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Dashankou Fauna: a Unique Window on the Early Evolution of Therapsids

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Dashankou Fauna: a Unique Window on the Early Evolution of Therapsids Vol.24 No.2 2010 Paleoherpetology Dashankou Fauna: A Unique Window on the Early Evolution of Therapsids LIU Jun* Institute of Vertebrate Paleontology and Paleoanthropology, CAS, Beijing 100044, China n the 1980s, the Institute of Geology, Chinese Academy of IGeological Sciences (IGCAGS) sent an expedition to the area north of the Qilian Mountains to study the local terrestrial Permian and Triassic deposits. A new vertebrate fossil locality, later named Dashankou Fauna, was discovered by Prof. CHENG Zhengwu in Dashankou, Yumen, Gansu Province in 1981. Small-scale excavations in 1981, 1982 and 1985 demonstrated that this locality was a source of abundant and diverse vertebrate fossils. In the 1990s, supported by the National Natural Science Foundation of China, the Fig. 1 Prof. LI Jinling in the excavation of 1995. She first summarized the known IGCAGS, the Institute of Vertebrate members of the Dashankou Fauna and brought it to light as the most primitive and abundant Chinese tetrapod fauna. Paleontology and Paleoanthropology (IVPP) under CAS, and the Geological Museum of China formed a joint team IVPP were productive and have since investigations were first disseminated to work on this fauna. Three large- unveiled an interesting episode in the to the public in 1995. In 2001, Prof. scale excavations, undertaken in transition from reptiles to mammals in LI Jinling summarized the known 1991, 1992, and 1995 respectively, as evolutionary history. members of the fauna and discussed well as the subsequent ones held by The results from these their features. She pointed out that * To whom correspondence should be addressed at [email protected]. Bulletin of the Chinese Academy of Sciences 83 古脊椎专题.indd 83 2010-6-30 11:44:46 BCAS Vol.24 No.2 2010 the fauna is dominated by therapsids to be the most basal therapsid. The and has been hotly debated as to (a kind of mammal-like reptiles) and paper was ultimately published in whether the clade had a Laurasian or is the most primitive and abundant Acta Palaeontologica Polonica. Gondwanan origin. Chinese tetrapod fauna. Recent studies However, given that the material is Biseridens qilianicus was initially of new therapsid materials from this fragmentary, stronger evidence for described as an eotitanosuchian based locality have cast some new light on its therapsid affinities is needed for on two fragmentary specimens, but therapsid origins and highlighted the further discussion. We hope that more later considered by some authors to importance of this fauna. complete specimens of this species be a basal anomodont closely related The rich, nearly continuous will come to light among the materials to Ulemica. Because of the poorly 315-million-year fossil record recently collected from the locality, preserved nature of the material, documents the evolutionary history of to provide more clues about the however, neither proposal could be a diverse clade of synapsid amniotes morphological changes accompanying verified until quite recently when an that includes extant mammals and their the pelycosaur-therapsid transition. additional specimen of Biseridens stem-group, often called “mammal- like reptiles.” One of the major open questions in synapsid research is the presence of a morphological and temporal gap (known as the “Olson’s Gap”) between the earliest therapsids and their supposed sphenacodontian- grade ancestors. Even at their first appearance in the fossil record, therapsids had already diversified into several distinct groups including small and large herbivores and predators, i.e., Biarmosuchia, Dinocephalia, Anomodontia, and Theriodontia. A remarkable specimen discovered at this locality, Raranimus dashankouensis (Liu, Rubidge & Li, Fig. 2 The fossil snout from the basalmost therapsid Raranimus dashankouensis in 2009) contributes to the diversity lateral view. This specimen enriches the morphological diversity of basal therapsids, a of this fauna and more importantly, clade dubbed “mammal-like reptiles,” and casts new light on how they have evolved from might help filling in the Olson’s Gap. their supposed sphenacodontian-grade ancestors. Comprising the partial snout of a tetrapod, the fragmentary specimen Aside from the primitive was recovered from the Dashankou demonstrates a unique combination therapsid Raranimus dashankouensis, locality. The new specimen shows a of therapsid and sphenacodontid the anomodont Biseridens qilianicus short snout, an elevated zygomatic features. The presence of a greatly Li & Cheng, 1997 also deserves extra arch, and a septomaxilla lacking elongated dorsal process of the attention. an elongated posterodorsal process premaxilla, a septomaxilla with a long