Paleontology and ichnology of the late Ediacaran Nasep–Huns transition (Nama Group, southern Namibia)

Abstract

The Nasep and Huns members of the Urusis Formation (Nama Group), southern Namibia, preserve some of the most diverse trace-fossil assemblages known from the latest Ediacaran worldwide, including potentially the world's oldest “complex” vertical sediment-penetrating burrows. These sediments record relatively diverse communities of bilaterian metazoans existing before the base of the Cambrian and an increase in the intensity of metazoan ecosystem engineering behaviors that could eventually produce profound changes in the character of the Phanerozoic sedimentary record (the “agronomic revolution”). Despite this, relatively little about this trace-fossil assemblage is known. We explore the Nasep–Huns transition at two localities in the Witputs sub-basin and describe the trace- and body-fossil diversity present in these horizons alongside a paleoenvironmental reconstruction. We document eight unique ichnotaxa from these localities, including well-preserved “probes” potentially left by priapulids. We also report the first occurrence of Corumbella from Namibia, helping to establish a biostratigraphic link between Namibia, Brazil, Paraguay, Iran, and the southwestern United States. Last, we find that several ichnotaxa, in particular small treptichnids, appear to be preferentially preserved on the bases of gutter casts, hinting at the potential existence of an unusual late Ediacaran preservational window with possible implications for timing the first appearance of key bilaterian behaviors.

Decline and fall of the Ediacarans: late‐Neoproterozoic extinctions and the rise of the modern biosphere

“…In support of models suggesting self‐amplifying bioturbation depth via sediment mixing or irrigation (Herringshaw, Callow & McIlroy, 2017; Budd & Jensen, 2017), Ediacaran burrowing reached its acme in the uppermost Nama group in the form of treptichnids (Fig. 3B; Darroch et al ., 2021; Turk et al ., 2022). These arcuate burrows radiate vertical probes reminiscent of Cambrian and Recent priapulids (Jensen & Runnegar, 2005; Vannier et al ., 2010), embodying a new, matground‐destabilising bioturbation mode qualitatively distinct from any associated with White Sea or Shibantan bilaterians.…”

“…These arcuate burrows radiate vertical probes reminiscent of Cambrian and Recent priapulids (Jensen & Runnegar, 2005; Vannier et al ., 2010), embodying a new, matground‐destabilising bioturbation mode qualitatively distinct from any associated with White Sea or Shibantan bilaterians. This Namibian bioturbation surge matches the ichnofossil signal from North America (Carbone & Narbonne, 2014; Tarhan et al ., 2020), South America (Parry et al ., 2017), Europe (Högström et al ., 2013), and China (Weber, Steiner & Zhu, 2007), and in light of its global spread and consistency it deserves serious scrutiny as an extinction driver (Darroch et al ., 2018, 2021; Turk et al ., 2022).…”

“…Given the taxonomic range and relative abundance of scavenged Ediacarans, the coincident lack of evidence for predation on them might reflect a genuine biotic signal. However such evidence, including hypothetical indirect signatures of predation in the form of damage repair (Kenchington, Dunn & Wilby, 2018), is absent even in the latest Nama interval, despite the appearance of priapulid‐grade burrows suggesting the existence of infaunal predators (Vannier et al ., 2010; Darroch et al ., 2021; Turk et al ., 2022). Phanerozoic priapulids, including Cambrian taxa, are known to feed on detritus or small, epibenthic animals such as molluscs, annelids, brachiopods (Vannier et al ., 2010) and possibly cnidarians (Han et al ., 2007).…”

“…Alongside a bioturbation surge and the appearance of possible priapulomorph burrowers, the Nama interval witnessed a global spread of biomineralised taxa (Fig. 3D–I; Penny et al ., 2014; Warren et al ., 2017; Cai et al ., 2019; Selly et al ., 2020; Turk et al ., 2022). Taxonomically overlapping faunas comprising rangeomorphs, erniettomorphs, and skeletal metazoans occur in Namibia (Darroch et al ., 2016), Nevada (Selly et al ., 2020), South America (Becker‐Kerber et al ., 2017), Iran (Vaziri et al ., 2018), and China (Xiao et al ., 2021; Chen et al ., 2022).…”

“…Taken together, the taxonomic, ecological, and macroevolutionary continuities among Ediacaran assemblages and the Cambrian world appear increasingly robust with enhanced sampling and novel analytical techniques (e.g. Eden et al ., 2022; Bowyer et al ., 2022; Hoyal Cuthill, 2022; Turk et al ., 2022). This does not mean that the possible roles of extrinsic catastrophic events such as flood basalt volcanism (Hodgin et al ., 2021) in sharpening or accelerating late‐Ediacaran turnover should be dismissed altogether.…”