Paterimitra pyramidalis from South Australia: scleritome, shell structure and evolution of a lower Cambrian stem group brachiopod

Abstract

Abstract:The tommotiid Paterimitra pyramidalis Laurie, 1986, is re-described based on well preserved material from the lower Cambrian Wilkawillina, Wirrapowie and Ajax limestones of the Flinders Ranges, South Australia. The material shows that the scleritome of Paterimitra pyramidalis includes three sclerite morphotypes (S1, S2 and L). Detailed shell microstructure studies shows striking similarities with both the paterinid brachiopod Askepasma toddense, as well as with the tommotiid Eccentrotheca helenia, which strengthens the suggested evolutionary link between tommotiids and brachiopods. Based on partly articulated specimens 2 and similarities in shell microstructure and sclerite morphology with Eccentrotheca, Paterimitra pyramidalis is reconstructed as a tube dwelling, epifaunal, sessile, filter feeder with an organic pedicle-like attachment structure. The proposed reconstruction of the scleritome comprises a basal unit composed of one S1 and one S2 sclerite, as well as an unresolved number of L sclerites lining a coniform tubular structure.

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).…”

The early Cambrian tommotiidKulparina rostratafrom South Australia

“…The sclerites of Kulparina rostrata show strong resemblance to sclerites of the recently re-described tommotiid Paterimitra pyramidalis (see Larsson et al, 2014). This is most obvious for sclerites originally described under the name K. rostrata.…”

“…Phoronida and Brachiopoda; Skovsted et al, 2008Skovsted et al, , 2011. Subsequently, the tannuolinid tommotiid Micrina etheridgei (Tate, 1892) has been shown to share many characters (shell morphology, ultrastructure and larval shell morphology) with Cambrian linguliform brachiopods , whilst a third tommotiid taxon, Paterimitra pyramidalis Laurie, 1986, appears to be closely allied with paterinid brachiopods (Skovsted et al, 2009a;Topper et al, 2013;Larsson et al, 2014), thus further strengthening the proposed tommotiid-lophophorate link. Laurie, 1986 exhibits three different sclerite types; a bilaterally symmetrical, pyramidal sclerite (S1), a triangular and bilaterally symmetrical (S2) sclerite, and laterally compressed asymmetrical (L) sclerites (Skovsted et al, 2009a;Larsson et al, 2014).…”

“…91) and is consequently incompatible with the reinterpreted bilaterally symmetrical sclerites of Kulparina. Due to the strong similarities with P. pyramidalis, it is more appropriate to apply the terminology developed for this tommotiid (see Skovsted et al, 2009a andLarsson et al, 2014) for Kulparina. This means that we refer to the symmetrical sclerites (K. rostrata sensu Bengtson et al, 1990) as S sclerites and the laterally compressed sclerites ('E.…”

“…The basal part of the scleritome consists of one S1 sclerite and one S2 sclerite defining a circular posterior opening, and an unresolved number of L sclerites lining the distal margin of the S1/S2-composite (Skovsted et al, 2009a;Larsson et al, 2014). The similarities between Paterimitra L sclerites and laterally compressed sclerites of Eccentrotheca suggest a similar arrangement of ring-like sclerites lining the distal margin of the Paterimitra scleritome (Skovsted et al, 2009a;Larsson et al, 2014). The similarities in mode of shell formation, scleritome construction and microstructure of Eccentrotheca and Paterimitra led Skovsted et al (2011) to informally establish a supergroup of tommotiid taxa referred to as the eccentrothecimorphs (a group that also includes Porcauricula, Sunnaginia, Micrina, Tannuolina and Kulparina) and are considered to represent a key stem group to the lophophorate phyla Brachiopoda and Phoronida (see Skovsted et al, 2011).…”