Susan L. Halgedahl

Exceptionally Preserved Jellyfishes from the Middle Cambrian

Cnidarians represent an early diverging animal group and thus insight into their origin and diversification is key to understanding metazoan evolution. Further, cnidarian jellyfish comprise an important component of modern marine planktonic ecosystems. Here we report on exceptionally preserved cnidarian jellyfish fossils from the Middle Cambrian (∼505 million years old) Marjum Formation of Utah. These are the first described Cambrian jellyfish fossils to display exquisite preservation of soft part anatomy including detailed features of structures interpreted as trailing tentacles and subumbrellar and exumbrellar surfaces. If the interpretation of these preserved characters is correct, their presence is diagnostic of modern jellyfish taxa. These new discoveries may provide insight into the scope of cnidarian diversity shortly after the Cambrian radiation, and would reinforce the notion that important taxonomic components of the modern planktonic realm were in place by the Cambrian period.

Magnetic domain patterns observed on synthetic Ti‐rich titanomagnetite as a function of temperature and in states of thermoremanent magnetization

The fundamental temperature dependence of magnetic domain structure has been recognized for several decades, but its relationship to thermoremanent magnetization (TRM) has been unexplored. As one step toward investigating this possible link, we have studied Bitter patterns on synthetic Ti-rich titanomagnetite (Al0.1Mg0.1Ti0.6Fe2.2O4; Curie point of approximately 75°C) as a function of temperature in a weak field (0.42 Oe), in states of weak field TRM, and after alternating field demagnetization. Two different types of patterns were studied with the following goals: (1) simple Kittel-like patterns of straight walls were studied in order to gain insight into the nature of TRM domain structures and the various possible mechanisms by which particles acquire weak field TRM; (2) mazelike patterns, which occupy regions of high surface stress, were studied in order to estimate the temperature dependence of magnetostriction constant in this material. Results of experiments to study Kittel-like patterns suggest that two different mechanisms, and two resulting species of domain structure, could account for fundamental differences between multidomain and pseudosingle-domain thermoremanence in titanomagnetite. In accordance with traditional models, typical multidomain TRM results from wall pinning in a particle that contains several domains whose volumes are approximately equal to each other. In contrast, pseudosingle-domain TRM is acquired when denucleation of domains and domain walls yields a domain structure with a large associated “intrinsic” moment. The following observations support these conclusions. In a particle with a Kittel-like pattern, the number of domains varies from one TRM experiment to the next. This variation indicates that a grain can acquire TRM in a range of local energy minimum states, rather than in one particular domain configuration of absolute minimum energy. Many such TRM patterns represent the near-ideal local energy minimum domain states, consisting of several domains of approximately equal widths, usually envisioned in models of multidomain thermoremanence. Observations suggest that a particle can maintain such states during cooling if weakly pinned walls adjust readily to demagnetizing fields generated during the denucleation process. In this case, TRM results from small displacements of pinned walls with respect to a state of minimum magnetostatic energy; several observations suggest that blocking of walls at pinning centers in the classical manner occurs mainly after the final nucleation event as the particle cools toward room temperature from the Curie point. In contrast, we have observed examples in which denucleation leaves behind an anomalously large domain where several approximately uniform domains existed prior to denucleation; several smaller domains survive elsewhere in the particle, virtually unaffected by the transition. Evidently, strong wall-pinning forces sometimes prevent surviving walls from responding to the change of internal field which must accompany the denucleation process. It is observed that denucleation can yield single-domain-like states during TRM acquisition. Thus denucleation, rather than complete failure to nucleate during TRM acquisition, is one mechanism for producing single-domain-like states in particles which ordinarily favor walls. Possibly, such states exist by virtue of strong pinning forces at surface defects, into which denucleating domains and domain walls collapse. Particles with an odd number of approximately uniform domains also are observed in TRM states, and such particles also may carry large intrinsic moments owing to domain imbalance; possibly, such domain configurations result from the final denucleation episode during cooling. Thus these observations indicate a strong link between TRM acquisition mechanisms, magnetic domain transitions, and the accessibility of domain states. We hypothesize that denucleation could account for the most desirable properties of TRM associated with pseudosingle-domain titanomagnetite particles, namely, high intensity and high thermal stability. The second class of patterns, mazelike patterns, was studied to estimate the temperature dependence of domain wall energy and magnetostriction constant from the observed temperature dependence of domain width. The temperature dependence of magnetostriction constant obtained in this manner for AMTM60 is similar to that obtained for magnetite in earlier studies.

