Deep-Water Incised Valley Deposits at the Ediacaran-Cambrian Boundary in Southern Namibia Contain Abundant Treptichnus Pedum

Abstract

Valley-filling deposits of the Nama Group, southern Namibia, record two episodes of erosional downcutting and backfill, developed close together in time near the Ediacaran-Cambrian boundary. Geochronological constraints indicate that the older valley fill began 539.4 ± 1 Ma or later; the younger of these deposits contains unusually well-preserved populations of the basal Cambrian trace fossil Treptichnus pedum. Facies analysis shows that T. pedum is closely linked to a nearshore sandstone deposit, indicating a close environmental or taphonomic connection to very shallow, mud-draped sandy seafloor swept by tidal currents. Facies restriction may limit the biostratigraphic potential of T. pedum in Namibia and elsewhere, but it also illuminates functional and ecological interpretation. The T. pedum tracemaker was a motile bilaterian animal that lived below the sediment-water interface-propelling itself forward in upward-curving projections that breached the sediment surface. The T. pedum animal, therefore, lived infaunally, perhaps to avoid predation, surfacing regularly to feed and take in oxygen. Alternatively, the T. pedum animal may have been a deposit feeder that surfaced largely for purposes of gas exchange, an interpretation that has some support in the observed association of T. pedum with mud drapes. Treptichnus pedum provides our oldest record of animals that combined anatomical and behavioral complexity. Insights from comparative biology suggest that basal Cambrian T. pedum animals already possessed the anatomical, neurological, and genetic complexity needed to enable the body plan and behavioral diversification recorded by younger Cambrian fossils.

Dynamic redox conditions control late Ediacaran metazoan ecosystems in the Nama Group, Namibia

“…A c c e p t e d M a n u s c r i p t 10 1995). In the Witputs Sub-Basin the Ediacaran-Cambrian (E-C) boundary is marked by a regionally extensive erosional unconformity (Germs, 1983;Saylor and Grotzinger, 1996;Narbonne et al, 1997;Grotzinger et al, 1995) overlain by an incised-valley fill sequence that contains the earliest Cambrian trace fossil Treptichnus pedum (Wilson et al, 2012).…”

“…However, ash beds in the overlying units (Figure 2) provide robust U-Pb zircon age constraints ranging from 548.8 ± 1 Ma in the Hoogland Member of the Kuibis Subgroup revised to 547.32 ± 0.31 Ma by Schmitz et al 2012) to 538.18 ± 1.24 Ma in the upper part of the Schwarzrand Subgroup (Grotzinger et al, M a n u s c r i p t 10 1995). In the Witputs Sub-Basin the Ediacaran-Cambrian (E-C) boundary is marked by a regionally extensive erosional unconformity (Germs, 1983;Saylor and Grotzinger, 1996;Narbonne et al, 1997;Grotzinger et al, 1995) overlain by an incised-valley fill sequence that contains the earliest Cambrian trace fossil Treptichnus pedum (Wilson et al, 2012). Therefore, the Nama Group section spans ~10 Myr and extends to beyond the ProterozoicCambrian transition (Saylor et al, 1998;Ries et al, 2010).…”

“…A c c e p t e d M a n u s c r i p t 10 1995). In the Witputs Sub-Basin the Ediacaran-Cambrian (E-C) boundary is marked by a regionally extensive erosional unconformity (Germs, 1983;Saylor and Grotzinger, 1996;Narbonne et al, 1997;Grotzinger et al, 1995) overlain by an incised-valley fill sequence that contains the earliest Cambrian trace fossil Treptichnus pedum (Wilson et al, 2012).Therefore, the Nama Group section spans ~10 Myr and extends to beyond the ProterozoicCambrian transition (Saylor et al, 1998; Ries et al, 2010).Each Sub-Basin has its own distinct stratigraphic framework, although key sequence boundaries and systems tracts can be identified in both, with Transgressive Systems Tracts (TSTs) generally being dominated by siliciclastic rocks whereas various carbonate facies distinguish Highstand Systems Tracts (HSTs; Saylor et al, 1995Saylor et al, , 1998; Figure 2). In general, the Kuibis Subgroup thins towards and completely disappears over the Osis Ridge (Germs, 1983(Germs, , 1995.…”

Biotic replacement and mass extinction of the Ediacara biota

“…An ash bed in the lower carbonate package of the Urusis Formation has been dated by U-Pb geochronology at 545.1 + 1 Ma, and an ash bed approximately 85 m below the investigated fossil beds at 543.3 + 1 Ma ([22]-see figure 1; recalculated to 540.61 + 0.67 Ma in [23]). An erosive unconformity overlain by complex valley-filling deposits of the earliest Cambrian Nomtsas Formation cuts down through the Ediacaran strata, although the physical unconformity itself is not well exposed on Swartpunt Farm [18,22,24]. Nomtsas strata in the Swartkloofberg Farm directly north of Swartpunt contain an ash bed dated to 539.4 + 1 Ma ( [22]; recalculated to 538.18 + 1.11 Ma in [23]).…”

On the ichnofossil Treptichnus pedum: inferences from the Nagaur Sandstone, Marwar Supergroup, India

“…These projections signify the behavioural movement of the animal. Wilson et al (2012) suggested the functional biology of priapulids in two ways: a) the animal might have lived infaunally to avoid predation or desiccation, and appeared on the surface episodically to feed and receive oxygen; and b) the animal might have been a deposit feeder that surfaced regularly to exchange gases and perhaps to disperse eggs, sperm or fertilized eggs. The latter appears more plausible because if the priapulid were able to find food on the surface then there is no valid reason for them to build three-dimensional burrows, although the safety from other predators could be the reason in that case the burrow system reflects the escape mechanism from predation.…”