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The Xiaoxiang Fauna (Ludlow, Silurian) - a window to explore the early diversification of jawed vertebrates
... Devonian osteichthyans [8]. A rich fossil invertebrate fauna, including corals, molluscs and trilobites, indicates a productive marine environment [5,7]. Kuanti osteichthyians include Guiyu oneiros [8], the large but fragmentary Megamastax amblyodus [9], and the scale-based Naxilepis gracilis [10]. ...
... Kuanti osteichthyians include Guiyu oneiros [8], the large but fragmentary Megamastax amblyodus [9], and the scale-based Naxilepis gracilis [10]. Other jawed fishes include undescribed acanthodians [5,6] along with various placoderms [10], including Silurolepis [11] and the two maxillate placoderms Entelognathus [12] and Qilinyu [13]. Prior to the discovery of the superbly preserved holotypes of Guiyu and Entelognathus, pre-Devonian gnathostomes were represented only by highly fragmentary material [1,2,10,14]. ...
... Of the two scale-based Xiaoxiang taxa, Naxilepis gracilis is present in the Kuanti and the overlying Miaokao Formation while Ligulalepis yunnanensis is restricted to the Miaokao [10]. Pridoli sediments from South China are not as well sampled, but material from the Yulungssu Formation includes an indeterminable osteichthyan [5] and Psarolepis romeri, a species also present in roughly contemporaneous deposits in Vietnam [5,16,21]. ...
Our understanding of early gnathostome evolution has been hampered by a generally scant fossil record beyond the Devonian. Recent discoveries from the late Silurian Xiaoxiang Fauna of Yunnan, China, have yielded significant new information, including the earliest articulated osteichthyan fossils from the Ludlow-aged Kuanti Formation. Here we describe the partial postcranium of a new primitive bony fish from the Kuanti Formation that represents the second known taxon of pre-Devonian osteichthyan revealing articulated remains. The new form, Sparalepis tingi gen. et sp. nov., displays similarities with Guiyu and Psarolepis, including a spine-bearing pectoral girdle and a placoderm-like dermal pelvic girdle, a structure only recently identified in early osteichthyans. The squamation with particularly thick rhombic scales shares an overall morphological similarity to that of Psarolepis. However, the anterior flank scales of Sparalepis possess an unusual interlocking system of ventral bulges embraced by dorsal concavities on the outer surfaces. A phylogenetic analysis resolves Sparalepis within a previously recovered cluster of stem-sarcopterygians including Guiyu, Psarolepis and Achoania. The high diversity of osteichthyans from the Ludlow of Yunnan strongly contrasts with other Silurian vertebrate assemblages, suggesting that the South China block may have been an early center of diversification for early gnathostomes, well before the advent of the Devonian “Age of Fishes”.
... However the contrasting tooth-form suggests a separation of activity (capture vs processing) that is broadly analogous to extant piscine durophages, possibly making it the earliest osteichthyan with specific adaptations for such a diet. The sub-tidal marine invertebrate fauna of the Ludlow of Yunnan included a rich variety of potential prey, including brachiopods, molluscs and trilobites 12,13,46 . Megamastax may have also consumed the heavily armoured fishes whose fossils are well represented in the Kuanti Formation (Fig. 4), including placoderms 19 and galeaspids 18 . ...
An apparent absence of Silurian fishes more than half-a-metre in length has been viewed as evidence that gnathostomes were restricted in size and diversity prior to the Devonian. Here we describe the largest pre-Devonian vertebrate (Megamastax amblyodus gen. et sp. nov.), a predatory marine osteichthyan from the Silurian Kuanti Formation (late Ludlow, ,423 million years ago) of Yunnan, China, with an estimated
length of about 1 meter. The unusual dentition of the new form suggests a durophagous diet which,
combined with its large size, indicates a considerable degree of trophic specialisation among early
osteichthyans. The lack of large Silurian vertebrates has recently been used as constraint in
palaeoatmospheric modelling, with purported lower oxygen levels imposing a physiological size limit.
Regardless of the exact causal relationship between oxygen availability and evolutionary success, this finding
refutes the assumption that pre-Emsian vertebrates were restricted to small body sizes.
