Atlantidosteus pacifica, Young, 2003

Young, Gavin C., 2003, A new species of Atlantidosteus Lelièvre, 1984 (Placodermi, Arthrodira, Brachythoraci) from the Middle Devonian of the Broken River area (Queensland, Australia), Geodiversitas 25 (4), pp. 681-694 : 684-689

publication ID	https://doi.org/ 10.5281/zenodo.4650651
persistent identifier	https://treatment.plazi.org/id/3D5187B5-AB22-5901-2B70-FE32A58A8AAF
treatment provided by	Felipe
scientific name	Atlantidosteus pacifica
status	sp. nov.

Treatment

Atlantidosteus pacifica n. sp. ( Figs 2 View FIG ; 3 View FIG ; 5B View FIG )

Atlantidosteus sp. – Young et al. 1993: 247.

“suborbital plate… resembling that of Homostius ”; “ Atlantidosteus ” – Turner et al. 2000: 497.

HOLOTYPE. — ANU V1033 (housed in the Geology Department, Australian National University, Canberra) is the only known specimen, a right SO plate from the cheek of a large arthrodire.

ETYMOLOGY. — An allusion to proximity of the fossil locality to the Pacific margin of the Palaeozoic Gondwana supercontinent.

DIAGNOSIS. — An Atlantidosteus in which the suborbital lamina of the suborbital plate decreases in height towards a pointed anterior margin, the postorbital lamina has a continuously curved dorsal margin, with no dorsal angle, and the anterior end of the suborbital section of the infraorbital sensory groove has a slight upward curvature.

REMARKS

Since only two of these highly distinctive SO plates have ever been found, it is possible that new material could indicate that the differences listed above could result from intraspecific variation. However it is considered more likely that this is a closely related species, of a slightly young- er age than the genotype, which is associated with mid-Emsian (laticostatus zone) conodonts ( Lelièvre 1984a: 197).

DESCRIPTION

This almost complete right SO plate has a preserved total length of 23.6 cm, and maximum depth of the postorbital lamina of 100 mm. It is thus of similar size to the SO of the type species (estimated length 23-25 cm), but considerably smaller than the corresponding bone of Homostius Asmuss, 1856 (about 40 cm long; Heintz 1934: fig 22). By comparison with a cast of the specimen of Homostius milleri Traquair, 1888 figured by Heintz (1934: pl. 14, fig. 1), in which the left SO is about 15.5 cm long on a skull roof some 33 cm in length, ANU V1033 could have come from a skull up to 50 cm long (assuming similar proportions). This would make it the largest arthrodire yet found in Australia ( Westralichthys Long, 1987 from Western Australia is a skull roof about 27 cm long). Apart from its large size, this is a highly distinctive SO plate ( Figs 2 View FIG ; 3 View FIG ), clearly very similar to the SO of the type species, A. hollardi Lelièvre, 1984 . Various differences in shape and development suggest that it is a separate species. The dorsal margin of its postorbital lamina has an even curvature over its entire length, in contrast to the type species, in which the anterior part of the dorsal margin is relatively straight, leading up to a dorsal angle ( Lelièvre 1984a: fig. 2A). Most of the posterior margin, which is missing in the type species, is broken and irregular in this new specimen, but given the thinness of the bone edge it has been assumed in the restoration that there is little missing (dashed lines, Fig. 3 View FIG ). The fine tubercular ornament is similar to that on the type species, and may be a generic characteristic. In both species the suborbital lamina lacks ornament, and presumably was embedded in soft tissue. A scattering of larger tubercles is developed beneath the supraoral sensory groove in the region of the ossification centre in ANU V1033, and the periphery of the dorsal and posterior margins is smooth ( Fig. 2A View FIG ). Posteriorly this may be partly due to abrasion, but along the anterodorsal margin it is natural, indicating either attachment of skin, or loose overlap of adjacent bones (the PtO and M plates of the skull roof).

