Osteoderm histology of basal Archosauriformes from the Middle-Upper Triassic of Argentina and Brazil

IGNACIO A. CERDA; JULIA B. DESOJO; JIMENA TROTTEYN

Jornada; Reunión Anual de Comunicaciones de la Asociación Paleontológica Argentina?, Noviembre de 2013; 2013

Postcranial osteoderms are commonly developed in the major lineajes of Archosauriformes, including forms as proterochampsids and doswelliids. Here we survey the histology of osteoderms of the doswelliids Archeopelta arborensis and Tarjadia ruthae, and the proterochampsid Chanaresuchus bonapartei. We studied osteoderms of C. bonapartei (PVL 6244) and T. ruthae (PULR 063), both from Los Chañares Formation (Ladinian?Carnian), Argentina, and A. arborensis (CPEZ 239a) from Santa Maria 1 Sequence (late Ladinian?early Carnian), Brazil. The osteoderms of C. bonapartei are compact structures composed by parallel-fibred bone. Radial and circumferential primary vascular spaces predominate. Few secondary osteons and resorption spaces are developed in the inner core. Doswelliid osteoderms possess a trilaminar organization, in which two distinct cortices (external and basal) can be differentiated from an internal core. The external cortex exhibits a distinct pattern of valleys and saddles and consists mainly of lamellar bone tissue formed during different cycles of erosion and deposition. The internal core shows cancellous bone with short thick trabeculae and small inter-trabecular spaces. These spaces are commonly coated by lamellar bone. Remaining patches of primary bone at the inner core are composed by parallel-fibred and woven bone tissue. The basal cortex is composed of poorly vascularised parallel fibred bone. The rather simple microstructure observed in C. bonapartei suggests that these elements grew at a constant, low rate. Conversely, the complex histology of the doswelliid osteoderms appears to be linked to variations in the growth rate during the osteoderm formation and also to the development of the external ornamentation.