Earliest ontogeny of Lower Palaeozoic brachiopods

Staff: M.G. Bassett & L.E. Popov

Introduction

Understanding of the early life history of fossil and recent brachiopods continues to be of great importance in phylogenetic reconstructions. However, the early ontogeny or development of many Early Palaeozoic rhynchonelliform brachiopods remains poorly understood. In particular, the ontogeny of strophomenates requires further study, because newly obtained data on the ontogeny of the Order Billingsellida, which is commonly considered to represent the ancestral stock of all strophomenates, suggests that they are fundamentally different from the Recent rhynchonellates. Not only did they have a planktotrophic larvae (at least in some taxa), but most importantly they exhibit a different sequence of mantle and shell formation, as well as adult attachment. The ontogeny of the Billingsellidawas (with the exception of having a planktotrophic larva) probably closely comparable to that described in recent Novocrania. It is possible that the sequence of changes from the larval attachment by the posterior part of the body to an adult attachment by the ventral side of the body may represent plesiomorphic characters inherited by both groups from the Chileata. Study of the rhynchonelliformean and craniiformean brachiopod brachiopod ontogeny and in particular time and characters of transition from planktotrophic to leicitotrophic larvae in various brachiopods has also important implication in understanding of the early Palaeozoic brachiopod biogeography.

The main difficulty in any study of the earliest ontogeny of fossil brachiopods is in discriminating between the embryonic, larval and post metamorphic shell. In linguliform brachiopods these growth stages are clearly delineated by discontinuities in shell growth, and frequently also by different textures of the larval and post metamorphic shell. There are, however, limitations in application of this method to the calcareous-shelled craniiformean and rhynchonelliform brachiopods defined by the different fossilisation potential of shells with different original chemical compositions. In spite of secondary alterations, it is occasionally possible to observe distinctive changes in the arrangement of secondary shell fibers in the umbonal area. In well preserved specimens there is an area, where the outer surface of the secondary layer is formed by mosaic of irregular tablets; this area usually forms a distinct halo. The secondary fibers outside the halo are radially arranged, suggesting that they were secreted by accretionary growth. The most probable explanation for that the secondary shell inside the halo was secreted simultaneously across the whole surface of the outer epithelium, whilst the shell outside the halo marks the onset of the secretion of primary and secondary shell layers. These creterea have been applied already to the study of early ontogeny of clitanbonitides billingsellides (Popov et al. 2007) siphonotretides (Popov et al. 2009, in press) and enigmatic Salanygolina, which is probably closely related to chileides (Holmer et al. 2009). These studies demonstrated for the first time that the development pattern and probably the larval body plan observed in the Recent Novocrania was spread far beyond recently recognised Subphylum Craniformea.

References

Popov, L. E., Egerquist, E., and Holmer, L. E. 2007. Earliest ontogeny of Middle Ordovician rhynchonelliform brachiopods (Clitambonitoidea and Polytoechioidea): implications for brachiopod phylogeny. Lethaia, 40, 85-96.

Holmer, L.E., Pettersson Stolk, S. Skovsted, C.B., Balthasar, U. and Popov, L.E. 2009. The enigmatic Early Cambrian Salanygolina – a stem group of rhynchonelliform chileate brachiopods? Palaeontology, 52, 1-10.