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A Systematic Study on Tooth Enamel Microstructures of Lambdopsalis bulla (Multituberculate, Mammalia)--Implications for Multituberculate Biology and Phylogeny - PubMed

Fangyuan Mao  1 ,

A Systematic Study on Tooth Enamel Microstructures of Lambdopsalis bulla (Multituberculate, Mammalia)--Implications for Multituberculate Biology and Phylogeny

Fangyuan Mao et al. PLoS One. .

Abstract

Tooth enamel microstructure is a reliable and widely used indicator of dietary interpretations and data for phylogenetic reconstruction, if all levels of variability are investigated. It is usually difficult to have a thorough examination at all levels of enamel structures for any mammals, especially for the early mammals, which are commonly represented by sparse specimens. Because of the random preservation of specimens, enamel microstructures from different teeth in various species are often compared. There are few examples that convincingly show intraspecific variation of tooth enamel microstructure in full dentition of a species, including multituberculates. Here we present a systematic survey of tooth enamel microstructures of Lambdopsalis bulla, a taeniolabidoid multituberculate from the Late Paleocene Nomogen Formation, Inner Mongolia. We examined enamel structures at all hierarchical levels. The samples are treated differently in section orientations and acid preparation and examined using different imaging methods. The results show that, except for preparation artifacts, the crystallites, enamel types, Schmelzmuster and dentition types of Lambdopsalis are relatively consistent in all permanent teeth, but the prism type, including the prism shape, size and density, may vary in different portions of a single tooth or among different teeth of an individual animal. The most common Schmelzmuster of the permanent teeth in Lambdopsalis is a combination of radial enamel in the inner and middle layers, aprismatic enamel in the outer layer, and irregular decussations in tooth crown area with great curvature. The prism seam is another comparably stable characteristic that may be a useful feature for multituberculate taxonomy. The systematic documentation of enamel structures in Lambdopsalis may be generalized for the enamel microstructure study, and thus for taxonomy and phylogenetic reconstruction, of multituberculates and even informative for the enamel study of other early mammals.

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Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Fig 1. General pattern of crystallite orientation in tooth enamel of Lambdopsalis bulla.

This pattern outlines the general enamel structures for all teeth with prisms that we examined, including incisors, premolars and molars. Sketch on the left shows the crystallite orientations on different sections of a sampled tooth enamel, including cross section (A), longitudinal section (B) and tangential section (C). The SEM-images on the right show the actual details of crystallite orientations on corresponding sections. The left sides of A and B are toward the OES and the right sides of them toward the EDJ. The upper side of C is towards the occlusal surface of a tooth and its bottom towards the cervical part of a tooth. Abbreviations: EDJ: enamel dentine junction; IPM: interprismatic matrix; OES: outer enamel surface; P: prism; S: sheath; Sm: seam; T: tubule.

Fig 2. Enamel prism shape in different teeth of Lambdopsalis bulla.

Cross-sections of (A) Left M2 (V 20298.7) and (B) Left M1 (V 20298.5) showing true outline of the sheath and shape of the seam; (C) The prism shape of the right di (V 20300.1); (D) Prisms of right m1 (V 20299–3) in insufficiently acid prepared section; E-H. Distortions of the prism shape caused by the cutting plane with an angle to the long axis of the prisms in the enamel of right I2 (V 20298.3), right m1 (V 20297.5), right p4 (V 20299–4) and right M2 (V 20298.8). All pictures are SEM-images except B, which is an optic image. Abbreviations: EDJ: enamel dentine junction; I: interprismatic matrix; OES: outer enamel surface; P: prism; S: sheath; Sm: seam; T: tubule.

Fig 3. Prism shape in cross sections of different teeth in Lambdopsalis bulla.

Although the prism shape in several pictures has some distortion, no distinct variation of the prism shape among prisms of different matured permanent teeth exist in the materials we examined. (A) Right I2 (V 20299-2); (B) Right i2 (V 20300.2); (C) Right P4 (V 20299-3); (D) Right p4 (V 20299-4); (E) Right M1 (V 20298.6); (F) Left M2 (V 20297.6); (G) Right m1 (V 20297.5); (H) Right m2 (V 20297.4).

Fig 4. Variation of prism shape in tooth enamel of Lambdopsalis bulla.

(A–B) Cross-sections of left M2 (V 20297.6) showing the atrophic prism in the outer part of mature enamel, Optic image (A) and SEM-image (B) for the same teeth; (C–D) Optic images of cross-section in different parts in the same right I2 (V 20298.2), cuspal area with prisms (C) and aprismatic enamel layer in cervical area of the tooth (D); (E) Optic image of cross-section in a right di (V 20300.1) with aprismatic enamel layer. C–E with the same magnification.

Fig 5. Cross section showing aprismatic layers in the cervical enamel of right DI (V 20299-1) of Lambdopsalis bulla.

Aprismatic enamel is formed by apatite crystallites in columns that are clearer on the transmission light image (A); the EDJ is more distinct under polarized light (B); (C) Aprismatic enamel lacks the trace of the sheath and seam under SEM images; (D) Close-up view showing the enamel portion of (C).

Fig 6. Schematic diagram showing the method used for calculate the prism density.

The cross-section of the enamel of left M2 (V 20297.6) is divided into the inner, middle and outer zones with equal width. Because the EDJ and particularly the OES are usually uneven, the boundary is drawn in a way that the empty region inside the line will be roughly compensated by the enamel outside of the line. The area of each zone is equal and the number of the prisms in each zone can be counted. For a prism that is crossed by the line as two roughly equal parts (a), then 0.5 prism will be counted for each zone on the left and right side of the line. If a prism is unevenly divided by the line (b), then the prism will be counted as belonging to the zone in which the larger part of the prism is located.

