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. 2015 Jul 28:5:12338.
doi: 10.1038/srep12338.

Developmental and evolutionary novelty in the serrated teeth of theropod dinosaurs

K S Brink  1 , R R Reisz  2 , A R H LeBlanc  1 , R S Chang  3 , Y C Lee  4 , C C Chiang  4 , T Huang  5 , D C Evans  6
Affiliations

Developmental and evolutionary novelty in the serrated teeth of theropod dinosaurs

K S Brink et al. Sci Rep. .

Abstract

Tooth morphology and development can provide valuable insights into the feeding behaviour and evolution of extinct organisms. The teeth of Theropoda, the only clade of predominantly predatory dinosaurs, are characterized by ziphodonty, the presence of serrations (denticles) on their cutting edges. Known today only in varanid lizards, ziphodonty is much more pervasive in the fossil record. Here we present the first model for the development of ziphodont teeth in theropods through histological, SEM, and SR-FTIR analyses, revealing that structures previously hypothesized to prevent tooth breakage instead first evolved to shape and maintain the characteristic denticles through the life of the tooth. We show that this novel complex of dental morphology and tissues characterizes Theropoda, with the exception of species with modified feeding behaviours, suggesting that these characters are important for facilitating the hypercarnivorous diet of most theropods. This adaptation may have played an important role in the initial radiation and subsequent success of theropods as terrestrial apex predators.

