Doolysaurus huhmini sp. nov.

Holotype.

KDRC -SA-V 001, is an associated partial skeleton preserved three-dimensionally in two contiguous blocks within a reddish, homogeneous, sandy mudstone (Figs 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13) consisting of a partial cranial skeleton including dentary, maxilla, maxillary and dentary teeth, quadrate, supraoccipital, fused exoccipital / opisthotic, basioccipital, basisphenoid, and postcranial elements including dorsal vertebrae, ribs, tibia, fibula, femur, metatarsals, and pedal phalanges (Figs 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13). While the cranial and axial elements are disarticulated, the hindlimb elements are well articulated. Within the block, the skeletal components are preserved in a sub-planar arrangement (Fig. 2). The specimen, which contains a small cluster of gastroliths, was discovered by one of the authors, H. J., and is housed at the Korea Dinosaur Research Center, Chonnam National University, Gwangju, Republic of Korea. KDRC -SA-V 001 is considered a juvenile individual based on its small size, unfused cranial elements and vertebrae, and histological assessment of a section of the femoral diaphysis (see discussion below).

Type locality and horizon.

Ilseongsan Formation (Albian – Cenomanian) cropping out on the southeastern coast of Aphae Island (Aphaedo), Shinan, Republic of Korea.

Etymology.

The generic name Doolysaurus honors “ Dooly the Little Dinosaur, ” an iconic Korean cartoon baby dinosaur character created by Soo-Jung Kim in 1983; saurus is from the Greek σαῦρος (sauros), meaning “ lizard. ” The specific name, huhmini, honors Professor Dr. Min Huh, a paleontologist who conducted research on a theropod fossil nest from the Aphaedo site, in recognition of his outstanding contributions to the study of dinosaurs in Korea over the past 30 years.

Nomenclatural acts.

This publication and the nomenclatural acts in the contents are registered in Zoobank. The publication is registered under LSID [urn:lsid:zoobank.org:pub: 83804EE9-28F5-42B8-AAA2-787D2BC337C6], the new genus Doolysaurus under LSID [urn:lsid:zoobank.org:act: 86C724BC-2A9E-47EB-A502-898E4FBAC0BD], and the new species Doolysaurus huhmini is under LSID [urn:lsid:zoobank.org:act: 895C6714-B565-4100-BE78-0F6AFE1537FE].

Diagnosis.

Doolysaurus huhmini is a small-bodied, early-diverging neornithischian dinosaur with the following unique combination of features recovered from analysis of the Fonseca et al. (2024) dataset, including one optimized autapomorphy (*): (1) the lateral condyle of the quadrate is larger than the medial condyle (shared with Orodromeus and Haya) (ch. 196: 2); (2) Exoccipital, relative positions of the exits of the hypoglossal nerve (XII) combined into a single exit (shared with Jeholosaurus) (ch. 254: 2); (3) Basioccipital, contribution to the border of the foramen magnum more than 1 / 3 its basioccipital condyle size (shared with Fona) (ch. 261: 0); (4) the apex of the maxillary teeth is located posterior to the center (shared with Zephyrosaurus) (ch. 431: 1); (5) the neural spine’s lateral expansion on the distal end of the dorsal vertebrae is absent (shared with Orodromeus) (ch. 517: 0); (6) Dorsal ribs, distal anteroposterior thickening present (shared with Koreanosaurus, Thescelosaurus neglectus, and Th. garbanii) (ch. 529: 1); (7) the maximum length of the distolateral femoral condyle as a percentage of its distal width is between 50 % and 40 % (shared with Koreanosaurus, Orodromeus, and Oryctodromeus) (ch. 831: 1); (8 *) the lateromedial widths of the crista tibiofibularis and the medial condyle of the distal femur are roughly equal near the base (ch. 833: 0); (9) the cross-section of the fibular shaft is D-shaped (shared with Albertadromeus, Koreanosaurus, Orodromeus, Oryctodromeus, Zephyrosaurus, and Fona) (ch. 855: 1); (10) extensor pits are present on the distal ends of the proximal phalanges of pedal digits II – IV (shared with Changmiania, Oryctodromeus, and Changchunsaurus) (ch. 914: 1). Diagnosis of the genus is as for the species.

Differential diagnosis.

