Specimens of Archaeopteryx resembling a flying squirrel. After conducting an embryological comparison with modern birds, he noticed a similarity between "Archaeopteryx" and the embryonic stage of birds, concluding that the former was likely an "archetype", or created kind, of the bird lineage. Owen's monograph presented a description of the osteology and the plumage, though he missed a number of elements. Owing to its overall similarity to the structure of modern birds, Owen postulated a toothless, horny beak-like instrument "fitted for preening", and misinterpreted the fragment of an upper jaw with teeth, located next to the pelvis on the specimen, as the remains of
Specimens of Archaeopteryx — limiting its ability to perch on branches and implying a terrestrial or trunk-climbing lifestyle. This has been interpreted as evidence of theropod ancestry. In 1988, Gregory S. Paul claimed to have found evidence of a hyperextensible second toe, but this was not verified and accepted by other scientists until the Thermopolis specimen was described. "Until now, the feature was thought to belong only to the species' close relatives, the deinonychosaurs." The Thermopolis Specimen was assigned to "Archaeopteryx siemensii" in 2007. The specimen is considered the most complete and well preserved "Archaeopteryx" remains yet. This specimen first came to light