Analysis of Mineral Segregation in Euzonus mucronata Burrow Structures: One Possible Method Used in the Construction of Ancient Macaronichnus segregates

Mineralogical segregation of sand grains distinguishes the trace fossil Macaronichnus segregatis, which is composed of a felsic burrow infill with a mafic-and mica-rich burrow mantle. This study focuses on determining the mechanism by which M. segregatis trace-makers segregated mineral grains during deposit feeding. A modern opheliid polychaete, Euzonus mucronata, from Pachena Bay, Vancouver Island (Canada), was examined to explain the activities of their ancient counterparts. Microscopic videotaping of deposit feeding allowed for collection of data on ingestion and excretion through visual grain counts of felsic, mafic, and shell components. Normalization of these grain counts to the composition of the host sediment illustrates preferential ingestion of felsic grains over mafic. Shell fragments were generally avoided and visually mantled the burrows, obscuring the paucity of mafic grains in burrow infills. The avoidance of shell fragments is potentially a function of the large grain size, angular shape, surface texture, and/or associated low nutritive value. The preferential ingestion of felsic grains is attributed to en masse feeding in felsic-rich locales identified through sediment probing. This form of mineral segregation likely reflects the specific nature of the sediment and worm population. Accordingly, en mass deposit feeding in selected felsic-rich localities is one possible mechanism used in the construction of Macaronichnus segregatis and M. segregatis-like structures.     

Extant Lizard Tracks: Variation and Implications for Paleoichnology

In this study, I collected tracks and trackways from nine species of extant lizards representing all five major lizard clades. Previously, tracks from species of only two of these clades were described. Lizard tracks conventionally are regarded as having curved digit imprints that progressively increase in length from digit I to IV, with a smaller digit V directing antero-laterally. However, the zygodactylous feet of chameleons (Calumma parsonii and Furcifer pardalis), the posteriorly directed digit V in the pes of ground-dwelling geckos (Eublepharis macularius) and the rounded feet of blue-tongued skinks (Tiliqua scincoides) did not make “typical” lizard tracks, and demonstrate that even within a limited taxonomic sample there can be considerable variation in the morphologies of lizard tracks. Among the lizards examined, mode of locomotion and how the feet function have more influence on the morphology of tracks than does the phylogenetic affinities of the trackmaker. This preliminary neoichnological study increases the known variation in lizard tracks and aids in interpreting the fossil trackway record by providing comparative information that can be used to identify fossil tracks made by lizards.     

Neoichnological activities of endobenthic invertebrates in downdrift coastal Ganges delta complex,India: Their significance in trace fossil interpretations and paleoshoreline reconstructions

Neoichnological studies of the downdrift coastal Ganges deltaic region (Ganges Delta Complex) indicate the nature and environmental zonation of the lebensspuren of common endobenthic invertebrates dominated by the brachyuran amphibious crab Family Ocypodidae (Ocypode spp., Ilyaplax pusillus and Uca marionis) and are utilized to interpret analogous trace fossils and paleoshoreline environments. The polycha‐ete Diopatra cuprea, the gastropods Turritella spp., Tele‐scopium telescopium and Cerithidea obtusum, some bivalves and the boring crab Charybdis rostrata also produce diagnostic lebensspuren.

The measured ichnoprofiles reveal the development of coast parallel Uca ‐Turritella (backswamps and salt‐marshes; 1 type burrows), Ocypode ‐ Ilyoplax (backshore to foreshore), Charybdis rostrata (foreshore relict woodground; boring structures), polychaete (middle‐lower foreshore; current ‐ oriented agglutinated burrows) and bivalve ‐ gastropod (lower foreshore; trails) ichnozones. Ocypode ‐ Ilyoplax, irrespective of ontogenic stages and sex, produce I, J, U, Y, and multibranched Y ‐ shaped burrows (juvenile ‐ old in backshore versus young ‐ adult in foreshore) in an orderly fashion. Their burrow density and diversity attain a maximum in backshore and upper foreshore respectively. Exceptionally, high burrow populations produce network burrow systems in the backshore. Juvenile pelletal designs (upper foreshore) and general landward burrow inclination are conspicuous.

