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Synchrotron X-ray micro-computed tomography enhances our knowledge of the skull anatomy of a Late Triassic ecteniniid cynodont with hypercanines

Leonardo Kerber

Corresponding Author

Leonardo Kerber

Centro de Apoio à Pesquisa Paleontológica da Quarta Colônia, Universidade Federal de Santa Maria (CAPPA/UFSM), São João do Polêsine, Brazil

Programa de Pós-Graduação em Biodiversidade Animal, Universidade Federal de Santa Maria, Santa Maria, Brazil

Correspondence

Leonardo Kerber, Centro de Apoio à Pesquisa Paleontológica da Quarta Colônia, Universidade Federal de Santa Maria (CAPPA/UFSM), São João do Polêsine, RS, Brazil.

Email: [email protected]

Contribution: Conceptualization, ​Investigation, Writing - original draft, Methodology, Validation, Visualization, Writing - review & editing, Software, Formal analysis, Project administration, Data curation, Resources, Funding acquisition, Supervision

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Rodrigo T. Müller

Rodrigo T. Müller

Centro de Apoio à Pesquisa Paleontológica da Quarta Colônia, Universidade Federal de Santa Maria (CAPPA/UFSM), São João do Polêsine, Brazil

Programa de Pós-Graduação em Biodiversidade Animal, Universidade Federal de Santa Maria, Santa Maria, Brazil

Contribution: Conceptualization, ​Investigation, Data curation, Formal analysis, Writing - review & editing, Writing - original draft

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Daniel de Simão-Oliveira

Daniel de Simão-Oliveira

Centro de Apoio à Pesquisa Paleontológica da Quarta Colônia, Universidade Federal de Santa Maria (CAPPA/UFSM), São João do Polêsine, Brazil

Contribution: Conceptualization, ​Investigation, Software

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Flávio A. Pretto

Flávio A. Pretto

Centro de Apoio à Pesquisa Paleontológica da Quarta Colônia, Universidade Federal de Santa Maria (CAPPA/UFSM), São João do Polêsine, Brazil

Programa de Pós-Graduação em Biodiversidade Animal, Universidade Federal de Santa Maria, Santa Maria, Brazil

Contribution: Conceptualization, ​Investigation, Formal analysis, Writing - original draft

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Agustín G. Martinelli

Agustín G. Martinelli

Sección Paleontología de Vertebrados, Museo Argentino de Ciencias Naturales “Bernardino Rivadavia” – CONICET, Buenos Aires, Argentina

Contribution: Conceptualization, ​Investigation, Funding acquisition, Writing - original draft, Methodology, Visualization, Writing - review & editing

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Iasmim M. Michelotti

Iasmim M. Michelotti

Centro de Apoio à Pesquisa Paleontológica da Quarta Colônia, Universidade Federal de Santa Maria (CAPPA/UFSM), São João do Polêsine, Brazil

Contribution: Conceptualization, ​Investigation, Software

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Julien Benoit

Julien Benoit

Universidade Federal do Rio Grande do Sul, Porto Alegre, Brazil

Contribution: Conceptualization, Writing - review & editing, Funding acquisition, Resources

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Pedro H. Fonseca

Pedro H. Fonseca

Evolutionary Studies Institute, University of the Witwatersrand, Johannesburg, South Africa

Contribution: Conceptualization, ​Investigation, Writing - review & editing, Visualization

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Romain David

Romain David

Centre for Human Evolution Research, The Natural History Museum, London, UK

Contribution: Conceptualization, ​Investigation, Writing - review & editing, Funding acquisition

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Vincent Fernandez

Vincent Fernandez

European Synchrotron Radiation Facility, Grenoble, France

Contribution: Conceptualization, ​Investigation, Software, Writing - review & editing, Writing - original draft

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Kenneth D. Angielczyk

Kenneth D. Angielczyk

Negaunee Integrative Research Center, Field Museum of Natural History, Chicago, Illinois, USA

Contribution: Conceptualization, Project administration, Funding acquisition, Writing - review & editing, Resources

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Ricardo Araújo

Ricardo Araújo

Instituto de Plasmas e Fusão Nuclear & Centro de Recursos Naturais e Ambiente (CERENA), Instituto Superior Técnico, Universidade de Lisboa, Lisboa, Portugal

Contribution: Conceptualization, Writing - review & editing, Project administration, Resources, Funding acquisition

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First published: 13 January 2025

Abstract

Hypercanines, or hypertrophied canines, are observed in a wide range of both extinct and extant synapsids. In non-mammaliaform cynodonts, the Permo-Triassic forerunners of mammals, long canines are not uncommon, appearing in several unrelated taxa within the clade. Among them is Trucidocynodon riograndensis, a carnivorous ecteniniid cynodont from the Late Triassic of Brazil, which exhibits a specialized dentition, including spear-shaped incisors, very long and narrow canines, and sectorial postcanines with distally oriented cusps, all of which have finely serrated margins. Recent synchrotron X-ray micro-computed tomography of a large specimen (CAPPA/UFSM 0029; Várzea do Agudo site, Brazil) provides new insights into its lower jaw and dentition, as well as offers the first digital endocast of an ecteniniid. Our study reveals the presence of (i) putatively opened-root canines in the adult stage and the possible presence of unresorbed remnant of an old canine, which may indicate that the specimen stopped replacing its canines; (ii) lower canines that are longer than the upper canines and, in occlusion, were kept inside deep paracanine fossae that perforated the dorsal surface of the rostrum; (iii) a diastema between the incisors and lower canine, which is absent in the holotype; (iv) advanced brain structures, such as the absence of a pineal body, presence of cerebral hemispheres divided by the interhemispheric sulcus and expanded laterally, and a higher encephalization quotient than non-mammaliaform prozostrodonts, reflecting the homoplastic evolution of relative brain sizes observed in Triassic cynodont lineages. Finally, the abundance of carnivorous and omnivorous species at the Várzea do Agudo site, where the specimen was found—including the archosauriforms Dynamosuchus collisensis and Stenoscelida aurantiacus—suggests a diverse predator guild that warrants further investigation from a paleoecological perspective.

Graphical Abstract