Comparative morphology, histology and growth of the dental plates of the Devonian dipnoan Chirodipterus

Developmental plasticity and disparity in early dipnoan (lungfish) dentitions

Although the lungfish (Dipnoi) belong within the Osteichthyes, their dentitions are radically different from other osteichthyans. Lungfish dentitions also show a uniquely high structural disparity during the early evolution of the group, partly owing to the independent variation of odontogenic and odontoclastic processes that are tightly and stereotypically coordinated in other osteichthyans. We present a phylogenetic analysis of early lungfishes incorporating a novel approach to coding these process characters in preference to the resultant adult dental morphology. The results only partially resolve the interrelationships of Devonian dipnoans, but show that the widely discussed hypothesis of separate tooth-plated, dentine-plated, and denticulated lineages is unlikely to be true. The dipnoan status of Diabolepis is corroborated. Lungfish dentitions seem to have undergone extensive and nonparsimonious evolution during the early history of the group, but much of the resulting disparity can be explained by a modest number of evolutionary steps in the underlying developmental processes, those for dental formation (odontogenic) and those for the remodeling of dentine tissue (odontoclastic). Later in lungfish evolution, this disparity was lost as the group settled to a pattern of dental development that is just as stereotypic as, but completely different from, that of other osteichthyans.

Prismatic dentine in the Australian lungfish, Neoceratodus forsteri (Osteichthyes: Dipnoi)

The Australian lungfish, Neoceratodus forsteri, has a dentition consisting of enamel, mantle dentine and bone, enclosing circumdenteonal, core and interdenteonal dentines. Branching processes from cells that produce interdenteonal dentine leave the cell surface at different angles, with collagen fibrils aligned parallel to the long axis of each process. In the interdenteonal dentine, crystals of calcium hydroxyapatite form within fibrils of collagen, and grow within a matrix of non-collagenous protein. Crystals are aligned parallel to the cell process, as are the original collagen fibrils. Because the processes are angled to the cell surface, the crystals within the core or interdenteonal dentine are arranged in bundles set at angles to each other. Apatite crystals in circumdenteonal dentine are finer and denser than those of the interdenteonal dentine, and form outside the fibrils of collagen. In mature circumdenteonal dentine the crystals of circumdenteonal dentine form a dense tangled mass, linked to interdenteonal dentine by isolated crystals. The functional lungfish tooth plate contains prisms of large apatite crystals in the interdenteonal dentine and masses of fine tangled crystals around each denteon. This confers mechanical strength on a structure with little enamel that is subjected to heavy wear.

Unique dentition of lungfish

Lungfish are a group of ancient fish, represented almost continuously in the fossil record from their first appearance in the Devonian to the present time. They have numerous unique characters in the dentition, found only among lungfish, as well as some that are shared with other groups of fish and with higher vertebrates, such as a thin layer of true enamel, instead of the enameloid found in the dentition of most fish. Unique characters include the diversity of forms of dentine in the tooth plates, as well as the organisation of the dentines and the unusual mode of continuous growth of the dentition. Their enamel is based on amelogenin formed by the ameloblasts of the dental lamina, and is mineralised by long crystals of calcium hydroxyapatite deposited in layers over the developing tooth. The different forms of dentine, which show progressive evolutionary change within the group, are based on an extracellular matrix of collagen, mineralised with calcium hydroxapatite of variable crystalline form. Cells that line the oral cavity produce the ameloblasts that are endodermal in origin, and cells that form the dental papillae are, like the other cells that secrete skeletal tissues, derived from head mesoderm, as in other vertebrates. Unique to all dipnoans is the fusion of the dental tissues to the underlying bone and the way in which bone, included within the tooth plate, remodels to permit growth in the whole dentition.

Morphology and phylogenetic position of the holodipteran dipnoans of the Upper Devonian Gogo Formation of northwestern Australia

