Skimming the surface with Burgess Shale arthropod locomotion

The first arthropod trackways are described from the Middle Cambrian Burgess Shale Formation of Canada. Trace fossils, including trackways, provide a rich source of biological and ecological information, including direct evidence of behaviour not commonly available from body fossils alone. The discovery of large arthropod trackways is unique for Burgess Shale-type deposits. Trackway dimensions and the requisite number of limbs are matched with the body plan of a tegopeltid arthropod. Tegopelte, one of the rarest Burgess Shale animals, is over twice the size of all other benthic arthropods known from this locality, and only its sister taxon, Saperion, from the Lower Cambrian Chengjiang biota of China, approaches a similar size. Biomechanical trackway analysis demonstrates that tegopeltids were capable of rapidly skimming across the seafloor and, in conjunction with the identification of gut diverticulae in Tegopelte, supports previous hypotheses on the locomotory capabilities and carnivorous mode of life of such arthropods. The trackways occur in the oldest part (Kicking Horse Shale Member) of the Burgess Shale Formation, which is also known for its scarce assemblage of soft-bodied organisms, and indicate at least intermittent oxygenated bottom waters and low sedimentation rates.     

Sediments of the Middle Cambrian Burgess Shale, Canada

The Phyllopod Bed of the Burgess Shale, in which Walcott found the famous soft bodied fossils, consists of thin graded beds of calcareous siltstone and mud-stone, which are probably turbidites. The Burgess Shale was deposited on a reef front submarine fan, and the preservation of the fossils is probably due to rapid burial.     

FOSSIL DIAGENESIS IN THE BURGESS SHALE

Abstract:  Current models for the exceptional preservation of Burgess Shale fossils have focused on either the HF-extractable carbonaceous compressions or the mineral films identified by elemental mapping. BSEM, EDX and microprobe analysis of two-dimensionally preserved Marpolia , Wiwaxia and Burgessia identifies the presence of both carbonaceous and aluminosilicate films for most features, irrespective of original lability. In the light of the deep burial and greenschist facies metamorphism documented for the Burgess Shale, the aluminosilicate films are identified as products of late-stage volatilization and coincident mineralization of pre-existing compression fossils, whereas the three-dimensionally preserved gut-caecal system of Burgessia is interpreted as an aluminosilicate replacement of a pre-existing carbonate phase. The case for late diagenetic emplacement of aluminosilicate minerals is supported by the extensive aluminosilicification of trilobite shell and (originally) calcareous veinlets in the Burgess Shale, as well as documentation of other secondarily aluminosilicified compression fossils. By distinguishing late diagenetic alteration from the early diagenetic processes responsible for exceptional preservation, it is possible to reconcile the range of preservational modes currently expressed in the Burgess Shale.     

From weird wonders to stem lineages: the second reclassification of the Burgess Shale fauna

The Burgess Shale, a set of fossil beds containing the exquisitely preserved remains of marine invertebrate organisms from shortly after the Cambrian explosion, was discovered in 1909, and first brought to widespread popular attention by Stephen Jay Gould in his 1989 bestseller Wonderful life: The Burgess Shale and the nature of history. Gould contrasted the initial interpretation of these fossils, in which they were 'shoehorned' into modern groups, with the first major reexamination begun in the 1960s, when the creatures were perceived as 'weird wonders', possessing unique body plans and unrelated to modern organisms. More recently, a third phase of Burgess Shale studies has arisen, which has not yet been historically examined. This third phase represents a revolutionary new understanding, brought about, I believe, by a change in taxonomic methodology that led to a new perception of the Burgess creatures, and a new way to comprehend their relationships with modern organisms. The adoption of cladistics, and its corollary, the stem group concept, has forged a new understanding of the Burgess Shale ... but has it also changed the questions we are allowed to ask about evolution?     

