Psittacosaurus houi, a longer snouted psittacosaurid from the Lower Cretaceous Lujiatun Unit of Yixian Formation, China, with the synonymy of the unresolved genus Hongshanosaurus revisited

Introduction

Psittacosauridae represents a family of basal ceratopsian dinosaurs with a bipedal posture and characteristic upper and lower jaws that form a beak-like rostrum similar to that of a parrot (Osborn, 1923). In the Lower Cretaceous, hundreds to thousands of psittacosaurid specimens have been reported from the Barremian to Albian of China, Mongolia and Russia (Sereno, 2010; Sereno, Zhao & Tan, 2010; Napoli et al., 2019; Podlesnov et al., 2023). The genus Psittacosaurus is one of the possible two genera that comprise the family Psittacosauridae, and the most species-rich genus within non-avian dinosaurs (Sereno, 2010).

In the past, multiple species of Psittacosaurus were recognized, and their validities have been continuously tested. Along with the description of the type species, Psittacosaurus mongoliensis (AMNH 6254), Osborn (1923) described a specimen from the Ondai Sayr locality (AMNH 6253) and assigned it to a new genus and species, Protiguanodon mongoliense. In a study that followed, these two genera were placed within a single new family Psittacosauridae based on the overall similarity between the two (Osborn, 1924). Additionally, two new species, P. osborni and P. tingi, were erected following Osborn’s studies (Young, 1931). In Young (1958), another new species, P. sinensis, was described, which was followed by still another species, P. youngi (Zhao, 1962). In the 1980s, three additional species were described, P. guyangensis, P. xinjiangensis, and P. meileyingensis (Cheng, 1983; Sereno & Chao, 1988; Sereno et al., 1988). Subsequently, eight new species were added to the genus by the end of 2010 (Russell & Zhao, 1996; Averianov et al., 2006; Zhou et al., 2006; Sereno et al., 2007; Sereno, Zhao & Tan, 2010), bringing the total number of potential species of Psittacosaurus to 16 (Sereno, 2010). On the other hand, a comparative study by Sereno (2010) recognized nine valid species: P. mongoliensis, P. neimongoliensis, P. sinensis, P. sibiricus, P. lujiatunensis, P. major, P. meileyingensis, P. xinjiangensis and P. gobiensis. More recently, Napoli et al. (2019) added P. amitabha as a new species, with the total number of Psittacosaurus species still being debated.

Another genus that constitutes the family Psittacosauridae is Hongshanosaurus, which contains a single species Hongshanosaurus houi. The species was described based on a juvenile skull (IVPP V12704, holotype) and an adult skull (IVPP V12617, referred specimen) (You, Xu & Wang, 2003; You & Xu, 2005). Hongshanosaurus was distinguished from Psittacosaurus by an oval-shaped external naris and orbit and a longer rostrum (You & Xu, 2005). However, a lack of definitive morphological differences between Hongshanosaurus and Psittacosaurus has led to multiple proposals that the former may be a junior synonym of the latter. For example, Sereno (2010) argued that the longer rostrum in Hongshanosaurus might come from the taphonomic deformation and synonymized H. houi with P. lujiatunensis. Additionally, Hedrick & Dodson (2013) supported the synonymy of H. houi with P. lujiatunensis based on a three-dimensional geometric morphometric analysis in which H. houi formed a morphological cluster with P. lujiatunensis and P. major. The study further tested the validity of the diagnosis of P. lujiatunensis, P. major, and H. houi, respectively, using 25 specimens of these psittacosaurids. The taxonomic analysis in Hedrick & Dodson (2013) showed that most of the diagnostic characters were intraspecifically and taphonomically variable, thus invalidating the diagnosis and leading to the conclusion that P. major and H. houi are junior synonyms of P. lujiatunensis. However, Napoli et al. (2019) criticized Hedrick & Dodson (2013) for failing to account for ontogenetic and taphonomic variation, as well as intraspecific variation found in modern animals, in their geometric morphometric analysis and argued that P. lujiatunensis and P. major were separate species because they did not form a sister clade in the phylogenetic analysis. While Napoli et al. (2019) did not consider the results of the taxonomic analysis by Hedrick & Dodson (2013), their phylogenetic analysis possibly suggests that H. houi is also distinct from P. lujiatunensis and P. major if it had been included as another operational taxonomic unit (OTU).

In this study, we describe a well-preserved adult skull (ZMNH M12414) of a psittacosaurid with a long snout resembling that of Hongshanosaurus houi. We test the purported validity of the genus Hongshanosaurus and address current Psittacosaurus taxonomy based on the morphology and the phylogenetic analysis. The application of computed tomography (CT) techniques has allowed the non-destructive rendering of individual skull elements, including those that are difficult to observe in articulated specimens (such as endocranial elements), and helped us to describe each of them. This leads to a revision of diagnostic characters based on the less deformed and articulated specimen and allows us to address the taxonomy and specific diversity within Psittacosauridae. While the synonymy of Hongshanosaurus with Psittacosaurus is supported, our study recognizes Psittacosaurus (formerly Hongshanosaurus) houi as a valid species that can be distinguished from all other Psittacosaurus species, including the coeval Psittacosaurus lujiatunensis.

Materials and Methods

Phylogenetic analysis

To assess the phylogenetic relationships among species of the genus Psittacosaurus, Psittacosaurus houi, P. sinensis and P. sibiricus were added to the character-taxon matrix of Napoli et al. (2019), which is based on the original matrix in Han et al. (2018). Psittacosaurus houi was scored based on the rendered three-dimensional computer model of ZMNH M12414 (Figs. 1–2, S3–S33) and the descriptions of IVPP V12617 in the previously published literature (You & Xu, 2005; Tanoue, You & Dodson, 2009; Taylor et al., 2017; Bullar et al., 2019; Landi et al., 2021; Han et al., 2018). Psittacosaurus sinensis was scored based on published literature for the holotype IVPP V738 (Young, 1958; Sereno, 1990; Tanoue, You & Dodson, 2009; Sereno, 2010) and a referred specimen BNHM BPV149 (Chao, 1962; Sereno, 2010). Psittacosaurus sibiricus was scored based on published literature for the holotype PMTGU 16/4–20 (Averianov et al., 2006) and referred specimens KOKM 22985/2 (Podlesnov et al., 2023) and PM TGU 16/0–15, 20, 30–39, 16/1–11, 51, 136, 137, 166, 167, 175, 176, 179, 200, 201–203, 209, 216, 223, 228, 258, 271, 274, 276, 281, 283 and 284 (Averianov et al., 2006).

