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 * The dermal bones and large rhomboid scales are ornamented with ganoine sculpturing of coarse, irregular or patterned longitudinal ridges, characteristic of lower actinopterygians and Ligulalepis.


 * The fish has a preserved length of, 26 cm and a preserved maximumdepth (although laterally compressed and distorted) of,11 cm. The head occupies about 23% of the total body length (to the base of the caudal fin), and the body is about 2.5 times as long as it is deep, giving Guiyu a fusiform streamlined appearance.


 * The skull roof is divided into the parietal and postparietal shields by the dermal intracranial joint (Fig. 4a). The parietal shield is about as wide as it is long, and has the supraorbital sensory canal medial to the anterodorsally facing anterior nostril.

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See discussions, stats, and author profiles for this publication at: http://www.researchgate.net/publication/24238222 The oldest articulated osteichthyan reveals mosaic gnathostome characters. Nature ARTICLE in NATURE · APRIL 2009 Impact Factor: 41.46 · DOI: 10.1038/nature07855 · Source: PubMed CITATIONS 93 READS 126 6 AUTHORS, INCLUDING: Min Zhu Institute of Vertebrate Paleontology and Pa… 134 PUBLICATIONS 1,867 CITATIONS SEE PROFILE Wenjin Zhao Chinese Academy of Sciences 40 PUBLICATIONS 411 CITATIONS SEE PROFILE Jing Lu Chinese Academy of Sciences 15 PUBLICATIONS 203 CITATIONS SEE PROFILE Tuo Qiao Chinese Academy of Sciences 18 PUBLICATIONS 213 CITATIONS SEE PROFILE Available from: Min Zhu Retrieved on: 16 October 2015 ARTICLES The oldest articulated osteichthyan reveals mosaic gnathostome characters Min Zhu1, Wenjin Zhao1, Liantao Jia1, Jing Lu1,2, Tuo Qiao1,2 & Qingming Qu1,2 The evolutionary history of osteichthyans (bony fishes plus tetrapods) extends back to the Ludlow epoch of the Silurian period. However, these Silurian forms have been documented exclusively by fragmentary fossils. Here we report the discovery of an exceptionally preserved primitive fish from the Ludlow of Yunnan, China, that represents the oldest near-complete gnathostome (jawed vertebrate). The postcranial skeleton of this fish includes a primitive pectoral girdle and median fin spine as in non-osteichthyan gnathostomes, but a derived macromeric squamation as in crown osteichthyans, and substantiates the unexpected mix of postcranial features in basal sarcopterygians, previously restored from the disarticulated remains of Psarolepis. As the oldest articulated sarcopterygian, the new taxon offers insights into the origin and early divergence of osteichthyans, and indicates that the minimum date for the actinopterygian–sarcopterygian split was no later than 419 million years ago. Osteichthyans, which fall into two major monophyletic groups, namely actinopterygians (bichirs, sturgeons, gars, bowfins and teleosts) and sarcopterygians (coelacanths, lungfishes and tetrapods), make up 98% of recognized living vertebrate species1. The rise of osteichthyans from other primitive gnathostomes is a key transition in vertebrate evolution, yet this transition is poorly documented by the fossil discoveries of the last century2.Majormorphological gaps existed between actinopterygians and sarcopterygians, and between osteichthyans and non-osteichthyan gnathostomes (chondrichthyans, acanthodians and placoderms). The past decade has seen the gap narrowing with fossil discoveries such as Psarolepis3–5, Ligulalepis6,7 and Dialipina8, which show fascinating combinations of osteichthyan and nonosteichthyan gnathostome characters, providing new opportunities for studying the polarity and evolution of these characters. However, the basal osteichthyan phylogeny remains uncertain4,6,9–11 owing to the large number of unknown character states in these early forms and the provisional assignment of disarticulated remains to a single taxon. A better understanding of these fishes is therefore crucial in reconstructing the part of phylogeny close to the split between actinopterygians and sarcopterygians. The fossil record of osteichthyans extends back to the Ludlow epoch of the Silurian period, ,422 million years (Myr) ago2,11, yet is documented by fragmentary fossils12–17. Dialipina salgueiroensis from the Emsian of the Canadian Arctic8 (,400 Myr ago, serotinus conodont zone18), referred to the actinopterygians2,8,9,11,19 or stem osteichthyans 10, represents the oldest articulated and complete osteichthyan known thus far. Two articulated stem lungfishes, Porolepis from Spitsbergen20 and Uranolophus from Wyoming21, are older (Pragian, ,411 Myr ago) but less complete than D. salgueiroensis. Psarolepis from the latest Pridoli to the Lochkovian of China and