Search

The presence of a petaloid pattern (previously known as ‘dark dentine’) in crosssections of teeth of the embolomere Pholiderpeton attheyi has been used as a synapomorphy of theembolomeres or of the embolomeres plus the stem tetrapod, Crassigyrinus scoticus . Among the taxastudied, dentine that appears dark results from closely packed dentine tubules and can be found inany part of a tooth section in which such crowding occurs. The petaloid pattern is restricted to toothsections of a particular diameter, and is obliterated in larger sections of teeth that show complexfolding. Petaloid dentine has been found in all tetrapod teeth with plicidentine that were sectioned inthis study, whether from stem tetrapods, the Embolomeri, Temnospondyli, or Stereospondyli, andhas been recognised in some sarcopterygian sh, an extant actinopterygian sh, ichthyosaurs, andVaranus . The presence of petaloid dentine is neither a synapomorphy of the tetrapod node nor of anynode within tetrapods.KEY WORDS: dark dentine, Embolomeri, petaloid dentine, Rhizodontida, stem tetrapods,Stereospondyli, Temnospondyli, tooth histology.The pattern known as ‘dark dentine’ in sections of earlytetrapod teeth with folded dentine (plicidentine) was rstrecognised in 1876 by Thomas Atthey when describing teeth ofthe Carboniferous embolomere, Pholiderpeton attheyi (Clack1987; =‘ Anthracosaurus russelli ’ in error in Atthey 1876 p162,pl. 11; Fig. 1a). Atthey described the appearance of thePholiderpeton attheyi tooth sections as stellate (=petaloid inthe present paper) with a number of fusiform bodies of lightcoloured dentine radiating from the pulp cavity to the circum-ference and embedded in dentine of a dark colour. The rstreference to ‘dark dentine’ was to the interstices between thefusiform bodies, but Panchen (1977, 1985) noted that thecolour arrangement could be reversed. Tooth sections thatshow ‘dark dentine’ may resemble either a light coloured daisyon a dark background when seen in transmitted light (e.g.Fig. 1a) or a dark coloured daisy on a light background whenseen in re ected light (e.g. Fig. 3a).The petaloid pattern is known in the early tetrapodliterature as ‘dark dentine’ (e.g. Panchen 1985). The presenceof ‘dark dentine’ has been used as a synapomorphy ofCrassigyrinus scoticus and embolomeres (Panchen 1985;Panchen & Smithson 1988), and of Ichthyostega stensioei andmost other tetrapods (Coates 1996), although Smithson(1980) implied that the phenomenon might be size-related,having been described in only the larger anthracosauroids(most embolomeres) with the condition unknown in smallerforms. The purpose of this paper is to review the pattern oftooth folding in early tetrapods, and to determine thedistribution of petaloid dentine among them. The Ducabrooklocality from the Lower Carboniferous of Queensland,Australia (Thulborn et al . 1996), yields histologically intactspecimens of both the stem tetrapod Ossinodus pueri(Warren & Turner 2004) and the rhizodontid Strepsodus sp.(Johanson et al . 2000) with which to test the hypothesis ofthe phylogenetic usefulness of tooth dentine characters, inparticular the phenomenon of petaloid dentine among earlytetrapods.

The lower jaw of the Devonian tetrapodAcanthostegais described for the first time. Redescriptions are provided for the lower jaws of the elpistostegidPanderichthys, the Devonian tetrapodsElginerpeton, Obruchevichthys, Metaxygnathus, VentastegaandIchthyostega, and the Carboniferous tetrapodsCrassigyrinus, MegalocephalusandGephyrostegus. The character distri- butions thus revealed differ considerably from previous accounts, particularly in the wide distribution of certain primitive characters. Meckelian ossification in the middle part of the jaw is widespread among Devonian tetrapods, being demonstrably absent only inAcanthostega.Among Carboniferous tetrapods, a tooth-bearing parasymphysial plate is shown to be present inCrassigyrinusandMegalocephalus(having already been demonstrated by other authors inWhatcheeriaandGreererpeton).A phylogenetic analysis of 26 early tetrapods including all the aforementioned genera, scored for 51 lower jaw characters, produces at least 2,500 equally parsimonious trees. However, the lack of resolution lies largely in a big top end polychotomy containing anthracosaurs, temnospondyls, seymouriamorphs, microsaurs and a nectridean-amniote clade. Below this polycho- tomy, which may correspond approximately to the tetrapod crown group, there is a well-resolved stem-group containing, in descending order,Megalocephalus, Greererpeton, Crassigyrinus, (jaws associated with)Tulerpeton, Whatcheeria, Acanthostega, Metaxygnathus, Ichthyostega, VentastegaandMetaxygnathus(unresolved), anElginerpeton-Obruchevichthysclade, andPanderichthys.This conflicts with recently published phylogenies by Coates and Lebedev & Coates, which placeTulerpetonand all post-Devonian tetrapods in the amphibian or amniote branches of the tetrapod crown group.

