Abstract
Twenty candidate fossils with claim to be the oldest representative of the Phylum Porifera have been re-analysed. Three criteria are used to assess each candidate: (i) the diagnostic criteria needed to categorize sponges in the fossil record; (ii) the presence, or absence, of such diagnostic features in the putative poriferan fossils; and (iii) the age constraints for the candidate fossils. All three criteria are critical to the correct interpretation of any fossil and its placement within an evolutionary context. Our analysis shows that no Precambrian fossil candidate yet satisfies all three of these criteria to be a reliable sponge fossil. The oldest widely accepted candidate, Mongolian silica hexacts from c. 545 million years ago (Ma), are here shown to be cruciform arsenopyrite crystals. The oldest reliable sponge remains are siliceous spicules from the basal Cambrian (Protohertzina anabarica Zone) Soltanieh Formation, Iran, which are described and analysed here in detail for the first time. Extensive archaeocyathan sponge reefs emerge and radiate as late as the middle of the Fortunian Stage of the Cambrian and demonstrate a gradual assembly of their skeletal structure through this time coincident with the evolution of other metazoan groups. Since the Porifera are basal in the Metazoa, their presence within the late Proterozoic has been widely anticipated. Molecular clock calibration for the earliest Porifera and Metazoa should now be based on the Iranian hexactinellid material dated to c. 535 Ma. The earliest convincing fossil sponge remains appeared at around the time of the Precambrian-Cambrian boundary, associated with the great radiation events of that interval.
| Original language | English |
|---|---|
| Pages (from-to) | 972-1004 |
| Number of pages | 33 |
| Journal | Biological Reviews |
| Volume | 89 |
| Issue number | 4 |
| Early online date | 29 Apr 2014 |
| DOIs | |
| Publication status | Published - Nov 2014 |
Bibliographical note
We gratefully acknowledge the following funding bodies for their support: The Royal Commission for the Exhibition of 1851 Research Fellowship and a Palaeontology Association research grant (both to J.B.A.) as well as the support of the Oxford University Department of Zoology and the Department of Earth Sciences; NERC (grants to MDB and RHTC). Norman Charnley is acknowledged for his expertise and patience with SEM‐EDX analyses. Owen Green is acknowledged for his extensive support in the lab over many years. We would also like to thank Paul Taylor and Allison Daley at the NHM‐London for help with access to fossils. We would like to thank Derek Siveter for help with access to fossil material at the OUMNH. We would also like to thank Françoise Debrenne for her hospitality over many years and for her tutelage concerning the Archaeocyatha. Philip Donoghue and Nick Butterfield are thanked for constructive discussion concerning this work. Allison Daley read a late draft of the manuscript and provided critical feedback. Finally, we would like to thank Greg Edgecombe and Graham Budd for their very constructive reviews which helped us to greatly improve the manuscript.Keywords
- Precambrian
- Porifera
- fossil calibration
- sponges
- Ediacaran
- origin of animals
- macroevolution
- Cambrian explosion