Vietnam3–5,17,22 is another early osteichthyan with the postcranial condition partially known, albeit based on disarticulated remains. Its tentative reconstruction4 shows an enigmatic fish bearing mosaic gnathostome features, but the association of parts fromdifferent individuals requires testing against articulated specimens. The newarticulated fish fromthe Silurian ofChina closely resembles Psarolepis in cranial features, although it bears ridged ornamentation, as present in basal actinopterygians, rather than the cosmine of early sarcopterygians9. This discovery extends the earliest record of nearcomplete articulated osteichthyans to at least 8Myr earlier in geological history, and significantly enhances our growing knowledge of the origin of osteichthyan morphology. Geological framework The new fish material was recovered from the muddy limestone of the Kuanti Formation immediately beneath the first appearance point of Ozarkodina crispa23 at a locality near Xiaoxiang Reservoir, Qujing, Yunnan, China (Fig. 1). The marine Silurian strata in Qujing are subdivided into four formations in ascending order: the Yuejiashan, Kuanti, Miaokao and Yulungssu formations24–26. Early fishes are recorded from the sequence in association with rich invertebrates such as corals, brachiopods, cephalopods, ostracods, bryozoans and trilobites25, and include Psarolepis, an indeterminable osteichthyan from the Yulungssu Formation15,17, and two ‘actinopterygians’ Naxilepis gracilis and Ligulalepis yunnanensis from the Miaokao and Kuanti formations16. The latter three forms are only represented by scales, and the assignment of Naxilepis and L. yunnanensis to the actinopterygians is doubtful due to the meagreness of available data. The new fish is found from the bed,140m below the lowest occurrence of Naxilepis, thereby representing the earliest record of osteichthyans in China. The fish fauna (here named the Xiaoxiang fauna, characterized by the early diversification of gnathostomes) from the site also includes the agnathans, placoderms and acanthodians under study. The late Ludlow (Silurian) age of the new fish bed is mainly derived from the conodont zonation27, although the earlier age assessment (early Ludlow or late Wenlock) of the Kuanti Formation had been proposed based on invertebrate or vertebrate assemblages25,26,28. Osteichthyes Huxley, 1880 Sarcopterygii (Romer, 1955) Guiyu oneiros gen. et sp. nov. Etymology. The generic name derives from gui (Chinese Pinyin), meaning ghost or secret, and yu (Chinese Pinyin), meaning fish. 1Key Laboratory of Evolutionary Systematics of Vertebrates, Institute of Vertebrate Paleontology and Paleoanthropology (IVPP), Chinese Academy of Sciences, PO Box 643, Beijing 100044, China. 2Graduate School, Chinese Academy of Sciences, Beijing 100039, China. Vol 458|26 March 2009| doi:10.1038/nature07855 469 © 2009 Macmillan Publishers Limited. All rights reserved The specific name is from the Greek oneiros, dream, alluding to a dream fish with mosaic gnathostome characters. Holotype. IVPP (Institute of Vertebrate Paleontology and Paleoanthropology, Beijing) V15541, a near-complete fish in part and counterpart. Referred material. Skulls, V15542.1-6; lower jaws, V15542.7-13; scales, V15542.14-16. Horizon and locality. Late Ludlow, Silurian, Kuanti Formation; Qujing, Yunnan, China. Diagnosis. Guiyu differs from Psarolepis3–5,17 and Achoania29,30 in its ganoine sculpturing of dermal skeleton, anterior nostril without dermintermedial process, no teeth on median rostral, premaxillary without inturned symphyseal process, presence of prerostral bone, dentary teeth reaching anterior end of lower jaw, and lacking infradentary foramina. Description The holotype in its part and counterpart represents an articulated and three-dimensionally preserved bony fish lacking only the caudal fin (Figs 2a, b and 3a, b). The dermal bones and large rhomboid scales are ornamented with ganoine sculpturing of coarse, irregular or patterned longitudinal ridges (Supplementary Information 2, section 9), characteristic of lower actinopterygians8,19,31–34 and Ligulalepis6,7,35. The ganoine-like tissue is also present in the surface covering of acanthodians and sarcopterygians, suggesting that ganoine might be plesiomorphic for crown osteichthyans36. The fish has a preserved length of,26 cm and a preserved maximumdepth (although laterally compressed and distorted) of,11 cm. The head occupies about 23% of the total body length (to the base of the caudal fin), and the body is about 2.5 times as long as it is deep, giving Guiyu a fusiform streamlined appearance. The skull roof is divided into the parietal and postparietal shields by the dermal intracranial joint (Fig. 4a). The parietal shield is