The myth of dorsal ribs in gnathostome vertebrates

Traditionally, two types of rib are distinguished in gnathostomes: dorsal (upper) and ventral (lower, pleural) ribs. They are defined according to their position in the connective tissue system of the body: dorsal ribs develop at the intersection of the serially arranged myosepta with the horizontal septum that separates epaxial from hypaxial musculature, whereas ventral ribs develop at the intersection of myosepta with the peritoneum and usually encircle the body cavity. Distribution of rib types among gnathostomes has traditionally been reported as follows: elasmobranchs have dorsal ribs; all Actinopterygii have only ventral ribs with the exception of polypterids, and two subgroups of teleosts, which supposedly also have dorsal ribs; within Sarcopterygii tetrapods have dorsal ribs, whereas dipnoans have ventral ribs. Here, we report the development of ribs in polypterids, a taxon playing a crucial role in discussions on rib homology. We demonstrate that putative dorsal ribs of polypterids have a unique ontogeny and represent an autapomorphy of this taxon. We discuss previous hypotheses of rib homology and offer a more plausible (i.e. more parsimonious) alternative to the conventional interpretation. We conclude that dorsal ribs do not exist and that ribs of gnathostomes are ventral ribs.     

陕西府谷古鳄类一新属

本文详细记述了陕西府谷下三叠统上部二马营组底部的一古鳄类化石.据其骨骼形态特征拟订为古鳄亚目(Proterosuchia)、引鳄科(Erythrosuchidae)的一新属种:石拐古城鳄(Guchengosuchus shiguaiensis gen. et sp. nov.),同引鳄科内已知属的对比,表明新属古城鳄与武氏鳄 Vjushkovia 的关系最为接近.     

云南东部中三叠世始鳍龙类—新属种(英文)

根据一保存完整的骨骼(包括头骨、下颌和全部头后骨骼)建立了鳍龙类的一新属新种——利齿滇东龙(Diandongosaurus acutidentatus gen.et sp.nov.)。标本采自云南省罗平县中三叠世安尼期关岭组上段。新属种既具有肿肋龙类(包括Dactylosaurus,Anarosaurus,Serpianosaurus和Neusticosaurus)吻部两侧不收缩、眶前区域长于眶后区域、眼眶大于上颞孔等典型特征,同时又具有幻龙类(包括Simosaurus和nothosaurians)前颌和下颌前部犬齿型齿发育、上颌具1或2个犬齿型齿等典型特征。新种的额骨和顶骨均愈合,额骨两后外侧支的末端后延超过上颞孔前缘,轭骨和鳞骨相交将眶后骨排除于下颞孔之外,方轭骨发育,锁骨前外侧缘具一突起,3对荐肋以及最前部尾肋的外侧端均未见明显收缩。这些特征也多表现出肿肋龙类和幻龙类的混合特征。此外,新种还具有一些较为独特的特征,包括前额骨和后额骨沿眼眶背缘相交,第3至第8对尾肋的长度超过荐肋,后肢末端发育异常膨大的爪趾骨。系统关系分析表明,滇东龙既不是肿肋龙类也不是幻龙类;它可能与由乌蒙龙、幻龙类和传统的肿肋龙类所构成的分支亲缘关系最近,为始鳍龙类基干类群。     

Leopard seals (Hydrurga leptonyx) use suction and filter feeding when hunting small prey underwater

Leopard seals (Hydrurga leptonyx) are unusual among apex predators in that they feed at both the top and near the bottom of marine food webs; they capture and consume marine amniotes (seals and penguins) as well as krill. This is thought to be achieved with their unusual dentition: rostral caniniform teeth function to grip large prey and tricuspate postcanines function to sieve krill. The use of canine teeth is known, yet until now, the function of the postcanines has never been documented. Here, we present the first direct observations of filter feeding in leopard seals. Suction was used to draw small prey into the mouth followed by expulsion of ingested seawater through the sieve formed by postcanine teeth. Individuals show abrasive wear on canines and incisors, but not postcanines. This suggests that postcanines are not systematically used for piercing prey during macrophagous feeding, confirming that the postcanines primarily serve a sieving function. Rather than being less efficient at feeding as a result of its polarized diet, the leopard seal is well adapted towards two disparate feeding modes.     

