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Dental caries in an Early CretaceousichthyosaurBenjamin P. Kear aa South Australian Museum, North Terrace, Adelaide, 5000 E-mail:b Vertebrate Palaeontology Laboratory, School of BiologicalSciences, University of New South Wales, Sydney, 2052Published online: 27 Nov 2008.

To cite this article: Benjamin P. Kear (2001) Dental caries in an Early Cretaceousichthyosaur, Alcheringa: An Australasian Journal of Palaeontology, 25:4, 387-390, DOI:10.1080/03115510108619228

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Dental caries in an Early Cretaceous ichthyosaur

BENJAMIN P. E A R

KEAR, B. P., 2002.01.31. Dental caries in an Early Cretaceous ichthyosaur. Aleheringa 25, 387-390. ISSN 0311-5518.

A single anterior dentary tooth, recovered with a near complete juvenile ichthyosaur skull from the Early Cretaceous (upper Albian) Hughenden-Richmond region of north- ern Queensland, has revealed the presence of a potential dental caries (tooth decay) infection. Dental caries is a disease affecting hard dental structures including the cemen- tum and results in cavitation. The implications of dental caries development on tooth replacement and feeding in Cretaceous ichthyosaurs are discussed.

Benjamin P. Kear, South Australian Museum, North Terrace, Adelaide 5000 (present address) and Vertebrate Palaeontology Laboratory, School of Biological Sciences, Uni- versity of New South Wales, Sydney 2052;received 7 May 2000, accepted 7 September 2001;[e-mail:kear.ben@saugov.sa.gov.au].

Keywords: dental caries, ichthyosaur, palaeopathology, dental morphology.

EXAMINATION of the dentition in a juvenile ichthyosaur skull (AM F98273), attributable to Platypterygius longmani Wade, 1990 from the Early Cretaceous of Queensland, has revealed the presence of a potential dental caries (tooth decay) infection. Dental caries is a progressive disease involving decalcification, decomposition of organic constituents and bacterial invasion of hard dental structures including the cementum (Kalnins 1965, Guinta 1984). It occurs in response to entrapment and decay of food particles and results in cavitation.

Dental caries is rare in fossil reptiles, probably because of cont inuous and rapid tooth r ep lacemen t (Rothschi ld & Tanke 1992, Rothschild & Martin 1993). Indeed, among Mesozoic marine repti les, the only well- documented case involves a dental abscess in the mandible of a Late Cretaceous mosasaur from Belgium (Moodie 1917). Pathological damage in the teeth of ichthyosaurs is generally restricted to crown breakage in response to feeding (Massare 1987). Well-developed dental caries therefore represent a previously unrecognised pathological phenomenon for the group. This paper describes the dental morphology and a

0311/5518/2001/04387-4 $3.00 © AAP

potential dental caries infection in the Australian Cre taceous i ch thyosaur P l a t y p t e r y g i u s longmani. The implications of dental caries infection on tooth replacement and feeding are discussed.

Materials and methods

AM F98273 consists of a near complete juvenile skull including in excess of 100 well preserved functional and replacement teeth from both the upper and lower jaws. Only the dental morphology of the specimen is described below.

AM F98273 is derived from the late Albian (Day 1969, Smart & Senior 1980) Toolebuc Format ion o f ' D u n r a v e n ' Stat ion, near Hughenden , nor th-cent ra l Queensland. Ichthyosaur dental terminology follows Edmund (1960, 1969), Kirton (1983), Mazin 1983, Bardet (1990) and Montani (1997a, b). Hard dental structures potentially open to dental caries infection are defined as including the cementum following Kalnins (1965) and Guinta (1984). Institutional abbreviation AM refers to the Australian Museum, Sydney. All measurements were taken using digital calipers and are in millimelres (nun).

