Milesacanthus cf. ancestralis (Fig. 8)

MATERIAL. — Nine scales, one of which is lost (M N H N. F.A L D -4 0), a n d o n e t h i n s e c t i o n MNHN.F.ALD-41.

DESCRIPTION

The crown of these four scales is diamond-shaped and its anterior part may be more rounded as in MNHN.F.ALD-38 (Fig. 8C 2). The convex base, also diamond-shaped, and with anterior and rounded posterior edges is located anteriorly.

The scales with a flat crown show sub-parallel grooves (18 for Fig. 8A 1, B 2 and possibly more), diverging in the front part (Fig. 8C 2), like in the material of Milesacanthus ancestralis from Saudi Arabia.The grooves are not really deep but narrower than the ridges. Both grooves and ridges run along the entire crown length of the scale. In the scale MNHN.F.ALD-36 (Fig. 8A 1), grooves and ridges start just at the front edge of the scale, whereas in the scale MNHN.F.ALD-37 (Fig. 8B 2) the ridges start at the vertical edge of the crown and the posterior edge is more crenulated.

One scale (ALD-38) shows 13 larger grooves (Fig. 8C 2), but only in its anterior part. Pore openings, located mainly at the beginning of the grooves, connect to an ascending canal system. The grooves begin after the anterior edge of the crown and most lateral grooves undulate, slightly converging in the beginning, then parallel to the diagonal, and stop in the posterior third of the crown. The neck is well marked with a row of foramina posteriorly and anteriorly (Fig. 8A 2, B 2, C 2). Furthermore, wart-like protuberances are visible on the posterior neck (Fig. 8A 2; more evident on Fig. 7F), visible on “young” scales according Burrow et al. (2006: 547).

HISTOLOGY

The thin section 9 MNHN.F.ALD-41 (Fig. 7G) shows a classical pattern for an acanthodian, with a primordium in the middle of the crown that is made of dentine covered with thin box-in-box growth zones continuing from the crown into the acellular bone base. Contrary to Milesacanthus ancestralis and M. antarctica (Young & Burrow 2004), no vascular canal was found, but only one thin section has been made.

DISCUSSION

The number and the development of ridges on the crown recalls the scales of M. antarctica, but the presence of wart-like bumps in the posterior part of the neck is more suggestive of M. ancestralis . Although wide canals were not detected here, they are present in both M. ancestralis and M. antarctica, and pore openings are visible at the surface of the scale MNHN.F.ALD-38 (Fig. 8C 2). The cone of the base inserted in the crown is, in our material, more developed. In conclusion, some acanthodian scales are closer to M. antarctica and the others to M. ancestralis but their histology is different.