Rhizodont crossopterygian fish from the Dinantian of Foulden, Berwickshire, Scotland, with a re-evaluation of this group

Abstract

Material of crossopterygian fishes from Foulden, Berwickshire, is important in including the first known entire specimen giving a clear picture of the body form of the Family Rhizodontidae (sensu Andrews 1973). This group is specialised in having numerous subsidiary lateral lines on the body, a ventrally expanded shoulder girdle, fins with peculiar stiffened structure, the pectorals forming greatly enlarged paddles and other fins being reduced in size. The Foulden remains fall into large and small size-ranges occurring largely at different levels in the sequence, but although these may represent growth in one taxon the available parts of the body are mostly complementary. Separate preliminary descriptions of the two size-ranges are therefore given, the small complete form being named ?Strepsodus anculonamensis sp. nov., and the large form remaining unnamed pending further study. An abridged historical account of previously named rhizodont genera (Rhizodus, Strepsodus, Sauripterus and Pycnoctenion) is given in order to explain why the new Foulden form(s) cannot at present be generically assigned more closely. A revised diagnosis of the Family Rhizodontidae leads to a discussion of the functional morphology and mode of life of fish of this type. Analogies with modern forms indicate that, like some sharks and crocodiles, they may have fed by tearing flesh off large prey, rotating or shaking against its inertia. Ways in which variously shaped rhizodont teeth (recurved, sigmoid etc.) may have functioned are suggested. Addenda deal with (1) the possible occurrence at Foulden of a large lungfish and (2) (after considering primitive and advanced rhizodont features) the renaming of Rhizodus ornatus Traquair 1878 from higher in the Scottish lower Carboniferous, as the type of a new genus, Screbinodus.

Cope's Rule and Romer's theory: patterns of diversity and gigantism in eurypterids and Palaeozoic vertebrates

“…During this time, agnathan diversity decreased and placoderms went extinct. With the beginning of the Carboniferous, chondrichthyans had diversified and sarcopterygian fishes such as rhizodonts had taken over the role of apex predators (Andrews 1985). …”

New information on the enigmatic tetrapodomorph fishMarsdenichthys longioccipitus(Long, 1985)

“…Photographs of the specimens were undertaken with a Panasonic Lumix DMC-FZ20 camera and a Canon EOS 5D with a 65-mm macro lens, with drawings produced based on the photographs. Terminology for describing general morphology will follow Ahlberg and Johanson (1997), whereas scale morphology follows Andrews (1985). SYSTEMATIC PALEONTOLOGY SARCOPTERYGII Romer, 1955TETRAPODOMORPHA Ahlberg, 1991MARSDENICHTHYS LONGIOCCIPITUS Long, 1985a Type Horizon-Avon River Group (Cas et al, 2003).…”

“…A prominent tearshaped median boss marks the internal surface (Fig. 6D), as in tristichopterids (e.g., Young et al, 1992), rhizodontids (e.g., Andrews, 1985), Medoevia (Lebedev, 1995), Canowindra (Thomson, 1973), and Rhiz odopsis (Williamson, 1837;Schultze and Heidtke, 1986: fig. 3b).…”

Devonian Vertebrates From Colombia

“…In addition, the enamel of the tooth surface shows extremely thin striae, as in Strepsodus. The parasymphysial tusk is rounded in section and thus different from the laterally compressed tusks of Rhizodus, Screbinodus and Notorhizodon (Andrews 1985;Young et al 1992). The dentary differs from that of Carboniferous rhizodontids known to date in being somewhat deeper.…”

“…5, ®gs 3±4), which frequently occurs in rhizodontids and tristichopterids (Jarvik 1980;Andrews 1985;Long 1989). The external ornamentation of these scales is variable.…”