3D meshes of H1 and H6 footprints at PK23 track site, Northern Chile

We report new ichnites corresponding to a trackway composed of three tetradactyl pedal footprints and plant remains near Southeastern Guatacondo, Tarapacá Region, within the transition of Majala and Chacarilla Formations (Late Jurassic-Early Cretaceous). The fossils were found in the context of a Copper Mining road construction at the PK23 site, Quebrada Blanca Phase 2 project, by the Teck Mining Company Quebrada Blanca S.A.

We performed stratigraphical, paleobotanical, and paleoichnological studies, along with the footprint characterization and comparisons by Principal Component Analysis, and Discriminant Analysis of I-II, II-III and III-IV interdigital angles, using a large tetradactyl footprint morphological database of potential producers (dinosaurs, crocodiles, lepidosaurs, and pterosaurs). Additionally, we performed 3D pedal computer modeling kinematic analysis of the Chilesaurus diegosuarezi’s 3D tetradactyl foot under two different sedimentary conditions (i.e. hard soil with superficial impression, and soft sediment with a penetrative impression), to test it as a potential trackmaker.

The stratigraphic section reveals an association of siliciclastic facies, formed by the intercalation of fine-grained sandstones and wackes, with ripple marks, cross and horizontal lamination, evidencing intermittent moderate to low sedimentation energy.

Among the plant material, several organs of the family Cheirolepidiaceae were identified such as, leaves and leafy branches of Brachyphyllum type, Classostrobus pollen cone, ovuliferous scales, and wood fragments related to Cupressinoxylon. The monodominance of Cheirolepidiaceae among the preserved macroflora allows inferring the characteristics of the environment in which the plants lived, highlighting halophytic, and temporarily arid conditions. Evidence as a whole, permits to interpret the depositional environment as an arid deltaic-littoral plain, temporarily covered with an ephemeral water layer associated with a stunted woody vegetation, in which terrestrial vertebrates walked by.

Despite the low predictive power of the multivariate analysis, due to the various sources of morphological variability inherent to footprint formation, the PK23 footprints seem to better correspond to Dinosauria or Avialae as the most likely producers, rather than to Lepidosauria or Crocodylia, discarding the dinosaur Chilesaurus due to very low interdigital mediolateral rotation capabilities.

Methods
This investigation was carried out in transitional strata between the Majala and the Chacarilla formations outcropping eight kilometers to the southeast of Guatacondo, Tarapacá Región in northern Chile, within the Quebrada Blanca Phase 2 project owned by Teck Mining Company Quebrada Blanca S.A (Fig. 1). Unfortunately, it is no longer possible to see the footprints in-situ, since the original site subsequently collapsed, so. However, prior to this event, we carried out three field trips in which it was possible to scan some sectors of the surface in 3D (Available from DRYAD WEBSITE), along with a collection of casts and rock samples. 

Six biogenic structures were initially recognized, in which three correspond to a single trackway. Two of them (H1-H6, virtual meshes available at the DRYAD WEBSITE), show good preservation of anatomical details. These footprints were scanned using Go!Scan20 for the generation of 3D models with the software Vxelements, CreaformMR. Additionally, rubber and plaster molds were made following Reuil and Muzzopappa (2019) protocols.

The collected material used to develop this study is stored and housed at the Municipal Museum of Pica, Tarapacá Region, Chile. Thus far, only field numbers have been assigned to the material.

All the paleontological interventions at PK23 fossiliferous outcrop were carried out under the voluntary commitments detailed in the Environmental Classification Ordinance (Resolución de Calificación Ambiental “RCA” in spanish) number 074/2018 CV-17, reviewed and mandated by The Council of National Monuments in Chile.