Dinosaur teeth from the Lower Cretaceous Jiufotang Formation of western Liaoning, China

Introduction

The Jehol Biota represents a Mesozoic terrestrial Lagerstätte, with its known fossil localities mainly distributed in the Lower Cretaceous of western Liaoning, northern Hebei, and southeastern Inner Mongolia in northeastern China (Pan et al., 2013; Zhou, 2014; Xu et al., 2020). This biota is renowned for yielding numerous many exceptionally well-preserved two-dimensional and three-dimensional dinosaur fossils, which are found in ascending stratigraphic order within the Dabeigou, Yixian, and Jiufotang formations (e.g., Sereno et al., 1988; Xu et al., 2002; Xing et al., 2019; Zhang et al., 2022; Yu et al., 2024). Notably, three-dimensionally preserved dinosaur fossils have been discovered at the Beipiao and Xidayingzi sites within the Lujiatun Bed of the Yixian Formation, as well as at the Huanghuagou (Shijianggou) site in the Huanghuagou Bed of the Jiufotang Formation (Chang et al., 2003; Duan et al., 2006; Zhang et al., 2022). Recently, two dinosaur teeth were recovered from a newly identified site of three-dimensionally preserved dinosaur fossils located in Shangshuiquan Village, Lianhe Town, Longcheng District, Chaoyang City, Liaoning Province of the Jiufotang Formation of the Jehol Biota. This discovery expands the known distribution of three-dimensionally preserved dinosaur fossil sites of this biota. The aim of this paper is to provide a comprehensive description of the two teeth and to explore their taxonomic implications.

Specifically, one of the two teeth is attributed to a theropod, whose dental taxonomy is primarily established through morphological comparisons, phylogenetic analysis, machine learning, and discriminant analysis (e.g., Hendrickx et al., 2019; Hendrickx et al., 2024; Hendrickx, Tschopp & Ezcurra, 2020; Wills, Underwood & Barrett, 2021; Wills, Underwood & Barrett, 2023). It is important to clarify that this study exclusively employs morphological comparisons to analyze the theropod tooth, as it only preserves the mesial half and lacks the diagnostic features characteristic of specific theropod subgroups.

Geological Setting

The fossil site containing dinosaur teeth was discovered at Shangshuiquan Village (41°31′6″N, 120°16′1″E), Lianhe Town, Longcheng District, Chaoyang City, western Liaoning Province during our field expeditions (Fig. 1A). The Lianhe area geographically belongs to the Lower Cretaceous Jiufotang Formation in the Dapingfang-Meileyingzi basin (Duan et al., 2006; Wang et al., 2009). The lithology of this section consists of a set of sedimentary rocks with a total thickness of 15.26 m. These sedimentary rocks are primarily composed of conglomerate and gravel-bearing sandstone, interbedded with gravel-bearing tuffaceous siltstone. Two distinct fossil layers have been identified, with layer 1 situated directly beneath layer 2, approximately 8.85 m apart (Fig. 1B). All specimens are preserved within the gravel-bearing tuffaceous siltstone (Figs. 1C and 1D), with a titanosauriform tooth found in fossil layer 1 and a theropod tooth found in fossil layer 2.

Materials & Methods

PMOL-ADt0006 (Fig. 2) and PMOL-ADt0007 (Fig. 3) are isolated teeth that likely represent shed teeth due to the short length of their preserved roots. These specimens were collected from the Jiufotang Formation of the Lower Cretaceous at Shangshuiquan Village, Lianhe Town, Longcheng District, Chaoyang City, western Liaoning Province, China (Fig. 1A), during our field expedition on April 26, 2024. They are housed in the Paleontological Museum of Liaoning (PMOL). In the description of PMOL-ADt0006, we adhere to the terminology and measurement methodology established by Hendrickx, Mateus & Araújo (2015). Measurements were conducted using the software ImageJ 1.54f.

To eliminate the matrix, a steel needle was utilized to mechanically prepare the specimens PMOL-ADt0006 and PMOL-ADt0007 under a Leica S6 microscope. Subsequently, the specimens were photographed using a three-dimensional super microscopy system (Olympus DSX1000), which is housed in PMOL.