Doolysaurus is differentiated from the two other Korean ornithischian species insofar as preserved elements can be compared. Direct comparison between Doolysaurus and Koreaceratops is not feasible because Koreaceratops preserves only a limited set of postcranial characters (Lee et al. 2011). Nevertheless, Doolysaurus can be distinguished from basal ceratopsians by cranial features including the presence of a fossa at the base of the posterior pterygoid wing on the quadrate (Ch. 195: 1); absence of a nuchal crest on the supraoccipital (Ch. 245: 0); pendent, ventrally extending distal ends of the paroccipital processes (Ch. 250: 1); and subtriangular unworn non-caniniform maxillary crowns (Ch. 428: 0). Koreanosaurus boseongensis (Huh et al. 2011) was discovered from the significantly younger Seonso Conglomerate (Santonian – Campanian) (Kim et al. 2008). However, specimens of the two taxa have limited anatomical overlap. Holotype and referred specimens of Koreanosaurus preserved the forelimbs, part of the axial skeleton, and the hindlimb, excluding the pes. Nevertheless, both specimens include the femur, tibia, fibula, and dorsal vertebrae.

Comparing shared elements and considering previously documented ontogenetic variation in neornithischians (e. g., Weishampel et al. 2003; Poole 2023; Pintore et al. 2025), we discuss notable differences both with Koreanosaurus and other taxa. Although both Doolysaurus and Koreanosaurus lack an anterior inner condyle groove on the femur (Ch. 826: 0), the morphology of the anterior margin differs markedly between the two taxa (Fig. 4). In Koreanosaurus, the anterior margin of the distal femur is distinctly flat and meets the similarly flattened medial margin at a defined angle (Fig. 4 A). In contrast, Doolysaurus exhibits a rounded anterior margin that transitions smoothly into the flat medial surface without a distinct angular boundary, similar to Haya (Makovicky et al. 2011; Barta and Norell 2021) (Fig. 4 B). On the distal femur, the medial condyle of Doolysaurus is symmetrical (Fig. 4 B), resembling the condition in Haya (Makovicky et al. 2011; Barta and Norell 2021) and Thescelosaurus assiniboiensis (Brown et al. 2011) (Fig. 4 C). By contrast, Koreanosaurus exhibits an asymmetrical medial condyle (Fig. 4 A), a condition similar to that observed in Jeholosaurus (Han et al. 2012) and Parksosaurus (Sues et al. 2023) (Fig. 4 C). In addition, the femoral shaft of Koreanosaurus appears slightly more bowed than that of Doolysaurus (Ch. 781: 0) (Fig. 4 A, B). However, this difference may reflect ontogenetic variation. Poole (2023) reported that in iguanodontians such as Tenontosaurus, Zalmoxes, and Camptosaurus, juvenile individuals tend to exhibit a bowed femur, which becomes straighter in adults (Weishampel et al. 2003). Conversely, Dryosaurus shows the opposite trend (Poole 2023).

The proximal tibia of Doolysaurus exhibits a sharply defined and prominent lateral condyle, in contrast to the more gently rounded condyle observed in Koreanosaurus (Huh et al. 2011) (Fig. 5). This pronounced lateral condyle margin in Doolysaurus closely resembles that of Albertadromeus (Brown et al. 2013), in contrast to Parksosaurus (Sues et al. 2023), Thescelosaurus assiniboiensis (Brown et al. 2011), and Orodromeus (Scheetz 1999) (Fig. 5 C). In addition, Doolysaurus lacks an accessory condyle on the proximal tibia (Ch. 844: 0) (Fig. 5 B), a condition shared with Parksosaurus (Sues et al. 2023) and Thescelosaurus assiniboiensis (Brown et al. 2011) (Fig. 5 C). In contrast, Koreanosaurus possesses a well-developed accessory condyle in the same region (Huh et al. 2011) (Fig. 5 A).

Doolysaurus differs from other Asian early-diverging neornithischian taxa in several cranial morphological characters. The apex of the maxillary crown is positioned posterior to the crown center (Ch. 431: 0), whereas in Jeholosaurus, Changchunsaurus, Haya, and Changmiania, it is centrally placed. Doolysaurus lacks a prominent labial ridge on the maxillary crown (Ch. 434: 0), unlike Changmiania, Haya, Changchunsaurus, and Jeholosaurus, which exhibit a triangular apicobasal ridge. In the tibia, Doolysaurus lacks an accessory condyle on the lateral proximal condyle (Ch. 844: 0), a feature present in Jeholosaurus, Changchunsaurus, and Haya. Doolysaurus differs from Yueosaurus in possessing posteriorly projecting neural spines on the preserved dorsal vertebrae (Ch. 561: 1), a round tibial midshaft (Ch. 846: 1), and distinct extensor pits on the distal ends of pedal phalanges II – IV (Ch. 914: 1).

