Giving the early fossil record of sponges a squeeze

Twenty candidate fossils with claim to be the oldest representative of the P hylum P orifera 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 P recambrian fossil candidate yet satisfies all three of these criteria to be a reliable sponge fossil. The oldest widely accepted candidate, M ongolian silica hexacts from c . 545 million years ago ( M a), are here shown to be cruciform arsenopyrite crystals. The oldest reliable sponge remains are siliceous spicules from the basal C ambrian ( Protohertzina anabarica Z one) S oltanieh F ormation, I ran, 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 F ortunian S tage of the C ambrian and demonstrate a gradual assembly of their skeletal structure through this time coincident with the evolution of other metazoan groups. Since the P orifera are basal in the M etazoa, their presence within the late P roterozoic has been widely anticipated. Molecular clock calibration for the earliest P orifera and M etazoa should now be based on the I ranian hexactinellid material dated to c . 535 Ma. The earliest convincing fossil sponge remains appeared at around the time of the P recambrian‐ C ambrian boundary, associated with the great radiation events of that interval.