Troodontids represent a clade of small theropod dinosaurs (Makovicky and Norell 2004) that display significant morphological similarities to birds (Xu et al. 2002). The teeth of later-diverging troodontids are unique in having proportionally large denticles that exhibit an apically hooked morphology, as well as a distinct constriction between the crown and the root (Currie 1987; Makovicky and Norell 2004). Over the past 170 years, a considerable number of isolated troodontid teeth and troodontid bones associated with teeth have been documented (e.g., Leidy 1856; Osborn 1924; Russell 1948; Barsbold 1974; Currie 1987; Currie et al. 1990; Russell and Dong 1993; Norell et al. 2000, 2009; Currie and Dong 2001; Xu et al. 2002, 2011; Makovicky et al. 2003; Xu and Norell 2004; Xu and Wang 2004; Ji et al. 2005; Averianov and Sues 2007; Gao et al. 2012; Goswami et al. 2013; Tsuihiji et al. 2014; Pei et al. 2022; Wang et al. 2025). Despite the potential phylogenetic implications of certain dental traits and the prevalence of tooth fossils associated with troodontids, a comprehensive examination of the anatomical features of their teeth, particularly with respect to internal structure, remains to be conducted.

The Upper Cretaceous Yuliangzi Formation, situated within the Sunwu–Jiayin Basin in Jiayin, Heilongjiang, is characterized by a notable diversity of dinosaur fossils, including the first dinosaur remains identified in China (Riabinin 1925). This formation is particularly recognized for its extensive collection of hadrosaurid fossils (Riabinin 1930; Godefroit et al. 2000, 2008; Xing et al. 2022), in addition to a significant number of isolated theropod teeth (Riabinin 1930; Lü and Han 2012; Yu et al. 2022). However, many of these theropod remains have not yet been subjected to thorough examination. Recently, a new troodontid tooth was discovered in the Yuliangzi Formation at Longgushan, Jiayin, Heilongjiang, China. This study utilizes computed tomography (CT) scanning technology to provide a comprehensive anatomical description of both the external and internal structures of this tooth, as well as to discuss its taxonomic classification. Furthermore, to establish a comparative baseline, the only other Troodon-morphotype tooth reported from this formation was also scanned and integrated (Yu et al. 2022), thereby providing the first comprehensive three-dimensional evaluation of all currently known troodontid teeth from the Yuliangzi Formation.

SC000900 and SC000908 are isolated right maxillary teeth. SC000908 was previously designated DMJY V-815 in the study by Yu et al. (2022), which described it based on optical observations. In contrast, the present study provides the first high-resolution micro-CT analysis of this specimen, revealing its previously undocumented internal structures and microanatomy. The specimens described in this study were collected from the lower member of the Upper Cretaceous Yuliangzi Formation at DMJY No. 4 (Yu et al. 2022; 48°51'26.0"N, 130°14'58.2"E), located in Longgushan, Jiayin County, Heilongjiang Province, China (Fig. 1). This member has been assigned in detail to an early Maastrichtian age (Sun et al. 2011; Sun 2022). However, it is noted that the precise chronostratigraphic position of this formation remains a subject of ongoing debate. Alternative hypotheses have suggested a late Maastrichtian age for these deposits (Van Itterbeeck et al. 2005; Nichols and Johnson 2008). Although this stratigraphic uncertainty is acknowledged, the early Maastrichtian assignment is tentatively retained as the primary chronological baseline for subsequent biogeographic interpretations, pending further high-resolution radiometric calibration. The teeth are currently housed in the Shenzhou Dinosaur Museum of Jiayin, Jiayin Dinosaur National Geopark, China. In the description of SC000900 and SC000908, the terminology and measurement methodologies established by Hendrickx et al. (2015) are followed.

A steel needle was employed for mechanical preparation of specimen SC000900 under a Leica S6 microscope. Subsequently, imaging of the specimen was conducted using a three-dimensional large depth-of-field microscopy system (Olympus DSX-1000) located at the Paleontological Museum of Liaoning. Specimen SC000900 was subjected to CT scanning at Yinghua NDT (Shanghai) Co., Ltd., using the Phoenix V|tome|x M240, which produced a total of 2,201 projections at 100 kV and 200 µA. The resultant voxel size was determined to be 3.54 × 3.54 × 3.54 µm. Specimen SC000908 was scanned at Yinghua NDT (Shanghai) Co., Ltd., using the Phoenix V|tome|x M neo 300/180, producing 2,200 projections at 120 kV and 160 µA. The voxel size obtained was 3.3 × 3.3 × 3.3 μm. Segmentation, three-dimensional reconstruction, and measurements were performed using VG STUDIO MAX 3.4 (Volume Graphics, Heidelberg, Germany).

Dinosauria Owen, 1842

Theropoda Marsh, 1881

Maniraptora Gauthier, 1986

Troodontidae Gilmore, 1924

Troodontidae indet.

Material. SC000900, one isolated tooth (Figs 2, 3, 4, 5, 6); SC000908, one isolated partial tooth crown (Figs 7, 8).

