Invertebrate Ichnology

The disappearance of the soft-bodied Ediacara biota at the Ediacaran–Cambrian boundary potentially represents the earliest mass extinction of complex life, although the precise driver(s) of this extinction remain unresolved. The ‘biotic replacement’ model proposes that an evolutionary radiation of metazoan ecosystem engineers in the latest Ediacaran profoundly altered marine palaeoenvironments, resulting in the extinction of Ediacara biota and setting the stage for the subsequent Cambrian Explosion. However, metazoan ecosystem engineering across the Ediacaran–Cambrian transition has yet to be quantified. Here, we test this key tenet of the biotic replacement model by characterizing the intensity of metazoan bioturbation and ecosystem engineering in trace fossil
assemblages throughout the latest Ediacaran Nama Group in southern Namibia. The results illustrate a dramatic increase in both bioturbation and ecosystem engineering intensity in the latest Ediacaran, prior to the Cambrian boundary. Moreover, our analyses demonstrate that the highest-impact ecosystem engineering behaviours were present well before the onset of the Cambrian. These data provide the first support for a fundamental prediction of the biotic replacement model, and evidence for a direct link between the early evolution of ecosystem engineering and the extinction of the Ediacara biota.

Environmental changes within a Neogene coastal dune system are recorded by endobenthic unioniform bivalves that lived in muddy or sandy interdune pond sediments. These bivalves were suspension-filter feeders that formed dense, almost monospecific communities in the wet-interdune deposits of the continental intervals of the Río Negro Formation (late Miocene-early Pliocene). Activity of unioniform bivalves appears to be related to sediment type; resting and locomotion traces dominate in the muddy heterolithic facies, whereas equilibrium/escape structures prevail in the sand-dominated heterolithic facies. These traces characterize two scenarios of the wet-interdune development. First, during high and/or relatively stable water levels, bivalves colonized the muddy bottom and produced resting and locomotion traces. When water level dropped due to desiccation, biogenic structures were impacted by the formation of mud cracks and subsequently covered by sand delivered by migrating dunes. Second, in spite of dune migration, some interdune areas remained wet or flooded and, in response to sediment aggradation, the bivalves produced equilibrium or escape structures, depending on the thickness of eolian sand cover. Only the integration of ichnologic and sedimentologic observations allows deciphering the evolution of the Neogene wet-interdune system in such a detail.

Eocene oceanic red beds that formed in a well-oxygenated setting at low sed-imentation rates below the calcite compensation depth are effectively barren of organic carbon in the present state. Recurrent distal low-erosive turbidites preserve the bioturbated zone underneath that documents seasonal and long-term fluctuating accumulation of considerable amounts of organic matter on the sea floor as evidenced by Scolicia; the producers of this trace fossil exploited nutritious organic matter conserved in turbidite-buried sea floor deposits. Over the long-term, slow average sedimentation of (hemi)pelagic oxic (red) mud led to long oxygen exposure times and low burial of organic matter. Consequently, trace fossils representing persistent sediment-feeding modes are of small size. Although the food-limited setting appears appropriate for producers of graphoglyptids, such 'stationary' burrows have not been encountered because seasonal deposition of organic matter fostered at least temporary surface layer feeding organisms, for instance producers of Nereites irregularis that intensively reworked the sediment and, hence, hindered graphoglyptid production. These findings confirm palaeoceanographic modelling results that suggest upwelling in the study area during the Eocene.

The association of vertebrate remains and invertebrate traces, although less studied than other bioerosion traces, provides important paleoecological information. This report describes Cubiculum ornatum Roberts, Rogers, and Foreman 2007, Osteocallis Roberts, Rogers, and Foreman 2007 and other unidentified insect traces from the dermal skeletal remains of glyptodonts found in Uruguay. They come from the Fray Bentos Formation (Late Oligocene), the Camacho Formation (Late Miocene) and the Dolores Formation (Late Pleistocene-Early Holocene). The reported traces were likely made by sarcosaprophagous beetles, which indicate depositional conditions with dry episodes in a warm climate for the referred stratigraphical units.

The mixed layer of modern oceans is a zone of fully homogenized sediment resulting from bioturbation. The mixed layer is host to complex biogeochemical cycles that directly impact ecosystem functioning, affecting ocean productivity and marine biodiversity. The timing of origin of the mixed layer has been controversial, with estimates ranging from Cambrian to Silurian, hindering our understanding of biogeochemical cycling and ecosystem dynamics in deep time. Here we report evidence from the Global Stratotype Section and Point (GSSP) of the basal Cambrian in the Burin Peninsula of Newfoundland, Canada, showing that a welldeveloped mixed layer of similar structure to that of modern marine sediments was established in shallow marine settings by the early Cambrian (approximately 529 million years ago). These findings imply that the benthos significantly contributed to establishing new biogeochemical cycles during the Cambrian explosion.

