Revision of the Lower Cambrian brachiopod Heliomedusa Sun & Hou, 1987

Jin Yugan & Wang Huayu 1992 01 15: Revision of the Lower Cambrian brachiopod Heliomedusa Sun & Hou, 1987. Lethaia , voi. 25. pp. 35–49. Oslo. ISSN 0024–1164.
Fossils referred to Heliomedusa , from the Chiungchussu Formation of Early Cambrian age in Yunnan. are extraordinarily preserved. They were previously considered to represent jellyfish. and have been suspected more recently of being an obolellid brachiopod. A study of the morphology of this genus indicates that it is related closely lo the non-pediculate craniopsids. which occurred from Early Ordovician to Carboniferous time. The soft parts of this brachiopod. such as setae and nerves, which usually are not preserved in fossils also are described. Brachiopoda, Inarticulata, Craniapsidue, anatomy, setae, nervous system, mantle canals, Early Cambrian .     

Structure of the brachiopod lophophore

Data on the development, structure, and functional morphology of the brachiopod lophophore are analyzed. The common origin of the tentacle apparatus in Lophophorata from the postoral ciliary band of the larva is shown. The brachiopod lophophore is based on the brachial axis consisting of the brachial fold running along the row of tentacles. The brachial axis may be attached to the brachial (dorsal) mantle lobe (trocholophe, schizolophe, and ptycholophe lophophores) or extend freely into the mantle cavity to form coiling brachia (spirolophe, zygolophe, and plectolophe lophophores). The circulation of water flows through the mantle cavity in the brachiopods with attached and free lophophores is described. A new hypothesis on the sorting of particles suspended in water during filtration is proposed.     

Comparative hydrodynamic stability of brachiopod shells on current-scoured arenaceous substrates

Selected brachiopod specimens were placed on a well-sorted medium grained sand (0.5 mm) in a recirculating flume and subjected to a constant unidirectional current of 26–27 cm/sec. The specimens were placed in each of six feasible life-orientations. In three orientations, the valves were reclining with either the lateral, anterior or posterior profile upcurrent. In three orientations, the commissural plane was perpendicular to the substrate with either the pedicle or brachial valve upcurrent, or a lateral margin upcurrent. Destabilization indices were calculated for each specimen in each orientation based on the elapsed time until the specimen reoriented. Nonstrophic biconvex brachiopods are most stable with the valves reclining on the substrate. Among orientations with the valves erect, the pedicle valve upcurrent and the brachial valve upcurrent are the most and least stable orientations, respectively. Shell stability increased among nonstrophic specimens that were more equi-biconvex. Biconvex strophic brachiopods are also more stable if the valves were equi-biconvex, cither compressed (biplanate) or inflated, provided that the specimen was propped up on its hingeline with a lateral margin upcurrent. Catacline interareas afforded greater stability than procline, apsacline or orthocline interareas when the valves were perpendicular to the substrate. Accentuated plication reduced shell stability. Alate biconvex forms were often more stable if the valves were vertical to the substrate rather than reclining. Alate biconvex geometries are more stable than nonstrophic and strophic biconvex forms if the commissural plane is perpendicular to the substrate and parallel to the current. Broad catacline interareas increased stability in all orientations. Plano-convex and dorsi-biconvex alate forms are usually less stable than equi-biconvex or ventri-biconvex alate geometries. Concavo-convex geometries are stable in all orientations except with the valves nearly vertical to the substrate and parallel to the current. Spines greatly retard sediment-scour and maintain concavo-convex specimens in orientations with the valves elevated above the substrates. D Brachiopoda, shell shape, hydrodynamic stability.     