Locality and horizon. Longgushan, Jiayin, Heilongjiang Province, China; lower member of the Yuliangzi Formation, early Maastrichtian.

Description of SC000900. SC000900 consists of an almost complete tooth crown accompanied by the basalmost portion of the root (Fig. 2). The limited preservation of the root suggests that this specimen may represent a shed tooth. Additionally, the tooth is characterized by a relatively short crown, exhibiting a crown height ratio of 1.3 (Table 1), following measurements in Hendrickx et al. (2015). The specimen is notably small, with a crown height of 9 mm (Table 1).

Tooth anatomy. The crown exhibits a recurved shape and is compressed labiolingually. It is categorized as folidont, characterized by a distinct constriction at its base (Fig. 2A, B). The apex of the crown shows signs of wear. The basal cross-section of the crown is lenticular in shape, with a slight concavity on the lingual side (Fig. 2K, L). The labial apex of this cross-section is situated relatively mesially. The labial surface of the crown is convex. Both the mesial and distal carinae are characterized by denticulation along the crown (Fig. 2A, B). The mesial carina exhibits slight abrasion, and the wear facet faces mesiolingually (Fig. 2H). It is recurved distally (Fig. 2I, J) and is inclined slightly toward the lingual side when viewed mesially (Fig. 2G, H). The distal carina is only weakly concave, appearing nearly straight in both labial and lingual views, and is almost perpendicular to the crown baseline. It is inclined slightly toward the labial side when viewed distally (Fig. 2C, D).

A lingual depression is observed (Fig. 2E, F), resembling the morphology observed in the previously reported troodontid tooth (see fig. 5B in Yu et al. 2022). This depression is centrally positioned and, in its preserved state, displays a subtriangular shape, with its base spanning approximately one-half of the length of the lingual surface. The apicobasal extent of the lingual depression occupies one-third of the crown, which is in contrast to the condition typically seen in most theropods, where such depressions are generally restricted to the basal region of the crown (Hendrickx et al. 2015).

The enamel, dentin, and pulp cavity were segmented in the crown (Fig. 3). The respective total volumes for the enamel, dentin, and pulp cavity are 2.83 mm³, 76.44 mm³, and 0.99 mm³. It is noteworthy that the pulp cavity is the smallest structure among the three components of the tooth.

The enamel exhibits a smooth texture (Fig. 3A, B). Its thickness remains relatively uniform around the circumference of the crown (Fig. 4E, F). However, it diminishes toward the base of the crown on both the labial and lingual surfaces (Fig. 4C). Specifically, the enamel thickness is measured as 31 μm, 29 μm, and 17 μm in the apical (Fig. 4D), middle (Fig. 4E), and basal (Fig. 4F) regions of the crown, respectively. Additionally, the enamel is consistently thicker on the labial surface of each distal denticle compared with the lingual surface (Fig. 4G, H). The enamel thickness exhibits a slight reduction on the labial side and an increase on the lingual side as it approaches the proximal base of a distal denticle. This pattern is observed across all distal denticles. For instance, in a middle distal denticle of the crown, the enamel thickness is measured as 36 μm, 35 μm, and 30 μm in the apical (Fig. 4G), middle (Fig. 4H), and basal (Fig. 4I) regions, respectively, on the labial side. Conversely, on the lingual side, the enamel thickness is recorded as 14 μm, 24 μm, and 28 μm in the apical (Fig. 4G), middle (Fig. 4H), and basal (Fig. 4I) regions, respectively. At the proximal base of the denticles, the enamel thickness is nearly uniform (Fig. 4I).

The dentin exhibits greater thickness on the labial side of the crown base compared with the lingual side, and it is also thicker on the distal side of the crown base than on the mesial side (Table 1). The morphology of the pulp cavity is characterized by a blade-like shape, demonstrating considerable labiolingual compression (Fig. 3D–G). It tapers sharply at the apex and exhibits a distal curvature, maintaining a central position in both mesial and distal views (Fig. 3F, G). Notably, the apex of the pulp cavity is situated significantly lower than that of the crown (Fig. 3D, E). The mesial margin of the pulp cavity is markedly apicobasally convex, while the distal margin is apicobasally concave. The central region of the mesial margin displays several projections, which may represent a natural anatomical feature, as indicated by the regular curvature of their mesial margins. The basal half of the distal margin exhibits nine projections that gradually decrease in length as they extend toward the crown apex (Figs 3D, 3E, 4J). The spacing between adjacent distal projections measures approximately 0.5 mm, while the proximal diameter of each projection is approximately 0.05 mm. Each projection extends toward the interdenticular diaphysis.

Denticle anatomy. The denticles are clearly visible at the base of the mesial carina and extend along the full length of the distal carina. They also appear to be present along the entire mesial carina, despite its slight erosion (Fig. 2H). The distal denticles protrude more in the proximodistal direction from the crown than their mesial counterparts (Fig. 2A, B). As a result, the distal denticles appear larger than the mesial denticles in both lingual and labial views. The mesial denticles are relatively small, with a maximum length of 0.3 mm, and at least five mesial denticles are clearly preserved.

