Arbitrary Paleontology and Monophyly

["W. J. T. Mitchell" <wjtm@midway.uchicago.edu>]

Jonathan Wagner wrote:

The odds that all dinosaurs had a common ancestor are probably
astronomically huge.

The thing that puzzles me still is, if the odds were so loaded in favor of a
common (dinosaur) ancestor, why was this not evident for such a long time,
to Harry Seeley in 1887, or to Colbert and Ostrom later on?  What changed in
dinosaur science to make something seem so certain (in a field that seems to
have its fair share of uncertainty)?  Was it the discovery of new evidence?
Or a new way of looking at the evidence?  Tom Mitchell