At the barest level, dinosaur bones record death and postmortem transformation. How the animal lived only becomes apparent through the clues we coax from prehistoric remains. Tracks and traces, on the other claw, are manifestations of life. Dinosaur sign, preserved as impressions in stone, record fleeting moments of anatomy in action. And within the library of fossilized behavior studied and cataloged so far, there is a small number of traces that record something that might seem unexpected within the imagery of the thoroughly modern dinosaur – tail marks.
Dinosaurs were not habitual tail-draggers. This isn’t news. In the 1970s, on the basis of anatomical and trackway evidence, paleontologists lifted the tails of dinosaurs to balance well clear of the ground. Some, such as the herbivorous, be-thumbspiked Iguanodon, had ossified tendons that added strength to a deep, rigid tail whereas the fleet carnivore Deinonychus made course corrections with the help of a tail stiffened by long, interlocking processes of the tail vertebrae. Even without such specializations, the osteology of dinosaurs clearly testified that all the members of this celebrated evolutionary group held their tails aloft. The conspicuous rarity of sinuous drags between dinosaur footprints confirmed that dinosaurs must have held their posteriors high.
And yet, as reviewed by paleontologists Jeong Yul Kim and Martin Lockley in a new Ichnos paper, there are multiple reported cases of dinosaur tail impressions of one sort or another. The question isn’t only what the dinosaurs might have been doing to leave behind a caudal signature, but how paleontologists can tell true tail impressions from other traces that can easily deceive.
Some of the traces mentioned by Kim and Lockley are clear and easily understood. In the roughly 198 million year old rock of southern Utah’s Moenave Formation, and currently enclosed in the St. George Dinosaur Discovery Site, there is a resting trace from a mid-sized theropod. This dinosaur, envisioned within the museum as a colorful Dilophosaurus, sat down, did a little shuffle forward, and sat again before walking off. The resting predator left behind footprints, hand impressions, indentations made by the hips, and short tail drags. In their description of this lovely set of traces, Andrew Milner, Lockley, and colleagues noted that the tail drags indicated that the dinosaur kept the front part of its tail elevated even as a rear portion touched the ground.
But not all fossils come to us in such clear resolution. Kim and Lockley mention that 38 instances of dinosaur tail traces have been reported since the first was described from the Connecticut Valley in 1859, but a significant number of these cases have since been tossed out. In addition to puzzling drags that might be dinosaur tail traces yet lack any footprints or other tell-tale clues to pin down their identity, Kim and Lockley mention a supposed tail drag that could be a rut made by a slipping dinosaur’s foot, another ambiguous drag that turned out to be associated with hand prints left behind by a 76 million year old pterosaur (which had no tail to leave such a mark), and a tail trace associated with dinosaur tracks that may have actually been made by a plodding croc. Out of 38 reports, there are only 24 in which distinctively dinosaurian tracks are matched with what appear to be tail impressions of animals in motion or at rest.
The tail drag count could be even lower still. Late in the study, Kim and Lockley write that “it is necessary to consider that some presumed tail traces may have been made by dinosaurs dragging their toes.” When dinosaurs walked through soft sediment, sinking their feet deep into the substrate, the animals would have left additional drag marks as their feet cleared the muck. While Kim and Lockley insist that dinosaur tail traces “are less rare than conventionally assumed”, a total collection of 24 cases – if that – is rather paltry compared to the thousands upon thousands of dinosaur footprints discovered so far.
Nevertheless, in the case of southern Utah’s crouching theropod and others, dinosaurs sometimes left tail impressions. And two particular dinosaur subgroups left the vast majority of these signs. To date, tail traces have been attributed to theropods (think Dilophosaurus and kin), ornithopods (such as the shovel-beaked hadrosaurs), and sauropods (Apatosaurus and allies). But the breakdown isn’t even. Kim and Lockley mention only two instances of sauropod tail traces (for some reason skipping an additional instance proposed for a historic site in the Early Cretaceous rock near Glen Rose, Texas), with the rest in a nearly even split between theropods and ornithopods.
The theropod and ornithopod tail traces bring us back to bones. These were bipedal dinosaurs capable of crouching, and when they did so they left impressions from parts of their posteriors. And with their bodies balanced around the hips, these bipedal dinosaurs could lean back in such a way to bring their tails in contact with the ground. (Sauropods, held aloft on column-like limbs, had to dip their tails down to drag.) Tail drags don’t alter our understanding of dinosaur anatomy, but instead are glimpses at dinosaur behavior – clues that put long-dead creatures into motion within our imagination.
Kim, J., Lockley, M. 2013. Review of dinosaur tail traces. Ichnos. 20, 3: 129-141
Milner, A., Harris, J., Lockley, M., Kirkland, J. Matthews, N. 2009. Bird-like anatomy, posture, and behavior revealed by an Early Jurassic theropod dinosaur resting trace. PLoS ONE. 4, 3: e4591. doi:10.1371/journal.pone.0004591