Carboniferous (Tournaisian) fish assemblages from the Isle of Bute, Scotland: systematics and palaeoecology

We describe fish assemblages from the C arboniferous (mid‐ to late T ournaisian) B allagan F ormation at two localities, H awk's N ib and M ill H ole, on the I sle of B ute, S cotland. Fossil material occurs in thin, locally reworked dolomitic limestone beds, interpreted as the deposits of very shallow lakes or lagoons, developed on, or adjacent to, a seasonally dry coastal plain. The mostly disarticulated fossils comprise isolated teeth, mandibles, scales, tesserae, dermal bones, lepidotrichia and vertebrae. The fauna includes rhizodonts (cf. A rchichthys portlocki , cf. S trepsodus sauroides ), lungfish ( S agenodus sp.), other sarcopterygians ( M egalichthys sp.), one shark ( A geleodus pectinatus ), climatiiform acanthodians and indeterminate actinopterygians. The Mill Hole assemblage is especially noteworthy because it includes some putative juvenile forms ( A rchichthys and S agenodus ). A critical review of the habitat preferences of the documented taxa suggests that most were either euryhaline or, in the case of A rchichthys , probably endemic to brackish or freshwater settings. The B ute fish beds fall within a crucial evolutionary period during which many fish and other animal groups were infiltrating nonmarine environments, either passively or actively. It may be that lakes and lagoons may have functioned as protected nurseries for juveniles during this wave of colonization.