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The Osteoderm Microstructure in Doswelliids and Proterochampsids and Its Implications for Palaeobiology of Stem Archosaurs

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The Osteoderm Microstructure in Doswelliids and Proterochampsids and Its Implications for Palaeobiology of Stem Archosaurs The osteoderm microstructure in doswelliids and proterochampsids and its implications for palaeobiology of stem archosaurs DENIS A. PONCE, IGNACIO A. CERDA, JULIA B. DESOJO, and STERLING J. NESBITT Ponce, D.A., Cerda, I.A., Desojo, J.B., and Nesbitt, S.J. 2017. The osteoderm microstructure in doswelliids and proter- ochampsids and its implications for palaeobiology of stem archosaurs. Acta Palaeontologica Polonica 62 (4): 819–831. Osteoderms are common in most archosauriform lineages, including basal forms, such as doswelliids and proterochamp- sids. In this survey, osteoderms of the doswelliids Doswellia kaltenbachi and Vancleavea campi, and proterochampsid Chanaresuchus bonapartei are examined to infer their palaeobiology, such as histogenesis, age estimation at death, development of external sculpturing, and palaeoecology. Doswelliid osteoderms have a trilaminar structure: two corti- ces of compact bone (external and basal) that enclose an internal core of cancellous bone. In contrast, Chanaresuchus bonapartei osteoderms are composed of entirely compact bone. The external ornamentation of Doswellia kaltenbachi is primarily formed and maintained by preferential bone growth. Conversely, a complex pattern of resorption and redepo- sition process is inferred in Archeopelta arborensis and Tarjadia ruthae. Vancleavea campi exhibits the highest degree of variation among doswelliids in its histogenesis (metaplasia), density and arrangement of vascularization and lack of sculpturing. The relatively high degree of compactness in the osteoderms of all the examined taxa is congruent with an aquatic or semi-aquatic lifestyle. In general, the osteoderm histology of doswelliids more closely resembles that of phytosaurs and pseudosuchians than that of proterochampsids. Key words: Archosauria, Doswelliidae, Protero champ sidae, palaeoecology, microanatomy, histology, Triassic, USA. Denis A. Ponce [[email protected]], Universidad Nacional de Río Negro, Estados Unidos 750, General Roca, 8332, Río Negro, Argentina. Ignacio A. Cerda [[email protected]], Instituto de Investigación en Paleobiología y Geología (IIPG), Uni- versidad Nacional de Río Negro, Av. Roca 1242, 8332, General Roca, Río Negro, Argentina; Museo “Carlos Ameghi- no”, Belgrano 1700, Paraje Pichi Ruca (predio Marabunta), Cipolletti, 8324, Río Negro, Argentina. Julia B. Desojo [[email protected]], División Paleontología Vertebrados, Museo de La Plata, Paseo del Bosque s/n, La Plata, B1900FWA, Buenos Aires, Argentina. Sterling J. Nesbitt [[email protected]], Department of Geosciences, Virginia Tech, Blacksburg, 24061, Virginia, USA. Received 9 May 2017, accepted 21 July 2017, available online 27 October 2017. Copyright © 2017 D.A. Ponce et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License (for details please see http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. sidae (Sues et al. 2013; Trotteyn et al. 2013). Dos welliids Introduction were medium size archosauriforms (Doswellia kalten bachi, Archosauriformes is a clade of diapsid reptiles that origi- ~2 m length) and had a wide distribution across Pangaea nated by the latest Permian and are extant as birds and cro- (America and Europe) during the Middle and Late Triassic codylians. Early in archosauriform evolution, osteoderms (Sues et al. 2013). The osteoderm arrangement consisted of appeared superficial to the vertebrae and, in some taxa, several rows of osteoderms with pronounced external orna- within the dermis surrounding the limbs. These skeletal el- mentation and smooth anterior edges (Sues et al. 2013). The ements consist of mineralized organs entrenched within the phylogenetic relationship of Vancleavea campi among other dermis (Francillon-Vieillot et al. 1990; Vickaryous and Hall non-archosaurian archosauriforms has been historically 2008). The function, size, shape, external ornamentation, difficult to assess because of a relatively few identifiable and ar ran gement of osteoderms vary among archosauri- parsimony-informative characters (Long and Murry 1995; forms (Vickaryous and Sire 2009). Among non-archosau- Parker and Barton 2008; Nesbitt et al. 2009). Vancleavea rian archosauriforms, or stem archosaurs, osteoderms are has been considered as an aquatic animal with a body (1.2 m commonly reported in the Doswelliidae and Protero champ- in length) completely covered by five different osteoderms Acta Palaeontol. Pol. 62 (4): 819–831, 2017 https://doi.org/10.4202/app.00381.2017 820 ACTA PALAEONTOLOGICA POLONICA 62 (4), 2017 A B Here, we further explore