Thursday, March 11, 2010

Transitional Forms

According to scientists, the Archaeopteryx lithographica (page 40) is the earliest and most primitive bird known. Certain features allow scientists to think that it is a transitional fossil between modern birds and reptiles, more specifically dinosaurs. What are these specific features that connect theropod dinosaurs with birds? Does the Archaeopteryx fossil resemble one of these animals more than the other? Also, have scientists found any other transitional forms between dinosaurs and birds that prove this relationship of evolution is true?


  1. Archaeopteryx is more reptile than bird because its skeleton is almost identical to that of some theropod dinosaurs. "All eight known fossils of Archaeopteryx, discovered between 1860 and 1992, were found in a 516 sq-mi (1,336 sq-km) area of the Solnhofen quarries. Classified as a bird because of the presence of feathers and the structure of the legs and wings, it nevertheless had many characteristics now found only in reptiles or in bird embryos. It was about the size of a pigeon or grackle." ( It is still debated whether Archaeopteryx was arboreal or a swiftly running terrestrial animal and poor flyer.Its reptilian features are a jaw with teeth, a long bony tail, claws, separate fingers on the wing, and a neck attached to the skull from behind instead of from below. The two birdlike traits are large feathers and an opposable big toe, probably used for perching. Scientists are unsure if this species could fly; however, the asymmetrical feathers with one side of each feather being larger than the other suggests that it could fly. It is mainly dinosaurian though. Not all of its features appear halfway between those of birds and reptiles and since it has only a few birdlike qualities, it is mostly reptilian. In the mid-1990s scientists found fossils in China that proved another example of reptile and bird relationships. These fossils found in lake sediments represented a veritable parade of feathered theropod dinosaurs. Some had very small filamentous structures covering the whole body, which were probably earlier feathers. One species was the Sinornithosaurus millenii. Its body was covered with long, thin feathers. The feathers were so small it couldn't have possibly helped it fly. Its teeth, claws, and long bony tail showed that it was far from being a modern bird."The study on the skull of Sinornithosaurus millenii [Xu, X. and Wu, X.-C. (2001) provides new information on the poorly known cranial anatomy of dromaeosaurid dinosaurs, revealing that dromaeosaurids share more similarities with Archaeopteryx than previously thought. It also provides evidence suggesting that early dromaeosaurids are more bird-like than the later, more derived ones, thus contradicting claims that more bird-like dinosaurs appeared later in fossil record than the earliest birds." ( Another species found was the Microraptor gui or four-winged dinosaur. Unlike any modern bird, it was a thirty-inch-long creature that had fully feathered arms and legs. "Xu Xing, a paleontologist at the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing, China, and colleagues suggest in the January 23 issue of the journal Nature that the species is an early ancestor of birds that probably used its feathered limbs, along with a long, feather-fringed tail, to glide from tree to tree.They argue that the animal represents an intermediate stage in the evolution of flight, from gliding much as flying squirrels do today to the active wing flapping of modern birds." (

  2. It was also discovered that many of these fossils showed that these animals behaved in birdlike ways as well. Mark Norell, an American paleontologist found one small feathered dinosaur sleeping with its head tucked under its folded, winglike forearm. This is exactly how modern birds sleep. The other fossil of a female theropod die while sitting on her nest of eggs, showing brooding behavior similar to that of birds. The fossils dated back between 135 and 110 million years ago, which means that they could not have been direct ancestors of Archaeopteryx, but possible cousins. it is difficult to find fossils that may be directly related to Archaeopteryx because feathers are only preserved in special sediments in fine grained silt of quiet environments like lake beds or lagoons. As the fossils discovered get younger, though, the reptilian tail seems to shrink, the teeth disappear, the claws fuse together, and the appearance of the a large breastbone to anchor flight muscles begins to arise. Although it is not one hundred percent certain, due to the evidence of multiple species with both reptilian and birdlike qualities, it can be assumed that reptiles did evolve into birds at some point in the past.