The Origin of the Brachiopod Bodyplan  

Brachiopods are bivalved, marine suspension feeders that dominated benthic communities in the Palaeozoic, and have remained moderately diverse into the present.  The Brachiopoda are the oldest of all modern phyla; their bivalved body plan evolved in the earliest Cambrian, and the brachiopod crown group was established, at the latest, in the second stage of the period (Adtabanian). The origin and early evolution of the phylum have long been shrouded in mystery.  The most popular theory, the ‘Brachiopod Fold Hypothesis’, contends that the bivalved condition arose through ventral folding of an elongated animal with two dorsal shells.  This hypothesis gained support with the description of Halkieria evangelista, which exhibits two brachiopod-like shells in addition to numerous spine-like calcareous sclerites.  However, new fossil discoveries have recently demonstrated that the bivalved condition of brachiopods evolved through a stepwise reduction of a multi-component tubular skeleton. 

The likely ancestors of brachiopods, tommotiids, are one of the most conspicuous organophosphatic components of the Cambrian ‘small shelly fossils’. Their small cone or cap-shaped sclerites are among the first skeletal fossils to appear in the lowermost Cambrian.  Until recently, tommotiids have only been known from disarticulated material and the gross morphology of the animal; the structure of the skeleton and the phylogenetic position has been subject to speculation.  Tommotiids were usually reconstructed as slug-like animals with a dorsal cover of imbricated sclerites.  The first articulated tommotiids, recently found in South Australia, exhibit a radically different body plan. 

The scleritome of Eccentrotheca is a slowly expanding cone with circular cross section, composed of a multitude of irregular cap shaped sclerites arranged in vertically stacked rings.  The tubiform skeleton displays wide variation in form, but was attached to hard substrates via organic structures at the perforated apex.   The animal was likely a sessile filter feeder.  A second tommotiid scleritome, Paterimitra, exhibit a modified version of the tubular model, with a multitude of Eccentrotheca-like sclerites surrounding two unequal, bilaterally symmetrical sclerites which define a tubular ‘pedicle’ opening. 

The proposed link between brachiopods and tommotiids was originally based on similarities in shell composition and microstructure, in particular the shared presence of shell penetrating setae in the Cambrian brachiopod Mickwitzia and the tommotiid Micrina. We now also know that tommotiids, like most brachiopods, were sessile filter feeders, and that they attached to the substrate via a pedicle-like organic structure.  We can identify how the two opposing shells of brachiopods evolved from cap-shaped sclerites through a series of intermediate steps.  The irregular basal sclerites of Eccentrotheca gave rise to the symmetrical sclerites of Paterimitra through numerical reduction and specialisation of function of sclerites in the basal sclerite ring.  Reduction in tube height through loss of sclerite rings led to a simplified scleritome with 3 or 4 sclerites (Tannuolina) and paired fusion of the remaining sclerites resulted in an effectively bivalved scleritome (Micrina).