Critical appraisal of vertebrate ichnotaxonomy

RICARDO NESTOR MELCHOR; FERNANDO GENISE, JORGE

Firenze

Congreso; 32 th International Geological Congress; 2004

The ichnotaxonomical approach of tetrapod tracks, mostly reflecting the influence of vertebrate paleontologists, is different from that of invertebrate trace fossils. The taxonomy of tetrapod tracks has been largely based on anatomic features of the supposed producers, whereas non-anatomic features (called “extramorphologic”) have been commonly ignored. The latter convey important information on behavior, gait and sustrate consistency. In contrast, even when there are no “universal” ichnotaxobases of application to invertebrate trace fossils, a consensus exist that the main parameters used for classification are morphologic features that reflect behavior. The main goal of vertebrate ichnotaxonomy is the identification of the producer, an aspect that is not considered in invertebrate ichnotaxonomy. Similar tetrapod traces of different age or geographic provenance are commonly recognized under different ichnotaxa. This bone-based reasoning relies on the fact that the vertebrate faunas have changed through geologic time, then the track producers should be different, and the track types are thus distinguished under different ichnotaxa. Provincial ichnotaxa result from incomplete communication between researchers or from extrapolations about the distribution of track taxa based on coetaneous vertebrate faunas. In contrast, the age or geographic location is not regarded as of taxonomical value for invertebrate trace fossils. The circular reasoning of using age as a valid, and commonly first-rank, ichnotaxobase undermines most ichnostratigraphical and ichnofacial approaches. In consequence, vertebrate ichnofacies are commonly restricted to a definite time interval (e.g., Late Permian Chelichnus ichnofacies). Tetrapod track ichnotaxonomy based on morphologic characters that reflect behavior (including “extramorphologic” features) will result in the synonymy of ichnogenera of similar morphology (e.g. Chelichnus, Laoporus, and Brasilichnium). In turn, it will facilitate the comparison of similar vertebrate and invertebrate ichnofaunas of different age occurring in the same sedimentary facies (e.g., eolian dune ichnofaunas). Studies on modern ichnocoenoses will facilitate the assessment of the behavioral and paleoecological meaning of each ichnofacies. The proposed changes are regarded as an essential step for the unification of the continental ichnofacial model considering both invertebrate and vertebrate trace fossils.