Comparative bone microstructure of two non-mammaliaform cynodonts from the Late Triassic (Carnian) Chañares formation of Northwestern Argentina

ABSTRACT

The aim of the present contribution is to examine the bone histology of two non-mammaliaform cynodonts recovered from the Late Triassic (Carnian) Chañares Formation, at Los Chañares locality, La Rioja province, Northwestern Argentina. The taxa analysed here are the Traversodontidae Massetognathus pascuali and the Probainognathidae Probainognathus jenseni, both forms known from multiple skeletons. Our osteohistological results reveal an uninterrupted growth pattern and high intraspecific histovariability in Massetognathus. On the other hand, Probainognathus presents cyclical growth, a cortex dominated by parallel-fibred bone tissue and scarce vascularisation. In spite of being derived non-mammaliaform eucynodonts, Massetognathus and Probainognathus show high growth plasticity, a feature usually regarded as plesiomorphic for cynodonts. Both taxa show a thick cortex in the long bones, a feature that could be linked to counteract the strong flexion forces during digging.

KEYWORDS:

• Cynodontia
• Probainognathus
• Massetognathus
• palaeohistology
• fossoriality
• histovariability

Acknowledgments

We thank Emily Jane Wickenhauser for the revision of the English in the present manuscript. We thank the partners of the LACEV (Laboratorio de Anatomía Comparada y Evolución de los Vertebrados, Museo Argentino de Ciencias Naturales “Bernardino Rivadavia”, Autonomous City of Buenos Aires, Argentina) for their comments and discussion. We thank Dr. Pablo Edmundo Ortiz for the access to the collections from Facultad de Ciencias Naturales e Instituto Miguel Lillo (Tucumán, Argentina). We especially thank Mirta González and Vicky Sánchez (Museo Argentino de Ciencias Naturales “Bernardino Rivadavia”) for their assistance in the use of the petrographic microscope. We thank Dr. Martinelli and the anonymus reviewers by the improvement of the MS. The research presented here has been supported by CONICET.

Disclosure statement

No potential conflict of interest was reported by the author(s).