Molly F. Miller

A semiquantitative field method for evaluating bioturbation on bedding planes

Amount of bioturbation on bedding planes records the relationship between physical and chemical depositional conditions and biological activity. We propose a simple and easy-to-use method for field estimation of horizontal disruption using bedding-plane bioturbation indices that allow rapid generation of large data sets. Photographs of a representative subset of observations can be evaluated in the laboratory using image analysis and statistical analysis programs, providing a check on accuracy and potentially yielding additional information on the size, shape, and distribution of burrows and bioturbated patches. Extent of bioturbation on bedding planes can be integrated with other sedimentologic and ichnologic characteristics to interpret the infaunal community paleoecology and to reconstruct the history of the depositional, erosional, and burrowing events. Furthermore, use of the bedding-plane bioturbation indices and ichnofabric indices on vertical surfaces facilitates comparison of the extent of bioturbation through time, within and between facies, and across and between basins. It therefore enhances recognition of significant changes in physical processes, chemical conditions, and biological activity, and is a powerful tool in identifying long-term biological trends.

Reevaluation of the timing and extent of late Paleozoic glaciation in Gondwana: Role of the Transantarctic Mountains

Evidence from Antarctica indicates that a 2000-km-long section of the Transantarctic Mountains—including Victoria Land, the Darwin Glacier region, and the central Transantarctic Mountains—was not located near the center of an enormous Car- boniferous to Early Permian ice sheet, as depicted in many paleo- geographic reconstructions. Weathering profiles and soft-sediment deformation immediately below the preglacial (pre-Permian) un- conformity suggest an absence of ice cover during the Carbonif- erous; otherwise, multiple glacial cycles would have destroyed these features. The occurrence of glaciotectonite, massive and strat- ified diamictite, thrust sheets, sandstones containing dewatering structures, and lonestone-bearing shales in southern Victoria Land and the Darwin Glacier region indicate that Permian sedimenta- tion occurred in ice-marginal, periglacial, and/or glaciomarine set- tings. No evidence was found that indicates the Transantarctic Mountains were near a glacial spreading center during the late Paleozoic. Although these findings do not negate Carboniferous glaciation in Antarctica, they do indicate that Gondwanan glacia- tion was less widespread, and, therefore, that glacially driven changes to other Earth systems (i.e., glacioeustatic fluctuations, cli- mate) were much smaller than previously hypothesized.

Tetrapod and Large Burrows of Uncertain Origin in Triassic High Paleolatitude Floodplain Deposits, Antarctica

Abstract Two types of large diameter burrows, recognized by non-overlapping size distributions, occur in high paleolatitude floodplain deposits of the Lower Triassic Fremouw Formation, Shackleton Glacier area, Antarctica. Type G (giant) burrows are gently dipping tunnels 8 to 19 cm in diameter. Type L (large) burrows are 2 to 6.5 cm in diameter, curved or subhorizontal tunnels that rarely branch; scratch markings on both burrow types generally are parallel or tangential to the long axis of the burrows. Type G burrows are interpreted as produced by tetrapods based on similarity in size, architecture, and surface markings to Permian burrows from South Africa that contain complete skeletons of therapsids. These are the first tetrapod burrows described from Antarctica. Type L burrows have characteristics of both fossil tetrapod and crayfish burrows, precluding identification of an unique producer. Triassic tetrapods, including therapsids, that lived in high southern latitudes probably burrowed to dampen the effects of seasonal environmental fluctuations, just as do many of their mammalian counterparts living today in high latitudes. The paleolatitudinal and paleooclimatic distributions of burrowing therapsids and their mammalian descendents can be assessed by focusing search efforts on very large burrows, and by identifying producers using criteria delineated herein; this will clarify the extent to which the burrowing habit originated and persisted in high latitudes.

