Xiao-Chun Wu

An ancestral turtle from the Late Triassic of southwestern China

The origin of the turtle body plan remains one of the great mysteries of reptile evolution. The anatomy of turtles is highly derived, which renders it difficult to establish the relationships of turtles with other groups of reptiles. The oldest known turtle, Proganochelys from the Late Triassic period of Germany, has a fully formed shell and offers no clue as to its origin. Here we describe a new 220-million-year-old turtle from China, somewhat older than Proganochelys, that documents an intermediate step in the evolution of the shell and associated structures. A ventral plastron is fully developed, but the dorsal carapace consists of neural plates only. The dorsal ribs are expanded, and osteoderms are absent. The new species shows that the plastron evolved before the carapace and that the first step of carapace formation is the ossification of the neural plates coupled with a broadening of the ribs. This corresponds to early embryonic stages of carapace formation in extant turtles, and shows that the turtle shell is not derived from a fusion of osteoderms. Phylogenetic analysis places the new species basal to all known turtles, fossil and extant. The marine deposits that yielded the fossils indicate that this primitive turtle inhabited marginal areas of the sea or river deltas.

Triassic marine reptiles gave birth to live young

Sauropterygians form the largest and most diverse group of ancient marine reptiles that lived throughout nearly the entire Mesozoic era (from 250 to 65 million years ago). Although thousands of specimens of this group have been collected around the world since the description of the first plesiosaur in 1821 (ref. 3), no direct evidence has been found to determine whether any sauropterygians came on shore to lay eggs (oviparity) like sea turtles, or gave birth in the water to live young (viviparity) as ichthyosaurs and mosasauroids (marine lizards) did. Viviparity has been proposed for plesiosaur, pachypleurosaur and nothosaur sauropterygians, but until now no concrete evidence has been advanced. Here we report two gravid specimens of Keichousaurus hui Young from the Middle Triassic of China. These exquisitely preserved specimens not only provide the first unequivocal evidence of reproductive mode and sexual dimorphism in sauropterygians, but also indicate that viviparity could have been expedited by the evolution of a movable pelvis in pachypleurosaurs. By extension, this has implications for the reproductive pattern of other sauropterygians and Mesozoic marine reptiles that possessed a movable pelvis.

An unusual archosaurian from the marine Triassic of China

A new Triassic archosaurian from China shows a number of aquatic specializations, of which the most striking is the extreme lateral compression of the long tail. Others that may also reflect aquatic adaptations include platelike scapula and coracoid, elongate neck with extremely long and slender ribs, and reduction of osteoderms. In contrast, its pelvic girdle and hind limb have no aquatic modifications. Anatomic features, taphonomy, and local geological data suggest that it may have lived in a coastal–island environment. This lifestyle, convergent with some Jurassic marine crocodyliforms that lived at least 40 million years later and the saltwater species of extant Crocodylus, contradicts with the prevailing view that Triassic archosaurians were restricted to nonmarine ecosystems. Its mosaic anatomy represents a previously unknown ecomorph within primitive archosaurians.

FIRST COMPLETE PISTOSAUROID FROM THE TRIASSIC OF CHINA

Triassic pistosauroids (Reptilia: Sauropterygia) are the closest relatives of the Plesiosauria, a group that achieved considerable taxonomic diversity and cosmopolitan distribution during the Mesozoic. Despite their importance for the understanding of the origin of the Plesiosauria, however, our knowledge on the Triassic pistosauroids (Augustasaurus Sander et al, 1997; Chinchenia Young, 1965; Corosaurus Case, 1936; Cymatosaurus Fritsch, 1894; Kwangsisaurus Young, 1959; Pistosaurus Meyer, 1939; Edinger, 1935; Rieppel, 1997, 1998, 1999; Storrs, 1991) has been limited due to the lack of well-preserved specimens. We report a new pistosauroid, Yunguisaurus liae gen. et sp. nov., from the Triassic of southern China. This taxon is represented by a virtually complete and articulated skeleton embedded in a limestone slab from the Falang Formation (upper Middle to lower Upper Triassic). Previously described Triassic pistosauroids are known from the upper Lower to lower Middle Triassic (Rieppel, 1999, 2000; Wang et al., 2001), and Y. liae is the youngest pistosauroid (excluding the Plesiosauria). Because of the rarity of pistosauroids, we are providing this preliminary report while the postcranial skeleton waits for further preparation and subsequent full description. A unique combination of primitive and derived traits is observed in this specimen, and a phylogentic analysis confirms its close relationship with derived pistosauroids.

