Retropinnidae

Retropinnidae View in CoL View at ENA ( Fig. 8 View Figure 8 )

In Retropinna tasmanica the diastema is narrow, but the edges of hypural 2 and 3 are rounded, thus forming an indentation in the otherwise almost continuous posterior margin of the hypural plates. Posterior to the hypurals, a single CMC is present; its dorsoventral extension ranges between 50 to 100% of the height of hypural 2 (in lateral view). The CMC is rounded to slightly triangular, pointing with its tip to the diastema ( Fig. 8A View Figure 8 ). It seems of minor importance for the support of fin rays; there may be some interaction with the uppermost ray of the lower lobe. A similar situation is present in a 28.0 mm SL specimen of R. semoni : here the cartilage is more elongated in the dorsoventral direction, thus having a more pronounced triangular shape and the position is more anterior with about half of its length anterior to the posterior margin of the hypurals. It almost reaches the level of the ossified parts of the hypural plates ( Fig. 8B View Figure 8 ). The dorsal and ventral facets of the triangular CMC exhibit emarginations into which the diastemal-directed edges of hypural 2 und 3 articulate. Its dorsal and ventral tips seem to be involved in the support of one fin ray each ( Fig. 8B View Figure 8 ). The situation becomes more complicated with a larger specimen, i.e. 40.3 mm SL, of the same species in which two separate CMCs are present in about the same position as the single cartilage in the smaller specimen ( Fig. 8C View Figure 8 ). Each cartilage is closely associated to one fin ray. Their facets directed to the hypurals are slightly concave enabling articulation. Independent of amount, size and exact position of the cartilages, their width in the horizontal plane is about the same or even slightly larger than that of the hypural plates. None of the caudal fin rays show medially directed articulatory pegs.