about as wide as it is long, and has the supraorbital sensory canal medial to the anterodorsally facing anterior nostril. The dermal bone pattern is more evidently shown in several isolated skull specimens (for example,V15542.1, Fig. 4b). Unlike Psarolepis and Achoania, the large median rostral does not form the outer dental arcade, and the premaxillary lacks the obvious inturned symphyseal process in anterior view (Fig. 4c). A separate triangular bone (preorbital, Fig. 4c) posterodorsal to the premaxillary, rather than the posterodorsal process of the premaxillary, encloses the posterior nostril; this resembles Onychodus37 and possibly Achoania29 but differs from Psarolepis3. The ‘lacrimal’ in Onychodus37, which has a more anterior position than the lacrimal of rhipidistians2, might be homologous with the preorbital of actinopterygians and Guiyu. Noteworthy is an unpaired tooth-bearing bone behind the premaxillaries in ventral view (Fig. 4e, f). This bone has never been found in known osteichthyans, yet topologically corresponds to the prerostral or premedian plate of placoderms2,38. The postparietal shield is as long as the parietal shield, as in some later sarcopterygians38. In Psarolepis, the parietal shield is about 1.5 times the postparietal shield in length. The middle and posterior pit-lines lie close to the midline of the shield, and the otic canal runs through the supratemporal and tabular, and connects the supratemporal commissural canal in the lateral extrascapular (Fig. 2b). Anterior to the extratemporal—which is a small bone lateral to the tabular and the anterior half of the lateral extrascapular— an elongate bone between the supratemporal and the opercular represents an accessory extratemporal. Three extrascapular bones are situated behind the postparietal shield; this resembles onychodonts and rhipidistians but differs from actinopterygians and most coelacanths. Concerning the neurocranial features, Guiyu shows a striking resemblance to Psarolepis, Achoania and Onychodus. They share large pear-shaped internasal cavities, a long and narrow internasal septum in front of a small triangular parasphenoid, and the same position of ethmoid articulation on the postnasal wall (Fig. 4e, f). The neurocranium of Guiyu is much less ossified than that of Psarolepis and Achoania, raising difficulties in locating the anatomical structures on its lateral sides (Supplementary Information 2, section 7). However, the postorbital pillar is distinguishable as a bridge connecting the basipterygoid process and the side of the neurocranium. The dermal bones of the cheek and operculo-gular system show a similar pattern to those of early actinopterygians31,32, Psarolepis and onychodonts. Three branchiostegal rays on the left side are preserved belowthe big opercular and subopercular bones. The large gular bones occupy most of the space between the lower jaws as in Onychodus. The lower jaws of the holotype are anteriorly dislocated and broken in their upper half; however, the posterior extremity of the left lower jaw retains its natural position to the upper jaw. As supplemented by some isolated specimens (Fig. 4g, j–m), the lower jaw of Guiyu is nearly identical in shape to that of Psarolepis, and is anteriorly reflexed. The mandibular sensory canal pierces through four infradentaries, leaving a series of openings on the external surface. Posteriorly, a shallow area devoid of ornamentation represents the part of the lower jaw overlapped by the maxillary, as in early sarcopterygians30. In early actinopterygians, this area is usually more extensive32. The dentary bears a row of sharp, conical but slender teeth on the dorsal margin D1 Xishancun Formation Yulungssu Formation Miaokao Formation Kuanti Formation Yuejiashan Formation miao Fm. Ludlow Pridoli Psarolepis sp. Osteichthyes indet. Naxilepis gracilis Ligulalepis yunnanensis Naxilepis gracilis 0 200 m C1 Last appearance point of Ozarkodina crispa First appearance point of Ozarkodina crispa Guiyu oneiros (Xiaoxiang fauna) Qujing Yunnan Lungwang- Figure 1 | Summary Silurian sequence in Qujing (Yunnan, China), showing the stratigraphic positions of Guiyu oneiros gen. et sp. nov. and other bony fishes. The stratigraphic data are taken from ref. 25. ARTICLES NATURE| Vol 458|26 March 2009 470 © 2009 Macmillan Publishers Limited. All rights reserved interior to a band of smaller tooth-like denticles that are present in Lophosteus, Andreolepis11 and many crown osteichthyans. Each marginal tooth has a large pulp cavity extending close to the tip, and lacks the acrodin (Supplementary Information 2, section 9c, e). Guiyu resembles Psarolepis in having five coronoids separating the dentary and prearticular bones, a supporting Meckelian lamina