Victoria E. McCoy

FACTORS CONTROLLING EXCEPTIONAL PRESERVATION IN CONCRETIONS

ABSTRACT Concretions are an important source of soft-bodied fossils. In order to determine the controls on exceptional fossilization within concretions, we scored 88 concretion-bearing sites for 11 environmental and compositional variables together with presence/absence of soft-tissue preservation and analyzed their relationships using multiple correspondence analysis (MCA) and qualitative logistic regression. Sites yielding exceptional fossils were distributed randomly through the 88 examples considered, suggesting that exceptional preservation can occur in almost any environment where concretions form. The patterns of interaction of the variables in the MCA revealed that the most important factors controlling exceptional preservation in concretions are related to (1) the potential for fossil preservation in the broader depositional setting (i.e., how likely fossils are to be preserved without concretions but in the same depositional environment) and (2) the rate of concretion growth. In an analysis of the contribution of individual variables, logistic regression showed that two features correlate with the presence of soft-tissue preservation in concretions: (1) fine-grained host lithology, and (2) relatively constant δ13C values. The role of the first of these can be tested experimentally. Decay experiments on fish tissue in glass beads of three different sizes and therefore of different permeability showed that decay is inhibited, and mineral precipitation enhanced, in low-permeability sediments. Thus a process of positive feedback promotes exceptional preservation where early cementation results in a rapid decrease in permeability during concretion formation. The second feature, a relatively constant δ13C trend, suggests that certain patterns of concretion growth, pervasive growth or concentric growth with one growth layer, are more conducive to fossilization than others.

The ‘Tully monster’ is a vertebrate

Problematic fossils, extinct taxa of enigmatic morphology that cannot be assigned to a known major group, were once a major issue in palaeontology. A long-favoured solution to the ‘problem of the problematica’, particularly the ‘weird wonders’ of the Cambrian Burgess Shale, was to consider them representatives of extinct phyla. A combination of new evidence and modern approaches to phylogenetic analysis has now resolved the affinities of most of these forms. Perhaps the most notable exception is Tullimonstrum gregarium, popularly known as the Tully monster, a large soft-bodied organism from the late Carboniferous Mazon Creek biota (approximately 309–307 million years ago) of Illinois, USA, which was designated the official state fossil of Illinois in 1989. Its phylogenetic position has remained uncertain and it has been compared with nemerteans, polychaetes, gastropods, conodonts, and the stem arthropod Opabinia. Here we review the morphology of Tullimonstrum based on an analysis of more than 1,200 specimens. We find that the anterior proboscis ends in a buccal apparatus containing teeth, the eyes project laterally on a long rigid bar, and the elongate segmented body bears a caudal fin with dorsal and ventral lobes. We describe new evidence for a notochord, cartilaginous arcualia, gill pouches, articulations within the proboscis, and multiple tooth rows adjacent to the mouth. This combination of characters, supported by phylogenetic analysis, identifies Tullimonstrum as a vertebrate, and places it on the stem lineage to lampreys (Petromyzontida). In addition to increasing the known morphological disparity of extinct lampreys, a chordate affinity for T. gregarium resolves the nature of a soft-bodied fossil which has been debated for more than 50 years.

What big eyes you have: the ecological role of giant pterygotid eurypterids

Eurypterids are a group of extinct chelicerates that ranged for over 200 Myr from the Ordovician to the Permian. Gigantism is common in the group; about 50% of families include taxa over 0.8 m in length. Among these were the pterygotids (Pterygotidae), which reached lengths of over 2 m and were the largest arthropods that ever lived. They have been interpreted as highly mobile visual predators on the basis of their large size, enlarged, robust chelicerae and forward-facing compound eyes. Here, we test this interpretation by reconstructing the visual capability of Acutiramus cummingsi (Pterygotidae) and comparing it with that of the smaller Eurypterus sp. (Eurypteridae), which lacked enlarged chelicerae, and other arthropods of similar geologic age. In A. cummingsi, there is no area of lenses differentiated to provide increased visual acuity, and the interommatidial angles (IOA) do not fall within the range of high-level modern arthropod predators. Our results show that the visual acuity of A. cummingsi is poor compared with that of co-occurring Eurypterus sp. The ecological role of pterygotids may have been as predators on thin-shelled and soft-bodied prey, perhaps in low-light conditions or at night.

SEDIMENT PERMEABILITY AND THE PRESERVATION OF SOFT-TISSUES IN CONCRETIONS: AN EXPERIMENTAL STUDY

Abstract Sediment permeability is hypothesized to affect soft tissue fossilization within concretions through its effects on organic decay and concretion growth. The role of permeability was tested in a series of experiments in which cod tissue was decayed in glass beads of varying permeabilities. Decay was measured using infrared gas analysis (IRGA), and mineral precipitation within the beads (a proxy for concretion growth) was measured using micro CT scanning. The interactions of the three variables—sediment permeability, decay, and mineralization—were assessed with MANOVA and with linear regressions of decay and precipitation per unit decay on permeability. These two linear regressions were combined into a more general, nonlinear expression of the relationship between permeability and total mineral precipitation. The results show that sediment permeability has two competing effects on precipitation, the strength of each varying dynamically depending on permeability. Low permeability environments inhibit decay, thus enhancing fossilization but inhibiting overall precipitation because a build-up of decay products is necessary to promote mineral formation (the “decay effect”). However, low permeability environments can also increase precipitation per unit decay by inhibiting the diffusion of decay products away from the carcass, allowing for a faster build-up of decay products (the “mineralization effect”). At low permeabilities, the decay effect dominates (decay controlled), and precipitation is positively correlated with permeability. At higher permeabilities, the mineralization effect dominates (permeability controlled), and precipitation is negatively correlated with permeability. The experiments show that fossilization within concretions is promoted by decay inhibition (at low permeabilities) and rapid concretion growth (at intermediate permeabilities). Thus, the effects of permeability on fossilization are complex, and influence the mechanism of fossilization.

