| Summary: | The bone histology of the Antarctic ornithopods Trinisaura santamartaensis and Morrosaurus antarcticus from the Upper Cretaceous (Campanian-Maastrichtian) is here studied. With the purpose to obtain information regarding their growth pattern, we analyse the bone histology of appendicular elements from the holotype specimens of both species. The cortical bone of the studied elements is constituted by fibro-lamellar tissue, with vascular canals mostly showing in a longitudinal and reticular arrangement, suggesting an accelerated growth. The cortical bone tissue is interrupted by lines of arrested growth (LAGs) and annuli. The study reveals that both the holotypes of Trinisaura and Morrosaurus were sexually mature subadults at the time of death (i.e. they died before reach somatic maturity). The growth pattern of these Antarctic ornithopods is similar to that reported for the related taxon Gasparinisaura, which lived in lower latitudes, as well as for the Australian ornithopods, taxa that inhabited polar latitudes during the Early Cretaceous. This contrasts with the pattern reported for more derived ornithopods (i.e. hadrosaurids) and ceratopsians from the Northern Hemisphere, whose growth dynamics vary in different latitudes. In this regard, Gondwanan ornithopods would present a preadaptation for endure in environments with great temperature changes along the year. Fil: García Marsa, Jordi Alexis. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Museo Argentino de Ciencias Naturales "Bernardino Rivadavia"; Argentina Fil: Cerroni, Mauricio Andrés. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Museo Argentino de Ciencias Naturales "Bernardino Rivadavia"; Argentina Fil: Rozadilla, Sebastian. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Museo Argentino de Ciencias Naturales "Bernardino Rivadavia"; ...
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