Pelmatozoan arms from the Middle Cambrian of Australia: bridging the gap between brachioles and brachials?

The early Middle Cambrian Monastery Creek Phosphorite (Beetle Creek Formation, Queensland, Australia) contains an assemblage of disarticulated echinoderm ossicles that are exquisitely preserved. Amongst this material we recognize pelmatozoan brachials, radials, basals and holomeric columnals. Although we cannot reconstruct the complete animal with precision, these elements represent the oldest known pelmatozoan with crinoid-like appendages. Key elements include isotomously to heterotomously branched uniserial appendage plates with a tripartite adoral food groove, a longitudinal central canal interpreted as housing entoneural nerve, and differentiated articulation facets. There are also epispire-bearing radials bearing one to four arm insertion-facets, each one pierced by a central neural canal. These canals run internal towards the oral area beneath the external food groove. Co-occuring material includes single truncated cone-shaped basals and holomeric columnals, both with a similar articulation pattern, and irregular, epispire-bearing thecal plates. This mosaic of crinozoan (uniserial isotomous to heterotomous arms with neural canal), blastozoan (epispire-bearing thecal plates, appendage leading to oral thecal food groove without direct connection with body cavity) and apomorphic characters (circumoral instead of basal entoneural plexus) is unexpected and demonstrates that crinoid-like pelmatozoans with uniserial, branched arms appeared significantly earlier than the Tremadocian, when the first articulated crinoid skeletons are found. It also raises questions about the polyphyletic appearance of feeding appendages among pelmatozoan echinoderms.     

Host-specific pit-forming epizoans on Silurian crinoids

Brett, Carlton E. 197807 15: Host-specific pit-forming epizoans on Silurian crinoids. Lethaia , Vol. 11. pp. 217–232. Oslo. ISSN 0024–1164.
Circular-parabolic pits occur commonly on the endoskeletal remains of certain Paleozoic crinoids. Detailed study of several hundred specimens, representing about 30 pelmatozoan species from the Upper Silurian Rochester Shale of New York and Ontario, reveals that such pits occur exclusively in seven species of crinoids. Furthermore, there are consistent differences in the morphology and orientation of holes occurring on the different crinoid species. This suggests that distinct epizoan species settled selectively on given hosts. The relationship between the hole-producing epizoans and crinoid hosts is inferred to have been a form of dependent commensalism. Preliminary surveys of other Paleozoic crinoid assemblages reveal similar host-selectivity by pit-producing epizoans. Crinoidepizoan pairs apparently co-evolved through considerable spans of geologic time as related genera and species of different ages, from Silurian to Pennsylvanian, exhibit similar pits.     

Cambrian spiral-plated echinoderms from Gondwana reveal the earliest pentaradial body plan

Echinoderms are unique among animal phyla in having a pentaradial body plan, and their fossil record provides critical data on how this novel organization came about by revealing intermediate stages. Here, we report a spiral-plated animal from the early Cambrian of Morocco that is the most primitive pentaradial echinoderm yet discovered. It is intermediate between helicoplacoids (a bizarre group of spiral-bodied echinoderms) and crown-group pentaradiate echinoderms. By filling an important gap, this fossil reveals the common pattern that underpins the body plans of the two major echinoderm clades (pelmatozoans and eleutherozoans), showing that differential growth played an important role in their divergence. It also adds to the striking disparity of novel body plans appearing in the Cambrian explosion.     

Colonization of substrates: Vendian sedentary benthos

Studies of fossils sampled from a single fossiliferous layer (monotopic series) and a summary of previous data on taphonomy and morphology of Vendian macroorganisms suggest that most strategies of attachment of sedentary benthos were formed in the Vendian. These included: (1) free living with possible organic gluing; (2) fixation on the substrate using sucker-like structures and various incrustations; (3) fixation of the basal parts by shallow submergence in the sediment; (4) anchoring into the sediment with discoidal and rhizoid-like holdfasts; and (5) partial submergence and infaunal lifestyle. The most widespread fixation in the Vendian was the attachment by discoid organs, which can be interpreted as symbiotrophic structures. Symbiotrophy in Vendian organisms submerged in the anoxic sediment can apparently be supported by indirect evidence: structural diversity of basal organs, their complex morphology, large area of contact with substrate, unlimited isometric growth, predominant fossilization of attachment discs; inconsistent systematization of organisms.     

