Holonematidae, Obruchev, 1932

Racheboeuf, Patrick, Phuong, Ta Hoa, Hung, Nguyen Huu, Feist, Monique & Janvier, Philippe, 2006, Brachiopods, crustaceans, vertebrates, and charophytes from the Devonian Ly Hoa, Nam Can and Dong Tho formations of Central Vietnam, Geodiversitas 28 (1), pp. 5-36: 19

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? Holonematidae   gen. et sp. indet. ( Fig. 10 View FIG )

An isolated, incomplete suborbital plate from locality 4 ( Fig. 1C View FIG ) is tentatively referred to a holonematid. Although its ornamentation, which consists of more or less parallel, sinuous rows of tubercles ( Fig. 10A View FIG ), recalls that of the arthrodire Lyhoalepis duckhoai   , previously described from the Da Nhay outcrop of Ly Hoa (locality 1, Fig. 1B View FIG ; Tong-Dzuy et al. 1994; Janvier et al. 1997), it is unlikely that this suborbital plate belongs to Lyhoalepis   . The latter displays essentially phlyctaeniid-like characters that do not accord with the holonematid-like characters of this suborbital plate, such as, the large embayment (orbn?, Fig. 10A View FIG 2 View FIG ) of its dorsal margin, which is suggestive of the posteriorly placed orbital emargination of the holonematid Holonema   ( Miles 1971: fig. 33A).

The external surface of the plate clearly shows three sensory-line grooves that meet anteriorly to its central area. The postorbital sensory-line groove enters the plate dorsally behind a dorsal process and runs anteroventrally (ioc, Fig. 10A View FIG 2 View FIG ), but its connection to its ventral, infraorbital portion cannot be observed, owing to the poor preservation of the plate. The infraorbital portion of this groove then bends anteriorly, much in the same way as in eubrachythoracid arthrodires. The supra-oral sensoryline groove is broad and posteroventrally directed (sorg, Fig. 10A View FIG 2 View FIG ). Immediately behind it, and close to the point of junction of the three grooves is a large, rounded sensory pit (cusoa, Fig. 10A View FIG 2 View FIG ). There is evidence neither for a post-suborbital groove, nor for a distinct post-suborbital plate. Unfortunately, the anterodorsal part of the plate is missing, and this would have been crucial to rule out a more anterior position of the orbital margin.

The internal surface of the plate shows a well marked, vertical ridge (ar, Fig. 10B View FIG 2 View FIG ), meeting the dorsal margin of the plate at the level of its anterodorsal process that bounds anteriorly the presumed orbital notch. Ventrally, it joins a crescentiform knob (th, Fig. 10B View FIG 2 View FIG ), which is surrounded anteriorly by a spongiose area of the plate surface. This knob probably corresponds to the anterior palatoquadrate thickening of Holonema   , for the attachment of the autopalatine.

Interestingly, this suborbital plate resembles somewhat that of Bimbianga burrinjuckensis Young, 2005   , an Early Devonian (Emsian), supposedly primitive, holonematid with tuberculate ornamentation, from Burrinjuck, Australia.