Ultrastructure of excretory system in <Emphasis Type="Italic">Bothrioplana semperi</Emphasis> (Platyhelminthes,Turbellaria)

The ultrastructure of flame bulbs and epithelium of excretory canals in Bothrioplana semperi (Turbellaria, Seriata) have been studied. The flame bulbs consist of two cells, the terminal cell and the proximal canal cell. The weir is formed by two rows of longitudinal ribs. The ribs of the internal row originate from the flame cell, external ribs are formed by the proximal canal cell. Each external rib has a remarkable bundle of microfilaments, originating in the cytoplasm of the first canal cell distally to the bases of external ribs. Membrane of internal ribs is marked by small electrondense granules, separate or fused to an electron-dense layer, continuous to dense “membrane,” connecting both external and internal ribs. Sparse internal leptotrichs originate from the bottom of the flame bulb cavity. External leptotrichs are lacking. Septate junction is present only in proximal canal cell at the level of tips of cilia. The apical surface of the canal cell bears rare short microvilli. The basal membrane of canal cells forms long invaginations that may reach nearly the apical membrane. The epithelium of excretory canals lacks the cilia. The ultrastructure of flame bulbs and epithelium of the excretory canals in B. semperi shares representatives of suborder Proseriata (Seriata). The contradiction exists in interpretation of the structure of flame bulbs in Proseriata. Ehlers and Sopott-Ehlers assumed that the external ribs are derivatives of the proximal canal cell and internal ones are outgrowths of the terminal cell, while Rohde has found conversely: the external ribs are outgrowths of the terminal cell, the internal ones are outgrowths of the proximal canal cell. However, the illustrations provided by Rohde do not enable to ascertain what cells the internal and external ribs derive from, while illustrations provided by Ehlers justify his interpretation. The order of weir formation in B. semperi confirms the viewpoint of Ehlers. The implication of ultrastructure of flame bulbs in Proseriata, especially of the order of flame bulb formation, in the Platyhelminthes phylogeny has been discussed.     

Unusual histology and morphology of the ribs of mosasaurs (Squamata)

We report the presence of two previously unrecognized features in the dorsal ribs of mosasaurs: first, the presence of extremely dense, pervasive extrinsic fibres (anchoring soft tissue to bone, sometimes called Sharpey's fibres); and second, high intraspecific variation in costal bone compactness. Extensive extrinsic fibres are developed in the dorsal ribs of the mosasaurs Tylosaurus proriger and Eonatator sternbergi. The dorsal ribs of these mosasaurs are also characterized by a longitudinally ridged texture that almost completely covers the bone. Pervasive extrinsic fibres and ridged textures are absent in the mosasaur Selmasaurus russelli as well as the dorsal ribs of extant semi‐aquatic reptiles (e.g. crocodyliforms) and mosasaurs' close extant relatives and analogues (e.g. snakes and varanids). Similar ridged textures characterize the dorsal ribs of several other mosasaur taxa but are developed to a lesser extent (e.g. Mosasaurus, Clidastes, Platecarpus and Ectenosaurus), but in no other taxa have pervasive extrinsic fibres been reported. We interpret these osteohistological features in T. proriger and E. sternbergi as evidence of tendinous attachment of extensive and highly differentiated axial musculature capable of producing great stresses, most likely related to stabilization of the trunk relative to contralateral movements of the tail during carangiform locomotion. We also report the compactness indices (percentage of space occupied by bone rather than cavities) for these large mosasaur ribs, which are much higher than previously reported. This suggests high intraspecific variation in bone compactness that complicates its use in reconstructing mosasaur palaeoecology.     

The taxonomic value of fruit structure in the Chinese endemic genus Dickinsia (Apiaceae)

The monotypic Dickinsia Franch. is misplaced in its current position in the tribe Hydrocotyleae and is transferred to the tribe Mulineae. Its mericarps are strongly dorsally compressed and the lateral ribs of each mericarp are much more prominent than the dorsal rib. The carpophore is free. These characters are typical for genera of the Mulineae but are obviously different from other genera of Hydrocotyleae, which all have laterally compressed mericarps, dorsal ribs which are more expanded than the lateral ribs and an absence of free carpophores.     

