云南昭通早泥盆世肉鳍鱼类—新属(英文)

记述了产于云南昭通早泥盆世布拉格期基干肺鱼形动物一新属、新种——多孔弓鱼(Arquatichthys porosus gen.et sp.nov.)。新材料包括一件较为完整的下颌和鳞片若干。与基干肺鱼形动物相近的特征有:第四下齿骨水平凹线呈"L"形;下颌表面可见许多不规则排列的感觉管开孔;具三块冠状骨;冠状骨侧部为宽阔的小齿带;侧联合齿骨附着区较小;颌收肌窝大;菱形鳞片具明显的前腹突。鉴别特征包括下颌背缘明显隆起,以及后缘具加长的被覆压区。多孔弓鱼的发现为探讨肉鳍鱼类的早期分化提供了新的资料,在早期肉鳍鱼类的系统发育关系框架下讨论了鳞片的特征演化序列。     

An onychodont fish (Osteichthyes,Sarcopterygii) from the Early Devonian of China,and the evolution of the Onychodontiformes

Although the superbly preserved specimens of Onychodus jandemarrai have greatly advanced our understanding of the Onychodontiformes, a primitive sarcopterygian group with large parasymphysial tooth whorls, the scarcity of the otoccipital material in the group hampers further morphological comparisons between onychodonts and other sarcopterygian groups. Here we report a new onychodont Qingmenodus yui gen. et sp. nov. from the Early Devonian (Pragian) of South China that comprises well-ossified otoccipital and upper and lower jaw material. As one of the oldest known onychodonts, Qingmenodus shows for the first time the nearly complete structure of the otoccipital in onychodonts and provides an additional basis to address the phylogenetic position of the group. Its elongated otic shelf exhibits the posterior shift of the attachment for the basicranial muscle as in coelacanths and sheds light on the feeding mechanism of onychodonts. Qingmenodus displays a mosaic of primitive and derived onychodont features. The phylogenetic analysis places Qingmenodus immediately basal to the clade comprising Onychodus and Grossius.     

Stem sarcopterygians have primitive polybasal fin articulation

Among osteichthyans, basal actinopterygian fishes (e.g. paddlefish and bowfins) have paired fins with three endoskeletal components (pro-, meso- and metapterygia) articulating with polybasal shoulder girdles, while sarcopterygian fishes (lungfish, coelacanths and relatives) have paired fins with one endoskeletal component (metapterygium) articulating with monobasal shoulder girdles. In the fin–limb transition, the origin of the sarcopterygian paired fins triggered new possibilities of fin articulation and movement, and established the proximal segments (stylopod and zeugopod) of the presumptive tetrapod limb. Several authors have stated that the monobasal paired fins in sarcopterygians evolved from a primitive polybasal condition. However, the fossil record has been silent on whether and when the inferred transition took place. Here we describe three-dimensionally preserved shoulder girdles of two stem sarcopterygians (Psarolepis and Achoania) from the Lower Devonian of Yunnan, which demonstrate that stem sarcopterygians have polybasal pectoral fin articulation as in basal actinopterygians. This finding provides a phylogenetic and temporal constraint for studying the origin of the stylopod, which must have originated within the stem sarcopterygian lineage through the loss of the propterygium and mesopterygium.     

云南昭通早泥盆世肉鳍鱼类一新属

记述了产于云南昭通早泥盆世布拉格期基干肺鱼形动物一新属、新种--多孔弓鱼(Arquatichthys porosus gen.et sp.nov.).新材料包括一件较为完整的下颌和鳞片若干.与基干肺鱼形动物相近的特征有:第四下齿骨水平凹线呈"L"形;下颌表面可见许多不规则排列的感觉管开孔;具三块冠状骨;冠状骨侧部为宽阔的小齿带;侧联合齿骨附着区较小;颌收肌窝大;菱形鳞片具明显的前腹突.鉴别特征包括下颌背缘明显隆起,以及后缘具加长的被覆压区.多孔弓鱼的发现为探讨肉鳍鱼类的早期分化提供了新的资料,在早期肉鳍鱼类的系统发育关系框架下讨论了鳞片的特征演化序列.     

