Burgess Shale Fossils Research Articles

Evolution: The arthropod brain — a saga in three parts

The arthropod brain comprises three segments and innervates three appendages. How it evolved has been a matter of intense debate. New Burgess Shale fossils preserving nervous tissue bring us closer to solving this mystery but evaluating the many hypotheses they could fit is not easy.

Contingency and convergence in the theory of evolution: Stephen Jay Gould vs. Simon Conway Morris

Debating the interpretation of the Burgess Shale fossil records, Stephen Jay Gould and Simon Conway Morris have formulated two conflicting theses regarding the nature of evolutionary processes. While Gould argued that evolution is essentially a contingent process whose outcomes are unpredictable, Conway Morris claimed that the omnipresence of convergence testifies that it is in fact deterministic, leading to predictable and inevitable outcomes. Their theses have been extensively researched from various perspectives. However, a systematic parallel analysis of the core arguments each of them offered in support of their thesis has been lacking. I argue Conway Morris has successfully exposed the core weaknesses of Gould's thesis and offered a comprehensive account in favor of the major role of convergence in evolutionary history. On the other hand, I will point out some of the weak points in the latter's arguments supporting the deterministic view of life's evolution. Although Conway Morris has been more successful in arguing for the deterministic nature of the evolutionary processes, both theses could be improved if their shortcomings are taken into consideration.

Early formation and taphonomic significance of kaolinite associated with Burgess Shale fossils

Abstract The role of minerals in Burgess Shale–type fossilization is controversial, particularly that of the clay mineral kaolinite. Kaolinite may have formed on carcasses or attached to them as they decayed, stabilizing organic matter. Alternatively, kaolinite may have formed during metamorphism, playing no role in the preservation of soft tissues. Evaluating the formation and taphonomic role of kaolinite is difficult, because the mineralogy of Burgess Shale–type fossils is incompletely known. We used in situ selected-area X-ray diffraction to constrain the mineralogy of fossils from the classic Burgess Shale Formation in British Columbia, Canada. Fossils can be distinguished from the matrix that surrounds them by the presence of dolomite, kaolinite, and pyrite. Chlorite may be more abundant in the matrix. The preferential survival of kaolinite in association with fossils provides evidence of early diagenetic clay-organic interactions that protected the clay from metamorphic transformation. Kaolinite likely played a crucial role in fossilization, inhibiting the growth of heterotrophic bacteria and aiding polymerization of soft tissue biomolecules. This may result in biases in soft-tissue preservation to areas and times where kaolinite was prevalent.

THE LIMITS OF BURGESS SHALE-TYPE PRESERVATION: ASSESSING THE EVIDENCE FOR PRESERVATION OF THE BLOOD PROTEIN HEMOCYANIN IN THE BURGESS SHALE

Cambrian deposits bearing exceptional preservation of soft-bodied fossils offer unique insights into the diversity and paleobiology of the Cambrian biota. In recent years, a growing number of studies have provided evidence that highly labile tissues, such as nervous tissues, may be more common in fossils from Burgess Shale-type deposits than previously appreciated. These discoveries have provided novel insights into the anatomy and phylogeny of Cambrian animals and have also challenged the basic understanding of the limits on the resolution of anatomical information that may be captured by Burgess Shale-type preservation. Evidence used to infer primary blood chemistry was recently reported from the iconic Burgess Shale fossil Marrella splendens, the first such report from a Cambrian Lagerstatte. Diminutive crystals of a copper sulfide mineral found in association with Marrella were interpreted as evidence that the animal's blood contained the Cu-bearing protein hemocyanin. Copper minerals in shales in general, and in the Burgess Shale in particular, however, may have a complex geologic history. Using microscopy and SEM-EDS elemental mapping of material from the Walcott Quarry as well as from the Burgess Shale fossil quarry at Marble Canyon, we assess the timing of emplacement of Cu-sulfides in fossil-bearing mudrocks of the Burgess Shale. These data demonstrate that copper minerals are most conspicuously concentrated within veins that are oriented sub-perpendicular to bedding. These late-stage fluid flow paths allowed Cu-mineralizing fluids to infiltrate bedding plane-parallel cracks, which commonly run through Burgess Shale fossils and extend into the matrix around them. Based upon the relationships of mineral phases associated with fossils, it appears that Cu-mineralization occurred after metamorphic volatilization of a large proportion of the carbonaceous material that originally comprised the fossils. We find that copper minerals in the Burgess Shale are of demonstrably post-Cambrian origin, a secondary overprint that cannot be used to infer original blood chemistry or tissue composition of its fossils.

