Micro-computed tomography imaging reveals the development of a unique tooth mineralization pattern in mackerel sharks (Chondrichthyes; Lamniformes) in deep time

The cartilaginous fishes (Chondrichthyes) have a rich fossil record which consists mostly of isolated teeth and, therefore, phylogenetic relationships of extinct taxa are mainly resolved based on dental characters. One character, the tooth histology, has been examined since the 19th century, but its implications on the phylogeny of Chondrichthyes is still in debate. We used high resolution micro-CT images and tooth sections of 11 recent and seven extinct lamniform sharks to examine the tooth mineralization processes in this group. Our data showed similarities between lamniform sharks and other taxa (a dentinal core of osteodentine instead of a hollow pulp cavity), but also one feature that has not been known from any other elasmobranch fish: the absence of orthodentine. Our results suggest that this character resembles a synapomorphic condition for lamniform sharks, with the basking shark, Cetorhinus maximus, representing the only exception and reverted to the plesiomorphic tooth histotype. Additionally, †Palaeocarcharias stromeri, whose affiliation still is debated, shares the same tooth histology only known from lamniform sharks. This suggests that †Palaeocarcharias stromeri is member of the order Lamniformes, contradicting recent interpretations and thus, dating the origin of this group back at least into the Middle Jurassic.


Results
tooth mineralization patterns in lamniform sharks. Micro-CT images of tooth files from the upper jaw (palatoquadrate cartilage, PC) of the basking shark (Cetorhinus maximus) (7-692/RZ) and the left upper jaw (LPC) of the crocodile shark (Pseudocarcharias kamoharai) (7-693/RZ) were 3D reconstructed and virtually sectioned to examine the tooth mineralization sequence of both species. The tooth mineralization sequences were consistent through all tooth files of each investigated species, with little variations due to different numbers of teeth per tooth file. The code specification of the abbreviations used here is depicted in the Material and Methods section.
Pseudocarcharias kamoharai has five to eight teeth in tooth files of the upper jaw, with zero to two functional teeth per tooth file. In the youngest developmental stages, the only mineralized structure of the tooth is the superficial enameloid. The enameloid first starts to mineralize at the apex (LPC1R6) and mineralization continues to the tooth crown base (LPC1R5). Mineralization of the enameloid is completed in LPC1R4. Until this position, the osteodentine formation has neither started in the root, nor in the crown and enameloid is the only mineralized structure. Osteodentine starts forming in the root and in the center of the crown simultaneously after completion of enameloid (LPC1R3), until it has fully filled the pulp cavity (LPC1R1 & LPC2R2). LPC1R1 is already fully mineralized, but not in a functional position and, therefore, regarded as the first replacement tooth. LPC1F1 and LPC1F2 are fully mineralized and are in an erect position on the outer edge of the jaw cartilage, allowing them to be utilized. During tooth development, no orthodentine can be identified at any stage, resulting in fully mineralized teeth consisting of only one sort of dentine -osteodentine (Fig. 1C,D). The virtual section of an isolated functional tooth also demonstrates the presence of only one layer of dentine, which is traversed by small canals and surrounded only by the hypermineralized enameloid (Fig. 2E). A manual tooth section confirms this, showing that dentinal osteons are spreading throughout the whole crown and are also present in close proximity or next to the enameloid (Fig. 2F). The presence of osteodentine and the absence of orthodentine implies the osteodont tooth mineralization pattern for teeth of P. kamoharai (Figs 1 and 2; Supplementary Tables S2 and S3).