The therapsid clade Anomodontia between the nasal and the maxilla, facial process, and a high maxilla (which includes the herbivorous indicating that Biseridens is indeed contacting the prefrontal distinguishes Dicynodontia) is one of the most an anomodont. Moreover, this well- the specimen as a therapsid. However, successful tetrapod taxa in terms of preserved specimen also offers some Raranimus also has a number of abundance of individuals, generic new clues for the origin and radiation sphenacodontid characters unknown diversity and temporal duration. of anomodonts, which have long in any other therapsid. This find helps Anomodonts display a global been obscured by the lack of basal us understand the morphological distribution, being one of the very anomodant fossils. transition from pelycosaurs (basal few tetrapod clades that occur on Traditionally, the southern mammal-like reptiles) to therapsids. If all continents, and have a long African therapsids were thought to the acclaimed oldest known therapsid stratigraphic range extending from the have descended from basal therapsids Tetraceratops (Laurin & Reisz, Middle Permian to the Triassic and that migrated into the region from the 1996) is indeed a sphenacodont as possibly even into the Cretaceous. The Northern Hemisphere. Building on the recently suggested, our phylogenetic early evolution of anomodonts has discovery of the primitive anomodont analysis shows the new specimen been the subject of recent attention, Patranomodon in Karoo of South 84 Bulletin of the Chinese Academy of Sciences 古脊椎专题.indd 84 2010-6-30 11:44:49 Vol.24 No.2 2010 Paleoherpetology Africa, some paleontologists proposed Dashankou Fauna can be correlated that anomodonts were dispersing freely to the Zone II of Russia and the between Europe and southern Africa Eodicynodon Assemblage Zone of in the Late Permian. Subsequent South Africa, though slightly earlier. research, especially following the The Dashankou fauna is thought to description of Anomocephalus from be the earliest therapsid fauna, dating South Africa, supported the idea from the Roadian Age of the Middle that anomodonts first evolved in Permian, and helps to fill in the African part of Gondwana. However, Olson’s Gap. The small bolosaurid research by my group at the IVPP Belebey, a primitive anapsid reptile, showed that Biseridens is the most is abundant and generally preserved basal known anomodont, indicating as jaw fragments at the Dashankou that anomodonts had a Laurasian locality. Previously this genus was rather than a Gondwanan origin. only known from the Middle Permian Meanwhile our studies highlighted of Russia. All known Dashankou the existence of two separate groups therapsids are phylogenetically basal: of anomodonts: the Venjukovioidea they include the very basal therapsid in Laurasia and the Chainosauria Raranimus, the basal anteosaurid in Gondwanan. This suggests that dinocephalians Stenocybus and there may have been two separate Sinophoneus, and the basalmost Fig. 3 The basalmost anomodont radiations of basal anomodonts Biseridens qilianicus from Dashankou anomodont Biseridens. from an ancestral Laurasian stock. Fauna has inspired research into the origin Our further research will Nevertheless, given that the most and early radiation of this genus. As shown incorporate more specimens in order primitive dicynodonts were all is its skull in lateral and ventral views. to describe the diversity of the fauna found in Gondwana, we proposed more sufficiently and determine the that dicynodonts originated within Anakamacops petrolicus Li & Cheng, phylogenetic positions of all known the Gondwanan clade and later 1999; the anthracosaurs Ingentidens taxa in it more clearly. The current dispersed across Pangaea. This corridoricus Li & Cheng, 1999 conclusion as to the age of the locality study was published in a special and Phratochronis qilianensis Li & is solely based on correlations using issue of the journal Proceedings Cheng, 1999; the captorhinomorph vertebrate fossils. In the course of of the Royal Society B: Biological Belebey chengi Müller, Li & Reisz, our future work we hope to find more Sciences, entitled ‘Recent advances 2008; and the dinocephalians evidence from geological data, and in Chinese palaeontology,’ and was Stenocybus acidentatus Cheng & Li, from correlations involving other also highlighted by Nature. 1997 and Sinophonus yumenensis groups such as fossil plants, that Other taxa identified within this Cheng & Ji, 1996. will enable us to corroborate our fauna so far include the temnospondyl Comparing with other faunas, the hypotheses. References Cheng, Z., P. Li, J. Li, S. Ji, and Y. Jin., 1995. Discovery of a new vertebrate fauna of Late Permian from western Gansu and its stratigraphic significance. Chinese Science Bulletin, 41(15): 1271–1275. Laurin, M., and R. R. Reisz., 1990. Tetraceratops is
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