Dynamic palaeoredox and exceptional preservation in the Cambrian Spence Shale of Utah

Garson, D.E., Gaines, R.R., Droser, M.L., Liddell, W.D. & Sappenfield, A. 2011: Dynamic palaeoredox and exceptional preservation in the Cambrian Spence Shale of Utah. Lethaia, Vol. 45, pp. 164–177. Burgess Shale‐type faunas provide a unique glimpse into the diversification of metazoan life during the Cambrian. Although anoxia has long been thought to be a pre‐requisite for this particular type of soft‐bodied preservation, the palaeoenvironmental conditions that regulated extraordinary preservation have not been fully constrained. In particular, the necessity of bottom water anoxia, long considered a pre‐requisite, has been the subject of recent debate. In this study, we apply a micro‐stratigraphical, ichnological approach to determine bottom water oxygen conditions under, which Burgess Shale‐type biotas were preserved in the Middle Cambrian Spence Shale of Utah. Mudstones of the Spence Shale are characterized by fine scale (mm‐cm) alternation between laminated and bioturbated intervals, suggesting high‐frequency fluctuations in bottom water oxygenation. Whilst background oxygen levels were not high enough to support continuous infaunal activity, brief intervals of improved bottom water oxygen conditions punctuate the succession. A diverse skeletonized benthic fauna, including various polymerid trilobites, hyolithids, brachiopods and ctenocystoids suggests that complex dysoxic benthic community was established during times when bottom water oxygen conditions were permissive. Burgess Shale‐type preservation within the Spence Shale is largely confined to non‐bioturbated horizons, suggesting that benthic anoxia prevailed in intervals, where these fossils were preserved. However, some soft‐bodied fossils are found within weakly to moderately bioturbated intervals (Ichnofabric Index 2 and 3). This suggests that Burgess Shale‐type preservation is strongly favoured by bottom water anoxia, but may not require it in all cases. □Anoxia, Burgess Shale, Burgess Shale type‐preservation, Langston Formation, Spence Shale Member, Utah.     

Regulation and regeneration in the ctenophore Mnemiopsis leidyi

Lobate ctenophores (tentaculates) generally exhibit a remarkable ability to regenerate missing structures as adults. On the other hand, their embryos exhibit a highly mosaic behavior when cut into halves or when specific cells are ablated. These deficient embryos do not exhibit embryonic regulation, and generate incomplete adult body plans. Under certain conditions, however, these deficient animals are subsequently able to replace the missing structures during the adult phase in a process referred to as "post-regeneration." We have determined that successful post-regeneration can be predicted on the basis of a modified polar coordinate model, and the rules of intercalary regeneration, as defined by French et al. (V. French, P. J. Bryant, and S. V. Bryant, 1976, Science 193, 969-981.) The model makes certain assumptions about the organization of the ctenophore body plan that fit well with what we have determined on the basis of cell lineage fates maps, and their twofold rotational ("biradial") symmetry. The results suggest that cells composing the ctenophore adult body plan possess positional information, which is utilized to reconstruct the adult body plan. More specifically, we have found that the progeny of three specific cell lineages are required to support post-regeneration of the comb rows (the e(1), e(2), and m(1) micromeres). Furthermore, post-regeneration of the comb rows involves a suite of cell-cell inductive interactions, which are similar to those that take place during their embryonic formation. The significance of these findings is discussed in terms of the organization of the ctenophore body plan, and the mechanisms involved in cell fate specification. This situation is also contrasted with that of the atentaculate ctenophores, which are unable to undergo post-regeneration.     

Skeletal Nets of the Ediacaran Fronds

The Ediacara fauna is traditionally regarded as the first complex, diverse and widespread macroscopic life. The uncertainty of systematic position of its members has led to very different views on the early evolution of metazoans. In part, this may be due to a lack of data on sclerotization: a hard skeleton is a part of the archetype of the most taxa known from the fossil record, whereas the Ediacara fauna as a whole is most often considered soft-bodied, which complicates comparison. Here we report the Late Precambrian frond-like fossils (Petalonamae) from the Vendian assemblage of the Southeastern White Sea area (∼555.3 Ma), which show evidence of elaborate skeleton composed of a regular meshwork reinforced by dense longitudinal and circular bands. Judging from the nature of preservation and the dynamics of the environment Ediacaran fronds secreted a relatively rigid but flexible skeleton. The fact that frond-like Petalonamae had a supporting structure similar to that of sponges and cnidarians seems to be a powerful argument in favor of their metazoan affinity. The new observations indicated also that the widespread skeletonization had occurred long before the “Cambrian skeletal revolution”.     