The basking shark, Cetorhinus maximus has five to seven teeth within tooth files of the fractional part of the upper jaw of which three to four can be regarded as functional teeth because of their position along the margin of the jaw cartilage. Teeth have already an enameloid cap covering the whole of the crown in the earliest mineralization stages. A thin layer of orthodentine is present and osteodentine in the root already has started to form at this stage (PCaR3). In the adjacent file, only enameloid is present at the corresponding position, but no dentine (PCbR3) (Supplementary Fig. S1). Orthodentine can be distinguished from enameloid due to different densities. Dense tissues are represented by lighter shades (the hypermineralized enameloid is typically white in our reconstructions) and less dense tissues by darker shades (dentine is typically grey in our reconstructions). Osteodentine starts forming in the root and basally in the crown along the walls underneath the orthodentine. A hollow pulp cavity remains in the center of the tooth (PCaR3). During the next stages, the orthodentine layer becomes thicker www.nature.com/scientificreports www.nature.com/scientificreports/ and is fully mineralized in the first replacement tooth PCaR1. After the formation of osteodentine in the earliest stages, it intrudes basally into the pulp cavity until it fully fills the center of the crown. The osteodentine in the root is fully mineralized at the same developmental stage as orthodentine (PCaR1), but not in the crown until the next stage (PCaF3). The first replacement tooth (PCaR1) is already in a functional position, but the osteodentine in the crown has not fully filled the pulp cavity at this point. Therefore, completion of the mineralization process in the crown determines the earliest functional tooth (PCaF3) (Fig. 1A,B; Supplementary Tables S2 and S4). Manual and virtual tooth sections of an isolated functional tooth confirm the presence of both types of dentine -a dentine core which is made up of both, dentinal osteons and interosteonal tissue, giving it a spongiose appearance (osteodentine), which is surrounded by a prominent layer of dentine, lacking any vertical canals or pores but horizontal tubules, which are arranged parallel to each other and give the dentine layer a very compact appearance (orthodentine). The development of both, orthodentine and osteodentine in the crown shows that C. maximus has a pseudoosteodont mineralization pattern (Figs 1 and 2; Supplementary Tables S2 and S4).
To clarify which of the two mineralization patterns is present among the remaining members of the order Lamniformes, teeth of nine additional extant species were micro-CT scanned and virtually sectioned to determine their tooth histology. Additionally, teeth of all extant specimens (except the megamouth shark, Megachasma pelagios) also were manually sectioned horizontally and compared to the high-resolution images of the virtually sectioned teeth. Both methods demonstrate the presence of only one layer of dentine -osteodentine. Therefore, all extant species (except for Cetorhinus maximus) display the osteodont tooth histotype (Figs 2-4, Supplementary  Fig. S2). tooth histology patterns in fossil lamniform sharks. To clarify the plesiomorphic condition for lamniform sharks, fossil teeth of seven selected taxa (including supposedly basal (i.e. Eoptolamnidae (sensu Kriwet et al. 35 ) and derived taxa) of extinct lamniform sharks were examined using micro-computed tomography images. Additionally, an isolated tooth of the extant species Megachasma pelagios from the Pliocene (2.5-5 mya) is included here as well. www.nature.com/scientificreports www.nature.com/scientificreports/ Rendered high resolution micro-CT images of the virtual sections (in axial and sagittal view) reveal the absence of orthodentine in teeth of †Leptostyrax sp. ( †Eoptolamnidae (sensu Kriwet et al. 35 )), †Palaeocarcharodon orientalis (Lamniformes incertae fam.), and Megachasma pelagios (Megachasmidae). The hypermineralized enameloid (white) is clearly distinguishable from the dentine (grey) as a result of density differences. From the core of the tooth to the enameloid-dentine border, the entire dentine layer is traversed by dentinal osteons (Fig. 4). The same histology pattern can be identified in teeth of †Scapanorhynchus rapax (Mitsukurinidae) and †Squalicorax pristodontus ( †Anacoracidae), which thus also exhibit the osteodont tooth histotype ( Supplementary Fig. S3).
