科学家在美国南犹他州大阶梯――埃斯卡兰特国家历史文物保护区（Grand Staircase-Escalante National Monument）三叠纪地层中发现了波波龙化石，通过研究表明：这种鳄鱼的早期祖先是用两条腿直立行走的。

图为波波龙复原图

波波龙（属名：Poposaurus）是距今2亿多年前三叠纪（triassic period）晚期生活在美国地区的一类早期爬行动物，属于劳氏鳄（rauisuchian，一种史前爬行类动物，最终演化成鳄鱼）中的第三类。

化石的时代可追溯到距今2.25亿年前。骨骼从颈部到尾部都完整保存下来，根据其独特的骨骼结构——四肢骨（并不是类似今天的鳄鱼一样位于身体的两侧），科学家推测该种波波龙应该为直立行走。

波波龙模式种为纤细波波龙（P. gracilis），全长约14英尺（约4.27米），由古生物学家M. G. Mehl在1915年描述。

而此次美国纽约自然历史博物馆（American Museum of Natural History in New York City）的S. Nesbitt和耶鲁大学（Yale University）古生物学家J. Gauthier对这具发现于2006年的波波龙化石解剖学特征进行了新的厘定。

经过研究，研究人员把劳氏鳄划分到奥氏灵鳄（Effigia okeeffeae）中，是少数表明鳄鱼祖先与恐龙发生相对独立演化的证据之一，同时也表明双足行动的方法可以使它们在捕猎中获益，使其能在激烈的生存竞争中存活下来。

推荐原文：

Earth-Science Reviews  doi:10.1016/j.earscirev.2010.04.001

The origin and early radiation of dinosaurs

Stephen L. Brusattea, Sterling J. Nesbitta, Randall B. Irmisd, Richard J. Butlere, Michael J. Bentonf and Mark A. Norell

Dinosaurs were remarkably successful during the Mesozoic and one subgroup, birds, remain an important component of modern ecosystems. Although the extinction of non-avian dinosaurs at the end of the Cretaceous has been the subject of intense debate, comparatively little attention has been given to the origin and early evolution of dinosaurs during the Late Triassic and Early Jurassic, one of the most important evolutionary radiations in earth history. Our understanding of this keystone event has dramatically changed over the past 25 years, thanks to an influx of new fossil discoveries, reinterpretations of long-ignored specimens, and quantitative macroevolutionary analyses that synthesize anatomical and geological data. Here we provide an overview of the first 50 million years of dinosaur history, with a focus on the large-scale patterns that characterize the ascent of dinosaurs from a small, almost marginal group of reptiles in the Late Triassic to the preeminent terrestrial vertebrates of the Jurassic and Cretaceous. We provide both a biological and geological background for early dinosaur history. Dinosaurs are deeply nested among the archosaurian reptiles, diagnosed by only a small number of characters, and are subdivided into a number of major lineages. The first unequivocal dinosaurs are known from the late Carnian of South America, but the presence of their sister group in the Middle Triassic implies that dinosaurs possibly originated much earlier. The three major dinosaur lineages, theropods, sauropodomorphs, and ornithischians, are all known from the Triassic, when continents were joined into the supercontinent Pangaea and global climates were hot and arid. Although many researchers have long suggested that dinosaurs outcompeted other reptile groups during the Triassic, we argue that the ascent of dinosaurs was more of a matter of contingency and opportunism. Dinosaurs were overshadowed in most Late Triassic ecosystems by crocodile-line archosaurs and showed no signs of outcompeting their rivals. Instead, the rise of dinosaurs was a two-stage process, as dinosaurs expanded in taxonomic diversity, morphological disparity, and absolute faunal abundance only after the extinction of most crocodile-line reptiles and other groups.