Earliest ontogeny of Early Palaeozoic Craniiformea: implications for brachiopod phylogeny

Abstract

Popov, L.E., Bassett, M.G., Holmer, L.E., Skovsted, C.B. & Zuykov, M.A. 2010: Earliest ontogeny of Early Palaeozoic Craniiformea: implications for brachiopod phylogeny. Lethaia, Vol. 43, pp. 323–333. Well preserved specimens of the Early Palaeozoic craniiform brachiopods Orthisocrania and Craniops retain clear evidence of a lecithotrophic larval stage, indicating the loss of planktotrophy early in their phylogeny. The size of the earliest mineralized dorsal shell was <100 μm across, and the well preserved shell structure in these fossil craniiforms allows their earliest ontogeny to be compared directly with that of living Novocrania, in which the first mineralized dorsal shell (metamorphic shell) is secreted only after settlement of the lecithotrophic larvae. Immediately outside this earliest shell (early post‐metamorphic or brephic shell) and in the rest of the dorsal valve the primary layer in both fossil and living craniiforms has characteristic radially arranged laths, which are invariably lacking in the earliest dorsal shell. The ventral valve of the fossil specimens commonly preserves traces of an early attachment scar (cicatrix), which is equal in size to the dorsal metamorphic shell, and the brephic post‐metamorphic ventral valve also has a primary shell with radially arranged laths. However, a primary shell with radial laths is completely lacking in the ventral valve of living Novocrania, indicating that heterochrony may have been involved in the origin of the encrusting mode of life in living craniids; the entire ventral valve of Recent craniids (with the possible exception of Neoancistrocrania) may correspond to the earliest attachment scar of some fossil taxa such as Orthisocrania. It is also probable that the unique absence of an inner mantle lobe as well as the absence of lobate cells in Novocrania could be the result of heterochronic changes. The dorsal valve of both fossil and living craniiforms has a marked outer growth ring, around 500 μm across, marking the transition to the adult, and a significant change in regime of shell secretion. The earliest craniiform attachment is considered to be homologous to the unique attachment structures described recently in polytoechioids (e.g. Antigonambonites) and other members of the strophomenate clade. However, unlike the craniiforms, polytoechioids and strophomenates all have planktotrophic larvae, and planktotrophy is most probably a plesiomorphic character for all Brachiopoda. □Brachiopoda, Craniiformea, Early Palaeozoic, ontogeny, phylogeny.

Post‐metamorphic allometry in the earliest acrotretoid brachiopods from the lower Cambrian (Series 2) of South China, and its implications

“…data), but following Popov et al . (, ) and Bassett & Popov (), the presence of these structures indicates that the previously termed acrotretoid ‘larval shell’ actually formed at the end of the acrotretoid metamorphosis, which involved the shedding of the larval setae (Lüter ), associated with a fundamental change in shell secretion and the development of adult marginal setae; this area is delineated by an interruption in growth marked by the halo, and we here suggest that the term ‘larval shell’ should be replaced by ‘metamorphic shell’ (Popov et al . , ; Ghobadi Pour et al .…”

Setatella significans, a new name for mickwitziid stem group brachiopods from the lower Cambrian of Greenland and Labrador

“…Subsequently, evidence for shell penetrating setae has been found in a wide range of brachiopods from Cambrian and Ordovician strata (Balthasar 2004;Holmer & Caron 2006;Holmer et al 2008a;Jin et al 2008;Popov et al 2009) and the structure of recently discovered articulated tommotiid scleritomes has further strengthened the tommotiid-brachiopod link (Skovsted et al 2008(Skovsted et al , 2009aHolmer et al 2008b;). The original specimens from North-East Greenland were referred to the genus Mickwitzia Schmidt, 1888 by Holmer et al (2002) and were considered close to Mickwitzia occidens Walcott, 1908 from the Lower Cambrian of Nevada by Skovsted & Holmer (2003).…”

The operculum and mode of life of the lower Cambrian hyolith Cupitheca from South Australia and North China

“…Cupitheca holocyclata is reminiscent of apical attachment scars (cicatrix) in ventral valves of 343 palaeozoic craniiform brachiopods (Popov et al 2010) and thecideidine brachiopods (Baker 1983) as 344 well as the attachment scar formed by the pedicle tube of polytechoid strophomenates (Popov et al 345 2007). The craniiform cicatrix, the thecideidine apical attachment and the polytechoid pedicle tube 346 all constitute scars, formed at settlement of the planktic brachiopod larvae and represents the 347 physical attachment of the ventral valve to the substrate (Baker 1983;Popov et al 2007Popov et al , 2010.…”

“…The craniiform cicatrix, the thecideidine apical attachment and the polytechoid pedicle tube 346 all constitute scars, formed at settlement of the planktic brachiopod larvae and represents the 347 physical attachment of the ventral valve to the substrate (Baker 1983;Popov et al 2007Popov et al , 2010. 348…”