Acanthicus, Agassiz, 1829

Bogan, Sergio & Agnolin, Federico L., 2020, First fossil of the Giant Armored Catfish Acanthicus (Siluriformes, Loricariidae) from the Late Miocene of Entre Ríos Province, Argentina, Zootaxa 4808 (3), pp. 571-581 : 572-575

Acanthicus sp.

Referred material. MACN Pv-16105, partial skull table composed of the parieto-supraoccipital, and incomplete left and right sphenotics and frontals ( Figure 2 ); MACN Pv-13307, posterior half of a parieto-supraoccipital; MACN

Pv-16070, three incomplete compound pterotics, and right frontal; based on size congruence and differential preservation, it is very probable that these elements belong to different individuals ( Figure 3 ).

Description. The skull roof is relatively thick and dorsally flat, showing a depressed area between the contact of the parieto-supraoccipital and frontals. It is dorsally ornamented by strong annular bases for odontodes.

Parieto-supraoccipital bone is very elongate anteroposteriorly (its length representing more than two and a half times its maximum width). It is roughly subrectangular in contour, with its maximum transverse width located at the level of the articulation with the sphenotics. Lateral margins of the bone are nearly straight. Posteriorly, gradually decreases in transverse width. Posterior rear of the bone shows a rounded and almost smooth posterior process that is centrally located. The process is laterally delimited by shallow concavities for articulation with bony plates. The anterior margin of the bone shows the articulation for sphenotics and frontal. The parieto-supraoccipital shows an anterior process that is subtriangular in contour and extends as a wedge between the frontal bones.

The dorsal surface of the parieto-supraoccipital shows a particular ornamentation. From the center of the element radiate several grooves and lines of annular bases of odontodes that are roughly anteroposteriorly oriented. It shows anterior and posterior grooves and odontoid rows that result in a hourglass-shaped pattern, that is attenutaed towards the middle portion of the bone. Towards the lateral sides, the ornamentation is shallower and the rows of grooves and odontodes are more separate from each other ( Figures 2A,B and 3A ).

The sphenotics are poorly preserved and show eroded lateral margins. They articulate between the frontal and the parieto-supraoccipital anteriorly and forming a deep and concave notch that shows a homogeneously convex margin. The bone is relatively anteroposteriorly elongate and is dorsally convex.

The suture between the frontals and with the parieto-supraoccipital is somewhat curly. From the central portion of the element radiate several strong lines of annular bases of odontodes that orientate towards the posterior and medial margins of the bone.

A more complete frontal bone (MACN Pv-16070) is available. The frontal is roughly T -shaped and shows an orbital process that forms part of the orbital margin. The orbital process is well-developed and subrectangular in contour, with the anterior and posterior margins gradually decreasing their width near the orbital margin. The orbital surface is slightly concave and forms a smooth step. The ornamentation of the orbital process is represented by notable odontodes that are randonly distributed over the surface of the bone.

The posterior process shows a convex posterior margin and is laterally delimited by a smooth surface for articulation with the sphenotic. The compound pterotic is represented by three incomplete specimens (MACN Pv-16070). It is a relatively large element and in ventral view is very complex. It shows the ossified transcapular ligament and a free posterior margin with a rounded lateral margin. There is a large and posterolaterally located opening for the postotic canal.

It is dorsally covered by odontode bases and a large number of pits that become more evident towards the anteromedial margin of the bone resulting in a honeycomb aspect. This fenestrate condition was regarded by Chamon (2011) as typical for Acanthicus and Megalancistrus . Posteriorly, the bone shows ornamentation composed by strong crests with well-developed annular bases for odontodes.

Figures

FIGURE 2. Acanthicus sp. MACN Pv-16105, partial skull table composed of the parieto-supraoccipital, and incomplete sphenotic and frontal bones in A, B, dorsal, C, ventral, D, left lateral, and E, right lateral views. Scale bar equals 1 cm.

FIGURE 3. Acanthicus sp. MACN Pv-13307, posterior half of a parieto-supraoccipital bone in A dorsal, B ventral, and C anterior views; D-G, MACN Pv-16070, three incomplete compound pterotic bones D, indeterminate side, E,F, left side, and G, right side in D-E,G, dorsal, and F, ventral views; H-I, right frontal in H dorsal, and I ventral views. Scale bar equals 1 cm.