Redescription of the lungfish Straitonia waterstoni from the Viséan of Lothian, Scotland

Abstract

The unique specimen of the Early Carboniferous lungfish Straitonia waterstoni is described in detail. As the only articulated lungfish currently described from the Viséan, it is of particular interest for lungfish evolution. It retains some plesiomorphic features of the skull, and shows some derived features in common with later Carboniferous forms. A thorough description is provided for future phylogenetic analyses. Straitonia has a number of diagnostic features. These include: a relatively narrow B-bone with a broad and curved anterior margin; an unpaired, parallel-sided C-bone whose margin is convex anteriorly; a B-bone approximately the same width as the single C-bone; the I-bones are relatively small compared with the B- and J-bones; the J-bones are tri-lobed; the XY bone is elongate along a posteromedial–anterolateral axis. Elongate fins dorsally and ventrally suggest a condition similar to those of later Carboniferous forms with confluent second dorsal, caudal and anal fins. It may thus be the earliest lungfish to show this condition. The ventral tail fin includes some fused elements, probably haemal and infrahaemal spines. The identity of one of the skull bones is discussed, and concluded to be a C-bone rather than a D-bone as previously described. The D-bone is absent, as in later Carboniferous forms. The history of the D-bone is considered.