Burgess Shale fossils shed light on the agnostid problem.

Agnostids (agnostinids and eodiscinids) are a widespread and biostratigraphically important group of Cambro-Ordovician euarthropods whose evolutionary affinities have been highly controversial. Their dumbbell-shaped calcified tergum was traditionally suggested to unite them with trilobites, but agnostinids have alternatively been interpreted as stem-crustaceans, based on Orsten larval material from the Cambrian of Sweden. We describe exceptionally preserved soft tissues from mature individuals of the agnostinids Peronopsis and Ptychagnostus from the middle Cambrian (Wuliuan Stage) Burgess Shale (Walcott Quarry and Marble Canyon, British Columbia, Canada), facilitating the testing of alternative hypotheses. The digestive tract includes conspicuous ramifying cephalic diverticulae. The cephalon carries one pair of elongate spinous antennules projecting to the front, two pairs of appendages with distally setose, oar-like exopods, and three pairs of presumably biramous appendages with endopods sporting club-shaped exites. The trunk bears five appendage pairs, at least the first two of which are similar to the posteriormost cephalic pairs. The combined evidence supports a nektobenthic and detritivorous lifestyle for agnostinids. A head with six appendiferous segments contrasts strikingly with the four known in trilobites and five typical of mandibulates. Agnostinids are retrieved as the sister group to polymeroid trilobites in our phylogeny, implying that crustacean-like morphologies evolved homoplastically. This result highlights the variability in segmental composition of the artiopodan head. Finally, our study emphasizes the continued role of Burgess Shale-type fossils in resolving the affinities of problematic biomineralizing taxa.

Burgess Shale fossils illustrate the origin of the mandibulate body plan.

Retracing the evolutionary history of arthropods has been one of the greatest challenges in biology. During the past decade, phylogenetic analyses of morphological and molecular data have coalesced towards the conclusion that Mandibulata, the most diverse and abundant group of animals, is a distinct clade from Chelicerata, in that its members possess post-oral head appendages specialized for food processing, notably the mandible. The origin of the mandibulate body plan, however, which encompasses myriapods, crustaceans and hexapods, has remained poorly documented. Here we show that Tokummia katalepsis gen. et sp. nov., a large bivalved arthropod from the 508 million-year-old Marble Canyon fossil deposit (Burgess Shale, British Columbia), has unequivocal mandibulate synapomorphies, including mandibles and maxillipeds, as well as characters typically found in crustaceans, such as enditic, subdivided basipods and ring-shaped trunk segments. Tokummia and its closest relative, Branchiocaris (in Protocarididae, emended), also have an anteriormost structure housing a probable bilobed organ, which could support the appendicular origin of the labrum. Protocaridids are retrieved with Canadaspis and Odaraia (in Hymenocarina, emended) as part of an expanded mandibulate clade, refuting the idea that these problematic bivalved taxa, as well as other related forms, are representatives of the basalmost euarthropods. Hymenocarines now illustrate that the subdivision of the basipod and the presence of proximal endites are likely to have been ancestral conditions critical for the evolution of coxal and pre-coxal features in mandibulates. The presence of crustaceomorph traits in the Cambrian larvae of various clades basal to Mandibulata is reinterpreted as evidence for the existence of distinct ontogenetic niches among stem arthropods. Larvae would therefore have constituted an important source of morphological novelty during the Cambrian period, and, through heterochronic processes, may have contributed to the rapid acquisition of crown-group characters and thus to greater evolutionary rates during the early radiation of euarthropods.

Paleocommunity Analysis of the Burgess Shale Tulip Beds, Mount Stephen, British Columbia: Comparison with the Walcott Quarry and Implications for Community Variation in the Burgess Shale

