Jim Liu Xiao Iin Wang

The Choristodera is a ciade of distinctive but poorly known aquatic reptiles. For more than one century, knowledge of this group was limited to two highly specialized genera: Champsosaurta and Simoedosanrus from North America and Europe. Over the last two decades, additional nine genera have been referred to the Choristodera. Thus, we now have a better understanding of these animals. They had a temporal range spanning at least 190 million years from the Late Triassic to the late Oligocene and a geographical distribution from western North America to Japan via Eurasia.

The Jehol Biota is famous for its fossil vertebrates found in recent years, especially birds and feathered dinosaurs. The earliest reported tetrapod of the Jehol Biota was a small reptile — Nionjurosuchussplendens (Fig. 118). It was named in 1940 by R. Endo, and was classified as a primitive archosaur (Thecodonria). The holotype of the taxon was reportedly lost during the WWII. R. Endo and T. Shikama named another taxon, Rbyncbosaurus orien talis, in 1942 based on a specimen from the same locality and horizon where the holotype of Monjumuchns splendens was found. This taxon was classified into Rhynchocephalia, a group related to lizards and represented today by the isolated New Zealand Tuarara, Sphenodon. These two kinds of reptiles were compared by F. F. von Huene (1942), who believed that they are the same taxon and should be classified into Rhynchocephalia. Monjnrosnchus was finally referred to Choristodera by Ke-qin Gao and others in 2000 based on rhe neorype specimen (Fig. 119) that they newly selected. They diagnosed the taxon by the following combination of characters: dorsoventrally flattened skull; a deeply incised posterior skull margin; parietal foramen absent; conical subthecodont teeth, with striae; three sacral vertebrae; fibula with wide distal

no plantar tubercles. Monjttrusuchus can be distinguished from all other chorisroderes by the combination of the small supratemporal fossa and closed subtemporal fossa.

Many specimens of Monjurosuchus have been discovered in western Liaoning in recent years, some of which bear exquisite integumentary impressions (Fig. 120). Gao and others suggested that the overall appearance of the integument may be like that of the living Chinese crocodile lizard, Shinisaurus crocodilurus (Fig. 121), a semi-aquatic lizard that feeds on small fish, amphibians and invertebrates, and Monjurositchi/s may have had a similar lifestyle.

There also is one kind of "long-necked" chorisroderes, the hyphalosaurs, in the Jehol Biora. The specimens of Hyphalosaurtts were excavated from Dawangzhangzi Bed of the Yixian Formation (ca. 123 Ma). It is referred to chorisroderes based on the following characteristics: vertebral centrum platycoelus; three sacral vertebrae; dorsal ribs pachyostotic; epipodial segments (radius, ulna, tibia, fibula) greatly shorter than the humerus and femur. But its neck is greatly elongated.

There are mainly two fossil localities for Hyphalosaurus, one of them lies in the north of Fanzhangzi of Lingyuan City, Liaoning Province. A nearly complete fossil skeleton was found here in August, 1998. Then it was studied by Gao and his collaborators from the 1VPP, CAS, and was named as Hyphalosattrus lingyuanensis (Figs. 122, 123). The paper was published in the journal Vertebral a PaiAsiatica of January, 1999. However, its counterpart was named as "Sinohydrosaiiri/s lingyuanensis" by Jian-jun Li and others from the Beijing Natural History Museum. J. B. Smith and J. D. Harris published a paper in Journal of Vertebrate Paleontology to discuss the validity of two names in 2001; he pointed out that the two names are actually a synonymy, and he-

Line drawing of the lost holotype of Munjurusmhus splendens (skull length 92 mm), recently identified as a choristodere, from Danangou (Tanankou) locality (middle pan ofYixian) in Lingyuan, Liaoning. (Art: Mick Ellison/AMNH; Courtesy: Ke-qin Gao/ PKU)

but narrow proximal head; fifth metatarsal with expanded proximal end bur chose Hyphalosaurus lingyuanensis as the valid name.

119 Neotype of Monjurosuchus splendens (skull length 58 mm) from Niuyingzi locality (middle part ofYixian l:ormation) in Lingyuan, Liaoning. (Photo: Mick Ellison/AMNH: Courtesy: Ke-qin Gao/ PKU)

Champsosaurus Skin Impression

120 Skin impressions on a specimen of Monjurosuclms splendens. (Photo: IVPP)

The holotype of Hyphalosaurus lingyuanensis is a 116 cm long specimen exposed in ventral view. It has a small skull, several needle-like teeth lying in the anterior part of the skull. There are at least 13 rows of ribs and more than 20 rows of gastralia ("abdomen ribs"). The vertebral column consists of 19 cervicals, 16—17 dorsals, three sacrals, and more than 55 caudals. Therefore, the most striking feature on this specimen is the presence of greatly elongated neck (about 20 cm long) and a long tail.

