The authors of the paper actually describe their hypothesis for how it occurred [0]. Basically perfect conditions were required to preserve them. In short the

The authors of the paper actually describe their hypothesis for how it occurred [0]. Basically perfect conditions were required to preserve them. In short the first tracks were made in dry sand, eroded a little, moistened by fog, dew, or light rain, and then covered by dry sand again. The second tracks were made through the dry sand into the moist layer which was then crusted over with calcite and covered. Only the toe markings from the second tracks were preserved. [0] https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0237636#sec014 https://journals.plos.org/plosone/article?id=10.1371/journal...