The root of the problem: palaeoecology of distinctive crinoid attachment structures from the Silurian (Wenlock) of Gotland

The holdfast (attachment structure) is the most understudied aspect of the palaeoecology of the endoskeleton of fossil crinoids. A new collection of well-preserved holdfasts from a recently reopened quarry at Hunninge, Gotland, in Homerian (upper Wenlock) strata includes several morphologies. The most common are terminal dendritic radicular holdfasts (TDRHs) that may be derived from the cladid Ennallocrinus d'Orbigny. These have a consistent morphology of five, equally spaced, long radices that spread across the sea floor. These crinoids were gregarious, and TDRHs in a group commonly show the same radice orientations. The radices have a large axial canal compared with those of modern crinoids; each included, at least, nervous tissues. Taken together, these features suggest that, apart from attachment, these distinctive TDRHs may have served a sensory function. Other holdfasts in this assemblage also show monospecific aggregations, perhaps suggesting biochemical attraction such as that shown by certain other sessile invertebrates such as barnacles.     

THE EARLY RADIATION AND PHYLOGENY OF ECHINODERMS

1. Living echinoderms are characterized by an extensive water vascular system developed from the larval left hydrocoel, a complex, multi-plated endoskeleton with stereom structure, and pentamery. Fossil evidence shows that stereom evolved before pentamery, but both were acquired during the Lower Cambrian. 2. Cladistic analysis of Lower Cambrian genera reveals very few characters in common between carpoids and true echinoderms, and that the split between them was the first fundamental evolutionary dichotomy within the Dexiothetica. 3. Helicoplacoids are stem group echinoderms with spiral plating and three ambulacra arranged radially around a lateral mouth. They are the most primitive echinoderms and the first to show a radial arrangement of the water vascular and ambulacral systems. Unlike later echinoderms, their skeleton shows no dorsal/ventral (aboral/oral) differentiation. They were probably sedentary suspension feeders. 4. Camptostroma is the most primitive known pentaradiate echinoderm and, in our view, possibly a common ancestor of all living groups. It had a short conical dorsal (aboral) surface with imbricate plating, a ridged lateral wall and a slightly domed ventral (oral) surface with five curved ambulacra in a 2-1-2 arrangement inherited from the triradiate pattern of the helicoplacoids. Interambulacral areas bore epispires and the CD interambulacrum contained the anus, hydropore and/or gonopore. All parts of the theca had plates in at least two layers. 5. All other echinoderms belong to one of two monophyletic subphyla, the Pelmatozoa and the Eleutherozoa. 6. Stromatocystites is the earliest known eleutherozoan and differs from Camptostroma in having a test with only one layer of plates and having lost the dorsal elongation. In Stromatocystites the dorsal surface is flat and the plating tesselate. Stromatocystites was an unattached, low-level suspension feeder. 7. The lepidocystoids are the earliest known pelmatozoans. They differ from Camptostroma in having an attached dorsal stalk which retained the primitive imbricate plating, and by developing erect feeding structures along the ambulacra. In Kinzercystis, the ambulacra are confined to the thecal surface and erect, biserial brachioles arise alternately on either side. Lepidocystis has a similar arrangement except that, the distal part of each ambulacrum extends beyond the edge of the theca as a free arm. 8. Pelmatozoans diverged more or less immediately into crinoids, with multiple free arms composed of uniserial plates, and cystoids sensu lato, which retained brachioles. Gogia (Lower to Middle Cambrian) is the most primitive known cystoid and differs from Kinzercystis principally in having all plating tesselate, while Echmatocrinus (Middle Cambrian) is the most primitive known crinoid and differs from Lepidocystis in lacking brachioles and in having more than five free arms with uniserial plates. 9. Post Lower Cambrian differentiation of pelmatozoan groups proceeded rapidly, exploiting the primitive suspension-feeding mode of life. Maximum morphological diversity was reached in the Ordovician, but thereafter crinoids progressively displaced cystoid groups and reached their peak diversity during the Carboniferous. The eleutherozoans were slower to diversify, but by the Arenig the earliest ‘sea-stars’ (in reality, advanced members of the eleutherozoan stem group) had reversed their living orientation and had begun to exploit a deposit-feeding mode of life. These in turn led to the ophiuroids, echinoids and holothuroids. 