Aeduella sp.

Aeduella sp.

( Figs 7; 8)

MATERIAL. — An isolated postrostral ( MHNT.PAL. 2023.9.19 [ Fig. 7A, B]) , an isolated frontal ( MHNT.PAL.2023.30.3 [ Fig. 7 E-G]) , an isolated operculum ( MHNT.PAL. 2023.9.21 [ Fig. 8A, B]) , a suboperculum associated with articulated scales ( MHNT.PAL. 2023.9.11 [ Fig. 8G, H]) , an isolated suboperculum ( MHNT.PAL. 2023.9.10 [ Fig. 8E, F]) and a suboperculum associated with a medial gular ( MHNT.PAL. 2023.9.12 [ Fig. 8C, D; 7C, D]) .

DESCRIPTION

MHNT.PAL. 2023.9.19 ( Fig. 7A, B) have a symetric hexagonal shape, with a rounded anterior and posterior edge. It is 15 mm long and 8 mm wide (length/width ratio 2.1), and both its left lateral and posterior edges are damaged. A deformation of the matrix cuts the bone in half and distorts its relief. An ornamentation of flat tubercles is visible on the lateral surface at the posterior edge, reducing towards the anterior edge. No sensory canal or pit lines are visible. This bone is identified here as a postrostral.

MHNT.PAL.2023.30.3 ( Fig. 7 E-G) is an elongated rectangular bone, identified here as a frontal. It is preserved in part and counter-part and measures 13 mm long by 6 mm wide (length/width ratio 2.2). It has V-shaped anterior and posterior margins and has an ornamentation made by light ridges. The supraorbital canal is well marked and runs along the lateral margin of the frontal, making a slight curve.

MHNT.PAL. 2023.9.21 ( Fig. 8A, B) is an operculum measuring 10 mm long and 20 mm high, with a bending angle of 151°. The ratio between the width of the ventral part and the width of the dorsal part is 0.9. Its bone structure is partially preserved, but its shape is well visible thanks to its imprint on the matrix, with the exception of its dorsal end: this shape is rectangular, with a straight anterior edge, the dorsal and posterior edges rounded dorsally and the ventral edge oblique posterodorsally. It is ornamented with ridges parallel to the bone margins (growth lines) and light, flat tubercles all along its lateral surface.

MHNT.PAL. 2023.9.10 (7.5 mm long, anterior part 6 mm high and posterior part 14 mm high; Fig. 8E, F), MHNT. PAL. 2023.9.11 (8.2 mm long, anterior part 7 mm high and posterior part 16 mm high; Fig. 8G, H) and MHNT. PAL. 2023.9.12 (10 mm long, anterior part 7 mm high and posterior part 16 mm high; Fig. 8C, D) correspond to suboperculum bones as evidenced by their trapezoidal shape, their ornamentation of small flat tubercles on their lateral surface, and their relatively large sizes. MHNT. PAL. 2023.9.10 and MHNT.PAL. 2023.9.11 are complete, whereas the dorsal end of MHNT.PAL. 2023.9.12 is missing but visible on the bone counterpart. The ratio between the depth along the anterior margin and the depth along the posterior margin is 2.3 for these three suboperculums. These suboperculums described here have a distinctly concave, sloping dorsal margin, while the anterior, posterior and ventral margins are straight.

The suboperculum MHNT.PAL. 2023.9.12 is associated with a bone 13 mm further away, identified here as a medial gular ( Fig. 7C, D). Its anterior and posterior ends are missing and the lateral margins are damaged, but it measures 9 mm long and 5.5 mm wide as preserved. It is elongated anteroposteriorly, with a slender anterior part and an enlarging medial part. A slightly V-shaped pit line is present in the centre of the gular and the bone is ornamented with flat tubercles over its entire lateral surface.

COMPARISONS

The postrostral MHNT.PAL. 2023.9.19 is a large bone, ornamented with tubercles on the lateral face of its posterior part, with a hexagonal shape and rounded edges.These features allow it to be attributed to the Aeduellidae .With its pointed anterior and posterior margins, MHNT.PAL. 2023.9.19 is similar to Aeduella blainvillei , Bourbonnella hirsuta and Neslovicella spp. (e.g. Heyler 1969; Štamberg 2007, 2010). Of these taxa, the hexagonal shape and anterior reduction of the ornamentation of MHNT.PAL. 2023.9.19 are identical to the postrostral of Aeduella blainvillei ( Heyler 1969; Štamberg 2018). Even if the latter is described as pentagonal by Heyler (1969), it has a hexagonal shape (e.g. Heyler 1969: figs 46, 55, 76). These characters suggest that MHNT.PAL. 2023.9.19 belongs to Aeduella blainvillei , but based on its isolated nature, we prefer cautiously refer it to Aeduella sp. only, pending more complete material from the Decazeville Basin.