Intermediary metabolism and shell growth in the brachiopod Terebratalia transversa

Juvenile Terebratalia transversa (Brachiopoda) metabolize carbohydrates in the anterior-most marginal mantle at a rate of 0.46 μM glucose/g/hr (in vitro incubation of mantle in C¹⁴-glucose in a carrying medium of 10^-3 M non-radioactive glucose). The rate declines to 0.18μM glucose/g/hr in full-grown specimens. Carbohydrate metabolism in the marginal (anterior-most) mantle averages approximately 3.7 times greater than metabolism in (a portion of the ‘posterior’) mantle situated between the coelomic canals and the marginal mantle. This ratio remains constant in specimens of all sizes (i.e. an ontogenetic trend in the ratio is absent at p≤ 0.05). Organic acids are not detectable within the mantle (HPLC techniques) even after simulated anoxia (N₂ bubbling during mantle incubation). Glucose metabolism in vitro declines in both the marginal and ‘posterior’ mantles during anoxia and the metabolic ratio between marginal/‘posterior’ mantles becomes 1/1. We found no difference (at p≤ 0.05) in mean metabolic activity or in sue-related metabolic trends among populations from depths ranging between mean sea level and 70 m. However, the activity within the ‘posterior’ mantle was more variable in specimens from 70 m than in those from shallower habitats (10 m - mean sea level). The size of the specimens analyzed was most variable in the groups obtained from the shallowest habitats and least variable at 70 m depth. Our results may help define the energetics of fossil as well as living brachiopod shell growth. Brachiopod shell growth is known to be very slow relative to that of bivalves and our results indicate that this is a result of the animals' slow metabolism. The inflation of the valves in T. transversa is, in part, a function of the high ratio of intermediary metabolism in the marginal vs‘posterior’ mantle (i.e. parallels the relative growth rates at the shell margin vs‘posterior’ areas). We found that the bivalve, Chlamys hastata, which is commonly associated with T. transversa, has a lower ratio of metabolic activities in the ventral/dorsal mantle areas than the brachiopod has in the anterior/posterior. The difference produces a flatter shell in the bivalve in accord with allometric principles. The higher metabolic rate in the marginal vs‘posterior’ brachiopod mantle and its more pronounced decline with anaerobiosis is reflected in the greater definition of growth increments in the outer shell layer. Our results do not support recent generalizations that correlate shell thickness of a wide variety of invertebrates inversely with metabolic rate. Growth rate as determined from width of shell growth increments is a better index of metabolic rate. Although the genetic basis of glucose metabolism is unknown, the observed metabolic variability is consistent with suggestions that populations of marine organisms living in stable offshore environments are genetically more variable but morphologically more uniform than populations from shallow water. Furthermore, our results support suggestions that bivalved molluscs and brachiopods are very different metabolically, but the data are neutral with respect to theories of competitive exclusion of the two taxa throughout geologic history.     

Supraordinal brachiopod classification

Three controversial problems of brachiopod supraordinal classification are discussed: the position of brachiopods in the classification of Metazoa, their classification at phylum and class level, and the classification of the articulate brachiopods. The position of brachiopods in the system of Metazoa remains uncertain. There are no strong reasons for changing the traditional division of the phylum Brachiopoda into the classes Inarticulata and Articulata. The class Articulata is divided into the subclasses Orthata, Strophomenata, Spiriferata, and Terebratulata.     

Discovery of ventrally directed spiralia in a Permian spiriferellid brachiopod and implications for its feeding system

