Describer

Sereno, Zhao & Tan 2010

Time

Cretaceous Early Aptian

Classification

Ornithischia Genasauria Cerapoda Marginocephalia Ceratopia Neoceratopia Psittacosauridae

Diet

Herbivore

Fossilsite

Bayin Gobi Formation, Gobi Desert, Nei Mongol Autonomous Region, China

Fall Under

Psittacosaurus

Info

Psittacosaurus (Osborn, 1923) = Protiguanodon (Osborn, 1923)
Psittacosaurus > Psittacosaurus guyangensis (Cheng, 1983)
Psittacosaurus > Psittacosaurus mongoliensis (Osborn, 1923) = Psittacosaurus protiguanodonensis (Young, 1958) > Protiguanodon mongoliensis (Osborn, 1923)
Psittacosaurus > Psittacosaurus osborni (Young, 1931) > Psittacosaurus tingi (Young, 1931)
Psittacosaurus > Psittacosaurus sinensis (Young, 1958) > > Psittacosaurus youngi (Chao, 1962)
Psittacosaurus > Psittacosaurus meileyingensis (Sereno, Chao, Cheng & Rao, 1988)
Psittacosaurus > Psittacosaurus xinjiangensis (Sereno & Chao, 1988)
Psittacosaurus > Psittacosaurus neimongoliensis (Russell & Zhao, 1996)
Psittacosaurus > Psittacosaurus mazongshanensis (Dong & Azuma, 1996)
Psittacosaurus > Psittacosaurus ordosensis (Russell & Zhao, 1996)
Psittacosaurus > Psittacosaurus lujiatunensis (Zhou, Gao, Fox, and Chen, 2006)
Psittacosaurus > Psittacosaurus sibiricus (Averianov, Voronkevich, Leshchinskiy, Fayngertz, 2006)
Psittacosaurus > Psittacosaurus major (Sereno, Zhao, Brown and Tan 2007)
Psittacosaurus > Psittacosaurus gobiensis (Sereno, Zhao & Tan 2010)

Abstract

We describe a new species of psittacosaur, Psittacosaurus gobiensis, from the Lower Cretaceous of Inner Mongolia and outline a hypothesis of chewing function in psittacosaurs that in many respects parallels that in psittaciform birds. Cranial features that accommodate increased bite force in psittacosaurs include an akinetic skull (both cranium and lower jaws) and differentiation of adductor muscle attachments comparable to that in psittaciform birds.

These and other features, along with the presence of numerous large gastroliths, suggest that psittacosaurs may have had a high-fibre, nucivorous (nut-eating) diet. Psittacosaurs, alone among ornithischians, generate oblique wear facets from tooth-to-tooth occlusion without kinesis in either the upper or lower jaws. This is accomplished with a novel isognathous jaw mechanism that combines aspects of arcilineal (vertical) and propalinal (horizontal) jaw movement.

Here termed clinolineal (inclined) jaw movement, the mechanism uses posteriorly divergent tooth rows, rather than kinesis, to gain the added width for oblique occlusion. As the lower tooth rows are drawn posterodorsally into occlusion, the increasing width between the upper tooth rows accommodates oblique shear. With this jaw mechanism, psittacosaurs were able to maintain oblique shearing occlusion in an akinetic skull designed to resist high bite forces.

Holotype

Skull and articulated postcranial skeleton with gastrolith mass, Long Hao Institute of Geology and Paleontology (LH) PV2, lacking only portions of the right manus, some of the sacrum and pelvic girdle, and the right hind limb and tail. The holotype and more fragmentary remains of additional individuals nearby were discovered in 2001 during the first Chinese-American Dinosaur Expedition.

The holotypic skeleton of P. gobiensis represents a fully mature individual with coossification ofmany cranial sutures as well as all neurocentral sutures in the axial column. The mandibular symphysis, for example, is solidly fused. The rostral–premaxillary and predentarydentary sutures, in addition, are obliterated by fusion, and many other cranial sutures, such as the frontal–frontal and frontal–parietal, can be traced but are fused.

The remains of other individuals discovered near the holotypic site also confirmthat P. gobiensis is a small-bodied species comparable to Psittacosaurus sinensis (Young 1958). The trunk measures only 40 cmin length, corresponding to a body length of approximately 1 m. The wellpreserved skull was turned to the side. The postcranial skeleton was preserved belly up with the pectoral girdles, forelimbs, sternal plates and ribcage preserved in threedimensional articulation, suggesting that it was buried as a dried carcass. The gastrolith mass, which is intact with facets of adjacent stones fitted to one another, is positioned to the side, possibly the result of evisceration by a scavenger before burial.

Etymology

Gobi, after the Gobi Desert; -ensis (Latin) belonging to.

Localities and horizon

The holotype and potential referred specimens were found southwest of Suhongtu (40859042.400 N, 10483053.800 E) in the western Gobi Desert, Nei Mongol Autonomous Region, People’s Republic of China in the Lower Cretaceous (Aptian, ca. 115 Ma; Gradstein et al. 2004) [Bayan Gobi Formation] Bayin Gobi Formation.

Diagnosis

Small-bodied psittacosaur characterized by autapomorphies including: (i) pyramidal horn on the postorbital bar composed almost entirely of the postorbital, (ii) postorbital–jugal fossa, (iii) minimum width of the postorbital bar approximately 50 per cent the width of the base of the process, (iv) retroarticular process deflected posteromedially at an angle of 408 from the axis of the mandible, and (v) thin and restricted enamel on medial and lateral aspects of the maxillary and dentary crowns, respectively. None of these autapomorphies appear to be transient during growth or characterize juveniles alone (Sereno 2009).

Other features with limited distribution among psittacosaur species include a preorbital snout 35 per cent the length of the skull, a ventrolaterally projecting pyramidal jugal horn, a low quadratojugal eminence, absence of an external mandibular fenestra and maxillary and dentary tooth rows limited to eight and nine teeth, respectively. A partial skull of the larger-bodied species Psittacosaurus mongoliensis (LH PV3) was also found in the same formation.

Its longer skull proportions, higher tooth count, relatively smaller crowns, upturned prefrontal edge, primitive postorbital shape and ornamentation, straight retroarticular process and open sutures (despite its size) clearly show that it does not pertain to the new species. Other species from other formations in Inner Mongolia include Psittacosaurus tingi, Psittacosaurus osborni, Psittacosaurus mazongshanensis, Psittacosaurus ordosensis and Psittacosaurus neimongoliensis. The first two species are described from very young juveniles and have been regarded as nomina dubia (Sereno 2009).

Psittacosaurus mazongshanensis (Xu 1997) differs from P. gobiensis by its larger size, longer skull proportions, ovate crowns and posteriorly directed retroarticular process, although the now damaged holotype has also been regarded as a nomen dubium (Sereno 2009). Psittacosaurus ordosensis (Russell & Zhao 1996) differs from P. gobiensis in having an unusually short lower jaw as well as other features but as documented cannot be distinguished from Psittacosaurus sinensis (Sereno 2009). Psittacosaurus neimongoliensis, known from a complete subadult skull and nearly complete skeleton (Russell & Zhao 1996), also has a relatively short lower jaw and lacks the unusual postorbital shape and ornamentation and broad crown proportions present in P. gobiensis (Sereno 2009).

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