for the parasymphyseal tooth whorl (albeit less ossified and developed), and a relatively short adductor fossa. The second to fourth coronoids carry Figure 2 | Guiyu oneiros gen. et sp. nov., holotype V15541. a, b, A nearcomplete fish in part and counterpart. c, Close-up view of the anterior portion of the trunk shield in dorsal view, showingMD1andMD2flanked by rhomboid scales. d, Close-up view of the dorsal fin spine. MD1, first median dorsal plate; MD2, second median dorsal plate. Scale bar, 1 cm. sop op f.gl f.add ano gu lj lj br sop mx ju pop op pa ppa n.sp eta et l.ext m.ext pt psc scl pcl cla cle sp vrs vrs vrs drs drs dsp tr 1 cm 1 cm a b mx Figure 3 | Guiyu oneiros gen. et sp. nov. a, Restoration of the entire fish in lateral view. b, Interpretive drawing of the holotype V15541. Areas shaded in grey are unknown, and are reconstructed from other early osteichthyans8,37. ano, anterior nostril; br, branchiostegal ray; cla, clavicle; cle, cleithrum; drs, dorsal ridge scale; dsp, dorsal fin spine; et, extratemporal; eta, accessory extratemporal; f.add, adductor fossa; f.gl, glenoid fossa; gu, gular; ju, jugal; l.ext, lateral extrascapular; lj, lower jaw; m.ext, median extrascapular; mx, maxillary; n.sp., spiracular notch; op, opercular; pa, parietal shield; pcl, postcleithrum; pop, preopercular; ppa, postparietal shield; psc, presupracleithrum; pt, post-temporal; scl, supracleithrum; sop, subopercular; sp., pectoral spine; tr, lepidotrichia; vrs, ventral ridge scale. NATURE| Vol 458|26 March 2009 ARTICLES 471 © 2009 Macmillan Publishers Limited. All rights reserved large tusks and a narrow lateral band with 2–4 denticle rows. The tusk has a simple histological structure and lacks any in-folding of dentine near its root, as documented by the fractured surfaces (Supplementary Information 2, section 9f, g). The pectoral girdle of Guiyu is remarkable in its separate fin spine (Fig. 2b), as in Psarolepis4,5, the early chondrichthyan Doliodus39, placoderms and acanthodians2. A small bone (psc, Fig. 3b) in front of the post-temporal corresponds to the presupracleithrum of early actinopterygians32,33. The extensive postbranchial lamina is covered with posteriorly pointed tubercles as in Psarolepis, actinopterygians and placoderms. Immediately behind the median extrascapular, the postcranial skeleton is conspicuous in having two large median dorsal plates (MD1, MD2, Fig. 2a, c) that are flanked by rhomboid scales. The first median dorsal plate, as large as the median extrascapular, is about one-third of the second median dorsal plate in length. In internal view, each of these three unpaired bones bears a keel structure as in the median dorsal plate of placoderms2,40. In external view, the second median dorsal plate bears a posterodorsally directed spine, like the median dorsal spine of some antiarch placoderms40. The median dorsal plates resemble the median ridge scales or fulcral scales preceding the bases of median fins in basal actinopterygians32,33,41. However, they differ in that the ridge scales aremuch smaller and lack strong keel structure. Although the scutes of sturgeons are like the ridge scales, they are obviously derived for actinopterygians41,42. The dorsal fin spine is exceptional in its association with other parts of the body (Fig. 2a, b, d), and forms the leading edge of the dorsal fin. It consists of a stout base with vermiform ridges and a slender, recurved spine ornamented with about eight longitudinal ridges. Its position in the rear half of the body indicates that the unpaired fin spine of Psarolepis4,17 might be more posteriorly located than previously restored.Nofin spine is found for the pelvic and anal fins.Alarge median ridge scale precedes the dorsal fin spine. Along the ventral surface of the body, a similar large ridge scale is found at a position anterior to the level of the pelvic fins. Between this large scale and the base of the pectoral fin, there are many oval ridge scales. The anal fin is not far from the pelvic fins, and between them are a few paired, elongate, oval cloacal scales (Fig. 2b) as present in Moythomasia32. The presence of a narrow ventral ridge scale at the posterior end of the holotype is similar to those found near the lower hypochordal lobe of basal actinopterygians and indicates the position of the missing caudal fin. The lepidotrichia are partially preserved in the holotype (Fig. 2a). The large rhomboid flank scales are arranged into at least 30 vertically oriented rows from the median extrascapular to the base of the dorsal fin spine. They bear a prominent anterodorsal process, and are ornamented with linear ridges that terminate in up to 30 posterior serrations. Internally, the scales show the peg-and-socket articulation