Spiralia are lophophore‐supporting, coiled internal structures developed in some extinct brachiopods. In spite of considerable variations in their orientation, the spiralia of most spiriferide and spiriferinide taxa are known to be laterally directed. Recent studies have shown that these brachiopods consistently have a median inhalant and lateral exhalant feeding system. Here, we report a Permian spiriferellid brachiopod fossil (Spiriferella protodraschei) bearing ventrally directed spiralia in its interior. Using the serial sections of the specimen, we have reconstructed the detailed morphology and orientation of the spiralia. Each spiralium in the specimen does not show the apically tapering pattern supposedly universal in all the known types of spiralia: instead it maintains a similar diameter even at its last whorl. The spiralia appear to have directly developed from strong and anteriorly extended crura, consisting of ten whorls in one side and 13 whorls in the other side. As the morphology and orientation of spiralia are immediately associated with the arrangement of spirolophous lophophore within the mantle cavity, the extraordinary orientation and form of the spiralia indicate that this brachiopod likely had developed a considerably modified feeding pattern with respect to most other spirolophous brachiopods. It is postulated that the inhalant/exhalant current circulation of the species (and its descendants) would be considerably different from that of other spiriferide taxa. In particular, the combination of the vertically oriented life posture (free‐lying with thickened ventral apex bottom) and ventrally directed spiralia resembles both fossil atrypide and modern rhynchonellide brachiopods in the orientation of spirolophe, suggesting that some spiriferellid brachiopods may have developed a lateral inhalant/median exhalant feeding current system. A few spiriferide and spiriferinide brachiopod taxa with a weakly transverse but strongly convex ventral valve are noted to exhibit similar modifications in their spiralia, possibly due to the spatial limitation of their mantle cavities.     

Neuromuscular development in Novocrania anomala: evidence for the presence of serotonin and a spiralian-like apical organ in lecithotrophic brachiopod larvae

SUMMARY The phylogenetic position of Brachiopoda remains unsettled, and only few recent data on brachiopod organogenesis are currently available. In order to contribute data to questions concerning brachiopod ontogeny and evolution we investigated nervous and muscle system development in the craniiform (inarticulate) brachiopod Novocrania anomala . Larvae of this species are lecithotrophic and have a bilobed body with three pairs of dorsal setal bundles that emerge from the posterior lobe. Fully developed larvae exhibit a network of setae pouch muscles as well as medioventral longitudinal and transversal muscles. After settlement, the anterior and posterior adductor muscles and delicate mantle retractor muscles begin to form. Comparison of the larval muscular system of Novocrania anomala with that of rhynchonelliform (articulate) brachiopod larvae shows that the former has a much simpler muscular organization. The first signal of serotonin-like immunoreactivity appears in fully developed Novocrania anomala larvae, which have an apical organ that consists of four flask-shaped cells and two ventral neurites. These ventral neurites do not stain positively for the axonal marker α-tubulin in the larval stages. In the juveniles, the nervous system stained by α-tubulin is characterized by two ventral neurite bundles with three commissures. Our data are the first direct proof for the presence of an immunoreactive neurotransmitter in lecithotrophic brachiopod larvae and demonstrate the existence of flask-shaped serotonergic cells in the brachiopod larval apical organ, thus significantly increasing the probability that this cell type was part of the bauplan of the larvae of the last common lophotrochozoan ancestor.     

Regional specification during embryogenesis in the craniiform brachiopod Crania anomala

A fate map has been constructed for the embryo of Crania. The animal half of the egg forms the ectodermal epithelium of the larva's apical lobe. The vegetal half of the egg forms endoderm, mesoderm, and the ectoderm of the mantle lobe. The vegetal pole is the site of gastrulation; this site becomes the posterior ventral region of the mantle lobe of the larva. The plane of the first cleavage goes through the animal-vegetal axis of the egg; it bears no relationship to the future plane of bilateral symmetry of the larva. The timing of regional specification was examined by isolating animal, vegetal, or meridional halves from oocytes, eggs, or embryos from prior to germinal vesicle breakdown through gastrulation. Animal halves isolated from oocytes formed either the epithelium of the apical lobe or a larva with all three germ layers. Animal halves isolated from unfertilized eggs and eight-cell embryos formed only apical lobe epithelium. Beginning at the blastula stage, animal halves formed mantle in addition to apical lobe epithelium. In animal halves isolated after gastrulation, the mantle lobe was always truncated. Vegetal halves isolated at all stages prior to gastrulation gastrulated and formed apical and mantle lobes with endoderm and mesoderm; however, the relative size of the apical lobe that formed decreased substantially when vegetal halves were isolated at later developmental stages. When meridional halves were isolated from unfertilized eggs and two- to four-cell embryos, both halves frequently formed normally proportioned larvae. Beginning at the blastula stage, a number of pairs frequently had a member that lacked dorsal setae on its mantle lobe while the other member of the pair formed setae, indicating that the dorsoventral axis had been set up. The process of regional specification in Crania is compared to those of Discinisca and Glottidia in the brachiopod subphylum Linguliformea and Phoronis in the phylum Phoronida.     