Morphology and paleoenvironmental distribution of Paleozoic Spirophyton and Zoophycos; implications for the Zoophycos ichnofacies

Spirophyton producers inhabiting ephemeral ponds on estuarine floodplains marginal to the (Devonian) Catskill sea covering present-day southern New York were subjected to salinity fluctuations and desiccation, whereas Zoophycos producers living farther offshore were buried by storm deposits. Producers of Zoophycos living in interdistributary bays on Pennsylvanian lower delta plains (Tennessee) contended with fluctuations in salinity and oxygen conditions and variations in sediment grain size and in rate of sedimentation. In each case the producer was an r-selected opportunistic species, as shown by the distribution of the Spirophyton or Zoophycos and the paucity of co-occurring trace fossils. Morphologic analysis indicates that the Paleozoic producers of these Zoophycos and Spirophyton: (1) transported sediment downward from overlying layers or the sediment surface, and (2) were capable of a variety of behavioral patterns, only some of which resulted in complex 3-dimensional structures.

Tetrapod Burrows From the Triassic Of Antarctica

Abstract Two new tetrapod trace fossil records from the Triassic of Antarctica are described. The first is a giant terminal chamber collected from the lower Fremouw Formation (Lower Triassic) at Wahl Glacier (Beardmore Glacier region, central Transantarctic Mountains). Comparison to South African burrows known to contain the cynodont Thrinaxodon liorhinus, suggests that the Antarctic fossil was created by a tetrapod of similar size. The second set of tetrapod burrow casts was collected from Member ‘A’ of the Lashly Formation (lower Middle Triassic) and, if correctly interpreted, represent the first evidence of tetrapods from the Middle Triassic of Victoria Land. Our findings demonstrate that tetrapods were present in Victoria Land during the Middle Triassic, despite not being recovered as body fossils until the Upper Triassic in that region. Comparison to morphologically similar South African trace fossils suggests that procolophonids might have produced the Antarctic burrows, but this attribution is necessarily speculative because burrow inhabitants have not been found in situ and no morphological feature uniquely ties procolophonids to this type of trace fossil. We propose that underground burrows and dens were important shelters for Antarctic terrestrial vertebrates during the Triassic, despite a relatively moderate climate.

Behavioral plasticity of modern and Cenozoic burrowing thalassinidean shrimp

Abstract The use of trace fossils as paleoenvironmental indicators is based on empirically-derived and tested links between environmental conditions, behavior, and trace fossil morphology. Four approaches were used to assess how faithfully and at what resolution trace fossils, as mirrors of behavior, reflect environmental change: (1) comparing the abundance and morphology of Ophiomorpha nodosa in tidal channel-margin and tidal channel-axis facies (Miocene, Delaware); (2) determining the range of morphology of O. nodosa produced under unchanging environmental conditions (within the channel-margin facies); (3) evaluating the behavioral response of the modern burrowing thalassinidean, Neotrypaea californiensis , in Mugu Lagoon, California to an environmental perturbation, namely the burial of layers of glass plates; and (4) assessing how ancient producers of Ophiomorpha dealt with obstacles presented by dense shell and coral accumulations (Miocene, Maryland; Pleistocene, Bahamas). Comparison of Ophiomorpha nodosa in channel-margin versus channel-axis facies indicated that O. nodosa was significantly more abundant in the channel-margin facies. However, there was no significant difference in burrow characteristics (exterior or interior diameter, wall thickness) between the facies. As recorded by O. nodosa , its thalassinidean producer did not modify its behavior in response to conditions in the tidal channel axis. Rather, the tracemakers tended to avoid the channel axis, as indicated by the reduced abundance of O. nodosa . In contrast, O. nodosa within the channel-margin facies was highly variable in degree of pelletization of the burrow wall, in burrow fill and definition of the burrow margin, and in architecture of the burrow system. Variation in O. nodosa found within the channel-margin facies reflected behavioral flexibility in the absence of environmental change. Modern burrowing shrimp adapt to barriers (layers of glass plates) implanted within the substrate. They alter the geometry and depth of their burrow systems; they may even share shafts that penetrate the barrier. Meters thick Miocene shell beds (Maryland) in which the shells are not densely packed contain Ophiomorpha and Thalassinoides ; the producing thalassinideans were able to penetrate and move through the shell bed. Similarly, Pleistocene thalassinideans maneuvered around coral rubble in Bahamian fossil coral reefs. However, Miocene decimeter-thick shell beds that are densely packed and well sorted are not penetrated by thalassinidean burrows, implying that thalassinidean behavioral flexibility was not sufficient to penetrate densely packed shell beds. Likewise, in the Bahamian reefal settings, Ophiomorpha producers formed extensive maze systems immediately above impenetrable lithified surfaces in the reefal sequence. Behavior of thalassinidean shrimp is neither tightly constrained nor highly programmed, and there is no indication that this has changed since Miocene time. Small changes in morphology of traces produced by thalassinideans cannot be used to identify subtle changes in environmental conditions. Shrimp vary behavior apparently “whimsically”, as well as in response to environmental change. If this is true of animals other than thalassinideans, the challenge to the ichnologist is to distinguish between “background” and “environmentally triggered” behavioral variation as recorded in the trace fossil record.