Description.

Skull. Most cranial elements are disarticulated and distributed in one area of the block (Fig. 6). The preserved two portions of the maxilla of KDRC -SA-V 001 represent parts of the left element, and the more complete of these bears six maxillary teeth and nine alveoli (Fig. 6 A). Because the posterior portion of the maxilla is incompletely preserved, the total number of maxillary teeth likely exceeded nine. No replacement teeth are present. The crowns are triangular in lingual and labial views, slightly asymmetrical, and laterally compressed, with a distinct constriction immediately above the root. Small denticles are developed along both the mesial and distal margins. The apices of the crowns, particularly in the posterior tooth row, are positioned posteriorly rather than centrally. A well-developed cingulum is present at the crown base, contrasting with the condition in Parksosaurus (Sues et al. 2023). Adjacent crowns are closely spaced without gaps, and the roots are straight in anterior and posterior views. The lateral maxillary surface exhibits a gentle dorsoventral concavity between the buccal ridge and the ventral margin of the external antorbital fenestra. A posterolateral process is present, resembling that of Jeholosaurus (Barrett and Han 2009; Bertozzo et al. 2025). Although the dorsal margin is damaged, the ventral edge is well preserved and displays a gentle slope.

The left dentary is preserved with 15 teeth and two replacement teeth, but the arrangement of alveoli suggests that the dentary could have accommodated at least 17 teeth (Fig. 6 B). The cranial portion is broken and displaced. The dorsal margin remains relatively intact, whereas the ventral margin is damaged, obscuring whether it was initially straight or convex. In ventral view, the rostromedial edge curves slightly medially. The lateral surface is dorsoventrally convex and lacks the prominent ridge seen in Changchunsaurus (Jin et al. 2010) and the reduced ridge of Changmiania (Yang et al. 2020), instead resembling the condition in Jeholosaurus specimen YLSNHM 01942 (Bertozzo et al. 2025). Several foramina are present on the lateral surface, including a prominent elliptical foramen positioned within the buccal emargination, just posterior to the midlength of the dentary. The Meckelian canal is long and shallow, restricted to the ventral border of the medial surface of the anterior dentary. The coronoid process is straight and well developed, and the depth of the mandible at the coronoid is more than 140 % of the depth of the mandible beneath the tooth row, resembling the condition in Fona (Avrahami et al. 2024), Oryctodromeus (Krumenacker et al. 2023), Orodromeus (Scheetz 1999), Haya (Makovicky et al. 2011; Barta and Norell 2021), Changchunsaurus (Jin et al. 2010), and Jeholosaurus (Barrett and Han 2009; Bertozzo et al. 2025). It is subtriangular in outline, posterodorsally oblique, and positioned posterior to the teeth, confluent with the axis of the tooth row.

An estimated tooth count of 17 dentary teeth would be similar to Fona (Avrahami et al. 2024), Haya (Makovicky et al. 2011; Barta and Norell 2021), Changchunsaurus (Jin et al. 2010), Jeholosaurus (Barrett and Han 2009; Bertozzo et al. 2025), and Hypsilophodon (Galton 1974), but differs from the higher count in Thescelosaurus neglectus, which possesses approximately 20 dentary teeth (Boyd 2014). This number also exceeds the 13–14 teeth reported for juvenile Jeholosaurus and more closely matches the 17 observed in mature individuals (Hu et al. 2024). Dysalotosaurus has fewer dentary teeth, with an estimated count of approximately 10 in juveniles and up to 13 in adults (Hübner and Rauhut 2010). Two replacement teeth are preserved, a condition also reported in Jeholosaurus (Hu et al. 2024; Bertozzo et al. 2025), Thescelosaurus (Morris 1976; Brown et al. 2011; Boyd 2014), Haya (Makovicky et al. 2011; Barta and Norell 2021), Hypsilophodon (Galton 1974), and Parksosaurus (Sues et al. 2023). The dentary crowns are triangular in lingual and labial aspects, distinctly constricted above the root, slightly asymmetrical, and laterally compressed. A pronounced primary ridge defines the apex, with denticles extending along both sides of the ridge. The cingulum is robust, and the roots are straight, unlike the more curved roots of Parksosaurus (Sues et al. 2023) and Hypsilophodon (Galton 1974). The anterior teeth are similar in morphology to more posterior teeth, resembling Jeholosaurus (Barrett and Han 2009; Bertozzo et al. 2025) but differing from Thescelosaurus neglectus (Boyd 2014), Haya (Makovicky et al. 2011; Barta and Norell 2021), and Changchunsaurus (Jin et al. 2010). The lingual surface of the crowns bears fewer than 10 secondary ridges. The primary ridge is markedly more prominent than the others, a condition shared with Fona (Avrahami et al. 2024), Oryctodromeus (Krumenacker et al. 2023), Thescelosaurus neglectus (Boyd 2014), Haya (Makovicky et al. 2011; Barta and Norell 2021), and Changchunsaurus (Jin et al. 2010), but distinct from Zephyrosaurus (Sues 1980), Jeholosaurus (Jin et al. 2010), and Parksosaurus (Sues et al. 2023). Wear facets appear developed along the entire tooth row, and the crowns exhibit mesiodistal expansion above the roots as well as weak labiolingual expansion, expressed as a distinct cingulum.