Protistan epibionts of the ctenophore Mnemiopsis mccradyi Mayer

Mnemiopsis mccradyi, a common coastal ctenophore, was observed to bear two distinct, exclusive assemblages of protistan epibionts. The mobiline peritrich, Trichodina ctenophorii (Estes et al., 1997), and small Flabellula-like gymnamoebae inhabited only the surface of the comb plates. By contrast, small Vexillifera-like gymnamoebae and large Protoodinium-like dinoflagellates were found on the ectoderm. The relationship of the epimicrobial protists with their host varied from possible mutualism (vexilliferids) to commensalism (trichodinids) to parasitism (flabellulids and protoodinids). Trichodinids may benefit from comb plate attachment by enhanced food capture. Although they did not obviously impair comb plate beating, they did distort the surface and appear to produce fissures in the comb plate surface, which could provide inroads for more severe comb plate damage by amoebae. By contrast, when flabellulid amoebae occurred in very high surface densities (up to 5000 mm^–2), they clearly damaged comb plates by eroding the surface. Where flabellulid pseudopodia invaded the comb plate, we observed local degradation of comb plate cilia, as evidenced by central pair disorientation and plasma membrane perturbation and overt phagocytosis of comb plate cilia. Ectodermal vexilliferids, which occurred at much lower densities, did not appear to have any degradative impact on the ctenophore. By contrast, clusters of ectodermal protoodinids were found in localized depressions most likely caused by invasive phagocytosis. The impact of the protistan assemblages on ctenophore populations is unclear, but under conditions of severe infestation they might depress ctenophore population density.     

CHARGE CONTRAST IMAGING OF EXCEPTIONALLY-PRESERVED FOSSILS

Abstract:  Charge contrast images are a variant of secondary electron images acquired by operating a variable pressure scanning electron microscope in low vacuum mode; i.e. a gas is present in the specimen chamber. Spatial variation in the amount of charge that accumulates on the surface of the specimen is expressed as differences in greyscale tone; areas that are charging less are darker in tone. The precise mechanisms by which charge contrast images are generated are not known fully. Various different properties of a mineral may create a charge contrast; electrical conductivity is known to be one potentially important variable. As carbon is highly conductive but typical host lithologies (carbonates, silicates) less so or dielectric, the technique is potentially very suitable for imaging organically preserved fossils such as those from the Solite and Jehol biotas. It can also be applied to Burgess Shale fossils: complex films comprising 'aluminosilicates' with or without carbonaceous remains. Charge contrast images reveal anatomical detail not visible using either optical or other scanning electron microscope-based imaging methods.     

贵州台江凯里动物群中的非钙质藻类化石

贵州台江凯里组的非钙质藻类化石在我国中寒武统尚属首次发现。国外主要见于北美。描述的非钙质藻类化石Ｍａｒｐｏｌｉａ ｓｐｉｓｓａ． Ｂｏｓｗｏｒｔｈｉａ ｓｉｍｕｌａｎｓ ,Ａｌｇａ ｇｅｎ． ｅｔ ｓｐ． ｉｎｄｅｔ, Ａ,Ａｌｇａ ｅｔｓｐ． ｉｎｄｅｔ, Ｂ等常见于北美布尔吉斯页岩动物群。当前中武寒武非钙质藻类化石的发现不仅填补了我国中寒武统非钙质藻类的空白,而且对于凯里动物群与布尔吉斯页斯     

The bivalved arthropods Isoxys and Tuzoia with soft‐part preservation from the Lower Cambrian Emu Bay Shale Lagerstätte (Kangaroo Island,Australia)

Abstract: Abundant material from a new quarry excavated in the lower Cambrian Emu Bay Shale (Kangaroo Island, South Australia) and, particularly, the preservation of soft‐bodied features previously unknown from this Burgess Shale‐type locality, permit the revision of two bivalved arthropod taxa described in the late 1970s, Isoxys communis and Tuzoia australis. The collections have also produced fossils belonging to two new species: Isoxys glaessneri and Tuzoia sp. Among the soft parts preserved in these taxa are stalked eyes, digestive structures and cephalic and trunk appendages, rivalling in quality and quantity those described from better‐known Lagerstätten, notably the lower Cambrian Chengjiang fauna of China and the middle Cambrian Burgess Shale of Canada.     

