Argentinidae (Argentiniformes)

Argentinidae (Argentiniformes) View in CoL View at ENA ( Fig. 5 View Figure 5 )

In Argentina silus there are two CMCs showing a clear difference in size: the lower is about double to triple the size of the upper one ( Fig. 5A–D View Figure 5 ). Together they support one fin ray of the dorsal lobe positioned in between both cartilages, plus a fin ray of the lower lobe in direct contact with the ventral part of the lower CMC. Both fin rays have dorsoventrally widened bases in comparison to the neighbouring rays. The fin rays contacting the upper or lower cartilage both show prominent medially directed pegs, but these pegs are more pronounced in the ray of the upper lobe ( Fig. 5E View Figure 5 ). The pegs of this fin ray originate on the dorsal margin of the hemitrichs and point to the area between the upper CMC and the cartilaginous distal part of hypural 3. This lowest fin ray of the upper lobe shows tighter connections to the upper cartilage. The cartilage is roughly rectangular in lateral view, almost round posteriorly and dorsally it becomes visible that the cartilage is convex on its distal part and concave on its proximal facet, allowing articulation on the lower corner of hypural 3 ( Fig. 5C View Figure 5 ). When seen posteriorly, this upper cartilage has the same width as the hypural plates. The bases of the associated fin ray do not contact this cartilage directly: the distance between these bases is larger than the width of the cartilage. However, both cartilages are embedded in stable connective tissue. The lower cartilage is clearly wider than the hypural plates and it makes direct contact with the bases of its associated fin ray. The lower CMC is somewhat triangular to rectangular when seen laterally, dorsoventrally flattened ovoid when seen posteriorly and dorsally somewhat boomerang-shaped ( Fig. 5D View Figure 5 ). This allows a stable articulation on the upper edge of hypural 2. In lateral reflected light, a strong connective tissue, maybe a ligament, becomes visible, stretching from the upper tip of the lower CMC to the upper CMC. This ‘ligament’ is about as wide as the upper CMC. In the other argentinid species studied, A. sphyraena , A. kagoshimae and Glossanodon semifasciatus , the situation for the CMCs and their associated fin rays seems to equal that of A. silus ( Fig. 5F, G View Figure 5 ). In A. kagoshimae there is a pronounced inward-directed peg at the base of the upper fin ray of the lower lobe ( Fig. 5F, G View Figure 5 ). In Glossandon semifasciatus ( Fig. 5H View Figure 5 ) the two CMCs are larger, filling a significant part of the diastema. Together they support the lowest fin ray of the upper lobe. The upper fin ray of the lower lobe is placed at the contact zone of the lower CMC and hypural 2. The upper CMC is roughly boomerang-shaped in dorsal view, with blunt tips facing anteriorly; it is about 1.5 times as wide as the hypurals in posterior view. The lower CMC resembles a bar, rectangular when seen posteriorly and almost square when viewed laterally, but with tips in the anterioventral corner and an articulatory groove for the upper margin of hypural 2 in between the tips of both sides. It is slightly wider than the upper CMC in posterior view. The articulatory pegs of the lower ray of the upper lobe are quite large, extending medially in a right angle and contacting the posterior border of the upper CMC.