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Abstract

Triassic tetrapod footprints have a Pangaea-wide distribution; they are known from North America, South America, Europe, North Africa, China, Australia, Antarctica and South Africa. They often occur in sequences that lack well-preserved body fossils. Therefore, the question arises, how well can tetrapod footprints be used in age determination and correlation of stratigraphic units?

The single largest problem with Triassic footprint biostratigraphy and biochronology is the non-uniform ichnotaxonomy and evaluation of footprints that show extreme variation in shape due to extramorphological (substrate-related) phenomena. Here, we exclude most of the countless ichnospecies of Triassic footprints, and instead we consider ichnogenera and form groups that show distinctive, anatomically-controlled features.

Several characteristic footprint assemblages and ichnotaxa have a restricted stratigraphic range and obviously occur in distinct time intervals. This can be repeatedly observed in the global record. Some reflect distinct stages in the evolutionary development of the locomotor apparatus as indicated by their digit proportions and the trackway patterns. Essential elements are archosaur tracks with Rotodactylus, the chirotherian ichnotaxa Protochirotherium, Synaptichnium, Isochirotherium, Chirotherium and Brachychirotherium, and grallatorids that can be partly linked in a functional-evolutionary sequence. Non-archosaur footprints are common, especially the ichnotaxa Rhynchosauroides, Procolophonichnium, Capitosauroides and several dicynodont-related or mammal-like forms. They are dominant in some footprint assemblages.

From the temporal distribution pattern we recognize five distinct tetrapod-footprint-based biochrons likened to the known land-vertebrate faunachrons (LVFs) of the tetrapod body fossil record: 1. Dicynodont tracks (Lootsbergian=Induan age); 2. Protochirotherium (Synaptichnium), Rhynchosauroides, Procolophonichnium (Nonesian=Induan–Olenekian age); 3. Chirotherium barthii, C. sickleri, Isochirotherium, Synaptichnium (‘Brachychirotherium’), Rotodactylus, Rhynchosauroides, Procolophonichnium, dicynodont tracks, Capitosauroides (Nonesian–Perovkan=Olenekian–early Anisian); 4. Atreipus–Grallator (‘Coelurosaurichnus’), Synaptichnium (‘Brachychirotherium’), Isochirotherium, Sphingopus, Parachirotherium, Rhynchosauroides, Procolophonichnium (Perovkan–Berdyankian=Late Anisian–Ladinian); 5. Brachychirotherium, AtreipusGrallator, Grallator, Eubrontes, Apatopus, Rhynchosauroides, dicynodont tracks (Otischalkian–Apachean=Carnian–Rhaetian).

Tetrapod footprints are useful for biostratigraphy and biochronology of the Triassic. However, compared to the tetrapod body fossil record with eight biochrons, the five footprint-based biochrons show less resolution of faunal turnover as ichnogenera and ichnospecies at best reflect biological families or higher biotaxonomic units. Nevertheless, in sequences where body fossils are rare, footprints can coarsely indicate their stratigraphic age.

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