All the better to see you with: eyes and claws reveal the evolution of divergent ecological roles in giant pterygotid eurypterids

Pterygotid eurypterids have traditionally been interpreted as active, high-level, visual predators; however, recent studies of the visual system and cheliceral morphology of the pterygotid Acutiramus contradict this interpretation. Here, we report similar analyses of the pterygotids Erettopterus, Jaekelopterus and Pterygotus, and the pterygotid sister taxon Slimonia. Representative species of all these genera have more acute vision than A. cummingsi. The visual systems of Jaekelopterus rhenaniae and Pterygotus anglicus are comparable to that of modern predatory arthropods. All species of Jaekelopterus and Pterygotus have robust crushing chelicerae, morphologically distinct from the weaker slicing chelicerae of Acutiramus. Vision in Erettopterus osiliensis and Slimonia acuminata is more acute than in Acutiramus cummingsi, but not to the same degree as in modern active predators, and the morphology of the chelicerae in these genera suggests a grasping function. The pterygotids evolved with a shift in ecology from generalized feeder to specialized predator. Pterygotid eurypterids share a characteristic morphology but, although some were top predators, their ecology differs radically between genera.

A Possible Tracemaker for Arthrophycus alleghaniensis

Abstract 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.

In Search of the Arthrophycus Parallelus Tracemaker

Abstract Arthrophycus is an iconic ichnogenus known from lower Paleozoic clastic strata worldwide, yet its origin remains controversial. A medial groove imparts a bilobed symmetry in some forms, implicating arthropods as the tracemaker. Other forms have regular annulae that evoke an annelid body plan. Transverse ridges in some bilobed forms of Arthrophycus, however, were deemed by some as too blunt to have been made by arthropods, and the annulation is unlikely to have anything but a superficial resemblance to “worms.” Recent work has converged on a nontrilobite, long-bodied arthropod as the likely Arthrophycus tracemaker. Given the breadth of morphological variation and range in size included in various ichnospecies of Arthrophycus, there are likely multiple tracemakers for this ichnogenus, and the tracemakers may belong to more than one phylum. Some ichnospecies of Arthrophycus may even represent the only physical record of an unknown or poorly fossilized group of organisms. Observations of the physiology and neoichnology of modern Arthrophycus tracemaker analogs support the conclusion that the maker of the Carboniferous ichnospecies Arthrophycus parallelus is neither an annelid nor a previously described arthropod but a yet unknown member of the Ecdysozoa.

TAPHONOMIC DISPARITY IN FORAMINIFERA AS A PALEO-INDICATOR FOR SEAGRASS

Abstract Seagrass meadows are a key component of marine ecosystems that play a variety of prominent geobiological roles in modern coastal environments. However, seagrass itself has low preservation potential, and consequently seagrass meadows are hard to identify in the rock record. In this study we combine observational taphonomic data from a modern sparse seagrass meadow with actualistic taphonomic experiments, in order to test whether taphonomic disparity (i.e., evenness in the distribution of taphonomic grades among multiple individuals) in the larger benthic foraminiferan Archaias angulatus has potential as a paleo-indicator for seagrass dominated communities. Our observational study demonstrates that sparse seagrass meadows possess a higher proportion of both pristine and highly altered tests than non-seagrass settings. Our taphonomic experiments, conducted over a six-month period, demonstrate a greater magnitude of bioerosion and diversity of bioerosion types in foraminifera deployed within sparse seagrass patches, than those deployed in patches without any seagrass cover. Although our experimental results in particular have high variability, these combined approaches provide a link between pattern (high taphonomic disparity) and process (higher rates of bioerosion) in developing the taphonomic signature of seagrass meadows. On the basis of these results we suggest several taphonomic criteria that could be used to identify seagrass meadows in the rock record. These criteria are potentially species-independent, and so may have greater utility as seagrass proxies than invertebrate indicator species that frequently have limited temporal or spatial distributions.

Modern Analogs for the Study of Eurypterid Paleobiology

Eurypterids are extinct, chelicerate arthropods whose life habits might be elucidated through comparison with living analogs. There are at least two potential eurypterid analogs, xiphosurans and arachnids (specifically, scorpions). Eurypterids and scorpions share striking morphologic and structural similarities despite their different habitats (aquatic vs. terrestrial); eurypterids and xiphosurans share numerous morphological characters and an aquatic habit. Despite the physiological differences inherent between aquatic and terrestrial chelicerates, the similarities in the basic body plan suggest that eurypterids and scorpions faced similar functional challenges during ecdysis, but eurypterid feeding was probably more similar to that of xiphosurans. For studies on the mechanical strength and functional morphology of the eurypterid exoskeleton, Limulus is the closer analog. The choice of modern analog for other aspects of eurypterid paleobiology, including reproduction and whether eurypterids were active predators, is a matter of discussion. The lack of a single, clear eurypterid analog from among extant chelicerates may reflect that eurypterids occupied an ecological niche intermediate between xiphosurans and arachnids. The search for a modern analog for eurypterids, then, is not likely to yield a single model organism.