AbstractThe Tulip Beds locality on Mount Stephen (Yoho National Park, British Columbia) yields one of the most abundant and diverse (~10,000 specimens in 110 taxa) Burgess Shale fossil assemblages in the Canadian Rockies. Detailed semi quantitative and quantitative analyses of this assemblage suggest strong similarities with the Walcott Quarry on Fossil Ridge. Both assemblages are dominated by epibenthic, sessile, and suspension feeding taxa, mostly represented by arthropods and sponges and have comparable diversity patterns, despite sharing only about half the genera. However, the Tulip Beds has a higher relative abundance of suspension feeders and taxa of unknown affinity compared to the Walcott Quarry. These biotic variations are probably largely attributable to ecological and evolutionary differences between the two temporally distinct communities that adapted to similar, but not identical, environmental settings. For instance, the Tulip Beds is farther away from the Cathedral Escarpment than the Walcott Quarry. The Tulip Beds and Walcott Quarry assemblages are more similar to each other than either one is to the assemblages of the Chengjiang biota, although the relative diversity of major taxonomic groups and ecological patterns are similar in all assemblages. The conserved diversity patterns and ecological structures among sites suggest that the ecological composition of Cambrian Burgess Shale-type communities was relatively stable across wide geographic and temporal scales.

Extraordinary fossils reveal the nature of Cambrian life: a commentary on Whittington (1975) 'The enigmatic animal Opabinia regalis, Middle Cambrian, Burgess Shale, British Columbia'.

Harry Whittington's 1975 monograph on Opabinia was the first to highlight how some of the Burgess Shale animals differ markedly from those that populate today's oceans. Categorized by Stephen J. Gould as a ‘weird wonder’ (Wonderful life, 1989) Opabinia, together with other unusual Burgess Shale fossils, stimulated ongoing debates about the early evolution of the major animal groups and the nature of the Cambrian explosion. The subsequent discovery of a number of other exceptionally preserved fossil faunas of Cambrian and early Ordovician age has significantly augmented the information available on this critical interval in the history of life. Although Opabinia initially defied assignment to any group of modern animals, it is now interpreted as lying below anomalocaridids on the stem leading to the living arthropods. This commentary was written to celebrate the 350th anniversary of the journal Philosophical Transactions of the Royal Society.

Really Old Hormones Up to New Tricks: Glycoprotein Hormone Subunits May Have Roles in Development

One of the great questions in the discipline of endocrinology is the evolutionary origin of the hormones and receptors at the foundation of endocrine regulation. The origin of the pituitary gland itself and its essential hormones has been debated for many decades (1, 2). For a long time, researchers at France’s Museum National d’Histoire Naturelle in Paris, i.e. M. M. Fontaine and Y. A. Fontaine and their students, have been interested in this very question. Now, continuing this tradition, Dos Santos et al. (3) deciphered the evolution of the glycoprotein hormones. Most of us know that LH and FSH control reproduction. These hormones are composed of a common glycoprotein -subunit and a dissimilar -subunit that confer the specific hormonal actions of these and other members of the family such as human chorionic gonadotropin and TSH. Querat (4) is well known for sorting out the messy phylogeny of the glycoprotein -subunits, and he also previously helped to resolve the issue of the number of gonadotropins present in fish species. It is clearly established that the piscine gonadotropic hormones previously known as gonadotropin-I and -II (GTH-I and GTH-II) are indeed the fish equivalents of classical mammalian FSH and LH, respectively (4, 5). More recently a putative hormone, thyrostimulin, was discovered by Hsueh and colleagues (6) and was so named because it activates TSH receptors and stimulates T4 production. Other reports since then support this function. We could refer to thyrostimulin as one of the newest hormones; however, this would not be correct, given the results of Dos Santos et al. (3). Extensive invertebrate and vertebrate genome searches and rigorous phylogenetic analyses indicate that the glycoprotein subunits making up thyrostimulin have been around since the origins of the complex bilateral animal body plan more than 500 million years ago in the early Cambrian period. Thyrostimulin is presumably composed of the heterodimer of glycoprotein A2 (GPA2) and glycoprotein B5 (GPB5). It is these hormone-like subunits that Dos Santos et al. (3) set out to study with an eye on possible functions. They discovered differences in tissue distribution, GPA2 being more widely expressed in the amphioxus body than GPB5, with GPA2 expression levels being up to 100 times higher than those of GPB5. Often GPA2 and GPB5 are not even coexpressed. This raises the important question of whether GPA2 and GPB5 follow the general rule of the other glycoprotein hormones LH, FSH, and TSH that require / dimerization and glycosylation to be fully functional in vivo. What are the possible functions of GPA2 and GPB5? Dos Santos et al. (3) used in situ hybridization and the amphioxus as the model system to explore developmental expression of GPA2 and GPB5 and their presumptive target, the glycoprotein-related receptor. Amphioxus is at the base of the vertebrate family tree and is a free-living invertebrate cephalochordate that has the main vertebrate characteristics as an adult. These animals are filter feeders and are also known as lancelets because of the sword-like body form. It should be noted that the ancestor of amphioxus, the famed Burgess Shale fossil, Pikaia gracilens, is dated at about 505 million yr old and resembles its modern-day cousin. There has been an interest in the endocrine systems of amphioxus since the 1960s. These curious animals have served science particularly well in studies of the evolution and developmental roles of HOX genes (7). In the embryonic amphioxus, expression of GPA2 was much more restricted than GPB5. One significant observation was the expression of GPA2 in the region of Hatshek’s pit. This structure forms as a dorsal evagination of cells from the mouth epithelium and has long been speculated to be a homolog of the vertebrate pituitary gland. This idea was strengthened by Gorbman’s observation of an infundibulum-like outgrowth along the right margin of the brain and notochord than makes contact with Hatchek’s pit (2). This system bears considerable resemblance to the vertebrate hypothalamo-adenohypophyseal system, perhaps most like that of the lamprey (1). It is possible that neurohormones could be delivered by diffusion to the cells in Hatchek’s pit expressing GPA2. However, the main interest lies in the hypothesis that GPA2 and GPB5 are developmental regulators. Expression studies in amphioxus embryos indicate that they are not always expressed at the same place or developmental stage, suggesting that dimer-

