RATES AND TRENDS IN ECOMORPHOLOGICAL DIVERGENCE DURING EARLY BIRD EVOLUTION

Adaptive radiations have long fascinating evolutionary biologists, and by presevering specimens from sequential time slices, the fossil record can yield unique insights into these phenomena. The spectacular Jehol Biota of China (Early Cretaceous, ~125mya) preserves a staggering array of early birds from three different formations. These birds fall into roughly three groups: a paraphyletic grade of basal birds (e.g., Sapeornis), a small radiation of birds close to the modern forms (e.g., Yanornis), and a diverse radiation of the enantiornithines ("opposite birds", e.g., Longipteryx). Previous workers have argued, based on qualitative interpretations, for niche partitioning between opposite birds and stem modern birds. Here, I report on the use of principal components analysis of six morphometric values across over 20 genera that, together, are known to be ecologically relevant in modern birds in an attempt to explore the ecomorphological dynamics of this radiation. I used a previously published matrix, and subsampled over 50 of the resulting most parsimonious trees and then fit different models of evolution (OU, Brownian Motion, delta, trend, white noise) to each tree in the subset to determine how phylogenetic uncertainty impacted our evolutionary interpretation, and to compare this to randomly resolving polytomies along a consensus tree. These model fitting results tend to support OU, and further analysis suggests that a two-peak OU model may be more appropriate, with separate peaks for opposite and modern birds, as they appear to diverge and evolve "away from" each other ecomorphologically. Inherant differences that did not allow for morphological overlap could explain this phenomena, but evidence from allopatric birds suggests that both groups were quite capable of evolving into any region of morphospace occupied by the other. This leaves niche paritioning as the most reasonable explanation for the division.