X-ray scan reveals hidden anatomy inside a Jurassic giant’s tooth

Deep in the Gobi Desert of Xinjiang, roughly eight kilometres south of Qiketai Town in China’s Turpan-Hami Basin, palaeontologists have extracted a single fossil tooth that is rewriting what scientists know about some of the most spectacular animals of the Jurassic world. The tooth, catalogued as PMOL-ADt0009, belonged to a mamenchisaurid, a family of long-necked sauropod dinosaurs famed for their extraordinarily elongated necks and colossal body sizes. Although mamenchisaurids dominated East Asian ecosystems during the Middle to Late Jurassic, the fine structure of their teeth has remained almost entirely a mystery, because well-preserved dental material is exceedingly rare. Now, a team working with high-resolution X-ray computed tomography has peeled back the enamel, virtually, to expose the internal architecture of a mamenchisaurid tooth for the very first time.

The discovery, published in the Swiss Journal of Palaeontology, is significant on two fronts. Externally, the tooth displays a unique combination of features that firmly anchor it within Mamenchisauridae, most notably a row of small serrations, called denticles, restricted to the front edge of the crown, and a rounded bump, known as a distolingual boss, on the tongue-side surface. Internally, the CT scans delivered something no sauropod study has achieved before: a complete three-dimensional map of how enamel is distributed across the tooth, revealing thickness patterns that echo adaptations seen in entirely different dinosaur lineages. The findings suggest that sauropod dental evolution was more experimentally diverse than the fossil record alone had hinted.

The tooth was recovered from the Upper Jurassic Qigu Formation, a rock unit laid down between roughly 163 and 155 million years ago during the Oxfordian and Kimmeridgian stages. Magnetostratigraphic and biostratigraphic studies date the top of the formation to approximately 155.3 million years before present. The Qigu Formation consists primarily of purplish-red and brownish-red mudstones and siltstones, interbedded with greyish-green silty mudstones and fine-to-medium-grained sandstones, deposits formed by rivers and lakes under semi-arid conditions. The middle member of the formation, where the tooth was found, is the most fossiliferous interval, having yielded abundant remains of sauropods, turtles, and bivalves. Dinosaur fossils, however, are scarce across the unit, making every new specimen a precious window into Jurassic ecosystems of northwestern China.

Before scanning, the specimen underwent careful mechanical preparation with a steel needle under a stereomicroscope to remove the surrounding matrix. Researchers then imaged the tooth with an Olympus DSX1000 digital microscope to capture fine external details such as the marginal denticles and enamel ornamentation. The critical step came at the Institute of Geology of the Chinese Academy of Geological Sciences in Beijing, where the tooth was CT-scanned with a Nikon XT H 225 ST scanner. The machine generated 3141 projections at an operating voltage of 110 kilovolts and a current of 115 microamperes, producing a reconstructed voxel size of just 26.41 micrometres per side. Using VG STUDIO MAX software, the team segmented the enamel, dentine, and pulp cavity into separate digital volumes, measured their dimensions, and calculated enamel wall thicknesses using the sphere method.

The numbers themselves tell a story. The tooth measures 46.21 millimetres in total length, with a crown height of 34.10 millimetres and a preserved root height of 12.11 millimetres. Its slenderness index, the ratio of crown height to maximum mesiodistal width, is 2.03, marking it as a relatively slender-crowned spatulate tooth. The compression index stands at 0.80, and the expansion index is 1.17, indicating sides that remain nearly parallel in labial view, a trait shared with the famous mamenchisaurid Mamenchisaurus sinocanadorum. Volumetrically, the crown contains 415.87 cubic millimetres of enamel and 2852.11 cubic millimetres of dentine, while the pulp cavity occupies a mere 73.12 cubic millimetres. In other words, the combined hard tissues outweigh the pulp space by roughly forty-five times, confirming the tooth’s extraordinarily solid construction and explaining its mechanical resilience under the crushing loads of sauropod feeding.

