The Colleplax – a Homologous Structure in the Ontogeny of Early Cambrian Stem Rhynchonelliform Brachiopods  

The ontogeny of fossil and recent brachiopods has been of great importance in deciphering phylogenetic relationships of the clade.  The resolution within the clade could be improved with a better understanding of the ontogeny of Early Cambrian brachiopods.  One poorly-understood Early Cambrian group is the order Chileida, which has been considered to belong within the calcareous-shelled crown group Rhynchonelliformea, in the class Chileata.  The chileids are amongst the earliest known brachiopods with a calcareous and strophic shell, but as they are known mostly from rather coarsely silicified shells, their early life history has been largely unknown.  A unique find of well-preserved phosphatized material of a chileid from the Flinders Ranges, South Australia, shows that their early ontogeny closely parallels that of the unusual Early Cambrian paterinate-like Salanygolina from Mongolia.  In both Salanygolina and the new chileid, the anterior margin of the well defined ventral larval shell is indented by an unrestricted notch that, through later ontogeny, develops into an umbonal perforation, directly anterior to the umbo.  In subsequent ontogenetic development, this sub-triangular perforation is enlarged by resorption and covered posteriorly by the colleplax – a triangular plate – in the umbonal perforation.  The colleplax structure was first described from the equally enigmatic Palaeozoic order Dictyonellida (Rhynchonelliformea, Chileata); the colleplax in Salanygolina is considered to be homologous with that of the chileates.  As proposed in the original description, the foramen and colleplax clearly represent integral parts of an attachment structure that developed early in the ontogeny and was retained in the adult.  This type of colleplax holdfast cannot be considered homologous with the pedicle of the Cambrian paterinids, such as Micromitra, which are famously found still-attached to sponges in the Burgess Shale.  However, it is possible that the earliest larval attachment of the chileids and Salanygolina can be compared with the paterinid pedicle, emerging between the valves; we infer that the first larval pedicle was thus homologous to the adult pedicle of paterinids. 

Uniquely preserved scleritomes of early Cambrian tommotiids from South Australia show that they represent sessile stem brachiopods.  Study of the early ontogeny of the derived tommotiid Paterimitra shows that its early life history closely parallels that of the brachiopods; it has a brachiopod-like bivalved larval shell, which is also provided with a colleplax structure, here considered to be homologous with that of Salanygolina and the chileids.  However, in Paterimitra the colleplax-structure does clearly not function as an adult holdfast.  We propose that Salanygolina and the chileids, as well as the tommotiid Paterimitra, belong to the stem of the crown group Rhynchonelliformea.