Histological analysis of post-eruption tooth wear adaptations, and ontogenetic changes in tooth implantation in the acrodontan squamate Pogona vitticeps

Abstract

Teeth have been a focus of research in both extinct and extant taxa alike; a significant portion of dental literature is concerned with dental patterning and replacement. Most non-mammalian vertebrates continuously replace their dentition but an anomalous group of squamates has forgone this process in only having one tooth generation; these squamates all have apically implanted teeth, a condition known as acrodonty. Acrodont dentition and various characteristics attributed to it, including a lack of replacement, have often been defined ambiguously. This study explores this type of implantation through histology in the ontogeny of the acrodont agamid Pogona vitticeps. The non-replacing teeth of this squamate provides an opportunity to study wear adaptations, maintenance of occlusion in a non-mammalian system, and most importantly post-eruption changes in the tooth bone interface. In this study the post-eruption changes combined with dental wear likely gives the appearance of acrodont implantation.

Keywords: Acrodont; Histology; Ontogeny; Squamate; Tooth wear.

Figure 1. A comparative figure showing the external morphological differences in the dentition and mandibles between juvenile and adult specimens of P. vitticeps.

(A) Right mandibular ramus of juvenile specimen ROM R8234; (B) Right mandibular ramus of adult specimen ROM R8507;(C) Closeup of wear facets on dentary in adult P.vitticeps (D) superimposed outlines of mandibles of varying ontogenetic stages, showing that most growth occurs in the posterior end of the mandible. Abbreviations; ad, acrodont dentition; cp, coronoid process; pd, pleurodont dentition; wf, wear facets. Arrows indicating un-ankylosed teeth. Scale bar = 1 cm.

Figure 2. Details of external morphology of the juvenile dentition of P. vitticeps.

(A) ROM R8234 in lingual view showing un-ankylosed dentition, the most posterior growing into the coronoid; (B) ROM R8418 scanning electron microscope image of incomplete juvenile mandible with; (C) closeup showing fibrous alveolar bone between the teeth; (D) closeup showing the lingual contribution of dentine to the teeth; (E) closeup showing the youngest tooth growing into and resorbing the coronoid. Abbreviations; ab, location of alveolar bone; cp, coronoid process; de, dentine. Arrows showing un-ankylosed teeth.

Figure 3. Longitudinal histological sections of the juvenile mandible of P. vitticeps with a focus on the dentition.

(A) External view of ROM R8234, box outlining the dentition cut; (B) a section of the posteriormost seven tooth positions. (C) a section of the posterior seven tooth positions in cross polarized light; (D) a schematic representation showing the distinct tissues as well as the progressive ankylosis of the teeth. Abbreviations; ab, alveolar bone; de, dentine; lb, lamallar bone; pc, pulp cavity. Red arrow indicates where the newest tooth is resorbing the previous tooth. Scale bar = 1,000 um.

Figure 4. Coronal sections of juvenile mandible of P. vitticeps with a special focus on tooth histology.

(A) Coronal section of the jaw and tooth of a juvenile specimen of P. vitticeps ROM R8510 showing minimal ossification of the jaw bone, and pleurodont tooth attachment; (B) close up of a juvenile specimen’s (ROM R8510) dentition, showing unworn morphology; (C) coronal section of a juvenile specimen (ROM R8234) showing pleurodont implantation, and remodeling; (D) closeup of the attachment site in ROM R8234, showing the labial side of the dentition being resorbed by an osteoclast; (E) coronal section of the coronoid process (ROM R8234) showing a tooth developing lingually inside the coronoid and dentary; (F) a close up of the tooth in the jaw bone, showing the resorption of the jawbone around newly developing tooth. Abbreviations: ab, alveolar bone; ac, acellular cementum; cb, cancellous bone; cp, coronoid process de, dentine; dt, developing tooth; lb, lamellar bone; en, enamel; ost, osteocyte lacunae; oc, osteoclast; rb, resorption bay; rl, reversal line. All un-labeled scale bars = 1 mm.

Figure 5. Longitudinal sections of adult mandible of P. vitticeps with a focus on tooth histology.

(A) External view of specimen ROM R8507, box outlining the dentulous area sectioned; (B) broad view showing the variation between the anterior and the posterior dentition; (C) a closeup of the worn dentition showing wear facets and remodeling zones; (D) broad view showing the variation between the anterior and the posterior dentition, showing the depth of wear facets and arrangement of vasculature; (E) a close up of the anterior dentition with open pulp cavities and associated vasculature. Abbreviations: de, dentine; jb, jaw bone; pc, pulp cavity; pp, pseudo-pedicle; vc, vascular canal; wf, wear facet. The arrows demarcate the extensive vasculature leading to pulp cavities. Scale bar = 1,000 um.

Figure 6. Coronal sections of adult mandible of P. vitticeps with a focus on tooth histology.

(A) Coronal section of the jaw and tooth of an adult specimen of P. vitticeps (ROM R8507) showing the maintained vasculature and worn enamel on the labial side, in cross polarized light; (B) Coronal section of ‘A’ in normal transmitted light; (C) Coronal section of the jaw and tooth of an adult specimen of P. vitticeps ROM R8507 showing the extensive remodeling of the tooth attachment site as well as the tooth, in cross polarized light; (D) Coronal section of ‘C’ in normal transmitted light. Abbreviations: de, dentine; lb, lamellar bone; en, enamel; vc, vascular canals; rz, remodeling zone. Scale bar = 500 um.

Figure 7. Schematic explanation of implantation changes through ontogeny and tooth wear.

(A) Three basic traditional implantation categories, without attachment tissue illustrated; (B) pleurodont impanated tooth is attached via alveolar bone, yellow arrows show direction of dentine deposition; (C) purple arrows show direction of dentine resorption by odontoclasts, blue arrow shows direction of bone deposition by odontoblasts; (D) blue arrow shows further bone deposition, trabecula in the dentary is reduced; (E) the tooth appears implanted at the apex of the dentary; (F) Shows the position of maxillary tooth; (G) the enamel is worn on the labial side; (H) with advanced wear, the enamel, dentine, and dentary bone are worn on the labial side.