A new dinosaur tracksite with small footprints in the Urbión Group (Cameros Basin, Lower Cretaceous, La Rioja, Spain)

I Díaz-Martínez,F Pérez-Lorente,E García-Ortiz

doi:10.5209/rev_jige.2015.v41.n1.48661

Abstract

A new dinosaur tracksite (La Rueda) with ten small tridactyl footprints (the length ranges between 9 and 15 cm) from the Urbión Group (Cameros Basin, Lower Cretaceous, La Rioja, Spain) is described. The footprints are approximately as long as wide and have high divarication angles between digits II-IV (~80º), some pad impressions on each digit, claw marks, a medial notch and a circular heel pad impression. They are here classified as indeterminate ornithopod footprints and contribute to the increase in the dinosaur ichnodiversity of the Urbión Group. Small dinosaur footprints are scarce in the worldwide fossil record. In the Urbión Group, large dinosaur tracks are much more frequent than small ones. This scarcity could be explained as ecological biases (dearth of small individuals in an area). Nevertheless, the number of small footprints in the Urbión Group is instead the product of by preservation biases (coarse grain sediments and fluvial erosive bases) and the weathering and erosion processes (brittle nature of the rock) that affect especially to small tracks than large ones identified in this Group.

Highlights

The Cameros Basin is a privileged place to study dinosaur footprints
The measurements taken were: footprint length (FL), footprint width (FW), pace length (PL), stride length (SL), trackway deviation (TD), external trackway width, pace angulation (ANG), footprint rotation (FR), digit divarication angles (II^III, III^IV, II^IV), and the extension of the digit III beyond a line drawn across the tip of the digits II and IV, measured down the axis of digit III (TE)
Digit III projection is from 5.1 to 6.5, which means that the footprint length is on the order of twice the projection (FL= 2TE)

Summary

Introduction

The Cameros Basin is a privileged place to study dinosaur footprints. The Lower Cretaceous of the Cameros Basin is traditionally divided into five lithostratigraphic groups: Tera, Oncala, Urbión, Enciso and Oliván. All the units except for the Oliván Group have yielded dinosaur footprints. The tracksites are distributed in the provinces of Burgos, Soria and La Rioja, in beds whose age ranges from Late Jurassic to Early Cretaceous (Tithonian-Albian). The La Rueda tracksite (LRU) is a new outcrop from La Rioja with ten small (length < 15 cm) tridactyl footprints. Leonardi (1981) suggested that there are just a few tracksites described with small dinosaur footprints in the world fossil record. The same occurs in the Cameros Basin when comparing them to the total number of tracksites.

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