Bone histology sheds light on the nature of the “dermal armor” of the enigmatic sauropod dinosaur Agustinia ligabuei Bonaparte, 1999

Abstract

Agustinia ligabuei is an Early Cretaceous sauropod dinosaur from the northwest of Patagonia that is currently the topic of debate with respect to its phylogenetic position and atypical dermal armor. The presence of four morphotypes of laminar and transversely elongated putative osteoderms was used to consider Agustinia as an armored sauropod. Regarding the different hypotheses about the identity of the bony structures of Agustinia (e.g., osteoderms, cervical or dorsal ribs, hypertrophied elements), a comparative histological analysis has been carried out. Histological evidence is presented herein and reveals that none of the morphotypes of Agustinia shows a primary bone tissue formed by structural fiber bundles as in other sauropod dinosaur osteoderms. Furthermore, on the basis of their gross morphology and microstructure, the bony structures originally classified as types 1 + 4 and 3 are more comparable respectively with dorsal and cervical ribs than any other kind of dermal or bony element. Due to poor preservation, the nature of the type 2 cannot be assessed but is here tentatively assigned to a pelvic girdle element. Although a phylogenetic reassessment of Agustinia is not the purpose of this paper, our paleohistological analyses have broader implications: by not supporting the dermal armor hypothesis for Agustinia, its inclusion in Lithostrotia is not justified in the absence of other diagnostic features.

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Abbreviations

MCF-PVPH:

Museo Municipal “Carmen Funes” de Plaza HuinculPalaeovertebrate collection Plaza Huincul, Neuquén Province, Argentina

MCS-Ph:

Museo de Cinco Saltos, Palaeohistological collection, Cinco Saltos, Rio Negro Province, Argentina

MCS-Pv:

Museo de Cinco Saltos, Palaeovertebrate collection, Cinco Saltos, Rio Negro Province, Argentina

MPCA-Ph:

Museo Provincial ‘Carlos Ameghino’, Palaeohistological collection, Cipolletti, Rio Negro Province, Argentina

MPCA-Pv:

Museo Provincial ‘Carlos Ameghino,’ Palaeovertebrate collection, Cipolletti, Río Negro Province, Argentina

PVL:

Paleovertebrate collection of Instituto Miguel Lillo, Tucumán Province, San Miguel de Tucumán, Argentina

PVLPh:

Paleohistology collection of Instituto Miguel Lillo, Tucumán Province, San Miguel de Tucumán, Argentina

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Acknowledgments

We thank J. Bonaparte and A. Martinelli for providing helpful information, comments, original field-work photos, and field notes, as well in helping us to reconstruct the taphonomy of holotype materials of Agustinia; the Dirección Provincial de Patrimonio Cultural of Neuquén and the Museo Municipal Carmen Funes of Plaza Huincul for their collaboration, as well as legal and logistic support; J. Curruinca, S. Adrian, Sr. Firelli, and the Municipalidad of Picún Leufú for allowing the access at the Cerro Leon locality and to support part of the field works; R.A. Coria of MCF for the collection access and E. Montes for preparing part of the material; D. Codega of USL for making the thin sections; J. Carballido, P. Mannion, M. D’Emic, and an anonymous reviewer for providing constructive criticism and suggestions that improved the original manuscript. J. Blanco kindly provided the Agustinia reconstruction for Fig. 1. A. Elbakyan, J. Bar, and Wikipaleo Group provided the literature. This project has been partially funded by Jurassic Foundation (Grant to F. B) and Agencia Nacional de Promoción Científica y Técnica (PICT 2015-1021 to I.A.C).

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Correspondence to Flavio Bellardini.

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Communicated by: Robert Reisz

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Bellardini, F., Cerda, I.A. Bone histology sheds light on the nature of the “dermal armor” of the enigmatic sauropod dinosaur Agustinia ligabuei Bonaparte, 1999. Sci Nat 104, 1 (2017). https://doi.org/10.1007/s00114-016-1423-7

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Keywords

  • Early cretaceous
  • Sauropoda
  • Osteoderms
  • Bone microstructure
  • Mineralized fibers
  • Metaplasia