The character-taxon matrix in Napoli et al. (2019) was further modified in the following perspectives. We chose to set the OTUs of P. mongoliensis and lujiatunensis species based on the holotype specimens. This was because it was possible that “Psittacosaurus mongoliensis composite” and “Psittacosaurus lujiatunensis composite” in the previous studies included the specimens belonging to different species. For P. major, LH PV1 and CAGS-IG-VD-004 were integrated into a single OTU. While character 103, the distinctive indentation on the midline of the posterior edge of the parietals, was scored as absent (1) in P. amitabha and P. major in Napoli et al. (2019), they were scored as present (0) in the present study as it is apparent in the dorsal views (You, Tanoue & Dodson, 2008; Napoli et al., 2019). For character 238, the presence of 14 dorsal vertebrae was included in the first derived (15 vertebrae) state defined by Han et al. (2018), allowing P. sibiricus with 14 dorsal vertebrae (Averianov et al., 2006) to be scored.

Following the previous studies (Han et al., 2018; Napoli et al., 2019), the matrix was analyzed using T.N.T. version 1.5 (Goloboff, Farris & Nixon, 2008), treating all characters as equally weighted and the following characters as ordered (additive): 2, 23, 31, 40, 126, 164, 196, 203, 204, 222, 227, 238, 243, 247, 268, 292, 296, 302, 306, 320, 361. The analysis was performed with the default settings, except that the maximum number of trees was set to 99,999. The traditional search was chosen to find the most parsimonious trees (MPTs) with the tree bisection-reconnection (TBR) algorithm, where 1,000 replicates of random stepwise addition and 100 trees held at each step. Then, a second TBR search was performed using the MPTs obtained in the previous procedure, where branches were collapsed if the minimal branch length was zero. The strict consensus tree was generated based on the MPTs found after the second TBR search. Bootstrap and Bremer support values were calculated as support indices.

Results

Description and comparisons

ZMNH M12414 measures 123 mm in skull length, 196 mm in width from the lateral-most extents of the jugal horns, and 125 mm in the height from the ventral-most dentary flange to the dorsal-most skull roof (Table S2).

The skull is divided into 32 elements by suture lines observable in the CT data (Fig. S34). The skull exhibits fractures in some elements, including the premaxillae, nasals, the right prefrontal, postorbital, squamosal, parietal, angular, surangular, left maxilla/jugal contact, splenial, and dentary. It also shows slight distortion in the right supratemporal fenestra and mandibular condyle. However, these deformations do not appear to affect intra- and interspecific morphological comparisons (see Discussion).

Histological evidence suggests that IVPP V12617, the other referred specimen of Psittacosaurus houi, is fully mature and at least 10 years old at the time of death (Zhao et al., 2013). Because the basal skull length of ZMNH M12414 is 139.1 mm long and almost equivalent to that of IVPP V12617 (143.7 mm long; Bullar et al., 2019), we tentatively regard the former as mature. This interpretation about the full maturity of ZMNH M12414 is also supported by the cranial sutures that are nearly obliterated in some parts.

In dorsal view, the skull is wider than long (Fig. 1E), as in Psittacosaurus sinensis (IVPP V738; Sereno, 2010) and P. lujiatunensis (Zhou et al., 2006). Notably, the skull exhibits a relatively large proportion for the preorbital length from the anterior-most extent of the rostral bone to the anterior-most orbital margin, against the total skull length (the preorbital length proportion, PLP). The PLP against the total skull length is approximately 50%, being similar to 52% of IVPP V12617 and 46% of P. amitabha (Napoli et al., 2019), and unlike those of other known Psittacosaurus species which exhibit less than 40% (Sereno, 2010).

Rostral—In anterior view, the rostral is subtriangular with a broad ventral margin, and the nasal process extends dorsally (Fig. S3C). The rostral is also triangular in lateral view, having a nearly vertical sutural contact with the premaxilla (Figs. S3A, S3B). Because the anterior-most ends of the nasals are not fully preserved, the articulation between the rostral and nasal is obscured (Figs. 1A–1C). As in Psittacosaurus mongoliensis (Osborn, 1923), P. sinensis (Sereno, 2010; Chao, 1962), P. amitabha (Napoli et al., 2019), P. major (Sereno et al., 2007; You, Tanoue & Dodson, 2008) and IVPP V12617 (You & Xu, 2005), the anterior margin formed by the rostral and nasal slopes posterodorsally (Figs. 1A, 1B). However, the margin slopes more gently at an angle of 30 degrees in ZMNH M12414, IVPP V12617 (You & Xu, 2005), and P. amitabha (Napoli et al., 2019). The anterior-most extent of the rostral is rounded in dorsal and ventral views (Figs. S3E, S3F), and neither pointed nor strongly bowed ventrally in lateral view (Figs. S3A, S3B).

Premaxilla—Both premaxillae are preserved and make up most of the lateral surfaces of the snout (Figs. 1A, 1B). The premaxilla contacts the rostral anteriorly, the nasal and prefrontal dorsally, and the lacrimal and maxilla posteriorly as in other species except for Psittacosaurus sinensis (Sereno, 2010), in which the premaxilla also meets the jugal posteriorly (Fig. S4). Unlike in other Psittacosaurus species (Sereno et al., 1988; Russell & Zhao, 1996; Zhou et al., 2006; Sereno et al., 2007; You, Tanoue & Dodson, 2008; Sereno, 2010; Sereno, Zhao & Tan, 2010), the sutural contact between prefrontal and premaxilla is relatively narrow in lateral view (Figs. 1A, 1B). A similar character, though even wider, is also found in P. major (LH PV1; Sereno et al., 2007), whereas the contact is even narrower in ZMNH M12414 and IVPP V12617 (You & Xu, 2005).

