August 2, 2011
Sometime between 145 million and 140 million years ago, in the vicinity of what is now Teruel, Spain, a small herd of sauropod dinosaurs traveled together near a shallow, sandy bay. We know this because they left their footprints in the rock record, and paleontologist Diego Castanera and colleagues have just released an in-press report about these significant trackways in Palaeogeography, Palaeoclimatology, Palaeoecology.
Fossilized sauropod footprints have been described from the region before, but trackways—especially those of multiple individuals—are rare. Moreover, trackways record prehistoric behaviors that we can’t observe from our 21st century perspective, so a collection of tracks left by several sauropods can offer insights into how the animals moved as well as their social lives.
Naturally, knowing the exact genus or species of dinosaur that created the tracks is impossible. Tracks don’t come with labels, and unless an animal literally dies in its tracks, determining the specific creature that created the traces is fraught with uncertainty. Nevertheless, the anatomy of tracks often allows paleontologists to narrow down the list of suspects to particular dinosaur subgroups. In this case, sauropods are the best fit for the kidney-shaped tracks left by the front feet and the roughly triangular prints left by the hind feet, especially given their distance from one another.
What kind of sauropods left the tracks? That’s difficult to say, but Castanera and co-authors propose that small titanosaurs might be the best fit. This widespread sauropod group—which included the gargantuan Argentinosaurus and the dwarf genus Magyarosaurus—was partly characterized by having wide chests, which gave their trackways a “wide gauge”—or a wider gap between the left and right limbs—that matches the pattern seen in the Teruel tracks. The problem is that the bones of titanosaurs are virtually unknown from the appropriate place and time period, so the trackways could have been left by another sort of sauropod which moved in a similar way.
Regardless of what sort of sauropod left the tracks, though, the most significant aspect of the site is that it preserves the tracks of six individual animals moving in the same direction, nearly parallel to each other. This pattern is typical of other trackways where groups of dinosaurs were moving together. The tracksite represents a herd and not simply a collection of unassociated tracks.
These sauropods were relatively small. The hind foot prints are between nine inches and a foot in length—these animals were not earth-shakers. Frustratingly, though, it is presently impossible to tell whether the track-makers were juvenile animals or just small sauropods. If all the animals were juveniles, then the trackway would throw support to the idea that young sauropods stuck together in small herds after they left the nest, but if the dinosaurs were dwarfed then the tracks may indicate a peculiar, isolated environment where isolated lineages of big dinosaurs evolved into small dinosaurs. Such tiny sauropods have been found in Romania, and represent a widespread but poorly understood phenomenon in which island habitats change organisms in strange ways. Whether the tracks found near Teruel represent another case of nano-sauropods remains to be seen.
Castanera, D., Barco, J., Díaz-Martínez, I., Gascón, J., Pérez-Lorente, F., & Canudo, J. (2011). New evidence of a herd of titanosauriform sauropods from the Lower Berriasian of the Iberian Range (Spain) Palaeogeography, Palaeoclimatology, Palaeoecology DOI: 10.1016/j.palaeo.2011.07.015
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