A total of 19 distal denticles have been preserved, although one distal denticle may not be preserved at the crown base. Four distal denticles located at the mid-crown exhibit incompleteness. The counts of distal denticles per 5 mm along the basal, middle, and apical crown regions are 12, 10.5, and 11, respectively. The average number of denticles per 5 mm along the distal carina is calculated to be 11.2. The distal denticles exhibit an apically inclined orientation and are relatively large (Fig. 5), with a maximum apicobasal length of 0.5 mm recorded at approximately the mid-length of the distal carina. This length exhibits a gradual reduction toward both the tooth apex and tooth base as the distance from the mid-crown increases. Furthermore, the distal denticles are elongated in the proximodistal direction compared with the apicobasal direction (Fig. 5A, B). The external margin of the denticles displays an apically hooked morphology (Fig. 5A, B). The interdenticular slit is subtriangular in shape, while the interdenticular space is both deep and large (Fig. 5A, B). Additionally, the interdenticular diaphysis is shallow (Fig. 5A, B).

Wear facet. The wear facet is situated on the lingual surface (Fig. 2E, F) and presents an elliptical shape with smooth margins (Fig. 6A, B). The longest axis measures 4.3 mm, while the shortest axis measures 1.0 mm, resulting in a consistently flat surface. Fine scratches have been identified on the polished surface, in addition to several coarse scratches that are likely indicative of dental microwear. However, it remains challenging to entirely rule out the possibility that these coarse scratches may have resulted from the fossilization process (Fig. 6A, B). The majority of these fine scratches exhibit a parallel orientation and are typically positioned slightly offset from the longest axis of the wear facet. Specifically, they are generally located more apically on the mesial side of the facet and more basally on the distal side (Fig. 6A, B).

Description of SC000908. SC000908 is an isolated, broken tooth crown lacking its apex. It shows a folidont morphology, with a distinct distal recurvature of the preserved crown and a pronounced basal constriction (Fig. 7A–D). Although more fragmentary than SC000900 due to this missing apical portion, the newly acquired high-resolution micro-CT datasets provide detailed insight into its internal structures and dental tissue distribution (Figs 7E–H, 8).

Tooth and denticle anatomy. Both the mesial and distal carinae are denticulated. The mesial carina bears a wear facet that faces mesiolingually along its apical one-third (Fig. 7B). A shallow, subtriangular, centrally positioned lingual depression is distinct on the basalmost preserved region of the lingual surface (Fig. 7B). Structurally, the enamel layer is well preserved but exhibits basal thinning, tapering from a maximum thickness of 25.9 μm near the apical breakage line to 15 μm near the base (Fig. 8G–I). The basal mesial denticles and the middle distal denticles show a pronounced, asymmetric enamel distribution, where the enamel on the labial surface of the denticle systematically exceeds that on the lingual surface (Fig. 8C, D).

Pulp cavity. The pulp cavity is characterized by a blade-like shape, demonstrating considerable labiolingual compression and tracking the distal curvature of the crown (Fig. 7E–H). The pulp cavity tapers apically toward its apex, which maintains a strictly central position in both mesial and distal views (Figs 7G, 7H, 8B). The mesial margin of the pulp cavity is markedly apicobasally convex, while the distal margin is apicobasally concave (Figs 7E, 7F, 8E, 8F). The central region of the mesial margin displays several minor projections with a regular curvature. In contrast, the basal half of the distal margin exhibits a series of nine finger-like vertical projections that gradually decrease in length as they extend toward the crown apex (Figs 7E, 7F, 8E). These distal projections are spaced at approximately 0.5 mm, with each projection extending toward the corresponding interdenticular diaphysis. The basal portion of the pulp cavity is distinctly expanded, bearing an elongated projection near its mesiobasal margin (Fig. 7E, F).

Wear facet. A flat, partially preserved wear facet is distinct on the lingual surface of the crown, with its upper apical portion truncated by the breakage of the specimen (Fig. 7B). The remaining well-polished area exhibits a semielliptical outline (Fig. 7B).

Troodontid teeth, including a newly discovered complete crown (SC000900) and a previously documented specimen (SC000908), are fully described from the Maastrichtian Yuliangzi Formation situated at Longgushan, Jiayin, Heilongjiang, China. These specimens offer novel insights into the internal morphology of troodontid teeth, particularly regarding the enamel thickness on the crown and denticles, as well as the features of the pulp cavity. The high-resolution CT data reveal a pronounced microstructural consistency between the two teeth, suggesting that these concealed dental characters may represent taxonomically informative features within this Late Cretaceous population. The identification of these specimens, alongside other theropod teeth, implies that the Yuliangzi Formation may also contain theropod body fossils in addition to isolated teeth, thereby suggesting the potential for future paleontological discoveries. Furthermore, the reported diversity of theropods from the Yuliangzi Formation might be exaggerated owing to the absence of comprehensive studies. Additionally, analysis of dinosaur faunal assemblages within Maastrichtian strata in China reveals the consistent presence of tyrannosaurids.