The Cambrian cirratuliform Iotuba denotes an early annelid radiation

Abstract

The principal animal lineages (phyla) diverged in the Cambrian, but most diversity at lower taxonomic ranks arose more gradually over the subsequent 500 Myr. Annelid worms seem to exemplify this pattern, based on molecular analyses and the fossil record: Cambrian Burgess Shale-type deposits host a single, early-diverging crown-group annelid alongside a morphologically and taxonomically conservative stem group; the polychaete sub-classes diverge in the Ordovician; and many orders and families are first documented in Carboniferous Lagerstätten. Fifteen new fossils of the ‘phoronid’ Iotuba (=Eophoronis) chengjiangensis from the early Cambrian Chengjiang Lagerstätte challenge this picture. A chaetal cephalic cage surrounds a retractile head with branchial plates, affiliating Iotuba with the derived polychaete families ‘Flabelligeridae’ and Acrocirridae. Unless this similarity represents profound convergent evolution, this relationship would pull back the origin of the nested crown groups of Cirratuliformia, Sedentaria and Pleistoannelida by tens of millions of years—indicating a dramatic unseen origin of modern annelid diversity in the heat of the Cambrian ‘explosion’.

First record of growth patterns in a Cambrian annelid

“…Annelids are a morphologically and ecologically diverse group that includes terrestrial species, such as earthworms and leeches, aquatic polychaetes and abyssal tubeworms. Annelids evolved during the Cambrian Explosion, and their earliest diversity is preserved in several Burgess Shale-type deposits [1][2][3][4][5][6][7][8][9]. Thirteen Cambrian polychaete species have been described so far from these deposits, six of them from the Burgess Shale alone [1][2][3].…”

“…Thirteen Cambrian polychaete species have been described so far from these deposits, six of them from the Burgess Shale alone [1][2][3]. Phylogenetically, most of the Cambrian species, except two recently described from China [4,5], were recovered as stem-group annelids [3,4,6,7,10]. The body plan of the ancestral annelid has also been discussed in phylogenomic studies, which are largely in agreement that it was a segmented polychaete-like worm with prominent parapodia and chaetae and a head with sensory organs [11][12][13].…”

New fossil of Gaoloufangchaeta advances the origin of Errantia (Annelida) to the early Cambrian

“…The placement of Guanshanchaeta in Phyllodocida importantly extends the fossil records of Phyllodocida and Errantia back to the early Cambrian during which representatives of other clades of annelids also appeared [ 14 , 18 ], thus providing further indirect support to the Ediacaran hypothesis for the origin of Annelida [ 4 , 5 ]. Even though the annelid affinity of Iotuba [ 19 ] is highly questionable, we would expect to find fossils representing Sedentaria from the exceptionally preserved biotas from the early Cambrian. Pelagic Gaoloufangchaeta , together with previously reported epibenthic stem-group annelids [ 7 – 10 ] and endobenthic palaeoannelids [ 14 ] from the early Cambrian, reveal the diversification of not only the morphology but also the lifestyle of annelids in the ‘Cambrian Explosion’.…”

“…The earliest robust evidence of crown annelids comes from the records of early Cambrian representatives of Palaeoannelida [ 14 ] and Sipuncula [ 18 ], both representing basally branching groups of Annelida. Iotuba from the early Cambrian Chengjiang Biota which has recently been interpreted as a taxon within Sedentaria [ 19 ] suspiciously lacks typical characteristics of annelids, such as segments, palps and chaetae, leaving its annelid affinity highly questionable. There has been no fossil record of the diversified Errantia until the Cambrian–Ordovician transition, as evidenced by the appearance of scolecodonts, which are interpreted as the jaws of Eunicida, Errantia [ 20 ].…”