INTRODUCTION

The early evolutionary history of endobenthic tiering and infaunalization is still poorly understood. An increase in burrowing depth and intensity through the Phanerozoic has been suggested (e.g., Thayer 1979, 1983). Earlier studies, focused on onshore-offshore trends in infaunalization, were based on analysis of carbonate ichnofabrics in the Great Basin of the western United States (Droser and Bottjer 1988, 1989). In these carbonate systems, intense bioturbation developed first in nearshore environments and in offshore settings later (Droser and Bottjer 1989). Inner-shelf carbonates of the Great Basin reveal two major increases in the extent and depth of bioturbation during the early Paleozoic: the first between pre-trilobite and trilobite-bearing Cambrian rocks, and the second between the Middle and Late Ordovician (Droser and Bottjer 1989). Comparatively little is known about the timing and paleoenvironmental extent of infaunalization in siliciclastic settings, although a gradual increase throughout the Early Cambrian has been suggested (McIlroy and Logan 1999). More recently, a number of studies have proposed that sediments were firm near the surface during the Cambrian and Early Ordovician, allowing preservation of shallow-tier trace fossils with well-preserved scratch marks (Droser et al. 2002a, 2002b, 2004; Jensen et al. 2005). Also, an increase in ethologic variability and depth of bioturbation in offshore siliciclastics by the Middle Ordovician was noted by Mángano and Droser (2004). However, detailed integrated ichnofabric and facies analysis of this critical interval remains to be performed.

The lower Paleozoic Basin of Northwest Argentina (Figure 1) contains extensive outcrops of shallow-marine deposits that display stacked offshore-to-nearshore shallowing-upward cycles spanning the Late Cambrian to Caradocian interval (Buatois and Mángano 2003; Astini 2003; Astini et al. 2004; Buatois et al. 2006). This recurrent stratal pattern allows the ichnofabric of the same shallow-marine subenvironments to be compared through time, therefore permitting evolutionary controls to be identified. In addition, characterization of these Cambrian-Ordovician ichnofaunas provides information on intermediate to high-latitude shallow-marine infaunal communities, allowing comparisons with those from other paleogeographic regions. In this paper, we suggest that (1) a significant increase in the exploitation of infaunal ecospace occurred in upper-offshore environments between the early and late Tremadocian in Gondwana, and (2) infaunalization in high- to intermediate-latitude Gondwanic settings was delayed with respect to other paleocontinents, such as Baltica and Laurentia.