The left quadrate (Fig. 7) is dorsoventrally elongated with a blade-like appearance and a gently curved shaft, resembling the condition in other early-diverging neornithischians. The shaft is gently curved, with its ventral portion oriented vertically. The quadrate head is recurved posteriorly relative to the main axis, rounded in lateral view, and triangular in dorsal view. A pronounced ridge extends along the posteroventral side of the proximal end. The lateral wing is directed posteriorly with a transversely narrow anterior margin, whereas the medial (pterygoid) wing forms a large anteromedially directed fan of bone that arises below the dorsal head. The pterygoid ramus is not dorsoventrally narrow, and no prominent oval fossa is developed, although a distinct fossa occurs at the base of the posterior side of the pterygoid wing. No hamular process is present, as in Changmiania (Yang et al. 2020) and Changchunsaurus (Jin et al. 2010), but this character is variable in Haya (Barta and Norell 2021). A pit at the base of the jugal wing is absent. The lateral condyle is rounded and projects slightly more ventrally than the medial one, which is flatter. Based on the preserved part, the lateral condyle is estimated to be larger than the medial one, resembling the condition in Orodromeus (Scheetz 1999) and Haya (Barta and Norell 2021). In posterior view, the articular surface of the condyles is nearly horizontal.

Only the left fused exoccipital / opisthotic is preserved, and it is disarticulated from the other cranial elements (Fig. 8 A). As in other early-diverging neornithischians, the left exoccipital / opisthotic of KDRC -SA-V 001 is completely fused, with no visible suture between them (Norman 2004; Bertozzo et al. 2025). It is slightly twisted mediolaterally along its main axis, and a broad, trapezoidal paraoccipital process extends laterally, resembling the condition in Jeholosaurus (Bertozzo et al. 2025). This process is anteroposteriorly expanded rather than pendent, terminating in a subtriangular ventral tip that projects ventrally, similar to Jeholosaurus (Bertozzo et al. 2025), but differing from the morphology reported in Haya (Barta and Norell 2021). The posteroventral margin of the paraoccipital process bears no distinct scars for epaxial muscle attachment. On the anterolateral surface, a rod-like crista interfenestralis projects anteriorly, comparable to that in Hypsilophodon (Galton 1974) and Jeholosaurus (Bertozzo et al. 2025). The recessus scalae tympani is expressed as a wide, circular opening piercing the dorsal position of the exoccipital. A posteriorly projecting boss occurs on the dorsomedial corner of the posterior surface. The anterodorsal surface forms a complex articulation with the supraoccipital and is perforated by the foramen for the posterior semicircular canal. The posttemporal foramen is fully enclosed within the paraoccipital process, transmitting the vena capitis dorsalis, as in Orodromeus (Scheetz 1999), Zephyrosaurus (Sues 1980), Haya (Makovicky et al. 2011; Barta and Norell 2021), Jeholosaurus (Barrett and Han 2009; Bertozzo et al. 2025), and Hypsilophodon (Galton 1974). Two foramina pierce the ventral surface of the exoccipital: the larger posterior opening transmits cranial nerve XII, whereas the smaller, more anterior one likely corresponds to cranial nerve XI. Additionally, a single foramen on the medial surface also transmits branches of cranial nerve XII, indicating that the hypoglossal nerve exits were confluent into a single canal.