and dual ridges as in Ligulalepis35. The isolated lateral-line scales were found in the collection (Fig. 4g–i); however, the path of the lateral line is faintly outlined in the flank of the holotype. Phylogenetic relationships To determine the placement of Guiyu and its impact on osteichthyan relationships, we constructed a new data matrix of 153 characters and a b l g j Co5 Co5 Co4 Median extrascapular Accessory extratemporal Co3 Co2 Dentary teeth Socket po Anterodorsal process Ridge e d i Peg po ar Attachment area terodorsal for tooth whorl Posterior Prerostral f Ethmoid articulation Anterior nostril Pineal foramen Premaxillary Co4 Co So h Ant Preorbital Dentary m Den Overlapped area by maxillary Lateral extrascapular Supraorbital canal Co1 k c terior nostril Preros Median rostral l en Postorbital pillar Extratemporal Figure 4 | Guiyu oneiros gen. et sp. nov. a, Close-up view of the head in dorsal view, holotype V15541. b–f, Isolated anterior cranial portions: b, c, dorsal and anterior views (V15542.1); d, e, dorsal and ventral views (V15542.2); f, ventral view (V15542.3). g, A lower jaw in internal view, V15542.7. h, i, An isolated scale in internal and external views, V15542.14. j–m, Lower jaws: j, internal view (V15542.8); k, internal view (V15542.9); l, external view (V15542.10); m, external view (V15542.11). Co1–5, first to fifth coronoid; po, pore of lateral line. Scale bar: 2mm(h, i); 5mm(a–g, j–m). ARTICLES NATURE| Vol 458|26 March 2009 472 © 2009 Macmillan Publishers Limited. All rights reserved 23 taxa including three non-osteichthyan taxa (Acanthodes, an acanthodian, Akmonistion, a chondrichthyan, and Dicksonosteus, a placoderm), mainly revised from references 9, 10, 29, 43. Phylogenetic analysis (Supplementary Information 1) yields a single most parsimonious tree of length 292, in which Guiyu is placed as the sister taxon to the clade comprising Psarolepis plus Achoania. Meemannia and Ligulalepis form successively more basal taxa among the Sarcopterygii. Lophosteus and Andreolepis represent the stem osteichthyans, and Dialipina forms the most basal taxon among the Actinopterygii. Dating the actinopterygian–sarcopterygian split The fossil record provides a key means of molecular clock calibration 44,45. Aminimumconstraint on the divergence of actinopterygians and sarcopterygians was previously based on the lowest occurrence of crown osteichthyan Psarolepis from the top of the Yulungssu Formation4,17,45 (Fig. 1), dating close to the Silurian/Devonian boundary, ,416Myr ago. The discovery of Guiyu, with accurate dating based on Silurian conodont zonation, provides a new minimum date for molecular-clock-based estimates of the osteichthyan crown node (Fig. 5). As the earliest known, yet not most basal, member of the crown osteichthyan lineage, Guiyu lived ,419Myr ago; thus, the actinopterygian–sarcopterygian split must have occurred no later than this time. The mosaic of gnathostome characters in Guiyu Gnathostomes have their possible earliest occurrence in the Late Ordovician2,46. The Silurian has yielded definite but fragmentary gnathostome remains, with the only articulated representative being a possible acanthodian from Australia47 that merely preserves the middle part of the body and adds little novel data to early gnathostome morphology. Until now, the oldest known near-complete gnathostomes were represented by diverse acanthodians2 and an antiarch placoderm48 from the earliest Devonian (Lochkovian). The discovery of Guiyu provides not only the exceptional corroboration of weak inferences based on disarticulated remains attributed to Psarolepis or Lophosteus, but also the near-complete restoration of a primitive fish with mosaic gnathostome characters. The trunk shield of early osteichthyans, including Andreolepis, Ligulalepis and Meemannia, might bear median fin spine and spine-bearing pectoral girdle as well. Isolated remains that were found in association with these early osteichthyans and referred to non-osteichthyan groups should be reviewed in the light of Guiyu. The phylogenetic position ofGuiyu and related taxa has a substantial impact on the understanding of key osteichthyan apomorphies. Our phylogenetic scenario, although it should be approached with some caution considering the lower Bremer indices at several nodes, offers new insights into the incremental acquisition of osteichthyan apomorphies, and suggests considerable parallelism between actinopterygians and sarcopterygians. For instance, the placement of Ligulalepis at the base of the Sarcopterygii conforms to the notion that its neurocranium reveals the primitive osteichthyan condition6,7. Guiyu, as well as Psarolepis and Achoania, is more derived with regard to its dermal and endoskeletal