Middle Cambrian Arthropods from Utah

Abstract The Middle Cambrian Spence Shale Member (Langston Formation) and Wheeler and Marjum Formations of Utah are known to contain a diverse soft-bodied fauna, but important new paleontological material continues to be uncovered from these strata. New specimens of anomalocaridids include the largest and smallest near complete examples yet reported from Utah. New material of stem group arthropods includes two new genera and species of arachnomorphs: Nettapezoura basilika and Dicranocaris guntherorum. Other new arachnomorph material includes a new species of Leanchoilia comparable to L. protogonia Simonetta, 1970; Leanchoilia superlata? Walcott, 1912; Sidneyia Walcott, 1911a; and Mollisonia symmetrica Walcott, 1912. L. protogonia from the Burgess Shale is confirmed as a separate species and is not a composite fossil. The first example of the trilobite Elrathia kingii preserving traces of the appendages is described. In addition, new material of the bivalved arthropods Canadaspis Novozhilov in Orlov, 1960; Branchiocaris Briggs, 1976; Waptia Walcott, 1912; and Isoxys Walcott, 1890 is described.

Low-temperature behavior of single-domain through multidomain magnetite

Abstract We have investigated the low-temperature behavior of a suite of ‘grown’ synthetic and natural magnetites that span single-domain (SD) and multidomain (MD) behavior. Synthetic samples had been grown in the laboratory either in an aqueous medium or in glass. Natural samples included SD magnetites occurring in plagioclase and truly MD magnetites in the form of large octahedra. In all experiments a sample was first given a saturation remanence at room temperature; next, moment was measured continuously during cooling and warming between 230 K and 60 K. Similar to results reported earlier by other workers, magnetic memory is large in SD samples, whereas truly MD samples are almost completely demagnetized by cycling between room temperature and 60 K. Pseudo-single-domain samples exhibit behavior that is intermediate with respect to that of the SD and truly MD states. When data from this study are combined with data obtained by Hartstra [10] from sized, natural magnetites, it is found that the percentage of total remanence that survives cycling between room temperature and 60 K decreases linearly with the logarithm of grain size and, thus, with increasing number of domains. This relation suggests that memory can provide a reasonable estimate of grain size in those magnetite-bearing rocks for which these samples provide good analogues. Remarkably, some of the large natural octahedra provide a magnified view of MD response to low temperatures and thus reveal two surprising and intriguing types of behavior. First, below approximately 180 K these octahedra demagnetize through a series of large Barkhausen jumps. Second, near 117 K these same octahedra exhibit a ‘wild zone’, where magnetic moment executes large, random excursions. We interpret these two phenomena as direct evidence for the unpinning and irreversible displacement of domain walls in response to the drop in coercivity and, possibly, the broadening of domain walls as temperatures drop toward the isotropic point. One implication of this behavior is that cooling to progressively lower temperatures could provide an effective method for stepwise removal of paleomagnetic components carried by MD grains, even without passage through the isotropic point of magnetite.

Experiments to investigate the origin of anomalously elevated unblocking temperatures

A fundamental problem in rock magnetism and paleomagnetism is the physical origin of anomalously elevated unblocking temperatures. To investigate this problem, we have studied viscous remanent magnetization (VRM) acquired at 225°C by 1–2 μm magnetite particles synthesized with the glass-ceramic method. Experiments were performed with two primary goals: (1) to determine which particular conditions produce anomalously elevated unblocking temperatures, and (2) to compare various observed aspects of viscous behavior to predictions based on two different physical mechanisms that might raise the unblocking temperature: the reduction of magnetic relaxation time by the applied field (the Neel medium-field mechanism), and directional ordering of defects within domains and domain walls. When compared to model predictions, the present results indicate that neither the Neel medium-field mechanism nor the directional ordering mechanism satisfactorily accounts for anomalously high unblocking temperatures. Surprisingly, however, the unblocking temperature spectrum of VRM is critically sensitive to a samples initial state. When samples first are thermally demagnetized from 580°C to 225°C, VRMs acquired subsequently at 225°C exhibit pronounced high-temperature “tails” extending to 450°C. Yet when samples first are demagnetized in an alternating field at room temperature before acquisition at 225°C, their VRMs unblock by approximately 300°C. Despite these two very different thermal responses, VRM acquisition curves at 225°C are virtually identical for the two initial states. Furthermore, zero-field storage prior to acquisition suppresses VRM acquisition coefficients by similar amounts for both initial states. Significantly, standard thermal demagnetization to room temperature greatly attenuates the high-temperature tail of a VRM acquired subsequently at 225°C. We conclude that thermal response of domain structure, as determined by thermomagnetic history, plays a critical role in the thermal unblocking process.

