Evaluating scenarios for the evolutionary assembly of the brachiopod body plan

Abstract

SUMMARY The fossil faunas of the Cambrian provide the only direct insight into the assembly of animal body plans. However, for many animal groups, their early fossil record is linked to disarticulated remains, interpretation of which is problematic since they possess few characters from which their affinity to phyla can be established and, indeed, few characters at all. One such group is the tommotiids, which has been interpreted, on the basis of skeletal anatomy, as a paraphyletic assemblage uniting brachiopods and phoronids, through the acquisition and subsequent modification, or loss, of an imbricated set of dorsal phosphatic sclerites. Here we present a reexamination of the fossil evidence uniting the tommotiids and brachiopods, supplemented with new anatomical data from synchrotron radiation X-ray tomographic microscopy of key tommotiid taxa. The characters used to support the complex hypothesis of character evolution in the brachiopod stem lineage relies on scleritome reconstructions and inferred mode of life which themselves rely on brachiopods being chosen as the interpretative model. We advocate a more conservative interpretation of the affinity of these fossils, based a priori on their intrinsic properties, rather than the modern analogue in whose light they have been interpreted.

A silicified tommotiid from the lower Cambrian of Greenland

“…This observation, and recently presented evidence that not all tommotiids may have had tubular scleritomes (Skovsted et al 2015a), strengthens suggestions that the evolution of the lophophorate stem group was more complex than previously imagined (Murdock et al 2014, Zhang et al 2014. The new material also opens up new avenues for future research into the distribution, structure and affinity of Small Shelly Fossils.…”

“…Based on recently discovered tommotiids from South Australia (Skovsted et al 2008(Skovsted et al , 2009a(Skovsted et al , b, 2011(Skovsted et al , 2015bHolmer et al 2008;Balthasar et al 2009;Larsson et al 2014), tommotiids have been interpreted to be basal members of the stem group leading to the lophophorate phyla Brachiopoda and Phoronida. This interpretation was based both on the morphology of their tubular scleritome structure (Skovsted et al 2008(Skovsted et al , 2009b(Skovsted et al , 2011Holmer et al 2008) and the remarkable similarities in ultrastructure between tommotiids and the earliest brachiopods (Balthasar et al 2009;Murdock et al 2012Murdock et al , 2014Larsson et al 2014).…”

The Evolution of Brachiopoda

“…Carlson, in preparation). Later studies (Murdock et al 2012(Murdock et al , 2014 recommended that a robust phylogenetic analysis was needed to test, with data, the various stem group hypotheses. Coupled with the establishment of phylogenetic definitions of Brachiopoda and Pan-Brachiopoda (Carlson & Cohen 2016) (Figure 2), which define the brachiopod crown and total clades, more such analyses will enable the many stem group hypotheses to be tested rigorously.…”

Brachiopoda

“…Paterinid shell ultrastructure differed from all other crown fossil brachiopods but shared some characteristics with the organophosphatic sclerites of the extinct metazoans known as tommotiids ). Whether or not particular tommotiid morphotypes that may have been sessile fi lter feeders (such as Eccentrotheca ; see Skovsted et al 2011 ) represent the stem lineage for crown phoronids, paterinids, and crown linguliform brachiopods remains unclear (Murdock et al 2014 ). Molecular estimates place the origin of the crown brachiopods in the Middle Cambrian, but the phoronidbrachiopod root is older, possibly originating as far back as the Ediacaran (Sperling et al 2011 ).…”