The proportionally small head, pointed snout, needle-like teeth and greatly elongated neck of Hyphalosaurus strongly indicate its fish-eating diet in life. The dorsal ribs are pachyostotic and thickened distally. Functionally, this thickening appears to increase the specific gravity of the body, enabling the animal to remain submerged with a minimum of effort. It also shows other morphological features that reflect an aquatic lifestyle, including platy-coelous vertebrae, poor ossification of the distal ends of the limb bones, reduced ossification of the carpals and tarsals and short epipodials.

It is interesting to note that at least six fossil fish of the genus Lycoptera were preserved on the same slab with the holotype of Hyphalosaurus. One of them is so close to the mouth of Hyphalosaurus (Fig. 123) as if it could not escape from being preyed on when Hyphalosaurus opened its mouth.

Many complete skeletons of Hyphalosaurus were found in Wangjiagou, Wanfotang and Hejiaxin ofYixian County in fall, 1999. This area became the second fossil locality of Hyphalosaurus.

All known specimens of Hyphalosaurus were excavated from tuffaceous shales deposited in lacustrine environment. These rocks recorded the frequent volcanic eruptions. One can imagine the scenery during that time: the volcanoes threw masses of ashes into the atmosphere, and as the ashes fell down, they spread out and buried everything. Volumes of greenhouse gases and toxic gases came along with volcanic eruptions; the habitat was devastated, and hundreds of thousands of animals died together. Some individuals of Hyphalosaurus were buried together, as seen in many specimens (Fig. 124).

The first specimen mentioned as choristodere in China was a snout fragment from the Otog Qi District in the Ordos Basin, Inner Mongolia studied by E. Buffetaut. It was revised by D. Sigogneau-Russell in 1981, and a new taxon, Ikechosaurus sunailinae was named based on that specimen. A few years later, many well-preserved, articulated specimens of Ikechosaurus were discovered in Luohandong Formation, Zhidan Group (Early Cretaceous)

during the fieldwork of China-Canada Dinosaur Project. Sigogneau-Russell had noted that in the presence of a broad snout and closely packed teeth with rectangular bases, lkechosaurus was most similar to the Paleocene genus Simoedosaurus, a view also held by several other researchers. However, still others thought lkechosaurus is more closely related to Champsosaurus than to Simoedosaurus. A new species, lkechosaurusgaoi (Fig. 125) was erected based on a fragmentary skeleton from the Jiufotang Formation of Chifeng, Inner Mongolia by Jun-chang Lii and others. This is the third choristodere reported from the Jehol Biota.

Many well-preserved skeletons have been found in Yixian and Chaoyang, Liaoning Province recently, which could be referred to lkechosaurus. Their skull is flat, having elongated snout and large temporal openings, with a skull length of 30 cm. The total length of these reptiles can be up to 2 m, and the length of the tail is more than half of the total length.

121 Shinisaurus crocodilurus. Chinese crocodile lizard, a living semi-aquatic lizard, to which Monjurosuchus may have a similar integumental overall appearance. (Photo: Chun-xuan Chen)

Yixian Lizard

122 Holotype of Hyphalosaums lingyuonensis (total body length 116 cm), a long-necked choristodere, from Fanzhangzi locality (middle part of Yixian Formation) in Lingyuan, Liaoning Province. (Photo: IVPP)

123 A close-up view of the skull of the holotype of llyphalosaurus lingyuonensis. showing a fossil fish near its mouth. (Photo: IVPP)

122 Holotype of Hyphalosaums lingyuonensis (total body length 116 cm), a long-necked choristodere, from Fanzhangzi locality (middle part of Yixian Formation) in Lingyuan, Liaoning Province. (Photo: IVPP)

123 A close-up view of the skull of the holotype of llyphalosaurus lingyuonensis. showing a fossil fish near its mouth. (Photo: IVPP)

1124 Four individuals of Hyphalosaurvs on one slab, with two adults and two young, as if one family died in a single accident. (Photo: IVPP)

Feathered Dinosaur Fossils
A crushed skull of the holotype of lkechosaurus gaoi (skull about 19 cm long), a gavial-like choristodere, from a locality of Jiufotang Formation in Chifeng, Inner Mongolia. (Photo: IVPP)
Yabeinosaurus

An undescribed fossil skeleton of Yabeinosaurus from Dawangzhangzi locality (middle part ofYixian Formation) in Lingyuan, Liaoning, red arrow denoting a possible condition of autotomy in the tail. (Photo: IVPPI

skeleton of Yabeinosaurus from Gezidong locality (jiulongshan Formation), Lingyuan, Liaoning. (Plioto: IVPP)

An undescribed fossil skeleton of Yabeinosaurus from Dawangzhangzi locality (middle part ofYixian Formation) in Lingyuan, Liaoning, red arrow denoting a possible condition of autotomy in the tail. (Photo: IVPPI

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