The described lebensspuren, having a wide range of ancient analogues, provide supportive evidence in the identification of lithified crab burrows, paleoshoreline environments and paleosealevel fluctuations.     

Determining Euzonus mucronata Burrowing Rates with Application to Ancient Macaronichnus segregatis Trace-makers

Assessing the temporal significance of invertebrate ichnofossils is essential in interpreting ancient organism behaviors, depositional settings, and bioturbation and sedimentation rates. The trace fossil, Macaronichnus segregatis, is known to represent the work of deposit-feeding polychaetes and commonly occurs as a pervasive structure in shallow-marine sandstone deposits. This study uses the polychaete Euzonus mucronata, which produces M. segregatis-like structures, as a modern analogue to the trace-making counterparts. Field measurements from Pachena Beach, Vancouver Island, Canada, included assessment of population densities and worm behaviors. Volumetric burrowing rates were obtained from a thin-walled aquarium constructed in the laboratory. The burrowing rate calculated for 5 Euzonus (0.089 cm³/hr) was extrapolated to populations (approximately 1,400–5,000 worms/m²) estimated from Pachena Beach, which require 70–300 days to completely rework 0.1 m³ of sediment. Calculated rates are dependent upon the limitations of simulating a natural setting in an aquarium, the population density assessment, and the particular characteristics of the worm population and foreshore at Pachena Bay. However, these initial estimates can still be applied to rock record examples such as the Macaronichnus segregatis found in the Appaloosa Sandstone of Alberta, Canada. In this unit, ancient worms persisted in dense populations and reworked sediment at a rate that exceeded deposition during overall foreshore aggradation.     

Between a rock and a hard place: arthropod trackways and ichnotaxonomy

Several challenges exist in ichnotaxonomy: overcoming the perceived distinction between invertebrate and vertebrate ichnotaxonomy, standardizing terminology, rationalizing the plethora of ichnotaxa already in existence, and developing principles for diagnosing new ichnotaxa. Ichnotaxa should be based on morphology, and this morphology incorporates three key components; the behaviour expressed, the producer, and the substrate. Invertebrate and vertebrate ichnotaxa can both be accommodated within this framework, but they differ in the relative contributions of these components. The key to justifying the synonymy of existing ichnotaxa is the recognition of intergrading specimens. However, this is only the case for minor morphological variants (i.e. those representing minor differences in behaviour, such as gait parameters or stance; or minor differences in preservation, such as undertrack fallout or slight differences in substrate conditions). Intergrading specimens should not be used to justify synonymy between major morphological variants (i.e. those representing major behavioural differences, defined herein as ethological categories; or major differences in preservation, such as formation in soup, soft and firmgrounds), and such specimens should be denoted as hybrids (e.g. Cruziana × Rusophycus ). New ichnotaxa should ideally be based on observations of large samples of material, so that recurrence is demonstrable, and morphological continuums, or subset relationships, representing minor morphological variation, are identified. Ichnotaxa may only be erected on the basis of limited material if they truly represent a unique morphology. These principles have been developed with arthropod trackways in mind, but it is hoped that they will be of more general utility.     

COMMENT ON MARDEN (2013): “REANALYSIS AND EXPERIMENTAL EVIDENCE INDICATE THAT THE EARLIEST TRACE FOSSIL OF A WINGED INSECT WAS A SURFACE SKIMMING NEOPTERAN”

Marden's (2013) reanalysis of Knecht et al. (2011) suggesting that specimen SEMC‐F97 is the result of the skimming behavior of a neopteran insect and, more importantly, fossil evidence of “… surface skimming as a precursor to the evolution of flight in insects” (Marden 2013) is found to be deficient on three fronts: (1) the principal specimen was never viewed firsthand which led to significant morphological misinterpretations; (2) poorly designed and executed neoichnological experiments led to incredulous results; and (3) the assumption that this specimen is fossil evidence supporting the surface skimming hypothesis of the origin of insect flight despite the fact that since its induction into the literature that hypothesis has been refuted based on significant paleontological, phylogenetic, genetic, and developmental evidence.