The supraoccipital is completely preserved (Fig. 8 B). In posterior view, the posterodorsal surface is rhomboidal in outline and bears a subtle midline elevation instead of a prominent nuchal crest, a condition resembling that of Fona (Avrahami et al. 2024), Thescelosaurus (Brown et al. 2011; Boyd 2014), and Haya (Makovicky et al. 2011; Barta and Norell 2021). Two shallow depressions are developed laterally, parallel to the midline, as also noted in Fona (Avrahami et al. 2024) and Thescelosaurus (Boyd 2014). Unlike Thescelosaurus neglectus (Boyd 2014) and T. assiniboiensis (Brown et al. 2011), the dorsal surface lacks foramina. In dorsal view, the element is subtriangular. The posteroventral margin, which forms the dorsal border of the foramen magnum as in other early-diverging neornithischians, is distinctly concave. The articular surfaces for the prootic, laterosphenoid, and fused exoccipital / opisthotic are rough and perforated by small foramina.

The basioccipital is intact but is disarticulated from the other cranial elements (Fig. 8 C). The occipital condyle is enlarged and vertically oriented, with considerable dorsoventral thickening and a slightly flattened articular surface. A wide U-shaped depression defines the ventral margin of the foramen magnum. The endocranial floor is slightly concave and bears a marked midline groove. This groove on the endocranial floor differs from that of other early-diverging neornithischians, which show either a midline ridge, such as Thescelosaurus assiniboiensis (Brown et al. 2011), Zephyrosaurus (Sues 1980), and Jeholosaurus (Bertozzo et al. 2025), or a flat endocranial surface, such as Fona (Avrahami et al. 2024), Haya (Makovicky et al. 2011; Barta and Norell 2021), Oryctodromeus (Krumenacker et al. 2023), and Orodromeus (Scheetz 1999). In ventral view, a well-developed keel divides the basal tubera, as seen in Jeholosaurus (Barrett and Han 2009; Bertozzo et al. 2025). The floor of the basioccipital is arched, resembling the condition in most early-diverging neornithischians. Although disarticulated, we estimate that the foramen magnum occupies more than 30 % of the dorsal margin of the occipital condyle. This condition resembles that of Fona (Avrahami et al. 2024), Oryctodromeus (Krumenacker et al. 2023), Th. assiniboiensis (Brown et al. 2011), Orodromeus (Scheetz 1999), and Haya (Makovicky et al. 2011; Barta and Norell 2021), but differs from Th. neglectus (Boyd 2014) and Th. garbanii (Morris 1976).

Only the posterior portion of the basisphenoid is preserved at the margin of the block (Fig. 8 D). The element is disarticulated, as are the other cranial components. The dorsoposterior surface is flat, bearing the posterior part of the sella turcica. The sella turcica is connected to two symmetrical grooves that extend along the ventral surface. The basipterygoid processes and the basioccipital articular surface are not preserved.

Vertebrae and ribs. The dorsal vertebrae of KDRC -SA-V 001 are represented by three complete isolated centra, two incomplete centra, and one complete neural arch (Fig. 9). All elements are unfused and disarticulated. The centra are amphicoelous (Fig. 9 B). Their articular surfaces are subrounded to triangular in outline, with a flat dorsal surface and weak ventral keels. The keels are subdued, and considering the condition in other early-diverging neornithischians, in which the ventral keel diminishes posteriorly along the dorsal series, this feature suggests that the preserved centra likely belong to the anterior dorsal region. The floor of the neural canal is not pierced by any large, anteroposteriorly elongated foramen, resembling the condition in Parksosaurus (Sues et al. 2023) and Thescelosaurus assiniboiensis (Brown et al. 2011).

The preserved transverse processes are thin, cylindrical, and extend horizontally and approximately perpendicular in anterior view (Fig. 9 A), as seen in Changmiania (Yang et al. 2020), Albertadromeus (Brown et al. 2013), Koreanosaurus (Huh et al. 2011), Haya (Makovicky et al. 2011; Barta and Norell 2021), Changchunsaurus (Jin et al. 2010), Jeholosaurus (Han et al. 2012), and Parksosaurus (Sues et al. 2023). The preserved prezygapophyses are short and process-like, whereas the postzygapophyses are damaged. The neural spines are positioned slightly posterior to the centra, a morphology seen in Koreanosaurus (Huh et al. 2011), Nevadadromeus (Bonde et al. 2022), Parksosaurus (Sues et al. 2023), and Thescelosaurus assiniboiensis (Brown et al. 2011). The tips of the neural spines show no lateral expansion, as in Orodromeus (Scheetz 1999).

Three ribs are preserved in sequential arrangement, but their proximal ends are eroded at the specimen margin, making it difficult to determine their anatomical position (Fig. 9 C). The rib shafts are strongly curved, and their cross sections are elliptical.