In three cases (teeth of †Dwardius woodwardi (Lamniformes incertae fam.), †Squalicorax sp. ( †Anacoracidae), and †Otodus megalodon ( †Otodontidae)) the presence or absence of orthodentine could not be properly identified based solely on micro-CT images. The enameloid-dentine border was less well defined in micro-CT images than in other examined teeth and some areas of dentine were indistinct, probably due to taphonomic processes, not allowing to identify the presence or absence of dentinal osteons (Fig. 5B,E,H). This was especially evident in †Otodus megalodon, where tooth enameloid could not be distinguished from the dentinal core and only a few coarse canals were visible (Fig. 5H). Therefore, the tooth crowns were manually sectioned horizontally and examined under a light microscope. These tooth sections demonstrate the presence of dentinal osteons, traversing the entire dentine core from the center to the enameloid-dentine border ( Fig. 5C,F,I). In †Dwardius woodwardi small dentinal tubuli are visible that originate in the osteons and penetrate the adjacent enameloid. Since they originate in the osteons they should be regarded as extensions of the osteon rather than representing an additional dentine layer. Orthodentine was not identified in any fossil tooth and all examined fossil taxa therefore have the osteodont tooth histotype. tooth histology of †Palaeocarcharias stromeri. The systematic position of †Palaeocarcharias stromeri still has to be considered as ambiguous despite all recent advances (see supplementary information (Supplementary Discussion 1) for a discussion of the most recent phylogenetic analysis 14 ). Here, tooth histology can provide additional and important information. For this, a tooth of the holotype (JME-SOS-2294) was scanned using micro-computed tomography. Virtual sections through three planes (frontal, sagittal, and axial) show a dentine core that is entirely traversed by a network of dentinal osteons (osteodentine) and covered by enameloid. The dentinal osteons are distributed within the entire dentine layer, from the center of the crown to close to the enameloid-dentine border. A compact layer of dentine (orthodentine) between the osteodont core and the hypermineralized enameloid is not present in the type specimen of †Palaeocarcharias stromeri, which therefore represents the osteodont histotype ( Fig. 6B,C,E).

Discussion
tooth mineralization patterns in sharks of the order Lamniformes. We identified two different tooth mineralization patterns within lamniform sharks resulting in the osteodont and pseudoosteodont histotypes. According to our results, the basking shark (Cetorhinus maximus) is the only lamniform shark having the pseudoosteodont tooth histotype, consisting of an osteodentine core that is covered by orthodentine. All other lamniform sharks displayed the osteodont tooth histotype, developing only one type of dentine (osteodentine). www.nature.com/scientificreports www.nature.com/scientificreports/ There was no lamniform species that showed the orthodont tooth histotype (a hollow pulp cavity surrounded by orthodentine) that is known from carcharhiniform sharks 25,26,[31][32][33][37][38][39][40][41] . Teeth of the great white shark (Carcharodon carcharias) 31,32,42,43 , shortfin mako (Isurus oxyrinchus) 26,34 , and the salmon shark (Lamna ditropis) 44 are also lacking orthodentine and thus 13 out of 14 examined extant lamniform shark species follow an osteodont tooth mineralization pattern. Unfortunately, there is no data for the longfin mako (Isurus paucus), but as all lamnids as well as its closest relative, Isurus oxyrinchus, have osteodont teeth 26,34 , it seems eligible to assume that its teeth also follow an osteodont mineralization pattern. Additionally, teeth of extinct species, from the assumed basal-most family †Eoptolamnidae (sensu Kriwet et al. 35 ) ( †Leptostyrax sp.) from the Albian/Cenomanian (Early Cretaceous, 94-113 mya) to †Otodus megalodon from the Miocene (Neogene, 5-23 mya) all showed the osteodont histotype. The latter species was previously described as being osteodont 45 , but also pseudoosteodont 46 . In both studies appropriate tooth sections were not prepared but fractured parts of the tooth were only superficially inspected. The tooth section prepared for this study unambiguously demonstrates that the whole interior of the crown is filled with osteodentine, while orthodentine is absent supporting †O. megalodon to be osteodont. The osteodont tooth histology for lamniforms is also confirmed by a number of publications: for a more detailed list, see Supplementary Table S1.  www.nature.com/scientificreports www.nature.com/scientificreports/ spp. (Carcharhiniformes) 25,26,40 , Lamniformes 26,31,34,43 . However, tooth crowns of these groups consist of both, ortho-and osteodentine and, therefore, should be referred to as being pseudoosteodont sensu Jambura et al. 26 . Consequently, Lamniformes (except for Cetorhinus) is the only group in which osteodentine alone constitutes the interior of the tooth crown and, therefore, should be the only group to be referred to as being osteodont.
