How Birds Came to Be Regarded as Dinosaurs

Controversy over the Origin of Birds in the Second Half of the 20th Century

Abstract

This paper examines the controversy over the origin of birds in the field of vertebrate paleontology during the second half of the 20th century. This debate has reached a tentative consensus among most experts that birds belong to a clade of dinosaurs called the theropod (thus, the theropod hypothesis). Henry Gee, a paleontologist and senior editor of Nature, regarded the controversy as a conflict between the “Romer-Simpson view” related to evolutionary scenarios and an alternative based on cladistics, arguing that the latter supported the theropod hypothesis. However, the process that led to the settlement of the controversy had other and more important aspects than the conflict between the two different points of view. The changing numbers of phenotypic characters supporting each phylogenetic hypothesis mostly determined the trend of the controversy. These debates based on characters often took place separately from the conflicts between the two viewpoints, which had much less impact on the early debates. As phenotypic characters supporting phylogenetic hypotheses other than the theropod theory became dismissed, proponents of those hypotheses much less often employed arguments based on characters. As a result, the controversy became a dichotomy between the theropod hypothesis based on characters and oppositions based on other arguments, such as evolutionary scenarios. Changes in the norms about research practice within the paleontological community were also significant. In the 1990s, cladistics became the standard methodology in vertebrate paleontology. Consequently, researchers who opposed the theropod hypothesis and argued for evolutionary scenarios irreconcilable with cladistics became marginalized.