Table of Content

15 June 2015, Volume 53 Issue 2

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A streamlined jawless fish (Galeapida) from the Lower Devonian of Yunnan, China and its taxonomic and paleoecological implications

GAI Zhi-Kun   ZHU Min   JIA Lian-Tao   ZHAO Wen-Jin

2015, 53(2):  93-109. 

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 A new genus and species of the Huananaspiformes (Galeaspida), Rhegmaspis xiphoidea, is described from the Posongchong Formation (Pragian, Early Devonian), Zhaotong, Yunnan Province, China. The new taxon is characterized by having a torpedo-shaped head-shield, a long rostral process, ventrolaterally set orbits, and ventrally curved branchial fossae, but no corners, inner corners, and ventral rim of head-shield. It is assigned to the family Gantarostrataspidae that include Gantarostrataspis and Wumengshanaspis. According to the new material and new observation, the Gantarostrataspidae is emended and a fresh look is prososed for Gantarostrataspis and Wumengshanaspis. As a streamlined jawless fish, Rhegmaspis displays an adaptation for a suprabenthic lifestyle with more active feeding behavior among galeaspids. The new form not only enriches the diversity of the Huananaspiformes, but also provides evidence for the last adaptive radiation of galeaspids by occupying an unexploited ecological niche during the Pragian of the Early Devonian.

A new species ofSiyingia from the Lower Devonian Xishancun Formation of Qujing, Yunnan

SI Chu-Dong, GAI Zhi-Kun, ZHAO Wen-Jin

2015, 53(2):  110-122. 

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This paper describes a new species of the Polybranchiaspiformes(Agnatha: Galeaspida), Siyingia perlatuspinosa sp. nov., from the Early Devonian Xishancun Formation in Qujing, East Yunnan, southwestern China. The diagnosis of the genus Siyingia is emended based on data from the holotype of the new species. A phylogenetic analysis, using an expanded version of the data matrix of Zhu and Gai defined in 2006, suggests thatSiyingia and Polybranchiaspis form the monophyletic sister group of a clade containing Laxaspis and Damaspis. This discovery increases the known Early Devonian diversity of polybranchiaspiforms, and helps to clarify the pattern of interrelationships within the Polybranchiaspiformes.

New discoveries from the Sinokannemeyeria-Shansisuchus Assemblage Zone: 2. A new species ofNothogomphodon (Therapsida: Therocephalia) from the Ermaying Formation of Shanxi, China

LIU Jun, Fernando ABDALA

2015, 53(2):  123-132. 

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A new therocephalian specimen collected from the upper part of the Ermaying Formation of Liulin, Shanxi, China is named asNothogomphodon sanjiaoensis sp. nov. This new species is differentiated from the type species Nothogomphodon danilovi by the following characters: dentary lower margin uneven; canine base is ovate rather than rounded; distinct diastema present between canine and first postcanine; absence ofaccessory cutting cusps on posterior border of the postcanines.Nothogomphodon has a postcanine morphology that resembles that of sectorial toothed basal cynodonts, which is the only evidence of therocephalians having developed this kind of complex sectorial dentition. Faunal analysis between the Russian Eryosuchus fauna and the ChineseSinokannemeyeria-Shansisuchus Assemblage indicates that onlyNothogomphodon is common to both faunas, whereas there is no common genus shared between the Chinese Ermaying and Karamayi formations. Therocephalians were present at a moderate level of diversity in Middle Triassic faunas from Laurasia, whereas nonmammaliaform cynodonts are only represented by two taxa. Conversely, in Gondwana cynodonts experienced an explosion in diversity in the Middle Triassic with the evolution of Gomphodontia, while therocephalians were scarcely represented in faunas of this age.

The gastral basket in basal birds and their close relatives: size and possible function

Jingmai K. O’CONNOR，ZHENG Xiao-Ting，WANG Xiao-Li，  ZHANG Xiao-Mei， ZHOU Zhong-He

2015, 53(2):  133-152. 

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Here we investigate the size and morphology of the gastral basket in non-neornithine Mesozoic birds and their close non-avian dinosaur relatives. The size of the gastral basket reportedly increases with body size in the Theropoda. However, contra earlier reports basal birds appear to have proportionately large gastral baskets compared to non-avian theropods, suggesting gastralia may have functioned as a compensatory feature for muscle attachment in the absence of a craniocaudally extensive sternum. As in crocodilians, the proximal row of gastralia in basal birds articulates with the sternum when present suggesting the two features may have functioned as a unit; this hypothesis is not supported by any scars to indicate the gastralia served as points of muscle attachment. Alternatively, the large size of the gastral basket may be related to its respiratory function and the high oxygen demands of powered flight in birds. Due to incomplete preservation of these delicate elements as a unit, sample size is too low to draw significant results from statistical analysis of this element.

Diverse stem cetaceans and their phylogenetic relationships with mesonychids and artiodactyls

GAO Hong-Yan      NI Xi-Jun* 

2015, 53(2):  153-176. 

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 The transition in the evolution of cetaceans from terrestrial life to a fully aquatic existence is one of the most enduring evolutionary mysteries. Stem cetaceans are quite diverse and well documented in the fossil record. Five family level clades form the paraphyletic stem cetaceans (‘archaeocetes’): Pakicetidae, Ambulocetidae, Remingtonocetidae, Protocetidae, and Basilosauridae. The most basal group, the pakicetids probably had a semi-aquatic life, living near the freshwater environment. They likely represent the initial step in the transformation of a terrestrial artiodactyl to an aquatic cetacean. The more derived ambulocetids exhibit more characters likely linked to increasing aquatic adaptation, and they probably were more adapted to the marine realm than to a freshwater environment. Remingtonocetids show evolution of balance organs and the sound transmission mechanism in the direction of modern cetaceans. Stable oxygen isotope analyses suggest that remingtonocetids were probably exclusively marine. Protocetids are very diverse, and they are the first cetacean group that acquired a global distribution. Protocetids retain well-developed hind limbs, but their pelvic-vertebra articulation became loose or completely lost. Basilosaurids are the extinct sister group to the crown cetaceans. The phylogenetic relationships between cetaceans and other mammals have long been debated. Molecular, paleontological, and morphological data and analyses all support a close evolutionary link between cetaceans and artiodactyls. However, the prevalent hippopotamus-cetacean hypothesis does not receive solid support from paleontological and morphological data. Detailed review and more extensive phylogenetic analyses on anthracotheriids and entelodontids will aid the clarification of uncertainties related to the hippopotamus-cetacean phylogenetic hypothesis. If the phylogenetic positions of extant cetaceans and artiodactyls were not constrained by molecular data, the traditional mesonychid-cetacean relationship would still have support from the analyses based solely on paleontological and morphological data.