Reply: Theropod-bird link reconsidered

Abstract

Norell et al. reply — First, maniraptoran theropods, and theropods with furculae, are known from the Late Jurassic period⁷,⁸, significantly closing the ‘gap’ discussed above. We did not say in our Scientific Correspondence¹ that Velociraptor was a direct ancestor, only that dromaeosaurs are related to birds. How do Feduccia and Martin explain the presence of primitive living mammals such as monotremes and marsupials 100 million years after they split from the line leading to ourselves?

Main

Second, exact similarity is not a requirement for homology — take, for example, the forelimb variation among the living orders of mammals. Bat wings, whale flippers and human arms are very different but homologous. The furculae of living birds are highly variable⁶ and often different from the furcula of Archaeopteryx, yet Feduccia and Martin do not question the homology between these two groups. These authors identify this element as a possible interclavicle, a bone that is extremely different in crocodylomorphs and not present in any ornithodiran⁹.

We do not think Feduccia and Martin can, from a small photograph, say with confidence that the furcula is articulated along the entire margin of the coracoid. Examination of the actual specimen shows that the furcula attaches to the scapula just as it does in modern birds. We discussed this feature in our Scientific Correspondence¹: “⃛the proximal process of the furcula tapers to a point where it contacts the scapulocoracoid”.

Third, we disagree with Feduccia and Martin's implication that preconceived process dictates the interpretation of pattern. Science is about discovery; by assuming less, we discover more. Feduccia and Martin believe that furculae cannot exist in the non-avian Maniraptora because they did not fly, but this view precludes the possibility of furculae ever being discovered in these animals.

Fourth, Longisquama is a poor-quality fossil, and the interpretation of single elements is controversial. Longisquama lacks other characters — present in non-avian Maniraptora — that would ally it with birds. Single features do not overturn a hypothesis that is strongly supported by a plethora of character evidence.

Ironically, if one does use Feduccia and Martin's reasoning that Longisquama is a close bird ‘ancestor’ as advocated elsewhere¹⁰, the temporal paradox increases. Longisquama comes from rocks about 220 million years old, creating a fossil-free gap of more than 80 million years before the appearance of Archaeopteryx. Any empirical measure of stratigraphic fit¹¹ will prefer a hypothesis of maniraptoran relationships over this one.