First report of deinonychosaurian tracks from the Upper Cretaceous Candeleros Formation of northwestern Patagonia, Argentina

HEREDIA, ARTURO M.; BELLARDINI, FLAVIO; BAIANO, MATTIA A.; MANIEL, IGNACIO J.; GARRIDO, ALBERTO C.; WINDHOLZ, GUILLERMO J.; GUEVARA, JAVIER E.; CIAFFI, ALESSIO; MESSINA, MATTIA Y.; DÍAZ-MARTÍNEZ, IGNACIO

Florianópolis

Congreso; 5th International Congress on Ichnology; 2024

5th ICHNIA organizing committee, International Ichnological Association

Several didactyl tracks from the Cenomanian Candeleros Formation (El Sauce locality, Neuquén Province, Argentina) are documented here. This record consists of at least six tracks preserved as negative epirelief in medium-grained sandstone from alluvial plain deposits. The tracks are strongly asymmetric, preserving only the impression of digits III and IV and a narrow and long claw mark corresponding to digit II. The tracks are between 9.6 and 10.3 cm long and between 4.8 and 5.9 cm wide. No clear trackway has been recognized, although five of the tracks have a similar orientation and only one is oriented in the opposite direction. Tracks do not preserve anatomical features, like digital pad impressions. The angle of divarication between the claw mark of digit II and the impression of digit III is considerably greater than the angle between the impressions of digits III?IV (e.g., in the best-preserved track the values are 16° and 9°, respectively). The Cretaceous record of didactyl tracks is poorly known in South America and has been assigned to dromaeosaurids (Deinonychosauria). It includes didactyl tracks documented in Bolivia from the Campanian Toro Toro Formation referred to as Dromaeopodus? isp. and undetermined tracks from the Maastrichtian El Molino Formation [1, 2]. Several ichnotaxa based on didactyl tracks and documented mainly in the Northern Hemisphere were related to deinonychosaurian theropods (e.g., Menglongipus sinensis, Velociraptorichnus sichuanensis, Dromaeosauripus yongjingensis, Dromaeosauripus hamanensis, Dromaeopodus shandongensis) [3, 4, 5, 6, 7]. The didactylid tracks from El Sauce show a smaller opening angle of digits III?IV than those of these ichnotaxa, except for D. shangdongensis which displays similar values. Nevertheless, unlike the last ichnospecies, the tracks studied here do not preserve the impression of the phalangeal pads. In addition, none of these ichnotaxa have documented the claw mark of the digit II as in the present tracks. The deinonychosaurian fossil record of the Upper Cretaceous of Argentina is restricted to the Neuquén Basin and includes the Unenlagiinae dromaeosaurids: Buitreraptor gonzalezorum from the Cenomanian Candeleros Formation; Unenlagia comahuensis, Unenlagia paynemili, Neuquenraptor argentinus and Pamparaptor micros from the late Turonian?early Coniacian Portezuelo Formation; and Austroraptor cabazai from the Campanian?Maastrichtian Allen Formation [8, 9, 10]. Taking into account this fossil record and the size of their feets, it is likely that the producer of the tracks presented here was a medium-sized Unenlagiinae like B. gonzalezorum. Furthermore, these tracks show that the producer had a developed ungual phalanx of digit II and the ability to rest it on the substrate, reflecting some kind of behavior (probably resting). Considering the above differences, the tracks were not assigned to any of the known ichnotaxa, and further studies are needed to determine if they correspond to a new ichnotaxon. The new findings from El Sauce improve not only our knowledge on the dromaeosaurids of northwestern Patagonia, but also increase the scarce deinonychosaurian track record of South America and particularly of the Cenomanian worldwide.