Underground Vendobionta From Namibia

The late Precambrian fossils from Namibia have generally been regarded as soft-bodied organisms whose three-dimensional preservation resulted from smothering in fluidized sand. The sedimentological context of Pteridinium and Namalia within a sandstone bed, however, allows us to distinguish between two taphocoenoses: (1) winnowed, laterally collapsed, current-transported specimens accumulated as a lag deposit of turbidite-like flows, and (2) specimens 'floating' in the top part of an event bed with their vanes extending upwards to the upper bedding surface. The second taphocoenosis is interpreted as an in situ preserved 'infaunal' community. The immobile underground life habit and the bizarre modes of growth of Pteridinium and Namalia do not fit any extinct or modern group of multicellular organisms. Similar statements can be made for Ernietta and Rangea , thus reviving the Vendobionta hypothesis.     

Interpreting ‘shelly’ fossils preserved as organic films: the case of hyolithids

Most studies of Burgess Shale‐type preservation have focussed on soft‐bodied organisms, but ‘shelly’ fossils are also preserved as carbonaceous films. These films are usually interpreted as coherent organic layers – often external sheaths or periostracal layers – that were present in the original mineralized elements. The example of hyolithids shows that the organic films of skeletal parts do not represent original ‘layers’, but a composite resulting from the coalescence, into a single carbonaceous film, of all the preservable organic matter present in the skeletal element. The diagenetic processes that led to Burgess Shale‐type preservation, which involve the polymerization of organic matter and the loss of original internal structure and chemical integrity of the original tissues, are entirely compatible with – and could account for – the characteristics observed in the fossil films of hyolithid skeletal elements. These observations have general implications for the interpretation of other organisms preserved as carbonaceous films, such as the diverse and often problematic Cambrian sponges.     

Comparative feeding behavior of planktonic ctenophores

The phylum Ctenophora (known as comb jellies) consists of gelatinousmarine carnivores found from the surface to several thousandmeters depth. Their morphology can be simple or complex, rangingfrom a sac-like shape with no tentacles to large lobed formswith sinuous "auricles," papillae, and two different kinds oftentacles. This diversity appears to reflect adaptations tomany different diets. For example, some species can continuously"graze" on small crustaceans or larvae, others engulf largerjellies, and some are able to snare individual larger prey througha variety of strategies. Thus feeding behavior can help explainthe high morphological diversity in this relatively small phylum.Because of their fragility, comb jellies are difficult to studyalive and the natural histories of many types, especially thosefound in the deep sea, have not been examined. This accountcategorizes ctenophore feeding methods using published reportsas well as new observations using submersibles and blue-waterscuba diving.     

Eoandromeda and the origin of Ctenophora

The Ediacaran fossil Eoandromeda octobrachiata had a high conical body with eight arms in helicospiral arrangement along the flanks. The arms carried transverse bands proposed to be homologous to ctenophore ctenes (comb plates). Eoandromeda is interpreted as an early stem‐group ctenophore, characterized by the synapomorphies ctenes, comb rows, and octoradial symmetry but lacking crown‐group synapomorphies such as tentacles, statoliths, polar fields, and biradial symmetry. It probably had a pelagic mode of life. The early appearance in the fossil record of octoradial ctenophores is most consistent with the Planulozoa hypothesis (Ctenophora is the sister group of Cnidaria + Bilateria) of metazoan phylogeny.     