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388 BENJAMIN P. KEAR A L C H E R I N G A

DC B

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Fig. 1. Dentition of juvenile ichthyosaur skull AM F98273. A, anterior dentary tooth showing dental caries-like cavitation. B, labelled diagram of same tooth, hatched area is damaged. C, complete anterior dentary tooth from adjacent tooth position. D, Scanning electron micrograph of potential dental caries infection area, top-right section of cavity damaged during analysis. Scale bars for A-C are 10 mm. Abbreviations: CM, cementum; DC, dental caries cavity; DR, exposed dentine ring; EL, crown enamel layer; RP, resorption pit.

Description

The dentition of AM F98273 consists of in excess of one hundred tightly interlocking functional

(non-replacement) teeth, present in both the upper and lower jaws. Implantation is aulacodont (sensu Mazin 1983, Montani 1997a) with teeth set into a common dental groove. Each individual tooth

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ALCHERINGA DENTAL CARIES IN AN EARLY CRETACEOUS ICHTHYOSAUR 389

(Fig. 1A-C) is conical with a longitudinally striated enamel crown. This is separated from the massive sub-rectangular root by an inset ring of exposed dentine. The external surface of the tooth root bears a thick layer of cementum, which when in situ, lies entirely beneath the bony jaw-line and is completely enclosed within the dental groove (suggesting probable coverage by gingiva in life). All of the recovered functional teeth exhibit well- developed resorption pits on the lingual surface of the root. These are commonly associated with partly formed replacement teeth.

The potential dental caries infection in AM F98273 is restricted to a single anterior dentary tooth (Fig. 1 A, B), which bears a prominent, U- shaped cavity (1.83 mm high, 3.97 mm wide), on its median-lingual surface. The cavity (Fig. 1D) occurs immediately ventral to the constricted dentine ring, passing through the cementum layer into the underlying orthodentine. There is no involvement of the enamel layer. The cavity is 'blind-ended' and has no contact with the pulp cavity nor is it associated with any post-mortem fracturing. This pattern resembles a cemental caries-type infection, developing in response to recession of the gingiva and exposure of the vulnerable tooth root (Kalnins 1965, Guinta 1984).

Discussion

Initiation of the disease process in AM F98273 may have resulted from a combination of factors including mode of tooth implantat ion, replacement and feeding habits.

The teeth in species of Platypterygius are numerous and densely packed along the dental groove, with characteristic robust rectangular roots (Bardet 1990). This arrangement leaves little space between tooth roots; however the conical crown bases remain separated by narrow crevice- like gaps, providing potent ial points for entrapment of food particles and initiation of dental caries.

Tooth replacement was a continuous process in post-Triass ic ichthyosaurs , involving development of the replacement tooth outside the pulp cavity (distolingual to the functional

tooth) and at least partial resorption of the functional tooth base (Edmund 1960, 1969; Montani 1997a, b). The timing of this process is unknown but slow or periodic replacement would have promoted dental caries by allowing infection to set in over an extended period of time. Retarded replacement rates may also be indicated by the presence of rounded broken teeth in many ichthyosaur specimens (Massare 1987), implying retention of teeth long enough for secondary wear to take place.

The dentition in species of Platypterygius corresponds to the "smash-guild" of Massare (1987), denoting a preference for fairly soft- bodied prey such as belemnites, thin-shelled ammonites and small fish. The robust conical tooth crowns, blunt tips, limited curvature and deep roots, however, suggest an ability to withstand high loadings, possibly incurred during processing of food by 'shake-feeding'. This involves the predator grasping part of the prey and shaking it from side-to-side thus using dynamic loading to deform and tear (Taylor l 987). Such active dismemberment could facilitate entrapment of food particles between the teeth and thus establish favourable conditions for development of dental caries.

Acknowledgments

Many thanks to Robert Jones of the Australian Museum for generous provision of specimens. Preparation facilities and materials were supplied by Prof. M. Archer (Australian Museum) and staff at the Vertebrate Palaeontology Laboratory, University of New South Wales. Scanning electron microscopy was conducted by Miles Davies of the University of Adelaide. This manuscript benefited greatly from the comments of Neville Pledge and a second anonymous reviewer. Financial support for this research was provided by the Australian Research Grant Scheme (grants to M. Archer), Origin Energy, The Advertiser, The Waterhouse Club and the Coober Pedy Tourism Association.

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