'The footprints are organised in two parallel rows, as expected if they were made by animals with paired appendages.
For comparison, non-bilateral animals include sponges, corals, jellyfish, and anemones.
Scientists have discovered the earliest footprint of an animal on Earth, dating back 541 million years. "Unless the animal died and was preserved next to its footprints", said Xiao, "it is hard to say who made the footprints".
The scientists weren't able to locate the body fossils of the animals that made these traces.
"Together, these trackways and burrows mark the arrival of a new era characterized by an increasing geobiological footprint of bilaterian animals", the researchers point out.
Still, this discovery means that paleontologists will have to revise their vision of how life developed in Earth's primordial oceans.
Created before the Cambrian Explosion-a sudden burst in animal diversity that took place about 541 million years ago-scientists think these are the oldest trackways ever discovered on Earth.
Researchers aren't exactly sure how many legs the creature had - it could be two, or it could be many more. "Also, they are organised in repeated groups, as expected if the animal had multiple paired appendages".
He added: "At least three living groups of animals have paired appendages (represented by arthropods, such as bumblebees; annelids, such as bristle worms; and tetrapods, such as humans)". In fact, the China discovery represents one of the earliest known records of animals evolving appendages.
"Arthropods and annelids, or their ancestors, are possibilities".
The trackways appear to be connected to burrows, suggesting that the animals may have periodically dug into sediments, perhaps to mine oxygen and food. Other research has suggested that the evolutionary roots of bilaterians should go back further than that, but fossils had never turned up until now.
National Oceanic and Atmospheric Administration made a decision to look back in time, to see what happened in the past. Between 1949 and 2016, tropical cyclone translation speeds declined 10 percent worldwide, the study says.
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