A relict trematosauroid (Amphibia: Temnospondyli) from the Middle Jurassic of the Junggar Basin (NW China)
A temnospondyl ilium from the uppermost Toutunhe Formation (Middle Jurassic, Callovian) of the southern Junggar Basin is described. Among the known temnospondyls it is very unusual in morphology because of its very long and slender shaft. It compares closely only to the ilium of one of the latest known trematosaurids from the Ladinian of southern Germany. The Toutunhe Formation has also yielded vertebrae and skull fragments of temnospondyls which belong to the brachyopid Gobiops from the Upper Jurassic of Mongolia. Brachyopoid ilia do not, however, display a morphology similar to that of the new specimen. It is therefore concluded that this specimen represents a second taxon of temnospondyl from the Toutunhe Formation, which probably represents the latest surviving trematosauroid. The Trematosauroidea, which was hitherto exclusively known from the Lower to early Upper Triassic, therefore joins the Brachyopoidea – and possibly the Capitosauroidea – as another group of temnospondyls which survived the end-Triassic mass extinction.
The Junggar Basin in the Xinjiang Uygur Autonomous Region of the People’s Republic of China has yielded one of the best records of Mesozoic vertebrates in central Asia (Dong 2001). The Jurassic and Cretaceous sediments of the southern Junggar Basin, which have so far been relatively neglected by previous research, were investigated in the years 1999–2002 by several joint Sino-German expeditions (Maisch et al. 2001, 2003a). The Middle Jurassic Toutunhe Formation yielded numerous taxa, including chondrichthyan and osteichthyan fish (Maisch et al. 2001, 2003a), temnospondyl amphibians (M.W. Maisch and A.T. Matzke, unpublished data), turtles (Matzke et al. 2004), crocodiles (Maisch et al. 2003b), dinosaurs (Maisch and Matzke 2003) and mammals (Pfretzschner 2003).
The richest fauna from the Toutunhe Formation, unfortunately only yielding isolated but well preserved teeth and bones, was recovered from a single locality and horizon, termed the turtle–archosaur–amphibian assemblage by Maisch et al. (2001, see also Maisch et al. 2003a for details). This assemblage has yielded remains of at least two different xinjiangchelyid turtles, a goniopholid crocodile (cf. Sunosuchus), and a large rhamphorhynchoid pterosaur, which have not yet been described. It also yielded rare remains of theropod dinosaurs (Maisch and Matzke 2003) and the brachyopid temnospondyl Gobiops desertus (M.W. Maisch and A.T. Matzke, unpublished data).
The purpose of this paper is to describe one of the most puzzling specimens from this locality, an isolated temnospondyl ilium which clearly does not agree with described ilia of brachyopoid or any other temnospondyls, but shows a striking similarity to the ilium of an undescribed trematosaur from the Lettenkeuper (Upper Ladinian) of southern Germany, one of the latest representatives of the group known so far.
Sino-German Project collection, currently housed at the University of Tübingen, Germany. The collection remains the property of the People’s Republic of China and will be transferred to a public Chinese collection after the scientific studies are completed. The final repository will be announced in an internationally accessible journal.
Staatliches Museum für Naturkunde Stuttgart, Germany.
Geologisch-Paläontologisches Museum, University of Tübingen, Germany.
As there is as yet no post-Triassic record of trematosaurs, it appears plausible that the specimen represents a new genus and species. Nevertheless its fragmentary nature precludes any further identification and we therefore refrain from erecting a new taxon.
The specimen is an undistorted right ilium. It has a proximodistal length of 40.5 mm and a proximal width of 15.5 mm. The ilium can be divided in a proximal acetabular portion and a distal shaft. The small acetabulum opens both laterally and ventrally. Its dorsal margin is well-defined and elevated as a rugose ridge. The contact surfaces for the pubis and ischium that are situated anterior, posterior and medial to the acetabular facet on the ventral surface of the bone are strongly rugose, indicating that a large amount of cartilage intervened between the individual pelvic bones. The pubic facet is mediolaterally expanded and slightly inclined anterodorsally. The ischium facet is inclined posterodorsally. It is incomplete posteriorly and ventrally.
