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Crystallite Orientation Discontinuities and the Evolution of Mammalian Enamel – Or, When Is a Prism?

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Crystallite Orientation Discontinuities and the Evolution of Mammalian Enamel – Or, When Is a Prism? Scanning Microscopy Volume 3 Number 2 Article 28 6-20-1989 Crystallite Orientation Discontinuities and the Evolution of Mammalian Enamel – Or, When is a Prism? K. S. Lester Westmead Hospital Dental Clinical School W. von Koenigswald University of Bonn Follow this and additional works at: https://digitalcommons.usu.edu/microscopy Part of the Life Sciences Commons Recommended Citation Lester, K. S. and von Koenigswald, W. (1989) "Crystallite Orientation Discontinuities and the Evolution of Mammalian Enamel – Or, When is a Prism?," Scanning Microscopy: Vol. 3 : No. 2 , Article 28. Available at: https://digitalcommons.usu.edu/microscopy/vol3/iss2/28 This Article is brought to you for free and open access by the Western Dairy Center at DigitalCommons@USU. It has been accepted for inclusion in Scanning Microscopy by an authorized administrator of DigitalCommons@USU. For more information, please contact [email protected]. Scanning Microscopy, Vol. 3, No. 2, 1989 (Pages 645-663) 0891-7035/89$3.00+.00 Scanning Microscopy International, Chicago (AMF O'Hare), IL 60666 USA CRYSTALLITEORIENTATION DISCONTINUITIES AND THE EVOLUTION OF MAMMALIANENAMEL - OR, WHENIS A PRISM? K.S. Lester • 1 and W. von Koenigswald 2 1 Westmead Hospital Dental Clinical School, Westmead, Australia 2 Paleontological Institute, University of Bonn, Bonn, West Germany (Received for publication January 19, 1989, and in revised form June 20, 1989) Abstract Introduction The nature and relationship of crystallite The existence of an additional crystallite domains have been explored in fossil and extant orientation discontinuity, minor boundary plane, enFels spanning an evolutionary period of 200 x or seam has been described and illustrated by 10 years. Minor crystallite orientation discon­ scanning electron microscopy as a consistent tinuities, either linear or planar, were found to feature of the enamel of many Chiroptera and of be consistent characteristics of all specimens the dermopteran Cynocephalus (Lester and Hand, examined. 1987; Lester, et al., 1988). The seam occurs where The earliest minor discontinuity is linear the horse-shoe shaped prism boundary is incomplete (convergence line), shown here in Oligokyphus and and contiguous with the interprism {Fig. 1). Eozostrodon. The convergence line would be the Crystallites on either side of the seam lie at an result of the occasional development of a conical angle to it within the longitudinal axis of the Tomes' process to the parent ameloblast. An prism so as to subtend an acute angle with the increase in number and regularity of convergence enamel-dentine junction (usually 65-70°). The lines, shown here in Haldanodon, marks the appear­ seams are not evident where the prism boundaries ance of a regular pseudoprismatic enamel struc­ are complete. which. in Chiroptera, is most often ture. in the outer one third where the enamel is thick. The second minor discontinuity to appear is The developmental basis for seams has been des­ planar (seam), shown here in a dryolestid eupanto­ cribed in terms of the morphology of the formative there. The seam has previously been deduced to front, which, for most practical purposes, may be relate developmentally to a central groove on the taken to be the same thing as the mineralizing sloping floor-wall of the Tomes' process pit. front. A seam may be related to a consistent Coincident with the appearance of the seam is groove in the most superficial part of the that of a rudimentary major planar discontinuity developing floor wall of the Tomes' process pit which does not enclose a domain to constitute what (Lester and Boyde, 1987). would normally be acknowledged as a prism. Its A clue to the possible significance of the developmental basis would be the establishment of seam was found in the enamel of the vampire bat a steep wall and floor (however partial in (Desmodus rotundus murinus) {Lester et al., 1988). circumference) to the Tomes' process pit. Here, the degree of prism development is relative­ The extent of the major planar discontinuities ly poor throughout, with normal prism demarcation was found to increase subsequently to enclose a progressively lost in the outer third of the classically recognizable prismatic domain, shown cuspal enamel and progressively in the thinning here in Amphiperatherium, Hassianycteris, Smilodon cervical enamel. The seams, however, present in and Felis. This would be consistent with the conjunction with the prisms, persist in a recog­ further development of a definitive floor and wall nizable form in the absence of definitive prisms to the Tomes' process pit. both cuspally and cervically (Fig. 2). The sequential