Perspectives on continental freshwater ichnology

RICARDO NESTOR MELCHOR

Sao Leopoldo

Simposio; Simposio Latinoamericano de Icnología; 2010

UNISINOS

Ichnology is an interdisciplinary science and can be accessed by resea rchers with different backgrounds and purposes. My approach to continental ichnology is from a geological background with a strong biological inclination and always considering invertebrate, vertebrate and plant trace fossil s. The purpose is an ethological, paleoecological, paleoenvironmental, and evolutionary interpretation. The starting point of any ichnological study is a bed by bed documentation of sedimentary features and the taxonomic identification of every ichnofossil present in a sedimentary succession. This information provides a basic framework for both ichnofacies and ichnofabric approaches. There is a recent agreement (2006) on the taxobases of common use on invertebrate ichnology, two editions of the Treatise on Invertebrate Paleontology has been published (1962, 1975) and a new version is in preparation, however, the ichnotaxonomic approach in invertebrate ichnology is still far from uniform. Tetrapod ichnotaxonomy is based largely on morphological features of footprints with an anatomical correlate, commonly ignoring those linked to behavior. It has been argued that these differences between vertebrate and invertebrate ichnotaxonomy are linked to . separate traditions and to larger size, greater complexity in anatomy and in behavior of tetrapods. An updated compilation of vertebrate footprint taxa is largely needed. Perhaps, a web based platform like MediaWiki© would be an ideal format to allow easy contribution and interaction by multiple researchers. Adequate interpretation of behavior, paleoecology, and possible trace maker affinity of any ichnofossil would be greatly aided by neoichnologic observations on a modern analog or homolog of the potential producer and the sedimentary environment. There are many proposed ichnofacies for continental freshwater settings, although only two of them are considered at present as archetypal, the Scoyenia and Mermia ichnofacies. Most tetrapod ichnofacies are not comparable with archetypal invertebrate ichnofacies, especially because of the reduced temporal duration of the former. Many tetrapod ichnofacies overlap with the Scoyenia ichnofacies and a compromise solution has not been agreed, although it is recognized that integration between invertebrate and vertebrate ichnofacies is desirable. Ichnofabric studies in continental freshwater sett ings are sca rce. This methodology can be very useful to discern suites of trace fossils, especially in highly bioturbated intervals. This is usually achieved by a routine documentation of abundance of each trace fossil and of the cross-cutting relationships between trace fossils. Fluvial or shallow lacustrine settings experience repeated flooding and deSiccation, resulting in overprinting of trace fossil suites. In absence of cross-cutting relationsh ips between trace foss ils the distinction of trace fossil suites related to water satu rated and desiccated substrates is difficult. Our knowledge about substrate properties adequate for preservation of a tetrapod footprint or bioglyphic pattern in burrow wa lls, in t erms of a range of shear strength, the effect of variable water saturation, grain size and different sediment composition (e.g., siliciclastic mud, lime mud, sand, volcanic ash), is highly rudimentary. Similarly, could it be possible to define a range of shear strength that typifies different substrate consistency (soupground, looseground, softground, and firmground) for use in the field or lab during neoichnological observations? A number of concepts and models have been proposed in the last years in the realm of continental ichnology, and there is still opportunity for significant developments. For the near future, th ere are, at least, six aspects that should be considered: 1) more studies with both a strong sedimentologic and ichnotaxonomic focus; 2) whiter ichnotaxonomy; 3) an objective characterization of substrate properties, mostly consistency; 4) application of ichnofabric studies; 5) neoichnological observations as inherent to an ichnological study; and 6) search for a common ichnofacies scheme integrating invertebrate, vertebrate and plant trace fossils.