Early ontogeny of the Trigoniida

The order Trigoniida is a major clade in the Bivalvia, dating back to the Silurian and surviving with a single relic genus, Neotrigonia, in Australian waters today. During the Mesozoic, the Trigoniida were truly cosmopolitan, highly diverse and abundant. Over 90% of the 2500 species-level and 300 genus-level names assigned to Trigoniida refer to Mesozoic taxa. Considering their remarkable fossil record, surprisingly little is known with regard to the early ontogeny of the Trigoniida. To date, the prodissoconch and early dissoconch are solely documented for a single species of Neotrigonia. We present well-preserved larval and post-larval growth stages of seven species of Mesozoic and extant Trigoniida, including representatives from Middle Jurassic (Bajocian-Bathonian), Early (Albian) and Late Cretaceous (Santonian-Campanian) strata in Poland, Madagascar and Antarctica, respectively. The specimens are assigned to five genera (Trigonia, Nototrigonia, Neotrigonia, Myophorella, Oistotrigonia) in three families (Trigoniidae, Myophorellidae, Pterotrigoniidae) and currently three superfamilies (Trigonioidea, Myophorelloidea, Megatrigonioidea). All species are characterised by rather simple, D-shaped prodissoconchs, which are variable with regard to size, but generally relatively large, and often lack a distinct P-2 stage. This, together with their moderately convex, oval shape, suggests a dominantly lecitotrophic development. Late Cretaceous Nototrigonia oliveiroi differs from all other taxa in its prolonged, morphologically distinct nepioconch stage. Early post-larval shells of Mid Jurassic Trigonia triangularis are wing-shaped in outline, and thus radically different from adults. All taxa exhibit a brief prosogyrate larval to post-larval growth stage. Transition to opisthogyrate growth likely concurs with the development of a parivincular ligament. Allometric growth is common, with adult shells being more elongate than juveniles. The onset of post-larval shell ornament is probably most revealing with regard to phylogeny. Morphologies within the Trigonioidea (Trigoniidae: Trigonia, Nototrigonia, Neotrigonia) seem more disparate than between Myophorelloidea (Myophorellidae: Myophorella) and Megatrigonioidea (Pterotrigoniidae: Oistotrigonia). However, broader taxon sampling, particularly within Megatrigonioidea, is needed to decide whether the similarity of juveniles in the latter two genera would suggest that Pterotrigoniidae should be transferred to Myophorelloidea. Moreover, the early ontogeny of Palaeozoic and Triassic Trigoniida is undocumented. Consequently, the plesiomorphic conditions of the prodissoconch, nepioconch and early dissoconch are unestablished.