10. The basic echinoderm ambulacrum was already present in the helicoplacoids. It had biserial, alternate flooring plates and complexly plated sheets of cover plates on either side. The radial water vessel lay in the floor of the ambulacrum, external to the body cavity, and gave rise ventrally to short, lateral branches (fore-runners of tube feet) that were used to open the cover plate sheets, and dorsally was connected to internal compensation sacs which acted as fluid reservoirs (and were preadapted for a role in gaseous exchange). Plating on the cover plate sheets was organized and reflected the positions of the lateral branches from the radial water vessel. In Camptostroma, the cover plate sheets had biserially aligned rows of cover plates associated with the lateral branches. 11. Brachioles arose by extension of the lateral branches of the radial water vessel and associated serially aligned cover plates found in Camptostroma. They bear a single alternate series of cover plates. In Lepidocystis the ambulacra extended beyond the edge of the oral surface as true arms. Brachial plates of arms are homologues of primary ambulacral flooring plates, and arms bear multiple series of cover plates. Uniserial ambulacral plating is a derived condition and evolved independently in crinoids, paracrinoids and isorophid edrioasteroids. Pinnules in crinoids arose independently in inadunates and camerates by a progressively more unequal branching of the arms. Thus all parts of the subvective system in crinoids are internally homologous, whereas in cystoids, brachioles and arms (or ambulacra) are not homologous structures. 12. The position of the hydropore is the best reference point in orientating echinoderms. Carpenter's system of identifying ambulacra by letters, arranged clock-wise in oral view with the A ambulacrum opposite the hydropore, is consistent in all echinoderm classes. In all Lower Cambrian pentaradiate echinoderms the anus, gonopore and hydropore lie in the CD interambulacrum and this is accepted as the primitive arrangement. In helicoplacoids we tentatively suggest that the A ambulacrum spiralled down from the mouth while the two ambulacra that spiralled up represent the B + C and D + E ambulacra combined. 13. The pelmatozoan stem arose from a polyplated stalk, via a meric stem to a true column with holomeric (single piece) columnals. This happened independently in the crinoids and the cystoids. 14. Our analysis of echinoderm phylogeny leads us to recommend the following changes to the higher level classification of echinoderms: The phylum Echinodermata includes only those groups with radial symmetry superimposed upon a fundamental larval asymmetry. It has a stem group that contains the triradiate helicoplacoids and a crown group to which all other (pentaradiate) echinoderms belong. The crown group contains two monophyletic subphyla, the Pelmatozoa and the Eleutherozoa, and the Pelmatozoa contains two superclasses, the Crinoidea which are extant and the Cystoidea, which are extinct.     

Settling strategy of stalked echinoderms from the Kaili Biota (middle Cambrian), Guizhou Province,South China

A total of 124 articulated echinoderms, including Sinoeocrinus lui (n = 30), “S. globus” (n = 92), one indeterminate gogiid, and one indeterminate edrioasteroid, from the Kaili Formation (Middle Cambrian), eastern Guizhou, China was examined. Among them, 73% of gogiid echinoderms are preserved attached to skeletal substrates. Suitable skeletal substrates for gogiid holdfasts in the Kaili Biota are organophosphatic brachiopods; large trilobite fragments, including cranidia, free cheeks, thoracic segments, and pygidia; hyoliths; and Scenella shells. The high frequency of “S. globus” attached to organophosphatic brachiopods is due to the high richness of organophosphatic brachiopods rather than a host-specific association in the Kaili Biota. However, the possible discrimination against attachment to small trilobites, such as Pagetia, requires further investigation. Based on 11 hand samples, echinoderm population density (EPD) varies from 0.3 to 4.5 echinoderms per 50 grind points (0.06 to 0.96 echinoderms per cm²), depending on the percent coverage of skeletal debris. Crowding effect is evident when EPD is approximately 0.32 echinoderms per cm² and the skeletal coverage is less than 20%.     