A new genus and three new species of dactylogyrids (Monogenea), gill parasites of the threadfin bass,Pronotogrammus multifasciatus Gill (Perciformes: Serranidae) in the Southeastern Pacific Ocean off Peru

Pronotogrammella n. g. is erected to accommodate Pronotogrammella boegeri n. sp. (type-species), Pr. scholzi n. sp. and Pr. multifasciatus n. sp. (Monogenea: Dactylogyridae). The new species are gill parasites of the threadfin bass Pronotogrammus multifasciatus Gill (Perciformes: Serranidae), a demersal teleost collected from off the coastal zone of Puerto Pizarro, Tumbes, Peru. Pronotogrammella n. g. is mainly characterised by having broadly fork-shaped dorsal anchors, which have an accessory anchor sclerite articulated to the tip of the superficial roots. Pronotogrammella n. g. is also characterised by having: (i) a tubular tapered-shaped male copulatory organ (MCO), filamentous distally, with a clockwise coil at distal end or not, lacking accessory piece; (ii) a dorsal bar with an anteromedial delicate umbelliform membrane supported by two processes; (iii) hooks with upright blunt thumb and uniform shank; (iv) a vaginal aperture dextrolateral; (v) a subquadrangular haptor, with inconspicuous lateral flaps and lacking haptoral reservoirs; and (vi) eye-spot or chromatic granules absent. Pronotogrammella boegeri n. sp. is characterised by its crosier-shaped MCO having a clockwise coil at distal end and by its dorsal bar with a straight anteromedial processes. Pronotogrammella scholzi n. sp. is typified by possessing of a dorsal bar with the anteromedial processes like cow horns, hoof-shaped deep roots of the dorsal anchors and a broader shaft of the MCO. Pronotogrammella multifasciatus n. sp. differs from all congeners by having a tubular MCO with twisted shaft and a base with a short and broad arm and by having an almost dumbbell-shaped ventral bar.

     

Postcanine microstructure in Cricodon metabolus,a Middle Triassic gomphodont cynodont from south‐eastern Africa

Cricodon metabolus is a trirachodontid cynodont from the Anisian (Middle Triassic) of eastern and southern Africa. It has labiolingually expanded (gomphodont) postcanines but also a sectorial tooth in the last postcanine locus. In this paper, we examine the crown microstructure of isolated sectorial and gomphodont postcanines belonging to the holotype specimen of this taxon using scanning electron microscopy. The enamel of both teeth is prismless and composed of discontinuous columnar divergence units, supporting the consistent presence of synapsid columnar enamel in cynognathians. Abundant tubules and numerous irregularly spaced incremental lines are also visible in the enamel and dentine layers in each tooth. This study reveals that the enamel thickness varies along the tooth row in Cricodon as the enamel layer of the gomphodont postcanines is 11.5 times thicker than that of the sectorial crown. It is likely that this difference reflects occlusal stresses and fewer replacements in gomphodont postcanines relative to sectorial teeth. Approximately 100 incremental growth lines of von Ebner are present in the dentine layer, indicating that the deposition of the dentine by odontoblasts occurred for three months before the animal's death.     

Mediolateral somitic origin of ribs and dermis determined by quail-chick chimeras

Somites are transient mesodermal structures giving rise to all skeletal muscles of the body, the axial skeleton and the dermis of the back. Somites arise from successive segmentation of the presomitic mesoderm (PSM). They appear first as epithelial spheres that rapidly differentiate into a ventral mesenchyme, the sclerotome, and a dorsal epithelial dermomyotome. The sclerotome gives rise to vertebrae and ribs while the dermomyotome is the source of all skeletal muscles and the dorsal dermis. Quail-chick fate mapping and diI-labeling experiments have demonstrated that the epithelial somite can be further subdivided into a medial and a lateral moiety. These two subdomains are derived from different regions of the primitive streak and give rise to different sets of muscles. The lateral somitic cells migrate to form the musculature of the limbs and body wall, known as the hypaxial muscles, while the medial somite gives rise to the vertebrae and the associated epaxial muscles. The respective contribution of the medial and lateral somitic compartments to the other somitic derivatives, namely the dermis and the ribs has not been addressed and therefore remains unknown. We have created quail-chick chimeras of either the medial or lateral part of the PSM to examine the origin of the dorsal dermis and the ribs. We demonstrate that the whole dorsal dermis and the proximal ribs exclusively originates from the medial somitic compartment, whereas the distal ribs derive from the lateral compartment.     