The anterodorsal portions of both premaxillae are fragmented and partially missing (Figs. S4A–S4D). The posterior expansion of the posterolateral process of the premaxilla excludes the maxilla from the external nares, which are bounded solely by the nasals and premaxillae, as in all other Psittacosaurus species (Sereno et al., 1988; Russell & Zhao, 1996; Zhou et al., 2006; Sereno et al., 2007; You, Tanoue & Dodson, 2008; Sereno, 2010; Sereno, Zhao & Tan, 2010; Napoli et al., 2019). The lateral surface of the premaxilla is smooth and slightly depressed below the external naris, but has multiple grooves and pits near its buccal margin, which is particularly evident on the right premaxilla (Fig. S4B) probably due to the erosion of the left premaxilla. A well-developed crest extends anterodorsally along the premaxillary-maxillary suture and a conspicuous groove crosses this suture horizontally just below the distal end of the premaxillary-maxillary ridge (Figs. S4A, S4B), as in Psittacosaurus meileyingensis (IVPP V7705; Sereno et al., 1988) and P. lujiatunensis (Zhou et al., 2006). In lateral view, the position of the ventral margin of the premaxilla is raised above maxillary tooth row (Figs. 1A, 1B), as in IVPP V12617 (You & Xu, 2005; Bullar et al., 2019). Similar features are present in P. sinensis (Sereno, 2010), P. mongoliensis (Sereno, 2010) and P. major (Sereno et al., 2007; You, Tanoue & Dodson, 2008), but the distance from the posterior end of the ventral margin of the premaxilla to the maxillary tooth row is the widest in ZMNH M12414 and IVPP V12617 (You & Xu, 2005; Bullar et al., 2019).

Maxilla—Both maxillae are preserved (Fig. S5). In lateral view, the maxilla is subtriangular and contacts the jugal posteriorly, the premaxilla anteriorly and the lacrimal dorsally as in other Psittacosaurus species (Sereno et al., 1988; Russell & Zhao, 1996; Zhou et al., 2006; Sereno et al., 2007; You, Tanoue & Dodson, 2008; Sereno, 2010; Sereno, Zhao & Tan, 2010; Napoli et al., 2019; Podlesnov et al., 2023). There are nine alveoli for each maxilla, with seven on the left and nine on the right occupied by the erupted teeth (Figs. S5C–S5F). The position of the external naris is located above the anterior part of the maxilla as in P. major (CAGS-IG-VD-004; You, Tanoue & Dodson, 2008), P. meileyingensis (Sereno et al., 1988), and P. amitabha (Napoli et al., 2019) (Figs. 1A, 1B). The dorsal part of the maxilla is tall, trapeziform and reaches to the level with the ventral border of the orbit (Figs. 1A, 1B) as in Psittacosaurus meileyingensis (Sereno et al., 1988), P. neimongoliensis (Russell & Zhao, 1996), P. lujiatunensis (Zhou et al., 2006), P. major (Sereno et al., 2007; You, Tanoue & Dodson, 2008), P. sibiricus (KOKM 22985/2; Podlesnov et al., 2023) and IVPP V12617 (You & Xu, 2005), but differing from the lower, triangular maxilla in all others. As in P. lujiatunensis (Zhou et al., 2006), P. mongoliensis (Sereno, 2010), P. amitabha (Napoli et al., 2019) and P. gobiensis (Sereno, Zhao & Tan, 2010), a sub-triangular maxillary fossa (Figs. S5A, S5B) is present on the lateral surface with a horizontal eminence located along its ventral margin. The fossa is anteroposteriorly longer than dorsoventrally deep and positioned anterior to the orbit as in P. amitabha (Napoli et al., 2019) and P. major (Sereno et al., 2007). Sereno (2010) describes the neurovascular foramina that open within or on the rim of the maxillary fossa and the anterolateral maxillary foramen located near or along the suture with the premaxilla in Psittacosaurus. The presence of these features is not confirmed in ZMNH M12414 due to limited resolution of CT images. Like most Psittacosaurus species (Sereno et al., 1988; Zhou et al., 2006; Sereno et al., 2007; Sereno, Zhao & Tan, 2010; Sereno, 2010), a maxillary protuberance is found posterior to the maxillary fossa at the posterior margin near the maxilla-jugal suture (Figs. S5A, S5B). In ventral view, the distance between the anterior ends of the tooth row is wide, as the ratio of the width across the anterior ends to that across the posterior ends is greater than 45% (Fig. 1F), as in P. amitabha (Napoli et al., 2019) and P. lujiatunensis (Zhou et al., 2006).

Lacrimal—Both lacrimals are preserved, with the right one being better preserved (Fig. S6). As in other Psittacosaurus species (Sereno et al., 1988; Russell & Zhao, 1996; Zhou et al., 2006; Sereno et al., 2007; You, Tanoue & Dodson, 2008; Sereno, 2010; Sereno, Zhao & Tan, 2010; Napoli et al., 2019; Podlesnov et al., 2023), the lacrimal forms the anterior margin of the orbit, and it is bounded by the premaxilla anteriorly, the prefrontal dorsally and the jugal ventrally with a relatively smaller trapezoid shape than other Psittacosaurus (Figs. 1A, 1B). It contributes to a small part of the anteroventral border of the orbit. The lacrimal foramen is located at the boundary between the lacrimal and the jugal (Fig. 1B, S6B), whereas the foramen is located along the orbital margin in other Psittacosaurus species (Zhao, 1962; Sereno et al., 1988; Sereno, 1990; Russell & Zhao, 1996; Zhou et al., 2006; Sereno et al., 2007; Sereno, Zhao & Tan, 2010; Sereno, 2010). This may be due to the partial breakage of the lateral surface of the right lacrimal, causing the lacrimal foramen to appear in a more anteroventral position. A right lacrimal canal continuous with the foramen can be seen in the CT image (Fig. S35), supporting the identification of the lacrimal foramen.

Nasal—Despite multiple fractures, the nasals retain their overall shape (Fig. S7). In dorsal view, the nasal is a long bone that flanks its counterpart in the anterior half including the rostroventral process, where it contacts the rostral bone ventrally (Figs. S7E, S7F). The posterior part of the internasal suture is unclear (Figs. S7E, S7F). The posterior part of the nasal expands laterally, reaching its maximum width where it meets the prefrontal. At its narrowest part, the main body of the nasal is about as wide as the prefrontal and it terminates posteriorly in contact with the frontals above the orbit (Fig. 1E) as in other Psittacosaurus species (Sereno et al., 1988; Russell & Zhao, 1996; Zhou et al., 2006; Sereno et al., 2007; Sereno, 2010; Sereno, Zhao & Tan, 2010).

Frontal—Both frontals are preserved (Fig. S8). The frontals are flat and form a broad central element of the skull roof, constituting the posterodorsal rim of the orbit (Fig. 1E). In dorsal view, both frontals are fused into a single unit along the midline as the interfrontal suture cannot be observed (Figs. S8E, S8F). The frontals contact the nasals anteriorly, the prefrontals laterally, the postorbitals posterolaterally, and the parietals posteriorly. The ventral surface of the frontals has distinct depressions for the olfactory bulbs anteriorly and for the cerebral hemispheres posteriorly (Fig. S8F).

