1 Conicet–Insugeo, Universidad Nacional de Tucumán. Casilla de Correo 1, 4000 San Miguel de Tucumán, Argentina. E–mail: ichnolog@infovia.com.ar

2 Department of Earth Sciences, University of California, Riverside, CA 92521, USA.

Key words: Ichnology. Paleoenvironments. Fossil record. Ecosystems. Ordovician radiation.

Knowledge of the Ordovician radiation comes essentially from the body fossil record (Sepkoski, 1995; Sheehan, 2001). However, the study of animal–substrate interactions based on the analysis of biogenic structures contributes significantly to an understanding of paleoecological breakthroughs associated with the Ordovician radiation (Mángano and Droser, 2004). Paleoecological levels allow for the ranking of ecological changes through the Phanerozoic (Droser et al., 1997). First level change, the highest category, indicates colonization of a new ecosystem and fourth level change, at the lowest category, indicates turnover at the community level. While body–fossil evidence reveals second– to fourth–level changes associated with the Ordovician biodiversification event, trace–fossil data point to first– and second–level changes, such as the colonization of terrestrial environments and the establishment of deep marine ecosystems of modern aspect. The aim of this paper is to discuss how ichnology can provide information about ecospace utilization and paleoenvironmental expansion during the Ordovician radiation and, therefore, shed light into the colonization history of ancient ecosystems.

A significant amount of data comes from shallow–marine siliciclastic environments. Analysis of ichnodiversity changes through the Ordovician (Mángano and Droser, 2004) does not support the widely accepted belief that shallow–water ichnofaunas were fully diversified by Cambrian times with no subsequent ichnodiversity increase (Seilacher, 1974, 1977; Crimes, 2001). Lower Ordovician shallow marine siliciclastic deposits display abundant, presumed, trilobite–produced biogenic structures. In peri–Gondwanan settings, the most significant trace fossil turnover event is recorded by the Cruziana ichnostratigraphy. Elements of the Cruziana semiplicata group (Upper Cambrian–Tremadocian) are replaced by elements of the Cruziana rugosa group (Arenigian–Llanvirnian) (Crimes, 1975; Seilacher, 1992). Other common components of the Cruziana ichnofacies are vermiform structures such as Planolites, Palaeophycus, Teichichnus, Phycodes and Helminthopsis. Middle to Late Ordovician shallow–marine ichnofaunas generally show more varied behavioral patterns. Trilobite traces are rarely the dominant component in wave–dominated open marine clastics, possibly reflecting the development of a deeper infauna (i.e., taphonomic bias). The dominant patterns include branched, spreiten burrow systems (e.g., Phycodes, Trichophycus), branched, constricted burrow systems (e.g., Arthrophycus), branched burrow mazes and boxworks (e.g., Thalassinoides), dumbbell–shaped traces (e.g., Arthraria, Bifungites), and chevronate trails (e.g., Protovirgularia). Most of these behavioral architectures were present in the Cambrian and Early Ordovician, but generally were subordinate in abundance and diversity to trilobite traces. In contrast to Cambrian faunas, shallow marine Ordovician biotas display more complex community structures, as reflected by the tiering structure of infaunal resident and opportunistic communities.

Unlike siliciclastic shallow marine settings, softgrounds in carbonate environments do not show a significant increase in trace fossil diversity through the Ordovician, but rather reveal increased ecospace utilization and tiering. Ichnofabric analyses of inner shelfal carbonate deposits of the Great Basin reveal two major increases in the extent and depth of bioturbation during the Early Paleozoic: the first one between pre–trilobite and trilobite–bearing Cambrian rocks and the second between the Middle and Late Ordovician (Droser and Bottjer, 1989). The Ordovician increase of bioturbation results in part from an increase in the size of discrete structures (Droser and Bottjer, 1989). Although Thalassinoides does occur in Cambrian and Lower Ordovician rocks, examples typically are less than 10 mm in burrow diameter, architecturally more simple, and commonly form two–dimensional networks (cf. Myrow, 1995). In contrast, Upper Ordovician Thalassinoides resemble modern structures produced by decapod crustaceans recording extensively reworking with severe obliteration of primary structures (Sheehan and Schiefelbein, 1984; Droser and Bottjer, 1989). Ichnofabric evidence also indicates an onshore–offshore pattern because extensive bioturbation first developed in shallow water settings and only later developed in more offshore settings (Droser and Bottjer, 1989). Significant changes in the evolution of macroboring organisms occurred during the Ordovician (Kobluk et al., 1978; Ekdale and Bromley, 2001; Wilson and Palmer, 2001). This significant rise in bioeroders probably occurred by the end of the Middle Ordovician and recently has been referred to as "the Ordovician bioerosion revolution" by Wilson and Palmer (2001).