Bitter patterns versus hysteresis behavior in small single particles of hematite

To establish links among domain images, volume changes in magnetization, and mechanisms for pseudosingle-domain (PSD) behavior, we have studied single platelets of natural hematite (Elba, Italy) in order to compare Bitter patterns resulting from high-field treatments to the hysteresis behavior of the same individual particles. There is excellent agreement between the fields that cause large changes of a particles Bitter pattern and the fields that cause large changes of a particles magnetic moment. When in the saturation remanent state, most particles exhibit patterns that suggest a state of near saturation. Such patterns change very little until the application of a critical back field triggers the appearance of a major Bitter line that nearly bisects the grain. Likewise, in the majority of particles the measured ratio of saturation remanence to saturation moment is large (≃0.7), and there is little change in saturation remanence until a critical back field equal to the coercive force (H c ) reverses the moment through a large Barkhausen jump. In most particles whose patterns have been compared to their measured behavior, this large jump on the hysteresis loop occurs in the same critical back field that causes the sudden appearance of a major Bitter line. Consequently, we interpret hysteresis to be governed by the nucleation of walls in the majority of hematite particles studied here. In such particles, H c increases as effective grain size decreases, according to H c d eff -0.64 , where d eff M s 1/3 and M s equals the particles saturation moment. This study leads to four conclusions: (1) Bitter patterns do represent volume domains and the magnetization processes that they imply, (2) nucleation is the dominant hysteresis mechanism in PSD particles of this natural hematite, (3) the grain size dependence of nucleation field H n in hematite is given by H n =H c d eff -0.64 , and (4) nucleation is a viable mechanism for explaining PSD behavior in other natural magnetic materials, such as magnetite, titanomagnetite, and pyrrhotite

Hysteresis properties of magnetic spherules and whole rock specimens from some Paleozoic platform carbonate rocks

It is widely established that many Paleozoic carbonate rocks were remagnetized during the late Paleozoic Kiaman reversed superchron. Yet the paleomagnetic recorders of this event have remained elusive. Magnetic spherules have been candidates for this role, because they are probably authigenic. Therefore we have studied the hysteresis properties of individual magnetic spherules from two formations, the remagnetized Onondaga (Devonian, New York) and the effectively unremagnetized Wabash (Silurian, Indiana). Also, we have compared the hysteresis properties of spherules to those of remagnetized limestones from the Onondaga, Trenton (Ordovician, New York), and Helderberg (Devonian, New York) formations and from the unremagnetized Wabash formation. The spherules studied here vary from about 20 µm to over 100 µm in diameter. Nevertheless, on the basis of hysteresis behavior this spherule population spans the full range of domain states, from truly multidomain (MD) to pseudo-single-domain (PSD) to single domain (SD). Because these spherules are so large, it is remarkable that more than half display the highly PSD-like behavior typical of many remagnetized carbonate rocks. Equally as remarkable is the lack of dependence on spherule size of various hysteresis properties such as coercive force (Hc), remanent coercive force (Hcr), the ratio of saturation remanence to saturation moment (Mrs/Ms), and the ratio of remanent coercive force to coercive force (Hcr/Hc). These findings dispel the preconception that large spherules are too magnetically soft to be stable carriers of ancient magnetization. Surprisingly, Onondaga and Wabash spherules yield virtually identical suites of hysteresis properties. On the one hand, this high degree of similarity could indicate that spherules are irrelevant to remagnetization. More intriguing, however, is that this similarity could mean that the Wabash formation, although effectively unremagnetized, nevertheless contains magnetic mineralogical traces of a regional remagnetization event. It is significant that for these spherules a bilogarithmic plot of Mrs/Ms versus Hcr/Hc does not define a simple linear trend. Instead, Mrs/Ms increases more rapidly with decreasing Hcr/Hc for spherules in the PSD range than in the MD range. When a power law is fit solely to the hysteresis ratios of PSD-like to SD-like spherules, extrapolation to the SD limit gives Mrs/Ms = 0.75. Similarly, a power law fit to the hysteresis ratios obtained here from remagnetized carbonate rocks yields Mrs/Ms = 0.72 at the SD boundary. These results could indicate that a large percentage of SD-like spherules, like the SD carriers in remagnetized carbonate rocks, are governed by magnetocrystalline anisotropy. Finally, the present data point to values of Hc near 350 Oe both for SD-like spherules and for the SD grains thought to carry remanence in remagnetized carbonates. Clearly, PSD- and SD-like spherules share several key rock magnetic properties with remagnetized carbonate rocks. Thus spherules still are likely candidates for carrying the remagnetization signal.

