ABSTRACT
Lungfi sh have a long evolutionary history, fi rst appearing in the Early Devonian, with three genera extant. Lungfi sh dentitions were particularly diverse and have been a focus of study for many years. Although diverse, all dentitions can be derived from a toothplated dentition, where components of this dentition, in terms of tooth structures and the processes controlling development, have become dissociated and free to vary. Despite previous suggestions that lungfi sh dentitions are not homologous to dentitions in other sarcopterygian and actinopterygian taxa (osteichthyans), new research on Neoceratodus forsteri indicates several shared similarities in terms of genes involved in dental patterning, tooth origin and positioning on the jaw, and contribution of neural crest cells to tooth development. Future research should expand on these early results, and continue particularly to study genes from the ‘core dental gene network’, found in other fi shes. Keywords: dentitions, evolution, tooth pattern, Necoceratodus, protopterus, Devonian
Here we will review the growth and construction of the lungfi sh dentition through evolutionary time of selected dipnoans both fossil and extant, but with a primary focus on the toothplated dentition. Th is represents the plesiomorphic lungfi sh dentition, and one from which all other lungfi sh dentitions can be derived, as all are based on some modifi cation of the teeth characterising toothplated lungfi sh. Th e teeth of these in turn are homologous to other osteichthyans, although patterning on the dentate bones has been highly modifi ed. Th e majority of lungfi sh species are supremely and uniquely adapted to crushing all food prey using paired, opposing tooth plates in upper and lower jaws (see Figure 1 for examples of these). Th is crushing action occurs several times between powerful jaws, equipped with these palatal and lingual tooth plates, before sucking in the partly regurgitated food mass to swallow. Th is behaviour is observed in the three genera now living in Australia, Africa and South American (Bemis and Lauder 1986). Although this is an extremely specialized dentition it can be shown to have formed early in the fossil record from a developmental dental pattern that has been conserved for 400 Myr since the early Devonian (Reisz and Smith 2001) and as the closest living relatives of Tetrapoda this raises some fundamental questions (Ahlberg et al. 2006; Hällstrom and Janke 2009). Exactly how did the evolutionary process of creating such a unique form of dentition come about, early in the history of the group? Th is has been hard to reconcile with the stereotypical osteichthyan pattern. Both the arrangement of teeth and the fact that all teeth are retained throughout life contrasts with the osteichthyan dentition (including tetrapods), with teeth arranged in conventional marginal linear rows. In the osteichthyan tooth rows, loss of the old tooth usually occurs in a regulated way and is linked with timed addition of teeth in each alternate tooth position in the jaws. Undoubtedly the evolutionary change characterizing lungfi sh dentitions occurred through extensive modifi cation of the developmental programme of the stereotypic osteichthyan dentition, with its marginal and palatal rows of replaceable unitary teeth. Nevertheless, similarities to the osteichthyan dentition can be demonstrated and some of these are conserved in the early pattern of the dentition (Smith et al. 2009). A generalised model for the development of lungfi sh tooth plates was proposed, through a study of the pattern of tooth development and growth in the larval tooth plates of the African species Protopterus aethiopicus and by comparison with Neoceratodus forsteri, the extant Australian lungfi sh (Smith 1985). At that time this accounted for all published data (Kemp 1977, 1979) but many papers on Neoceratodus have since been published. However, recent insights into developmental regulation of the patterning process have been off ered by in situ gene expression data combined with skeletal preparations, in a new analysis of the early larval stages of Neoceratodus forsteri (Smith et al. 2009). We also now know that despite earlier reports to the contrary (Kemp 1995b), very early in embryonic
development, neural crest cells participate in tooth development and this odontogenic fate of crest-derived mesenchyme (ectomesenchyme) is conserved at least from lungfi sh to tetrapods (Kundrat et al. 2008). It is especially informative to compare detailed data on tooth development in extant forms with early growth stage information obtained from Devonian juvenile forms and late growth stages in the adult (Bemis and Lauder 1986; Reisz and Smith 2001; Smith and Krupina 2001; Smith et al. 2002). Th is type of data on developmental pattern can be obtained from amongst fossil dipnoans as the dentition is retained for life without shedding any of the separate teeth so that each tooth plate retains a history of its development (Denison 1974; Smith 1986) (see Figure 1F, G, H illustrating Devonian juvenile and adult plates of Andreyevichthys epitomus and Gogodipterus paddyensis). Adults have tooth plates on the palate and the lingual side of the lower jaws (prearticular), formed from anterior or lateral addition of teeth to radial rows diverging from the oldest teeth at the medial side of each (see Figure 1C, D, E for examples of Devonian and extant forms with new teeth added at the lateral margins). It has long been understood that individual teeth are consolidated into dental plates, some called tooth plates, without loss of teeth through shedding (statodont dentition: compare juvenile and adult form of the Devonian Andreyevichthys in Figure 1F, G). Th ese retained teeth (Figure 1F, G, I, J), or fl attened and worn tooth tissues, (as in Neoceratodus adult, Figure 1A, B) have extensive and continuous growth of their dentine below the biting surface. Not only does the dentine grow invasively into the bone (Smith 1985, 1986) but also it is formed of a special and unique extra hard type called petrodentine (Smith 1984). In this statodont dentition all teeth are retained and those formed at the earliest stages of construction of the tooth plates (as in the Devonian juvenile and adult plates, Figure 1F, G) were patterned in early development by regulatory systems (several aspects of these that are in common with other osteichthyan fi shes are discussed below). Th ese fi rst teeth are oft en left in situ to allow one to interpret order of tooth initiation from their relative sizes, degree of wear and retained positions (Figure 1H). By comparison between the Late Devonian hatchling tooth plates of Andreyevichthys epitomus and the living form Neoceratodus forsteri (Smith and Krupina 2001; Smith et al. 2002), it was proposed that developmental
constraints conserve this evolutionary pattern. Th at is, one where teeth are only added to one end of each radial row. Th is particular pattern has been conserved since the Early Devonian form Diabolepis speratus, recognized as the sister group of the ‘undisputed’ stem+crown dipnoans (Ahlberg et al. 2006, also see Johanson and Ahlberg, this volume). It has recently been proposed from the study of early tooth patterning in the lower jaw of Neoceratodus (Smith et al. 2009) that the fi rst three teeth in the marginal row form in a pattern linked with that of stereotypic osteichthyans (these are illustrated in Figure 2C-H). Th is provides the basis for how this ancient dipnoan pattern was modifi ed very early in its evolutionary history, at least in the lower jaw marginal pattern, and how this may have evolved is explored further in the sections below.