The traditional view of deep marine environments – that of a virtually empty terminal Proterozoic and Cambrian environment and an Early Ordovician colonization event of the deep marine biotope (Crimes, 1974; Seilacher, 1974) – is being replaced by the idea of a protracted process initiated during the terminal Proterozoic (MacNaughton et al., 2000; Crimes, 2001; Orr, 2001). During the Early Ordovician the main lineages of deep marine traces (i.e., rosette, meandering, patterned, spiral) were established in deep sea sediments. In contrast, Cambrian deep marine ichnofaunas are composed mostly of ethologically simple, facies–crossing ichnogenera (e.g., Palaeophycus, Planolites, Helminthoidichnites) and shallow–water components, such as "presumed trilobite traces". Lower Ordovician flysch ichnofaunas seem to be moderately diverse and fodinichnia usually dominates. In contrast, Upper Ordovician–Lower Silurian ichnofaunas are considerably more diverse and both pascichnia and agrichnia are well represented (Orr, 1996, 2001). Trace fossil evidence records that the advent of a deep marine ecosystem of modern aspect originated during the Ordovician, representing a second–level change in terms of evolutionary paleoecology. Compared with late Mesozoic to Cenozoic assemblages, however, Ordovician assemblages are significantly less diverse and display less complex ecologic structures.

Relatively little is known about the colonization history of marginal marine, brackish water environments, but the available information is growing rapidly. While Cambrian brackish water ichnofaunas are restricted to the outermost regions of estuaries and bays, Ordovician assemblages seem to reflect a landward expansion into inner areas of marginal marine systems. Analysis of Ordovician marginal marine ichnofaunas demonstrates that some trilobites and soft–bodied animals were able to invade estuarine and other marginal marine settings (Buatois et al., 2001). As in the case of open marine ecosystems, trilobites were among the dominant tracemakers in Cambrian to Lower Ordovician estuarine ichnofaunas, but their importance apparently declines in Upper Ordovician assemblages.

Ichnofossil occurrences predate data from the body fossil record and demonstrates that the terrestrial invasion had already started by the Ordovician (Buatois et al., 1998). Recent research suggests that arthropods walked on land as early as the Late Cambrian to Early Ordovician as indicated by trackways produced by an amphibious animal in coastal eolian dunes (MacNaughton et al., 2002). Retallack and Feakes (1987) and Retallack (2001) discussed the presence of backfilled traces attributed to millipedes. Johnson et al. (1994) documented arthropod trackways (Diplichnites, Diplopodichnus) undoubtedly produced by myriapod-like invertebrates in pond deposits that were periodically desiccated, suggesting the presence of an incipient Scoyenia ichnofacies.

Ichnologic evidence indicates that biotas experienced significant paleoenvironmental expansion and increasing ecospace utilization during the Ordovician, recording paleoecologic changes of first to fourth orders. The bulk of information comes from shallow– and deep–marine settings, but a growing body of knowledge in marginal–marine and continental environments is providing a more complete picture of Ordovician ichnofaunas. In contrast to Cambrian faunas, shallow–marine Ordovician biotas display more complex community structures. Ichnologic data from shallow–marine siliciclastic deposits suggests an increase in the complexity of tiering structure of both infaunal resident and opportunistic communities. In perigondwanic settings, significant trilobite turnover events are recorded by the replacement of elements of the Cruziana semiplicata group by elements of the Cruziana rugosa group. Ichnofabric analysis in shallow–marine carbonate deposits of the Great Basin reveals an increase in extent and depth of bioturbation between the Middle and Late Ordovician. Ichnofabric evidence also indicates an onshore–offshore pattern because increase of bioturbation moved from onshore to offshore through the Ordovician. Subsequent to incipient colonization of slope to deep–sea environments during the Vendian and Cambrian, a major colonization episode of deep–marine biotopes took place during the Early Ordovician. Diverse ichnofaunas from a number of deep–marine successions indicate that soft–bodied organisms were widespread in slope and base–of–slope tubidite systems. Complex behavioral patterns, that typify the Nereites ichnofacies and occur in deep marine turbidites during the Ordovician, were present in shallow–water environments during the Vendian and Cambrian, suggesting retreat of sophisticated grazers and farmers. Presence of low–diversity ichnofaunas in nearshore, shelf and slope deposits, accumulated in areas adjacent to volcanic arcs, reveals the capacity of soft–bodied animals and trilobites to cope with stressful conditions in unstable settings. Trace fossil evidence also records colonization of marginal–marine ecosystems, particularly estuarine systems. Brackish–water ichnofaunas are known since the Cambrian, but become more widespread during the Ordovician when the Curvolithus association first appeared. Trilobite traces are dominant in moderate to low energy areas of estuarine complexes, particularly tidal flats and interbar zones, reflecting the ability of arthropods to foray into marginal–marine environments. During the Late Ordovician, a significant invertebrate invasion of continental environments took place, as recorded by myriapod–like traces in paleosols and lake–margin facies, following an initial episode of colonization during the Cambrian–Ordovician transition.

Mángano thanks the following granting agencies for financial support Grants–in–Aid–of Research by Sigma Delta Epsilon, the Antorchas Foundation, the Percy Sladen Memorial Fund, the National Agency of Science and Technology and the Argentinean Research Council (CONICET). The National Science Foundation provided funding to Droser (EAR–9219731). Richard Bromley, Luis Buatois, Sören Jensen, Patrick Orr and Mark Wilson provided useful comments.