Dinosaurs and Birds
Aug 25, The relationship between birds and theropods, the group of dinosaurs that preceded them, "really illustrates well how evolution works," said. Using proper terminology, birds are avian dinosaurs; other dinosaurs are between birds and other coelurosaurs that has helped to illuminate the evolutionary history between birds and the theropod dinosaurs (especially the coelurosaurs). Systematics (the science of evolutionary relationships) has undergone a major change between the skeletons of meat-eating dinosaurs (theropods) and birds.
A study comparing embryonic, juvenile and adult archosaur skulls concluded that bird skulls are derived from those of theropod dinosaurs by progenesisa type of paedomorphic heterochronywhich resulted in retention of juvenile characteristics of their ancestors. O'Connor of Ohio University. In theropod dinosaurs carnivores that walked on two legs and had birdlike feet flexible soft tissue air sacs likely pumped air through the stiff lungs, as is the case in birds.
From these methods, the authors found that: There was one possible patch with animal cellular structures. The authors found their data supported identification as a concretion of sand from the burial environment, not the heart, with the possibility that isolated areas of tissues were preserved.
Reproductive biology[ edit ] When laying eggs, female birds grow a special type of bone in their limbs.What Is a Bird? - Evolutionary Relationship Between Birds and Reptiles
This medullary bone forms as a calcium-rich layer inside the hard outer bone, and is used as a calcium source to make eggshells. The presence of endosteally derived bone tissues lining the interior marrow cavities of portions of a Tyrannosaurus rex specimen's hind limb suggested that T. Because the line of dinosaurs that includes Allosaurus and Tyrannosaurus diverged from the line that led to Tenontosaurus very early in the evolution of dinosaurs, this suggests that dinosaurs in general produced medullary tissue.
A dinosaur embryo was found without teeth, which suggests some parental care was required to feed the young dinosaur, possibly the adult dinosaur regurgitated food into the young dinosaur's mouth see altricial.
This behaviour is seen in numerous bird species; parent birds regurgitate food into the hatchling's mouth. Gizzard stones[ edit ] Both birds and dinosaurs use gizzard stones. These stones are swallowed by animals to aid digestion and break down food and hard fibres once they enter the stomach.
When found in association with fossilsgizzard stones are called gastroliths. Molecular evidence[ edit ] On several occasions, the extraction of DNA and proteins from Mesozoic dinosaurs fossils has been claimed, allowing for a comparison with birds. Several proteins have putatively been detected in dinosaur fossils,  including hemoglobin.
Mary Higby Schweitzer and her team announced the discovery of flexible material resembling actual soft tissue inside a million-year-old Tyrannosaurus rex leg bone of specimen MOR from the Hell Creek Formation in Montana. The seven collagen types obtained from the bone fragments, compared to collagen data from living birds specifically, a chickensuggest that older theropods and birds are closely related. This study utilized eight additional collagen sequences extracted from a femur of the "mummified" Brachylophosaurus canadensis specimen MORa hadrosaur.
No other peptides of a Mesozoic age have been reported. Init was suggested that the presumed soft tissue was in fact a bacterial microfilm. Origin of avian flight Debates about the origin of bird flight are almost as old as the idea that birds evolved from dinosaurswhich arose soon after the discovery of Archaeopteryx in Two theories have dominated most of the discussion since then: A more recent theory, "wing-assisted incline running" WAIRis a variant of the cursorial theory and proposes that wings developed their aerodynamic functions as a result of the need to run quickly up very steep slopes such as trees, which would help small feathered dinosaurs escape from predators.
In Marchscientists reported that Archaeopteryx was likely capable of flightbut in a manner substantially different from that of modern birds. This hypothesis proposes that some fast-running animals with long tails used their arms to keep their balance while running.
Modern versions of this theory differ in many details from the Williston-Nopcsa version, mainly as a result of discoveries since Nopcsa's time. Nopcsa theorized that increasing the surface area of the outstretched arms could have helped small cursorial predators keep their balance, and that the scales of the forearms elongated, evolving into feathers.
The feathers could also have been used to trap insects or other prey. Progressively, the animals leapt for longer distances, helped by their evolving wings. Nopcsa also proposed three stages in the evolution of flight.
How Dinosaurs Shrank and Became Birds
These dinosaurs were only distantly related to modern snakes, lizards, and turtles, groups that had split off at different times. But 65 million years ago there was a massive extinction event, and all dinosaurs were killed except for a single group of feathered dinosaurs. These evolved over the next 65 million years into modern birds. So birds aren't just closely related to dinosaurs, they really are dinosaurs! This is what most people mean when they say that birds are reptiles, although technically according to the phylogenetic system mammals are also reptiles.
You may wonder why biologists have two systems of classification.
How Dinosaurs Shrank and Became Birds - Scientific American
One reason, of course, is the history behind them, but they are also both useful in their own ways. The phylogenetic system is useful for understanding the relationships between animals, while the Linnaean system is more useful for understanding how animals live.
It's sort of like cooking. If you organized all your ingredients phylogenetically, you would put everything that was made from peanuts on the same shelf. Then you could see that peanut butter, peanut oil, and peanut brittle are related to each other. But when you really want to cook, you would use something like the Linnaean system and put all your oils together, all your dry goods together, etc. So both systems have their uses.
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