Birds are the only dinosaurs that have survived into modern times. Why is that? Of all thedinosaur species, how did they manage to make it through thecatastrophic events of 65 million years ago, whereas all theirfellow dinos perished? A new study, published May 27 in Nature , hints at an evolutionary phenomenon that may have played tobirds’ advantage: They are, it seems, baby dinosaurs whose biologyprevents them from ever growing up. ( Scientific American is part of Nature Publishing Group.) Bhart-Anjan Bhullar and his doctoral advisor, Arkhat Abzhanov,posit that birds may have evolved from dinosaurs by a process knownas paedomorphosis, whereby an organism retains juvenile traits evenafter it becomes sexually mature. “They certainly look to havesome paedomorphic characteristics,” says Jack Horner, aMontana State University paleontologist who studies dinosaur growth and development and was not involved in the study. Paedomorphosis “is notuncommon in evolution and speciation,” he adds. Welding Turning Rolls
Bhullar and Abzhanov reached this conclusion by comparing theskulls of birds and dinosaurs across phylogenies, or relatedgroups, and at different developmental stages. To quantitativelycompare cranial geometries, they scanned the skulls of theropoddinosaurs (which are thought to be birds’ ancestors), crocodilesand alligators (dinosaurs’ cousins), early transitional birds suchas Archaeopteryx , and modern birds. Then they created digitized versions of eachskull and mapped out cranial landmarks, such as nostril tips, eyesocket dimensions and places where bones meet. Their measurements showed that whereas a typical non-avian dinosaurbegan life with a round head, large eyes and a big brain (relativeto the rest of the body) then later developed an elongated snoutand smaller relative brain size, birds kept their baby faces. China Electric Plate Compactor
If birds did evolve by paedomorphosis, they join species such as axolotls . These salamanders evolved to retain tadpolelike gills and finsand, unlike most other amphibians, remain aquatic into adulthood.This feature appears to be due to a hormone disruption. By addingthyroid hormone into their water , researchers have caused axolotls to metamorphose into terrestrial salamanders . But why would it be advantageous for adult animals to look like kids? Greg Erickson, who studies evolutionarymorphology at The Florida State University and was not involved inthe study, says that paedomorphosis can help a species to developnew adaptations and exploit new niches. China Concrete Cut Saw
In particular, he suggeststhat paedomorphosis may have enabled birds to develop larger eyes,which aid in spatial assessment during flight as well as a highbrain-to-body-mass ratio, which may contribute to intelligence. An even simpler explanation is that kids are small and, in times ofenvironmental stress , small is good. Bhullar cites an example ofTemnospondyli—large primitive amphibians that were commonbefore 120 million years ago. Catastrophic events killed off mostof the temnospondyli, except for a few paedomorphic species.”The interesting thing to me is that [after the catastrophe]these little paedomorphic animals were at the base of a giantradiation,” Bhullar says. He suggests that a similarphenomenon may have occurred during the catastrophic events that killed the dinosaurs—being small may have been anadvantage, because smaller animals require less food and can moreeasily hide.
“Everything that lived on land and weighed morethan one kilogram perished,” Bhullar says. “The onlydinosaurs that survived were the paedomorphic ones.” And aftermany of those larger species went extinct, the little dino may havebeen better placed to exploit the new niches that opened up.