Rhizodus

Abstract: Well-preserved material is described of several specimens of Screbinodus ornatus, a small (approximately 1.5 metres long) rhizodontid sarcopterygian, from the Visean (Mississippian, Carboniferous) of Scotland. This species shows numerous ‘typical’ rhizodontid characteristics, such as post-parietals with a median ‘tail’, pectoral lepidotrichia with elongated basal segments, large symphysial tusks on the dentary and a ‘reverse’ overlap relationship between the cleithrum and clavicle. It also shows several characters unknown in other rhizodontids, such as the loss of the extratemporal and subopercular, and the probable exclusion of the postorbital from the orbital margin. A phylogenetic analysis finds that rhizodontids are the sister group to all other members of the tetrapod total group (with the exception of Kenichthys). This confirms that resemblances between derived rhizodontids and digited stem-tetrapods such as Acanthostega (noted by several previous authors) are due to convergence. A second analysis f...

Abstract: All living tetrapods have a one-to-two branching pattern in the embryonic proximal limb skeleton, with a single element at the base of the limb (the humerus or femur) that articulates distally with two parallel radials (the ulna and radius or the tibia and fibula). This pattern is also seen in the fossilized remains of stem-tetrapods, including the fishlike members of the group, in which despite the absence of digits, the proximal parts of the fin skeleton clearly resemble those of later tetrapods. However, little is known about the developmental mechanisms that establish and canalize this highly conserved pattern. We describe the well-preserved pelvic fin skeleton of Rhizodus hibberti, a Carboniferous sarcopterygian (lobe-finned) fish, and member of the tetrapod stem group. In this specimen, three parallel radials, each robust with a distinct morphology, articulate with the femur. We review this unexpected morphology in a phylogenetic and developmental context. It implies that the developmental patterning mechanisms seen in living tetrapods, now highly constrained, evolved from mechanisms flexible enough to accommodate variation in the zeugopod (even between pectoral and pelvic fins), while also allowing each element to have a unique morphology.

Abstract: The Rhizodontida are an extinct order of large, predatory, lobe-finned fishes, found world-wide in Devonian and Carboniferous freshwater deposits. They are thought to be basal members of the tetrapod lineage. In this paper the pectoral fin skeleton of Rhizodus hibberti, a derived member of the group, is described in detail for the first time. It shows that muscular processes of the humerus (the deltoid and supinator processes) may have appeared later in tetrapod evolution than previously thought. The rhizodontids share an unusual fin-ray morphology, with highly elongated basal hemisegments, overlapping extensively with the endoskeleton. This morphology raises the possibility that segmentation and endoskeletal overlap share some common upstream genetic control during lepidotrichial development. The relative size of the lepidotrichia and the complexity of the endoskeleton does not fit with a «clockface» model of limb developmental evolution.

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.