Faunal distribution and colonization strategy in a Middle Ordovician hardground community

The attached fauna of one of the many hardgrounds from the Galena Group (Trentonian Substage) of the Upper Mississippi Valley is described. The fauna is composed of three principal elements, viz. (1) borers, including Cicatricula retiformis ichnogen. et ichnosp. nov., (2) pelmatozoans with encrusting holdfasts, and (3) bryozoans. Analysis of the distribution of members of each population on the hardground shows that most are strongly aggregated. The nature of, and reasons for, such aggregations are considered in the light of comparable Recent shallow-water marine populations. The community on this hardground, and those on other Galena Group hardgrounds, are immature. This is a consequence of frequent and damaging scour, which these organisms were poorly adapted to resist.     

Zooid orientation structures and water flow patterns in Paleozoic bryozoan colonies

Anstey, Robert L. 1981 12 15: Zooid orientation structures and water flow patterns in Paleozoic bryotoan colonies. Lethaia . vol. 14, pp. 287–302. Oslo. ISSN 0024–1164.
By means of direct physical evidence provided by zooecial orientation structures, active water flow systems in Paleozoic bryozoans are inferred to be variously centripetal, centrifugal, or basipetal. Monticules, previously assessed as excurrent water outlets, fall into three additional functional types: incurrent, bypassed, and funnel. In one species circular zoarial fenestrations served as excurrent water outlets. Water flow patterns are strongly correlated with zoarial growth form, which vanes in a general way with inferred habitat conditions in ancient environments. Monticular astogeny and phylogeny include a graded series of sizes, types, and functions. Analogy with zooidal polarities in extant stenolaemates suggests that colony bases and centripetal monticules in the Paleozoic orders were anally budded, but that erect branches and centrifugal monticules were orally budded, a character shared only by the freshwater Phylactolaemata. * Bryozoa, Stenolaemata, functional morphology, monticule function, hydrodynamics, feeding currents, Palaeozoic .     

Late Cambrian hard substrate communities from Montana/Wyoming: the oldest known hardground encrusters

Hardground surfaces from the Late Cambrian Snowy Range Formation in Montana/Wyoming are the oldest known non-reefal hard substrates exhibiting encrusting fossils. These surfaces range in age from Early Franconian to early Trempealeauan. Hardgrounds were developed on slightly hummocky to planar, truncated surfaces of glauconite-rich, carbonate, flat pebble conglomerates, which were deposited during episodes of storm scouring in shallow subtidal environments of the Montana/Wyoming shelf. Snowy Range hardgrounds are encrusted by a low diversity assemblage of fossils dominated by simple discoidal holdfasts of pelmatozoans, probably crinoids, and including small conical spongiomorph algae? and probable stromatolites. Macroborings (e.g. Trypanites) are notably absent from all hardground surfaces, although sharp-walled, vertical, cylindrical holes (borings?) occur in micrite clasts imbedded in certain flat pebble conglomerates. No evidence of faunal succession or microecologic partitioning of irregular surfaces was observed on these Cambrian hardgrounds.     

Dumbbell-shaped gogiid clusters: the oldest evidence of secondary tiering for stalked echinoderms

Among 381 specimens of Cambrian stalked echinoderms from eastern Guizhou, China examined, several slabs ( n  = 19) contain either dumbbell-shaped or v-shaped echinoderm clusters. Four slabs of Globoeocrinus globules Zhao, Parsley & Peng, 2008 from the middle-upper part (Cambrian Series 3 portion) of the Kaili Formation are prepared to reveal the attachment sites. Articulated gogiid echinoderms are reported to be attached to both sides of inarticulate (organophosphatic) brachiopods; thus, allowing me to interpret that the larvae of these gogiids were capable of attaching to live benthic brachiopods. This study documents the one of the earliest examples of echinoderms employing secondary tiering, which elevates an organism higher into the benthic boundary layer. Many of the gogiid echinoderm pairs attached to a live brachiopod are similar in size, indicating they were from a single larval spatfall.     