The frontal MHNT.PAL.2023.30.3 can be confidently assigned to Aeduellidae due to the presence of the supraorbital canal running close to the lateral edge of the bone ( Poplin & Dutheil 2005). The V-shaped margins of MHNT.PAL.2023.30.3 are only found on Neslovicella spp. , Bourbonnella hirsuta and Aeduella blainvillei ( Heyler 1969; Poplin & Dutheil 2005; Štamberg 2007, 2010). MHNT. PAL.2023.30.3 differs from B. hirsuta and Neslovicella spp. in its elongated shape, whereas the frontals of Neslovicella spp. and B. hirsuta are broad and short ( Štamberg 2007, 2010). The B. jocelynae frontal has also a V-shaped posterior margin, but is too poorly preserved for further comparison ( Mickle 2011). In contrast, the frontals of Aeduella blainvillei are elongated and very similar to MHNT.PAL.2023.30.3. These characters suggest that MHNT.PAL.2023.30.3 belongs to Aeduella blainvillei , but, again, its isolated nature push us to refer it cautiously to Aeduella sp. only.

The operculum MHNT.PAL. 2023.9.21 can be assigned to Aeduellidae on the basis of its large size, rectangular global shape and ornamentation (e.g. Heyler 1969; Štamberg 2018). With its posterodorsally oblique ventral margin and 151° bending angle, MHNT.PAL. 2023.9.21 falls within the morphological variation of operculums of Bourbonnella spp. , Neslovicella spp. , Spinarichthys dispersus , Westollia crassa and Aeduella blainvillei ( Heyler 1969; Poplin 2001; Štamberg 1986, 2007, 2010, 2018, 2024). Aeduella and Decazella are the only Aeduellidae currently reported from the Bourran Formation, which suggests that MHNT.PAL. 2023.9.21 can be attributed to Aeduella sp.

The suboperculums MHNT.PAL. 2023.9.10 -12 are attributable to Aeduellidae due to their trapezoidal shape and ornamentation (e.g. Poplin & Dutheil 2005). The trapezoidal shape and the presence of a distinctly concave, sloping dorsal margin are characteristic of Aeduella blainvillei and Westollia crassa (e.g. Štamberg 2018, 2024). Among the Aeduellidae specimens with a preserved suboperculum already described from the Bourran Formation, some of them have an identical shape to those described here and were attributed to Aeduella sp. by Heyler (1969: 183, fig. 132, pl. XLIV, fig. 6 [see Fig. 8L]). It is very likely that MHNT.PAL. 2023.9.10 -12 belong to the same taxon. The characteristics of MHNT.PAL. 2023.9.10 -12 allow them to be assigned to the genus Aeduella , but their isolated nature suggests caution about specific attribution. We therefore refer them here to Aeduella sp.

The medial gular of MHNT.PAL. 2023.9.12 is attributable to Aeduellidae on the basis of its ornamentation and presence of a distinct pit line (e.g. Heyler 1969; Štamberg 2018). In most Aeduellidae , the medial gulars are poorly known because they are either masked by other skull bones in articulated specimens, or not preserved as isolated bones in disarticulated specimens (e.g. Heyler 1969; Heyler & Poplin 1983; Štamberg 2007). Among the Aeduellidae , Aeduella blainvillei is the only known taxon in which the median gular has a sharply tapered anterior end and an enlarged posterior part ( Štamberg 2018). Although the median gular of Decazella vetteri is unknown, the difference between the median gular of A. blainvillei and that of other aeduellids suggests that this morphology is diagnostic of the species (e.g. Heyler 1969; Štamberg 2018, 2024; Štamberg & Werneburg 2023). The characters of this specimen and its isolated nature therefore allow us to assign it to Aeduella sp .. This attribution is consistent with that of the associated suboperculum on MHNT. PAL. 2023.9.12, and could possibly suggest that they belong to the same individual.