地球上最古老的动物或许可以追溯到距今5.85亿年前，比原先认为的5.55亿年前要早3000万年。
来自加拿大艾伯塔大学等机构的研究人员在新一期《科学》杂志上报告说，他们在南美国家乌拉圭发现了有动物爬行痕迹的化石，经年代测定，留下痕迹的动物应生活在距今5.85亿年前。
这种动物在堆积物上留下了宽2毫米至3毫米的爬行痕迹。研究人员据此推断这种动物是一种类似蛞蝓的动物，拥有左右对称的身体，体长1厘米左右。
细菌等原核生物在地球上存在的时间超过30亿年，而此前科学家找到的证据显示，最古老动物生活在距今5.55亿年前。（生物谷Bioon.com)
doi:10.1126/science.1216295
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Bilaterian Burrows and Grazing Behavior at >585 Million Years Ago
Ernesto Pecoits1，*， Kurt O. Konhauser1， Natalie R. Aubet1，2， Larry M. Heaman1， Gerardo Veroslavsky2， Richard A. Stern1， Murray K. Gingras1
Based on molecular clocks and biomarker studies， it is possible that bilaterian life emerged early in the Ediacaran， but atpresent， no fossils or trace fossils from this time have been reported. Here we report the discovery of the oldest bilaterianburrows in shallow-water glaciomarine sediments from the Tacuarí Formation， Uruguay. Uranium-lead dating of zircons in cross-cuttinggranite dykes constrains the age of these burrows to be at least 585 million years old. Their features indicate infaunal grazingactivity by early eumetazoans. Active backfill within the burrow， an ability to wander upward and downward to exploit shallowlysituated sedimentary laminae， and sinuous meandering suggest advanced behavioral adaptations. These findings unite the paleontologicaland molecular data pertaining to the evolution of bilaterians， and link bilaterian origins to the environmental changes thattook place during the Neoproterozoic glaciations.