A new reconstruction of the skull ofCrassigyrinus scoticus(Lydekker) is provided, based on newly prepared material. A revised interpretation of the naris shows it in most respects to be like that of other Carboniferous tetrapods, lacking both an anterior tectal and a lateral rostral. However it does incorporate an enlarged septomaxilla, and part of the dorsal surface of the vomer is also visible within the naris of most specimens. Prominent paired buttresses run along the midline of the snout, and a single bowl-shaped interpremaxillary fenestra pierces the snout and palate but is not confluent with the naris or choana. A preopercular is probably absent. A new cladistic analysis placesCrassigyrinuswith the Viséan genusWhatcheeria, and puts those two taxa as a sister-clade to the anthracosaursProterogyrinusandPholiderpeton.Temnospondyls form a clade with loxommatids, and these two arms form a dichotomy crownward toGreererpeton, IchthyostegaandAcanthostegawhich are successive stemward plesions.

The postcranial skeleton ofAcanthostega gunnarifrom the Famennian of East Greenland displays a unique, transitional, mixture of features conventionally associated with fishand tetrapod-like morphologies. The rhachitomous vertebral column has a primitive, barely differentiated atlas-axis complex, encloses an unconstricted notochordal canal, and the weakly ossified neural arches have poorly developed zygapophyses. More derived axial skeletal features include caudal vertebral proliferation and, transiently, neural radials supporting unbranched and unsegmented lepidotrichia. Sacral and post-sacral ribs reiterate uncinate cervical and anterior thoracic rib morphologies: a simple distal flange supplies a broad surface for iliac attachment. The octodactylous forelimb and hindlimb each articulate with an unsutured, foraminate endoskeletal girdle. A broad-bladed femoral shaft with extreme anterior torsion and associated flattened epipodials indicates a paddle-like hindlimb function. Phylogenetic analysis placesAcanthostegaas the sister-group ofIchthyostegaplus all more advanced tetrapods.Tulerpetonappears to be a basal stemamniote plesion, tying the amphibian-amniote split to the uppermost Devonian.Caerorhachismay represent a more derived stem-amniote plesion. Postcranial evolutionary trends spanning the taxa traditionally associated with the fish-tetrapod transition are discussed in detail. Comparison between axial skeletons of primitive tetrapods suggests that plesiomorphic fish-like morphologies were re-patterned in a cranio-caudal direction with the emergence of tetrapod vertebral regionalisation. The evolution of digited limbs lags behind the initial enlargement of endoskeletal girdles, whereas digit evolution precedes the elaboration of complex carpal and tarsal articulations. Pentadactylous limbs appear to have stabilised independently in amniote and amphibian lineages; the colosteidGreererpetonhas a pentadactylous manus, indicating that basal amphibian forelimbs may not be restricted to patterns of four digits or less.

SynopsisWell preserved material of the crossopterygian fishEusthenopteronenables fresh reconstructions and interpretations of its postcranial skeleton to be given. Comparisons throughout with other bony fishes show that it may be primitive in many features. Similarities with early amphibians such as the screw-shaped glenoid, the form of the humerus (on which an attempt to restore the pectoral musculature is based), the dorsal bicipital ribs and the possibility of a sacral attachment, throw much light on the origin of the tetrapod postcranial skeleton, particularly of the cheiropterygium. A functional analysis of the skeleton ofEnsthenopteronis attempted, suggesting that it resembled the pike (Esox) in its mode of life and that it may have been capable of short journeys “walking” overland. The possible selective factors stimulating the evolution of such a fish, and further evolution to the tetrapod stage are discussed.