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

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.

The evolution of ornithischian quadrupedality

“…Computer modelling of trackmaker/substrate interactions suggests that the absence of manus impressions from large 'bipedal' ornithopod trackways might result from differential loading, whereby the forelimb supports a smaller proportion of the total body mass than the hind limb: consequently, the manus indents the sediment less frequently and/or less conspicuously than the pes (Falkingham et al 2011). This suggestion is supported by tracksites preserving both pes-only and manus-pes ornithopod trackways, with this differential preservation reflecting the mechanical properties of the sediment, rather than representing genuine locomotor differences (Castanera et al 2013). As a result, trackway evidence now complements the osteological evidence in suggesting that large-bodied ornithopods were quadrupedal, while smallbodied taxa were bipeds.…”

A walk in the maze: Variation in Late Jurassic tridactyl dinosaur tracks - A case study from the Late Jurassic of the Swiss Jura Mountains (NW Switzerland)

“…In the case of the Ajoie ichnoassemblages, there is no evidence of manus impressions and we can rule out a manus preservation bias (e.g. : Castanera et al, 2013a) as the tracks were excavated level-by-level. Only trackway BEB500-T120 has a FL/FW ratio and mesaxony that fall within the parameters of certain ornithopod ichnotaxa (Lockley, 2009;Castanera et al, 2013b, Fig.…”

The theropod that wasn't: an ornithopod tracksite from the Helvetiafjellet Formation (Lower Cretaceous) of Boltodden, Svalbard

“…As noted elsewhere (Milan & Bromley 2006;Jackson et al 2009;Razzolini et al 2014), collapse and flow of sediment with a high moisture content can significantly alter track morphology, resulting in erroneous conclusions regarding the trackmaker and other interpretations based on morphology. Castanera et al (2013b) recently discussed the problems of discerning ornithopod and theropod tracks, noting that the association of pes prints with manus prints (which are easily overlooked) can also provide important evidence in support of an ornithopod trackmaker. Our field investigations indicate that many of the morphotype A tracks, including those described in Edwards et al (1978), are also associated with small, round to oval prints that we interpret as manus tracks.…”