Tullock formations, except for three specimens that are from the Lance Formation of Wyoming (UCMP locality V5620).…”

Section: Introductionmentioning

confidence: 99%

Mammalian distal humerus fossils from eastern Montana, USA with implications for the Cretaceous-Paleogene mass extinction and the adaptive radiation of placentals

DeBey

Wilson

2017

Palaeontologia Electronica

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“…Fritz Quarry, Polecat Bench Formation, Park County, Wyoming, USA. The Fritz Quarry is assigned to the Tiffanian 5 land mammal age [18] Late Palaeocene (approximately 58-57 Myr ago).…”

Section: Systematic Palaeontologymentioning

confidence: 99%

Biogeography of worm lizards (Amphisbaenia) driven by end-Cretaceous mass extinction

et al. 2015

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Worm lizards (Amphisbaenia) are burrowing squamates that live as subterranean predators. Their underground existence should limit dispersal, yet they are widespread throughout the Americas, Europe and Africa. This pattern was traditionally explained by continental drift, but molecular clocks suggest a Cenozoic diversification, long after the break-up of Pangaea, implying dispersal. Here, we describe primitive amphisbaenians from the North American Palaeocene, including the oldest known amphisbaenian, and provide new and older molecular divergence estimates for the clade, showing that worm lizards originated in North America, then radiated and dispersed in the Palaeogene following the Cretaceous-Palaeogene (K-Pg) extinction. This scenario implies at least three trans-oceanic dispersals: from North America to Europe, from North America to Africa and from Africa to South America. Amphisbaenians provide a striking case study in biogeography, suggesting that the role of continental drift in biogeography may be overstated. Instead, these patterns support Darwin and Wallace's hypothesis that the geographical ranges of modern clades result from dispersal, including oceanic rafting. Mass extinctions may facilitate dispersal events by eliminating competitors and predators that would otherwise hinder establishment of dispersing populations, removing biotic barriers to dispersal.

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3. Paleocene Biochronology: The Puercan Through Clarkforkian Land Mammal Ages

Cited by 111 publications

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A role for suppressed incisor cuspal morphogenesis in the evolution of mammalian heterodont dentition

A role for suppressed incisor cuspal morphogenesis in the evolution of mammalian heterodont dentition

Mammalian distal humerus fossils from eastern Montana, USA with implications for the Cretaceous-Paleogene mass extinction and the adaptive radiation of placentals

Biogeography of worm lizards (Amphisbaenia) driven by end-Cretaceous mass extinction

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