CONCLUSIONS

1. Ichnofabric analysis of upper-offshore deposits in northwest Argentina shows a sharp contrast between Furongian to lower Tremadocian and upper Tremadocian to Caradocian units.

2. Furongian to lower Tremadocian upper-offshore deposits are dominated by very shallow-tier structures produced by trilobites and shallow-tier worm structures (e.g., Arthrophycus minimus, Palaeophycus tubularis, Phycodes isp.), with little disturbance of the primary fabric (BI = 0-1).

3. Upper Tremadocian-Caradocian upper-offshore deposits show a remarkable increase in degree of bioturbation (BI = 2-4, locally 5), and are characterized by the mid-tier Trichophycus venosus cross-cutting the more shallowly emplaced trilobite and worm trace fossils.

4. The early to late Tremadocian transition seems to have been a turning point for exploitation of the infaunal ecospace in high- to intermediate-latitude siliciclastic basins of Gondwana.

5. Similar ichnofabrics, however, appear to have been present since the Early Cambrian in other paleocontinents, such as Baltica and Laurentia, suggesting that infaunalization and the development of a mid-tier benthos in these regions were a consequence of the Cambrian explosion. The delayed appearance of Trichophycus in Gondwana may reflect a subsequent dispersal and migration, or alternatively, behavioral convergence by different producers during the Ordovician diversification event.