Milner Andrew R C, Harris Jerald D, Lockley Martin G, Kirkland James I, Matthews Neffra A
St George Dinosaur Discovery Site at Johnson Farm, St George, Utah, United States of America.
PLoS One. 2009;4(3):e4591. doi: 10.1371/journal.pone.0004591. Epub 2009 Mar 4.
Fossil tracks made by non-avian theropod dinosaurs commonly reflect the habitual bipedal stance retained in living birds. Only rarely-captured behaviors, such as crouching, might create impressions made by the hands. Such tracks provide valuable information concerning the often poorly understood functional morphology of the early theropod forelimb.
METHODOLOGY/PRINCIPAL FINDINGS: Here we describe a well-preserved theropod trackway in a Lower Jurassic ( approximately 198 million-year-old) lacustrine beach sandstone in the Whitmore Point Member of the Moenave Formation in southwestern Utah. The trackway consists of prints of typical morphology, intermittent tail drags and, unusually, traces made by the animal resting on the substrate in a posture very similar to modern birds. The resting trace includes symmetrical pes impressions and well-defined impressions made by both hands, the tail, and the ischial callosity.
CONCLUSIONS/SIGNIFICANCE: The manus impressions corroborate that early theropods, like later birds, held their palms facing medially, in contrast to manus prints previously attributed to theropods that have forward-pointing digits. Both the symmetrical resting posture and the medially-facing palms therefore evolved by the Early Jurassic, much earlier in the theropod lineage than previously recognized, and may characterize all theropods.
非鸟类兽脚亚目恐龙留下的化石足迹通常反映了现存鸟类所保留的习惯性双足站立姿势。只有很少被捕捉到的行为，比如蹲伏，可能会留下手部造成的印记。这些足迹提供了有关早期兽脚亚目前肢功能形态的宝贵信息，而这些形态往往鲜为人知。
方法/主要发现：在此，我们描述了犹他州西南部莫纳维组惠特莫尔角段下侏罗统（约1.98亿年前）湖相海滩砂岩中一条保存完好的兽脚亚目恐龙足迹化石。该足迹化石包括典型形态的脚印、间歇性的尾部拖痕，以及不同寻常的、动物以与现代鸟类非常相似的姿势趴在基质上留下的痕迹。这些休息痕迹包括对称的足部印记以及由双手、尾巴和坐骨胼胝体留下的清晰印记。
结论/意义：手部印记证实，早期兽脚亚目恐龙和后来的鸟类一样，手掌朝内，这与之前归为兽脚亚目恐龙的脚印中脚趾向前的情况不同。因此，对称的休息姿势和手掌朝内的特征在侏罗纪早期就已演化形成，在兽脚亚目恐龙谱系中比之前认为的要早得多，并且可能是所有兽脚亚目恐龙的特征。