intracranial joints, although it shares many primitive features with non-osteichthyan gnathostomes. The resemblance of the cheek and operculo-gular bone pattern between actinopterygians and stem sarcopterygians denotes a primitive condition for osteichthyans. The anterodorsal process of the scale might be a synapomorphy of crown osteichthyans rather than the actinopterygians. The discovery of Guiyu offers an exceptional example of a primitive fish close to the split of crown osteichthyans. However, our understanding of the stemsection of the Osteichthyes phylogenetic tree still remains vague owing to the rarity of relevant fossils. More evidence on Silurian placoderms, acanthodians, chondrichthyans and the fragmentary osteichthyan taxa that are basal to Guiyu will provide tests of the present phylogeny and document further the origin of the Osteichthyes. METHODS SUMMARY We adopted the traditional apomorphy-based definition of the Osteichthyes2, considering the unresolved deep phylogeny of gnathostomes. Phylogenetic analysis was performed in PAUP4.0b10 (ref. 49) using a branch-and-bound search with default setting, with Dicksonosteus, Akmonistion and Acanthodes specified as the outgroup. Life reconstruction (Supplementary Information 3) was drawn by B. Choo (Victoria Museum) through communication with M.Z. Early Devonian Carboniferous Ludlow Permian Pridoli Loch. Prag. Ems. Eif. Giv. Fras. Fam. Silurian Gor. Lud. Middle Late (Myr ago) Actinopterygians Sarcopterygians Crown osteichthyans crispa conodont biozone Osteichthyans Crown sarcopterygians -- -- Acanthodes Dicksonosteus Lophosteus Andreolepis Dialipina Cheirolepis Osorioichthys Howqualepis Moythomasia Mimia Ligulalepis Meemannia Guiyu Psarolepis Achoania Onychodus Miguashaia Styloichthys Diabolepis Youngolepis Powichthys Kenichthys Akmonistion Stem sarcopterygians 2 1 1 1 2 1 1 1 1 3 3 2 2 2 4 4 1 1 1 1 57 77 88 73 91 91 73 94 52 -- -- -- -- -- -- -- 50 Minimal divergence actinopterygians–sarcopterygians (~419 Myr ago) -- 422.9 421.3 418.7 416.0 411.2 407.0 397.5 391.8 385.3 374.5 359.2 299.0 251.0 Figure 5 | Timing of earliest evolution of the Osteichthyes. The topology is the most parsimonious tree arising from a matrix of 23 taxa coded for 153 morphological characters (tree length5292, consistency index50.572, retention index50.737, rescaled consistency index50.421). The numbers at nodes indicate bootstrap support (where the value is greater than 50%) and Bremer decay index (bottom and top numbers, respectively). The absolute age assessment is from ref. 50. Eif., Eifelian; Ems., Emsian; Fam., Famennian; Fras., Frasnian; Giv., Givetian; Gor., Gorstian; Loch., Lochkovian; Lud., Ludfordian; Prag., Pragian. NATURE| Vol 458|26 March 2009 ARTICLES 473 © 2009 Macmillan Publishers Limited. All rights reserved Received 5 October 2008; accepted 23 January 2009. 1. Nelson, J. S. Fishes of the World (Wiley, 2006). 2. Janvier, P. Early Vertebrates (Clarendon Press, 1996). 3. Yu, X.-B. A new porolepiform-like fish Psarolepis romeri gen. et sp. nov. (Sarcopterygii, Osteichthyes) from the Lower Devonian of Yunnan, China. J. Vert. Paleontol. 18, 261–274 (1998). 4. Zhu, M., Yu, X.-B. & Janvier, P. A primitive fossil fish sheds light on the origin of bony fishes. Nature 397, 607–610 (1999). 5. 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PAUP*: Phylogenetic Analysis Using Parsimony (* and other methods), Version 4.0b 10 (Sinauer Associates, 2003). 50. Gradstein, F. M. et al. A Geologic Time Scale 2004 (Cambridge Univ. Press, 2004). Supplementary Information is linked to the online version of the paper at www.nature.com/nature. Acknowledgements We thank M.-M. Chang, P. Janvier, J. Long, P. Ahlberg, X.-B. Yu and B. Choo for discussions, X.-B. Yu and B. Choo for improving the manuscript, J. Zhang and C.-H. Xiong for field work, X.-F. Lu and C.-H. Xiong for specimen preparation, W.-D. Zhang and W. Wang for making thin sections, B. Choo for life restoration, J.-L. Huang for illustrations, and J. Zhang for photography. This work was supported by the Major State Basic Research Projects (2006CB806400) of MST of China, the Chinese Academy of Sciences, the National Natural Science Foundation of China, and the CAS/SAFEA International Partnership Program for Creative Research Teams. M.Z., J.L. and T.Q. thank G. Young and J. Long for the examination of Australian specimens with the support of an Australian Research Council Discovery Grant (DP0772138). Author Information Reprints and permissions information is available at www.nature.com/reprints. Correspondence and requests for materials should be addressed to M.Z. (zhumin@ivpp.ac.cn). ARTICLES NATURE| Vol 458|26 March 2009 474 © 2009 Macmillan Publishers Limited. All rights reserved