Forelimb. The forelimb elements of KDRC -SA-V 001 are poorly preserved. Only the distal portion of the humerus (Fig. 10 A) and possible fragments of the scapula (Fig. 10 B) and coracoid (Fig. 10 C) are exposed along the lower margin of the block. Most parts that would permit identification of diagnostic anatomical features are not preserved, obscuring detailed morphological assessment.

Hind limb. The distal left femur is preserved to the midshaft and remains roughly articulated with the tibia and fibula (Fig. 11). The shaft is slightly bowed anteriorly in lateral view and broadens distally. In cross-section, the midshaft is roughly circular to subelliptical, with the remnants of a posterolaterally projecting fourth trochanter. The fourth trochanter is prominent, but its distal end is damaged, making it difficult to confirm whether it has a pendant morphology similar to that of other early-diverging neornithischians. At the base of the fourth trochanter, an elongated shallow depression is present on the medial surface of the shaft, representing the insertion scar of the m. caudofemoralis longus.

The distal end of the femur is expanded anteroposteriorly, with the medial and lateral condyles separated by a fully open posterior (flexor) intercondylar groove. The medial condyle is larger than the lateral condyle, projecting posteriorly and not enclosing the groove. The posterior intercondylar groove extends for less than one-quarter of the femoral length, a condition comparable to that of Koreanosaurus (Huh et al. 2011), Oryctodromeus (Krumenacker et al. 2023), Jeholosaurus (Han et al. 2012), Parksosaurus (Sues et al. 2023), and Thescelosaurus (Gilmore 1915; Morris 1976; Brown et al. 2011), but differing from Orodromeus (Scheetz 1999) and Haya (Makovicky et al. 2011; Barta and Norell 2021). In contrast, an anterior (extensor) intercondylar groove is absent, distinguishing this specimen from Zephyrosaurus (Sues 1980) and Thescelosaurus neglectus (Gilmore 1915). The distal condyles are oriented perpendicularly relative to the shaft axis and are subequal in anteroposterior expansion. However, the medial condyle is slightly broader than the lateral one, resembling the condition in Parksosaurus (Sues et al. 2023), while the other early-diverging neornithischians exhibit subequal condyles. The transverse widths of the medial condyle and the crista tibiofibularis are subequal, differing from the condition in Koreanosaurus (Huh et al. 2011), Orodromeus (Scheetz 1999), Zephyrosaurus (Sues 1980), Haya (Makovicky et al. 2011; Barta and Norell 2021), Parksosaurus (Sues et al. 2023), and Thescelosaurus neglectus (Gilmore 1915). The surface between the lateral condyle and the crista tibiofibularis is smooth to shallowly grooved, a morphology shared with Koreanosaurus (Huh et al. 2011), Oryctodromeus (Krumenacker et al. 2023), Haya (Makovicky et al. 2011; Barta and Norell 2021), Jeholosaurus (Han et al. 2012), Thescelosaurus neglectus (Gilmore 1915), and Th. assiniboiensis (Brown et al. 2011).

The left tibia is preserved from the midshaft to the proximal end (Fig. 12 A). In cross-section, the tibial shaft is rounded, resembling the condition in Koreanosaurus (Huh et al. 2011), Jeholosaurus (Han et al. 2012), and Parksosaurus (Sues et al. 2023), but differing from the triangular morphology of Fona (Avrahami et al. 2024), Oryctodromeus (Krumenacker et al. 2023), Thescelosaurus (Gilmore 1915; Morris 1976; Brown et al. 2011), Orodromeus (Scheetz 1999), and Haya (Makovicky et al. 2011; Barta and Norell 2021).

The proximal end of the tibia has well-developed lateral and medial condyles that are distinctly separated by a deep notch (Fig. 12 A). The lateral condyle is offset anteriorly relative to the medial condyle and is larger or subequal in width to the posterior condyle, as in Albertadromeus (Brown et al. 2013), Koreanosaurus (Huh et al. 2011), Jeholosaurus (Han et al. 2012), and Parksosaurus (Sues et al. 2023), but unlike Orodromeus (Scheetz 1999), Oryctodromeus (Krumenacker et al. 2023), and Changchunsaurus (Butler et al. 2011). In anteroposterior view, the lateral condyle defines an abrupt overhanging buttress with a subhorizontal ventral margin, a feature shared with Fona (Avrahami et al. 2024), Oryctodromeus (Krumenacker et al. 2023), and Orodromeus (Scheetz 1999), but absent in Changchunsaurus (Butler et al. 2011) and Parksosaurus (Sues et al. 2023). No accessory condyle is developed on the lateral side, similar to the condition in Parksosaurus (Sues et al. 2023) and Thescelosaurus assiniboiensis (Brown et al. 2011). The cnemial crest is prominent, curving anterolaterally and appearing rounded in lateral view, a morphology typical of early-diverging neornithischians. The medial surface of the proximal tibia forms a continuous arc along the anteroposterior axis in proximal view.