Dorsal to the acetabular region, the ilium narrows rapidly to form an exceedingly long and slender shaft. The anterior margin of the shaft is markedly concave, whereas the posterior margin is equally strongly convex, so that the entire shaft bows posteriorly, although its anteroposterior width remains quite constant around 6 mm. The distal end is only very slightly expanded anteroposteriorly, being 7 mm in width. It ends in an elliptical, rugose surface. The lateral surface of the shaft is quite flat, whereas the medial one is slightly convex. Strong striations which probably represent muscle scars are found in places, particularly on the dorsolateral and dorsomedial portions of the ilium shaft and right above the acetabulum. An identification of the muscles seems futile without the corresponding pelvic and limb elements being available.
Temnospondyl ilia are relatively scarce, as most of the known taxa are represented only by cranial material, or the postcranial material remains inadequately described. The only temnospondyl subgroup that needs to be discussed here are the stereospondyls, because they alone are known to have survived into the Late Triassic and beyond. Schoch and Milner (2000) subdivided the Stereospondyli into two grades, stem-stereospondyls (including among others, the archegosaurs) and basal stereospondyls (rhinesuchids, rhytidosteids and their relatives), as well as two derived superfamilies, the Trematosauroidea and Capitosauroidea. Their scheme is followed here, except that the chigutisaurids are here regarded as close relatives of the brachyopids, and both families are united in a third derived superfamily, the Brachyopoidea, following Warren and Marsicano (2000) and Yates and Warren (2000). Only two of these groups, the Brachyopoidea – and possibly the Capitosauroidea – are so far known to have survived beyond the Triassic–Jurassic boundary (Warren et al. 1998). Within the Capitosauroidea, the ilium is best known in the paracyclotosaurids Paracyclotosaurus davidi (Watson 1958) possibly from the Middle Triassic of New South Wales and Stanocephalosaurus pronus from the Middle Triassic of Tanzania (Howie 1970), as well as in the eryosuchid Eryosuchus garjainovi from the Middle Triassic of Russia (Ochev 1972) and the mastodonsaurid Mastodonsaurus giganteus (Schoch 1999) from the Middle Triassic of Germany. In all four genera, representing three different families, the ilium is a dorsoventrally short element with a moderately elongated, flattened shaft that is considerably expanded distally in a blade-like shape. The proximal end, which bears the acetabulum, is also quite expanded. The ilium is even shorter in the paracyclotosaurids than it is in the other two genera. From the available evidence, it therefore appears highly unlikely that the element belongs to a capitosauroid.
In the Brachyopoidea, to which all other post-Triassic temnospondyls belong (see Warren et al. 1997; Warren and Marsicano 2000; M.W. Maisch and A.T. Matzke, unpublished) the pelvis is inadequately known in most taxa. In the brachyopid Batrachosuchus a partial ilium is mentioned by Warren and Marsicano (2000) but is not described. This is, apparently, the only available material for that family at the moment! In the chigutisaurids complete ilia have been described in Pelorocephalus from the Late Triassic of Argentina (Marsicano 1993), Compsocerops cosgriffi from the Late Triassic of India (Sengupta 1995) and Siderops kehli from the Early Jurassic of Australia (Warren and Hutchinson 1983). The latter two forms show the usual morphology of sterospondyl ilia, i.e. they are relatively short and markedly widened and blade-like distally (cf. Warren and Snell 1991). In Pelorocephalus the ilium looks rather like those of metoposaurids (Sawin 1945; Dutuit 1976; Warren and Snell 1991; Marsicano 1993; Warren and Marsicano 2000), i.e. it is more elongated and does not distinctly flare distally. This ilium morphology is closer to that of the new specimen, but it is still distinctly different, the iliac shaft being much wider. According to Marsicano (1993: figure 10), the ilium of Pelorocephalus has a shaft with a width (measured at mid-shaft length) to length (measured from the dorsal margin of the acetabulum to the distal tip of the iliac shaft) ratio of 0.25. In the Toutunhe specimen this ratio is 0.16, and in the Lettenkeuper trematosaurid it is 0.125. The similarity between the two trematosaur ilia and the dissimilarity to Pelorocephalus is even more striking, if one compares the maximum width (i.e. proximal width) to the total length of the bone. This yields a ratio of 0.51 for Pelorocephalus, whereas in the Lettenkeuper and the Toutunhe trematosaurs the ratio is 0.39 and 0.38, respectively, the lowest of any known stereospondyls. It therefore appears unlikely that the specimen represents a brachyopoid.
