In the Liaoning Province of The People’s Republic of China, researchers are combing an extremely rich fossil site, the Yixian Formation. It contains remarkably well-preserved remains of plants, insects, invertebrates, fish, amphibians, reptiles, birds, mammals, and dinosaurs.1 Arguably, the most interesting fossils from the site are theropod dinosaurs–-some of which have been interpreted as bearing feathers.2-3 These specimens have been touted by evolutionary biologists as key transitional intermediates linking birds to dinosaurs.
The initial dating of the The Yixian Formation seemed to support that possibility. Using the method called biostratigraphic correlation, scientists identified the fossils as late Jurassic (more than 135 million years old), making the “feathered” theropod dinosaurs contemporary with and thus a possible progenitor of archeopteryx, the oldest true bird. (This dating technique uses index fossils to estimate the age of one formation by comparing it with a formation of known age that contains the same index fossils.)4
Because of its importance, several research teams have focused with more robust and demanding methodologies on the dating of the Yixian Formation. Two recent studies used a radiometric dating technique, 40Ar-39Ar. Each study measured the age of the Yixian Formation to be between 121 and 125 million years old.5-6
These new dates place the Yixian Formation within the early Cretaceous period, making archeopteryx at least 20 million years older than the so-called “intermediates” leading up to it. The theropods from the Yixian Formation, like all theropods, now fall within the “temporal paradox.” That is, all theropods, despite their declared status as progenitors of birds, show up in the fossil record well after the first appearance of birds.7
Order of appearance in the fossil record remains important in establishing the validity of the evolutionary paradigm. Thus, the re-dating of the Yixian Formation has profound implications. It significantly weakens the theropod dinosaur-to-bird connection and renders the origin of birds unaccounted for by the evolutionary model.
Endnotes
Paul M. Barrett, “Evolutionary Consequences of Dating the Yixian Formation,” Trends in Ecology and Evolution, 15 (2000): 99-103.
P-J Chen et al ., “An Exceptionally Well-Preserved Theropod Dinosaur from the Yixian Formation of China,” Nature, 391 (1998): 147-152.
Q. Ji et al ., “Two Feathered Dinosaurs from Northeastern China,” Nature, 393 (1998): 753-761.
Barrett, 99-103.
Barrett, 99-103.
Carl C. Swisher III, et al., “Cretaceous Age For the Feathered Dinosaurs of Liaoning, China,” Nature, 400 (1999): 58-61.
Alan Feduccia, The Origin and Evolution of Birds , 2nd edition (New Haven: Yale University Press, 1999): 382.
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