|Calvapilosa kroegeri |
Vinther, Parry, Briggs & Van Roy, 2017
Exceptionally preserved fossils provide crucial insights into extinct body plans and organismal evolution. Molluscs, one of the most disparate animal phyla, radiated rapidly during the early Cambrian period (approximately 535–520 million years ago (Ma))2. The problematic fossil taxa Halkieria and Orthrozanclus (grouped in Sachitida) have been assigned variously to stem-group annelids, brachiopods, stem-group molluscs or stem-group aculiferans (Polyplacophora and Aplacophora), but their affinities have remained controversial owing to a lack of preserved diagnostic characters. Here we describe a new early sachitid, Calvapilosa kroegeri gen. et sp. nov. from the Fezouata biota of Morocco, (Early Ordovician epoch, around 478 Ma). The new taxon is characterized by the presence of a single large anterior shell plate and polystichous radula bearing a median tooth and several lateral and uncinal teeth in more than 125 rows. Its flattened body is covered by hollow spinose sclerites, and a smooth, ventral girdle flanks an extensive mantle cavity. Phylogenetic analyses resolve C. kroegeri as a stem-group aculiferan together with other single-plated forms such as Maikhanella (Siphogonuchites) and Orthrozanclus; Halkieria is recovered closer to the aculiferan crown. These genera document the stepwise evolution of the aculiferan body plan from forms with a single, almost conchiferan-like shell through two-plated taxa such as Halkieria, to the eight-plated crown-group aculiferans. C. kroegeri therefore provides key evidence concerning the long debate about the crown molluscan affinities of sachitids. This new discovery strongly suggests that the possession of only a single calcareous shell plate and the presence of unmineralised sclerites are plesiomorphic (an ancestral trait) for the molluscan crown.
Mollusca Linnaeus, 1758
Sachitida He, 1980
(This clade includes Aculifera plus the stem taxa Halkieriidae, Orthrozanclus, Siphogonuchitidae)
Siphogonuchitidae Qian, 1977
(This clade contains Calvapilosa, Siphogonuchites, Dabashanites, Drepanochites, Lomasulcachites, Lopochites and Quadrosiphogonuchites)
Calvapilosa kroegeri gen. et sp. nov.
Etymology. The generic name derives from the Latin calva, scalp and pilosus, hairy/shaggy for the head plate covered by densely spiculated mantle. Species named after Björn Kröger for ‘discovering’ the holotype in the collections at Yale.
Holotype. Yale Peabody Museum (YPM) 237255
|Figure 1: Calvapilosa kroegeri, holotype YPM 237255, from the Lower Ordovician epoch (Tremadocian age) Fezouata formation, near Zagora, Morocco.|
Locality and horizon. Calvapilosa can be found throughout the classical collecting area of the Fezouata biota to the north of Zagora. The holotype was collected in the Bou Zorgan area. The other specimens were found at additional excavations in this area, near Bni Zoli, in the Bou Glf and Tamagroute areas and on the western flank of Jbel Tigzigzaouine. All localities belong to the Fezouata Formation, Araneograptus murrayi graptolite biozone, upper Tremadocian stage (Lower Ordovician series). Detailed locality data are curated with the specimens.
|Figure 3: Reconstruction of Calvapilosa kroegeri, juvenile.|
a, Dorsal view. b, Ventral view. c, Lateral view. Colour scheme is speculative, whereas soft tissues are inferred by phylogenetic bracketing.
Resin model constructed by E. Horn (http://www.10tons.dk/).
|Figure 4: Time tree of aculiferan evolution based on a Bayesian total evidence analysis.|
J. Vinther, L. Parry, D. Briggs and P. Van Roy. 2017. Ancestral Morphology of Crown-group Molluscs revealed by A New Ordovician Stem Aculiferan. Nature. DOI: 10.1038/nature21055
Spiny, armoured slug reveals ancestry of molluscs Bristol.ac.uk/news/2017/february/mollusc-evolution.html