Comparative anatomy, homologies and evolution of the pectoral muscles of bony fish and tetrapods: a new insight

The Osteichthyes, including bony fishes and tetrapods, is a highly speciose group of vertebrates, comprising more than 42,000 living species. The anatomy of osteichthyans has been the subject of numerous comparative studies, but most of these studies concern osteological structures; much less attention has been paid to muscles. The most detailed comparative analyses of osteichthyan pectoral muscles that were actually based on a direct observation of representatives of various major actinopterygian and sarcopterygian groups were provided several decades ago by authors such as Howell and Romer. Despite the quality of their work, these authors did not have access to much information that is now available. In the present work, an updated discussion on the homologies and evolution of the osteichthyan pectoral muscles is provided, based on the authors' own analyses and on a survey of the literature, both old and recent. It is stressed that much caution should be taken when the results obtained in molecular and developmental studies concerning the pectoral muscles of model actinopterygians such as the teleostean zebrafish are discussed and compared with the results obtained in studies concerning model sarcopterygians from clades such as the Amphibia and/or the Amniota. This is because, as shown here, as a result of the different evolutionary routes followed within the actinopterygian and the sarcopterygian clades none of the individual muscles found, for example, in derived actinopterygians such as teleosts is found in derived sarcopterygians such as tetrapods. It is hoped that the information provided in the present work may help in paving the way for future analyses of the pectoral muscles in taxa from different osteichthyan groups and for a proper comparison between these muscles in those taxa.     

Oldest coelacanth, from the Early Devonian of Australia

Coelacanths are well-known sarcopterygian (lobe-finned) fishes, which together with lungfishes are the closest extant relatives of land vertebrates (tetrapods). Coelacanths have both living representatives and a rich fossil record, but lack fossils older than the late Middle Devonian (385-390 Myr ago), conflicting with current phylogenies implying coelacanths diverged from other sarcopterygians in the earliest Devonian (410-415 Myr ago). Here, we report the discovery of a new coelacanth from the Early Devonian of Australia (407-409 Myr ago), which fills in the approximately 20 Myr 'ghost range' between previous coelacanth records and the predicted origin of the group. This taxon is based on a single lower jaw bone, the dentary, which is deep and short in form and possesses a dentary sensory pore, otherwise seen in Carboniferous and younger taxa.     

Homology of fin lepidotrichia in osteichthyan fishes

Lepidotrichia are dermal elements located at the distal margin of osteichthyan fins. In sarcopterygians and actinopterygians, the term has been used to denote the most distal bony hemisegments and also the more proximal, scale-covered segments which overlie endochondral bones of the fin. In certain sarcopterygian fishes, including the Rhizodontida, these more proximal, basal segments are very long, extending at least half the length of the fin. The basal segments have a subcircular cross section, rather than the crescentic cross section of the distal lepidotrichial hemisegments, which lack a scale cover and comprise short, generally regular, elements. In rhizodonts and other sarcopterygians, e.g. Eusthenopteron, the basal elements are the first to appear during fin development, followed by the endochondral bones and then the distal lepidotrichia. This sequence contradicts the 'clock-face model' of fin development proposed by Thorogood in which the formation of endochondral bones is followed by development of lepidotrichia. However, if elongate basal 'lepidotrichia' are not homologous with more distal, jointed lepidotrichia and if the latter form within a distal fin-fold and the former outside this fold, then Thorogood's 'clock-face' model remains valid. This interpretation might indicate that the fin-fold has been lost in early digited stem-tetrapods such as Acanthostega and Ichthyostega and elongate basal elements, but not true lepidotrichia, occur in the caudal fins of these taxa.     

