Nicholas J. Minter

Morphogenesis of an extended phenotype: four-dimensional ant nest architecture.

Animals produce a variety of structures to modify their environments adaptively. Such structures represent extended phenotypes whose development is rarely studied. To begin to rectify this, we used micro-computed tomography (CT) scanning and time-series experiments to obtain the first high-resolution dataset on the four-dimensional growth of ant nests. We show that extrinsic features within the environment, such as the presence of planes between layers of sediment, influence the architecture of Lasius flavus nests, with ants excavating horizontal tunnels along such planes. Intrinsically, the dimensions of the tunnels are associated with individual colonies, the dynamics of excavation can be explained by negative feedback and the angular distribution of tunnels is probably a result of local competition among tunnels for miners. The architecture and dynamics of ant nest excavation therefore result from local interactions of ants with one another and templates inherent in the environment. The influence of the environment on the form of structures has been documented across both biotic and abiotic domains. Our study opens up the utility of CT scanning as a technique for observing the morphogenesis of such structures.

Skimming the surface with Burgess Shale arthropod locomotion

The first arthropod trackways are described from the Middle Cambrian Burgess Shale Formation of Canada. Trace fossils, including trackways, provide a rich source of biological and ecological information, including direct evidence of behaviour not commonly available from body fossils alone. The discovery of large arthropod trackways is unique for Burgess Shale-type deposits. Trackway dimensions and the requisite number of limbs are matched with the body plan of a tegopeltid arthropod. Tegopelte, one of the rarest Burgess Shale animals, is over twice the size of all other benthic arthropods known from this locality, and only its sister taxon, Saperion, from the Lower Cambrian Chengjiang biota of China, approaches a similar size. Biomechanical trackway analysis demonstrates that tegopeltids were capable of rapidly skimming across the seafloor and, in conjunction with the identification of gut diverticulae in Tegopelte, supports previous hypotheses on the locomotory capabilities and carnivorous mode of life of such arthropods. The trackways occur in the oldest part (Kicking Horse Shale Member) of the Burgess Shale Formation, which is also known for its scarce assemblage of soft-bodied organisms, and indicate at least intermittent oxygenated bottom waters and low sedimentation rates.

Augerinoichnus helicoidalis, a new helical trace fossil from the Nonmarine Permian of New Mexico

New Mexico contains a significant record of trace fossil assemblages, in terms of both abundance and ichnodiversity, from Lower Permian non-marine depositional settings. Most notable amongst these are the trace fossil assemblages in the Robledo Mountains Formation of the Robledo Mountains in Dona Ana County, southern New Mexico, as recognized by the recent proposal to designate this area as a national monument. These trace fossil assemblages formed on a tidal flat under largely non-marine conditions (Mack and James, 1986; Hunt et al., 1993; Lucas et al., 1995a, 1998) and are dominated by the trackways of tetrapods and arthropods, yielding important information on the paleoecology, diversity and behavior of late Paleozoic arthropods (Braddy and Briggs, 2002; Minter and Braddy, 2006a), as well as evidence of specialized foraging strategies (Minter et al., 2006). Additional Lower Permian trace fossil assemblages occur at a number of localities in New Mexico and represent a variety of non-marine depositional settings. The material described herein represents a new ichnogenus of helical burrow, named Augerinoichnus helicoidalis, and is recurrent across Lower Permian localities in New Mexico (Fig. 1). Augerinoichnus occurs in tidal flat settings from the Robledo Mountains Formation of the Robledo Mountains (Mack and James, 1986; Hunt et al., 1993; Lucas et al., 1995a, 1998), fluviodeltaic coastal plain settings from the Robledo Mountains Formation of the Don˜a Ana Mountains in southern New Mexico (Lucas et al., 1995b), and floodplain sandflat settings from the Abo Formation of Can˜oncito de la Uva in the Joyita Hills of central New Mexico (Hunt et al., 1995), the McLeod Hills of central New Mexico (Lucas et al., 1995c), and the Fra Cristobal Mountains of central New Mexico (Lucas et al., 2005). The report of this new ichnogenus provides important paleoecological information on foraging strategies in nonmarine paleoenvironments, as well as further evidence for the occurrence of trace fossils in non-marine settings similar to those considered indicative of deep marine depositional settings.

Spiral-shaped graphoglyptids from an Early Permian intertidal flat

Spiral-shaped foraging trace fossils, assigned to the graphoglyptid cf. Spirorhaphe azteca, are reported from an Early Permian intertidal flat in the Robledo Mountains of southern New Mexico, USA. Remarkably similar spiral-shaped structures are produced in modern intertidal flats by the paraonid polychaete Paraonis fulgens, and function as traps to capture mobile microorganisms migrating in the sediment in response to tides. We envisage a similar function for the Early Permian trace fossils. Previous studies have suggested that the lack of P. fulgens‐type traces from ancient intertidal deposits indicates that such behavior only evolved geologically recently in such settings. However, this report demonstrates that such specialized foraging behavior was present in intertidal settings by at least the Early Permian. Graphoglyptids are typical of deep-marine settings, and characteristic of the Nereites ichnofacies. This represents their first undoubted occurrence in intertidal facies in the geological record. We postulate that the occurrence of graphoglyptids in deep-marine and intertidal settings is related to the predictability of resources. The scarcity of intertidal graphoglyptids in the geological record is most likely a preservational effect.