The skeletal kinematics of lung ventilation in three basal bird taxa (emu, tinamou, and guinea fowl)

In vivo visceral and skeletal kinematics of lung ventilation was examined using cineradiography in two palaeognaths, the emu (Dromaius novaehollandiae) and the Chilean tinamou (Nothoprocta perdicaria), and a basal neognath, the helmeted guinea fowl (Numida meleagris). Upon inspiration, the thorax expands in all dimensions. The vertebral ribs swing forward and upward, thereby increasing the transverse diameter of the trunk. The consistent location of the parapophysis throughout the dorsal vertebral series, ventral and cranial to the diapophysis, ensures a relatively uniform lateral expansion. An increase in the angle between the vertebral and the sternal ribs causes the sternal ribs to push the sternum ventrally. Owing to the greater length of the caudal sternal ribs, the caudal sternal margin is displaced further ventrally than the cranial sternal margin. When observed in lateral view, sternal movement is not linear, but elliptical. The avian thorax is highly constrained in its movement when compared with crocodylians, the other extant archosaur clade. Birds lack a lumbar region and intermediate ribs. Sternal ribs are completely ossified, and have a bicondylar articulation with the sternum. Considering the importance of pressure differences between cranial and caudal air sac complexes for the generation of unidirectional air flow in the avian lung, it is hypothesized that a decrease in the degrees of freedom of movement of the avian trunk skeleton, greater expansion of the ventrocaudal trunk region, and elliptical sternal movement may represent specific adaptations for fine-tuned control over air flow within the complex avian pulmonary system.     

江西上三叠统安源组叶肢介一新属*

香溪叶肢介亚科(Xiangxiellinae)以壳瓣胎壳较大、壳上具有一条或两条短脊为特征,包括两个属:原单脊叶肢介属(ProtomonocarinaTasch,1962)和香溪叶肢介属(XiangxiellaShen,1976)。该亚科化石广布于世界各地,最早出现于我国晚石炭世,三叠纪末消失。本文建立的新属叉脊叶肢介(Dicerestheriagen.nov.),以胎壳上具有两条呈“人”字形向前后分叉的、粗壮的短脊为特征,与香溪叶肢介的构造特征最为接近。推测新属壳瓣胎壳脊的形成与闭壳肌附着引起的壳瓣变异或与生态环境变化有关。化石产于江西抚州崇礼县礼陂煤矿,上三叠统安源组徐坊段。湘赣一带安源组为一套含煤地层,属于滨海、近海、沼泽沉积区,产半咸水类型的双壳类,时代认为卡尼及早诺利期。植物化石Ptiozamites–Anthrophyopsis组合是诺利期的代表,适宜于热带、亚热带气候,滨海环境生长的群落。因此推测新属是一个生活在炎热、潮湿的热带-亚热带气候近海沼泽环境的类型。     

A thin-shelled reptile from the Late Triassic of North America and the origin of the turtle shell

A new, thin-shelled fossil from the Upper Triassic (Revueltian: Norian) Chinle Group of New Mexico, Chinlechelys tenertesta, is one of the most primitive known unambiguous members of the turtle stem lineage. The thin-shelled nature of the new turtle combined with its likely terrestrial habitat preference hint at taphonomic filters that basal turtles had to overcome before entering the fossil record. Chinlechelys tenertesta possesses neck spines formed by multiple osteoderms, indicating that the earliest known turtles were covered with rows of dermal armour. More importantly, the primitive, vertically oriented dorsal ribs of the new turtle are only poorly associated with the overlying costal bones, indicating that these two structures are independent ossifications in basal turtles. These novel observations lend support to the hypothesis that the turtle shell was originally a complex composite in which dermal armour fused with the endoskeletal ribs and vertebrae of an ancestral lineage instead of forming de novo. The critical shell elements (i.e. costals and neurals) are thus not simple outgrowths of the bone of the endoskeletal elements as has been hypothesized from some embryological observations.     

The structures of dorsal and ventral regions of a dragonfly retina

Summary The apposition eyes of the corduliid dragonfly Hemicordulia tau are each divided by pigment colour, facet size and facet arrangement into three regions: dorsal, ventral, and a posterior larval strip. Each ommatidium has two primary pigment cells, twenty-five secondary pigment cells, and eight receptor cells, all surrounded by tracheae which probably prevent light passing between ommatidia, and reduce the weight of the eye. Electron microscopy reveals that the receptor cells are of two types: small vestigial cells making virtually no contribution to the rhabdom, and full-size typical cells. The ventral ommatidia have a distal typical cell (oriented either horizontally or vertically), four medial typical cells, two proximal typical cells and one full-length vestigial cell. The dorsal ommatidia have only four full-length typical cells, and one distal and three vestigial full-length cells. The cross-section of dorsal rhabdoms is small and circular distally, but expands to a large three-pointed star medially and proximally. The tiered receptor arrangement in the ventral ommatidia is typical of other Odonata but the dorsal structure has not been fully described in other species. Specialised dorsal eye regions are typical of insects that detect others against the sky.     