Prefrontal—Both prefrontals are preserved, but the right element exhibits some fractures (Fig. S9). They are narrow and meet the premaxillae and lacrimals ventrally, forming the anterodorsal corner of the orbit in left lateral view (Figs. 1A, 1B). In dorsal view, the prefrontal contacts the nasal anteromedially and the frontal posteriorly (Fig. 1E). The maximum width of the prefrontal is nearly as wide as that of the nasal (Fig. 1E) as commonly seen in other Psittacosaurus species (Sereno et al., 1988; Russell & Zhao, 1996; Sereno et al., 2007; You, Tanoue & Dodson, 2008; Sereno, 2010; Sereno, Zhao & Tan, 2010).

Parietal—Both parietals are preserved (Fig. S10); however, a fracture on the right side near the parietal frill (Fig. S10E) resulted in the ventral distortion of the right supratemporal fenestra (Fig. 1D). The parietals form the posterior-most element of the central skull roof and fuse along the midline to form a low sagittal crest (Figs. S10A, S10B and S10E), as in other Psittacosaurus (Sereno et al., 1988; Averianov et al., 2006; Zhou et al., 2006; Sereno et al., 2007; You, Tanoue & Dodson, 2008; Sereno, 2010; Sereno, Zhao & Tan, 2010; Napoli et al., 2019; Podlesnov et al., 2023). The parietal contacts the frontal and the postorbital anteriorly, the laterosphenoid anteroventrally, the squamosal laterally and the supraoccipital ventrally. In dorsal view, the posterior margin of the parietal is almost linear and runs perpendicular to the sagittal crest (Fig. S10E) as in IVPP V12617 (Bullar et al., 2019) and in Psittacosaurus meileyingensis (Sereno et al., 1988), while all other species have a distinct incised margin at the middle of the posterior end of parietals. In P. meileyingensis (Sereno et al., 1988), the lateral process of the parietal shows a distinctive slope angled posterodorsally, whereas the parietal broadly contacts the squamosal in IVPP V12617 (You & Xu, 2005) and ZMNH M12414 (Fig. 1E).

Postorbital—Both postorbitals are preserved (Fig. S11). The postorbital is a three-pronged element composed of the bar on the skull roof, the temporal bar and the jugal bar that separates the orbit from the infratemporal fenestra (Figs. S11A–S11D). The left postorbital is present in its original position, whereas the right postorbital is fractured at the center of its triradiate form, and the temporal bar is disarticulated from the squamosal (Figs. S11B, S11D). This results in a slight ventral distortion of the right supratemporal fenestra along with a fracture in the parietals, as seen in posterior view (Fig. 1D). The jugal bar is thick and relatively long, compared with that of all other Psittacosaurus species, overlapping the jugal and forming most of the posterior border of the orbit (Figs. 1A, 1B). The jugal bar is the longest prong in the postorbital and is nearly twice as long as the temporal bar (Figs. S11A–S11D). This condition appears to be an autapomorphy of P. houi, shared with the holotype (IVPP V12704; You, Xu & Wang, 2003) and another referred specimen (IVPP V12617; You & Xu, 2005). In contrast, the length of the jugal bar is shorter than or nearly equal to that of the temporal bar in other Psittacosaurus species (Russell & Zhao, 1996; Averianov et al., 2006; Zhou et al., 2006; Sereno et al., 2007; You, Tanoue & Dodson, 2008; Sereno, 2010; Sereno, Zhao & Tan, 2010; Napoli et al., 2019). The distal-most extent of the left jugal bar is missing (Figs. S11A, S11B, S11I and S11K). The jugal bar is expanded anteroventrally (Figs. S11A–S11D) as in Psittacosaurus lujiatunensis (Zhou et al., 2006) and P. meileyingensis (Sereno et al., 1988), differing from the narrow tip in all others. The temporal bar of the postorbital laterally overlaps the postorbital bar of the squamosal and forms the entire dorsal border of the infratemporal fenestra in lateral view (Fig. 1A). The bar on the skull roof is shorter than the temporal and jugal bars (Figs. S11A–S11D), as in other Psittacosaurus species (Russell & Zhao, 1996; Zhou et al., 2006; Averianov et al., 2006; Sereno et al., 2007; You, Tanoue & Dodson, 2008; Sereno, 2010; Sereno, Zhao & Tan, 2010; Napoli et al., 2019). In lateral view, the bar on the skull roof extends along the margin of the orbit, contributing to form the posterodorsal rim of the orbit (Figs. 1A, 1B) as in other Psittacosaurus species (Russell & Zhao, 1996; Zhou et al., 2006; Averianov et al., 2006; Sereno et al., 2007; You, Tanoue & Dodson, 2008; Sereno, 2010; Sereno, Zhao & Tan, 2010; Napoli et al., 2019; Podlesnov et al., 2023).

Squamosal—Both squamosals are preserved (Fig. S12). They are nearly complete except for the missing anterior ramus on the right squamosal (Figs S12B, S12D, S12F, S12H, S12J and S12L). The squamosal is a tetraradiate bone as in other Psittacosaurus species (Zhou et al., 2006; Sereno et al., 2007; You, Tanoue & Dodson, 2008; Sereno, 2010; Sereno, Zhao & Tan, 2010; Napoli et al., 2019) and is located at the upper posterolateral corner of the skull. Anteriorly, the squamosal contacts the postorbital to form a bar separating the upper and lower temporal fenestrae. The anterior ramus of the right squamosal is missing due to a local fracture (Figs. S12B, S12H), along with the cracked postorbital, both of which likely contributed to the ventral distortion of the right supratemporal fenestra. The anterior ramus of the left squamosal is completely preserved but short in dorsal view (Fig. 1E), as in Psittacosaurus lujiatunensis (Zhou et al., 2006), P. major (CAGS-IG-VD-004; You, Tanoue & Dodson, 2008) and P. amitabha (Napoli et al., 2019). This contrasts with most other Psittacosaurus species, in which the anterior-most end of the ramus reaches the anterior margin of the supratemporal fenestra (Russell & Zhao, 1996; Sereno et al., 2007; Sereno, 2010; Sereno, Zhao & Tan, 2010). The ventral ramus is relatively short and does not contact the quadratojugal (Figs. 1A–1C). Although Hedrick & Dodson (2013) argue that the ventral ramus of the squamosal and the dorsal ramus of the quadratojugal are almost always broken in P. lujiatunensis, P. major, and Hongshanosaurus houi (=Psittacosaurus houi, IVPP V12617), our CT images and rendered model show no evidence of such damages. The medial ramus projects medially to meet the parietal, with which it forms the posterior margin of the supratemporal fenestra (Figs. 1E, S12G, S12H). Further ventrally, the ventromedial ramus of the squamosal contacts the exoccipital (Figs. S12I, S12J).