THE FINE STRUCTURE OF TWO UNUSUAL STALKED BACTERIA

Two strains of bacteria that produce slender appendages (pseudostalks) from their lateral surfaces were studied using the electron microscope. The pseudostalks were shown to be extensions of the cytoplasm and peripheral membranes of the cell proper. Both strains of bacteria produce holdfasts at the poles of the cells by the means of which attachment can take place. The pseudostalks are not involved in the attachment of cells. No specialized intracytoplasmic structures are present at the point of juncture of pseudostalk and cell. A discussion of the possible functions of the pseudostalks, based on the electron microscope findings, is presented.     

A hexamer origin of the echinoderms' five rays

Of the major deuterostome groups, the echinoderms with their multiple forms and complex development are arguably the most mysterious. Although larval echinoderms are bilaterally symmetric, the adult body seems to abandon the larval body plan and to develop independently a new structure with different symmetries. The prevalent pentamer structure, the asymmetry of Lovén's rule and the variable location of the periproct and madrepore present enormous difficulties in homologizing structures across the major clades, despite the excellent fossil record. This irregularity in body forms seems to place echinoderms outside the other deuterostomes. Here I propose that the predominant five-ray structure is derived from a hexamer structure that is grounded directly in the structure of the bilaterally symmetric larva. This hypothesis implies that the adult echinoderm body can be derived directly from the larval bilateral symmetry and thus firmly ranks even the adult echinoderms among the bilaterians. In order to test the hypothesis rigorously, a model is developed in which one ray is missing between rays IV-V (Lovén's schema) or rays C-D (Carpenter's schema). The model is used to make predictions, which are tested and verified for the process of metamorphosis and for the morphology of recent and fossil forms. The theory provides fundamental insight into the M-plane and the Ubisch', Lovén's, and Carpenter's planes and generalizes them for all echinoderms. The theory also makes robust predictions about the evolution of the pentamer structure and its developmental basis.     

Characterization of the Adhesive Holdfast of Marine and Freshwater Caulobacters

Caulobacters are prosthecate (stalked) bacteria that elaborate an attachment organelle called a holdfast at the tip of the cellular stalk. We examined the binding of lectins to the holdfasts of 16 marine Caulobacter strains and 10 freshwater species or strains by using a panel of fluorescein-conjugated lectins and fluorescence microscopy. The holdfasts of all the marine isolates bound to only wheat germ agglutinin (WGA) and other lectins that bind N-acetylglucosamine (GlcNac) residues. The freshwater caulobacters showed more variability in holdfast composition. Some bound only to WGA and comparable lectins as the marine strains did. Others bound additional or other lectins, and some did not bind to the lectins tested. The binding of WGA appeared to involve the regions of the holdfast involved with adhesion; a holdfast bound to WGA was significantly less adhesive to glass. Competition experiments with WGA-binding holdfasts and oligomers of GlcNac demonstrated that trimers of GlcNac (the preferred substrate for WGA binding) were more effective than dimers or monomers in preventing WGA binding to holdfasts, suggesting that stretches of contiguous GlcNac residues occur in the WGA-binding holdfasts. In addition, differences between freshwater and marine holdfasts in the strength of WGA binding were noted. The effect of a number of proteolytic and glycolytic enzymes on holdfast integrity was examined; the proteases had no effect for all caulobacters. None of the glycolytic enzymes had an effect on marine caulobacter holdfasts, but chitinase and lysozyme (both attack oligomers of GlcNac) disrupted the holdfasts of those freshwater caulobacters that bound WGA. Despite some similarity to chitin, holdfasts did not bind Calcofluor and no measurable effects on holdfast production were detectable after cell growth in the presence of diflubenzuron or polyoxin D, inhibitors of chitin synthesis in other systems. Finally, the holdfasts of all caulobacters bound to colloidal gold particles, without regard to the coating used to stabilize the gold particles. This binding was stronger or more specific than WGA binding; treatment with colloidal gold particles prevented WGA binding, but the reverse was not the case.     