Many Palaeozoic and hybodontiform sharks (the putative sister group to modern sharks 50,51 ) are also referred to as being osteodont 22,[52][53][54][55] according to the traditional definition of histotypes. However, as in modern groups (except for the lamniform sharks) both, orthodentine and osteodentine form the interior of the crown and, therefore, they have the pseudoosteodont histotype. There seems to be only one exception: apparently the teeth of †Aztecodus harmsenae, a Devonian chondrichthyan, also lack orthodentine 53 . The presence of pseudoosteodonty in Palaeozoic sharks, †Hybodontiformes and in many extant elasmobranchs (sharks, rays and skates) strongly indicates that this is the plesiomorphic condition for the modern sharks, and not a modification of the orthodont tooth histotype as previously suggested 26,40 . The osteodont tooth histotype exclusively found in sharks of the order Lamniformes represents a highly derived synapomorphic condition for this group within the elasmobranch fishes (Fig. 7). www.nature.com/scientificreports www.nature.com/scientificreports/ The basking shark Cetorhinus maximus however, represents a deviation from the general lamniform pattern in that it displays the pseudoosteodont tooth histology. In phylogenetic analyses based solely on tooth morphology, C. maximus and Megachasma pelagios formed a clade at the base of the lamniform sharks, representing a primitive sister group to other lamnid sharks 56 . Although this would explain a plesiomorphic state of the tooth histology in Cetorhinus to some extent, it conflicts the derived state in its putative sister group Megachasma. Furthermore, other morphological characters and molecular data suggest C. maximus to be a highly derived lamniform shark [4][5][6]8 , indicating that teeth of both planktivores became vestigial and, therefore, dental characters were reduced or lost completely 56,57 . This might also be the case for the tooth histology of C. maximus which reverted to a plesiomorphic state. However, M. pelagios, the second planktivorous species of this order still shares its derived tooth histology with all other lamniform sharks and, therefore, does not support an ecophenotypic link between tooth histology and feeding behaviour. What is also curious about the tooth histology in Cetorhinus maximus is that it was not reverted to the most probable ancestral state of lamniform and carcharhiniform sharks (orthodont), but to the ancestral state of all galeomorph sharks (pseudoosteodont) (Fig. 7). The reason for this reversal of the tooth mineralization pattern in C. maximus remains ambiguous, but seemingly is neither linked to its phylogenetic position, nor to its planktivorous feeding behaviour. †Palaeocarcharias stromeri and the origin of lamniform sharks. High resolution micro-CT images of a tooth of the holotype specimen of †Palaeocarcharias stromeri (JME-SOS-2294) display two types of tissues -a prominent outer layer of hypermineralized tissue (enameloid) and a core of less dense tissue (dentine). Dentinal osteons traversed the whole dentine core and were also present very close to the enameloid layer, which is characteristic for osteodentine. There were no signs of a compact orthodentine layer which can be found in pseudoosteodont teeth, thus teeth of †P. stromeri consist of only one layer of dentine (osteodentine) and display the osteodont histotype, which is characteristic for lamniform sharks 26,31,34,41 .
Our findings contradict the original description of the tooth histology of †P. stromeri by Beaumont 10 , who indicated the presence of three layers of dentine -a core of "trabecular dentine" (osteodentine) that was surrounded by a mantle of orthodentine, which again was covered by vitrodentine 10 (vitrodentine is one of many synonyms that were used for enameloid before its true nature was resolved 58 ). The presence of both, orthodentine and osteodentine, within the crown would suggest that †P. stromeri displays the pseudoosteodont histotype. According to the most recent study on †P. stromeri, vitrodentine and orthodentine were misinterpreted in the original work and instead are components of the multi layered enameloid 14 . Vitrodentine resembles the SCE ('Single Crystallite Enameloid') unit, while the tissue described as orthodentine in fact was the BCE ('Bundled www.nature.com/scientificreports www.nature.com/scientificreports/ Crystallite Enameloid') unit of the enameloid. Misinterpretations of the BCE unit (or parts of it) to be orthodentine occurred also in other studies, i.e. for Lamna sp. 59 and Lamna nasus 60 . Berkovitz and Shellis 61 reported the presence of orthodentine in Carcharias sp. However, dentinal tubules in the tissue they identified as orthodentine in their figure (figure 11.30) 61 do not originate in this tissue but come from the adjacent osteodentine which is known to occur at the dentine-enameloid junction 31 . Furthermore, the presence of a sharp junction between osteodentine and "orthodentine", which is known from the enameloid-dentine border, but not from dentine-dentine borders 31,61 is another indicator for this misinterpretation. Therefore, it is apparent that Carcharias sp. has the osteodont tooth histotype reported here for Carcharias taurus and other lamniform sharks.