Jugal—Both jugals are present (Fig. S13). The jugal is large and consists of infraorbital, infratemporal, and dorsal rami, and the jugal horn (Figs. 1A, 1B). The anteromedially curved infraorbital ramus, the dorsal ramus, and the posteromedially sloped infratemporal ramus are plate-shaped. The ventral surface of the jugal is flat (Figs. S13E, S13F). A gentle ridge runs posterolaterally from the apex of the dorsal process to the lateral end of the jugal horn, dividing the lateral aspect of the jugal into anterior and posterior surfaces. In lateral view, the area of the anterior surface is much larger than that of the posterior surface, as in Psittacosaurus major (Sereno et al., 2007; You, Tanoue & Dodson, 2008) and P. sibiricus (Podlesnov et al., 2023), which is thought to be due to the shorter infratemporal ramus than in other Psittacosaurus (Russell & Zhao, 1996; Sereno, 2010; Sereno, Zhao & Tan, 2010; Napoli et al., 2019) (Figs. S13A, S13B). Additionally, ZMNH M12414 exhibits a relatively larger anterior proportion of the jugal than IVPP V12704 (holotype; You, Xu & Wang, 2003). The infraorbital ramus of the jugal is deeper dorsoventrally than the infratemporal ramus, leaving the ventral margin of the orbit slightly higher than the ventral margin of the infratemporal fenestra (Figs. S13A–S13D). The anterior surface of the jugal is smooth and slightly concave in the middle as in P. lujiatunensis (Zhou et al., 2006). The dorsal process of the jugal is overlapped by the ventral ramus of the postorbital, forming the bar between the orbit and infratemporal fenestra (Figs. S13A, S13B). The axes of the dorsal process of the jugal and maxillary tooth row are oriented at an angle of approximately 135 degrees in lateral view (Figs. 1A, 1B), resulting in the lateral surface of the jugal being gently inclined, in contrast to the more acute angle observed in all other species. As a result, the surface area of the jugal is very large in dorsal view (Fig. 1E), and this feature is only found in ZMNH M12414 and IVPP V12617 (You & Xu, 2005; Bullar et al., 2019) among Psittacosaurus species. The infraorbital ramus forms the ventral margin of the orbit and anteriorly contacts the lacrimal and the maxilla (Figs. 1A, 1B). The infratemporal ramus bifurcates posteriorly as in other species of Psittacosaurus (Sereno et al., 1988; Sereno & Chao, 1988; Russell & Zhao, 1996; Zhou et al., 2006; Averianov et al., 2006; Sereno et al., 2007; You, Tanoue & Dodson, 2008; Sereno, 2010; Sereno, Zhao & Tan, 2010; Napoli et al., 2019; Podlesnov et al., 2023), although this feature is not seen in the incomplete left jugal (Figs. S13D, S13L). The dorsal part of the bifurcated infratemporal ramus is large and overlaps the quadratojugal, but never reaches the quadrate (Fig. 1B). The infratemporal ramus forms the anteroventral and ventral margins of the infratemporal fenestra, without expanding posterodorsally to form the posteroventral margin (Figs. 1A, 1B) as in most other Psittacosaurus species (Sereno et al., 1988; Sereno & Chao, 1988; Russell & Zhao, 1996; Zhou et al., 2006; Averianov et al., 2006; Sereno, 2010; Sereno, Zhao & Tan, 2010; Napoli et al., 2019; Podlesnov et al., 2023). The stout jugal horn protrudes laterally, forming the well-developed horn that is sub-triangular in dorsal view. The sagittal section of the horn is also sub-triangular. The ventral surface of this horn is flat and lies more dorsal than the level of the maxillary tooth row (Figs. 1A, 1B, S13E, S13F).

Quadratojugal—Both quadratojugals are preserved, while the left one is partly fractured (Fig. S14). The quadratojugal is subtriangular in lateral view (Fig. S14B) and mediolaterally flattened in posterior view (Fig. S14L). The posterior margin of the quadratojugal extensively overlaps the quadrate, while the quadrate condyle is exposed in lateral view (Figs. 1A–1C, 1E) as in Psittacosaurus lujiatunensis (Zhou et al., 2006) and P. gobiensis (Sereno, Zhao & Tan, 2010). As in most other species of Psittacosaurus (Sereno et al., 1988; Sereno & Chao, 1988; Russell & Zhao, 1996; Sereno et al., 2007; You, Tanoue & Dodson, 2008; Sereno, 2010; Sereno, Zhao & Tan, 2010), the dorsal process of the quadratojugal does not contact the ventral process of the squamosal, only slightly contributing to the posterior rim of the infratemporal fenestra (Figs. 1A–1C, 1E). In P. meileyingensis (Sereno et al., 1988), P. sinensis (BNHM BPV149; Sereno, 2010), and IVPP V12617 (Bullar et al., 2019), the quadratojugal prominence is present in the ventral part, whereas only a weak eminence (bump) is observed in the right quadratojugal of ZMNH M12414 (Figs. S14B, S14F). The ventral part of the quadratojugal extends anteriorly to the level of the posterior margin of the laterotemporal fenestra (Figs. 1A, 1B) as in P. sinensis (IVPP V738; Sereno, 2010), P. neimongoliensis (Russell & Zhao, 1996), P. mongoliensis (Sereno, 2010), P. sibiricus (Averianov et al., 2006; Podlesnov et al., 2023), P. gobiensis (Sereno, Zhao & Tan, 2010), P. major (CAGS-IG-VD-004; You, Tanoue & Dodson, 2008), P. lujiatunensis (Zhou et al., 2006) and IVPP V12617 (You & Xu, 2005; Taylor et al., 2017).

