The Tetrapod Ichnogenus Protochirotherium Fichter and Kunz 2004, a Characteristic Early Triassic Morphotype of Central Pangea

摘要

Early Triassic chirotherian footprint assemblages from Poland, Germany, and Morocco are important for understanding archosaur evolution in the aftermath of the Permian-Triassic crisis. However, their ichnotaxonomy is confusing because various authors have interpreted their diversity differently. After an analysis and ichnotaxonomic re-assessment, the presence of the ichnogenera Brachychirotherium, Isochirotherium, and Chirotherium in these assemblages is not supported. Distant similarities with these ichnotaxa are functions of extra morphological variation and substrate-related factors. Instead, Early Triassic chirotherian footprints described under these names are assigned here to the ichnogenus Protochirotherium and to a more slender morphotype identified as Synaptichnium. In particular, Protochirotherium appears to be more widely distributed in central Pangea as a characteristic morphotype reflecting a distinct stage in archosaur evolution. Trackmakers were nonarchosaurian archosauriforms or, alternatively, stem-group crocodylians. Morphologically and temporally these footprints match the hypothetical ancestor of the Chirotherium barthii trackmaker. Chirotherium barthii appears by the beginning of the Middle Triassic. Because of its restricted stratigraphic range, and its wider distribution in central Pangea, Protochirotherium also has biostratigraphic significance for this region and can be considered as an indicator of Early Triassic-aged strata.