Holodipterus (Holodipterus) gogoensis was first described by Miles ( Zool. J. Linn. Soc . 61, 1-328 (1977)) from eight specimens from the Frasnian Gogo Formation of the Canning Basin, Western Australia. In 1991 H. longi was described by Campbell & Barwick from a single specimen from the same locality. Subsequent collecting has produced two other new species which we assign to two new subgenera, described herein as H. (Holodipteroides) elderae and H.(Asthenorhynchus) meemannae . Diagnoses of the two new subgenera are given. A small isolated palate, which provides new information on the mode of growth of the group, is described as Holodontid gen. et sp. indet. It is shown that all these holodipterans have a distinctive mode of dental growth involving periodic and extensive resorption of denticles, teeth and calluses. Hypermineralized dentine occurs in both the teeth and the calluses. Smith (in Campbell & Smith, Rec. Aust. Mus . 39(3), 131-167 (1987); and in Smith, Mém. Mus. natn. Hist, nat., Paris C53, 179-194 (1988)) has argued that because teeth are developed only in dental laminae, which are complicated structures unlikely to have been evolved more than once, they are reliable guides to understanding phyletic relationships. Consequently she considered that holodipterans should be regarded as derivatives of a Speonesydrion-ike dipnoan that retained and developed numbers of shedding denticles. Campbell (in Campbell & Smith 1987), and Campbell & Barwick (in Early vertebrates and related problems of evolutionary biology (ed. M.-m. Chang et al .), Beijing Science Press (1991)) argued that teeth have evolved several times in dipnoans and that their presence in holodipterans is not indicative of relationship. The latter authors have given reasons for considering Holodipterus to be a member of the so-called ‘rasping’ or ‘denticle shedding’ lineage which can be traced back to the Early Devonian Uranolophus . The newly described subgenera add information vital for clarifying the position of the holodipterans. H. ( Holodipteroides ) had no true teeth, but it produced radial ridges on the palate and prearticulars simply by adding new raised tissue at the end of the radial ridges, and it had denticulated plates attached to the surface of the basihyal. H. (Asthenorhynchus) had a pectoral girdle of the same kind as Griphognathus and a number of dermopalatines and other denticulated plates at the anterior end of the palate, as does H. (Holodipterus) gogoensis and Griphognathus whitei , which is the best documented member of the ‘denticle-shedding’ lineage. It is also shown that holodipterans and Griphognathus shared a distinctive pustulose/ perforate structure of the head bones. We therefore conclude that holodipterans, Griphognathus (and perhaps other rhynchodipterids) developed from some unspecified Middle Devonian stock that underwent rapid radiation either at the end of the Middle Devonian or in the early Late Devonian (Frasnian).

The dipnoan species, Sorbitorhynchus deleaskitus , from the Lower Devonian of Guangxi, China

A dipnoan species from the Emsian of Guangxi, China, Sorbitorhynchus deleaskitus , which has previously been diagnosed in the appendix to a more general paper on dipnoan phylogeny (Cambell & Barwick 1990), is described in detail. The head is large and heavily ossified, with cosmine on the external dermal bones. The dental plates are thick and have few poorly defined large tuberosities towards their anterolateral edges. It is the only Early Devonian dipnoan for which the full hyoid arch, gular, basibranchial, and submandibular plates are preserved, and in which the parasphenoid is clearly defined. In many features the genus is more advanced than Dipnorhynchus , but in none is it more advanced than Dipterus , apart from the loss of the buccohypophysial foramen. It gives a new perspective on late Emsian evolution of the group, and reinforces our concept of an independently developing dentine-plated line contemporaneous with a true tooth-plated line.

The relationships of Uronemus: a carboniferous dipnoan with highly modified tooth plates

Previous accounts of the dentition of the Carboniferous dipnoan Uronemus have stressed the significance of the scattered small denticles. These, together with the marginal teeth and ridges, have been interpreted as primitive characters of the dipnoan dentition shared with three other genera: the Devonian Uranlophus and Griphognathus and the Carboniferous to Permian Conchopoma. Genera with tooth plates have been considered to be a monophyletic group in which tooth plates are a derived character; Uronemus has been excluded from this group in all previous investigations dealing with the significance of the dentition for determining relationships among dipnoans. The macromorphology of the dentition of Uronemus has been re-examined and correlated with the histology of all the dental tissues. Optical study of thin sections and scanning electron microscope study of the adjacent cut surfaces has shown that the hard, wear-resistant dentine of the teeth and ridges is petrodentine. The arrangement, growth, wear and histology of the dental tissues have been compared with those of denticulated and tooth-plated genera. The arrangement of new teeth relative to the tooth ridge, the pattern of wear along the ridge, and the type of dentine and its growth indicate that the dentition of Uronemus is best interpreted as a tooth plate with one long lingual tooth ridge and reduced lateral tooth rows. Therefore the marginal tooth ridges are not considered to be homologous with those of denticulate dipnoans such as Uranolophus. The presence of petrodentine, a tissue type only found in forms with tooth plates, is consistent with the view that the dentition is derived by modification of a radiate tooth plate. The denticles covering restricted regions of the palate and lower jaw are considered to have been a secondary acquisition. The suggestion that Conchopoma is a close relative of Uronemus is not accepted, and possible new relationships have been proposed. New data on Scaumenacia and Phaneropleuron, two other genera previously compared with Uronemus, are presented. Rhinodipterus, a form with elongate lingual ridges, is also discussed. Phaneropleuron is shown to have radiate tooth plates and not a marginal row of conical teeth as previously described. It is proposed that the tooth plate of Uronemus is derived from a dipterid type of plate. A discussion of some of the other factors involved in determining the relationships of the genus is given.(ABSTRACT TRUNCATED AT 400 WORDS)