英国研究人员报告最新成果说，在约2．5亿年前一次生物大灭绝事件以前，所有爬行动物都是腿在身体两侧“趴”着行走，而在此之后，大中型爬行动物大都“站”了起来。
英国布里斯托尔大学15日发布新闻公报称，该校研究人员和日本同行分析了大灭绝事件前后的460件爬行动物足迹化石。结果发现大灭绝事件之前所有爬行动物的腿脚都是长在身体两侧，而之后大中型爬行动物的腿脚几乎都长到身体正下方的位置，从“趴”进化到“直立”。
研究人员说，“直立”与“趴”相比有非常大的生理优势。直立行走时膝关节等处所受的压力较小，且步幅较大。更关键的是，腿脚在身体正下方，就可以像柱子一样支撑起更重的身体，如恐龙和大象。
此前人们认为生物从“趴”到“直立”的进化经历了很长时间，但这次研究显示大灭绝事件可能促使动物在短时间内迅速完成了这种进化过程，后来出现的许多爬行动物都直立行走，如此后不久出现的恐龙。
有研究称，在约2．5亿年前地球上发生了最大一次生物灭绝事件，地球上超过90％的物种消失。一般认为这次大灭绝是环境突变造成的，但具体原因尚不明确。
相关研究报告发表在《古生物学》杂志上。（生物谷Bioon.com）
生物谷推荐原始出处：
Palaeontology Volume 52 Issue 5, Pages 1029 - 1037
Tetrapod postural shift estimated from Permian and Triassic trackways
by TAI KUBO* and MICHAEL J. BENTON
*Department of Earth and Planetary Science, University of Tokyo, 7-3-1, Hongo, Bunkyouku, Tokyo 113-0033, Japan
Abstract: The end-Permian mass extinction, 252 million years (myr) ago, marks a major shift in the posture of tetrapods. Before the mass extinction, terrestrial tetrapods were sprawlers, walking with their limbs extended to the sides; after the event, most large tetrapods had adopted an erect posture with their limbs tucked under the body. This shift had been suspected from the study of skeletal fossils, but had been documented as a long process that occupied some 15–20 myr of the Triassic. This study reads posture directly from fossil tracks, using a clear criterion for sprawling vs erect posture. The track record is richer than the skeletal record, especially for the Early and Middle Triassic intervals, the critical 20 myr during which period the postural shift occurred. The shift to erect posture was completed within the 6 myr of the Early Triassic and affected both lineages of medium to large tetrapods of the time, the diapsids and synapsids.
