Search

Tullimonstrum gregarium is an iconic soft-bodied fossil from the Carboniferous Mazon Creek Lagerstatte (Illinois, USA) (1). Despite a large number of specimens and distinct anatomy, various analyses over the past five decades have failed to determine the phylogenetic affinities of the 'Tully monster', and although it has been allied to such disparate phyla as the Mollusca (2), Annelida (3,4) or Chordata (5), it remains enigmatic (1-5). The nature and phylogenetic affinities of Tullimonstrum have defied confident systematic placement because none of its preserved anatomy provides unequivocal evidence of homology, without which comparative analysis fails. Here we show that the eyes of Tullimonstrum possess ultrastructural details indicating homology with vertebrate eyes. Anatomical analysis using scanning electron microscopy reveals that the eyes of Tullimonstrum preserve a retina defined by a thick sheet comprising distinct layers of spheroidal and cylindrical melanosomes. Timeof-flight secondary ion mass spectrometry and multivariate statistics provide further evidence that these microbodies are melanosomes. A range of animals have melanin in their eyes, but the possession of melanosomes of two distinct morphologies arranged in layers, forming retinal pigment epithelium, is a synapomorphy of vertebrates. Our analysis indicates that in addition to evidence of colour patterning (6), ecology (7) and thermoregulation (8), fossil melanosomes can also carry a phylogenetic signal. Identification in Tullimonstrum of spheroidal and cylindrical melanosomes forming the remains of retinal pigment epithelium indicates that it is a vertebrate; considering its body parts in this new light suggests it was an anatomically unusual member of total group Vertebrata., The enigmatic T. gregarium from the Carboniferous Mazon Creek Lagerstatte (307 million years ago) is among the world's most controversial fossils. Familiar to millions of people as the official state [...]

The Soom Shale Member of the Cedarberg Formation (South Africa) is a key early Paleozoic Lagerstatte that directly overlies glacigenic deposits of the Late Ordovician glacial maximum; it was deposited on the Gondwanan craton during deglacial transgression. We show that it contains a substantial coarse silt to fine sand component that occurs in discrete laminae intimately associated with plankton-derived organic material. We interpret this component as loess, sourced from glacially derived debris and blown into the sea either directly or across seasonal sea ice, as happens today on the McMurdo Ice Shelf, Antarctica. Falling through the water column, this material likely stimulated production of phytoplankton, which then sank as sediment-loaded aggregates. In marked sedimentary partitioning, these silt and/or organic laminae alternate with fine, organic-poor mud laminae that likely represent (river-derived?) nepheloid plumes and that sporadically thicken into centimeter-scale mud turbidites. Sustained eolian input directly into the surface water may have been key to maintaining the high productivity of the Soom sea and then, via eutrophication and anoxia, to the exceptional preservation of its biota. doi: 10.1130/G31426.1

Exceptional preservation of soft-bodied Cambrian chordates provides our only direct information on the origin of vertebrates (1,2). Fossil chordates from this interval offer crucial insights into how the distinctive body plan of vertebrates evolved, but reading this pre-biomineralization fossil record is fraught with difficulties, leading to controversial and contradictory interpretations (3,4). The cause of these difficulties is taphonomic: we lack data on when and how important characters change as they decompose, resulting in a lack of constraint on anatomical interpretation and a failure to distinguish phylogenetic absence of characters from loss through decay (3). Here we show, from experimental decay of amphioxus and ammocoetes, that loss of chordate characters during decay is non-random: the more phylogenetically informative are the most labile, whereas plesiomorphic characters are decay resistant. The taphonomic loss of synapomorphies and relatively higher preservation potential of chordate plesiomorphies will thus result in bias towards wrongly placing fossils on the chordate stem. Application of these data to Cathaymyrus (Cambrian period of China) and Metaspriggina (Cambrian period of Canada) highlights the difficulties: these fossils cannot be placed reliably in the chordate or vertebrate stem because they could represent the decayed remains of any non-biomineralized, total-group chordate. Preliminary data suggest that this decay filter also affects other groups of organisms and that 'stem-ward slippage' may be a widespread but currently unrecognized bias in our understanding of the early evolution of a number of phyla., The fossil record offers unique and crucial insight into many important episodes in the evolution of life, and allows reconstruction of the extinct relatives that comprise the stem lineages of [...]

