The bone microstructure of polar “hypsilophodontid” dinosaurs from Victoria, Australia

Abstract

High-latitude (i.e., “polar”) Mesozoic fauna endured months of twilight and relatively low mean annual temperatures. Yet non-avian dinosaurs flourished in this taxing environment. Fossils of basal ornithopod dinosaurs (“hypsilophodontids”) are common in the Early Cretaceous high-latitude sediments of Victoria, Australia, and four taxa have been described; although their ontogenetic histories are largely unexplored. In the present study, eighteen tibiae and femora were utilized in the first multi-specimen ontogenetic histological analysis of Australian polar hypsilophodontids. The sample consists of eleven individuals from the Flat Rocks locality (Late Valanginian or Barremian), and five from the Dinosaur Cove locality (Albian). In both groups, growth was most rapid during the first three years, and skeletal maturity occurred between five and seven years. There is a weak asymptotic trend in a plot of growth mark count versus femur length, with considerable individual variation. Histology suggests two genera are present within the Dinosaur Cove sample, but bone microstructure alone could not distinguish genera within the Flat Rocks sample, or across the two geologically separate (~ 26 Ma) localities. Additional histologic sampling, combined with morphological analyses, may facilitate further differentiation between ontogenetic, individual, and species variation.

High-latitude neonate and perinate ornithopods from the mid-Cretaceous of southeastern Australia

“…Previous histological work on a subset of nine ornithopod femora from the Eumeralla and Wonthaggi formations documented between 0 and 8 cyclical growth marks (CGMs), regarded as a record of annual growth 43,44 . In addition, the presence of an external fundamental system (EFS) in some of the largest specimens marks the cessation of appreciable growth and the onset of skeletal maturity 25 . The smallest of these femora (NMV P216768; length≈48 mm) lacks CGMs and displays an abrupt transition from fibro-lamellar to parallel-fibred tissues hypothesized to be a ‘hatching line’ 25 , similar to that seen in a neonate sauropod from Madagascar 45 .…”

“…Work on Victorian ornithopod material has previously identified juvenile specimens, although implications for high-latitude breeding were not explicitly discussed. Recent histological investigations on a large sample of Victorian material identified neonate ornithopods from the Wonthaggi Formation based on the highly vascular woven bone texture, the presence of an apparent ‘hatching’ line in one specimen (indicated by a sudden transition from fibro-lamellar to parallel-fibred bone texture), and the absence of cyclical growth marks 25,26 . A summary of small ornithopod material from the Eumeralla and Wonthaggi formations mentions a 29 mm long femur (NMV P208159) 24 .…”

“…Due to the lack of available characters, the specimen has not been described here, although we consider it plausibly represents an embryonic ornithopod femur. Despite the implications for interpreting high-latitude breeding, these previous studies presented an overview of material 24 and focused largely on the relative growth rates of these animals rather than potential breeding strategies 25,26 .…”

“…Here, we describe two miniscule and partial ornithopod femora from the Griman Creek Formation (GCF; Cenomanian 9 ) in central-northern New South Wales, Australia, and build upon previous descriptions of femora from the Eumeralla and Wonthaggi formations. Although histological analysis is obviated by the pseudomorphic opalization of some fossils from the Griman Creek Formation (opalised fossils typically do not preserve bone microstructure) 22 , gross anatomical comparisons with the histologically sampled specimens from the Eumeralla and Wonthaggi formations 25,26 indicate that the GCF specimens represent perinate (around the point of hatching) individuals. Together, these specimens constitute the first evidence of perinatal dinosaurs from Australia and, more broadly, the first insights into the high-latitude breeding preferences of non-iguanodontian ornithopods in Gondwana.…”

The palaeoenvironment of the Upper Cretaceous (Cenomanian–Turonian) portion of the Winton Formation, Queensland, Australia

“…Ornithopods have been shown to exploit a range of humid region habitats including wetlands ( Nadon, 1993 ), lacustrine ( Platt & Meyer, 1991 ), fluvial and riparian settings ( Vila et al, 2013 ; Herne et al, 2018 ), and coastal environments ( Diedrich, 2011 ) including lagoons ( Vila et al, 2013 ). Both large and small ornithopods have been found in polar environments, with some potentially overwintering there, through to temperate climates ( Rich & Rich, 1989 ; Chinsamy, Rich & Vickers-Rich, 1998 ; Chinsamy et al, 2012 ; Herne et al, 2018 ; Woodward, Rich & Vickers-Rich, 2018 ). Ornithopods appear to dominate high latitude to temperate localities, while saurichians, particularly sauropods, seem to dominate low latitude ‘tropical’ climate zones ( Matsukawa, Lockley & Jianjun, 2006 ), although both extend beyond these ranges, as demonstrated by Antarctic ( Cerda & Chinsamy, 2012 ) and Patagonian ( Apesteguía, 2004 ; Salgado, Apesteguía & Heredia, 2005 ; Filippi & Garrido, 2012 and others) sauropods.…”