A Tale of Two Ichnologies: The Different Goals and Potentials of Invertebrate and Vertebrate (Tetrapod) Ichnotaxonomy and How They Relate to Ichnofacies Analysis

Abstract

Invertebrate and vertebrate (tetrapod) ichnology have evolved as separate disciplines, largely due to differences in the morphology and behavior of the tracemakers and the practical impossibility of treating them as uniform subjects of analysis. Thus, invertebrate ichnotaxa reflect behavior to a much greater degree than they reflect tracemaker morphology. The situation is essentially reversed in the case of vertebrates, where morphology, especially of the foot, plays an important primary role in ichnotaxonomy (and a secondary role in trackmaker identification), whereas behavior is of much lesser importance and in many cases can compromise consistent and effective ichnotaxonomy. Inherent differences in morphological and behavioral complexity also have significant implications for our concept of the different potentials of vertebrate and invertebrate ichnotaxonomy and ichnofacies analysis—as shown in distinctions made between biotaxonichnofacies and ethoichnofacies, respectively. Vertebrate tracks not only inform us about morphology, but in the most general sense also have made huge contributions to our understanding of vertebrate behavior that have only limited supra-morphological relevance to ichnotaxonomy or ichnofacies analysis. These contributions have done much to settle and/or stimulate debate about the posture, speed, abundance, spatio-temporal distributions and social behavior of dinosaurs, pterosaurs, birds and other vertebrates, and so have had a dynamic impact on thinking in the field of vertebrate paleontology and even biostratigraphy. Nonetheless, this influence may affect ichnotaxonomy as in the case of trackway evidence of posture that proved important in differentiating the wide- and narrow-gauge trackways Brontopodus and Parabrontopodus. By contrast, the impact of invertebrate ichnology on our understanding of invertebrate behavior is complexly interwoven with an ichnotaxonomy that has a much less obvious relationship to tracemaker morphology. Thus, the use of invertebrate traces to decipher the evolution of fossil behavior has proved an ambiguous and subtle pursuit that does not readily demonstrate parallels with the evolution of easily identified taxonomic groups. Because vertebrates (especially tetrapods) are inherently complex organisms, in comparisons with invertebrates, their traces play a less ambiguous and more significant role in helping us understand major phases in vertebrate evolution. In many cases they have pointed to interpretations of vertebrate paleobiology that were not confirmed by other lines of evidence until generations later (e.g., polar dinosaur migrations). When vertebrate tracks are used for palichnostratigraphy or as evidence to support faunal turnover and extinction events, ichnotaxonomy assumes a significant role in applied biostratigraphy. Such biostratigraphic utility is far more ambiguous and much less significant in the case of invertebrate ichnology. Despite these inherent differences, in the field of ichnofacies analysis, invertebrate and vertebrate ichnology come together as integrated components of facies analysis, and potentially have great synergistic utility in paleoecological and paleoenvironmental analysis. Nevertheless vertebrate (tetrapod) ichnofacies nomenclature, and terrestrial ichnofacies analysis in general, is not yet well-established and is currently generating disparate schools of thought. It is argued that tetrapod ichnofacies are inherently more diverse and differentiated than invertebrate ichnofacies.