Giant kelp vegetative propagation: Adventitious holdfast elements rejuvenate senescent individuals of the Macrocystis pyrifera “integrifolia” ecomorph

Recent findings on holdfast development in the giant kelp highlighted its key importance for Macrocystis vegetative propagation. We report here for the first time the development of adventitious holdfasts from Macrocystis stipes. Swellings emerge spontaneously from different areas of the stipes, especially in senescent or creeping individuals. After being manually fastened to solid substrata, these swellings elongated into haptera, which became strongly attached after 1 month. Within 4 months, new thalli increased in size and vitality, and developed reproductive fronds. Our results suggest the usage of these structures for auxiliary attachment techniques. These could act as a backup, when primary holdfasts are weak, and thus improve the survival rate of the giant kelp in natural beds.     

Diversity dynamics of echinoderms and evolution of marine communities

A compound analysis of two global paleontological databases (Sepkoski??s database (SDB) and The Paleobiology Database) allowed the recognition of a number of previously undescribed trends in the evolution of the phylum Echinodermata. Paleozoic echinoderms, dominated by sessile epibenthic filter feeders, played an important role in benthic communities, especially in the Ordovician and Carboniferous. Paleozoic echinoderms typically showed an increased rate of genus renewal, which significantly decreases in the Meso-Cenozoic. After the P-T crisis the echinoderms became dominated by motile taxa, while the role of infaunal forms increased. During the global turnover in the benthic communities at the K-T boundary, which was accompanied by a sharp increase in the mean alpha-diversity, many marine organisms became inhabitants of much richer (compared to the Mesozoic) communities. However, of all echinoderms, this trend is observed only in crinoids. In contrast to most large taxa, echinoderms do not show positive correlation between the duration of genera and alpha-diversity of communities, which included these genera. During the Phanerozoic the geographical distribution of echinoderms showed a sharp paleolatitudinal gradient, i.e., each period was characterized by one paleolatitudinal zone with the maximum diversity of echinoderms, and the diversity rapidly decreasing to the north and to the south of this zone. The zone of the maximum diversity of echinoderms, like of entire marine biota, during the Phanerozoic gradually moved from the tropics of the southern hemisphere to the middle latitudes of the northern hemisphere.     

Variations in holdfast attachment mechanics with developmental stage, substratum-type, season, and wave-exposure for the intertidal kelp species Hedophyllum sessile (C. Agardh) Setchell

Biomechanical models that describe physical and biological interactions on wave-exposed shores typically assume that a species' attachment properties are similar between seasons and sites. We tested this assumption using Hedophyllum sessile to investigate how macroalgal biomechanical attachment properties vary with developmental stage, substratum-type, season, and wave-exposure. Hedophyllum sessile is an intertidal kelp species that is able to survive in wave-exposed areas in the Northeast Pacific. For both juveniles and adults, holdfast attachment force and strength were measured at a wave-exposed and wave-protected site in Barkley Sound, British Columbia, Canada. Substratum and wave-exposure effects on attachment properties were tested in juvenile populations. Adult populations were sampled prior to (in July 1996) and after (in November 1996) a series of storms. Site and seasonal wave-exposure effects on attachment properties were tested in these adult populations. Comparisons to known attachment properties of other temperate macroalgal species were also made. Causes for these patterns are discussed but were not isolated in these studies. Juveniles' attachment properties differed on different substrata types and between wave-exposures, with the highest attachment forces and the most attached juveniles in articulated coralline algal turfs. Adult attachment is firm ( approximately 100 N), but relatively weak ( approximately 0.07 MNm(-2)). Adult attachment did not vary with site wave-exposure, but there was a shift within each site to more resistant holdfasts after a series of early winter storms. Seasonal increases in storm swells correlated to more thallus tattering and selected against large, loose holdfasts. The data presented here suggest that results from holdfast attachment field studies in one season cannot be extrapolated to another due to a complex set of dynamics. This is the first documentation of seasonal patterns in macroalgal attachment properties.     

