SARCOPTERYGII Romer, 1955

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 : 20-21

Infraclass SARCOPTERYGII Romer, 1955 ( Figs 13-15 )

Apart from isolated scales, sarcopterygian remains are rare in the Ly Hoa fauna, but two new specimens are worth describing. Specimen BT 207n shows the internal impression of a large dermal plate ( Fig. 13 ), whose margins still retain a very small strand of cosmine-covered bone. Owing to the lack of landmarks, such as the sensory-line canals or large sensory pores, the identity of this plate is quite difficult to determine. Considering its size and overall shape, it could be either an opercular, or a “cheek plate” (composed by the fused squamosal, quadratojugal and preopercular in certain basal dipnomorph and tetrapodomorph sarcopterygians, such as the Youngolepididae and Kenichthys ). In the latter case, its outline would best agree with the cheek plate of Kenichthys ( Chang & Zhu 1993: fig. 7F) and differs notably from those of Youngolepis described to date, whatever their wide range of individual variation (Tong-Dzuy & Janvier 1990: fig. 32; Chang 1991: pl. 2A, B; Zhu & Fan 1995: fig. 2A). However, it shows no evidence of anterior overlap areas for the postorbital, jugal and maxillary, and its internal surface is only hollowed by a small socket (s, Fig. 13B ), in which an adjacent bone may have been inserted. In addition, its margin shows no evidence of openings for the jugal canal and the characteristic three sensory pits, and its internal impression shows no internal mark of this canal (which usually forms a slight ridge on the internal surface of the bone). Assuming that it is an opercular, its shape is quite different from that of the opercular of both Kenichthys and Youngolepis (Tong-Dzuy & Janvier 1990: fig. 31A; Chang & Zhu 1993: fig. 13A). Only an isolated bone, presumably from the opercular series of Youngolepis and figured by Tong-Dzuy & Janvier (1990: pl. 7:7), bears some resemblance to this specimen.

The second sarcopterygian specimen is an isolated jaw bone ( Fig. 14 ), interpreted here as either a dermopalatine or an ectopterygoid. It shows a large tusk insertion pit and one poorly preserved tusk (pt, t, Fig. 14B ). The latter is sectioned near its base and displays a polyplocodont structure (pld, Fig. 14B ). The lateral margin of this bone is covered with fields of denticles (dtc, Fig. 14B ), as in the dermopalatine, ectopterygoid and coronoids of Youngolepis ( Chang 1991) . In addition, many cosmine-covered scales occur in the Ly Hoa Formation. As a whole, they agree with those of Youngolepis , and generally show series of relatively large cosmine pores and dentine ridges in the anterior part of their external surface ( Fig. 15 ).

Figures

FIG. 13. — Sarcopterygii gen. et sp. indet., Ly Hoa Formation,?Lower Devonian,Ly Hoa,Quang Binh Province,Vietnam,imprint of a cheek plate or an opercular with remnants of cosmine along the margins (BT 207n), photograph in immersion in water (A) and camera lucida drawing (B). Cosmine stippled. Abbreviation: s, small socket in the ventral margin. Scale bar: 10 mm.

FIG. 14. — Youngolepididae gen. et sp. indet., Ly Hoa Formation,?Lower Devonian,Ly Hoa,Quang Binh Province,Vietnam,toothbearing bone from the dermopalatine-ectopterygoid series or from the coronoid series (BT 207o), photograph (A) and camera lucida drawing (B). Abbreviations: dtc, denticles on the lateral margin; pld, plicidentine of polyplocodont type; pt, pit for tusk insertion; t, tusk. Scale bar: 5 mm.

FIG. 15. — Youngolepididae gen. et sp. indet., Ly Hoa Formation,?Lower Devonian,Ly Hoa,Quang Binh Province,Vietnam,isolated scales in external view: A, caudal scale (BT 207p); B, larger flank scale with large cosmine pores (BT 207q). Scale bars: 5 mm.