Pennsylvanian sea level cycles, nutrient availability and brachiopod paleoecology

As with modern organisms, the spatial and temporal distribution of fossil communities was controlled by both the physical setting in which the organisms lived and by the organisms' physiology and interactions. By studying the sedimentological and geochemical context of fossil communities, it is possible to assess the relative importance of the physical setting and the organisms' physiology. Comparison of Pennsylvanian brachiopod associations with changing sedimentological context (water depth/facies) and nutrient availability indicates that body size is a function of water depth and nutrient availability for most spire-bearing (athyridids and spiriferids) brachiopods but rarely for productid brachiopods. Spire-bearing brachiopods dominate the associations in high-nutrient settings, and productid brachiopods dominate the associations in low-nutrient settings. This difference suggests that physiological differences between brachiopod orders, such as lophophore filtering efficiency, play an important role in controlling their distribution.     

Regional specification during embryogenesis in the inarticulate brachiopod Discinisca.

The process of embryogenesis is described for the inarticulate brachiopod Discinisca strigata of the family Discinidae. A fate map has been constructed for the early embryo. The animal half of the egg forms the dorsal ectoderm of the apical and mantle lobes. The vegetal half forms mesoderm and endoderm and is the site of gastrulation; it also forms the ectoderm of the ventral regions of the apical and mantle lobes of the larva. The plane of the first cleavage goes through the animal-vegetal axis of the egg along the future plane of bilateral symmetry of the larva. The timing of regional specification in these embryos was examined by isolating animal, vegetal, or lateral regions at different times from the 2-cell stage through gastrulation. Animal halves isolated at the 8-cell and blastula stages formed an epithelial vesicle and did not gastrulate. When these halves were isolated from blastulae they formed the cell types typical of apical and mantle lobes. Vegetal halves isolated at all stages gastrulated and formed a more or less normal larva; the only defect these larvae had was the lack of an apical tuft, which normally forms from cells at the animal pole of the embryo. When lateral isolates were created at all developmental stages, these halves gastrulated. Cuts which separated presumptive anterior and posterior regions generated isolates at the 4-cell and blastula stages that formed essentially normal larvae; however, at the midgastrula stage these halves formed primarily anterior or posterior structures indicating that regional specification had taken place along the anterior-posterior axis. The plane of the first cleavage, which predicts the plane of bilateral symmetry, can be shifted by either changing the cleavage pattern that generates the bilateral 16-cell blastomere configuration or by isolating embryo halves prior to, or during, the 16-cell stage. These results indicate that while the plane of the first cleavage predicts the axis of bilateral symmetry, the axis is not established until the fourth cleavage. The development of Discinisca is compared to development in the inarticulate brachiopod Glottidia of the family Lingulidae and to Phoronis in the phylum Phoronida.     

Functional disposition of the lophophore in living Brachiopoda

Emig, C. C. 1992 07 15: Functional disposition of the lophophore in living Brachiopoda.
The shape and disposition of adult brachiopod lophophores relate to in- and excurrent apertures. to the internal water irrigation system, to shell orientation at substratum and to near-bottom currents. The arrangement of the mantle canals and gonads of different lophophores are induccd by water circulation. The trocholophe (2% of living species) is considered as a plesiomorphic character which represents the basic plan of the lophophore, shared by all Lophophorata. Three different types of schizolophe (10%) are represented in terebratuloids, thecidioids and discinids. The spirolophe (19%), characteristic of rhynchonellides and most inarticulate brachiopods, except the schizolophe Pelagodiscus , has evolved divergently into specific arrangements of the mantle canals and gonads. The zygo-plectolophe (67%) is characteristic of most Terebratulida. The ptycholophe (2%) probably evolved independently in Megathlris and the thecidioids. The mesolophe, known in the fossil chonetdceans, is considered to be a primitive zygo-plectolophe. The median brachiopod sulcus increases the efficiency of the excurrent system and is considered as an evolved character but a homoplasy within the brachiopods. The characteristics of Recent lophophore types have to be taken into account when reconstructing the lophophore in fossil forms. Brachiopoda, Lophophorata, lophophore, water system, orientation, evolution .     