云南武定中泥盆世骨鳞鱼目一新种

本文记述的是采自云南武定中泥盆统地层中的一个总鳍类、骨鳞鱼目(Osteolepiformes Jarvik 1942)、骨鳞鱼科(Osteolepididae Cope 1889)的瑟索鱼属(Thursius)化石新种——武定瑟索鱼(Thursius wudingensis sp. nov.).对骨鳞鱼化石的记述在国内尚属首次.本文除了对化石新种进行描述和分类之外,还对武定瑟索鱼的生物地理意义以及它与杨氏鱼的关系作了尝试性的探讨.     

The braincase and palate of the tetrapodomorph sarcopterygian mandageria fairfaxi: morphological variability near the fish–tetrapod transition

The braincase of the Late Devonian tristichopterid sarcopterygian Mandageria fairfaxi , from Canowindra, NSW, Australia, differs radically from the conservative pattern present in other 'osteolepiforms' (stem–group tetrapodomorph fishes) and non–dipnoan sarcopterygian fishes in general. The basioccipital region is short, displaced anteriorly, and either unossified or loosely articulated to the exoccipital, leaving most or all of the notochordal tunnel open ventrally. The exoccipital complex, which is developed into a large saddle that would have rested on top of the notochord, carries large, triangular articular facets on its posterior face and appears to have formed part of a functional neck joint, a synovial articulation between the skull and vertebral column that allows the former to rotate against the latter. Such a joint is characteristic of post–Devonian tetrapods, but unknown in other sarcopterygians. We infer that the ventrally open notochordal tunnel allowed gentle flexion of the cranial notochord during (predominantly vertical) rotational movement at the occiput; this is a mechanically unique solution to the problem of creating a mobile neck. Other unusual features of Mandageria include a posteriorly located lateral commissure, and structures on the entopterygoid and lateral commissure that may have been associated with an elaborate spiracular tract.     

A general scenario of Hox gene inventory variation among major sarcopterygian lineages

Background  

H ox genes are known to play a key role in shaping the body plan of metazoans. Evolutionary dynamics of these genes is therefore essential in explaining patterns of evolutionary diversity. Among extant sarcopterygians comprising both lobe-finned fishes and tetrapods, our knowledge of the Hox genes and clusters has largely been restricted in several model organisms such as frogs, birds and mammals. Some evolutionary gaps still exist, especially for those groups with derived body morphology or occupying key positions on the tree of life, hindering our understanding of how Hox gene inventory varied along the sarcopterygian lineage.     

Evolutionary origin of the Otx2 enhancer for its expression in visceral endoderm

In the mouse, the Otx2 gene has been shown to play essential roles in the visceral endoderm during anterior-posterior axis formation and head induction. While these are primary processes in vertebrate embryogenesis, the visceral endoderm is a tissue unique to mammals. Two enhancers (VE and CM) have been previously found to direct Otx2 expression during early embryogenesis. This study demonstrates that in anterior visceral endoderm the CM enhancer does not have an activity by itself, but enhances the activity of the VE enhancer. These two enhancers also cooperate for the activities in anterior mesendoderm and cephalic mesenchyme. Comparative studies suggest that VE enhancer function was most likely established before the divergence of sarcopterygians into Actinistia, Dipnoi and tetrapods, while the nucleotide sequence corresponding to the VE enhancer was already present in the last common ancestor of bony fishes. The CM enhancer sequence and function would have been also established in ancestral sarcopterygians. The VE/CM enhancers and their gene cascades in the ancestral sarcopterygian head organizer would then have been co-opted by amphibian deep endoderm cells and mammalian visceral endoderm cells for the head development.     

