Bone microstructure of the pareiasaur Provelosaurus americanus from the Middle Permian of southern Brazil

ABSTRACT

Provelosaurus americanus from the Guadalupian of Brazil, is the only species of pareiasaur known from South America and its studies are limited to anatomical descriptions. Here, we examined the microstructure of limb bones, a rib fragment and osteoderms of P. americanus, aiming to answer questions related to its paleobiology. The bone tissues of this specimen comprise poorly vascularised parallel-fibred bone interrupted by growth marks indicating slow, cyclical growth. This is consistent with the pattern found in other pareisaurs from South Africa. However, the space between the LAGs are irregular and there is no clear decreasing in vascularity toward the periphery of the bone, suggesting that it did not reach skeletal maturity. The highest number of LAGs was found in the rib, suggesting that our sample lived a minimum of fifteen years. The osteoderms present a trilaminar structure consisting of a diploe composed of a cancellous core surrounded by two cortical layers. A diploe is absent from the osteoderms of previously studied South African pareiasaurs. The osteogenesis of the osteoderms is intramembraneous, but a metaplastic ossification during the early developmental stage cannot be entirely discarded, due to extensive remodeling in the central core that could have destroyed potential metaplastic tissue.

KEYWORDS:

• Parareptilia
• Pareiasauria
• osteohistology
• life history
• growth record
• dermal armor

Acknowledgments

We are very grateful to Jennifer Botha (National Museum, Bloemfontein) for the major revisions that she made in earlier versions of the manuscript, and also the reviewer Ignacio A. Cerda and an anonymous reviewer for the comments and suggestions that greatly improved the MN. Fábio Veiga is acknowledged for teaching the histological procedure. We thank Marcelo Campos and Lucas Medeiros (Laboratório de Preparação de Amostras, Instituto de Geociências, Universidade Federal do Rio Grande do Sul-UFRGS, Brazil) for the mechanical preparation of limb bones and Luiz Lopes (UFRGS) for photographing the elements and the whole cross sections. We are grateful to Agustin Martinelli (UFRGS) for the fruitful discussions and sharing of important papers. Léo A. Hartmann (UFRGS) was helpful in the manuscript structure.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) [under the grant PROEX-0497] to BDMF; Pró-Reitoria de Pesquisa, Universidade Federal do Rio Grande do Sul [under the grants 2017-17285 and 2018-19696] to BDMF; Pró-Reitoria de Pesquisa (PROPESQ), Universidade Federal do Rio Grande do Sul [2017 - 17285,2018 - 19696];