Morphometric analysis as a tool to investigate swimming/wading tetrapod traces: a case of study from Patagonia

Abstract

Seven isolated tetrapod swimming-wading tracks are thoroughly described and analyzedmorphometrically for the first time. These tracks impressed in levels of the Agrio and Mulichincoformations (Lower Cretaceous, Neuquén Basin, northern Patagonia), are specifically documented frommarginal-marine deposits strongly affected from tidal influence. They represent bi or tridactyl traces,preserved as subparallel elongated ridges in positive hyporelief. Most are straight, but some are slightlycurved; some show a faintly more accentuated curvature in the proximal and/or distal end, rangingfrom 48 mm to 18 cm in length and from 17 mm to 9 cm in width. Each track was measured, and a 3Dmodel was created using photogrammetry. Morphometrically, the tracks were categorized into twodistinct morphotypes: Morphotype I exhibits "widespread" (with greater interdigit space) digitimpressions, while Morphotype II shows more "serrated" (with little interdigit space) digitimpressions. The metrics and characteristics used to differentiate between these morphotypes arediscussed here. To identify a potential tracemaker, we compiled a dataset of swimming tracks producedby theropods, crocodiles, and turtles-groups considered plausible producers based on the fossil recordof the basin and the morphology of the tracks sourced from several global sites. Such a dataset iscurrently lacking in the literature, making this work an essential step toward standardizing the studyof these morphologically heterogeneous tracks, though it is anticipated that the dataset will undergofurther refinement. Using ordination analysis, we created morphospaces to explore possibleconnections between the tracks and one or more of the groups mentioned above. This analysis showsa likely connection between Morphotype I and theropod swimming tracks, while Morphotype II seemsmore confidently ascribed to turtles. Although these taxa are well documented in the Cretaceouscontinental record of the basin, all are considered terrestrial and/or freshwater species, making therecord in a marginal-marine environment unexpected. For theropods, tracksites are known from theAgrio Formation in such type of depositional setting while clearly showing signs suggesting shorelinefluctuations and subaerial exposure. Additionally, this work provides a methodological framework forstudying morphologically diverse tracks and highlights the challenges of applying morphometricapproaches to the analysis of swimming-wading traces.