The ear region of Latimeria chalumnae: functional and evolutionary implications

The anatomy of Latimeria chalumnae has figured prominently in discussions about tetrapod origins. While the gross anatomy of Latimeria is well documented, relatively little is known about its otic anatomy and ontogeny. To examine the inner ear and the otoccipital part of the cranium, a serial-sectioned juvenile coelacanth was studied in detail and a three-dimensional reconstruction was made. The ear of Latimeria shows a derived condition compared to other basal sarcopterygians in having a connection between left and right labyrinths. This canalis communicans is perilymphatic in nature and originates at the transition point of the saccule and the lagena deep in the inner ear, where a peculiar sense end organ can be found. In most gnathostomes the inner ears are clearly separated from each other. A connection occurs in some fishes, e.g. within the Ostariophysi. In the sarcopterygian lineage no connections between the inner ears are known except in the Actinistia. Some fossil actinistians show a posteriorly directed duct lying between the foramen magnum and the notochordal canal, similar to the condition in the ear of Latimeria, so this derived character complex probably developed early in actinistian history. Because some features of the inner ear of Latimeria have been described as having tetrapod affinities, the problem of hearing and the anatomy of the otical complex in the living coelacanth has been closely connected to the question of early tetrapod evolution. It was assumed in the past that the structure found in Latimeria could exemplify a transitional stage in otic evolution between the fishlike sarcopterygians and the first tetrapods in a functional or even phylogenetic way. Here the possibility is considered that the canalis communicans does not possess any auditory function but rather is involved in sensing pressure changes during movements involving the intracranial joint. Earlier hypotheses of a putative tympanic ear are refuted.     

Lung ventilation in salamanders and the evolution of vertebrate air-breathing mechanisms

Functional analysis of lung ventilation in salamanders combined with historical analysis of respiratory pumps provides new perspectives on the evolution of breathing mechanisms in vertebrates. Lung ventilation in the aquatic salamander Necturus maculosus was examined by means of cineradiography, measurement of buccal and pleuroperitoneal cavity pressures, and electromyography of hypaxial musculature. In deoxygenated water Necturus periodically rises to the surface, opens its mouth, expands its buccal cavity to draw in fresh air, exhales air from the lungs, closes its mouth, and then compresses its buccal cavity and pumps air into the lungs. Thus Necturus produces only two buccal movements per breath: one expansion and one compression. Necturus shares the use of this two-stroke buccal pump with lungfishes, frogs and other salamanders. The ubiquitous use of this system by basal sarcopterygians is evidence that a two-stroke buccal pump is the primitive lung ventilation mechanism for sarcopterygian vertebrates. In contrast, basal actinopterygian fishes use a four-stroke buccal pump. In these fishes the buccal cavity expands to fill with expired air, compresses to expel the pulmonary air, expands to fill with fresh air, and then compresses for a second time to pump air into the lungs. Whether the sarcopterygian two-stroke buccal pump and the actinopterygian four-stroke buccal pump arose independently, whether both are derived from a single, primitive osteichthyian breathing mechanism, or whether one might be the primitive pattern and the other derived, cannot be determined. Although Necturus and lungfishes both use a two-stroke buccal pump, they differ in their expiration mechanics. Unlike a lungfish (Protopterus), Necturus exhales by contracting a portion of its hypaxial trunk musculature (the m. Iransversus abdominis) to increase pleuroperitoneal pressure. The occurrence of this same expiratory mechanism in amniotes is evidence that the use of hypaxial musculature for expiration, but not for inspiration, is a primitive tetrapod feature. From this observation we hypothesize that aspiration breathing may have evolved in two stages: initially, from pure buccal pumping to the use of trunk musculature for exhalation but not for inspiration (as in Necturus); and secondarily, to the use of trunk musculature for both exhalation and inhalation by costal aspiration (as in amniotes).     

Distribution and variation in enamel structure in the oral teeth of sarcopterygians: Its significance for the evolution of a protoprismatic enamel

The property of tooth enamel to resist alteration during fossilization, is used to analyse the unique arrangements of biological crystallites amongst genera of Paleozoic sarcopterygians, with both polarized light and s.e.m. Previous concepts of crystallite organization in reptiles and mammal‐like reptiles are evaluated. Two of the Devonian sarcopterygians, are shown to exhibit a protoprismatic pattern, identical with that of a stem group therian. The patterns of crystallites, together with the arrangement of incremental lines establish that this tissue is solely an ectodermal product; monotypic enamel, in contrast to bitypic enamel with two cell products contributing to it as in enameloid or acrodin. Each genus examined has a different pattern, of significance in considering relationships amongst sarcopterygians. Recent information on ganoine and some new findings on enamel in extant lungfishes have led to the conclusion that types of monotypic enamel are present in both actinopterygians and sarcopterygians, and challenges the use of monotypic enamel as a synapomorphy of sarcopterygians in cladistic analyses.     

