Sinornithomimus dongi, Kobayashi & Lü, 2003

Kobayashi, Yoshitsugu & Lü, Jun-Chang, 2003, A new ornithomimid dinosaur with gregarious habits from the Late Cretaceous of China, Acta Palaeontologica Polonica 48 (2), pp. 235-259 : 237-244

Sinornithomimus dongi sp. nov.

Etymology: Named after Professor Zhi−Ming Dong, who discovered these skeletons and made great contributions to the Mongol Highland International Dinosaur Project.

Holotype: IVPP−V11797−10; a nearly complete skeleton of a subadult individual with a 32 cm long femur (estimated body length is 2.5 m) but lacks the posterior caudal vertebrae.

Locality and age: Skeletons were discovered from a locality (41°17’10’’N, 103°52’38’’W) near Ulan Suhai in Alashanzuo Banner , Nei Mongol Autonomous Region of China. The locality is in the Ulansuhai Formation and its age is considered to be early Late Cretaceous .

Diagnosis.—An ornithomimid dinosaur with the following apomorphies: depression on dorsolateral surface of posterior process of parietal, fenestra within quadratic fossa divided into two by vertical lamina, low ridge on ventral surface of parasphenoid bulla, and loss of posterolateral extension of the proatlas.

Description.—The holotype skull is transversely crushed. It is intact except for some elements within the orbital region and the mandibular fenestrae that are somewhat displaced from their original positions ( Fig. 5 ). Some braincase (laterosphenoid, prootic, and orbitosphenoid) and palatal (vomer and basipterygoid) bones are crushed or not exposed.

Skull length is less than half of the length of the cervical series ( Table 1). The orbit is slightly longer than the antorbital fossa. The anterior border of the antorbital fenestra is straight and vertical as in Garudimimus brevipes and Ornithomimus sp. (TMP 95.110.1). In IVPP−V11797−11 ( Fig. 6 ), the oval (anteroposteriorly long axis) supratemporal fenestra is enclosed by the parietal and squamosal and opened posterodorsally. The supratemporal fossa is large and extends onto the posterior portion of the frontal.

The edentulous premaxilla ( Figs. 5 , 6 ) has thin and posteriorly narrowing nasal and maxillary processes that terminate anterior to the antorbital fossa. The dorsal edge of the

Skull length (premaxilla−squamosal) 183.1

Skull height (including mandible) at the orbit 65.7

Orbit, anteroposterior length of orbit 53.1 Antorbital fossa, length and height 48.5 × 29.4 Antorbital fenestra, anteroposterior length 30.2 Cervical vertebral series, total length 410

Dorsal vertebral series, total length 510 Forelimb, total length 540 Hindlimb, total length 1040

KOBAYASHI AND LÜ—NEW GREGARIOUS ORNITHOMIMID FROM CHINA 239

maxillary process contacts the nasal, separating the maxilla from the external narial opening. Anteriorly, the ventral bor− der of the premaxilla curves dorsally, which leaves a gap with the ventrally curved anterior portion of the dentary. In dorsal view, the outer edge of the premaxilla is U−shaped unlike Struthiomimus and other North American taxa (Makovicky et al. in press). A series of foramina is present along the ventral edge of the premaxilla. There is a foramen associated with a groove at the base of the internarial plate. The ventral surface of the premaxilla takes part in the palate, and the peripheral margin extends more ventrally than the palate, forming a bony beak.

The maxilla ( Figs. 5 , 6 ) has long, thin dorsal and posterior processes. The dorsal process contacts the anterior process of the lacrimal at the midpoint of the antorbital fenestra, and the ventral process thins posteriorly and meets the jugal. There are no foramina posterior to the premaxilla−maxillary suture along the ventral margin of the maxilla as seen in Gallimimus bullatus (GIN 100/1133). The convex ventral margin of the main body of the maxilla expands ventrally as strongly as in Garudimimus brevipes and Gallimimus bullatus . The expansion meets the dorsomedially directed dorsal margin of the dentary to form a cutting edge. Within the antorbital fossa, there are two accessory (promaxillary and maxillary) fenestrae.

The nasal ( Figs. 5 , 6 ) is anteroposteriorly long and transversely narrow. The anterior border of this element is concave where it forms the posterior border of the external narial opening. The nasals contact each other along a straight suture, but the posterior nasals ends diverge lateral to the anterior ends of the frontals and terminate anterior to the prefrontal−lacrimal contact.

In dorsal view, the frontals are triangular ( Figs. 5 , 7 ). Each is slightly shorter anteroposteriorly than the nasal, and is the widest close to the posterior edge along the frontal−parietal suture. The posterior quarter (behind the posterior end of the orbit) is inclined ventrally and is domed on each side. The dome of one frontal is separated from the other by a slight depression as in Gallimimus bullatus ( Osmólska et al. 1972) . The lateral slope of the dome forms part of the anterior portion of the supratemporal fossa although the frontal does not participate in the supratemporal fenestra.

