Milesacanthus cf. ancestralis, Burrow, Lelievre & Janjou, 2006
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publication ID |
https://doi.org/ 10.5252/g2011n2a1 |
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DOI |
https://doi.org/10.5281/zenodo.4596946 |
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persistent identifier |
https://treatment.plazi.org/id/CA2B87AB-6C41-FFA9-F8B6-E909918EC99E |
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treatment provided by |
Felipe |
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scientific name |
Milesacanthus cf. ancestralis |
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Milesacanthus cf. ancestralis ( Fig. 8 View FIG )
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 View FIG 2 View FIG ). 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 View FIG 1 View FIG , B 2 View FIG and possibly more), diverging in the front part ( Fig. 8C View FIG 2 View FIG ), 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 View FIG 1 View FIG ), grooves and ridges start just at the front edge of the scale, whereas in the scale MNHN.F.ALD-37 ( Fig. 8B View FIG 2 View FIG ) 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 View FIG 2 View FIG ), 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 View FIG 2, B 2, C 2 View FIG ). Furthermore, wart-like protuberances are visible on the posterior neck ( Fig. 8A View FIG 2 View FIG ; more evident on Fig. 7F View FIG ), visible on “young” scales according Burrow et al. (2006: 547).
HISTOLOGY
The thin section 9 MNHN.F.ALD-41 ( Fig. 7G View FIG ) 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 View FIG 2 View FIG ). 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.
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