First discovery of a primitive coelacanth fin fills a major gap in the evolution of lobed fins and limbs

The fossil record provides unique clues about the primitive pattern of lobed fins, the precursors of digit-bearing limbs. Such information is vital for understanding the evolutionary transition from fish fins to tetrapod limbs, and it guides the choice of model systems for investigating the developmental changes underpinning this event. However, the evolutionary preconditions for tetrapod limbs remain unclear. This uncertainty arises from an outstanding gap in our knowledge of early lobed fins: there are no fossil data that record primitive pectoral fin conditions in coelacanths, one of the three major groups of sarcopterygian (lobe-finned) fishes. A new fossil from the Middle-Late Devonian of Wyoming preserves the first and only example of a primitive coelacanth pectoral fin endoskeleton. The strongly asymmetrical skeleton of this fin corroborates the hypothesis that this is the primitive sarcopterygian pattern, and that this pattern persisted in the closest fish-like relatives of land vertebrates. The new material reveals the specializations of paired fins in the modern coelacanth, as well as in living lungfishes. Consequently, the context in which these might be used to investigate evolutionary and developmental relationships between vertebrate fins and limbs is changed. Our data suggest that primitive actinopterygians, rather than living sarcopterygian fishes and their derived appendages, are the most informative comparators for developmental studies seeking to understand the origin of tetrapod limbs.     

CHARACTER DIAGNOSIS, FOSSILS AND THE ORIGIN OF TETRAPODS

I. The traditional view of the origin of tetrapod vertebrates is that they are descendants of fossil osteolepiform fish, of which Eusthenopteron is best known. In recent years both that conclusion and the methodology by which it has been reached have been challenged by practitioners of cladistic analysis. Particularly a recent review by Rosen et al. (1981) claims that Dipnoi (lungfish) are the sister-group of the Tetrapoda, that Osteolepiformes is a non-taxon and that Eusthenopteron is more distant from tetrapods than are Dipnoi, coelacanths and probably the fossil Porolepiformes. We attempt to refute all these concludions by use of the same cladistic technique. 2. We accept that all the above-mentioned groups, together with some less well-known taxa, can be united as Sarcopterygii by means of shared derived (apomorph) characters. We also agree that Porolepiformes and Actinistia (coelacanths) can be characterized as valid taxa. The primitive and enigmatic fossil fish Powichthys is accepted as representing the plesiomorph sister-group of true porolepiforms. 3. Only two apomorph features, the course of the jaw adductor muscles and the position of incurrent and excurrent nostrils, appear to unite all the fish, living and fossil, currently regarded as Dipnoi. The characteristic tooth plates and the presence of petrodentine both exclude important primitive fossil forms. 4. Contrary to the opinion of Rosen et al., Osteolepiformes can be characterized — by the arrangement of bones forming the cheek plate, the presence of basal scutes to the fins and by the unjointed radials of the median fins. However, if these are true autapomorphies they exclude any osteolepiform from direct tetrapod ancestry. 5. Tetrapoda is a monophyletic group characterized by ten or more autapomorphies, including the bones of the cheek plate, a stapes and fenestra ovalis, and a series of characters of the appendicular skeleton. 6. Tetrapods have a true choana (internal nostril). We accept that the posterior (excurrent) nostril of Dipnoi is the homologue of the tetrapod choana. However, we assert that the posterior nostril of all bony fish is the homologue of the choana. This assertion would be refuted if any fish showed separate posterior nostril and choana. We reject the claim that this ‘three nostril condition’ occurred in porolepiforms and osteolepiforms. The evidence for a choana in porolepiforms is inadequate. Osteolepiforms had a true choana, characterized as in tetrapods by its relationship to the bones of the palate, but no third nostril. Dipnoans are not choanate. 7. Following cladistic practice, the relationship of the extant taxa is established first. Dipnoi are thus shown to be the living sister-group of tetrapods, but only on ‘soft anatomy’ characters unavailable in fossils. Coelacanths are the living sister-group of the taxon so formed. 8. The relationship of the fossil taxa to the extant sarcopterygians is then considered. The synapomorphy scheme proposed by Rosen et al. is discussed at length. Virtually all the characters they use to exclude close relationship of Eusthenopteron (and hence all osteolepiforms) to tetrapods, in favour of coelacanths and dipnoans, are invalid. 9. A series of synapomorphies uniting osteolepiforms and tetrapods is proposed, including a true choana (hence the taxon Choanata), the histology of the teeth, and a number of characters of the humerus. The recently discovered fossil Youngolepis, which lacks a choana, represents the sister-group of the Choanata, and is not uniquely close to Powichthys. The latter, as a porolepiform (s.l.) is a member of the sister-group to Choanata plus Youngolepis. 10. Our cladistic analysis suggests that all the extinct taxa considered are more closely related to tetrapods than are the Dipnoi. Moreover fossil evidence suggests that Dipnoi, considered as an extant taxon, may not even be the living sister-group of Tetrapoda. Early fossil dipnoans appear to have been marine fish without specific adaptations for air breathing. If so the apparent synapomorphies of Dipnoi and Tetrapoda may be homoplastic — the insistence on grouping extant taxa first would then have yielded an invalid inference.     