The anterior section of the dorsal margin shows a slight groove for contact with the PtO and/or M plate (gr.PtO/M, Fig. 3 View FIG ). The suborbital section of the infraorbital sensory canal (ioc.sb, Fig. 3A, C View FIG ) has a posterior inflection enclosing an angle of no more than 70°. It terminates anteriorly in the unornamented extension of the suborbital lamina (so.la, Fig. 3A View FIG ), which was presumably embedded in soft tissue. This is somewhat different to the typical brachythoracid arrangement where the suborbital lamina is in contact with the ventral skull-roof margin in front of the orbit, for example the PN plate of Dunkleosteus Lehman, 1956 (e.g. Heintz 1932: fig. 34). Beneath the angular inflection is a small row of pits (sbsp, Fig. 3A View FIG ), and the supraoral sensory canal (sorc) runs obliquely downwards and backwards to pass off the ventral margin just behind a ventral unornamented flange. All of these features are similarly developed on the type species, but the groove along its anterodorsal margin is more extensive, reaching almost to the dorsal corner (b.pto, Lelièvre 1984a: fig. 2A).

Two distinct shallow grooves in the ornament of ANU V1033 have the appearance of healed wounds or lesions (le, Fig. 3A View FIG ). The longer groove has intermittent enlarged tubercles around its margins, with a few scattered across it ( Fig. 2A View FIG ), confirming that it could not be a post-mortem feature. It is of interest that Lelièvre (1984a) also described an irregular depression at the start of the suborbital lamina in A. hollardi , which was similarly interpreted as the healed scar of a wound. From its size and depth this was evidently a more serious injury that the shallow lesions on ANU V1033. Observed differences on the external surface of ANU V1033 compared to the preserved part of the type species include the more continuous horizontal canal, which forms a short narrow posterodorsal groove (hc, Fig. 3A View FIG ), and the distinct cutaneous sensory pit (cuso, Fig. 3A View FIG ). This is presumably the homologue of the anterior pit on the SO of Buchanosteus Stensiö, 1945 (e.g. Young 1979: fig. 13) or Coccosteus Miller, 1841 ( Miles & Westoll 1968: fig. 14). It was not identified in the type species, but the SO is cracked right through at the appropriate position ( Lelièvre 1984a: fig. 2), so it may have been present but not preserved. The lower section of the infraorbital sensory canal in ANU V1033 curves slightly upward to the front, where it expands to merge with the unornamented anterior surface of the suborbital lamina (ioc.sb, Figs 2A View FIG ; 3A View FIG ). The thickened exposed part of the external surface of the SO forms an irregular pointed anterior margin to the ornamented surface between the two sections of the infraorbital sensory groove. In front of this the surface is stepped in by some 4 mm, and has a spongy open texture. In anterior view this stepped surface appears as two slightly concave facets. The clear delineation between exposed and non-exposed areas of the external surface compares closely with the SO of Homostius , in which Heintz (1934: fig. 22) identified several “limits” subdividing the “handle” of the SO from the “blade”, and dividing the “blade” into two portions. The important margin is that labelled a-d-c ( Fig. 4A View FIG ), which defines the anterior limit of the exposed part of the external surface. In Homostius the SO differs from that described here in having a much more extensive area covered dorsally in a loose overlap by the PtO and M plates (oaPtO+M, Fig. 4A View FIG ). Since the orbit in Homostius is completely enclosed by the PrO and PtO, it is likely that the PrO also overlapped the SO anteriorly. In Atlantidosteus the overlap for the PtO appears in lateral view as a narrow flange only at the anterior end of the blade portion (oaPtO, Fig. 3 View FIG ), although it is in fact a broad upwardly facing surface. The corresponding part in A. hollardi was identified as an “encoche orbitaire” by Lelièvre (1984a: fig. 2A), but part of this margin is not complete ( Lelièvre 1984a: fig. 2C, pl. 1B), so it is possible that it was similarly developed as an overlap for the PtO (shaded area on upper margin, Fig. 5A View FIG ).