Inflorescence morphology of some South American Anacardiaceae and the possible phylogenetic trends

Barfod, A. 1988. Inflorescence morphology of some South American Anacardiaceae and the possible phylogenetic trends. - Nord. J. Bot. 8: 3–11. Copenhagen. ISSN 0107–055X.
The inflorescences of 15 native and one introduced species of South American Anacardiaceae belonging to the genera Anacardium, Loxopterygium, Mangifera, Mauria, Mosquitoxylon, Schinus, Spondias, Tapirira and Toxicodendron are described according to the terminology of Troll and coworkers. The tribal divisions in the Anacardiaceae are supported by the inflorescence morphology. Tribe Spondia-deae has paniculate inflorescences whereas tribe Anacardieae and tribe Rhoeae both have thyrsoids. Toxicodendron is exceptional for the tribe Rhoeae having axillary panicles. Thyrosids are hypothesized derived from panicles by a two step process including neoteny and enriching by cymose branching.     

Phenotypic variability, deformations and post-traumatic reactions of the stem ofEncrinus cf.liliiformis Lamarck (Crinoidea) from the Middle Triassic of Qingyan, south-western China

Well-preserved stem remains ofEncrinus cf.liliiformis Lamarck 1801 from the Upper Anisian and lowest Ladinian (Middle Triassic) of Qingyan, Guizhou Province, south-western China, are described. The characteristic morphological features of the columnals, especially the sculptural characteristics of the articular facets, vary in the different parts of the stem. Numerous fine, short, peripheral crenulae and perilumina with a tendency to pentalobate shapes in the proximal stem region change to few long, coarse crenulae and circular perilumina in the distal stem part. The holdfasts exhibit various differentiations: the basic discoid type occurs on flat surfaces, encrusting holdfasts are characteristic of irregular substrates; juvenile holdfasts often show an irregular outline, adult ones have a more even, circular shape. Some special and unusual structures allow palaeoecological interpretations: Holdfasts and stems joined by callous growth probably result from the lack of suitable hard substrates for attachment on the soft seafloor; the larvae often had to settle very closely to each other on a relatively small substrate. After breakage of stem,Encrinus was able to continue living despite the traumatic loss of its basal fixation. Broken ends underwent skeletal regeneration. Some specimens strongly suggest that juvenile individuals sometimes even were able to re-attach secondarily, e.g. by coiling around stems of other crinoids and cementing to these by callous outgrowths.     

Attachment of the adhesive holdfast organelle to the cellular stalk of Caulobacter crescentus

Caulobacters attach to surfaces in the environment via their holdfasts, attachment organelles located at the base of the flagellum in swarmer cells and later at the end of the cellular stalk in the stalked cells which develop from the swarmer cells. There seems to be little specificity with respect to the types of surfaces to which holdfasts adhere. A notable exception is that the holdfast of one cell does not adhere to the cell surface of another caulobacter, except by joining holdfasts, typically forming "rosettes" of stalked cells. Thus, the localized adhesion of the holdfasts to the cells is in some way a specialized attachment. We investigated this holdfast-cell attachment by developing an adhesion screening assay and analyzing several mutants of Caulobacter crescentus CB2A selected to be defective in adhesion. One class of mutants made a normal holdfast by all available criteria, yet the attachment to the cell was very weak, such that the holdfast was readily shed. Another class of mutants made no holdfast at all, but when mixed with a wild-type strain, a mutant of this class participated in rosette formation. The mutant could also attach to the discarded holdfast produced by a shedding mutant. In addition, when rosettes composed of holdfast-defective and wild-type cells were examined, an increase in the number of holdfast-defective cells was correlated with a decrease in the ability of the holdfast material at the center of the rosette to bind colloidal gold particles. Gold particles are one type of surface to which holdfasts adhere well, suggesting that the stalk end and the colloidal gold particles occupy the same sites on the holdfast substance. Taken together, the data support the interpretation that there is a specialized attachment site for the holdfast at the base of the flagellum which later becomes the end of the stalk, but not a specialized region of the holdfast for attachment to this site. Also, attachment to the cell is accomplished by bond formations that occur not only at the time of holdfast production. Thus, we propose that the attachment of the holdfast to the cell is a true adhesion process and that the stalk tip and base of the flagellum must have compositions distinctly different from that of the remainder of the caulobacter cell surface.