Paleozoic-Mesozoic crayfish from Antarctica: Earliest evidence of freshwater decapod crustaceans

Discovery of an Early Permian claw from Antarctica extends the fossil record of crayfish by ∼65 m.y. and demonstrates that decapod crustaceans had radiated into freshwater habitats by the late Paleozoic. Burrows in Lower Triassic rocks of Antarctica are among the oldest apparently constructed by crayfish. Their morphology is similar to modern crayfish burrows, and this demonstrates that burrowing behavior was established early in the evolution of this group. The new discoveries show that the earliest Permian crayfish were distributed in high paleolatitudes of southernmost Pangea, where they lived in freshwater lakes fed by glacial meltwater. Modern crayfish habitat, used as a guide to crayfish temperature tolerance, indicates that summer temperatures of streams and lakes near the South Pole that supported the crayfish probably reached 10–20 °C during Permian-Triassic interglacial intervals.

Abundant and diverse early Paleozoic infauna indicated by the stratigraphic record

Marine benthic animals have lived within the sediment since the late Precambrian. An abundant early Paleozoic infauna is indicated by bioturbated zones and diversity of deposit-feeder and suspension-feeder trace-fossil genera. Burrow depths extended tens of centimetres below the sediment-water interface. Trace-fossil evidence of an abundant early Paleozoic infauna conflicts with evidence from the shelled fauna, which lacks infauna. This requires reinterpretation of early Paleozoic community structure and raises new questions about interactions between early Paleozoic soft-bodied and skeletonized animals.

Paleoenvironmental distribution of trace fossils in the catskill deltaic complex, New York State

Abstract The Devonian Tully clastic equivalents and associated rocks of east-central New York State were deposited in fluvial to offshore environments associated with the Catskill delta. These depositional environments have been well defined by Johnson and Johnson and Friedman. The compositional and textural immaturity of the sandstones (immature to submature graywacke) and nearly ubiquitous micro-cross-lamination indicate that rate of sediment supply exceeded rate of reworking. Four of the ichnofossils, including Arenicolites (“small form”), ? Teichichnus, Spirophyton , and large vertically oriented burrow-like structures of probable biological origin are found in narrow paleoenvironmental ranges, within alluvial, tidal, and nearshore facies. However, the other eleven trace fossils, including the most abundant types [ Arenicolites (“large form”), arthropod traces ( Diplichnites, Cruziana, ?Isopodichnus, Rusophycus), Bifungites, ?Chondrites, Skolithos (2 types), Zoophycos , large primarily intrastratally-produced burrows, and simple surficial trails ] occur in rocks deposited under diverse environmental conditions. Their distribution is not clearly related to environment or substrate texture. Factors contributing to the broad lateral distribution of most of the biogenic structures include: (1) the continual influx of sediment from the east, which may have excluded some forms and inhibited the development of stable, environmentally distinct ichnocoenoses, and (2) gradual changes in optimal behavioral patterns resulting from environmental transitions occurring over large geographic distances. The gradational nature of these changes may have exaggerated the broad environmental ranges of the trace-producing animals.

Hardly used habitats: Dearth and distribution of burrowing in Paleozoic and Mesozoic stream and lake deposits

Limited use of substrate ecospace by a meager infauna in streams and lakes is indicated by analysis of more than 10 000 observations of bioturbation in Permian through Jurassic freshwater deposits. In contrast, marine substrates have been inhabited and bioturbated since the Cambrian; colonization of freshwater substrates differed fundamentally from that of marine substrates. Bioturbation-enhanced flux of dissolved materials across the sediment-water interface was not operating until after the Paleozoic in freshwater ecosystems.