and analyse the microstructure of the osteoderms of doswelliids (D. kaltenbachi and V. campi) and one proterochampsid (the holotype of C. bona- partei). The goals of this contribution are to recognize the general histological pattern in osteoderms of non-archosau- rian archosauriforms, provide information about the origin and development of the osteoderms in doswelliids and pro- 5mm terochampsids by testing previous hypothesis (e.g., Cerda 5mm et al. 2015) and to demonstrate the value of osteoderms for D2 future skeletochronological studies. Finally, we perform the C first quantitative analysis of microanatomy in osteoderms of non-archosaurian archosauriforms to provide testable hy- 10 mm potheses about the palaeoecology of stem archosaurs. D1 Institutional abbreviations.—CPEZ, Coleção Municipal, São Pedro do Sul, Brasil; GR, Ruth Hall Museum of Pale- ontology at Ghost Ranch, USA; PULR, Museo de Ciencias 3mm 40 mm Naturales de la Universidad Nacional de La Rioja, Argentina; Fig. 1. Osteoderms examined in this study. A. Doswelliid Doswellia kalten- PVL, Instituto Miguel Lillo, Tucumán, Argentina; PVSJ, bachi Weems, 1980, USNM PAL 244214 from Near Doswell, Virginia, División Paleontología de Vertebrados, Museo de Ciencias USA, Carnian. B, C. Doswelliid Vancleavea campi Long and Murry, 1995 Naturales, Universidad Nacional de San Juan, Argentina; from Coelophysis Quarry, “Siltstone Member” (GR 138) and Hayden USNM, National Museum of Natural History (formerly Quarry, Petrified Forest Member (H4-102-08), Chinle Formation, New United States National Museum), USA. Mexico, USA, Early Norian. B. GR 138 (V1). C. H4-102-08 (V2). D. Pro- tero cham psid Chanaresuchus bonapartei Romer, 1971, PULR 07 from La Other abbreviations.—LAGs, lines of arrested growth. Rioja province, Ladinian–Carnian; D1, general view of a single row of osteo- derms associated with presacral vertebrae; D2, detailed view of the sampled osteoderm (box inset in D1). All elements in external view (except D, later- al-medial view), with the anterior margin facing the upper part of the figure. Material and methods morphotypes (Nesbitt et al. 2009). Proterochampsids, which For this study osteoderms of Doswellia kaltenbachi, Van- inhabited South America during the Late Triassic (Trotteyn cleavea campi, and Chanaresuchus bonapartei were sam- et al. 2013), resemble crocodylians in general morphology pled. The identification, locality, and age of these speci- (Trotteyn et al. 2013), and their osteoderms consist of a sin- mens are compiled in Table 1, and the photographs of most gle row on the midline that fused to neural spines (Trotteyn osteoderms prior to sectioning are presented in Fig. 1. There et al. 2013). is no photographic material of the osteoderms 1 and 2 of Most studies of the histology and microanatomy of ar- the GR 138 specimen. The specimens were to provide into chosauriform osteoderms have concentrated on archosaurian thin sections based on the methodology of Chinsamy and groups, such as Phytosauria (Scheyer et al. 2014), Aetosauria Raath (1992). Transverse sections were obtained from all (Parker et al. 2008; Cerda and Desojo 2011; Taborda et al. osteoderms, in addition to some longitudinal and parasag- 2013, 2015; Scheyer et al. 2014), “Rauisuchia” (Scheyer and ittal sections (Table 1). Two specimens of V. campi were Desojo 2011; Cerda et al. 2013) and Dinosauria (e.g., Scheyer sectioned and, therefore, the successive cross sections are and Sander 2004; Cerda and Powell 2010; Hayashi et al. identified as 1, 2 and V1 for GR 138 specimen and V2 for 2012; Burns and Currie 2014). Osteoderms of Doswelliidae H4-102-08 (uncatalogued, Hayden Quarry, Ghost Ranch, and Proterochampsidae species have been only recently ana- New Mexico, USA) specimen. In this survey, we follow the lysed (Scheyer et al. 2014; Cerda et al. 2015). recent phylogenetic analysis of Ezcurra (2016), in which Table 1. Histological sections of osteoderms referred in the the current study. Taxon Specimen Section type Locality Horizon References Weems 1980; Long and Doswellia USNM PAL transversal, Poor Farm Member, Falling Near Doswell, Virginia, USA Murray 1995; Desojo et al. kaltenbachi 2214 (holotype) parasagital Creek Formation; Carnian 2011; Sues et al. 2013 Coelophysis Quarry (GR 138) “Siltstone Memeber” (GR 138) Long and Murray 1995; Vancleavea GR 138 and and Hayden Quarry (H4- and Petrified Forest Member transversal Parker and Burton 2008; campi H4-102-08 102-08), Ghost Ranch, New (H4-102-08), Chinle Formation; Nesbitt et al. 2009 Mexico, USA Early Norian Chanaresuchus PULR 07 transversal, La Rioja province, Chañares Formation; Romer 1971; Trotteyn et al. bonapartei (holotype) longitudinal Argentina Ladinian–Carnian 2013 PONCE ET AL.—OSTEODERM MICROSTRUCTURE IN DOSWELLIIDS AND PROTEROCHAMPSIDS 821 CO: 0.85 CO: 0.93 ABCO: 0.82 C 0.6 mm
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