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Figures

Figure 1
Figure 1. Microanatomy of ziphodont archosaur teeth with deep interdental folds.
(A) Skull of Gorgosaurus libratus, a tyrannosaurid dinosaur, drawn by Danielle Dufault. (B) Complete tooth (ROM 31535) and sagittal thin sections through distal and mesial carinae of a maxillary cf. Gorgosaurus sp. (ROM 57981) tooth. (C–H), carinae of ziphodont teeth under SEM (left) and in thin section (right). (C), Indeterminate phytosaur, ROM 7981, mesial carina. (D) Coelophysis bauri, CM 76863, distal carina. (E) Allosaurus fragilis, ROM 66106, distal carina. (F) Carcharodontosaurus saharicus, ROM 52017 (left) and ROM 47691 (right), mesial carina. (G) cf. Gorgosaurus libratus, ROM 57981, mesial carina. (H) Tyrannosaurus rex, ROM 66108, mesial carina. Abbreviations: dej, dentine-enamel junction; e, enamel; if, interdental fold; is, interdental sulcus; pd, primary dentine.
Figure 2
Figure 2. Microanatomy of ziphodont teeth lacking deep interdental folds.
(A) Carcharodon megalodon (ROM 30530). (B) Smilodon sp. (ROM 3288) carina, lacking denticles (not a true ziphodont animal as the carina is composed of enamel only). (C) Varanus komodoensis (ROM R7565), distal carina, with unevenly sized denticles. (D) Hadrosaurid (ROM 58205) labial denticles. (E) Sagittal thin section through the distal carina of a maxillary tooth of Dimetrodon grandis (ROM 6039), showing inconsistently-sized denticles. Thin section (top), SEM (bottom). (F) Troodon formosus (ROM 05089), mesial carina. Thin section (top), SEM (bottom).
Figure 3
Figure 3. Microanatomy of interdental folds.
(A) Sagittal thin section of an interdental fold below the DEJ in Carcharodontosaurus saharicus (ROM 52037). Carina to the top. (B) Oblique transverse thin section through an interdental fold and denticle in a maxillary tooth of C. saharicus (ROM 52037) under cross-polarized light. Carina to the right, pulp cavity to the left. (C) SEM image of an interdental fold in a cf. Gorgosaurus (ROM 57981) maxillary tooth in coronal section. Abbreviations: dej, dentine-enamel junction; dt, dentine tubules; e, enamel; gd, globular dentine; if, interdental fold; pd, primary dentine; sd, sclerotic dentine.
Figure 4
Figure 4. Dentine-Enamel Junction of archosaur teeth.
(A) Triceratops sp. (ROM 67669) with enamel spindles and no globular mantle dentine. (B) Hadrosaurid (ROM 58205) with enamel spindles and no globular mantle dentine. (C) Spinosaurus aegyptiacus (ROM 65613) carina with dentine, globular dentine, and enamel ornamentations, but lacking true denticles. (D) Indeterminate phytosaur (ROM 7981) with globular mantle dentine. (E) cf. Gorgosaurus (CMN 2225) unerupted, newly forming tooth with globular mantle dentine, primary dentine, and incompletely mineralized enamel. (F) Cretaceous crocodilian (ROM 67512) with globular mantle dentine. (G) Coelophysis bauri (CM 87671) unerupted tooth, carina to the top right corner, showing a lack of globular dentine within the denticle. The globular dentine can be seen behind the dentine tubules of the denticle in the interdental space. (H) Sagittal thin section of the DEJ at a denticle tip in C. saharicus (ROM 52037), showing enamel spindles. Abbreviations: dt, dentine tubule; e, enamel, es, enamel spindle; gd, globular dentine; pd, primary dentine.
Figure 5
Figure 5. Microanatomy of unerupted cf.
Gorgosaurus sp. teeth. (A) Distal carina of CMN 2225 showing the deep folds at the base of each interdental sulcus. (B) Detail of interdental fold from (A) showing absence of sclerotic dentine. (C) Dentine tubules in an erupted tooth (ROM 57981) showing peritubular dentine in each hollow tubule. (D) Dentine tubules in an unerupted tooth (CMN 2225) showing less peritubular dentine when compared to erupted tooth in (C). (E) Transverse section between two neighbouring denticles (CMN 2225), carina to the top. (F) Transverse section through a denticle showing absence of globular dentine in the denticle tip. Carina to the right. (G) Unerupted tooth of cf. Gorgosaurus libratus (RTMP 2012.012.0029), with interdental folds. (H) Unerupted tooth of Allosaurus fragilis (UMNH.VP 23761), with interdental folds. Abbreviations: dt, dentine tubule; e, enamel; gd, globular dentine; if, interdental fold; is, interdental sulcus; pd, primary dentine.
Figure 6
Figure 6. SR-FTIR spectra of enamel and dentine of a cf.Gorgosaurus tooth, ROM 30582.
(A–F), region of interdental fold, (A) optical image, (B) reflectance image, (C) CO2 at 2345 cm−1 present in enamel and interdental fold. (D) cHAP-A at 879 cm−1 in the globular dentine of the interdental fold. (E) cHAP-B at 867 cm−1 in the globular dentine of the interdental fold. (F) Alkyl at 3000-2800 cm−1, representing organic material. Colour bar at the far right with blue as the lowest and red as the highest concentration. (G) FT-IR spectra and assignments. (H) FT-IR spectrum of enamel. (I) FT-IR spectrum of dentine. (J) Deconvolution of cHAP Type A and Type B of enamel. (K) Deconvolution of -CH3 and -CH2 of preserved organics in dentine.
Figure 7
Figure 7. Proposed model for the development of theropod ziphodont teeth, drawn by Danielle Dufault.
(A) Mineralization of the tooth tissues from tip towards the root. (B) Detail of two denticles, showing deposition of tooth tissues in 1) sagittal and 2) transverse views. Dental epithelium folds to form the shape of the tooth prior to differentiation of tooth tissues. (C) Dentine deposition by odontoblasts. Globular mantle dentine deposition in the interdental fold and the remainder of the tooth, and mantle dentine in the denticle tip. (D) Enamel deposition by ameloblasts at each denticle tip; primary dentine deposition. (E) Enamel mineralization into each interdental sulcus, stopping before closing the channel at the interdental fold in the dentine. Tooth eruption occurs at this stage. (F) Functional tooth with sclerotic dentine deposition. Abbreviations: e, enamel; ep, epithelium; if; interdental fold; is, interdental sulcus; pd, primary dentine; sd, sclerotic dentine. Colors: blue, enamel; purple, epithelium; red, dentine; white, sclerotic dentine.
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References

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