Manus track preservation bias as a key factor for assessing trackmaker identity and quadrupedalism in basal ornithopods

Abstract

Background: The 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 findings: Detailed 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/significance: Several 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.

Figure 1. Detailed overview map of the Las Cerradicas site.

A) Sketch of the site showing the occurrence of the different levels (modified from [35]). Scale  = 1 m. B) Stratigraphic log of the layers that crop out at the Las Cerradicas site.

Figure 2. Panoramic pictures of the Las Cerradicas site showing the different levels.

A) Picture from the old part of the tracksite showing the levels 1, 2 and 3. Note the “island” just in the middle of the picture. B) Picture from the new part of the tracksite showing the levels 1, 3 and 4.

Figure 3. Measurements taken from the tracks.

A) Measurements taken for the whole trackway (trackway redrawn and modified from [22]). B) Measurements taken for the individual tracks. Abbreviations: see text in Materials and Methods.

Figure 4. Digital Outcrop Model of the site made with the software Geomagic® Studio 10 and Petrel

(® Schlumberger). Note that the Model has the same orientation than the sketch of the Figure 1. Scale  = 1 m. The contour-line spacing is 5 mm.

Figure 5. Trackway LCR8 preserved in level 3 as true tracks.

A) Sketch of the trackway LCR8 with preserved manus tracks (redrawn from [22]). B) Picture and outline drawing of the pes-manus set LCR8.7. C) Picture and outline drawing of the pes-manus set LCR8.5. Scale (card)  = 8 cm.

Figure 6. Trackway LCR16 preserved in level 3 as true tracks.

A) Sketch and pictures of individual pes-manus sets for the trackway LCR16 with preserved manus tracks. Scale in the pictures 8 cm (card) and 15 (scale bar). B) Picture of the aforementioned trackway and the isolated small tracks LCR17. Note that some of the tracks are infilled by the overlying level 4.

Figure 7. Pictures of some of the best preserved tracks.

A) Natural cast of the track LCR15.1p. B) LCR15.4p preserved as a true track. C) Natural cast of the track LCR16.2p. D) Natural cast of the track LCR16.4p. E) LCR17.2pm? preserved as true track. F) Natural cast of the track LCR17.2p. G) LCR18.6p preserved as true track. H) Natural cast of the track LCR18.8?p. I) LCR8.4m preserved as true track with the infilling of the overlying level. J) LCR15.2m preserved as true track. Scale in A, C, D, F, H, J  = 15 cm. Scale in B, G, I  = 8 cm. Scale in E  = 5 cm.

Figure 8. Track LCR1.7 preserved in layer 1 as undertrack.

A) Picture of the track. Scale bar  = 15 cm. B) Photogrammetric 3D depth analysis model. The white line represents the longitudinal cross section that crosses the track from the “heel” pad through the digit III to the manus print. The contour-line spacing is 3 mm. The depth units are also mm. C) Cross section profile.

Figure 9. Track LCR3.3 preserved in layer 3 as true track.

A) Picture of the track. Note the levels 1 and 2 in the left part of the picture. Scale (card)  = 8 cm. B) Photogrammetric 3D depth analysis model. The white line represents the longitudinal cross section that crosses the track from the “heel” pad through the digit III to the manus print. The contour-line spacing is 3 mm. The depth units are also mm. C) Cross section profile.

Figure 10. Track LCR8.7 preserved in layer 3 as true track

(compare with the figure 5B ). A) Photogrammetric 3D depth analysis model. The white line represents the longitudinal cross section that crosses the track from the “heel” pad through the digit III to the manus print. The contour-line spacing is 3 mm. The depth units are also mm. B) Cross section profile.

Figure 11. Comparison with some ichnotaxa.

A–E) Tridactyl ichnotaxa sharing features with Las Cerradicas tracks: Toyamasauripus , Dinehichnus , Asianopodus , Therangospodus or Kalohipus . F–I: main “large ornithopod” ichnotaxa from the Early Cretaceous: Iguanodontipus , , Amblydactilus or Caririchnum .

Figure 12. Rose plot showing the orientation of the ornithopod and sauropod trackways from Las Cerradicas site.

The orientation unit in the rose diagrams is 5° and the circular lines correspond to one unit (1 trackway).

Figure 13. Overprinting relationships between sauropods and ornithopods.

A) LCR8.7p overprinting LCR13.6p. B) Detailed of LCR8.7p- LCR13.6p C) LCR6.2p overprinting LCR9.3m.