Tracks and trackways of a range of Pleistocene megafauna can be found in White Sands National Monument, New Mexico, U.S.A. These tracks occur is several forms, not all of which are visible and some of which are only intermittently visible depending on lighting and moisture conditions. Here we present the result of a successful test of cesium vapor magnetometry to detect a known Columbian mammoth trackway. This initial test found that not only the known mammoth tracks were easily detected by the method, but that the tracks of additional species, though not visible to the eye, were detected in the vicinity of the mammoth tracks, including likely giant sloth tracks. Our initial results indicate that resolution may be suitable to distinguish between the tracks of various species, including possibly humans which are known archaeologically to have overlapped temporally with these species in the southwestern U.S. This preliminary result has immediate implications for the detection and documentation of Pleistocene track sites, and further refinement of the procedure is planned in the coming months.

The Rockslide Formation (middle Cambrian, Drumian, Bolaspidella Zone) of the Mackenzie Mountains, northwestern Canada, hosts the Ravens Throat River Lagerstätte, which consists of two, 1-m thick intervals of greenish, thinly laminated, locally burrowed, slightly calcareous mudstone yielding a low-diversity and low-abundance fauna of bivalved arthropods, 'worms', hyoliths, and trilobites. Also present are flattened, circular, black carbonaceous objects averaging 15 mm in diameter, interpreted as coprolites preserved in either dorsal or ventral view. Many consist of aggregates of ovate carbonaceous flakes 0.5–2 mm long, which are probably compacted fecal pellets. Two-thirds contain a variably disarticulated pair of arthropod valves, and many also contain coiled to fragmented, corrugated 'worm' cuticle, either alone or together with valves. A few contain an enrolled agnostoid. In rare cases a ptychoparioid cranidium, agnostoid shield, bradoriid valve, or hyolith conch or operculum is present; these are taken to be due to capture and ingestion of bioclasts from the adjacent seafloor. Many of the coprolites are associated with semi-circular spreiten produced by movement of the worm-like predator while it occupied a vertical burrow. Its identity is unknown but it clearly exhibited prey selectivity. Many coprolites contain one or more articulated hyoliths, ptychoparioid trilobites, or outstretched agnostoid arthropods oriented dorsal side up. These are interpreted as opportunistic coprovores drawn to the organic-rich fecal mass while it was lodged near the entrance to the burrow. This argues that hyoliths were mobile detritivores, and agnostoids were mainly nektobenthic or benthic, like the ptychoparioid trilobites. Fecal matter was probably an important source of nutrition in the Cambrian food web.

Edward Hitchcock was one of the most influential 19 th century ichnologists. In a career that lasted nearly three decades, he established 31 invertebrate ichnogenera for Early Jurassic traces from the Hartford and Deerfield Basins of Connecticut and Massachusetts, some of which are in widespread use today. Upon reexamining some of Hitchcock's specimens, we have identified a number of fossils that are similar to the well-known ichnogenus Treptichnus. Modern ichnologists generally interpret non-marine occurrences of these burrows as the traces of larval insects. In some instances, Hitchcock ascribed his fossils to the activities of annelids or larval insects, and his descriptions of their mode of formation are similar to those presented in much more recent works. However, he also interpreted some treptichnids as algal/ plant fossils. We suspect that his varied interpretations reflect differences in morphology and preservation among specimens. Hitchcock worked in the 19 th century, a time known as the " Age of Fucoids " , and his comments also reflect the prevailing uncertainty on trace fossil origins. RIASSUNTO-[Le tracce fossili treptichnidi di Hitchock (Giurassico, Massachusetts, USA): interpretazioni contrastanti nell' " Era dei Fucoidi " ]-Edward Hitchcock è stato un influente geologo americano del XIX secolo. In una carriera durata quasi tre decenni, scrisse numerosi articoli e pubblicò due volumi fondamentali per la nascente scienza dell'icnologia: " Ichnology of New England " e " Supplement to the Ichnology of New England ". Ha istituito 31 icnogeneri di tracce di invertebrati sulla base di materiale giurassico proveniente dai bacini Hartford e Deerfield (Connecticut e Massachusetts, Stati Uniti). Alcuni di questi icnogeneri sono ampiamente utilizzati ancora oggi. Esaminando la raccolta di Hitchcock, che ha sede presso il Beneski Museum of Natural History (Amherst College) e il Museum of Comparative Zoology (Harvard University), abbiamo identificato diverse tracce simili al noto icnogenere Treptichnus, istituito nel 1889, 25 anni dopo la morte di Hitchcock. Questi campioni sono stati trovati su lastre etichettate ACM ICH 14/4, 16/4, 33/35, 55/20 e MCZ 114312. Al momento della loro descrizione iniziale, Hitchcock ordinò queste tracce in due gruppi. Egli ha attribuito alcuni di questi fossili al suo icnogenere Halysichnus, pensando ad anellidi o insetti larvali come produttori. Le deduzioni di Hitchock non sono lontane dall'attuale interpretazione di queste tane, attribuite dalla maggior parte degli autori al lavoro di insetti larvali. Ha dibattuto, tuttavia, se il secondo gruppo fosse di origine animale oppure se fossero " fucoidi " , ossia piante o alghe fossili. Pensiamo che le sue varie interpretazioni riflettano differenze nella morfologia e nella preservazione delle tracce. Ad esempio, Hitchcock considerava le biforcazioni come una caratteristica di piante e alghe fossili. Di conseguenza, quelle tracce che mostravano biforcazioni erano classificate come fucoidi, mentre quelle che non presentavano questa caratteristica erano considerate tane. I commenti di Hitchock riflettono la sua epoca, nota come " Era dei Fucoidi " , durante la quale le origini di molte tracce fossili erano incerte. A causa della sua prolifica produzione e della volontà di attribuire molte tracce fossili al lavoro di invertebrati, Hitchock può essere considerato come una figura di transizione nella storia dell'icnologia.

