The Early Cambrian tommotiidMicrina, a sessile bivalved stem group brachiopod

Abstract

The tannuolinid Micrina belongs to the tommotiids-a common and widely distributed, but poorly understood, group of Early Cambrian fossil metazoans with multiple external organophosphatic sclerites. Recent findings of sessile articulated tommotiid scleritomes indicate that previous reconstructions of tommotiids as sluglike bilaterians with a dorsal cover of sclerites require detailed re-evaluation. Comparative ultrastructural work has already indicated that the tommotiids might be a sister group to the Brachiopoda, with Micrina representing the most derived and brachiopod-like bimembrate tommotiid. Here we further develop and strengthen this controversial phylogenetic model with a new reconstruction of Micrina, where the two types of sclerites-mitral and sellatebelong to a near bilaterally symmetrical bivalved sessile organism. This new scleritome configuration was tested by recreating an articulated bivalved Micrina from isolated mitral and sellate sclerites; both sclerites have muscles that would have enabled movement of the sclerites. The mitral and sellate sclerites of Micrina are considered to be homologous with the ventral and dorsal valves, respectively, of organophosphatic linguliform brachiopods, indicating that a simple type of filter-feeding within an enclosed bivalved shell had started to evolve in derived tannuolinids. The new reconstruction also indicates that the phylogenetic range of 'bivalved', sessile lophophorates is larger than previously suspected.

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

“…The main reason for this is the presence of similar shell penetrating setae in tannuolinid tommotiids which also show some morphological similarities to brachiopods (Williams & Holmer 2002;Holmer et al 2008b). The discovery that shell penetrating setae are more widespread among Cambro-Ordovician brachiopods than previously thought illustrates the importance of this feature in the early evolution of the phylum (Jin et al 2008;Holmer et al 2008a) and the differences between Mickwitzia and Setatella described herein should be interpreted in the light of these findings.…”

“…Micrina as a brachiopod-like bivalved shell (Holmer et al 2008b) suggests that the linguliform stem group should include a range of 'mickwitziid' taxa exhibiting shell penetrating setae. Although we can identify basal (i.e.…”

“…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).…”

Metamorphosis in Craniiformea revisited: Novocrania anomala shows delayed development of the ventral valve

“…The hypothesis that Halkieria evangelista belongs within the brachiopod stem, has since been questioned by several authors and other Lower Cambrian fossils (e.g. the tommotiid Micrina) with a different reconstructed body plan have been put forward as potential brachiopod stem groups (Holmer et al, 2008;Vinther and Nielsen, 2005;Williams and Holmer, 2002).…”

Trachyplax arctica, a New Multiplated Problematic Fossil from the Lower Cambrian of North Greenland

“…All of their five reconstructions involve antero−posterior rows (between 1 and 4 in number) of the two sclerites types in an elongate, bi− laterally symmetrical, slug−like animal. None of them bears any similarity to the reconstruction of Micrina as a bivalved, stem group brachiopod suggested by Holmer et al (2008) on the basis of articulated Eccentrotheca Landing, Nowlan, andFletcher, 1980 (Skovsted et al 2008) and Paterimitra Laurie, 1986 ).…”