The postorbital ( Fig. 5 ) is dorsoventrally elongate with an anteroposteriorly expanded dorsal end. The posterior side (anterior border of the infratemporal fenestra) of the element is concave dorsally and convex ventrally. Ventrally, it narrows along the medial side of the jugal. The posterodorsal process contacts the lateral side of the anterior process of the squamosal.

Medially the parietals ( Fig. 7 ) are horizontally flat, forming the posterior part of the skull table. A posterolateral process extends beyond the posterior end of the skull table. The dorsolateral surface of the process has a depression unlike Struthiomimus sp. (TMP 90.26.1), and the distal tip fits onto the top of the paroccipital process. The lateral side of the skull table and the lateral surface of the posterior process form the concave margin of the supratemporal fossa.

The main body of the squamosal ( Figs. 6 , 7 ) has medial, anterior, ventral, and posterior processes. The medial process contacts the parietal and forms the posterior boundary of the supratemporal fenestra. The long anterior process fits onto the medial side of the posterodorsal process of the postorbital. The ventral process, exposed laterally, is as long as the anterodorsal process and terminates between the quadrate and quadratojugal ( Fig. 7A, E ). The posterior process is short, and its ventral border is concave for articulation with the quadrate. At the base of the posterior process, the lateral surface of the left squamosal in IVPP−V11797−31 preserves the squamosal recess as in Dromiceiomimus brevitertius Parks, 1926 and tyrannosaurids ( Witmer 1997).

The lacrimal is almost L−shaped with long anterior and ventral processes and a short posterior process ( Figs. 5 , 6 ). The posterior part of the lacrimal overlies and fits into a depression in the prefrontal. The prefrontal is nearly equal to the lacrimal in size in dorsal view and has posterior and ventral processes of sub−equal length. The anterior end contacts the nasal, separating the lacrimal from the frontal. The posterior process of the prefrontal plugs into a depression on the ventrolateral surface of the anterior portion of the frontal. The ventral process narrows and is sutured onto the medial side of the ventral process of the lacrimal.

The jugal is anteroposteriorly long with an expanded posterior end ( Figs. 5 , 6 ). The anterior end is not bifurcated for its contacts with the maxilla and lacrimal in contrast to Struthiomimus sp. (TMP 90.26.1) and Ornithomimus sp. (TMP 95.110.1). The expanded posterior end has an anteroposteriorly elongate depression for the quadratojugal. A long process near the posterior end of the jugal extends posterodorsally and meets the posterior side of the postorbital. The posterior border of the process forms the concave ventral border of the infratemporal fenestra.

The quadratojugal, well preserved in IVPP−V11797−31, is L−shaped with dorsal and anterior processes ( Fig. 7 ). The dorsal process is much longer than the anterior process and contacts the ventral process of the squamosal anteriorly. The process does not bifurcate at the dorsal end, and the ventral half of the process forms the weakly concave anterior border of the paraquadratic foramen, but lacks a distinct notch such as found in Ornithomimus sp. (TMP 95.110.1) (Makovicky et al. in press). The posteroventral part of the element forms a square corner, and is sutured to the lateral side of the accessory condyle of the quadrate.

The mandibular condyles of the quadrate ( Figs. 5–7 ) are roughly equal in size and are well separated by an anteroposterior sulcus. Lateral to the lateral condyle there is an accessory condyle. The accessory condyle is more dorsally positioned than the mandibular condyles and is contoured to the dorsally expanded region of the surangular. In posterior view, there is a slight concavity for the paraquadratic foramen. This concavity is dorsal to the accessory condyle ( Fig. 7D, H ). On the mid−posterior surface of the main body, an oval−shaped fossa is present as in other ornithomimosaurs ( Makovicky and Norell 1998). A fenestra found within the quadratic fossa, is roughly half of the fossa in size, and is divided by a vertical lamina ( Fig. 7D, H ). The quadrate has a large pterygoid wing. Its anteroposterior length is roughly 40% of the quadrate height. The ventral portion of the pterygoid wing forms a medially extending shelf, where it contacts the pterygoid.

Dorsally, the supraoccipital is flat and lies between the posterior processes of the parietals ( Figs. 5 , 7 ). The posterior surface has a vertical ridge as in Struthiomimus altus ( Lambe, 1902) (AMNH 5355), but unlike Gallimimus bullatus ( Makovicky and Norell 1998) . In dorsal view, the dorsal process is thin and U−shaped.

