Palaeontology: Last march of an early worm

The fossils of a worm-like animal that lived approximately 551 to 539 million years ago are presented in Nature. The study includes details of a trail produced by the animal shortly before death, known as a mortichnium, which indicates that this species was motile. Only a few animals from this period have been demonstrated to be capable of movement to date.

It is thought that motility emerged in animals in the late Ediacaran period (around 580 - 539 million years ago). Evidence for this is provided by trace fossils that include trails, trackways and burrows. However, with a few exceptions, the animals that produced these tracks are unknown.

Shuhai Xiao and colleagues describe the fossil species, Yilingia spiciformis, from the Dengying Formation in the Yangtze Gorges of South China. The authors collected 35 fossils of this species and estimate that the animal was around 5 - 26 mm in width and up to 27 cm in length, consisting of approximately 50 segments. They also found 13 trace fossils, including a mortichnium that was directly connected to a body fossil. Features of the mortichnium trail, including a width of 25 mm, indicate that this was made by the movement of the Yilingia animal.

The findings provide insights into the identity of animals responsible for the abundant trace fossils from this period, the authors conclude.