Yu Xiaobo, Zhu Min & Zhao Wenjin: The Origin and Diversification of Osteichthyans and Sarcopterygians: Rare Chinese Fossil Findings Advance Research on Key Issues of Evolution. Bulletin of the Chinese Academy of Sciences, Paleoichthyology, Vol.24 No.2 2010

The pectoral girdle of Guiyu is remarkable in its separate fin spine (Fig. 2b), as in Psarolepis4,5, the early chondrichthyan Doliodus39, placoderms and acanthodians2. A small bone (psc, Fig. 3b) in front of the post-temporal corresponds to the presupracleithrum of early actinopterygians.

To determine the placement of Guiyu and its impact on osteichthyan relationships, we constructed a new data matrix of 153 characters and 23 taxa including three non-osteichthyan taxa (Acanthodes, an acanthodian, Akmonistion, a chondrichthyan, and Dicksonosteus, a placoderm),

Guiyu is placed as the sister taxon to the clade comprising Psarolepis plus Achoania

For instance, the placement of Ligulalepis at the base of the Sarcopterygii conforms to the notion that its neurocranium reveals the primitive osteichthyan condition6,7. Guiyu, as well as Psarolepis and Achoania, is more derived with regard to its dermal and endoskeletal intracranial joints, although it shares many primitive features with non-osteichthyan gnathostomes.