In the Trematosauroidea, the postcranial skeleton is also incompletely known. Ilia are, however, present in at least some members of any major subgroup. These include the benthosuchid Benthosuchus sushkini and the wetlugasaurid Wetlugasaurus sp. (Bystrov and Efremov 1940) from the Lower Triassic of Russia, the metoposaurids Buettneria (Sawin 1945) and Apachesaurus (Hunt 1993) from the Upper Triassic of the USA, Dutuitosaurus (Dutuit 1976) from the Upper Triassic of Morocco, and the as yet undescribed taxa from the Middle Triassic of southern Germany labelled the “Lettenkeuper almasaurid” and the “Lettenkeuper trematosaurid” by Schoch and Milner 2000 (R.R. Schoch, personal communication 2003; personal observation).
The ilium of Benthosuchus is like that of most other stereospondyls, except that some specimens develop a distinctive anterodorsal process (Bystrov and Efremov 1940), whereas small ilia of Wetlugasaurus are slender and elongated, although with a distinctly posteriorly inclined and slightly distally expanded shaft (Bystrov and Efremov 1940). The metoposaurid ilium is similar to that of Pelorocephalus as it does not distinctly flare distally and is elongated (with a width–length ratio of 0.57 in Dutuitosaurus). However, it is still much shorter and more robust than that of the new specimen (Sawin 1945; Dutuit 1976; Hunt 1993).
The most striking similarity is found with the ilium of the Lettenkeuper trematosaurid (Fig. 2b), which in morphology and proportions (see above) is almost indistinguishable from the Toutunhe specimen. Unfortunately, ilia are not known for other trematosaurids so far (Steyer 2002). The only apparent difference is that the cross-section of the shaft is strongly compressed in the Lettenkeuper trematosaurid. This is, however, most probably due to diagenetic distortion. Personal observations on two undescribed associated skeletons of the Lettenkeuper almasaurid in the SMNS and GPIT collections shows an even more slender and delicate ilium in this form. It therefore seems highly plausible that a slender and elongate iliac shaft might be a synapomorphy of the metoposaurs, almasaurids and trematosaurs. Of course too little is known at present about the postcranial skeleton of most of these forms, but from the available evidence it seems highly likely.
The presence of a trematosauroid in the turtle–archosaur–amphibian assemblage of the Toutunhe Formation appears surprising, as other amphibian remains encountered therein can mostly be referred with certainty to the brachyopoid Gobiops desertus (M.W. Maisch and A.T. Matzke, unpublished). Nevertheless nothing precludes the presence of two temnospondyl amphibians in the assemblage. As all known brachyopoids are short-snouted, dorsoventrally compressed animals with wide skulls, all known trematosaurs have elongated, crocodile or gharial-like skulls with long snouts. It is thus possible that two animals with such a divergent skull morphology occupied different ecological niches and lived sympatrically. It is also possible that some small, non-stereospondylous vertebral intercentra referred to juvenile Gobiops specimens (M.W. Maisch and A.T. Matzke, unpublished) belong to the trematosauroid. What is known about the postcranial skeleton of these animals, particularly due to the exceptionally well-preserved Lettenkeuper trematosaurid which possesses crescent-shaped intercentra (R.R. Schoch, personal communication) and articulated material of Wantzosaurus elongatus from Madagascar (Steyer 2002) does not preclude such an identification.
Our sincere thanks go to the team of the Geological Survey No. 1 and our Chinese co-workers from the Nanjing Institute and Jilin University, for their hospitality, friendship and logistic help. H. Stöhr (Tübingen, who also skilfully prepared the fossil), F. Lörcher (Dotternhausen) and F. Grossmann (Tübingen) assisted in the excavations. Dr. R.R. Schoch (Stuttgart) is particularly thanked for interesting discussions and suggestions and for providing unpublished data on the Lettenkeuper trematosaurid. This project was financed by the DFG, Max Planck Society and NSFC, whose financial support is gratefully acknowledged. W. Gerber (Tübingen) took the photographs, which are of his usual high quality.