Ankylosaurids were among the most heavily armored Cretaceous dinosaurs. However, it seems all that protective plating was not their only defense, with evidence that members of the family had forefeet suited to digging, probably to carve out protective hollows.
Digging burrows has been such a successful move for so many mammals, we might expect some dinosaurs would have gone down that path as well. Indeed, the ecosystem as a whole often benefits from species that dig, whether it be providing a source of water in dry times – which wombats have been found to offer – or when echidnas refresh the soil.
However, evidence that dinosaurs adopted similar niches has been sparse. Dr Yuong-Nam Lee of Seoul National University examined a specimen from the Baruungoyot Formation, Mongolia, and found an arc shape to the bones of its front feet suited to shifting earth.
The specimen, stuck with the unromantic designation MPC-D 100/1359 rather than a name like many famous dinosaur fossils, was discovered in the 1970s, but not examined in detail until now. MPC-D 100/1359 also had several fused vertebrae and fewer bones in its hindfeet than most counterparts. These would have maximized its capacity to hold its ground, for example if it needed to firmly plant its back feet and dig ferociously.
All this does not necessarily mean MPC-D 100/1359 was the equivalent of modern-day moles or meerkats, species with a heavily subterranean lifestyle. In Scientific Reports Lee and co-authors propose some Asian ankylosaurids were able to dig for roots or groundwater, and may have protected their vulnerable undersides by excavating shallow pits that made them hard for predators to flip when they hunkered down.
Although the ankylosaurs were close relatives of easily the best-preserved non-avian dinosaur we have ever found, most of the nine described Mongolian species are known only from skulls. MPC-D 100/1359, on the other hand, has an unusually complete torso. Ribs, vertebrae, sternal plates, limb bones, and dermal armor have been found, although there is no trace of the head or tail. Some theropod finger and toe bones were mixed in, but whether the two creatures killed each other in an epic battle or died at different times is unknown.
Despite this unusual treasure trove, Lee and co-authors couldn't match MPC-D 100/1359 confidently to any of the ankylosaurs named on the basis of their skulls. It may be a new species, or could belong to a couple of existing ones.
MPC-D 100/1359 had a trunk two meters (7 feet) long, so allowing for the usual ankylosaur proportions, would have been around 5 meters (16.4 feet) from nose to tail. There has previously been speculation as to whether ankylosaurs dug, with some paleontologists dismissing the idea because of their size. However, the paper notes the largest ground sloths achieved similar proportions (minus the tail) and they are thought to have been mighty excavators.