The Xiaoxiang Fauna, characterized by the early diversification of gnathostomes.

The discovery of Guiyu, with the accurate dating based on Silurian conodont zonation (Walliser & Wang, 1989), provides not only the near-complete restoration of a primitive fish with mosaic gnathostome characters...

Guiyu sides with osteichthyans by bearing derived macromeric scales, yet it resembles non-osteichthyan chondrichthyans, placoderms and acanthodians by having a primitive pectoral girdle and median fin spine.

from the 419-Myr-old muddy limestone in eastern Yunnan

Three other fossil f ishes from Yunnan (Meemannia, Psarolepis and Achoania) cluster with Guiyu as basal or stem sarcopterygians, but they come from younger ages and consist of less complete materials when compared with Guiyu.

a unique combination of gnathostome vs non-gnathostome characters as well as sarcopterygian vs actinopterygian characters―all these make Guiyu highly significant for the global study on the origin of gnathostomes, the origin and diversification of osteichthyans, and the origin and diversification of sarcopterygians.

During the Late Silurian–Lochkovian periods ( a b o u t 4 11 – 4 2 2 mi l l i o n y e a r s ago), the South China region was a separate continent adjacent to eastern Gondwana, and its vertebrate fauna as a whole was highly endemic to the region, indicating little or no exchange between this region and other areas before the Middle Devonian period (397 million years ago).

Guiyu (and Psarolepis) suggest that early osteichthyans might carry median fin spine and spine-bearing pectoral girdles previously thought to exist only in non-osteichthyan groups.

the Late Silurian Xiaoxiang fauna