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 films ('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-specific variations in their composition, purportedly reflecting 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 reflects stepwise maturation of their constituent kerogens in an evolving hydrothermal fluid, with different phyllosilicates 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 reflecting variations in the maturation of their constituent tissues. Thus, their clay templates seem unremarkable, forming too late to account for exceptional preservation.

The Chengjiang biota of Yunnan, China, documents the earliest extensive radiation of the Metazoa recorded in the fossil record. Gauging preservational bias is crucial in providing an assessment of the completeness of this biota and thereby elucidating whether it represents a comprehensive depiction of Early Cambrian life. We here present a model to explain the nature of the exceptional preservation of the Chengjiang biota and details of the decay process. This study indicates that Chengjiang fossils were preserved through two taphonomic pathways that may have captured tissues of distinct compositions, and this finding should provide a foundation for the interpretation of Chengjiang fossils. Many Chengjiang fossils are preserved by pyrite (later pseudomorphed by iron oxides); the clay-rich host sediment was deficient in organic carbon but replete in available Fe, and this composition ensured that a decaying carcass acted as a local substrate for Fe- and S-reducing bacteria. Pyrite morphology probably reflects contrasts in the decay rate, and hence the [H.sub.2]S production rate, of different tissues in a carcass. Reactive, rapidly decaying tissues would have quickly supplied [H.sub.2]S, producing many pyrite nuclei, resulting in framboidal habits. More recalcitrant tissues would have produced [H.sub.2]S more slowly, so that crystal growth operated on fewer nuclei, resulting in larger euhedral pyrite crystals. Reflective films, especially common on Chengjiang arthropods, represent the remains of degraded carbon. Keywords: Cambrian, Chengjiang, decay, preservation, pyrite.

The Upper Ordovician (Ashgill) Soom Shale lagerstatte preserves chitinozoans on the bedding planes as scattered individuals and, more significantly, in linked chains, in aggregated masses and associated with organic envelopes. Chitinozoan affinites are problematical, but the Soom Shale has a restricted biota, providing constraints on hypotheses of relationship. No graptolites or gastropods, which are possible contenders for chitinozoan production, occur. The chitinozoans are, however, associated with conodonts and orthocone cephalopods and, of all the known Soom Shale biota, these organisms present the most likely candidates for chitinozoan origination. Keywords: Chitinozoa, Ordovician, South Africa, affinities, palynomorphs.

More than 500 million years ago the first vertebrate made its appearance in the sea. It had no hard skeleton and fossil specimens are consequently unknown. Because of this, theories of vertebrate origins are controversial, but recently new light has been shed on this old problem. The evidence comes from research into the fossilized remains of conodonts, a long-extinct and enigmatic group of animals.

Chancelloriids are an extinct group of spiny Cambrian animals of uncertain phylogenetic position. Despite their sponge-like body plan, their spines are unlike modern sponge spicules, but share several features with the sclerites of certain Cambrian bilaterians, notably halkieriids. However, a proposed homology of these 'coelosclerites' implies complex transitions in body plan evolution. A new species of chancelloriid, Allonnia nuda, from the lower Cambrian (Stage 3) Chengjiang Lagerstätte is distinguished by its large size and sparse spination, with modified apical sclerites surrounding an opening into the body cavity. The sclerite arrangement in A. nuda and certain other chancelloriids indicates that growth involved sclerite addition in a subapical region, thus maintaining distinct zones of body sclerites and apical sclerites. This pattern is not seen in halkieriids, but occurs in some modern calcarean sponges. With scleritome assembly consistent with a sponge affinity, and in the absence of cnidarian- or bilaterian-grade features, it is possible to interpret chancelloriids as sponges with an unusually robust outer epithelium, strict developmental control of body axis formation, distinctive spicule-like structures and, by implication, minute ostia too small to be resolved in fossils. In this light, chancelloriids may contribute to the emerging picture of high disparity among early sponges. [ABSTRACT FROM AUTHOR]