The paroccipital process ( Figs. 5 , 7 ) extends lateroventrally, and its ventral border is lower than the foramen magnum. The exoccipital is pneumatic with a large foramen at mid−length, whereas basally it is penetrated on the anterior side as seen in Gallimimus bullatus (GIN 100/987) (fig. 1F in Makovicky and Norell 1998). The foramen at the mid−length is also seen on the posterior surface of the paroccipital process in Gallimimus bullatus (GIN 100/1133) and Struthiomimus sp. (TMP 90.26.1). A posteroventral process, extending from the base of the paroccipital process, borders the lateral side of the foramen magnum. Its ventral end forms a dorsal portion of the occipital condyle. The metotic strut extends ventrally from the base of the paroccipital process. Its lateral surface is smooth. Ventrolateral to the occipital condyle and medial to the metotic strut, a depression is associated with foramina for the vagus (X) and hypoglossal (XII) nerves. The foramen for the vagus nerve (X) is more laterally positioned and slightly larger than the ones for nerve XII. Pneumatic structures anterior to the metotic strut in IVPP−V11797−10 are not visible because of crushing.

The parasphenoid is partially exposed in IVPP−V11797−31 ( Figs. 7 , 8 ). It has the bulbous structure typical of ornithomimosaurs and troodontids. It is wide posteriorly and has an anterior process (parasphenoid rostrum) as in Gallimimus bullatus ( Osmólska et al. 1972) . The bulbous portion has a flat ventral surface. The parasphenoid rostrum is roughly half of the height of the bulbous portion of the element in lateral view, and it becomes narrower anteriorly in ventral view. In lateral view, the process has a horizontal dorsal edge at the same level as the dorsal border of the bulbous portion. Dorsoventrally, it narrows dramatically as the ventral border curves anterodorsally, whereas the anterior process narrows gradually in Garudimimus brevipes and Gallimimus bullatus as well as in the troodontids Saurornithoides and Troodon ( Osmólska et al. 1972; Barsbold 1981; Currie 1985). The base of the anterior process has a low ridge unlike Gallimimus bullatus .

The posterior portion of the pterygoid is preserved in IVPP−V11797−31. Posteriorly the pterygoid contacts the medial surface of the pterygoid wing of the quadrate. At the base of the quadrate wing, a short basipterygoid process extends posteromedially. In IVPP−V11797−10, although the main body of the ectopterygoid is crushed, the hook−shaped jugal process of the element is exposed. The distal end of the process reaches the middle of the jugal.

The anterior portions of the dentaries curve ventrally ( Figs. 5 , 6 ). The symphyseal region of the paired dentaries is U−shaped in ventral view. The radius of the dorsal margin of the arc is less than that of the ventral margin of the joined premaxillae. The lateral side of the dentary, below the ventral expansion of the maxilla, has a foramen as in Gallimimus bullatus ( Hurum 2001) but lacks the series of foramina that are found in Pelecanimimus polyodon Perez−Moreno, Sanz, Buscalloni, Meratalla, Ortega, and Rasskin−Gutman, 1994 , Ornithomimus sp. (TMP 95.110.1), Struthiomimus sp. (TMP 90.26.1), and Gallimimus sp. (GIN 950818). The dorsal edge of the dentary is sharp in the anterior two−thirds of the element and more rounded in the posterior third. A ventral process at the posterior end laterally overlaps the anterior process of the angular. The splenial and prearticular are not exposed.

The dorsal border of the surangular is convex in lateral view and has an anteroposteriorly oriented ridge anterior to the retroarticular process for an articulation with the accessory condyle of the quadrate ( Figs. 5 , 7 ). The posterior surangular foramen is absent. The suture with the angular originates at the posterior end of the mandibular fenestra and extends to the posterior end of the retroarticular process as in Ornithomimus sp. (TMP 95.110.1) and Struthiomimus sp. (TMP 90.26.1). The lateral surface of the long anterior process of the angular has a shallow groove for the surangular contact. The articular is not well exposed.

The posterior part of a thin and long hyoid ( Fig. 5 ) is preserved ventral to the angular in a similar position to that described in Pelecanimimus polyodon ( Pérez−Moreno et al. 1994) . It is slightly curved in lateral view, following the outline of the ventral edge of the lower jaw, and expands slightly at its posterior end. The anterior portion of the hyoid is preserved in IVPP−V11797−31. It is thicker than in IVPPV11797−10 and curves medially and anteriorly.

Figures

Fig. 5. Skull of Sinornithomimus dongi gen. et sp. nov. (IVPP−V11797−10) in left lateral view. Photograph (A) and explanatory drawing of the same (B). Scale bar 5 cm.

Fig. 6. Juvenile skull of Sinornithomimus dongi gen. et sp. nov. (IVPP−V11797−11) in right lateral view. Photograph (A) and explanatory drawing of the same (B). Scale bar 5 cm.

Fig. 7. Juvenile skull of Sinornithomimus dongi gen. et sp. nov. (IVPP−V11797−31) in lateral (A, E), dorsal (B, F), occipital (C, G), and posterolateral views, showing the structure of the quadrate region (D, H). Scale bar below C represents 3 cm and is for A–C and E–G. Scale bar left of D is 2 cm.

Fig. 8. Stereographs (CT images) of the bulbous parasphenoid of Sinornithomimus dongi gen. et sp. nov. (IVPP−V11797−31). Scale bar 3 cm.