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.

Fossil Chaetognaths from the Burgess Shale: A Reply to Conway Morris (2009)

Walcott (1911) erected the new genus and species Oesia dis− juncta and assigned them to the polychaete annelids, based on a small collection of similar fossils from the famous Middle Cambrian Burgess Shale. In 2002 I suggested that the species is “possibly related to chaetognaths” (Szaniawski 2002: 405). Later, after obtaining new photos of the specimens and mak− ing comparative investigations with the extant chaetognaths, I was able to describe many significant similarities, and came to the conclusion that O. disjuncta indeed is an ancestral chaetognath (Szaniawski 2005). This interpretation already has been accepted in several publications (Vannier et al. 2005; Ball and Miller 2006; Hu et al. 2007. Giribet 2008). Ball and Miller (2006: 594) confirmed not only its “... remarkable re− semblance to modern chaetognaths” but also correctness of recognition of all its organs. They even reproduced a part of my illustration showing them (Ball and Miller 2006: fig. 2). Vannier et al. (2006: 629) combined the problemwith the open question of the systematic position of another Burgess Shale fossil Amiskwia sagittiformis Walcott, 1911, and expressed their reservation based on “...the lack of clear evidence of a grasping apparatus...”. Only Conway Morris (2009) firmly disagreed with this diagnosis and even devoted a special “dis− cussion” article addressing the issue. However, that article contains several ambiguities and misunderstandings which need clarification.

Ubiquitous Burgess Shale–style “clay templates” in low-grade metamorphic mudrocks

Despite the Burgess Shale’s (British Columbia, Canada) paleobiological importance, there is little consensus regarding its taphonomy. Its organic fossils are preserved as compressions associated with phyllosilicate fi lms (“clay templates”). Debate focuses on whether these templates were fundamental in exceptional preservation or if they formed in metamorphism, meaning that it is important to establish the timing of their formation relative to decay. An early diagenetic origin has been proposed based on anatomy-specifi c variations in their composition, purportedly refl ecting contrasts in decay. However, we demonstrate that these films bear a remarkable similarity to those that occur on organic fossils in graptolitic mudrocks and form as a normal product of low-grade metamorphism. Such phyllosilicates may also occur within voids created by volume loss in maturation, a process that may have aided their formation. In bedding-plane assemblages from graptolitic mudrocks, different taxa are associated with distinct phyllosilicates. This likely refl ects stepwise maturation of their constituent kerogens in an evolving hydrothermal fl uid, with different hyllosilicates forming as each taxon progressively underwent maturation. These observations provide an analogue for the distribution and composition of phyllosilicates on Burgess Shale fossils, which we interpret as refl ecting variations in the maturation of their constituent tissues. Thus, their clay templates seem unremarkable, forming too late to account for exceptional preservation.

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.

Reconstructing early sponge relationships by using the Burgess Shale fossil Eiffelia globosa, Walcott.

