Temporal paradox (paleontology)

The temporal paradox, or time problem is a controversial issue in the evolutionary relationships of birds. It was described by paleornithologist Alan Feduccia.Feduccia, Alan (1994) "The Great Dinosaur Debate" Living Bird. 13:29-33.] Feduccia, Alan (1996) "The Origin and Evolution of Birds." Yale University Press. New Haven, Conn. USA.]

The concept of a "temporal paradox" is based on the following facts. The consensus view is that birds evolved from dinosaurs, but the most bird-like dinosaurs, and those most closely related to birds (the maniraptorans), are known mostly from the Cretaceous, by which time birds had already evolved and diversified. If bird-like dinosaurs are the ancestors of birds they should be older than birds, but "Archaeopteryx" is 155 million years old, while the very bird-like "Deinonychus" is 35 million years younger. This idea is sometimes summarized as "you can't be your own grandmother".

Numerous researchers have discredited the idea of the temporal paradox. Witmer (2002) summarized this critical literature by pointing out that there are at least three lines of evidence that contradict it. First, no one has proposed that maniraptoran dinosaurs of the Cretaceous are the ancestors of birds. They have merely found that dinosaurs like
dromaeosaurs, troodontids and oviraptorosaurs are close relatives of birds. The true ancestors are thought to be older than "Archaeopteryx", perhaps Early Jurassic or even older. The scarcity of maniraptoran fossils from then is not surprising since fossilization is a rare event requiring special circumstances, and we may never find fossils of animals in sediments from ages that they actually inhabited. Second, fragmentary remains of maniraptoran dinosaurs actually have been known from Jurassic deposits in China, North America, and Europe for many years. The femur of a tiny maniraptoran from the Late Jurassic of Colorado was reported by Padian and Jensen in 1989. Jensen, James A. & Padian, Kevin. (1989) "Small pterosaurs and dinosaurs from the Uncompahgre fauna (Brushy Basin member, Morrison Formation: ?Tithonian), Late Jurassic, western Colorado" Journal of Paleontology Vol. 63 no. 3 pg. 364 - 373] A therizinosaur called "Eshanosaurus" has been described from the Early Jurassic, a troodontid known as WDC DML 001 has been reported from the Middle Jurassic, and teeth of dromaeosaurids and troodontids are known from Jurassic England.Witmer, L.M. (2002). “The Debate on Avian Ancestry; Phylogeny, Function and Fossils”, “Mesozoic Birds: Above the Heads of Dinosaurs” pp.3-30. ISBN 0-520-20094-2] Then there is "Epidexipteryx", a feathered maniraptoran that is older than Archaeopteryx. It was reported by Zhang et al. in 2008 from the Middle - Late Jurassic Daohugou Beds in northern China. It is a close relative to birds, a member of the Avialae.Zhang, Fucheng, Zhou, Zhonghe, Xu, Xing, Wang, Xiaolin, and Sullivan, Corwin. A bizarre Jurassic maniraptoran from China with elongate ribbon-like feathers. Available from Nature Precedings (2008)] Third, if the temporal paradox would indicate that birds should not have evolved from dinosaurs, then what animals are more likely ancestors considering their age? Brochu and Norell (2001) analyzed this question using several of the other archosaurs that have been proposed as bird ancestors, and found that all of them create temporal paradoxes - long stretches between the ancestor and "Archaeopteryx" where there are no intermediate fossils - that are actually worse. Thus, even if one used the logic of the temporal paradox, one should still prefer dinosaurs as the ancestors to birds.Brochu, Christopher A. Norell, Mark A. (2001) "Time and trees: A quantitative assessment of temporal congruence in the bird origins debate" pp.511-535 in "New Perspectives on the Origin and Early Evolution of Birds" Gauthier&Gall, ed. Yale Peabody Museum. New Haven, Conn. USA.]

ee also

* Origin of birds

References