The transition from planktotrophy to lecithotrophy in larvae of Lower Palaeozoic Rhynchonelliform brachiopods

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Abstract
Criteria are established for defining the presence of protegula formed on embryonic or larval mantle in representative genera of Lower Palaeozoic Obolellata, Strophomenata and Rhynchonellata. Width was used to define protegular type. Taxa with only an embryonic protegulum are inferred to have had lecithotrophic larvae while taxa with a larval protegulum or an embryonic protegulum surrounded by a larval protegulum are inferred to have had planktotrophic larvae. All or most of the taxa examined in the Obolellata, the Strophomenata and the orders Protorothida and Orthida in the Rhynchonellata had planktotrophic larvae. In the Pentamerida a minority of genera had only a larval, or an embryonic and a larval protegulum while a majority had protegular widths indicating lecithotrophy. In the orders Rhynchonellida, Atrypida, Athyrida and Spiriferida derived from the Pentamerida (with the exception of one species in the Atrypida) a number of the genera had protegular widths indicating lecithotrophy. It is suggested that the onset of lecithotrophy in the Pentamerida was associated with a developmental innovation in which the mantle lobe of the larva was reflected over the apical lobe during the process of metamorphosis. This evolutionary innovation probably occurred during the late Cambrian or early Ordovician and was subsequently inherited during the process of cladogenesis.