Geoscience Reference
In-Depth Information
flecting how lift forces gradually took the bird farther upward into the wild blue (or
mild gray) yonder.
Thecontroversyabouttheevolutionofflightindinosaursisnotwhetherornot
it happened, but how it happened. At first only two hypotheses were offered, with
pithy summary titles: “ground up” and “trees down.” The “ground up” hypothesis
says that certain lineages of non-avian theropods at first were ground dwellers but
later evolved flight through a combination of natural selection for longer arms, fast
running,lighterbodyframes,andflightfeathers.The“treesdown”hypothesisstates
that those same originally ground-dwelling dinosaurs climbed trees and otherwise
became more arboreal, and at first were gliders. But then natural selection favored
those that could flap under their own power instead of just glide.
Fortunately for scientists who do not like dichotomous arguments, a third hy-
pothesis was offered just recently, and it synthesized elements of the other two with
the special bonus of sounding more technical. Called the “wing-assisted incline
running” (WAIR) hypothesis, the researchers who developed this model—mostly
Kenneth Dial and his colleagues—noticed how baby birds, with only half-formed
wings,flapthesewhilerunningupinclines,includingverticallyorientedtreetrunks.
Thus,flappingcombinedwithrunninghelpsthebirdstogetupthosesurfacesmuch
more easily than if they did not flap at all. Half-formed wings also come in handy
whenbabybirdsfalloutofnests,asflappingslowsthemdownenoughthattheyare
more likely to survive once they strike the ground.
This is where trace fossils could help to test and otherwise augment these hy-
potheses, especially in Middle through Late Jurassic rocks, which is when paleon-
tologists suspect that birds began separating from dinosaurs, evolutionarily speak-
ing. Track evidence supporting ground-up flight would show small theropod tracks
with take-off patterns similar to those we see in modern birds, but probably more
typical of those that need a little more of a running or hopping start before be-
coming airborne. Track evidence for trees-down or incline-assisted flight is a little
more challenging from a trace fossil standpoint, as this would require paleontolo-