Another indicator for the multi-layered enameloid being misinterpreted as orthodentine in the original work of †P. stromeri is given by our micro-CT images. Computed tomography discriminates between tissues of different densities -in our case, the very dense hypermineralized enameloid appears white, while the less dense dentine is grey. If we compare the virtual section in sagittal view (Fig. 6C) with the original tooth section (also in sagittal view) (Fig. 6D), we can see that the hypermineralized tissue in the virtual section (white) resembles the combined layers of "vitrodentine" and "orthodentine" in the original tooth section (dark) in thickness. If only the thin layer of "vitrodentine" in the original tooth section was enameloid, the hypermineralized structure in the micro-CT scan would be much thinner and less prominent, since the enameloid has a higher mineral content and, thus, density than dentine 58,62,63 , which is also visible on the micro-CT images 26,34 . Therefore, the micro-CT images prove the misinterpretation of orthodentine in †P. stromeri, which means that it does not display the pseudoosteodont tooth histotype, but the osteodont histotype, which only occurs in lamniform sharks.
Although †P. stromeri is known from well-preserved articulated material, only a few studies were conducted to resolve the systematic position of this species, which was in debate for decades 12,13 . In the original description, †P. stromeri was placed at the base of the Lamniformes 10 . Other authors agreed that the tooth morphology is characteristic for lamniform sharks 11,16,17 but the body form was similar to that of orectolobiform sharks 11 , making it a transitional taxon between both clades or the basal sister group of lamniforms 12,13 . In the most recent work on the phylogenetic position of †P. stromeri, it was suggested to be the sister to the clade Carcharhiniformes + Lamnifor mes 14 . Unfortunately, this phylogenetic hypothesis is inconclusive (Supplementary Discussion S1). Consequently, it is most parsimonious to assume that the osteodont tooth histotype is a unique feature for Lamniformes and thus we conclude that the osteodont tooth histology of †P. stromeri adds very strong support of this shark being www.nature.com/scientificreports www.nature.com/scientificreports/ the oldest known lamniform shark. Therefore, the origin of this group dates back at least into the Middle Jurassic (Bathonian) 64 , a period when major diversifications of elasmobranch fishes took place 65,66 . strengths and weaknesses of micro-Ct imaging. Similar to previous studies, micro-CT imaging turned out to be a powerful tool for non-invasive investigations of internal structures 26,31,43,67 . This is especially evident in examining tooth mineralization processes within tooth files in extant lamniform sharks. However, our study also points out weaknesses of micro-CT imaging, especially for fossil specimens related either to taphonomic alterations or to insufficient resolution of the CT scan. This is especially apparent in the tooth of †Otodus megalodon, in which the enameloid was not distinguishable from the dentine and the peripheral vascular system with its tiny canaliculi was not detectable in the micro-CT images, but was visible in manual tooth section. The latter effect is easily explained by the low resolution (around 30 µm), which was caused by the size of the tooth. In extant sharks, enameloid has a much higher degree of crystallinity and a very low organic content compared to dentine 62,63 , which makes it appear denser in micro-CT images 26,34 . The poor results for fossil teeth can be distorted by diagenetic processes leading to changes in the chemical constitution of enameloid 63,68 resulting in less differences of densities between enameloid and dentine. Nonetheless, in most cases micro-CT scanning generated images that sufficiently resolved the internal structures of both, extant and fossil shark teeth without damaging or destroying the material and therefore, should be regarded as a reliable non-invasive alternative to conventional thin sectioning.

