Comment on "Bilaterian Burrows and Grazing Behavior at >585 Million Years Ago"

Abstract

Pecoits et al . (Reports, 29 June 2012, p. 1693) describe bilaterian trace fossils and assign them an Ediacaran age based on the age of a granite interpreted as intrusive. We argue that the granite is not intrusive but in fact represents the basement of the sedimentary succession. Moreover, we show that identical trace fossils occur in nearby Carboniferous-Permian glacigenic rocks.

Response to Comment on "Bilaterian Burrows and Grazing Behavior at >585 Million Years Ago"

“…This is one of the many examples we have previously illustrated (2) where our conclusions support that the igneous intrusion (our 585-million-year-old granite) is younger than the sedimentary rock (the 600-million-year-old trace-bearing strata). None of our ages have been questioned in Gaucher et al (1).…”

“…This is not true, because not only have we extensively described and illustrated the presence of xenoliths [ figure S9 in (2)], but we provided further evidence supporting an intrusive relationship between the granite and the Tacuarí Formation, namely: (i) pushed-up and folded country rocks; (ii) granite chilled margins at the contact; (iii) baked contacts in adjacent sedimentary rocks that show bleaching, silicification, hematitization, and occasional quartz-bearing cavities; (iv) partial assimilation of country rocks; (v) discordant contacts that truncate sedimentary layering and cleavage; and (vi) satellite dykes that are rooted in the main pluton and penetrate the host rock. All of these features were illustrated in figures S3 to S9 (2), yet they are disregarded by Gaucher et al (1). They then go on to state that the interpretation of the granite as dykes is not supported by their observations (1).…”

“…G aucher et al (1) call into question the stratigraphic relationship between the granite and the trace fossil-bearing rhythmites described by Pecoits et al (2). Gaucher et al (1) suggest that the granite is not intrusive but rather the basement of the sedimentary succession.…”

“…Gaucher et al (1) suggest that the granite is not intrusive but rather the basement of the sedimentary succession. They base their conclusions on the following arguments: (i) the illustrations provided by Pecoits et al (2) do not demonstrate an intrusive relationship through the sedimentary strata; (ii) the rhythmites do not show recrystallization or metamorphism as a consequence of the intrusion; (iii) the cleavage described is localized and brittle in nature and was not produced by the Sierra Ballena Shear Zone [active until 551 to 537 million years ago (Ma)], and hence, the Tacuarí Formation must postdate shearing; and (iv) outcrops corresponding to the Carboniferous/Permian San Gregorio Formation show identical trace fossils indistinguishable from those of the Tacuarí Formation.…”

“…This is not a trivial aspect because the San Gregorio Formation is characterized by its diverse and ubiquitous fossiliferous content (6)(7)(8)(9)(10)(11), whereas the Tacuarí Formation is not (Table 1). Avoiding this incongruence, Gaucher et al (1) suggest that freshwater and glacial conditions were responsible for the lack of fossils in the Tacuarí Formation. Surprisingly, the Tacuarí Formation does contain fossils-the same fossils (acritarchs) that Gaucher and collaborators have previously described in other Neoproterozoic successions from Uruguay, Argentina, Brazil, South Africa, and Namibia (12)(13)(14)(15).…”