A QUIET WALK BY A LARGE THEROPOD DINOSAUR: A CASE STUDY OF THE LONGEST BIPEDAL TRACKWAY FROM THE CRETACEOUS OF PATAGONIA

HEREDIA, ARTURO; PINO, DIEGO; BELLARDINI, FLAVIO; WINDHOLZ, GUILLERMO; BAIANO, MATTIA

Florianópolis

Congreso; ICHNIA 2024.The 5th International Congress on Ichnology; 2024

The Cenomanian Candeleros Formation exhibits one of the highest diversities of dinosaur tracks from the Late Cretaceous of Argentina. Here we report two exceptional trackways of bipedal animals from the El Chocón Medio locality (Candeleros Formation) in northwestern Patagonia, consisting of 30 and 13 successive tridactyl footprints (called here T1 and T2, respectively). Tracks of T1 and T2 are preserved in medium-grained sandstones from alluvial deposits and expressed as true tracks on the same tracking surface. T1 has a length of 40.4 m, and its footprints have an average length of 52.1 cm (a range of 50?60 cm in length), while T2 has a length of 15.6 m, and its footprints have an average length of 52.8 cm (a range of 51?55 cm in length). Strides displays constant values throughout both trackways, with T1 showing an average stride of 276.3 cm and T2 an average stride of. 275.4 cm. These trackways are separated by about 75 m and are almost parallel showing a similar orientation, the last 10 tracks of T1 have a 76°N orientation while T2 has a 75°N orientation. Some indeterminate trackways of smaller size (less than 35 cm long) have also been recorded crossing almost perpendicularly to T2. No intra-trackway variation was recognized in T1 and T2, suggesting no changes in the substrate properties such a moisture content, along the tracking surface. Ichnospecies attributable to theropod dinosaurs known from the Candeleros Formation include: Abelichnus astigarrae, Bressanichnus patagonicus, Deferrariischnium mapuchensis, Picunichnus benedettoi, and Candeleroichnus canalei. The footprints studied here show several differences with all these ichnotaxa, and considering that they are poorly preserved, they were not assigned to any particular ichnotaxon. The Candeleros Formation presents one of the most diverse theropod faunas from the Cretaceous of South America, including carcharodontosaurids, abelisaurids, basal coelurosaurs, alvarezsaurids and dromaeosaurids. However, taking into account their size, only a carcharodontosaurid or abelisaurid could have produced the current T1 and T2 footprints. Considering that the footprints of both parallel trackways have identical morphologies and sizes, the stride length and orientation of both trackways are similar, and that they are found in the same stratigraphic level, it is proposed that T1 and T2 would belong to the same large trackway. Thus, this trackway could reach 121 m in length, constituting the longest trackway documented of a large theropod in the Candeleros Formation and also in Patagonia. The longest known dinosaur trackway worldwide is that of a small theropod from the Late Cretaceous Cal Orcko tracksite of Bolivia, and it is 581 m long. Although some trackways of large theropods have been documented, such as the 311 m trackway attributed to Megalosauripus from the Late Jurassic of Turkmenistan, and the 180 m trackways from the Middle Jurassic White Limestone Formation of England, as far as we know no Cretaceous trackway of a large theropod as long as the one in the present study have been reported. Finally, the fact that the producer maintained the same constant speed throughout the entire trackway indicates a preferred walking gait that probably represents the energetic optima.