At the Carreras Pampa site, located in the Torotoro National Park, in central Bolivia, approximately 16,600 fossil footprints of dinosaurs and birds have been discovered, organized into 1,321 paths and 289 isolated tracks of various nature. The great variety and quality of the footprints makes the site a unique example, capable of providing unprecedented information on the behavior of dinosaurs.
The dinosaur tracks, dated between 70 and 66 million years ago, show different levels of depth and styles of preservation on the sediment: shallow and deep imprints, many of which preserve details such as fingernails, big toe impressions or lateral reliefs of displaced sand. The analytical study on the tracks was conducted by a joint US-Bolivian research team, led by Raul Esperante of the Geosciences Institute in Loma Linda, California. Tail tracks are particularly abundant and well preserved, suggesting that some dinosaurs rested their tails on the ground to stabilize themselves on a soft substrate.
The footprints indicate that most of the dinosaurs present were small to medium sized, with a hip height between 65 and 115 cm, and that they moved mainly in parallel or semi-parallel directions. At the end of the Cretaceous, the period to which the footprints date back, this area was probably a coastal-lagoon or coastal environment, which would explain the direction of the movements. This pattern, along with overlapping and crossing paths, suggests that dinosaurs moved through this environment in groups or in multiple events close together in time, indicating forms of gregarious behavior.
Alongside the walking footprints, the site also features swim marks, i.e. tracks left by dinosaurs that swam or waded, with their hind limbs barely touching the bottom and leaving alternating scratches on the submerged sediment. Some of these swims overlap with walking tracks, confirming that dinosaurs used the same area multiple times under different tidal conditions. Bird footprints are also numerous and well preserved, often associated with those of dinosaurs, and the presence of small fossil gastropods is also attested.
The substrate on which the traces were left was variable: sand and silt transported by the water formed a coherent but deformable terrain, with softer areas and others more compact. Local variations in the consistency of the sediment explain the presence of very deep footprints alongside shallow footprints in the same path (some footprints were left on a wet shoreline, while others on drier sand). The motion of the waves and the runoff of the water have left visible traces on the sediment, indicating that the area was originally a lake transition zone between fresh and marine waters, subject to slight oscillations in the water level. The rapid initial cementation of the sediments and the deposition of clay layers (as often happens in these natural contexts) ensured the exceptional preservation of the traces.
The Carreras Pampa dinosaur tracks offer a unique insight into life at the end of the Cretaceous, showing diverse dinosaurs active in a lake-coastal environment, engaging in walking, swimming and group movement, with a substrate soft enough to preserve extraordinary detail.