The relationships of the sponge classes are controversial, particularly between the calcareous and siliceous sponges. Specimens of the putative calcarean Eiffelia globosa Walcott from the Burgess Shale show the presence of diagnostic hexactinellid spicules integrated into the skeletal mesh. The arrangement of these spicules in Eiffelia is shown to be precisely equivalent to that of early protospongioid hexactinellids, and sponge growth occurred through an identical pattern to produce identical skeletal body morphology. The difference in spicule composition of the classes is interpreted through the observation of taphonomic features of Eiffelia that suggest the presence of at least two mineralogically distinct layers within the spicules. These results support molecular analyses that identify the calcarean-silicisponge transition as the earliest major sponge branch and suggest that the heteractinids were paraphyletic with respect to the Hexactinellida.

Mechanisms of fossilization of the soft-bodied and lightly armored faunas of the Burgess Shale and of some other classical localities

The splendid preservation of the Middle Cambrian Burgess Shale fauna, a fauna of exceptional importance for our understanding of the evolution of life, has not been adequately explained. Preservation of diagenetically altered rem- nants of the original organic tissues and formation of chlorite/illite coatings and cuticle replacements, both documented in the Burgess Shale fossils though not necessarily occurring together, can be understood as products of the same mechanism of fossilization of soft tissues. It is argued here that this mechanism consists of the following steps: (1) adsorption on structural biopolymers such as chitin, cellulose, and collagens of Fe²⁺ ions released during the oxidation of organic matter by iron(III)- reducing bacteria, (2) inhibition by the adsorbed Fe²⁺ ions of further bacterial decomposition of these biopolymers, which enables them to persist and later become kerogens; (3) in some microenvironments, nucleation of crystals of an iron(II)-rich clay mineral, a berthierine or a ferroan saponite, on the Fe²⁺ ions adsorbed on the preserved biopolymers and growth of such clay-mineral crystals to form a coating on the organic remains and/or to replace parts of the organism. The critical factors in the Burgess Shale-type preservation of Early and Middle Cambrian soft-bodied and lightly armored animals were probably: (1) rapid transport of live or freshly killed organisms into suboxic water, (2) extensive suboxic diagenesis in a sediment of high iron(III)/ (organic carbon) ratio, and (3) curtailment of the supply of sulfate ions shortly after the onset of pyritization. The proposed model of early diagenesis that results in Burgess Shale-type fossil preservation critically depends on the availability of steady suboxic depositional environments in open oceanic settings at depths of the order of 100 m in which iron(III)-rich fine-grained sediments, rapidly deposited with the entrained animals by turbidity currents, could accumulate without being disturbed by storm waves and deep currents. Evidence discussed in the present paper suggests that such conditions were common in the Early and Middle Cambrian. Adsorption of Fe²⁺ ions on structural biopolymers as a means of protecting organic fossil remains from decomposition by bacterial enzymes is a novel suggestion and needs to be demonstrated by direct experimentation. It is based on the following considerations. First, Fe²⁺ ions are strongly adsorbed on chitin under experimental conditions comparable to those in pore waters of suboxic iron-rich sediments, and while data on Fe²⁺ ion adsorption on collagen and cellulose seem to be lacking, other heavy-metal ions are strongly adsorbed on these biopolymers under appropriate conditions. Second, Fe²⁺ ions bonded with functional groups of chitin, collagen, or cellulose would prevent the very specific configuration and bonding which a biopoly- mer strand has to achieve within the active-site cleft of the appropriate bacterial enzyme to make enzymatic hydrolysis possible. Close examination of two other mechanisms recently proposed for Burgess Shale-type preservation of soft tissues shows that they are implausible: preservation by inactivation of extracellular enzymes on clay minerals would require a maladaptive reliance of tissue-decomposing bacteria on free extracellular enzymes, and preserva- tion by attachment of pre-existing clay-mineral particles would require a sequence of physically improbable events. It is argued here that adsorption of Fe²⁺ ions on structural biopolymers was the first step not only in Burgess Shale-type preservation of soft-bodied and lightly armored fossils but also in the preservation of such fossils by pyritization in Beecher’s Trilobite Bed in the Upper Ordovician Frankfort Shale in upstate New York and in the Lower Devonian Hunsrück Slate in Rhineland-Palatinate and in the preservation of such fossils within siderite concretions in various localities. It was probably the first step in the preservation (obscured by alunitic weathering) of soft-bodied and lightly armored fossils in the Soom Shale of the Cape Province, South Africa, and in the preservation of Ediacaran soft-bodied fossils in the classical localities of the Ediacaran Range of Australia and Mistaken Point, Newfoundland.

