title: 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.
authors.affiliation: Museo Municipal "Carmen Funes", Av. Córdoba 55, CP8318, Plaza Huincul, Neuquén, Argentina. flaviobellardini@gmail.com.|Dirección Provincial de Patrimonio Cultural, Vuelta de Obligado 50, CP8300, Neuquén, Argentina. flaviobellardini@gmail.com.
authors.forename: Flavio
authors.identifier: http://orcid.org/0000-0003-1616-8548
authors.initials: F
authors.lastname: Bellardini
authors.affiliation: CONICET, Instituto de Investigaciones en Paleobiología y Geología, Universidad Nacional de Río Negro, Museo Carlos Ameghino, Belgrano 1700, Paraje Pichi Ruca (predio Marabunta), CP8324, Cipolletti, Río Negro, Argentina.
authors.forename: Ignacio A
authors.initials: IA
authors.lastname: Cerda
mesh_terms: D000818:Animals; D001118:Argentina; D001842:Bone and Bones / Q000033:anatomy & histology* / Q000166:cytology; D025061:Dinosaurs / Q000033:anatomy & histology / Q000145:classification*; D005580:Fossils; D010802:Phylogeny; D013045:Species Specificity
keywords: Bone microstructure; Early cretaceous; Metaplasia; Mineralized fibers; Osteoderms; Sauropoda