(7) Neoproterozoic Environment and Evolution of Animal Life
Complex animals first evolved during the Ediacaran period, between 635 and 542 million years ago, when the oceans were just becoming fully oxygenated. In situ fossils of the mobile forms of these animals are associated with microbial sedimentary structures, and the animal’s trace fossils generally were formed parallel to the surface of the seabed, at or below the sediment–water interface. This evidence suggests the earliest mobile animals inhabited settings with high microbial populations, and may have mined microbially bound sediments for food resources. In search of a modern analogue, we reported the association of mobile animals—insect larvae, oligochaetes and burrowing shore crabs—with complex microbial mats in a modern hypersaline lagoon in Venezuela (Gingras et al., 2011). The mats are characterised by an uppermost population of cyanobacteria overlying a mixed chemolithoautotrophic/phototrophic community of sulphide-oxidizing bacteria. Interestingly, the animals living within the mat community maintained no irrigation burrows to the sediment-water interface, thus surviving independently of the overlying lagoonal waters. We found, however, that during the day, O2 levels in the biomats were four times higher than in the overlying water column, while at night hydrogen sulphide reaches up to the sediment-water interface. The complete absence of perforations and burrows through the mat confirms that the animals hibernate beneath the mat throughout the high-sulphide evening cycle, a behaviour documented in hypoxia-tolerant insect larvae. This behaviour may have been similarly displayed by some of the horizontal burrows found in Ediacaran-aged rocks, suggesting that early mobile animals may have evolved in similar environments during the Ediacaran, effectively exploiting oases rich in O2 that formed within low oxygen settings.
(Left Figure). The microbial mats found in Los Roques, Venezuela. (Right Figures) Schematic showing behaviours associated with oxygen oases. (a) Modern animal–biomat associations from Los Roques. (b) Potential Upper Ediacaran and Lower Cambrian associations. Various lifestyles are illustrated: (i) free swimming animals that dive into zones of higher oxygen content; (ii) animals that reside in burrows isolated from the water column; iii) open, branching networks provide access to the sediment–water interface and permit sub-mat feeding and exploitation of O2 above and below the biomat; (iv) burrows that maximise their surface area below biomats stand to increase O2 flux through the burrow wall; and (v) bottom-wall hugging animals residing within the oxycline. From Gingras et al. (2011).
More recently, in a different study led by my former PhD student (Ernesto Pecoits, now a Professor at the Uruguay University of Technology), we reported on the discovery of the oldest evidence for bilaterian life in the form of burrows in shallow-water glaciomarine sediments from the Tacuarí Formation, Uruguay (Pecoits et al., 2012; Pecoits et al., 2013). Uranium-lead dating of zircons in cross-cutting granite dykes constrains the age of these burrows to be at least 585 million years old, 30 million years older than any bilaterians previously reported. Active backfill within the burrow, an ability to wander upward and downward to exploit shallowly situated sedimentary laminae, and sinuous meandering suggest that these bilaterians displayed advanced behavioural adaptations. Importantly, these findings unite the paleontological and molecular data pertaining to the evolution of bilaterians, and link bilaterian origins to the environmental changes that took place during the Neoproterozoic "Snowball Earth” glaciations.
(Left Figures) Trace-fossil locality showing contact relationships with the granite. The contact between the granite and the Tacuarí Fm is traced in red. (A) General view of the outcrop. Notice the local deformation phase (anticline) along the granite contact and the clear crosscutting (i.e., discordant) relationship on the right limb. (B) Detail of the right limb dipping 45-50° E and being intruded by the granite at 70-80° E (see location of picture in Fig. A). (C) Close-up view of Fig. B showing a sandstone layer and the trace-fossil bearing rhythmites (see Figs. E and F). The inset shows the discordance between the granite and sandstone which has undergone a strong ferruginisation (D) Close-up view of Fig. C showing deeply silicified rhythmites immediately overlying the ferruginised sandstone layer (see location of picture in Fig. C). (E) Location of trace fossils at site B. The trace fossils are located in the rhythmites approximately at 1 m from the contact with the granite (see location of picture in Fig. C). (F) At this site, the rhythmites are only slightly silicified and the trace fossils are well preserved. (a), (b), (c) and (d) correspond to the fossil bearing slabs shown in Fig. E. (Right Figure) Close-up photograph of the Tacuarí trace fossils showing the typical bilobate furrow with beaded backfill/meniscae visible (black arrows). From Pecoits et al. (2012).