Results

Discussion

PMOL-ADt0006 is labiolingually compressed, distally recurved and serrated, suggesting its classification as a theropod (Xu & Clark, 2008; Han et al., 2011; Hendrickx & Mateus, 2014; Yu et al., 2023). This tooth is classified as either a ziphodont or pachydont tooth due to its distal curvature and the absence of constriction at the cervix (Hendrickx, Mateus & Araújo, 2015). Additionally, it remains challenging to ascertain whether the specimen is a mesial or lateral tooth, considering PMOL-ADt0006’s incompleteness. To date, theropod clades recovered from the Jiufotang Formation include Tyrannosauroidea, Ornithomimosauria, and Microraptorinae (Xu, Zhou & Wang, 2000; Xu et al., 2003; He, Wang & Zhou, 2008; Ji, Ji & Zhang, 2009). PMOL-ADt0006 can be distinctly differentiated from both Ornithomimosauria and Microraptorinae, as it possesses a minimum crown height of 3.6 cm, whereas the crowns of Ornithomimosauria and Microraptorinae are smaller, measuring less than one cm and three cm, respectively (Hendrickx et al., 2019). Furthermore, PMOL-ADt0006 features a serrated crown, in contrast to the unserrated crowns characteristic of Ornithomimosauria (Hendrickx et al., 2019). In terms of size, PMOL-ADt0006 is comparable to the teeth of the proceratosaurid Sinotyrannus (Ji, Ji & Zhang, 2009; Brusatte et al., 2010), which is the only known tyrannosauroid from the Jiufotang Formation. It is improbable that PMOL-ADt0006 represents a lateral tooth of Sinotyrannus, as it exhibits 19.5 mesial denticles per five mm, whereas the mesial teeth of Sinotyrannus possess 15 or 16 mesial denticles (Ji, Ji & Zhang, 2009) per five mm. Unfortunately, detailed information regarding the mesial teeth of Sinotyrannus is lacking (Ji, Ji & Zhang, 2009). Despite this limitation, it is unlikely that PMOL-ADt0006 is a mesial tooth of Sinotyrannus, as it displays a slightly twisted mesial carina, whereas tyrannosauroids typically possess mesial teeth with a strongly mesial carina that extends onto the lingual surface (Hendrickx et al., 2019). Moreover, PMOL-ADt0006 does not exhibit the braided texture commonly found in Proceratosauridae (Hendrickx et al., 2019). The classification of PMOL-ADt0006 within theropods remains challenging due to the preservation of only the mesial half of the tooth.

Additionally, as far as we know, the pulp cavity is only exposed in basal view, and its internal structure is little exposed within theropod teeth. PMOL-ADt0006 reveals new information of this structure in theropods. The pulp cavity exhibits a distal curvature. It progressively enlarges beneath the cervix, culminating in a sudden increase in diameter.

PMOL-ADt0007 exhibits a wrinkled enamel, an absence of denticles on the crown, a lingual concavity, a D-shaped mid-crown cross-section, and a labial groove. These traits are characteristic of eusauropods (Upchurch, 1998; Wilson & Sereno, 1998; Wilson, 2002; Upchurch, Barrett & Dodson, 2004). Consequently, it is reasonable to classify PMOL-ADt0007 within the clade Eusauropoda. The presence of wear facets, a feature commonly observed in eusauropods (Wilson, 2005a), further supports this classification. Within the eusauropods, PMOL-ADt0007 can be specifically assigned to Titanosauriformes based on a suite of characteristic features of this clade: the presence of labial and lingual grooves, a lingual bulge, and a slightly mesiodistally expanded apex (Upchurch, Barrett & Dodson, 2004; Mocho et al., 2017). It can be definitively excluded that PMOL-ADt0007 belongs to Titanosauria, as it possesses a SI value of 1.67, along with the labial groove and lingual concavity, whereas Titanosauria is characterized by a SI value exceeding 4.0 (Upchurch, 1998) and has secondarily lost both the labial grooves and the lingual concavity (Upchurch, Barrett & Dodson, 2004). It is noteworthy that PMOL-ADt0007 exhibits a minimum SI of 1.67 (Table 2), in contrast to the known early-diverging titanosauriforms, which possess the SI values ranging from 2.0 to 4.0 (Barrett et al., 2002; Mocho et al., 2017). However, the SI value for PMOL-ADt0007 may underestimate the original crown height due to the significant wear observed on its crown apex. Consequently, we classify PMOL-ADt0007 as a non-titanosaurian titanosauriform. This classification aligns with the observation that all sufficiently documented Cretaceous Asian sauropods are titanosauriforms (Barrett et al., 2002; Wilson, 2005b). In the Jehol Biota, the titanosauriform fossils comprise three genera along with several isolated teeth (Barrett & Wang, 2007; Wang et al., 2007; Zhou et al., 2018; Mo et al., 2023; Zhang et al., 2024). Among these three genera, only Liaoningtitan has been found to preserve teeth (Zhou et al., 2018). The specimen PMOL-ADt0007 exhibits distinct characteristics compared to the teeth of Liaoningtitan and the known isolated titanosauriform teeth, specifically in possessing both an apical wear facet and a distal wear facet (Barrett & Wang, 2007; Zhou et al., 2018; Zhang et al., 2024).

