Ediacaran Ecosystems and the Dawn of Animals

Slime travelers: Early evidence of animal mobility and feeding in an organic mat world

“…This necessitates that the organism that made Helminthoidichnites moved in a particular direction, signifying anterior/posterior differentiation, and likely had a hydrostatic skeleton (Budd & Jensen, ). These features demonstrate that Helminthoidichnites, and similar, contemporaneous trace fossils, represent the earliest definitive evidence for bilaterians in the fossil record (e.g., Jensen et al, ; Erwin et al, ; Buatois & Mángano, ). The recent discovery of Helminthoidichnites penetrating body fossils of large discrete taxa suggests that these organisms may have been scavengers, feeding on decayed organic matter from both the mat and buried members of the Ediacara Biota (Gehling & Droser, ).…”

“…A major consequence of an Ediacaran seafloor covered in an organic substrate is the preferential preservation of certain trace fossils (Droser et al, ). The cohesiveness of Ediacaran mats created a preservational paradox: Their ubiquitous presence likely generated the exceptional conditions promoting the preservation of trace fossils, but such structures required specific, energy‐intensive activity to be produced in relatively firm organic substrates (Buatois & Mángano, ; Dzik, ; Gehling et al, ; Seilacher, ). This issue is particularly relevant when considering the early record of animal mobility, where the ability to move evolved in environments characterized by abundant organic mats.…”

“…While many structures originally interpreted as trace fossils are now recognized as body fossils (e.g., Droser and Gehling, ; Sappenfield, Droser, & Gehling, ; Buatois & Mángano, ), there are Ediacaran trace fossils that definitively represent interactions between mobile metazoans and mats. Helminthoidichnites (Figure a) is a curvilinear trace fossil typically a few millimeters in diameter that occurs in both positive relief and negative relief on the tops and bottoms of fossiliferous sandstone beds (Droser et al, ; Gehling & Droser, ; Jensen, ; Jensen et al, ).…”

“…There are three hypotheses for footprint formation. The most widely accepted is that they are the result of dickinsoniomorphs that remained stationary for a period of time, removed the organic mat beneath them, and then moved from that area leaving a depression (Figure ; Buatois & Mángano, ; Dzik, ; Fedonkin, ; Gehling et al, ; Ivantsov, , , ; Ivantsov & Malakhovskaya, ; Sperling & Vinther, ). In this scenario, evidence of mobility between footprints is not preserved because the act of movement did not sufficiently disturb the organic mat (Coutts et al, ; Gehling et al, ).…”

“…The Ediacara Biota is classically divided into three temporally sequential assemblages: the oldest Avalon, followed by the White Sea, and youngest Nama (Waggoner, ). The earliest evidence of animal movement is reported from the Avalon assemblage (571–557 Ma; Boag, Darroch, & Laflamme, ) of Newfoundland (Liu, McIlroy, & Brasier, ; Menon, McIlroy, & Brasier, ; Liu, McIlroy, Matthews, & Brasier, ; Liu & McIlroy, ; although see Buatois & Mángano, ; Droser, Tarhan, & Gehling, ). In contrast, the latest Ediacaran, concurrent with the Nama assemblage (551–539 Ma; Boag et al, ; Linnemann et al, ), contains multiple, distinct trace fossil genera at numerous localities worldwide (Buatois, Mángano, Olea, & Wilson, ; Chen, Chen, Zhou, Yuan, & Xiao, ; Chen et al, ; Darroch et al, ; Jensen, Droser, & Gehling, ; Mángano & Buatois, ; Meyer et al, ).…”