Braincase of a Polycotylid Plesiosaur (Reptilia: Sauropterygia) from the Upper Cretaceous of Manitoba, Canada

ABSTRACT The braincase of a polycotylid plesiosaur referable to Dolichorhynchops from the Niobrara Formation (Santonian to the earliest Campanian) in Manitoba, Canada, is described. The well-preserved material without matrix permits observation of various anatomical features of the polycotylid braincase with unprecedented clarity. A virtual reconstruction of the braincase was created by using a three-dimensional scanning technique that produces accurate reconstruction of missing or damaged elements and demonstrates the three-dimensional relationships among the elements. Comparison with other plesiosaurian braincases revealed previously unrecognized characters and character states. The I-beam-shaped parasphenoid, the relatively short pituitary fossa, and the anterior process of the prootic are possible synapomorphies of the Polycotylidae or of its subclades. It could be a challenge to confirm the presence of the supraoccipital process and its median ridge, as well as the presence of the fenestra ovalis, in polycotylid skulls because of the location and orientation of these structures. Variable morphology of basioccipital-basisphenoid contact and the paroccipital process in Dolichorhynchops and related taxa require further investigation. The I-beam of the parasphenoid and the wide contact surfaces of the basioccipital and neighboring elements provide more resistance to flexion of the braincase. Distribution of the extensive pterygoid-basicranium contact, the shape of foramen magnum, anterior process of the prootic, and the short pituitary fossa do not confirm the current phylogenetic hypotheses. The location and orientation of the fenestra ovalis suggest the lack of a tympanic ear in this plesiosaur.

A new Triassic marine reptile from southwestern China

ABSTRACT A new genus and species of marine reptile is described from the Pelsonian (Anisian, Middle Triassic) of Luoping County, Yunnan Province, southwestern China. This diapsid reptile with a secondarily closed upper temporal fossa is the sister taxon to Saurosphargis from the lower Muschelkalk (Anisian, Middle Triassic) of central Europe. It further emphasizes the close faunal affinities between the Eastern and Western Tethyan realm. The new taxon is convergent on cyamodontoid placodonts in having developed a dorsal body armor composed of small osteoderms. The underlying ribs are transversely broadened so as to establish contact along their length, thus forming a closed dorsal ‘rib basket,’ a unique morphology shared with Saurosphargis.

Osteology of Yunguisaurus Cheng et al., 2006 (Reptilia; Sauropterygia), a Triassic Pistosauroid from China

Abstract. Triassic pistosauroids are closely related to the Plesiosauria which flourished later in the Mesozoic, but their fossil record has been poor due to the fragmentary nature of the known specimens. Yunguisaurus liae Cheng et al. (2006) (Reptilia, Sauropterygia) from China was the first Triassic pistosauroid represented by an almost complete skeleton, and we provide a full description of the holotype specimen based on the result of complete preparation of the skull and postcranium. A revised diagnosis characterizes Y. liae by a mixture of primitive and derived features for sauropterygians, such as the high number of cervical vertebrae (similar to the Plesiosauria) with large zygapophyses (shared with basal sauropterygians). The holotype skeleton likely represents a juvenile individual. In a revised phylogeny, relationships among the Triassic pistosauroids are fully resolved but weakly supported; the revised phylogeny differs from the existing hypothesis on a sauropterygian relationship, likely due to the previous reliance on fragmentary taxa. The long neck and hyperphalangy give an impression that Yunguisaurus is very “plesiosaurian,” but detailed comparison revealed morphological differences in various parts of the postcranial. These differences, in combination with the revised phylogenetic hypothesis, suggest that Yunguisaurus was not necessarily comparable to the Plesiosauria in terms of body plan and mode of swimming.