The periesophageal celom of the articulate brachiopod Hemithyris psittacea (Rhynchonelliformea,Brachiopoda)

The celomic system of the articulate brachiopod Hemithyris psittacea is composed of the perivisceral cavity, the canal system of the lophophore, and the periesophageal celom. We study the microscopic anatomy and ultrastructure of the periesophageal celom using scanning and transmission electron microscopy. The periesophageal celom surrounds the esophagus, is isolated from the perivisceral cavity, and is divided by septa. The lining of the periesophageal celom includes two types of cells, epithelial cells and myoepithelial cells, both are monociliary. Some epithelial cells have long processes extending along the basal lamina, suggesting that these cells might function as podocytes. The myoepithelial cells have basal myofilaments and may be overlapped by the apical processes of the adjacent epithelial cells. The periesophageal celom forms protrusions that penetrate the extracellular matrix (ECM) of the body wall above the mouth and the ECM that surrounds the esophagus. The canals of the esophageal ECM form a complicated system. The celomic lining of the external circumferential canals consists of the epithelial cells and the podocyte‐like cells. The deepest canals lack a lumen; they are filled with the muscle cells surrounded by basal lamina. These branched canals might perform dual functions. First, they increase the surface area and might therefore facilitate ultrafiltration through the podocyte‐like cells. Second, the deepest canals form the thickened muscle wall of the esophagus and could be necessary for antiperistalsis of the gut. J. Morphol., 2011. © 2010 Wiley‐Liss, Inc.     

Blood cells and coelomocytes of the inarticulate brachiopod Lingula anatina

Three cell types are described from the coelomic cavity of the pedicle of the brachiopod Lingula anatina . Erythrocytes are abundant in the blood vessels of the mantle and also occur, in reduced numbers, in the pedicle. Phagocytic amoebocytes, characterized by a variable number of electrondense, homogeneous granules are common in the coelomic fluid of the pedicle. The enigmatic spindle bodies described by earlier authors constitute the most common cell type encountered in the pedicle coelom of aquarium-maintained specimens. The origin of spindle bodies from muscle cells is suggested.     

The nature of siliceous mosaics forming the first shell of the brachiopod Discinisca

The juvenile shell of the brachiopod Discinisca consists of a mosaic of micrometer-sized siliceous tablets embedded in a chitinous substrate. The first-formed tablets are secreted on glycocalyx by a newly differentiated collective of outer epithelial cells. They are mainly rhombic but may also be ellipsoidal, discoidal, or deformed and sporadically overlap one another. On the surrounding juvenile shell, secreted by an incipient outer mantle lobe, the tablets are nearly all perfect rhombic plates in rhombic arrays. Their constant size, arrangement, and centripetal crystallization suggest intracellular assembly. The tablets, which are normally bilamellar, consist of discrete aggregates of crystalline spherules of silica in rhombic arrays within an organic matrix of fibrous protein and, presumably, a soluble polysaccharide(s). Mosaic secretion ceases at about the time when juveniles settle on the sea bed, which more or less coincides with the secretion of a ring of lamellae around the mosaic, induced by rapid advances and retractions of the outer mantle lobe prior to deposition of the organophosphatic mature shell. Energy dispersion X-ray analyses of pelagic and newly settled juveniles show that phosphatic secretion, even in the site of the first-formed outer epithelial collective, does not begin until all siliceous secretion has ceased.     

