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.
Highlights
Australia was located within the Antarctic Circle during the Early Cretaceous[7,8], and basal ornithopod (“hypsilophodontid” sensu Norman et al.[9] and Horner et al.10) fossils are common in sediments of the Early Cretaceous (Valanginian-Albian) Otway Group[11] along the coast
The Dinosaur Cove locality is considered Albian in age, and hypsilophodontid taxa include Fulgurotherium australe, Leaellynasaura amicagraphica and Atlascopcosaurus loadsi[7,13,14]
Detailed qualitative bone microstructural descriptions of each thin section slide can be found in Supplementary Information online, and a summary of the bone tissue microstructures observed in the Flat Rocks and Dinosaur Cove samples follows here
Summary
Australia was located within the Antarctic Circle during the Early Cretaceous[7,8], and basal ornithopod (“hypsilophodontid” sensu Norman et al.[9] and Horner et al.10) fossils are common in sediments of the Early Cretaceous (Valanginian-Albian) Otway Group[11] along the coast. The present study differs from Woodward et al.[6] in part because here the bone microstructure of 18 of those hypsilophodontid specimens are described more fully (Table 1) This includes a more thorough description of the hypsilophodontid femur figured in the first Victoria dinosaur histology study[4] as well as a pathologic specimen previously only described morphologically[12]. In addition to documenting ontogenetic growth in polar dinosaurs, the femur and tibia sample from Woodward et al.[6] will be utilized here to independently assess Victoria hypsilophodontid taxonomic diversity: variation in annual individual growth rates is to be expected[16], -sized elements with widely varying growth mark counts or primary bone tissue organization may suggest the presence of more than one hypsilophodontid taxon within the sample from Victoria
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