Diversity of dinosaur tracks and swim traces in a new site in the Upper Cretaceous El Molino Formation, Torotoro National Park, Bolivia

Abstract

In January 2021, a new site containing dinosaur trackways was uncovered at kilometer 99 during the Anzaldo to Torotoro highway construction. The trackways were found on two beds of ooid and ostracod-bearing quartz arenites of about 160 m2, each separated by roughly 2 m of calcareous siltstone. In the lower layer, five theropod and five sauropod trackways were identified, three trackways of uncertain identity, and three trackways of didactyl tracks. Due to time constraints, only the upper layer containing a rich assemblage of theropod tracks and traces was thoroughly studied before road construction removed the bed. The upper bed contained 493 tracks in 88 trackways. Of those trackways, 21 consisted of a single, isolated track. In addition to tracks, 73 swim traces from 14 trackways were characterized. We distinguished five track morphotypes based on the morphology of the digits and pedal impression. Swim traces occurred in three distinct morphotypes. Invertebrate burrowing and sediment collapse, after the kick-off phase of track formation, partially modified most tracks. Before substrate stabilization, the water level rose, and dinosaurs swam in the area, leaving characteristic swim trackways with their feet scratching the bottom. This report records abundant theropod tracks and swim traces in a fortuitous exposure that is no longer accessible and comments on behavioral and paleoenvironmental implications. This location adds to the significance of the dinosaur track record of the Parque Nacional Torotoro (Torotoro National Park), Bolivia.