The proximal left fibula is preserved from the midshaft in articulation with the tibia (Fig. 12 B). However, much of the detailed anatomical morphology of the proximal end of the fibula is difficult to discern. It is expanded both medially and laterally, and its anterior edge arcs anteromedially, a condition typical of early-diverging neornithischians. In cross-section, the fibular shaft is D-shaped, which contrasts with the more rounded condition described in Thescelosaurus neglectus (Gilmore 1915) and Th. garbanii (Morris 1976).

Metatarsals I-III of the left foot are partially preserved in KDRC -SA-V 001 (Fig. 13). Considering the typical early-diverging neornithischian digit formula (2-3 - 4 - 5 - 0) (Avrahami et al. 2024) and that the length of phalanx II- 1 exceeds that of IV- 1, the articulated metatarsals and digits are identified as belonging to the left pes. They are closely appressed, as in other early-diverging neornithischians. However, metatarsal I is highly damaged, providing little morphological information. Metatarsal II is represented only by its distal portion. Its ventral surface is gently arched dorsally. A shallow collateral ligament pit is visible, but the ginglymoid distal articular surface of this metatarsal is damaged. The distal half of metatarsal III is preserved and broadens slightly toward the distal end. In cross-section, it is elliptical, with the right lateral surface curving more medially than the left. Its ginglymoid articular surface is more strongly expressed ventrally than dorsally; a shallow collateral ligament pit is present.

All phalanges of pedal digits II and III are well preserved and articulated, whereas digit I preserves only one phalanx, and those of digit IV are not preserved (Fig. 13). Digits II and III consist of three and four phalanges, respectively, consistent with the typical early-diverging neornithischian phalangeal formula of 2-3 - 4 - 5 - 0 (Avrahami et al. 2024). All preserved phalanges are elongated and cylindrical, with expanded proximal and distal ends and a relatively thin midshaft. The first phalanges (II- 1 and III- 1) are longest, with subsequent phalanges decreasing in length. The distal articular surfaces bear well-developed condyles, more distinct ventrally, and possess deep collateral ligament pits laterally and medially. Notably, the dorsodistal surface of phalanx II- 1 bears a distinct fossa (Fig. 13; ep). This fossa is also present in Changmiania (Yang et al. 2020), Oryctodromeus (Krumenacker et al. 2023), and Changchunsaurus (Butler et al. 2011). The proximal articular facets change in morphology from a single surface on the first phalanges to bifaceted surfaces separated by a medial ridge on the second and third phalanges.

The ungual of digit III is preserved in partial articulation, whereas an isolated ungual likely belongs to digit II based on size (Fig. 13). Both unguals are claw-like, being longer than wide, with pointed distal ends and dorsally arched dorsal and ventral margins. Longitudinal grooves are present on the medial and lateral surfaces. The ungual of digit II is narrower and more slender than that of digit III, whereas the ungual of digit III exhibits a more strongly curved ventral surface. Flexor tubercles are absent or expressed only as a low mound adjacent to the articular surface.

Gastroliths. Forty to fifty small and well-sorted subangular pebbles and similarly sized discoid stones occur in a cluster between the vertebrae and femur of specimen KDRC -SA-V 001 (Fig. 14 A, B). These pebbles exhibit smooth surfaces and consist of diverse lithologies, including quartzite and volcanic rocks. Some of the dark volcanic pebbles are oblong in shape, whereas those of quartzite tend to exhibit a more spherical morphology. The size of individual pebbles ranges from 2 to 10 mm in diameter, and the cluster forms an approximately ellipsoidal mass with principal axes of 12 mm, 12 mm, and 31 mm. The total volume of the gastroliths is estimated at 9.3 mL. Considering that several pebbles are exposed at the surface, the actual volume was likely greater. Based on the average density of silicate rocks (3.3 g / cm 3), the total mass of the gastroliths is estimated at a minimum of 30.7 g. Comparable occurrences of clustered gastroliths have been documented in closely related taxa such as Haya (Makovicky et al. 2011; Barta and Norell 2021) and Changmiania (Yang et al. 2020), but have not been reported in any North American thescelosaurid taxa.

Histological data.