The inner surface of ANU V1033 is much more complete posteriorly than in the type species ( Lelièvre 1984a: pl. 1A), although the posterior part of the bone shows few features. Radiating striations on the abraded posterodorsal margin ( Fig. 2B View FIG ) indicate the anteriorly placed ossification centre, as in A. hollardi , where the striations are more prominent anteriorly in an area which is

smooth in A. pacifica n. sp. This difference may be due to better preservation. A depressed groove on the anterodorsal margin of the type species is interpreted by Lelièvre (1984a: fig. 2B) as an area for attachment of the palatoquadrate. In the SO of Buchanosteus ( Young 1979: pl. 3B) the palatoquadrate attachment is similarly developed, as an area delimited by two ridges (remnants of the perichondral ossification). However in the new specimen the corresponding region is just a smooth thickened area (?att.pq, Fig. 3B View FIG ), with no ridges, possibly suggesting that the palatoquadrate was not perichondrally ossified. Similarly, the attachment for the autopalatine (att.aup, Fig. 3B View FIG ) is less developed than in A. hollardi , with a smooth, gently concave surface in place of the irregular ridges of that species. In both species this area has a very similar convex dorsal margin forming a crest (cr2, Fig. 3B, C View FIG ). This crest encloses a groove (gr.mx, Fig. 3B, C View FIG ) probably for the maxillary branch of the trigeminal nerve, together with the buccalis lateralis nerve passing anteriorly, and the mandibular branch of the trigeminal nerve passing posteriorly ( Lelièvre 1984a: fig. 2C). In Buchanosteus a groove possibly for the maxillary branch of the orbital artery was also identified running anteriorly beneath the orbit ( Young 1979: fig. 15). The autopalatine attachment in ANU V1033 can be assumed to extend forward to the extremity of the suborbital lamina, where its ventral limit is marked by a groove (gr, Fig. 3B, C View FIG ), also as preserved in Buchanosteus ( Young 1979: pl. 3B). The corresponding groove is evidently more pronounced in Homostius , with an extensive ventral lamina (R4 of Heintz 1934: fig. 23). In A. pacifica n. sp. the crest identified as a linguiform process in A. hollardi is not developed, and the shape of the suborbital lamina is different, tapering to a point anteriorly ( Fig. 5A, B View FIG ). Its ventral margin is gently convex anteriorly, and concave posteriorly ( Fig. 2 View FIG ). In contrast, in A. hollardi the suborbital lamina is expanded at its anterior preserved end, which is broken. Its anterior angle may have narrowed to a point, as in A. pacifica n. sp., in which case it seems that the suborbital lamina would have been proportionately longer in A. hollardi (about 35% of total length). Beneath the linguiform process there is an irregular ventral flange ( Lelièvre 1984a: pl. 1A), and a similar flange may be developed in Homostius (fl, Fig. 4B View FIG ). A posterior thickening in ANU V1033 (pvpr, Fig. 3B View FIG ) may correspond to that supporting the posteroventral process of the palatoquadrate in Buchanosteus , but its surface is somewhat abraded ( Fig. 2B View FIG ). This structure may be less well developed in A. hollardi , but is also abraded. Again, a similar but more prominent ridge is developed in the SO of Homostius (R3 of Heintz 1934: fig. 23), but neither species of Atlantidosteus shows the vertical ridge (R1 of Heintz), although the bone is thickened in this region.

In ANU V1033 the dorsal aspect of the area transitional between the suborbital and postorbital laminae is similar to that illustrated by Lelièvre (1984a: fig. 2C), but more completely preserved. Direct comparison with a resin cast of A. hollardi (ANU V3030) shows some minor differences. The inner crest (cr2, Fig. 3B, C View FIG ) is convex mesially when viewed from above, whereas in A. hollardi it follows the inner bone surface, and continues farther posteriorly to define a mandibular groove of some width (g.n.md, Lelièvre 1984a: fig. 2C). In A. pacifica n. sp. this groove is less well defined, forming only a shallow depression. The outer crest (cr1, Fig. 3B, C View FIG ) is also convex mesially (not preserved in A. hollardi ). Lateral to this the anterior end of the groove for the PtO plate is expanded as a shallow concave depression supported by the maximum thickness of the bone (gr.PtO, Fig. 3C View FIG ).