Changhsingian brachiopod communities along a marine depth gradient in South China and their ecological significance in the end-Permian mass extinction

Diversity indices (dominance and evenness) and ecological spatial structure (lifestyles and relative abundances) are important features of Changhsingian brachiopod communities prior to the end-Permian mass extinction (EPME) and could predict temporal and spatial extinction patterns during the EPME. In South China, Changhsingian brachiopod communities show higher diversity than other contemporaneous brachiopod communities in the world and have been reported from a variety of sedimentary environments. In this paper, brachiopods from 18 sections in South China were selected to divide communities and compare their ecological structure. Based on the results of network analysis, cluster analysis and quantitative data from the selected sections, we show that Changhsingian brachiopod communities in South China can be categorized into three assemblages along a marine depth gradient: the Neochonetes–Fusichonetes–Paryphella Assemblage from the shallow-water clastic-rock facies, Spinomarginifera–Peltichia–Oldhamina Assemblage from the shallow-water carbonate platform facies and Fusichonetes–Crurithyris Assemblage from the deep-water siliciclastic intracontinental basin facies. Compared with communities from carbonate platform facies, the communities from siliciclastic facies were characterized by high dominance, low evenness and low lifestyle diversity, which might be important biotic factors leading to earlier extinctions. After the extinction began in all environments, the whole earliest Triassic brachiopod community was first dominated by Fusichonetes and then by Crurithyris. These patterns of domination and replacement could be explained by morphological and ecological advantages. The domination of these two genera, which were already adapted to the oxygen and food-limited deep-water habitat, indicates that the cooler deep-water environment might have been a relatively less stressed habitat after the beginning of the EPME. This suggests that global warming might be the main trigger among the previously proposed synergistic environmental stresses, while anoxia might not, at least for the beginning of EPME.     

The Early Silurian brachiopod Eocoelia from the Hudson Bay Basin, Canada

Eocoelia akimiskii sp. nov. from the Lower Silurian (upper Telychian) Attawapiskat Formation of Akimiski Island, James Bay, Nunavut, is the first and oldest Eocoelia to be described from the Hudson Bay Basin, one of three largest inland basins of North America. The new species lacks dental plates, dental cavities, and marginal deflection or lip, which indicates a post-Aeronian form of the well-known Eocoelia lineage. In rib numbers, Eocoelia akimiskii falls between E. curtisi and E. sulcata , being closer to E. sulcata . In rib strength, however, the new species is more closely allied to E. curtisi . Although Eocoelia is currently assigned to the Rhynchonellida because of its lack of spiralia, several features (particularly the lack of a septalium, the presence of a unique notothyrial platform and cardinal process, and dense, free-hanging fibrous growth frills) of the genus and other leptocoeliids are distinctly atypical of the rhynchonellides. The new species occurs in an inter-reef, shelly packstone facies within the Attawapiskat Formation, which is characterized by coral-stromatoporoid reefs with abundant, diverse, reef-dwelling brachiopods and other shelly organisms. The close association of Eocoelia akimiskii with the Attawapiskat reefs supports a shallow subtidal (BA2) setting generally assigned to the Eocoelia Community. The reefs themselves, however, host an extremely abundant brachiopod fauna dominated by Pentameroides , Trimerella , Septatrypa , and Gypidula . Four species of Clorinda are also common elements of the reef-dwelling brachiopods. This demonstrates that the concept of the classic Early Silurian level-bottom brachiopod communities cannot be directly applied to reef-dwelling brachiopod communities.     

An ecologically mixed brachiopod fauna from Changhsingian deep-water basin of South China: consequence of end-Permian global warming

The Late Permian Shaiwa Group of the Ziyun area of Guizhou, South China is a deep-water facies succession characterized by deep-water assemblages of pelagic radiolarians, foraminifers, bivalves, ammonoids and brachiopods. Here we report 20 brachiopod species in 18 genera from the uppermost Shaiwa Group. This brachiopod fauna is latest Changhsingian in age and dominated by productides. The palaeoecologic and taphonomic analysis reveals that the brachiopod fauna is preserved in situ. The attachment modes and substratum preference demonstrate that the Shaiwa brachiopod fauna comprises admixed elements of deep-water and shallow-water assemblages. The presence of the shallow-water brachiopods in the Shaiwa faunas indicates the involuntary settlement of shallow-water brachiopods. The stressed ecologic pressure, triggered by warming surface waters, restricted ecospace and short food sources, may have forced some shallow-water elements to move to hospitable deep-water settings and others to modify their habiting behaviours and exploit new ecospace in deep-water environments. We infer that the end-Permian global warming and subsequent transgression event may have accounted for the stressed environmental pressure in the shallow-water communities prior to the end-Permian mass extinction.     