NEW INFORMATION ON WUMENGOSAURUS DELICATOMANDIBULARIS JIANG ET AL., 2008 (DIAPSIDA: SAUROPTERYGIA), WITH A REVISION OF THE OSTEOLOGY AND PHYLOGENY OF THE TAXON

ABSTRACT Wumengosaurus delicatomandibularis Jiang et al., 2008, is restudied on the basis of three new specimens. Many elements of the skeleton that were either briefly or not described at all in the original study are detailed. With the additional information derived from the new specimens, we have revised the diagnosis of W. delicatomandibularis. A phylogenetic analysis further demonstrates that the Pachypleurosauria is not a monophyletic group and questions that W. delicatomandibularis is closely related to the Pachypleurosauria. W. delicatomandibularis might represent the basal-most member of a monophyletic clade that includes the Nothosauroidea and those taxa assigned to the ‘Pachypleurosauria.’ The five known specimens show morphological variation in W. delicatomandibularis, as in Keichousaurus and in pachypleurosaurs such as Neusticosaurus and Serpianosaurus.

A New Archosaur (Diapsida, Archosauriformes) from the Marine Triassic of China

ABSTRACT A new Middle Triassic archosaur, Diandongosuchus fuyuanensis, gen. et sp. nov., is described on the basis of a skeleton from the Zhuganpo Member (Ladinian) of the Falang Formation, eastern Yunnan Province, China. It is primarily characterized by the nasal process of the premaxilla extending posteriorly well beyond the external naris, the super-sized coracoid foramen laterally bordered by the scapula, the ischium with a strongly expanded medial portion anteroposteriorly longer than the proximodistal height of the bone, and anteriorly notched cervical osteoderms. D. fuyuanensis is a pseudosuchian on the basis of the crocodile-normal tarsal joint and other features, such as the distal end of the ulna in posterolateral view squared off, osteoderms with a distinct anterior process, the presacral vertebrae dorsally covered by more than one osteoderm, dorsal osteoderm alignment dorsal to presacrals 10–24 staggered, the pubis-ischium contact reduced to a thin proximal contact, and the medial contact of the ischia extensive but the dorsal margins separate. It is from a marine deposit but shows few morphological adaptations of the postcranial skeleton for a semiaquatic way of life when compared with Qianosuchus from the Anisian limestone of the same area. A phylogenetic analysis derived from an existing data matrix suggests that the new archosaur occupies the basal-most position in Poposauroidea and further confirms the poposauroid status of Qianosuchus. On the basis of current information, the discovery of Diandongosuchus does not firmly underscore the affinity of the semiterrestrial vertebrate faunas between the eastern and western regions along the northern coastline of the Tethys.

A new eosauropterygian (Diapsida, Sauropterygia) from the Triassic of China

ABSTRACT A new Middle Triassic sauropterygian, Qianxisaurus chajiangensis, gen. et sp. nov., is described on the basis of a skeleton from the Zhuganpo Member (Ladinian) of the Falang Formation, Xingyi, southwestern Guizhou Province, China. It is primarily characterized by a premaxilla with eight teeth, marginal teeth with a slightly constricted peduncle and short conical crown, snout longer than the rest of skull, postorbital with a truncated dorsal process and forked posterior process, a very small supratemporal fenestra, a short retroarticular process posteriorly truncated and lacking a dorsal concavity, an elongate trunk with 28 vertebrae, a stout sacrum with four vertebrae, a longitudinal groove on proximodorsal surface of the dorsal ribs, and a coracoid with a narrow lateral portion and a foot-shaped medial portion. Q. chajiangensis appears similar to the European pachypleurosaurs, but a global phylogenetic analysis failed to support close relationships with these taxa. Instead, Q. chajiangensis is an eosauropterygian and most probably the sister group of a clade consisting of the European pachypleurosaurs and the Nothosauroidea. The discovery of Q. chajiangensis certainly enriches our knowledge of the early history of the Eosauropterygia and leads to a better understanding of the phylogenetic relationships among known eosauropterygians.