Damiani Ross, Modesto Sean, Yates Adam, Neveling Johann
Bernard Price Institute for Palaeontological Research, University of the Witwatersrand, Wits 2050, Johannesburg, South Africa.
Proc Biol Sci. 2003 Aug 22;270(1525):1747-51. doi: 10.1098/rspb.2003.2427.
A 251 million year old partial burrow cast containing an articulated skeleton of the mammal-like carnivore Thrinaxodon liorhinus is the oldest evidence for burrowing by a cynodont synapsid. The burrow cast comes from terrestrial flood plain sediments close to the Permian-Triassic boundary in the Karoo of South Africa. Together with those of the later cynodont Trirachodon, the Thrinaxodon burrow cast indicates that burrow-making was broadly distributed in basal synapsids and has a long history in non-mammalian synapsid evolution. A reconstruction of its appendicular skeleton in cross-section indicates that Thrinaxodon was able to adopt a facultatively mammalian stance within its burrow shaft. Burrows of cynodont design are more common in Triassic rocks than previously realized, and suggest that burrowing may represent an adaptive response by cynodonts to the environmental conditions associated with the mass extinction event that punctuated the end of the preceding Permian period. The widespread occurrence of burrowing among extant mammals implies that the ancient synapsid ability to burrow conferred a strong adaptive value in the evolution of Mammalia.
一个有2.51亿年历史的部分洞穴铸型，其中包含似哺乳类肉食动物三尖叉齿兽（Thrinaxodon liorhinus）的一具连接完整的骨骼，这是犬齿兽类合弓纲动物挖掘行为的最古老证据。该洞穴铸型来自南非卡鲁地区靠近二叠纪-三叠纪边界的陆地洪泛平原沉积物。与后来的犬齿兽类三瘤齿兽（Trirachodon）的洞穴铸型一起，三尖叉齿兽的洞穴铸型表明挖掘行为在基干合弓纲动物中广泛存在，并且在非哺乳类合弓纲动物的进化中有着悠久的历史。对其附肢骨骼横截面的重建表明，三尖叉齿兽能够在其洞穴通道内采取一种兼性的哺乳动物姿态。犬齿兽类设计的洞穴在三叠纪岩石中比以前认识到的更为常见，这表明挖掘行为可能代表了犬齿兽类对与二叠纪末期大灭绝事件相关的环境条件的一种适应性反应。现存哺乳动物中广泛存在挖掘行为意味着古代合弓纲动物的挖掘能力在哺乳动物的进化中赋予了强大的适应性价值。
