Structure, ontogeny and evolution of the patellar tendon in emus (Dromaius novaehollandiae) and other palaeognath birds
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Abstract
The patella (kneecap) exhibits multiple evolutionary origins in birds, mammals, and lizards, and is thought to increase the mechanical advantage of the knee extensor muscles. Despite appreciable interest in the specialized anatomy and locomotion of palaeognathous birds (ratites and relatives), the structure, ontogeny and evolution of the patella in these species remains poorly characterized. Within Palaeognathae, the patella has been reported to be either present, absent, or fused with other bones, but it is unclear how much of this variation is real, erroneous or ontogenetic. Clarification of the patella’s form in palaeognaths would provide insight into the early evolution of the patella in birds, in addition to the specialized locomotion of these species. Findings would also provide new character data of use in resolving the controversial evolutionary relationships of palaeognaths. In this study, we examined the gross and histological anatomy of the emu patellar tendon across several age groups from five weeks to 18 months. We combined these results with our observations and those of others regarding the patella in palaeognaths and their outgroups (both extant and extinct), to reconstruct the evolution of the patella in birds. We found no evidence of an ossified patella in emus, but noted its tendon to have a highly unusual morphology comprising large volumes of adipose tissue contained within a collagenous meshwork. The emu patellar tendon also included increasing amounts of a cartilage-like tissue throughout ontogeny. We speculate that the unusual morphology of the patellar tendon in emus results from assimilation of a peri-articular fat pad, and metaplastic formation of cartilage, both potentially as adaptations to increasing tendon load. We corroborate previous observations of a ‘double patella’ in ostriches, but in contrast to some assertions, we find independent (i.e. unfused) ossified patellae in kiwis and tinamous. Our reconstructions suggest a single evolutionary origin of the patella in birds and that the ancestral patella is likely to have been a composite structure comprising a small ossified portion, lost by some species (e.g. emus, moa) but expanded in others (e.g. ostriches).
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2014. Structure, ontogeny and evolution of the patellar tendon in emus (Dromaius novaehollandiae) and other palaeognath birds. PeerJ PrePrints 2:e404v1 https://doi.org/10.7287/peerj.preprints.404v1note This preprint is not peer-reviewed. You may wish to reference the subsequent peer-reviewed version of this article.
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Competing Interests
John R. Hutchinson is an Academic Editor for PeerJ. There are no other competing interests.
Author Contributions
Sophie Regnault performed the experiments, analyzed the data, wrote the paper, prepared figures and/or tables.
Andrew A Pitsillides analyzed the data, contributed reagents/materials/analysis tools, reviewed drafts of the paper.
John R. Hutchinson conceived and designed the experiments, analyzed the data, reviewed drafts of the paper.
Animal Ethics
The following information was supplied relating to ethical approvals (i.e., approving body and any reference numbers):
Only cadaver material was used in this study; no ethical approval was required (the animals were not killed for this study).
Grant Disclosures
The following grant information was disclosed by the authors:
Leverhulme Trust grant number RPG-2013-108
Funding
This research was funded by a grant from the Leverhulme Trust (RPG-2013-108) to J.R.H. and A.P., and J.R.H.’s role was enabled by a Senior Research Fellowship awarded by the Royal Society Leverhulme Trust in 2012. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.