The external morphology reveals a tooth built for a precise cutting job. The crown curves lingually, with a strongly convex labial surface and a concave lingual face that together produce the characteristic D-shaped cross-section of spatulate sauropod teeth. Three wear facets record its working life: a nearly circular apical facet oriented lingually and inclined at roughly 25 degrees to the crown’s long axis, plus V-shaped mesial and distal facets along the crown margins. The mesial facet is well developed and confluent with the apical facet, while the distal facet is only in an early stage of attrition, indicating the tooth was still actively functioning when it was shed or the animal died. Along the apical fifth of the mesial margin, eight denticles were identified; the first three were worn away, leaving circular concavities about 0.65 millimetres across, while the remaining four are well preserved with blunt, coarsely granular tips.

The CT data exposed internal structures that external study could never resolve. The prominent lingual ridge, a sub-vertical apicobasal ridge dividing the concave lingual surface into two depressions, forms from the thickening of both enamel and dentine. By contrast, the distolingual boss, a feature long treated as taxonomically important across mamenchisaurids, euhelopodids, and various Asian sauropod teeth, arises solely from the lingual expansion of the dentine, with the overlying enamel remaining uniformly thin. This mirrors a recent micro-CT finding in an Euhelopus-like titanosauriform from the Lower Cretaceous Yixian Formation, suggesting that dentine-driven boss formation may be homologous across at least some of these lineages rather than independently evolved. The lingual ridge itself is considered plesiomorphic for sauropods, yet its histological composition had never been directly demonstrated until now.

Perhaps the most striking internal discovery concerns enamel thickness. In the apical half of the crown, the labial enamel is significantly thicker than the lingual enamel, with the thickest zones exceeding 0.56 millimetres concentrated centrally on the labial surface. On the lingual side, comparable thickness is confined exclusively to the lingual ridge. Toward the base of the crown, the asymmetry fades and thickness becomes progressively more symmetrical. This labiolingual asymmetry carries evolutionary weight: basal sauropods such as Camarasaurus show plesiomorphically uniform enamel around the tooth circumference, whereas derived neosauropods like Nigersaurus and Diplodocus exhibit pronounced asymmetry linked to herbivory in large-bodied dinosaurs. Finding a similar pattern in a mamenchisaurid implies that this adaptation evolved convergently, independently, in multiple sauropod branches, and possibly even in ornithischian clades such as iguanodontians and ceratopsians, where asymmetry is even more pronounced.

The pulp cavity adds a final architectural surprise. Three-dimensional reconstruction shows a centrally positioned cavity exceeding 4.1 centimetres in length, curving lingually in step with the recurved crown. Along its length, the cavity undergoes a dramatic volumetric transition: basally, the root canal is enlarged and bulbous with an approximately circular cross-section, but apically it narrows sharply under severe labiolingual compression, becoming a thin, laminar structure with a slit-like basal cross-section that tapers to a small circular tip. This bulbous-to-laminar configuration resembles the condition recently documented in the Euhelopus-like titanosauriform tooth, hinting at deeper shared developmental rules governing sauropod tooth formation.

Taxonomically, the tooth complicates an already murky picture. It differs clearly from the only other reported sauropod tooth from the Qigu Formation, specimen SGP 2002/005, which has a smaller crown of 17.8 millimetres, lacks denticles, a labial groove, and a distinct lingual boss, and displays an irregular anastomosing enamel pattern. The team provisionally assigns PMOL-ADt0009 to Mamenchisauridae indeterminate, noting that it may plausibly belong to Xinjiangtitan shanshanensis, a gigantic mamenchisaurid recovered from the same locality and horizon, though no dental material of that taxon is preserved for confirmation. Whatever its exact owner, the tooth demonstrates that at least two sauropod species coexisted within the Qigu Formation, a diversity long underestimated. As palaeontologists continue scanning fossils that look unremarkable from the outside, the hidden anatomy of Jurassic giants keeps proving that the most important features of ancient life were often the ones no eye could see.

Subject of Research: The anatomy of a mamenchisaurid tooth informed by digital reconstruction

Article Title: The anatomy of a mamenchisaurid tooth informed by digital reconstruction

Article References: Sui, Z.-X., Shao, S., & Yin, Y.-L. (2026). The anatomy of a mamenchisaurid tooth informed by digital reconstruction. Swiss Journal of Palaeontology, 145, 475-486. https://doi.org/10.3897/sjp.145.187923

Image Credits: AI Generated

DOI: 10.3897/sjp.145.187923

Keywords: anatomy, mamenchisaurid, tooth, informed, digital, reconstruction, scientific research