A reevaluation of Wiwaxia and the polychaetes of the Burgess Shale

Wiwaxia corrugata and the indisputable polychaetes of the Middle Cambrian Burgess Shale, particularly Canadia spinosa, have figured prominently in recent hypotheses about the early evolution of polychaete annelids. Based on similarities between the sclerites of Wiwaxia and the notochaetae of Canadia with the broad notochaetae (paleae) of Recent chrysopetalid polychaetes, these two fossil taxa have been variously treated as closely related to the most highly derived stem forms of the polychaete (and annelid) crown group or as members of a specific, Recent subgroup within Polychaeta, the order Phyllodocida. Chrysopetalidae is a member of Phyllodocida, which is part of the major polychaete clade Aciculata; the latter two taxa are distinguished by four and six well defined autapomorphic characters, respectively. The best preserved or otherwise appropriate fossils of Wiwaxia corrugata, Canadia spinosa and the other polychaetes of the Burgess Shale have been studied in detail in order to determine whether they possess any characters that could support the homology of wiwaxiid sclerites, canadiid notochaetae and chrysopetalid paleae. Most of these fossil taxa have significant autapomorphies but the specific characters of the Aciculata and Phyllodocida are entirely absent. It is demonstrated that constraining cladograms in such a manner that wiwaxiid sclerites, canadiid notochaetae and chrysopetalid paleae are homologous leads to results that are markedly unparsimonious. Furthermore, Canadia and the other polychaetes of the Burgess Shale cannot be referred to any extant subgroup within the Polychaeta and cannot be used to polarize character evolution within the annelid crown group. Apart from its dubious sclerites, Wiwaxia has no characters that could indicate any close relationship with Polychaeta or Annelida.     

Beyond the Burgess Shale: Cambrian microfossils track the rise and fall of hallucigeniid lobopodians

Burgess Shale-type deposits are renowned for their exquisite preservation of soft-bodied organisms, representing a range of animal body plans that evolved during the Cambrian ‘explosion’. However, the rarity of these fossil deposits makes it difficult to reconstruct the broader-scale distributions of their constituent organisms. By contrast, microscopic skeletal elements represent an extensive chronicle of early animal evolution—but are difficult to interpret in the absence of corresponding whole-body fossils. Here, we provide new observations on the dorsal spines of the Cambrian lobopodian (panarthropod) worm Hallucigenia sparsa from the Burgess Shale (Cambrian Series 3, Stage 5). These exhibit a distinctive scaly microstructure and layered (cone-in-cone) construction that together identify a hitherto enigmatic suite of carbonaceous and phosphatic Cambrian microfossils—including material attributed to Mongolitubulus, Rushtonites and Rhombocorniculum—as spines of Hallucigenia-type lobopodians. Hallucigeniids are thus revealed as an important and widespread component of disparate Cambrian communities from late in the Terreneuvian (Cambrian Stage 2) through the ‘middle’ Cambrian (Series 3); their apparent decline in the latest Cambrian may be partly taphonomic. The cone-in-cone construction of hallucigeniid sclerites is shared with the sclerotized cuticular structures (jaws and claws) in modern onychophorans. More generally, our results emphasize the reciprocal importance and complementary roles of Burgess Shale-type fossils and isolated microfossils in documenting early animal evolution.     

Saccoglossus testa from the Mazon Creek fauna (Pennsylvanian of Illinois) and the evolution of acorn worms (Enteropneusta: Hemichordata)

The limited fossil record of enteropneust hemichordates (acorn worms) and the few external features that distinguish the four families have provided a challenge to our understanding of the evolution of the group and their various feeding adaptations. The middle Pennsylvanian Saccoglossus testa sp. nov. from the Mazon Creek, Westfalian D Carbonate Formation, Francis Creek Shale of northern Illinois provides evidence for the exploitation of surface sediments. Saccoglossus testa has a long proboscis characteristic of the extant genus Saccoglossus, a specialist in surface deposit feeding. The collar is as long as it is wide. The anterior trunk lacks a distinctively wide branchial region. These three features distinguish it from its sympatric enteropneust species Mazoglossus ramsdelli Bardack that has a proboscis characteristic of an infaunal deposit feeder. It is the seventh known species of fossil enteropneust, including a resting trace of a Lower Triassic fossil that has collar lips that characterize the extant deep‐sea family Torquaratoridae, and which represents a second parallel evolution of surface deposit feeding. An analysis of the seven fossils shows that the earliest Enteropneusta had a relatively simple harrimaniid‐like body plan, and that the spengelid, the torquaratorid and lastly the most complex ptychoderid body plan appeared in that chronological order.