Quadrate—Both quadrates are preserved (Fig. S15). As in other Psittacosaurus species (Russell & Zhao, 1996; Zhou et al., 2006; Sereno et al., 2007; Averianov et al., 2006; Sereno, 2010; Podlesnov et al., 2023), the quadrate shaft is oriented anterolaterally in dorsal view (Figs. 2E, S15G, S15H) and the condyle expands transversely to form a broad articular surface in ventral view (Figs. 1F, S15E, S15F). The shaft is slightly arched along its posterior margin (Figs. S15A, S15B) as in Psittacosaurus mongoliensis (Sereno, 2010), P. sinensis (IVPP V738; Sereno, 2010), P. neimongoliensis (Russell & Zhao, 1996) and IVPP V12617 (You & Xu, 2005; Bullar et al., 2019), but unlike the strongly concave condition of P. sinensis (BNHM BPV149; Sereno, 2010), P. meileyingensis (Sereno et al., 1988) and P. lujiatunensis (Sereno et al., 1988). The quadrate shaft is oriented at an angle of approximately 45 degrees in lateral view, as is the postorbital jugal bar (Figs. 1A, 1B), a feature shared with ZMNH M12414, IVPP V12617 (You & Xu, 2005; Bullar et al., 2019) and P. amitabha (Napoli et al., 2019). Dorsal to the condyles, the quadrate is not exposed in lateral view just posterior to the quadratojugal-quadrate suture (Figs. 1A, 1B), as in P. mongoliensis (Sereno, 2010), P. sinensis (Sereno, 2010) and P. meileyingensis (Sereno et al., 1988), also in P. major (Sereno et al., 2007; You, Tanoue & Dodson, 2008) and P. lujiatunensis (Zhou et al., 2006) to a lesser degree. The pterygoid wing developed as a typically broad and thin bone and completes the medial wall of the laterotemporal fossa as far anterior as the main body of the postorbital (Figs. S15G–S15L). This medial wall obscures the ventral portions of the braincase in lateral view (Figs. 1A, 1B), including most of the cranial nerve foramina.

Palate—The palate consists of premaxillae, maxillae, vomers, palatines, pterygoid and ectopterygoid (Figs. 1F, S4, S5 and S16–S19). The short secondary palate is formed by the palatal processes of premaxillae and maxillae and is well-exposed anteriorly in ventral view (Fig. 1F). The choanae (internal nostrils) are bordered anteriorly and laterally by the maxillae, medially by the vomers, posteriorly by the palatines and posteromedially by the pterygoids (Fig. 1F). In ZMNH M12414, the choanae are fully exposed in ventral view as large elliptical openings into the oral cavity, which is one of the major differences in the palate between psittacosaurids and basal neoceratopsians (Dodson, You & Tanoue, 2010). Anterior to the internal naris, the rostral, maxillae and premaxillae form a gentle vault (arched cavity), and the incisive foramen is bounded by the same three elements. The internal naris extends anteriorly to the level of the anterior edge of maxillary tooth row (Fig. 1F) as in other species of Psittacosaurus (Zhou et al., 2006).

Palatine—Both palatines are well-preserved in ZMNH M12414 (Fig. S17). The palatine contacts the maxilla and the jugal laterally, the pterygoid posteriorly, and the vomer medially. In some Psittacosaurus species, the palatine does not extend dorsally enough to contact the vomers, which are instead embraced by the pterygoids (Dodson, You & Tanoue, 2010; Podlesnov et al., 2023). In contrast, the palatine contacts the vomer in ZMNH M12414 (Fig. S17). However, it should be noted that the exact point of contact is not discernible. Whether this is due to the CT image resolution or obliterated sutures remains unclear.

Vomer—The long, fused vomer is a paired bone located along the midline of the skull, contacting the maxillae anteriorly and the pterygoids posteriorly and forming the medial margins of the choanae (Fig. 1F). The bone consists of a plate-like posterior vertical part and a rod-like, slightly arched vomerine bar (Figs. S16A, S16B). The anterior end of the vomer does not contribute to the formation of the secondary palate but lies dorsally, in a subnarial position (Figs. 1F, S16). This feature is one of the major differences in palate between psittacosaurids and basal neoceratopsians (Dodson, You & Tanoue, 2010). The vomer is in contact with the palatine laterally and the pterygoid posteriorly (Figs. S16C, S16D).

Pterygoid—Both pterygoids are preserved, and a well-developed pterygoid is triradiate, with a posterolaterally directed quadrate ramus, a posteroventrally directed mandibular ramus and an anterodorsally extending palatal ramus (Fig. S18). The quadrate ramus is Y-shaped and the largest part of the pterygoid. The quadrate ramus is thin and broadly meets the pterygoid ramus of the quadrate and the prootic posterolaterally (Figs. S18A–S18D). Medially, there is a large cup-like facet for the basipterygoid process of the basisphenoid, which is adjacent to the dorsal margin of the quadrate ramus (Figs. S18G, S18H). The pterygoid forms the posterior half of the palate. The mandibular ramus is one of the diagnostic characters of the genus Psittacosaurus (Sereno, 2010), where the ventral-most extent is elongated to form the mandibular process (Sereno, 2010). In ZMNH M12414, the mandibular process is short and mediolaterally broad, which can be seen from the lateral and ventral views (Figs. S18A–S18D). The mandibular ramus is in contact with the maxilla anterolaterally (Figs. S18A–S18B). The palatal ramus is larger than the mandibular ramus and contacts the vomers and the palatine anteriorly (Figs. S18A, S18B). As in other Psittacosaurus (Zhou et al., 2006; You, Tanoue & Dodson, 2008; Sereno, 2010; Podlesnov et al., 2023), the anterior end of the pterygoid (Figs. S18E, S18F) forms a very short median joint between the left and right pterygoids, which is located anteriorly, at approximately the midpoint of the skull (Fig. 1F). This feature is noted as one of the major differences in the palate between psittacosaurids and basal neoceratopsians (Dodson, You & Tanoue, 2010).

Ectopterygoid—Both ectopterygoids are well-preserved in ZMNH M12414 (Fig. S19). The outline of the ectopterygoid is a tall triangle in dorsal and ventral view, and it contacts the jugal anterolaterally and the maxilla ventrally (Figs. S19G–S19L). The ectopterygoid is separated from the palatine by the maxilla as in Psittacosaurus sinensis (Sereno, 1987) and P. neimongoliensis (Russell & Zhao, 1996), while the bone meets the palatine in P. lujiatunensis (Zhou et al., 2006) and P. mongoliensis (Russell & Zhao, 1996). The maxillary articular surface is flat (Figs. S19E, S19F), while the dorsal surface forms a gentle ridge (Figs. S19C, S19D).

