Mandibles of rhizodontids: anatomy, function and evolution within the tetrapod stem-group

Abstract

The Rhizodontida (Pisces: Sarcopterygii) is a clade of predatory fishes from the Upper Devonian (Aztecia; ?Givetian of Antarctica) through to the Upper Carboniferous (Strepsodus; Moscovian of northern Europe and North America). They form the most basal plesion within the tetrapod stem-lineage. The mandibles were dominated by large symphysial tusks on the dentary. Not much else is known of the mandibles in primitive rhizodontids. However, later forms show several derived characters: the mandible is very deep dorsoventrally and narrow linguolabially; the coronoid fangs bear only a single fang and no other dentition; the Meckelian element was unossified, leaving the adductor fossa unfloored by bone; the prearticular produced a large dorsal process lingual to the adductor fossa, presumably for muscle attachment. These and other characters are discussed in the context of the evolution of the tetrapod stem-group.The mandible appears to have been split into two functional units, one comprising the firmly sutured prearticular, coronoids and dentary, the other comprising the firmly sutured infradentaries. The connection between the two units was weak, suggesting a longitudinal intramandibular hinge. The possibility that this acted as a ‘torsion grip’ during feeding is discussed.

Cranial morphology of the Carboniferous rhizodontidScrebinodus ornatus(Osteichthyes: Sarcopterygii)

“…12A-C) shows parts of both vomers, a single ectopterygoid and a fragment of the left entopterygoid. No endoskeletal bone is visible, suggesting that bones such as the pterygoquadrate and autopalatine were not ossified (a similar situation is seen with the Meckelian cartilage on the mandible of Screbinodus and Rhizodus; see Jeffery 2003). The vomers on SME 4714 ( Fig.…”

“…This paper follows Jeffery (2003) in referring to these (from largest to smallest) as tusks, fangs, teeth and denticles, while noting that "this does not necessarily imply differences in morphology or ontogeny (particularly between the tusks, fangs and teeth) but conveniently distinguishes the size ranges" (Jeffery 2003, p. 356). …”

Upper Devonian osteichthyan remains from the Genoa River, Victoria, Australia

“…The polyplocodont (NMV P229479) and dendrodont (NMV P229477) teeth were sectioned by Ian Stewart, and photographed with a Leica DFC500 camera using a Leica M205C microscope. Anatomical terminology follows that of Jeffery (2003).…”

“…2e) by lacking a rounded posteroventral margin and large dorsoventral depth. Among tetrapodomorphs, NMV P198470 differs from rhizodontids by lacking an unossified articular and other meckelian elements (Jeffery, 2003) (fig. 2f); Devonian tetrapods, such as Ventastega (Ahlberg and Clack, 1998) (fig.…”

“…However, the lingual surface of NMV P229479 differs from that of rhizodontids, such as Barameda (Holland et al, 2007) in lacking fine, raised striae on the enamel. Gardiner, 1984); d, Miguashaia (modified from Forey et al, 2000); e, Holoptychius (modified from Jarvik, 1972); f, Rhizodus (modified from Jeffery, 2003); g, Ventastega (modified from Ahlberg and Clack, 1998); h, Ectosteorhachis (modified from Thomson, 1964); i, Eusthenopteron specimen NRM PZ P35; j, Gyroptychius specimen NRM PZ P1409; k, tristichopterid specimen MNHN n° MCD 42 (modified from Lelièvre and Janvier, 1986). Scale = 1 cm; scale not included for c-h; all jaws excluding j are in lingual view; j is in labial view.…”