Fig 7. Enamel tubules in the tooth cross-section of Lambdopsalis bulla.

(A–B) Enamel tubules near the EDJ of left M2 (V 20298.7); (C) Enamel tubules in the enamel of right M2 (V 20298.8) after acid treatment, which are found to be present in the prism cores (right) as well as in interprismatic enamel (left); (D) Enamel tubules still with peritubular dentine in left M2 (V 20298.7) with insufficient acid treatment. (E) Dentinal tubules of left M2 (V 20298.7) with same peritubular dentine like the enamel tubules. (F) Enamel tubule in left M2 (V 20298.7) with a bulbous terminal expansion is perpendicular to the cross-sections of prisms and paths through the enamel layer. (G) Distribution and density variation in the entire enamel of right m2 (V 20297.4) in which enamel tubules are denser in the inner zone and have a random distribution. The tubules have no one to one relation with prisms. Fig 7A is an optic image and the rest are SEM images.

Fig 8. Optic images showing enamel tubules in the longitudinal section of tooth in Lambdopsalis bulla.

(A) Right i2 (V 20297.3); (B) Right di (V 20300.1); (C) Right P4 (V 20299-3); (D) Right p4 (V 20299-4); (E) Right m1 (V 20297.5); (F) Left M1 (V 20298.6); (G) Right M2 (V 20297.8); (H) Right m2 (V 20297.4). The spiral centrifugal course and branches of enamel tubules are shown in (A). (B) and (F) show that the lengths of enamel tubules can relatively consistent in the same tooth; (D) and (H) show that the tubules can have unstable and inconsistent lengths; (C) and (E) show that the enamel tubules begin in the middle of the enamel layer and extend outward.

Fig 9. SEM images showing enamel type in enamel of incisors in Lambdopsalis bulla.

(A–B) Right DI (V 20299-1); (C) Right di (V 20299-5); (D) Left di (V 20301.1); (E) Right I2 (V 20298.2); (F) Right I2 (V 20298.3); (G–H) Right i2 (V 20300.2). The images on the left are cross-sections and those on the right are the longitudinal sections.

Fig 10. SEM images showing enamel type in enamel of premolars in Lambdopsalis bulla.

(A–B) Right P4 (V 20299-3); (C–D) Right p4 (V 20299-4). The images on the left are cross-sections and those on the right are the longitudinal sections.

Fig 11. SEM images showing enamel type in enamel of molars in Lambdopsalis bulla.

(A) Left M1 (V 20298.5); (B) Right M1 (V 20298.6); (C) Left M2 (V 20298.7); (D) Right M2 (V 20298.8); (E–F) Right m1 (V 20297.4); (G–H) Right m2 (V 20297.4). The images on the left are cross-sections and those on the right are the longitudinal sections.

Fig 12. SEM images showing enamel type in the cross-section of right i2 (V 20300.2) in Lambdopsalis bulla.

(A) Enamel type changing from the EDJ (bottom) to the OES (top); (B–D) Details showing enamel type of outside (B), middle (C) and inside (D). B–D with the same magnification. Incremental lines are marked by a series of white arrows. Abbreviations: HP: hypogenetic prism.

Fig 13. Polarized light images showing irregular decussation in the cross-section of the tooth in Lambdopsalis bulla.

(A) Right i2 (V 20300.2); (B) Left M2 (V 20298.7); (C) Right P4 (V 20299-3); (D) Right I2 (V 20299-2). ID: irregular decussation.

Fig 14. Schmelzmuster of the deciduous incisor in Lambdopsalis bulla.

(A1-2) Optic and polarized light images of the cross sectional views of the right lower deciduous insisor (V 20300.1); (B1-2) Optic and polarized light images of the cross sectional views of the right upper deciduous incisor (V 20299-1); (C) Right upper deciduous incisor (V 20715.1) and (D) Right lower deciduous incisor (V 20300.1) showing the distribution of the enamel on the teeth. The arrow marks the distal ending of the enamel. A and B are composed from images of partial views of the cross section.

Fig 15. Schmelzmuster of the left I2 (V 20298.1) of Lambdopsalis bulla.

Optic image (upper) and polarized light image (lower) through the longitudinal section. The images are composed from several images showing partial section.

Fig 16. Schmelzmuster of the incisors in Lambdopsalis bulla.

(A1-2) Optic and polarized light images of the cross sectional views of the right I2 (V 20298.2); (B1-2) Optic and polarized light images of the cross sectional views of the right i2 (V 20300.1); (C) Right I2 (V 20298.2), and (D) Left lower i2 (V 20716.1) showing the positional relationship of the distal edge of the enamel with the margin of alveolus. These teeth are not from the same individual.

Fig 17. Schmelzmuster of the premolar in Lambdopsalis bulla.

(A) Right P4 (V 20299-3); (B) Right p4 (V 20299-4). 1. Optic images; 2. Polarized light images. The two teeth are from the same individual.

Fig 18. Schmelzmuster of the upper molar in Lambdopsalis bulla.

(A) Left M2 (V 20298.7); (B) Left M1 (V 20298.5); 1. Optic images; 2. Polarized light images. All pictures through the transverse section. The two teeth are not from the same individual.

Fig 19. Schmelzmuster of the lower molars in Lambdopsalis bulla.

(A) Right m2 (V 20300.3); (B) Right m1 (V 20298.4); 1. Optic images; 2. Polarized light images. All pictures through the transverse section. The two teeth are not from the same individual.

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