Late Ediacaran Trackways Produced by Bilaterian Animals with Paired Appendages

1. What they did

Molecular evidence suggests that most groups of bilaterian animals (animals with a left and right side, such as arthropods and worms) arose in the Ediacaran Period, 635 to 541 million years ago. Yet hardly any Ediacaran body fossils can be firmly linked to these groups, and none clearly has paired appendages (legs or leg-like parts). Ediacaran trace fossils have so far been simple surface trails and shallow burrows. Trackways, rows of imprints left by legs or leg-like parts, were unknown before the Cambrian.

The authors excavated a slab of limestone from the Shibantan Member at Wuhe in the Yangtze Gorges area of South China, dated to about 551 to 541 million years ago. It held two possible trackways (TW1 and TW2) and three burrows (UB1 to UB3). Because they collected the slab in place, with both the top and bottom surfaces, they knew which way up each impression was. They photographed the specimens under natural light and low-angle light from different directions, made a latex mould of the bottom surface, and measured each trackway and burrow. They compared the trackways with younger trackways and with other trace fossils on the same surface.

2. What they found

Why these are trackways

• Each trackway is two rows of shallow pits on the top surface, matched by two rows of bumps on the bottom surface.
• The sediment between and beside the rows is undisturbed, and TW2 crosses two burrows (UB1 and UB2) without cutting into them. Trails or burrows would have disturbed that sediment and cut into the burrows they crossed.
• The authors conclude the impressions are most likely tracks made by paired appendages. They may be undertracks: imprints pressed into the sediment below a microbial mat by an animal walking on top of it.

The trackways

• TW1: about 70 mm long, 13.8 mm wide on the outside and 4 mm on the inside. Tracks are 1 to 2 mm across and about 1 mm deep. At least four sets of tracks can be vaguely made out. Each set has two opposite series at about 15° to the midline, each series 17 to 18 mm long with four or more tracks. The apparent stride is about 13.7 mm. Faint marks along the midline and oblique scratches on one side may be present, but this is uncertain.
• TW2: about 91 mm long, 16 mm wide on the outside and 4 mm on the inside, with tracks of similar size. The two rows are clear, but series are not. Repeated groups of five imprints seem to appear, and small dimples run along the inner side of one row, making the trackway lopsided. Groups and dimples are spaced about 8 to 10 mm apart. There are no midline marks.
• The tracks are much less regular than most later trackways of arthropods and annelids. They look most like Danstairia, an irregular trackway from Early Devonian rocks in Scotland.

The burrows and possible connections

• The burrows are 9.5 to 11.5 mm wide; UB1 is 133 mm long and curved, UB2 and UB3 are 42 and 21 mm long and fairly straight. They were dug beneath microbial mats and are filled with carbonate sediment.
• TW2 seems to join UB3, and TW1 seems to join UB1, with the same direction and width, though with a slight offset at each join.
• Two other trace fossils on the same surface (Lamonte sp.) also seem to show possible trackways turning into burrows.

3. What they concluded

• These are the oldest known trackways made by bilaterian animals with paired appendages.
• The discrete tracks rule out legless worms moving by muscle waves. The lack of a body drag mark, and the way TW2 crosses burrows without cutting them, suggest the animal held its body up on its appendages (or pushed its appendages through a microbial mat).
• The trace maker was probably wider than 13.8 mm and may have had at least four, perhaps five, pairs of appendages. The authors caution that false series or groups of tracks can sometimes appear.
• The irregular tracks suggest poorly coordinated leg movements, unlike the smooth wave-like leg rhythm of modern arthropods. Modern arthropods are useful comparisons, but the trace maker cannot be placed in any phylum: all that can be said is that it was probably a bilaterian with paired appendages.
• If the links between trackways and burrows are confirmed, the same animal may have walked on top of microbial mats and burrowed beneath them, a moderately complex behaviour.
• Small bilaterian animals with paired appendages were beginning to play a growing role in shaping their environment through their interactions with microbial mats.

4. How tracking was or was not used

The study is a careful reading of fossil tracks and signs. The authors identified the impressions as trackways rather than trails or burrows from what the sediment showed: undisturbed sediment between the two rows, no drag mark, pits on the top surface matched by bumps on the bottom, and a trackway crossing burrows without cutting them. They measured trackway width, track size and depth, the angle and length of track series, and stride, and they read track groups to estimate the number of appendage pairs. They photographed the tracks with low-angle light from several directions to bring out faint imprints. They used Trewin’s (1994) system for describing arthropod trackways. No trailing in the modern sense was involved.

5. Tracker or observer certification

Not reported. The interpretations were made by the authors (Zhe Chen and Shuhai Xiao), who are palaeontologists; no test or certification of track-reading skill is described.

6. Other technologies, skills or innovations

• Photography with a Nikon D700 camera under natural light and low-angle light with controlled lighting directions.
• A latex mould of the bottom surface, photographed the same way.
• Excavation in place, with both top and bottom surfaces, so there was no doubt about which way up each impression was.
• Petrographic thin sections (shown in the supplementary materials) of other burrows in the Shibantan Member.