appearance of minor linear, We were subsequently impressed by the minor planar and major planar discontinuities in similarity of these non-prismatic areas of the crystallite orientation is seen as fundamental to enamel of Desmodus to the enamel of some fossil the evolution of mammalian enamel structure. mammals we had begun to examine. Our aim in this paper is to demonstrate the presence, and discuss the possible evolutionary significance of, both minor and major crystallite orientation KEYWORDS: enamel, evolution, mammals, prisms, discontinuities in the enamel of a range of fossil pseudoprisms, convergence lines, seams. and extant mammals and in an advanced therapsid. The description of these features, together with a •Address for Correspondence: recent analysis of Procerberus enamel which Keith S. Lester, Westmead Hospital Dental Clinical displays prismatic, pseudoprismatic and aprismatic School, Westmead, N.S.W. 2145 AUSTRALIA. Phone No: forms in the one surface (Lester, 1989b), prompt a {02)633.7173 fresh look at our conceptualization of the development and evolution of enamel. 645 K.S. Lester and W. von Koenigswald A scheme of descriptive terms, with suggested Materials and Methods preferred terminology in italics and alternative terms (in brackets), is offered below together Enamel from teeth or tooth fragments of the with some definitions. following taxa were examined. Crystallite orientation in enamel may be: Oligokyphus sp., ictidosaurian therapsid, continuous - aprismatic enamel (non-prismatic, Rhaeto-Liassic, Mendip Hills, Somerset, England. prisrnless) Several teeth provided by T. Rich, Melbourne. or Eozostrodon parvus, rnorganucodontid mammal, discontinuous - pseudoprismatic enamel# (pre­ Rhaeto-Liassic, Ewenny Bridgend, Glamorgan, Wales. prisrnatic); prismatic enamel Several teeth provided by K.A. Joysey, Cambridge. if discontinuous, the discontinuity may be: Haldanodon expectatus, docodont mammal, Kirnrne­ linear - convergence line* ridgian, Guimarota coal mine, Portugal. Several or tooth fragments provided by B. Krebs, Berlin. planar Eupantothere, dryolestid mammal, Kirnrneridgian, if planar, the discontinuity may be: Guirnarota coal mine, Portugal. Several tooth frag­ Geological\ minor - seam• (minor boundary plane) ments provided by B. Krebs, Berlin. or Amphiperatherium sp., didelphid marsupial, time major¢ Middle Oligocene, Moehren 13, Bavaria, W. Germany. if major, the discontinuity may be: Teeth provided by K. Heissig,. Muenchen. rudimentary Hassianycteris messelensis, palaeochiropteran, partial - partial prism boundary Middle Eocene, Messel near Darmstadt, W. Germany. definitive - prism boundary One tooth provided by G. Storch, Frankfurt. (major boundary plsne, border \ discontinuity, prism sheath, prism border) Definitions # Pseudoprisrnatic enamel: is a discontinuous Fig. 1. Syconycteris australis (chiropteran) enamel characterised by repetitive domains related enamel: oblique transverse section of prisms in a developmentally to conical Tornes' processes. Each polished, etched specimen showing the consistency domain (pseudoprisrn) is organised between minor and regularity of the seams (at arrows) in linear discontinuities in crystallite orientation association with the open ends of the horseshoe­ (convergence lines) each of which traces the path shaped prisms (p) and contiguous interprisrnatic of the tip of the Tornes' process of the parent enamel (ip). Bar= 10 µrn. ameloblast through the enamel during formation. * Convergence line: is a minor linear discon­ Fig. 2. Desmodus rotundus murinus (chiropteran) tinuity in crystallite orientation and appears as enamel: longitudinal section of outer third of a convergence of crystallite tips on a linear cuspal enamel in a polished, etched specimen focus. It is related developmentally to and traces showing the persistence of a seam (? convergence the withdrawal of the conical tip of the Tornes' line) (arrowed) in the absence of a definitive process of the ameloblast through enamel during prism. Bar= 1 µrn. development. + Seam: is a minor planar discontinuity in crys­ Fig. 3. Oligokyphus sp. (ictidosaurian therapsid) tallite orientation and appears as a convergence enamel: etched transverse fracture in cervical of crystallites to form a minor boundary plane region showing convergence lines (at arrows}, with often in association with a typical horseshoe­ associated angled crystallites, extending through shaped prism. It is related developmentally to the the bulk of the enamel thickness (edj enamel­ occurrence of a central groove on the more super­ dentine junction; oes outer enamel surface). ficial part of the sloping floor-wall of the Compare with Fig. 2. Bar= 10 µm. Tornes' process pit. ~ A major planar discontinuity in crystallite Fig. 4. Eozostrodon parvus (morganucodontid orientation is a plane in enamel at which crystal­ mammal) enamel: etched transverse fracture
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