Manus Track Preservation Bias as a Key Factor for Assessing Trackmaker Identity and Quadrupedalism in Basal Ornithopods

Diego Castanera,Peter L Falkingham,José I Canudo,Àngel Galobart,Novella L Razzolini,Bernat Vila,Phillip L Manning,Richard J Butler

doi:10.1371/journal.pone.0054177

Abstract

BackgroundThe Las Cerradicas site (Tithonian–Berriasian), Teruel, Spain, preserves at least seventeen dinosaur trackways, some of them formerly attributed to quadrupedal ornithopods, sauropods and theropods. The exposure of new track evidence allows a more detailed interpretation of the controversial tridactyl trackways as well as the modes of locomotion and taxonomic affinities of the trackmakers.Methodology/Principal FindingsDetailed stratigraphic analysis reveals four different levels where footprints have been preserved in different modes. Within the tridactyl trackways, manus tracks are mainly present in a specific horizon relative to surface tracks. The presence of manus tracks is interpreted as evidence of an ornithopod trackmaker. Cross-sections produced from photogrammetric digital models show different depths of the pes and manus, suggesting covariance in loading between the forelimbs and the hindlimbs.Conclusions/SignificanceSeveral features (digital pads, length/width ratio, claw marks) of some ornithopod pes tracks from Las Cerradicas are reminiscent of theropod pedal morphology. This morphological convergence, combined with the shallow nature of the manus tracks, which reduces preservation potential, opens a new window into the interpretation of these tridactyl tracks. Thus, trackmaker assignations during the Jurassic–Cretaceous interval of purported theropod trackways may potentially represent ornithopods. Moreover, the Las Cerradicas trackways are further evidence for quadrupedalism among some basal small- to medium-sized ornithopods from this time interval.

Highlights

Morphological similarities between ornithopod and theropod dinosaur tracks have often led to the interchange of taxonomic affinities
In this paper we aim to describe the preservation of the tracks present at Las Cerradicas, and to explore the bias in exposure potential experienced by the shallower manus tracks, compared to the deeper pes impressions
Two types of tracks and trackways are exposed in Las Cerradicas site: 1) trackways of tridactyl pes tracks with occasional manus tracks, and 2) trackways with small pes and manus tracks of sauropod affinity

Summary

Introduction

Morphological similarities between ornithopod and theropod dinosaur tracks have often led to the interchange of taxonomic affinities. Leonardi [24] suggested that many bipedal trackways of ornithopod dinosaurs might belong to facultative or obligate quadrupedal trackmakers He suspected that all large ornithopods were quadrupedal, at least facultatively, and that they would produce very shallow and small manus tracks that would be highly susceptible to preservation bias, weathering and erosion after exposure, or oversight. This phenomenon was demonstrated experimentally in both sauropods and ornithopods by Falkingham et al [25,26], who showed that variations in centre of mass position, combined with differential foot surface areas between manus and pes could, under specific substrate conditions, result in extremely shallow manus impressions from obligate quadrupeds, even when the substrate allowed pes tracks of considerable depth. The exposure of new track evidence allows a more detailed interpretation of the controversial tridactyl trackways as well as the modes of locomotion and taxonomic affinities of the trackmakers

Objectives

Methods

Results

Discussion

Conclusion