Monday, July 19, 2010
Image courtesy of NERC
The exaggerated head crests and elaborate sail-like structures of fossil reptiles developed to attract the opposite sex and had little to do with temperature regulation, say researchers.
Why flying reptiles – or pterosaurs – had huge crests on their heads and eupelycosaurs had massive sail-like structures on their backs has long been a mystery among palaeontologists.
Scientists have come up with a host of possible explanations for what they were for. Some say head crests could have acted like a rudder during flight, while others have said they could have been feeding aids.
But the idea favoured by most researchers is that they were important for keeping the creatures that sported them at the right temperature.
'People have touched on the idea that crests and sails could have been to do with sexual selection, but until now, no-one's taken it further,' says Dr Joseph Tomkins from the University of Western Australia (UWA), lead author of a study published in The American Naturalist.
Tomkins, an expert in so-called sexual selection – how different traits have evolved to attract a mate – says, 'While I was doodling a sail-backed lizard, it suddenly struck me that the sail had to be sexually selected.'
He reasoned that the sails on creatures like Dimetrodon and the head crests on Pteranodons were probably too big to serve any other purpose.
To test this idea, Tomkins and colleagues from UWA and the Universities of Portsmouth and Sheffield analysed the published data on crest length and eye socket size in Pteranodon, while for Dimetrodon, they compared sail height with body weight.
The researchers wanted to find out how the size of these structures compared with the creatures' overall body size to see if they could have been sexually selected.
'We used the laws of physics to work out what we'd expect if crests and sails were thermoregulators,' explains Tomkins.
'If these structures were only for temperature control, you can predict what sort of scaling relationship the trait should have in relation to body size,' he adds.
Tomkins and his colleagues found that the relationship between body size and crest or sail size was very similar to what you'd expect from a sexual trait in a creature alive today, but even more so.
'We expected there to be a relationship, but we were really surprised at how exaggerated the structures of these fossil creatures were in comparison to existing animals.'
'In red deer, large males have large antlers. Also in stag beetles, bigger males have big mandibles. It's even the same in stalk-eyed flies: the stalks on the largest males are disproportionately large. But the head crest of pteranodon in particular outclassed these traits,' says Tomkins.
Peacocks' elaborate tails are considered to be one of the most extreme examples of sexual display in any creature alive today.
'As a comparison, pterosaurs put even more effort into attracting a mate than peacocks did. Peacocks also shed their plumage every year, so it's only a burden some of the time. But pterosaurs had to carry their crest around all the time,' says co-author Dr Dave Martill from the University of Portsmouth.
The researchers also found that smaller ancestors of Dimetrodon would've been too small to even need organs to heat or cool them down.
'Pteranodon crests and eupelycosaur sails are some of the earliest and most extreme examples of elaborate sexual signals in the evolution of terrestrial vertebrates,' says Tomkins.
Joseph L Tomkins, Natasha R LeBas, Mark P Witton, David M Martill, and Stuart Humphries, Positive Allometry and the Prehistory of Sexual Selection, The American Naturalist, published online June 21 2010, doi: 10.1086/653001
Natural Environment Research Council - http://www.nerc.ac.uk/