It seems like forever since I posted about the very old putative bilaterian burrows Pecoits et al. (2012) reported in Science. I read the paper, thought about the implications, wrote the post and then filed the whole thing away in the giant messy cabinet at the back of my mind.
But a big claim like the one Pecoits et al. made – burrows from bilateran animals that appear before the first Ediacaran fossils! – is unlikely to go unchallenged by the scientific community. Now the argument has broken out. Gaucher et al. (2013) wrote a comment in Science criticising the reasoning that put such an old date on the formation where the burrows were found. Pecoits et al. (2013) responded. The plot is thickening!
The main bone of contention seems to be whether the huge body of granite that gave the actual radiometric date of 585 million years lies below the burrow-bearing formation (in which case it must be older than the fossils) or cuts through it (in which case it’s younger). The other question is whether the fossils and the rocks they’re found in actually belong to another nearby formation that is thought to be Permian in age. Burrows in Permian rocks would be no surprise at all . By that time reptiles and the ancestors of mammals walked the earth, insects of all kinds flew over it, and armadas of worms had been boring through soft sediments for hundreds of millions of years. Burrows that far into the Precambrian, on the other hand…
The argument is all very geological, and as I repeatedly said, I’m not much of a geologist. Looking at the figures wouldn’t help me decide who to believe at all. I’m rather amused by some of the snark that gets into the text, though. I have this feeling that Pecoits et al. are annoyed. Watch this, for example:
In this case, Gaucher et al. (1) take no notice of the outcrop-scale relationships and instead prefer to show five photographs from just one hand sample that they assigned to fossil site C to discredit the intrusive nature of the granite [figure 1, B to F, in (1)]. We do not want to speculate on the origin of this sample, but we see no evidence that it comes from fossil site C; it is not the ferruginized basal sandstone we previously documented [figure S3C in (2)].
Oh, yeah. “We do not want to speculate,” but we think something’s fishy with your evidence, only we’re too polite to say it in so many words!
Tee-hee. Academia’s version of an online flame war.
Gaucher C et al. (2013) Comment on “Bilaterian burrows and grazing behavior at >585 million years ago”. Science 339:906
Pecoits E et al. (2012) Bilaterian burrows and grazing behavior at >585 million years ago. Science 336:1693-1696
Pecoits E et al. (2013) Response to comment on “Bilaterian burrows and grazing behavior at >585 million years ago”. Science 339:906