Five types of coprolites, represented by 40 specimens from the Cambrian (Series 2-3) Burgess Shale-type deposits in the Pioche Shale of Nevada and the Spence Shale of Utah, are described. They are preserved in finely laminated deep-water calcareous mudstones. Round to ellipsoid features 13–42 mm in diameter consisting of black carbon film and variable amounts of skeletal fragments are interpreted as coprolites that were originally deposited in a burrow. Two kinds of elongated coprolites are also preserved and either consist of small pellets or skeletal debris. The pellets are typically 0.5 to 2 mm across and have a round to ellipsoid outline. Two different types of pellet-filled burrows are also present. The presence of organic tissue and skeletal fragments in some coprolites provides direct evidence of predatory or scavenging activity , and may advance understanding of the food chain in these Cambrian deposits. •

Organism-substrate interactions and their products – biogenic structures – are important biosignatures on Earth. This study discusses the application of ichnology – the study of organism-substrate interactions – to the search for present and past life beyond Earth. Three main questions are addressed: (1) Why to look for biogenic structures (i.e. traces and ichnofabrics) beyond Earth? (2) What biogenic structures to expect on other planets, moons and asteroids? (3) How to study extraterrestrial biogenic structures? Review of terrestrial evidence highlights a set of properties that make traces and ichnofabrics important for the search of potential extraterrestrial life: trace fossils preserve the activity of soft-bodied organisms; biogenic structures are resilient to processes that obliterate other biosignatures (i.e. mechanical and chemical degradation , diagenesis, tectonism and metamorphism); traces are very visible biosignatures; traces indicate environment and behaviour; traces can be universal biosignatures, i.e., biosignatures ideally suited for detecting any type of life. A model of organism-substrate interactions beyond Earth is here proposed. Expected extraterrestrial traces are those that manifest behaviours that allow to maintain homeostasis: excavations, meandering traces and biodeposition structures. Most of the existing rovers and orbiters provide basic instruments for searching these traces. It is here suggested that the search for extraterrestrial biogenic structures by rovers would also benefit from artificial adjustable lighting, GPR, LiDAR, and drilling equipment with optical televiewer. In this study, open-access databases of rover and orbiter imagery have been searched for traces and ichno-fabrics, but no unquestionable evidence of biogenic structures beyond Earth has been found besides those produced by humans. This sounds along the lines of the famous Fermi Paradox: if the universe is teeming with aliens, where are their traces? Results of this search show that habitable environments are not the only place to look for biogenic structures; non-habitable environments such as moons without atmosphere can favour the preservation of shallow-tier traces. The better preservation potential of traces compared to other biosignatures greatly widens the issue of planetary protection, including the interaction between astronauts or vehicles and the substrate may produce disturbances. Although this study highlights a new direction of study with the tools and concepts of ichnology, dialogue MARK between the astrobiological and ichnological communities is needed to use its full potential and possibly answer one of the major questions of science: Does life exist beyond Earth?