In Doolysaurus, the transverse femoral thin section reveals a thin cortical wall measuring 1.2–1.6 mm in thickness (Fig. 15). Although the outermost cortex in some areas is damaged, the preserved regions allow limited assessment of the primary histological structure. The cortical tissue consists of parallel-fibered bone containing primarily longitudinal vascular canals. The density of vascular canals shows no significant difference between the inner and outer cortex. A faint, lighter-colored transverse line occurs near the mid-cortex, but its alignment with a crack in the section and the differing texture surrounding it make its interpretation unclear; it may represent a line of arrested growth (LAG) (Fig. 15 B, C). There is no indication of significantly reduced vascularization toward the external surface, suggesting that the individual was still undergoing relatively rapid growth at the time of death.

Phylogenetic results.

Both parsimony and tip-dated Bayesian analyses of the Fonseca et al. (2024) dataset recover Thescelosauridae as a monophyletic group and Doolysaurus huhmini within this clade (Suppl. material 1: figs S 1 – S 4). In the Bayesian analysis, Doolysaurus is recovered in Thescelosaurinae as the sister taxon of a clade including Parksosaurus, Fona, and Thescelosaurus (Fig. 16). Parsimony analysis yielded 381 equally most parsimonious trees (tree length of 7081). In the strict consensus of these trees, relationships within Thescelosauridae are unresolved; Doolysaurus forms part of a large polytomy (Suppl. material 1: fig. S 1). In the 50 % majority-rule consensus, Doolysaurus also lies within a monophyletic Thescelosaurinae that contains Parksosaurus, Fona, and Thescelosaurus. This clade also includes additional small-bodied species from Asia (Suppl. material 1: fig. S 2); Changmiania, rather than being recovered as the sister taxon to all other Thescelosauridae (Fig. 16), is placed within Thescelosaurinae as the most basally divergent taxon within that clade (Suppl. material 1: fig. S 2). A clade comprising other Asian taxa (Jeholosaurus, Changchunsaurus, Haya, and Yueosaurus), like that reported by Fonseca et al. (2024), is recovered as the sister taxon to a Fona, Parksosaurus, and Thescelosaurus clade within Thescelosaurinae (Suppl. material 1: fig. S 2). In both the Bayesian analysis and the 50 % majority-rule consensus tree from the parsimony analyses, Koreanosaurus is recovered within Orodrominae as the only Asian taxon in this clade. This position has been recovered in previous analyses (Boyd 2015; Fonseca et al. 2024; Avrahami et al. 2024).

To test the sensitivity of the recovered phylogenetic position of Doolysaurus huhmini, we additionally employed the recently published matrix of Avrahami et al. (2024). In tip-dated Bayesian analyses, the topology is poorly resolved, showing multiple polytomies within Neornithischia (posterior probability> 0.5). Under a relaxed threshold (posterior probability> 0.1), Thescelosauridae is resolved as a monophyletic group, and Doolysaurus falls within this clade. The parsimony analysis yielded 12 equally most parsimonious trees with a best score (TBR) of 1834. Thescelosauridae is not recovered as monophyletic in the strict consensus tree but forms a clade in the 50 % majority-rule consensus. In this tree, Jeholosaurus, Parksosaurus, Yueosaurus, and Changchunsaurus are placed outside Thescelosauridae. Doolysaurus forms a node with Diluvicursor as a basal member of Orodrominae within Thescelosauridae, while Koreanosaurus forms a node with Micropachycephalosaurus within Thescelosaurinae. Notably, in both the Fonseca et al. (2024) and Avrahami et al. (2024) datasets, in all analyses in which Thescelosauridae is resolved, the Asian taxa, including Doolysaurus, consistently occupy basal positions within the group or form basal branches within either Orodrominae or Thescelosaurinae.

Doolysaurus shares two synapomorphies of Thescelosaurinae proposed by Fonseca et al. (2024): a fossa on the base of the pterygoid wing of the quadrate [ch. 195: 1] and an overhanging tibia lateral condyle [ch. 842: 1]. Although the precise composition of this clade has varied across studies and analyses, it generally includes the North American taxa, including Thescelosaurus, Parksosaurus, and Fona, and more basal Asian taxa, including Haya, Changchunsaurus, and Jeholosaurus (Boyd 2015; Fonseca et al. 2024). In contrast, Koreanosaurus has consistently been recovered as a member of Orodrominae (Boyd 2015; Madzia et al. 2018; Barta and Norell 2021; Krumenacker et al. 2023; Sues et al. 2023; Avrahami et al. 2024; Fonseca et al. 2024), and in our revised analysis, based on the modification of new characters, was again nested within Orodrominae.