Budd Graham E
Department of Earth Sciences, Palaeobiology, Uppsala University, Villavägen 16, Uppsala 752 36, Sweden.
Philos Trans R Soc Lond B Biol Sci. 2008 Apr 27;363(1496):1425-34. doi: 10.1098/rstb.2007.2232.
The fossil record of the earliest animals has been enlivened in recent years by a series of spectacular discoveries, including embryos, from the Ediacaran to the Cambrian, but many issues, not least of dating and interpretation, remain controversial. In particular, aspects of taphonomy of the earliest fossils require careful consideration before pronouncements about their affinities. Nevertheless, a reasonable case can now be made for the extension of the fossil record of at least basal animals (sponges and perhaps cnidarians) to a period of time significantly before the beginning of the Cambrian. The Cambrian explosion itself still seems to represent the arrival of the bilaterians, and many new fossils in recent years have added significant data on the origin of the three major bilaterian clades. Why animals appear so late in the fossil record is still unclear, but the recent trend to embrace rising oxygen levels as being the proximate cause remains unproven and may even involve a degree of circularity.
近年来，一系列惊人的发现，包括从埃迪卡拉纪到寒武纪的胚胎化石，让最早动物的化石记录变得鲜活起来。但许多问题，尤其是年代测定和解读方面，仍存在争议。特别是，在对最早化石的亲缘关系做出论断之前，需要仔细考虑其埋藏学的各个方面。然而，现在有理由认为，至少基础动物（海绵动物以及可能的刺胞动物）的化石记录可以追溯到寒武纪开始之前的一段相当长的时间。寒武纪大爆发本身似乎仍然代表着两侧对称动物的出现，近年来许多新化石为三大两侧对称动物类群的起源增添了重要数据。动物为何在化石记录中出现得如此之晚仍不清楚，但最近将氧气水平上升视为直接原因的趋势尚未得到证实，甚至可能存在一定程度的循环论证。
