The scleritome ofPaterimitra: an Early Cambrian stem group brachiopod from South Australia

Abstract

Early Cambrian tommotiids are problematic fossil metazoans with external organophosphatic sclerites that have been considered to be basal members of the lophophorate stem group. Tommotiids are almost exclusively known from isolated or rarely fused individual sclerites, which made previous reconstructions of the actual organism highly conjectural. However, the recent discovery of the first articulated specimens of the tommotiid Eccentrotheca revealed a tubular sclerite arrangement (scleritome) that limited the possible life habit to sessile filter feeding and thus further supported a lophophorate affinity. Here, we report the first articulated specimens of a second tommotiid taxon, Paterimitra from the Early Cambrian of the Arrowie Basin, South Australia. Articulated specimens of Paterimitra are composed of two bilaterally symmetrical sclerite types and an unresolved number of small, asymmetrical and irregular crescent-shaped sclerites that attached to the anterior margin of the symmetrical sclerites. Together, the sclerites form an open cone in which the symmetrical sclerites are joined together and form a small posterior opening near the base of the scleritome, while the irregular crescent-shaped sclerites defined a broad anterior opening. The coniform scleritome of Paterimitra is interpreted to have attached to hard substrates via a pedicle that emerged through the small posterior opening (sometimes forming a tube) and was probably a sessile filter feeder. The scleritome of Paterimitra can be derived from the tubular scleritome of Eccentrotheca by modification of basal sclerites and reduction in tube height, and probably represents a more derived member of the brachiopod stem group with the paired symmetrical sclerites possibly homologous to brachiopod valves.

Chronology of early Cambrian biomineralization

“…These fossils have been interpreted as sclerites of larger scleritomes of problematic animals (Bengtson, 1970(Bengtson, , 2004Holmer et al 2008;Skovsted et al 2008Skovsted et al , 2009b. The taxonomy of tommotiids was discussed by Landing (1984Landing ( , 1995, Bengtson (1986), Laurie (1986), Bengtson et al (1990), Conway Morris & Chen (1990), Esakova & Zhegallo (1996) and Skovsted et al (2009a).…”

“…Based on the organophosphatic shell composition and morphological similarities, tommotiids have been regarded as closely related to brachiopods (Williams & Holmer, 2002). Recent discoveries of articulated tommotiids show distinct affinities to lophophorates, including a sessile habit and brachiopod-like shells within the scleritome (Skovsted et al 2008(Skovsted et al , 2009bHolmer et al 2008).…”

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

“…The discovery of the scleritome of Paterimitra (Skovsted et al 2009a) indicate that a second lineage of tommotiids exhibiting brachiopod-like sclerites but lacking setigerous tubes, evolved simultaneously with the tannuolinid-linguliform lineage. The morphology, micro ornament and shell structure of Paterimitra is very similar to that of Askepasma, a paterinid brachiopod from the Lower Cambrian of South Australia ).…”

“…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).…”

Depth related brachiopod faunas from the lower Cambrian Forteau Formation of southern Labrador and western Newfoundland, Canada

“…The first representatives appear in the Tommotian Stage of Siberia and Mongolia (Terrenuvian Series, Unnamed Stage 2; Skovsted et al, 2014bSkovsted et al, , 2015 and the brachiopod stem group (i.e. tommotiids; Skovsted et al, 2008Skovsted et al, , 2009Skovsted et al, , 2011Skovsted et al, , 2014aLarsson et al, 2014) is known from even older strata of the Fortunian Stage (Maloof et al, 2010;Kouchinsky et al, 2012). Brachiopods diversified rapidly during the early Cambrian and most of the main Palaeozoic lineages (Class) had appeared by the middle Cambrian (Cambrian Series 3) by which time brachiopods were well represented in most benthic communities worldwide (Ushatinskaya, 2001).…”