The earliest fossil record of the animals and its significance

Abstract

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.

The Weng'an Biota (Doushantuo Formation): an Ediacaran window on soft-bodied and multicellular microorganisms

“…The age of the biota has been debated (Budd 2008;Erwin & Valentine 2013;Xiao et al 2014a;Yin et al 2015), with arguments focusing on the correlation of two karstic surfaces, one at the top of Unit 3 and the other within Unit 5 (for detailed discussion of Doushantuo correlation see Zhu et al (2007), Zhu et al (2013) and Yang et al (2015)). If the lower surface is correlated to the c. 582 Ma Gaskiers glaciation (Condon et al 2005) then the biota would be younger than 582 Ma.…”

Molecular clocks and the early evolution of metazoan nervous systems

“…Although what nineteenth century geologists referred to as the Cambrian era has since undergone considerable revision, one important fact has not changed: rocks older than the Cambrian contain very few bilaterian fossils [6,7]. Darwin was famously aware of this seemingly sudden origin of diverse animals in the fossil record and considered it among the most serious objections to his theory of natural selection [8].…”

“…Third, early bilaterians were probably small, perhaps with body sizes on the scale of one to a several millimetres. Small body size seems likely based on low available oxygen [99] and the paucity of trace fossils that are unambiguously attributable to bilaterians prior to the Cambrian [6].…”

“…The absence of trace fossils that can unambiguously be attributed to bilaterians prior to the Cambrian means that the negative evidence needs to be taken seriously [6]. This argument rests, however, on the assumption that early animals were large-bodied and would have burrowed into or ploughed across the sea floor.…”

Decoupling of body-plan diversification and ecological structuring during the Ediacaran–Cambrian transition: evolutionary and geobiological feedbacks

“…This debate has mostly revolved around two different datasets: the bodyfossil record and molecular clocks. Although the body-fossil record supports the notion that the Cambrian event of diversification is real and unparalleled in the history of life [9,10], interpretation of molecular clock evidence has been quite controversial. Earlier studies did not support the explosion scenario, suggesting deeper divergence among animals [11,12].…”