封面文章：恐龙缓慢占据地球生物主导地位
新研究提出，恐龙成为占主导地位动物是一个渐进的过程，而不是突然发生的。恐龙在距今大约2亿到1.5亿年前的侏罗纪是最常见的陆地动物，但是到达这个阶段之前的恐龙化石记录相对稀少，使确定恐龙是如何成功的变得困难。有些研究人员曾提出，真恐龙相对快速地取代了在它们之前的主要动物—— 一种名为dinosauromorph的类似恐龙的动物。根据这种“碰巧好运”情景，恐龙或是在资源竞争中战胜了它们的亲戚，或是在某种灭绝事件减少了dinosauromorph的数量之后进入其生态位。Randall Irmis和同事描述了一批来自美国新墨西哥州的化石，至少在侏罗纪之前的三叠纪期间，对碰巧好运的情景提出了挑战，他们提出恐龙与dinosauromorphs在三叠纪末期曾共存了1500万到2000万年。这批化石中有各种恐龙，包括食肉的theropods(霸王龙属于这类恐龙)、早期dinosauromorphs(包括一种新确定的名为Dromomeron romeri的物种)以及其他鱼类、两栖类和鳄鱼的近亲。
原始出处：
Science 20 July 2007:
Vol. 317. no. 5836, pp. 358 - 361
DOI: 10.1126/science.1143325
Randall B. Irmis,1* Sterling J. Nesbitt,2,3* Kevin Padian,1 Nathan D. Smith,4,5 Alan H. Turner,3 Daniel Woody,6 Alex Downs7
It has generally been thought that the first dinosaurs quickly replaced more archaic Late Triassic faunas, either by outcompeting them or when the more archaic faunas suddenly became extinct. Fossils from the Hayden Quarry, in the Upper Triassic Chinle Formation of New Mexico, and an analysis of other regional Upper Triassic assemblages instead imply that the transition was gradual. Some dinosaur relatives preserved in this Chinle assemblage belong to groups previously known only from the Middle and lowermost Upper Triassic outside North America. Thus, the transition may have extended for 15 to 20 million years and was probably diachronous at different paleolatitudes.
1 Museum of Paleontology and Department of Integrative Biology, University of California, Berkeley, CA 94720-4780, USA.
2 Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964, USA.
3 Division of Paleontology, American Museum of Natural History, New York, NY 10024, USA.
4 Committee on Evolutionary Biology, University of Chicago, Chicago, IL 60637, USA.
5 Geology Department, Field Museum of Natural History, Chicago, IL 60605, USA.
6 Department of Geological Sciences, University of Colorado, Boulder, CO 80309-0399, USA.
7 Ruth Hall Museum of Paleontology, Ghost Ranch Conference Center, Abiquiu, NM 87510-9601, USA.
* To whom correspondence should be addressed. E-mail: irmis@berkeley.edu (R.B.I.); snesbitt@ldeo.columbia.edu (S.J.N.)
