The advent of animals: The view from the Ediacaran

Abstract

Patterns of origination and evolution of early complex life on this planet are largely interpreted from the fossils of the Precambrian soft-bodied Ediacara Biota. These fossils occur globally and represent a diverse suite of organisms living in marine environments. Although these exceptionally preserved fossil assemblages are typically difficult to reconcile with modern phyla, examination of the morphology, ecology, and taphonomy of these taxa provides keys to their relationships with modern taxa. Within the more than 30 million y range of the Ediacara Biota, fossils of these multicellular organisms demonstrate the advent of mobility, heterotrophy by multicellular animals, skeletonization, sexual reproduction, and the assembly of complex ecosystems, all of which are attributes of modern animals. This approach to these fossils, without the constraint of attempting phylogenetic reconstructions, provides a mechanism for comparing these taxa with both living and extinct animals.

Keywords: Ediacara; Ediacaran; South Australia; animals; fossils.

Fig. 1.

Common Ediacara biota fossils from the Ediacara Member, Rawnsley Quartzite, Flinders Ranges, South Australia. (A) Dickinsonia costata with precontraction outline. (B) Funisia dorothea preserved as body casts and external molds where casts have been lost. (C) Two specimens of Parvancorina minchami. (D) Kimberella quadrata. (E) Multilayered sandstone case of Bradgatia sp. (F) Sandstone cast of scratch traces, Kimberichnus teruzzii, produced by K. quadrata. (G) Spriggina floundersi. (H) Internal cast of three walls of Pteridinium simplex. (I) Helminthoidichnites isp., groove and levee traces preserved on a bed base. Specimens D and G are from the Ediacara Conservation Park; other specimens are from the Nilpena Heritage Site. (Scale bar: 1 cm.)

Fig. 2.

The relative abundance of different fossils on the excavated beds from South Australia. The total number of fossils found on the bed is given in parentheses after the bed name. All the beds shown, with the exception of the beds represented by the four right columns, represent deposition between fair-weather and storm wave base [the wave-base sand facies of Gehling and Droser (19)]. The four beds on the right are within the sheet-flow sand facies. The body fossil Funisia is preserved on beds MM2, Matt, and STC-X in abundances that range in the thousands, and thus, these fossils actually dominate beds. However, their dense packing and typically poor preservation do not allow for accurate counts on these beds.

Fig. 3.

C. acula represents the oldest evidence of skeletonization. (A) A reconstruction of C. acula, after Clites and colleagues (27). This reconstruction is based on known specimens that have only up to four spicules, but it is likely it had more. (B) The holotype SAM P43257 of C. acula. The central depression is the mold of the thimble-like main body of the specimen. Note the rigid spicules that radiate from the main body.