The establishment of continental ecosystems

The colonization of land was a major evolutionary transition. Following a protracted prelude to the terrestrial invasion during the Ediacaran to Ordovician, the remainder of the Paleozoic experienced an explosion of diversity and the expansion of benthic biotas into new environments through the creation of new niches. This expansion progressed from coastal settings into rivers, floodplains, deserts, and lakes, as well as increasing colonization of infaunal ecospace. A pattern emerges in which colonization of a new environment is followed by rapid filling of available ecospace, after which animals establish new behavioral programs. These programs are represented initially by the creation of original architectural designs, and subsequently modified by a proliferation of ichnogenera representing variation upon these established themes. The overall pattern is consistent with the early burst model of diversification that has been identified for various animal and plant clades, wherein there is a decoupling as an initial expansion in disparity is followed by an increase in diversity.

A Marine Incursion In the Lower Pennsylvanian Tynemouth Creek Formation, Canada: Implications For Paleogeography, Stratigraphy and Paleoecology

Abstract We document the occurrence of a marine bed, and its associated biota, in the Lower Pennsylvanian (Langsettian) Tynemouth Creek Formation of New Brunswick, and discuss its implications for paleogeography, stratigraphy, and paleoecology. This is only the second marine interval found in the entire Pennsylvanian fill of the Maritimes Basin of Canada, the other being recently found in the broadly same-age Joggins Formation of Nova Scotia. Evidence for the new marine transgression comprises an echinoderm-rich limestone that infills irregularities on a vertic paleosol surface within the distal facies of a syntectonic fluvial megafan formed under a seasonally dry tropical climate. Gray, platy ostracod-rich shales and wave-rippled sandstone beds that directly overlie the marine limestone contain trace fossils characteristic of the Mermia Ichnofacies, upright woody trees, and adpressed megafloras. This association represents bay-fills fringed by freshwater coastal forests dominated by pteridosperms, cordaites, and other enigmatic plants traditionally attributed to dryland/upland habitats. The fossil site demonstrates that marine transgressions extended farther into the interior of Pangea than has previously been documented, and may allow correlation of the Tynemouth Creek and Joggins Formations with broadly coeval European successions near the level of the Gastrioceras subcrenatum and G. listeri marine bands. It also helps explain the close similarity of faunas between the Maritimes Basin and other paleotropical basins, if transgressions facilitated migration of marine taxa into the continental interior.

Nest-seeking rock ants (Temnothorax albipennis) trade off sediment packing density and structural integrity for ease of cavity excavation

We investigated excavation and nest site choice across sediment-filled cavities in the ant Temnothorax albipennis. Colonies were presented with sediment-filled cavities, covering a spectrum from ones that should be quick to excavate but will form a weak enclosing wall to those that should be slow to excavate but form a strong wall. Overall, colonies only showed a significant preference for cavities that were fastest to excavate over those that were slowest. The speed of decision making and moving appears paramount over the suitability of the sediment for forming an enclosing wall. The mechanism behind the choice is the differential between the rates at which alternatives are excavated and accumulate ants. The rates for a particular type of cavity were unaffected by the type with which it was paired. This suggests that there is no significant competition between sites during the decision-making process. Certain colonies were able consistently to discriminate across more closely matched alternatives. These colonies required a greater number of ants to be present and took longer before beginning to move. A race is run between alternatives to become habitable but the process may be tuned across colonies such that it may run for longer and an incorrect or split decision is less likely.

Lithographus, an Abundant Arthropod Trackway from the Cretaceous Haenam Tracksite of Korea

Trackways ascribed to Lithographus hieroglypichus and attributed to pterygote insects are described from the Cretaceous Uhangri Formation of Korea. The locality is part of the Haenam Tracksite at the Uhangri Dinosaur Museum Complex, which is famous for dinosaur, pterosaur and bird tracks. This represents the first report of the arthropod trackway Lithographus from the Cretaceous of Korea. The trackways are preserved in cherty mudstones that formed in the margins of an alkaline lake in the vicinity of active volcanoes. Numerous trackways are preserved at a single horizon. This probably reflects a brief period of exposure of balanced-fill lake margin sediments, which provided a window of opportunity for the production and preservation of trackways of insects that inhabited the region rather than a sudden influx of insects into the area related to volcanism and a productivity bloom.

Palaeoecological implications of the preservation potential of soft-bodied organisms in sediment-density flows: testing turbulent waters

Interpreting how far organisms within fossil assemblages may have been transported and if they all originated from the same location is fundamental to understanding whether they represent true palaeocommunities. In a three-factorial experimental design, we used an annular flume to generate actualistic sandy sediment-density flows that were fast (2 ms−1) and fully turbulent in order to test the effects of flow duration, sediment concentration, and grain angularity on the states of bodily damage experienced by the freshly euthanized polychaete Alitta virens. Results identified statistically significant effects of flow duration and grain angularity. Increasing sediment concentration had a statistically significant effect with angular sediment but not with rounded sediment. Our experiments demonstrate that if soft-bodied organisms such as polychaetes were alive and then killed by a flow then they would have been capable of enduring prolonged transport in fast and turbulent flows with little damage. Dependent upon sediment concentration and grain angularity, specimens were capable of remaining intact over flow durations of between 5 and 180 min, equating to transport distances up to 21.6 km. This result has significant palaeoecological implications for fossil lagerstätten preserved in deposits of sediment-density flows because the organisms present may have been transported over substantial distances and therefore may not represent true palaeocommunities.