Contributed by Zhonghe Zhou, August 3, 2011 (sent for review July 13, 2011)
The crop is characteristic of seed-eating birds today, yet little is known about its early history despite remarkable discoveries of many Mesozoic seed-eating birds in the past decade. Here we report the discovery of some early fossil evidence for the presence of a crop in birds. Two Early Cretaceous birds, the basal ornithurine Hongshanornis and a basal avian Sapeornis, demonstrate that an essentially modern avian digestive system formed early in avian evolution. The discovery of a crop in two phylogenetically remote lineages of Early Cretaceous birds and its absence in most intervening forms indicates that it was independently acquired as a specialized seed-eating adaptation. Finally, the reduction or loss of teeth in the forms showing seed-filled crops suggests that granivory was possibly one of the factors that resulted in the reduction of teeth in early birds.
Author contributions: X.Z., L.D.M., and Z.Z. designed research; X.Z., L.D.M., Z.Z., and D.M. performed research; X.Z., D.A.B., and F.Z. contributed new reagents/analytic tools; D.A.B. and F.Z. analyzed data; and L.D.M., Z.Z., and D.M. wrote the paper.
The authors declare no conflict of interest.