A new genus and species of agnathan Eugaleaspidiformes (Galeaspida), Yongdongaspis littoralis gen. et sp. nov., is described from the Llandovery (lower Silurian) Huixingshao Formation at Yongdong township, Xiushan County, Chongqing, southwestern China. The new taxon morphologically exhibits some transitional states between Sinogaleaspidae and a cluster of higher eugaleaspidiforms containing Tridensaspidae, Eugaleaspidae, Yunnanogaleaspis, and Nochelaspis, which we term here as the ‘eugaleaspid cluster’. Phylogenetic analysis of an extended character matrix of Galeaspida reveals Yongdongaspis, on which Yongdongaspidae fam. nov. is erected, as the sister taxon of this ‘eugaleaspid cluster’, supported by two synapomorphies, the presence of one median transverse canal, and two lateral transverse canals leaving from the infraorbital canal. As the first fish described from the Llandovery Huixingshao Formation in Chongqing, Yongdongaspis provides new fossil evidence to the subdivision and correlation of the Upper Red Beds in South China.
The Silurian fishes from north-western Hunan, China are characterised by the earliest known galeaspids Dayongaspis Pan and Zeng, 1985 and Konoceraspis Pan, 1992, and the earliest known antiarch Shimenolepis Wang J.-Q., 1991, as well as rich sinacanth fin spines. Shimenolepis from Lixian County in north-western Hunan, which was dated as the Telychian (late Llandovery), has long been regarded as the oldest representative of the placoderms in the world. As such, in addition to eastern Yunnan and the Lower Yangtze Region, north-western Hunan represents another important area in South China that yields important fossil material for the research of early vertebrates and related stratigraphy. Here we summarise the Silurian fishes known in north-western Hunan so far, and classify them into three vertebrate assemblages (i.e., the Wentang, Maoshan, and Yangtze assemblages). Based on the updated Silurian vertebrate and stratigraphic databases, the Silurian fish-bearing strata in north-western Hunan can be subdivided into the Rongxi, Huixingshao, and Xiaoxi formations in ascending chronological order, which can be correlated with the Lower Red Beds, the Upper Red Beds, and the Ludlow Red Beds in South China, respectively. A new look at the Silurian strata in Lixian suggests that the age of Shimenolepis is late Ludlow rather than late Llandovery as previously suggested. The research on Silurian fishes and biostratigraphy in north-western Hunan not only provides morphological data of early vertebrates, but also offers new palaeoichthyological evidence for the subdivision, correlation, and age assignment of the Silurian marine red beds in South China. The establishment of a related high-precision Silurian stratigraphic framework in north-western Hunan will help to elucidate the temporal and spatial distribution of Silurian fossil fishes, deepen the understanding of the evolution of early vertebrates, and unravel the coevolution between Silurian vertebrates and the palaeoenvironment.
The study of basal sarcopterygians is crucial to an understanding of the relationships and interrelationships of sarcopterygians, including their relationship to tetrapods. The new material from Qujing, Yunnan, southwestern China, represents the oldest known sarcopterygian fish and extends the record of sarcopterygians to the Late Silurian, or about 410 Ma. The new form is close to Youngolepis and Powichthys at the base of the Crossopterygii. Similarities among the lower jaws of onychodonts, porolepiforms, Youngolepis, Powichthys and the new form support a position of onychodonts within the Crossopterygii. Four characters in the character matrix of Cloutier & Ahlberg (1996, in Stiassny et al: Interrelationships of Fishes, Academic Press) are reviewed, and sarcopterygian interrelationships are studied on the basis of their data with minor modifications. The new scheme of sarcopterygian interrelationships differs markedly from Cloutier & Ahlberg's scheme. Neither actinistians nor onychodonts are situated at the base of Sarcopterygii, but within the Crossopterygii. Youngolepis and Powichthys are at the base of the Crossopterygii, instead of being the sister group of dipnoans plus Diabolepis.
Cambrian and Silurian Formations of Malung and Chutsing Districts
TING V.-K. & WANG Y.-L. (1937). Cambrian and Silurian Formations of Malung and Chutsing Districts,
Yunnan. Bulletin of the Geological Society of China, 16: 1-28.
Age of the Guandi Formation in Qujing District
WANG C.-Y. (2001). Age of the Guandi Formation in Qujing District, E. Yunnan. Journal of Stratigraphy, 25:
125-127.
The oldest articulated osteichthyan reveals mosaic gnathostome characters
- Zhu M Zhao W.-J
ZHU M., ZHAO W.-J., JIA L.-T., LU J., QIAO T. & QU Q.-M. (2009). The oldest articulated osteichthyan reveals
mosaic gnathostome characters. Nature. (doi: 10.1038/nature07855) (To be published on March 26,
2009).