科學家在美國南猶他州大階梯――埃斯卡蘭特國家歷史文物保護區（Grand Staircase-Escalante National Monument）三疊紀地層中發現了波波龍化石，通過研究表明：這種鱷魚的早期祖先是用兩條腿直立行走的。
圖為波波龍復原圖
波波龍（屬名：Poposaurus）是距今2億多年前三疊紀（triassic period）晚期生活在美國地區的一類早期爬行動物，屬於勞氏鱷（rauisuchian，一種史前爬行類動物，最終演化成鱷魚）中的第三類。
化石的時代可追溯到距今2.25億年前。骨骼從頸部到尾部都完整保存下來，根據其獨特的骨骼結構——四肢骨（並不是類似今天的鱷魚一樣位於身體的兩側），科學家推測該種波波龍應該為直立行走。
波波龍模式種為纖細波波龍（P. gracilis），全長約14英尺（約4.27米），由古生物學家M. G. Mehl在1915年描述。
而此次美國紐約自然歷史博物館（American Museum of Natural History in New York City）的S. Nesbitt和耶魯大學（Yale University）古生物學家J. Gauthier對這具發現於2006年的波波龍化石解剖學特徵進行了新的釐定。
經過研究，研究人員把勞氏鱷劃分到奧氏靈鱷（Effigia okeeffeae）中，是少數表明鱷魚祖先與恐龍發生相對獨立演化的證據之一，同時也表明雙足行動的方法可以使它們在捕獵中獲益，使其能在激烈的生存競爭中存活下來。（生物谷Bioon.com）
生物谷推薦原文：
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.