Theropod Troodon clutch. Credit: Charles Deeming The eggs of amniotes - mammals, reptiles and birds – come in a remarkable variety of shapes and sizes. 
 
 Evolutionary biologists have now addressed shape variety in terrestrial vertebrates' eggs, pinpointing morphological differences between the eggs of birds and those of their extinct relatives, the theropod dinosaurs. 
 
 Researchers from the University of Lincoln, UK, examined eggshell geometry from the transition of theropods - a sub-order of the Saurischian dinosaurs - into birds, based on fossil records and studies of their living species. 
 
 The results suggest that the early birds from the Mesozoic (252 to 66 million years' ago) laid eggs that had different shapes to those of modern birds. This may suggest that egg physiology and embryonic development was different in the earliest birds and so this may have implications for how some birds survived the Cretaceous-Palaeocene extinction event that wiped out the dinosaurs. 
 
 Their findings are published in the journal Royal Society Open Science. 
 
 Author Dr Charles Deeming, from Lincoln's School of Life Sciences, explained: "These results indicate that egg shape can be used to distinguish between different types of egg-laying vertebrates. More importantly they suggest Mesozoic bird eggs differ significantly from modern day bird eggs, but more recently extinct Cenozoic birds do not. This suggests that the range of egg shapes in modern birds had already been attained in the Cenozoic." 
 
 The origin of the amniotic egg (an egg which can survive out of water) is one of the key adaptations underpinning vertebrates' transition from sea to land more than 300 million years ago. Modern amniotic eggs vary considerably in shape and size and it is believed this variety may reflect the different patterns of egg formation and development in these taxa. 
 
 Dr Deeming added: "From a biological perspective, it is self-evident that different egg shapes by birds, both past and present, might be associated with different nesting behaviours or incubation methods. However, hardly any research has been carried out on this topic and fossil data are insufficient to draw firm conclusions. We hope that future discoveries of associated fossil eggs and skeletons will help refine the general conclusions of this work." 
 
 Dr Deeming and co-author Dr Marcello Ruta, also from the University of Lincoln, are now investigating how the highly variable amounts of yolk and albumen (egg white) in eggs of different species could be a possible determinant of bird egg shape. 
 
 Explore further: Fossilised bird egg offers clues to Brazil's prehistoric past 
 
 More information: D. Charles Deeming and Marcello Ruta 'Egg shape changes at the theropod-bird transition, and a morphometric study of amniote eggs' Royal Society Open Science 
 
 DOI: 10.1098/rsos.140311 
 
 

 Image copyright Charles Deeming Image caption Fossil records of theropod eggs allowed scientists to analyse their geometric properties 
 
 The shape of birds' eggs could have helped them survive the mass extinction event that killed off the dinosaurs, new research proposes. 
 
 A team analysed the geometric properties of eggs from 250 million years ago (Mesozoic Era) to today. 
 
 Before the extinction event about 65 million years ago, eggshells had notable differences to the lineage that survived. 
 
 It is these survivors that all modern day birds descend from. 
 
 But the authors note that egg shape is but a "small piece of the puzzle" of the evolutionary conundrum of why one lineage of birds made it through the mass extinction event, whereas others did not. Their findings are published in the Royal Society journal Open Science. 
 
 The analysis found that Mesozoic eggs were elongated and significantly more symmetrical than all other bird eggs. Mesozoic bird eggshells were also more porous than expected for their size. 
 
 Lead author of the work, Dr Charles Deeming from Lincoln University in the UK, found that fossil remains of eggs from 65 million years ago onwards were indistinguishable from modern bird eggs. The Mesozoic eggs, however, differed significantly. 
 
 "This implies that Mesozoic birds were doing something more akin to theropod-like ancestors than to modern birds," he said. 
 
 Image copyright Spl Image caption Microraptor is thought to be an evolutionary link between birds and dinosaurs 
 
 Theropod dinosaurs include the largest carnivorous predators which dominated the Earth during the Jurassic and Cretaceous periods. 
 
 A branch of these, referred to as avian theropods are on the lineage that gave rise to birds. 
 
 Although eggshell shape could be a contributing factor, how exactly one group survived still largely remains a mystery. 
 
 "We don't know why this iconic group disappeared and the relatives of modern birds survived," Dr Deeming told BBC News. "We're not claiming in our paper that this tells us the answer." 
 
 It could also be down to their behaviour, he explained. 
 
 "I suspect modern birds survived the major extinction event because they used contact incubation that's prevalent now; they built nests and sat on their eggs whereas most of the birds prior to that were burying the eggs like their theropod ancestors, potentially making them more vulnerable." 
 
 Image copyright Spl Image caption Scientists compared modern eggs to extinct eggs from early birds and feathered dinosaurs 
 
 Dr Gareth Dyke at the University of Southampton specialises in the evolution of birds. Commenting on the work, he said it was "perhaps intuitive" that eggshells differed in shape and that it was interesting to show this for the first time. 
 
 "We know that in the late Cretaceous there were at least two or three other lineages of flying birds that were anatomically quite similar to the group that went on to radiate and become all of the living [bird] species after the extinction event." 
 
 Dr Dyke added that the differences between these groups represented one of the big unanswered questions for palaeontologists. 
 
 "It's got to be some biological or ecological reason. If it was more random you would expect that stragglers from some groups in the Cretaceous would have limped across the extinction events, but that didn't happen." 
 
 Image copyright Spl Image caption Modern birds like guillemots incubate their eggs 
 
 Expanding on this point, Gary Kaiser, a biologist at the Royal British Columbia Museum in Canada, said that egg shapes were certainly important. 
 
 "We know that some dinosaurs incubated eggs but we also know from fine structure of the bones that their hatchlings were small and took a long time to reach adult size. A slow or extended reproductive period may have been the type of problem that led to the extinction of the dinosaurs." 
 
 Prof Xu Xing from the Chinese Academy of Sciences in Beijing commented: "The authors have done a nice piece of work demonstrating the shape changes among amniote eggs. 
 
 "However, I could not find how this study helps to understand the big extinction. Because the authors group birds based on geological time, they effectively eliminate the possibility of finding any pattern for birds surviving the extinction event." 
 
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– So why did birds survive the mass extinction that wiped out their dinosaur relatives? A new study suggests that the shape of their eggs played a role, reports the BBC. Scientists aren't sure exactly what that role was, but they do know that eggs of survivor birds were shaped differently than eggs of dinosaurs and birds that got wiped out. Among other things, the non-survivor eggs were more elongated and "significantly more symmetrical," says the news service. The difference in shape suggests that the birds themselves differed in physiology, reports phys.org, presumably in ways that gave them an advantage when the mass extinction took place 65 million years ago. One theory is that the differences in shape may have something to do with crucial differences in hatching and nesting behavior. "I suspect modern birds survived the major extinction event because they used contact incubation that's prevalent now," says one researcher from the UK's University of Lincoln. "They built nests and sat on their eggs whereas most of the birds prior to that were burying the eggs like their theropod ancestors, potentially making them more vulnerable." (Click to read how hummingbirds aren't as gentle as you might think when it comes to mating rituals.)