Shell and long-bone histology, skeletochronology, and lifestyle of Araripemys barretoi (Testudines: Pleurodira), a side-necked turtle of the Lower Cretaceous from Brazil

Abstract

In this study we provide a comprehensive investigation of the microanatomical and microstructural aspects of the carapace and limb bones of the Early Cretaceous side-necked turtle, Araripemys barretoi, from the Araripe Basin, Brazil. Inter-elemental histovariability reveals different secondary remodelling of the skeletal elements within the same individual. The vascularisation is scarce and mainly longitudinal, also it ceases towards the bone surface, forming an avascular parallel-fi bred bone with closely spaced LAGs. These traits indicate a late ontogenetic stage and a slow growth rate for one of the two A. barretoi specimens. The high cortical thickness of the costal plate suggests an increase of the shell stiffness. The elevated relative bone wall thickness of the ulna compared to other limb bones indicates a case of local pachyosteosclerosis, possibly to improve body stability in the aquatic environment.

Osteohistological characterization of notosuchian osteoderms: Evidence for an overlying thick leathery layer of skin

“…Most of the osteoderms investigated in this study possess a diploe structure containing two cortices, the external and the basal cortices, and a core between them. This structure is similar to that observed in xenarthran osteoderms (Vickaryous & Hall, 2006), early tetrapods (Witzmann & Soler-Gijón, 2010), the aetosaurs Calyptosuchus wellesi and Stagonolepis olenkae ; raiusuchians (Scheyer & Desojo, 2011) and turtle shell bones (e.g., Sena et al, 2021;Skutschas et al, 2017). This pattern differs from that observed in the Aetosaurinae specimens described by (Cerda & Desojo, 2011) and in the doswelliid Tarjadia ruthae dermal plates (Ponce et al, 2017), which lack the cancellous bone layer, and from the pareiasaurians (Scheyer & Sander, 2009), which exhibit a spongy osteoderm.…”

“…The osteoderms of Uberabasuchus terrificus , Itasuchus jesuinoi , Armadillosuchus arrudai , Mariliasuchus amarali , and Baurusuchidae indet. (LPRP/USP 0642) have basal cortices similar to those of aetosaurs, other pseudosuchian, and turtle shell plates (Scheyer et al, 2007 ; Sena et al, 2020 ., 2021 ). They are composed of parallel‐fibered bone tissue poorly vascularized interrupted by growth marks.…”

“…osteoderms present an isolated and enlarged circular space situated in the keel (Figures 7b and 9a ). Such vascular canals are interpreted as the ossification centers of the osteoderms and are seen in shell plates of the extinct Araripemys barretoi (Sena et al, 2021 ), Heckerochelys romani (Scheyer et al, 2014 ), the testudinatan Condorchelys antiqua (Cerda et al, 2016 ) and the extant Podocnemis expansa (Vieira et al, 2016 ) turtles. Osteoderms have a late ontogeny; they develop after the postcranial skeleton has formed within the well‐differentiated dermis (Vickaryous & Hall, 2006 ).…”

“…indet. (LPRP/USP 0642) have basal cortices similar to those of aetosaurs, other pseudosuchian, and turtle shell plates Sena et al, 2020Sena et al, ., 2021. They are composed of parallelfibered bone tissue poorly vascularized interrupted by growth marks.…”

“…Osteoderms are widespread integumentary features that occur in diverse extinct and extant amniotes, including turtles, nonavian archosaurs, lepidosaurs, and cingulatan mammals (Burns et al, 2013 ; Hill, 2005 ; Moss, 1972 ; Romer, 1956 ; Vickaryous & Sire, 2009 ; Witzmann, 2009 ). Osteoderms can bear superficial ornamentation in the dorsal surface, which may show different morphological patterns, such as tubercles in lizards, turtles, temnospondyls, and armadillos (de Buffrénil et al, 2011 ; Scheyer et al, 2007 ; Sena et al, 2021 ; Vickaryous & Hall, 2006 ; Witzmann & Soler‐Gijón, 2010 ), and pits and grooves in crocodylomorphs (Marinho et al, 2006 ; Montefeltro, 2019 ). The osteohistology of osteoderms is heterogeneous (Vickaryous & Sire, 2009 ) comprising blood vessels, nerves, yellow marrow, a collagenous meshwork, and cancellous and cortical bone (Kirby et al, 2020 ; Moss, 1972 ; Vickaryous & Sire, 2009 ).…”