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Nemavermes mackeei Schram, 1973, found in the Mazon Creek fossil site and the Bear Gulch Limestone, was described initially as a free-living marine nematode. Here we investigate 13 specimens of N. mackeei from the Mazon Creek to reassess its morphology and identity, and also two specimens originally identified as Gilpichthys greenei Bardack and Richardson, 1977. Based on the extensive morphological variation among these specimens, N. mackeei encompasses multiple species that are only distantly related. The holotype of N. mackeei is a proboscis of Tullimonstrum gregarium Richardson, 1966, making N. mackeei a junior synonym of T. gregarium. However, the other specimens that we investigated could not be attributed to T. gregarium. We name a new species from these specimens: Squirmarius testai new genus new species, a cyclostome. One specimen is likely a juvenile G. greenei. Other specimens were not identified during this study but represent a variety of vermiform bilaterians.
Fossils in amber are a particularly important and unique palaeobiological resource. Amber is best known for preserving exceptionally life-like fossils, including microscopic anatomical details, but this fidelity of preservation is an end-member of a wide spectrum of preservation quality. Many amber sites only preserve cuticle or hollow moulds, and most amber sites have no fossils at all. The taphonomic processes that control this range in preservation are essentially unknown. Here, we review the relationship between amber groups and fossil preservation, based on published data, to determine whether there is a correlation between resin type and aspects of preservation quality. We found that ambers of different chemistry demonstrated statistically significant differences in the preservational quality and the propensity of a site to contain fossils. This indicates that resin chemistry does influence preservational variation; however, there is also evidence that resin chemistry alone cannot explain all the variation. To effectively assess the impact of this (and other) variables on fossilisation in amber, and therefore biases in the amber fossil record, a more comprehensive sampling of bioinclusions in amber, coupled with rigorous taphonomic experimentation, is required.
Arthrophycus alleghaniensis is a well-known trace fossil common in the lower Silurian of the Appalachian Basin, eastern U.S.A. Despite the distinctive morphology of this trace, with few exceptions, hypotheses about the nature of the tracemaker have not extended beyond that of a long-bodied, segmented organism. A single organic compression of a long-bodied arthropod discovered in shale interbedded with sandstones containing A. alleghaniensis in the Silurian (Llandovery) Tuscarora Formation at Mann Narrows, Pennsylvania is described. The specimen preserves evidence of two trunk tagmata: an anterior tagma with tergites extending into broad, rectangular pleurae, and a posterior tagma bearing long, curved spines. Head and appendages are not preserved. The new arthropod, Pleuralata spinosa n. gen. n. sp., matches the size and general morphology required for an A. alleghaniensis tracemaker. Precise systematic affinities of this new arthropod could not be determined. This discovery supports the conclusion that the tracemakers of various Arthrophycus ichnospecies are likely poorly preserved, and presently unknown, members of the Ecdysozoa.
Carbonate concretions may preserve exceptional soft-tissue fossils. Organismal decay in sediment can produce HCO3− faster than it diffuses away, creating a local microenvironment of high alkalinity around the decaying organism that promotes carbonate precipitation. Infilling of sediment pore-space around the decaying organism decreases permeability and inhibits decay, thus increasing preservation potential within the concretion and promoting soft-tissue preservation. Different patterns of concretion growth may promote different mechanisms of soft-tissue preservation. Other factors such as shifting salinity and clay mineral chemistry, which increase preservation potential in the depositional environment, also promote soft tissue preservation within concretions. The rate of concretion nucleation and growth affects preservation; the faster concretions nucleate and grow, the better the preservation. Concretionary preservation is biased both by concretion-promoting environments and organism size effects on concretion formation.