Studies on skeleton formation in reptiles, v. Patterns of ossification in the skeleton of Alligator mississippiensis DAUDIN (Reptilia, Grocodylia)

Patterns of ossification are described in the endo-and exoskeleton of Alligator mississippiensis. The occurrence of a dermo-supraoccipital is discussed in light of the independence of dermal and endochondral bone. The development of the bony secondary palate is discussed in light of Haeckelian recapitulation. The sequence of ossification in the limb skeleton is shown to differ from the sequence of chondrification of the cartilaginous precursors. Patterns of ossification in Alligator are compared to lepidosaurs in terms of sequence and timing. Important differences relate to ossification patterns in the limb skeleton: lepidosaurs show a dominance of digit III > IV > II > I > V, whereas Alligator shows a dominance of digits III > II> IV > I > V in the ossification process. Ontogenetic repatterning in the ossification of the axial skeleton is discussed as it bears on the serial homology of dorsal ribs, sacral ribs and caudal ribs (transverse processes).     

The heteromorph ammonite genus Ancyloceras (Ancyloceratidae) in the Paris Basin (lower Aptian,Lower Cretaceous,NE France)

In the present work, we examine a sample of 54 specimens of the heteromorph ammonite genus Ancyloceras d’Orbigny, 1842 from the Argiles à Plicatules Formation (lower Aptian, Lower Cretaceous) of the eastern Paris Basin (NE France). The results are as follows: (1) the sample is regarded as monospecific and assigned to the well-named species Ancyloceras varians d’Orbigny, 1842; (2) its variability is continuous and concerns as well as adult size, coiling and shell ornamentation. Adult size varies by a factor of two from one individual to another. The angle between the proversum and the retroversum is variable. The ornamentation on the spiral and the proversum varies between robust morphotype with trituberculate strong ribs separated by few smooth intercalaries ribs and slender morphotype with feebly tuberculate ribs. There are no arguments in favor of sexual dimorphism in Ancyloceras varians.     

The scapulocoracoid of the Queensland lungfish Neoceratodus forsteri (Dipnoi: Sarcopterygii): morphology, development and evolutionary implications for bony fishes (Osteichthyes)

Among bony fishes, the ontogenetic sequence by which the actinopterygian scapulocoracoid develops has been well described, but that of the sarcopterygian scapulocoracoid is poorly known, as the majority of taxa are only known from fossils. To rectify this, the cartilaginous scapulocoracoid of the extant lungfish Neoceratodus forsteri is examined. In initial stages of its development, the scapulocoracoid of Neoceratodus has a simple rounded shape, and supports the glenoid fossa. It appears nearly contemporaneously with the proximal endochondral element (humerus) of the pectoral fin. Pectoral fin elements develop by segmentation from a continuous field of cartilaginous precursor cells extending distally from the glenoid region of the scapulocoracoid. Subsequent scapulocoracoid development produces a ventromedial process, which is not associated with this field of precursor cells. A dorsal process also develops outside this field. Thus, the scapulocoracoid of Neoceratodus may consist of at least two developmentally distinct regions; (1) the ventromedial being homologous with the coracoid of actinopterygians, tetrapods and other jawed vertebrates and (2) a smaller dorsal process, homologous to the scapular region. The two, together with the glenoid region, give an overall triangular shape. The scapulocoracoids of fossil lungfish and other sarcopterygian fishes are also triangular and are composed of scapular and coracoid regions, rather than the 'buttresses' associated with scapulocoracoids of the Actinopterygii and Tetrapoda.     

Étude morphologique et anatomiquede Rhynchonelloidea ruthenensis (Reynès, 1868) justification de la distinction des genres Rhynchonelloidea Buckman, 1914 et Homoeorhynchia Buckman, 1914 (Brachiopoda)

In the course of researches on the fauna of «calcaires bajociens á bryozoaires of Aveyron départment (France), numerous collects of shells named Rhynchonelloidea ruthenensis (Reynès), type-species of genus RhynchonelloideaBuckman, in two localities enabled to do a morphologic study of this species with populations analysis. Internal characters have been shown both by serial sections and by direct observation of dissected shells. A comparison of genus Rhynchonelloidea and Homoeorhynchia, till now either distinct or considered synonymous according to authors, has been realised. Internal characters of Homoeorhynchia acuta (Sow.) have been precised. The two genus are distinct by beak and foramen's characters, by presence or lack of costulation on the beak and dorsal umbo, by the number of ribs, by presence or lack of septalium and by the development of median dorsal septum.The stratigraphical range of R. ruthenensis is:Upper Toarcian, Aalensis zone-Lower Aalenian, Opalinum zone.