Archaeopteryx compared to its body length. It could reach up to in body length, with an estimated mass of . "Archaeopteryx" feathers, although less documented than its other features, were very similar in structure to modern-day bird feathers. Despite the presence of numerous avian features, "Archaeopteryx" had many non-avian theropod dinosaur characteristics. Unlike modern birds, "Archaeopteryx" had small teeth, as well as a long bony tail, features which "Archaeopteryx" shared with other dinosaurs of the time. Because it displays features common to both birds and non-avian dinosaurs, "Archaeopteryx" has often been considered a link between them. In the 1970s, John Ostrom,
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