*Corresponding authorswith Tullimonstrum being a notable anomaly in this respect. Tullimonstrum, a monotypic taxon known from several hundred specimens, is preserved as stains with some relief within Mazon Creek siderite nodules. Despite the uncertainty about its position in the tree of life, there is a surprisingly high level of agreement regarding the arrangement and shape of anatomical features ( Fig.1 and Table. 1). The anatomical complexity, evident cardinal axes and the bilateral symmetry demonstrates that Tullimonstrum is a bilaterian 1-3,5 but beyond this, it has defied systematic placement. Given the consensus regarding the shape and anatomical disposition of body parts this might seem perplexing but the issue is in fact quite simple: there is little agreement about its affinities because no study has identified unequivocal homologies/synapomorphies upon which to base a solid comparative anatomical interpretation. This is a classic example of how, without the criterion of topological relations between body parts as a potential falsifier of character hypotheses, testing of alternative hypotheses becomes problematic 9,10 . Different choices of extant anatomical comparator result in radically different hypotheses of homology and affinity for Tullimonstrum ( Table. 1) but evidence to test which hypothesis is correct remains elusive.Where topological data in fossils are equivocal, other homology criteria, normally subordinate to topology assume greater importance 9-11 . Here we apply the criterion of the intrinsic properties of body parts (also referred to as 'special qualities'' 10 or 'correspondence of composition' 11 ) allowing us to resolve the phylogenetic placement of Tullimonstrum.One of the defining characters of Tullimonstrum is the transverse bar. Associated with this in many specimens is a pair of dark structures which, regardless of the orientation of the fossil, occur at the distal ends of the bar (Figs 1-2; Extended Data Fig. 1). The transverse bar is relatively straight, although it bends forwards or backwards in some specimens 3 ; it is preserved in relief, suggesting a relatively recalcitrant structure, but there is no evidence that it was biomineralised 3 . Scanning electron microscopy and EDS reveal that the dark structures comprise thick, multi-layered masses of tightly-packed, micron-sized bodies composed of carbonaceous material (Fig. 2). They exhibit two distinct morphologies: The composition, anatomical localisation and fabrics indicate that the cylindrical and oblate bodies are layers of melanosomes; the range of shape and size compares closely with extant and fossilised melanosomes 6 . To further test this hypothesis we employed TOF-SIMS and principal component analyses (PCA) to compare the relative intensity distribution of the melanin-specific peaks originating from fresh, artificially matured, fossil melanin and non-melanin samples (Extended Data Fig. 3). Spectra from Tullimonstrum and pure melanin samples 12 shows a similar spectral composition (Fig. 3, Extended Data Fig.3). PCA...

Coleoid cephalopods are characterized by internalization of their shell, and are divided into the ten-armed Decabrachia (squids and cuttlefish) and the eight-armed Vampyropoda (octopuses and vampire squid). They have a rich fossil record predominantly of the limited biomineralized skeletal elements they possess: arm hooks, statoliths, mouthparts (the buccal mass) and internal shell (gladius or pen), although exquisitely preserved soft tissue coleoids are known from several Lagerst€ atten worldwide. Recent studies have shown that although morphological similarities between extant decabrachian gladii and fossil examples exist, no known examples of fossil decabrachians are currently known. However, molecular clock data and phylogenetic bracketing suggest that they should be present in Lagerst€ atten that are rich in vampyropod soft tissue fossils (i.e. Hâkel and Hâdjoula Lagerst€ atten, Cretaceous, Lebanon). We propose that a hitherto unknown taphonomic bias pertaining to the differing methods of buoyancy control within coleoid groups limits preservation potential. Both negatively and neutrally buoyant decabrachians use chemical buoyancy control (ammonia) whereas vampyropods do not. In the event of rapid burial in an environment conducive to exceptional preservation, ammonia dramatically decreases the ability of the decabrachian carcass to generate the required pH for authigenic calcium phosphate replacement, limiting its preservation potential. Moreover, the greater surface area and comparatively fragile dermis further decrease the potential for fossilization. This taphonomic bias may have contributed to the lack of preserved labile soft-tissues in other cephalopods groups such as the ammonoids.

One of the best records of late Paleozoic ecosystems, the Mazon Creek Lagerstätte is world famous for its striking flora and fauna preserved within siderite concretions. Distinct from other late Carboniferous concretionary Lagerstätten because of the remarkable fidelity of soft tissues and pigments that are frequently preserved, the Mazon Creek has seen a revival in investigations during the last 10 years using modern palaeontological techniques. However, many of these modern investigations build upon a literature that incorrectly interprets the palaeoenvironment of the Mazon Creek and the separate biotas: there is a lack of evidence to support a distinct freshwater fauna. Here, we present a detailed overview of the Mazon Creek Lagerstätte, including the palaeoenvironmental conditions, organisms present and the complex taphonomic processes involved in fossil formation. Investigation into the formation of siderite concretions and the complex taphonomic processes controlling soft-bodied preservation are still continuing but are reviewed in detail.

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