Reconstructing pectoral appendicular muscle anatomy in fossil fish and tetrapods over the fins‐to‐limbs transition

The question of how tetrapod limbs evolved from fins is one of the great puzzles of evolutionary biology. While palaeontologists, developmental biologists, and geneticists have made great strides in explaining the origin and early evolution of limb skeletal structures, that of the muscles remains largely unknown. The main reason is the lack of consensus about appendicular muscle homology between the closest living relatives of early tetrapods: lobe‐finned fish and crown tetrapods. In the light of a recent study of these homologies, we re‐examined osteological correlates of muscle attachment in the pectoral girdle, humerus, radius, and ulna of early tetrapods and their close relatives. Twenty‐nine extinct and six extant sarcopterygians were included in a meta‐analysis using information from the literature and from original specimens, when possible. We analysed these osteological correlates using parsimony‐based character optimization in order to reconstruct muscle anatomy in ancestral lobe‐finned fish, tetrapodomorph fish, stem tetrapods, and crown tetrapods. Our synthesis revealed that many tetrapod shoulder muscles probably were already present in tetrapodomorph fish, while most of the more‐distal appendicular muscles either arose later from largely undifferentiated dorsal and ventral muscle masses or did not leave clear correlates of attachment in these taxa. Based on this review and meta‐analysis, we postulate a stepwise sequence of specific appendicular muscle acquisitions, splits, and fusions that led from the ancestral sarcopterygian pectoral fin to the ancestral tetrapod forelimb. This sequence largely agrees with previous hypotheses based on palaeontological and comparative work, but it is much more comprehensive in terms of both muscles and taxa. Combined with existing information about the skeletal system, our new synthesis helps to illuminate the genetic, developmental, morphological, functional, and ecological changes that were key components of the fins‐to‐limbs transition.     

Primitive bony fishes, with especial reference to Cheirolepis and palaeonisciform actinopterygians

The relationships of the Devonian palaeonisciform fish Cheirolepis are examined and the early evolutionary trends within the Actinopterygii and the Osteichthyes are considered.
Cheirolepis is the most primitive known actinopterygian. The contemporary stegotrachelid palaeonisciforms are more advanced in their cranial and locomotor anatomy. The general directions of these advances are similar to those subsequently displayed by later palaeonisciforms over the stegotrachelids themselves. Cheirolepis , furthermore, possesses many characters which can be logically interpreted as primitive for the Osteichthyes by extrapolation of trends in actinopterygian and sarcopterygian lineages. 11 is the most primitive known osteichthyan.
The Osteichthyes are considered to have arisen from a micromerically-scaled acanthodian or acanthodian-like ancestor at the end of the Silurian period.