Congruence and conflict: case studies of morphotaxonomy versus rDNA gene tree phylogeny among articulate brachiopods (Brachiopoda: Rhynchonelliformea), with description of a new genus

We present five case studies among articulate (rhynchonelliform) brachiopods, i.e. of Rhynchonellida, Cancellothyridoidea, Terebratuloidea, Dyscolioidea, Laqueoidea, and various terebratulids with modified long‐loops, in an attempt to illustrate and better understand congruence and conflict between morpho‐classification and rDNA‐based molecular clade structure, having been prompted to address these issues by difficulties encountered when describing the newly collected brachiopod, E biscothyris bellonensis gen. et sp. nov. The five studies reveal dramatic conflict in the Rhynchonellida and Terebratuloidea/Dyscolioidea, good congruence in the Cancellothyridoidea and Laqueoidea, and fair congruence (albeit with weak phylogenetic signal) in the long‐looped terebratulids. We suggest that the leading cause of the observed conflict lies in the use of inadequately specific morphological characters and morpho‐classification. Phylogenetic systematic (cladistic) analyses of Rhynchonellida also conflict markedly with the rDNA gene tree, leading us to recognize that such analyses are not only conceptually circular (using morphological characters to assess a morphological classification) but also to propose that they are biased by the act of classification that necessarily precedes the identification of putatively homologous characters; when the prior classification does not reflect evolutionary history, phylogenetic analysis will do likewise. In addition, we propose that the brachiopod community has overlooked the significance of two sources of morphological homoplasy affecting brachiopod systematics: (1) the loss of co‐adapted genomic complexes caused by mass extinctions at the end of the Permian; and (2) the pervasive consequences of developmental integration and constraint resulting from the integrated roles of the outer mantle epithelium in shell deposition and growth that underly the determination of form and the shell‐based classification. © 2015 The Linnean Society of London     

贵州安顺募役剖面晚二叠世腕足动物群落及其生态特征*

本文对贵州募役剖面长兴期腕足动物群进行了系统分类鉴定和古生态分析,发现其与前人所报道的华南同时期腕足动物群存在着显著的差别,以Spiriferellina为绝对优势属,包含少量华南长兴期的常见分子,如Fusichonetes、Araxathyris和Peltichia等。募役剖面的腕足类化石主要产自碎屑岩层位,与华南同期碎屑岩相剖面的腕足动物群落相比,该动物群中的优势分子个体较大、壳体较厚且发育较粗大的壳疹,这可能与长兴期募役剖面动物群所处的特殊栖息地环境(浅水碳酸盐岩台地与深水硅质碎屑岩盆地之间的过渡地带)有关。二叠纪末期大规模的火山作用导致水体中碎屑物质含量增加,影响腕足类的滤食效率,而募役剖面清澈、水动力弱的水体环境,为滤食性的腕足动物提供了有利的环境条件。最后,通过与华南长兴期不同沉积相区腕足动物群落(六枝剖面、稻堆山剖面、中寨剖面、新民剖面、马家山剖面和仁村坪剖面群落)的对比分析,发现募役剖面腕足动物群与毗邻的六枝剖面腕足动物群在属级组成上的相似度较高,且过渡岩相栖息地环境下的腕足动物群在生物灭绝事件前也呈现出高优势度、低均匀度的群落结构特征,指示海洋底栖生物群落已经先于二叠纪末期生物集群灭绝事件出现了早期危机信号。