In comparison to theropods and ornithischians, sauropods exhibit a more limited stratigraphical distribution within the Jehol Biota (Benton, 2023). Over the past eighteen years, sauropod fossils have been exclusively documented in the Yixian Formation of western Liaoning (Barrett & Wang, 2007; Wang et al., 2007; Zhou et al., 2018; Mo et al., 2023) and southeastern Inner Mongolia (Zhang et al., 2024). The titanosauriform tooth described in this study represents the youngest fossil record of sauropods within the Jehol Biota.

By January 12, 2025, ten dinosaur species have been documented from the Jiufotang Formation (Sereno et al., 1988; Xu, Zhou & Wang, 2000; Czerkas & Yuan, 2002; Xu et al., 2003; You & Dodson, 2004; He, Wang & Zhou, 2008; Ji, Ji & Zhang, 2009; Gong et al., 2012; Han et al., 2014; Poust et al., 2020). These discoveries suggest the presence of theropods, including Tyrannosauroidea (Ji, Ji & Zhang, 2009), Ornithomimosauria (He, Wang & Zhou, 2008), and Microraptorinae (Xu, Zhou & Wang, 2000; Xu et al., 2003), as well as ornithischians such as Ankylosauria (Han et al., 2014) and Ceratopsia (Sereno et al., 1988) within the Jiufotang Formation. The current study reports two types of dinosaur teeth from the Jiufotang Formation of Longcheng, Chaoyang, western Liaoning, China, representing Theropoda and Titanosauriformes. The theropod tooth constitutes a distinct taxon, separate from previously recognized theropods. Additionally, the isolated titanosauriform tooth provides the first evidence of sauropod presence in this formation, indicating that skeletal fossils of sauropods may be recoverable from the Jiufotang Formation. These findings enhance the known dinosaur diversity in the Jiufotang Formation from ten taxa to 12; however, this remains significantly lower than the diversity observed in the Yixian Formation of the Jehol Biota, which comprises 39 species (Benton, 2023). Nevertheless, the discovery of these dinosaur teeth suggests that the Jiufotang Formation may still yield valuable insights into dinosaur diversity.

The Jehol Biota is renowned for its extensive collection of dinosaur fossils, both two-dimensionally and three-dimensionally preserved (e.g., Chang et al., 2003; Zhou, Barrett & Hilton, 2003; Xu & Norell, 2006; Xu et al., 2020). Among these, three-dimensionally preserved dinosaur fossils are of particular interest due to their capacity to retain significant biological information during fossilization. However, the number of fossil sites that yield three-dimensionally preserved dinosaur fossils is significantly lower compared to those that produce two-dimensionally preserved specimens within the Jehol Biota. To date, only three fossil sites have been identified that contain three-dimensional dinosaur fossils: the Beipiao and Xidayingzi sites of the Lujiatun Bed in the Yixian Formation, and the Huanghuagou (Shijianggou) site of the Huanghuagou Bed in the Jiufotang Formation (Chang et al., 2003; Duan et al., 2006; Zhang et al., 2022). The recent discovery of a new site featuring three-dimensionally preserved dinosaur fossils in Shangshuiquan, Lianhe, Longcheng, Chaoyang, Liaoning, within the Jiufotang Formation, significantly expands the geographical distribution of three-dimensionally preserved dinosaur fossil sites in the Jehol Biota.

Conclusion

A theropod tooth and an early-diverging titanosauriform tooth have been identified from the Jiufotang Formation located in Shangshuiquan Village, Lianhe Town, Longcheng District, Chaoyang City, western Liaoning Province, China. This site is particularly significant due to the three-dimensionally preserved dinosaur fossils in the Jehol Biota. The theropod tooth represents a taxon that is distinct from Sinotyrannus, which is the only large theropod dinosaur from the Jiufotang Formation. The titanosauriform tooth constitutes the youngest sauropod fossils identified within the Jehol Biota. These two findings highlight the diversity of dinosaurs present in the Jiufotang Formation. The availability of more complete fossil skeletons is crucial for advancing research on the dinosaurs within this formation.