The Burgess Shale fossils at the Natural History Museum, London

The fossils from Burgess Shale (British Columbia) and other exceptionally preserved Cambrian faunas have been the focus of intensive research in the last couple of decades. They reveal insights into a time and into a world where animals began to thrive more than 500 million years ago. They give palaeontologists a more complete picture of the diversity of the Middle Cambrian biota, where soft-bodied animals were surprisingly more numerous than shelly organisms. The Natural History Museum, London contains important palaeontological reference collections of worldwide significance. Among these were found and studied sixty-four specimens that came from the Burgess Shale site.

A reassessment of the enigmatic Burgess Shale fossil Wiwaxia corrugata (Matthew) and its relationship to the polychaete Canadia spinosa Walcott

The enigmatic fossil Wiwaxia corrugata is organically preserved in the Burgess Shale (Middle Cambrian, British Columbia) and is therefore extractable by careful acid maceration of the mineralic matrix. High magnification transmitted light microscopy and SEM of macerated Wiwaxia sclerites reveal a substantial amount of previously undescribed structural and microstructural detail. Anatomical and histological comparison with modern organisms indicates that Wiwaxia sclerites are polychaete paleae (flattened setae) and that Wiwaxia was a jawed annelid broadly related to the extant polychaete families Chrysopetalidae and/or Aprhoditidae (Palmyra). Canadia spinosa, an uncontested fossil polychaete from the same beds, shows a paleal microstructure identical to that of Wiwaxia, as well as a closely comparable gross anatomy and taphonomic grade. The unique combination of taxonomically significant characters shared by Wiwaxia and Canadia suggest that they are more closely related to each other than either is to any other fossil or extant polychaete. Thus they constitute a separate superfamily, Canadiacea superfam. nov., in the order Phyllodocida.

Organic preservation of non-mineralizing organisms and the taphonomy of the Burgess Shale

Organic preservation of non-mineralizing animals constitutes an important part of the paleontological record, yet the processes involved have not been investigated in detail. Organic-walled fossils are generally explicable as a coincidence of original, relatively recalcitrant, extra-cellular materials and more or less anti-biotic depositional circumstances. One of the most pervasive natural inhibitors of biodegradation results from substrate and enzyme adsorption onto, and within, clay minerals; such interactions are likely responsible for many of the organic-walled fossils preserved in clastic sediments. Close examination of the fossilLagerstätteof the Burgess Shale (Middle Cambrian, British Columbia) reveals that most of its so-called soft-bodied fossils are composed of primary (although kerogenized) organic carbon. Their preservation can be attributed to pervasive clay-organic interactions as the organisms were transported in a moving sediment cloud and buried with all cavities and spaces permeated with fine grained clays. The organic-walled Burgess Shale fossils were studied both in petrographic thin section and isolated from the rock matrix, following careful acid maceration. Isotopic analysis of bulk organic and carbonate carbon yielded values consistent with a normal marine paleoenvironment. Anatomical and histological consideration of the enigmatic Burgess wormAmiskwiasuggest that it may in fact be a chaetognath, while the putative chordatePikaiaappears not to be related to modern cephalochordates.

New Burgess Shale Fossil Sites Reveal Middle Cambrian Faunal Complex

Soft-bodied and lightly sclerotized Burgess shale fossils have been found at more than a dozen new localities in an area extending for 20 kilometers along the front of the Cathedral Escarpment in the Middle Cambrian Stephen Formation of the Canadian Rockies. Five different fossil assemblages from four stratigraphic levels have been recognized. These assemblages represent distinct penecontemporaneous marine communities that together make up a normal fore-reef faunal complex.

Addenda to Descriptions of Burgess Shale Fossils. Charles D. Walcott

Previous articleNext article No AccessReviewsAddenda to Descriptions of Burgess Shale Fossils. Charles D. Walcott C. C.C. C. Search for more articles by this author PDFPDF PLUS Add to favoritesDownload CitationTrack CitationsPermissionsReprints Share onFacebookTwitterLinkedInRedditEmail SectionsMoreDetailsFiguresReferencesCited by The Journal of Geology Volume 40, Number 1Jan. - Feb., 1932 Article DOIhttps://doi.org/10.1086/623925 Citations: 1Citations are reported from Crossref PDF download Crossref reports the following articles citing this article:Richard Taibi 1930s—The Best of Times, (Nov 2016): 173–250.https://doi.org/10.1007/978-3-319-44518-2_5