20 June 2012
Title:Does morphological convergence imply functional similarity? A test using the evolution of quadrupedalism in ornithischian dinosaurs
Authors: Susannah C. R. Maidment and Paul M. Barrett
Journal: Proceedings of the Royal Society B
Extinct animals cannot be fully understood by the shape of their bones according to new research published in Proceedings of the Royal Society B. The research, conducted by the Natural History Museum, concludes that reconstructions of soft tissues, such as muscles, are also needed in order to understand them fully.
The only parts of dinosaurs that remain are their bones, so it is unsurprising that palaeontologists have previously drawn conclusions about the lifestyles of the extinct beasts using only their fossilised skeletons. In fact the idea that animals with similar body shapes have similar lifestyles has long been central to the interpretation of fossils.
Dr Susannah Maidment and Dr Paul Barrett studied the way that four-legged dinosaurs stood and found that different dinosaur limb bones can be similar, even when their muscles were not, suggesting that these species stood and walked differently from each other.
It is a widely held assumption that similar function can be inferred from similar skeletons. This means that when palaeontologists find two similar fossils, they might assume that the features evolved independently; with the convergence deriving from the same selective pressures and mechanical factors. However, Maidment and Barrett’s findings suggest that these kinds of assumptions are unfounded.
Maidment and Barrett examined the bones of approximately 200 specimens (90 species) of extinct and extant archosaurs (four-limbed animals whose living representatives consist of birds and crocodilians). They correlated each bone with high-tech reconstructions of the related muscles. They then constructed the stepwise evolutionary changes in both bones and muscles, along different lineages in which quadrupedality evolved.
They found that a variety of different skeletons can produce the same functional outcome, implicitly suggesting that convergent skeletal structure is not a good predictor of functional similarity. This presents a major problem for palaeobiologists, who seek to understand the life history of extinct organisms based only on bones. These findings instead advocate an approach whereby function and functional similarity is determined by studying both bone and muscle reconstruction.