Braincase—The braincase of ZMNH M12414 includes a supraoccipital, partly fractured exoccipitals, basioccipitals, opisthotics, laterosphenoids, a basisphenoid and prootics (Fig. S20). In posterior view, the foramen magnum has a diameter of approximately 18.3 mm, bordered by the supraoccipital dorsally, the exoccipitals laterally and the basioccipital ventrally (Fig. S20D). The supraoccipital is dorsoventrally deep and subtriangular in posterior view (Fig. S20) unlike the diamond-shaped ones in other Psittacosaurus species (Sereno et al., 1988; Napoli et al., 2019; Podlesnov et al., 2023). Similarly, IVPP V12617 seems to have a tall subtriangular-shaped supraoccipital (You & Xu, 2005; Bullar et al., 2019), although Bullar et al. (2019) notes that the fused and obscured sutures between the supraoccipital and the parietal or the exoccipital make the exact location of the contact difficult to determine. Because the supraoccipital of ZMNH M12414 is undeformed and there is no evidence of fracture, we conclude that this character is an autapomorphy of ZMNH M12414 and IVPP V12617. The supraoccipital contributes only to the middle portion of the dorsal border of the foramen magnum and is covered dorsally by the parietals. The exoccipitals borders the foramen magnum laterally. The exoccipital and opisthotic are fused to form the paroccipital process, which extends posterolaterally but lacks the distal end (Fig. S22). As in other Psittacosaurus (Zhou et al., 2006; You, Tanoue & Dodson, 2008; Sereno, 2010; Bullar et al., 2019; Podlesnov et al., 2023), the basal tubera are present as paired processes projecting ventral to the occipital condyle (Fig. S23). The basioccipital borders the foramen magnum ventrally (Fig. S20D) and forms the occipital condyle that is directed posteriorly and slightly ventrally (Fig. S23) as in other Psittacosaurus species (Zhou et al., 2006; You, Tanoue & Dodson, 2008; Sereno, 2010; Bullar et al., 2019; Podlesnov et al., 2023). The condyle is about 12.5 mm wide in diameter with a smooth hemisphere. The basioccipital fuses with the basisphenoid anteroventrally and exoccipital-opisthotic dorsolaterally. The latter suture is almost completely fused and is difficult to discern. There is no foramen or fossa along the midline between the basal tubera and the occipital condyle (Fig. S20F), unlike those in Psittacosaurus amitabha (Napoli et al., 2019) and P. lujiatunensis (Zhou et al., 2006). The laterosphenoid contacts the prootic posteroventrally and the frontal anterodorsally (Fig. S24) as in other Psittacosaurus species (Zhou et al., 2006; You, Tanoue & Dodson, 2008; Sereno, 2010; Napoli et al., 2019; Podlesnov et al., 2023). The trigeminal foramen (CN V) is 5 mm in diameter and bounded by the laterosphenoid, the prootic and the basisphenoid, which is visible on both lateral sides of the bones (Figs. S20A–S20C). The basisphenoid is well-preserved and in contact with the prootic dorsally, the pterygoid anteriorly and the basioccipital posteriorly (Fig. S25). In Hedrick & Dodson (2013), a separation of the braincase from the palate was shown as the evidence for dorsoventral compressive deformation. However, in ZMNH M12414, the braincase and palate are in contact (Fig. S25). The basal tubera has a subcircular and subvertical posterior surface, with a round, rough ventral margin that is located slightly ventral to the occipital condyle (Figs. S20D, S20F, S23). The basipterygoid processes are elongated to reach 14.4 mm in length, which is subequal to the length of the basisphenoid body measured from the notch between the processes to the basal tubera (13.5 mm), as in P. major (Sereno et al., 2007; Sereno, 2010). Both prootics are preserved, although their boundaries to the exoccipital and basisphenoid are obscured (Fig. S26). The prootic forms the lateral wall of the braincase and appears to have a contact with the laterosphenoid dorsally, the parietal posterodorsally, the pterygoid ventrally and the exoccipital posteriorly. The shape of the prootic is sub-rectangular in lateral view (Figs. S26A, S26B).