Material and Methods
Material. Teeth and jaws of 11 extant and seven extinct taxa of sharks of the order Lamniformes were examined. Additionally, a tooth of the holotype specimen of †Palaeocarcharias stromeri (JME-SOS-2294) housed in the Jura Museum Eichstätt, Germany, the putative basal most lamniform shark from the Upper Jurassic (Tithonian), is included in this study. www.nature.com/scientificreports www.nature.com/scientificreports/ Extant material consisted of two jaws (basking shark Cetorhinus maximus (Inv.nr. 7-692/RZ), crocodile shark Pseudocarcharias kamoharai (Inv.nr. 7-693/RZ)) and isolated teeth of all 11 species. The fossil material consisted exclusively of isolated teeth and includes species from the Mesozoic and Cenozoic Era (five and three species respectively). Daggers preceding taxon names denote extinct species (Supplementary Table S5). tooth terminology. To specify the tooth position within the jaw, we employed a previously published code 26,34 . The first four letters define the position of the tooth file (the developmental sequence of replacement and functional teeth sensu Moyer et al. 31 ), if it is right (R) or left (L) of the symphysis or coming from the upper (palatoquadrate PC) or lower jaw (Meckel's cartilage MC). The number following the first three letters determines the position of the file distally to the symphysis. For example, the tooth file illustrated in Fig. 1B is the first file right to the symphysis in the lower jaw and, therefore, coded as RMC1. We distinguished between functional (F) and replacement teeth (R), with functional teeth being fully mineralized and in an erect or semi-erect position, allowing them to be utilized for food gathering (e.g. cutting, grasping, etc.). Replacement teeth are located lingually to the functional teeth and are not fully mineralized at this point of development. The tooth position within the tooth file is numbered, thus the first functional tooth of the RMC1 file is RMC1F1, the first replacement tooth of the same file is RMC1R1 (Fig. 8).
Micro-Ct scanning and imaging. Tooth mineralization and histology patterns for each species were investigated using a SkyScan1173 micro-CT device (Bruker/Skyscan, Kontich, Belgium) at the Department of Palaeontology (University of Vienna, Austria). Settings for each specimen are provided in the Supporting Information section (Supplementary Table S6). The generated slice file stacks were loaded into the software package DataViewer (version 1.5.1.2 (64 bit), SkyScan (Bruker micro-CT, Kontich, Belgium)) and Amira software package (version 5.4.5, FEI Visualization Sciences Group, Oregon, USA) to visualize and investigate 2D and 3D images of the studied material. This allowed us to set clipping planes through the jaws and teeth with different angles and digitally dissect through the material to examine the internal anatomy. Editing colour balance, contrast and labeling of the resulting 2D images was conducted in Adobe Photoshop CS6 (version 13.0, Adobe Systems, San José, USA). tooth sectioning. The quality of the results was tested by comparing the digital sections with actual tooth sections. For this, tooth sections were prepared for all extant specimens and three fossil specimens ( †Dwardius woodwardi, †Otodus megalodon, †Squalicorax sp.). For better handling of the small teeth, they were embedded in an adhesive medium using the two-component adhesive Araldite 2020/A and Araldite 2020/B, which were merged with a ratio of 100:30. The embedded teeth were cut horizontally through the crown and the exposed www.nature.com/scientificreports www.nature.com/scientificreports/ surface was polished using grinding powder (grain size 600 and 1000). Afterwards, the surface was treated with a 2 molar HCl solution for 10-60 sec and examined under the digital microscope Keyence VHX-6000 (Keyence International, Belgium). Pictures of the teeth were taken prior the sectioning process with an Olympus-OMD E5 mirrorless camera or the digital microscope. phylogenetic tree and ancestral state reconstruction. A phylogeny for 26 galeomorph sharks with known tooth histology was built from whole mitochondrial DNA sequences retrieved from the nucleotide database in GeneBank (accession number of the sequences can be found in the supplementary file (Supplementary  Table S7)). A complete sequenced genome of Heterodontus portusjacksoni was not available in the database, therefore, two other species of Heterodontus (H. francisci and H. zebra) were used as an outgroup of the clade consisting of [Orectolobiformes + [Carcharhiniformes + Lamniformes]. The sequences were aligned in MAFFT 69 and a matrix of approximately 16.5kbp resulted after trimming the edges. To construct the phylogeny the GTR + G + I substitution model was employed. The alignment was used in BEAST 2 software 70 , two parallel MCMC runs were performed over 10,000,000 generations sampling every 1000 generations.10% of the generations were set as a burn-in on TreeAnnotator 70 to obtain a maximum credibility tree on which the topology was used for stochastic character mapping with the make.simmap function in phytools 71 to perform an ancestral state reconstruction on the tooth histotype. Data for the tooth histology of the 26 species was retrieved from this study and the literature 22,26,31,34,43,44,49 . The final tree was edited in FigTree (v. 1.4.4).