Mandible—The mandible lacks the predentary and is composed of the following elements: left and right dentaries, left and right surangulars, left and right angulars, left and right splenials, left and right coronoids, left and right prearticulars and left and right articulars (Figs. 2, S27–S33). In lateral view, the mandible is strongly curved dorsally along its ventral margin, with the dentaries and angulars forming the anterior and posterior ends of the arc, respectively (Figs. 2A, 2B). The mandible is dorsoventrally deep relative to the anteroposterior length as in Psittacosaurus lujiatunensis (Zhou et al., 2006), P. major (Sereno et al., 2007; You, Tanoue & Dodson, 2008) and IVPP V12617 (You & Xu, 2005; Landi et al., 2021). Posteriorly, the articular and surangular form the retroarticular process (Figs. 2A–2D) as in other Psittacosaurus species (Sereno et al., 1988; Russell & Zhao, 1996; Zhou et al., 2006; Averianov et al., 2006; Sereno et al., 2007; You, Tanoue & Dodson, 2008; Sereno, 2010; Sereno, Zhao & Tan, 2010; Podlesnov et al., 2023). The height of the retroarticular process is approximately at the same level as the dentary tooth row (Fig. 2C) as in IVPP V12617 (You & Xu, 2005; Landi et al., 2021) and P. neimongoliensis (Russell & Zhao, 1996). In contrast to other Psittacosaurus species (Sereno et al., 1988; Russell & Zhao, 1996; Zhou et al., 2006; Averianov et al., 2006; Sereno et al., 2007; You, Tanoue & Dodson, 2008; Sereno, 2010; Sereno, Zhao & Tan, 2010; Podlesnov et al., 2023), the retroarticular process is reduced so that the mandibles appear dorsoventrally deep in lateral view (Figs. 2A–2D). The dentary is the largest bone of the mandible, extending for about half of its length (Figs. 2A–2D). The dentary contacts the prearticular and coronoid medially and the surangular and angular posteriorly. The dentary is convex lingually (Figs. S27E–S27H), causing the toothrow to be slightly convex medially in dorsal view (Figs. S27G and S27H). There are nine alveoli in the dentary, which are fully occupied by the erupted teeth on the left, while only six alveoli are occupied on the right (Figs. S27A–S27D, S27G and S27H). The ventral border of the dentary is strongly curved and possesses a prominent dentary flange at its posteroventral corner, as in P. meileyingensis (Sereno et al., 1988), P. lujiatunensis (Zhou et al., 2006) and P. major (Sereno et al., 2007; You, Tanoue & Dodson, 2008). Although this feature is particularly evident in the right dentary, the left dentary flange is poorly preserved (Figs. S27A, S27B). The surangular forms the posterodorsal portion of the lower jaw in lateral view (Figs. 2A, 2B), and contacts the dentary anteriorly with forming the posterior half of the coronoid process (Fig. S28). The surangular extends posteroventrally to the end of the mandible, covering the angular underneath. The posterior portions of the right surangular and angular are fractured (Figs. 2B, 2D), and the right mandibular condyle is somewhat tilted posterodorsally. In addition, the distortion is indicated by the asymmetry between the height of the left and right mandibular condyles in posterior view (Fig. 2F). The obliteration of the sutural boundaries between the right surangular and articular, which led to the incomplete restoration of these bones in the CT images (Figs. S28, S29), may be a result of these deformations. Although the central sutural boundary between the dentary, surangular and angular is fenestrated in P. meileyingensis (Sereno et al., 1988), P. mongoliensis (Sereno, 2010) and P. sinensis (Sereno, 2010), forming the external mandibular fenestra, its presence remains unclear in ZMNH M12414. The articular is mediolaterally broad, but dorsoventrally thin where it articulates with the robust quadrate (Fig. S29). The articular contacts the prearticular anteriorly and the surangular and angular laterally, forming the retroarticular process. In lateral view, the anteroposterior length of the articular is relatively shorter than those of other Psittacosaurus species including P. lujiatunensis (Zhou et al., 2006), P. major (You, Tanoue & Dodson, 2008), P. sibiricus (Podlesnov et al., 2023) and IVPP V12617 (Landi et al., 2021). This difference appears to be due to post-depositional deformation or intraspecific variation in P. houi (see Discussion). The angular is well-exposed laterally with a sheet-like process that covers the ventral margin of the mandible. The angular contacts the dentary anteriorly, the surangular dorsally, and the splenial, prearticular and articular medially (Fig. S30). The posterior part of the right angular has some fractures, likely contributing to the medial displacement of the posterior part (Figs. S30B, S30D, S30F, S30H, S30J and S30L). In medial view, the angular is medially covered by the splenial anteriorly and the prearticular dorsally (Figs. 2C, 2D). The ventral margin of the angular is sinusoidal in lateral and medial views as in P. meileyingensis (Sereno et al., 1988), P. lujiatunensis (Zhou et al., 2006), P. major (Sereno et al., 2007; You, Tanoue & Dodson, 2008), P. sinensis (IVPP V738; Sereno, 2010), P. gobiensis (Sereno, Zhao & Tan, 2010), P. sibiricus (Podlesnov et al., 2023), and IVPP V12617 (Landi et al., 2021). The splenial is a thin plate-like bone on the medial side of the mandible. The splenial covers the dentary anteriorly and the angular posteriorly, contacting the prearticular posterodorsally and coronoid dorsally (Fig. S31). The anterior process of the left splenial is fractured and failed to reach the anterior-most part of the dentary, while the process of the right extends to cover the ventromedial margin of the bone (Figs. S31C, S31D). The depth of the splenial seems to be short in ZMNH M12414, while the depth is tall in P. lujiatunensis (Zhou et al., 2006), P. major (You, Tanoue & Dodson, 2008), P. sibiricus (Podlesnov et al., 2023), and IVPP V12617 (Landi et al., 2021). However, the splenials of ZMNH M12424 are asymmetrical (Fig. S31), suggesting that the thin plate-like bones may have been fractured and the splenial was not fully preserved. Both coronoids are preserved, but the left one is better preserved (Fig. S32). The coronoid process is a small thin plate, almost rhombic in shape, attaching by the dorsal part of its medial side to the coronoid process of the dentary (Figs. 2C, 2D). The position of the coronoid process is on the main axis of the dentary in dorsal view and posterior to the tooth row in lateral view as in P. major (You, Tanoue & Dodson, 2008) and P. sibiricus (Podlesnov et al., 2023). This condition differs from IVPP V12617 (Landi et al., 2021) but it appears to be influenced by post-depositional deformation or intraspecific variation in P. houi (see Discussion). The prearticular is more complete on the left side, while the right side is missing the anterior process (Figs. S33A, S33B). The prearticular is a thin, strip-like bone that curves ventrally, extending along the medial side of the lower jaw. The prearticular contacts the splenial and the angular ventrally, the splenial anteriorly, and the coronoid dorsally (Fig. S33). The articular and splenial form the internal mandibular fenestra on the medial side of the lower jaw in the left mandible (Fig. 2C) as seen in P. lujiatunensis (Zhou et al., 2006), P. major (You, Tanoue & Dodson, 2008) and P. sibiricus (Podlesnov et al., 2023). However, due to the partially fractured splenial and prearticular, which fail to form the complete rim of the fenestra, it remains unclear whether the fenestra observed in ZMNH M12414 is homologous to those of other species.

Discussion

The present study reveals that the three psittacosaurid specimens, IVPP V12704, ZMNH M12414 and IVPP V12617, are assignable to a single species, namely Psittacosaurus (formerly Hongshanosaurus) houi based on the shared autapomorphy: long jugal bar of the postorbital process nearly twice the length of the temporal bar of the bone (You, Xu & Wang, 2003; You & Xu, 2005). The following discussion pertains to ZMNH M12414 and IVPP V12617, the referred specimens of P. houi in terms of the taxonomic comparisons because IVPP V12704, the holotype, represents a juvenile that is inadequate for taxonomic comparisons.

Conclusions

In the present study, a new specimen of a psittacosaurid skull, ZMNH M12414, was described, leading to a taxonomic reevaluation of the previously coined species “Hongshanosaurus” houi. Anatomical observations and phylogenetic analyses involving ZMNH M12414 and other previously described “Hongshanosaurus” specimens suggest the following: (1) ZMNH M12414 exhibits the features most consistent with those found in the previously described psittacosaurid “Hongshanosaurus houi”, (2) “Hongshanosaurus” is to be synonymized with Psittacosaurus, and (3) Psittacosaurus houi is a valid species. Psittacosaurus is known from numerous specimens from many localities, but its diversity remains controversial. The recognition of Psittacosaurus houi supports the highly diverse nature of the genus. The present study leaves some questions for future work regarding the taxonomy and morphological characters of the genus, such as intraspecific variation and ontogeny. Revisiting other previously described species of Psittacosaurus based on additional specimens is necessary, and disputed anatomical questions may be addressed through CT analyses as demonstrated in the present work.

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