Barkhausen jumps in large versus small platelets of natural hematite

To better understand the physical links among hysteresis properties, defect distributions, and grain size, Barkhausen jumps have been studied in individual platelets of natural hematite from Elba, Italy during hysteresis. Both Bitter patterns and hysteresis curves have been investigated in two very different groups of platelets: large, ∼1-mm-sized platelets with coercive forces (Hc) in the range of ∼20–50 Oe (2–5 mT), and much smaller, ∼100 μm-sized platelets, with Hc in the 100–140 Oe range (10–14 mT). Single platelets were cycled through both major and minor hysteresis, in order to determine (1) the changes of magnetic moment caused by Barkhausen jumps and (2) how these changes may depend on grain size and the critical field Hcrit required to unpin a wall from a defect. Results of these experiments lead to the following conclusions. First, the steepest part of the major hysteresis loop is dominated by large-scale wall motion through that part of the grain where the combination of wall nucleation and wall propulsion across defects requires the lowest fields anywhere in the particle. In the large platelets, this “soft” region (e.g., where Hcrit≤∼50 Oe, or 5 mT) can amount to as much as 25% of the grains volume; within this region, a wall can be driven well over 100 μ by fields commensurate with Hc. In the large platelets, the defects within this “soft” region appear to have quite variable volume densities. By contrast, in the small platelets, defects with comparably soft wall-pinning strengths appear to be distributed much more uniformly. Second, in both platelet groups, most of the relatively “hard” defects (e.g., Hcrit≥∼80 Oe) which have the greatest impact on wall pinning during hysteresis generally appear to be concentrated within a localized portion of the particle, rather than being distributed randomly throughout the volume. This “hard” region can amount to several tens of percent of a grains volume, but within it, the “hard” defects appear to be distributed rather homogeneously. It is through this “hard” region that the wall is forced, as fields stronger than Hc drive the platelet toward saturation. Consequently, these results lead to a very different picture of defect distributions than envisioned by previous models, based on random defects. We propose that this nonrandom spatial distribution of “hard” versus “soft” defects could originate from internal strain generated when these platelets cleaved naturally from their much larger, parent crystals.

Rare primitive deuterostomes from the Cambrian (Series 3) of Utah

Abstract. The fossil record of early deuterostome history largely depends on soft-bodied material that is generally rare and often of controversial status. Banffiids and vetulicystids exemplify these problems. From the Cambrian (Series 3) of Utah, we describe specimens of Banffia episoma n. sp. (from the Spence Shale) and Thylacocercus ignota n. gen. n. sp. (from the Wheeler Formation). The new species of Banffia Walcott, 1911 shows significant differences to the type species (B. constricta Walcott, 1911) from the Cambrian (Series 3, Stage 5) Burgess Shale, notably in possessing a prominent posterior unit but diminished anterior section. Not only does this point to a greater diversity of form among the banffiids, but also B. episoma indicates that the diagnostic median constriction and crossover of either side of the body are unlikely to be the result of taphonomic twisting but are original features. Comparisons extend also to the Cambrian (Series 2) Heteromorphus Luo and Hu in Luo et al., 1999 and, collectively, these observations support an assignment of the banffiids to the vetulicolians. The new taxon T. ignota represents the first discovery of a vetulicystid from beyond China and also significantly extends its stratigraphic range from Series 2 Cambrian into Series 3 Cambrian. Despite overall similarities in bodyplan, T. ignota differs from other vetulicystids in a number of respects, notably the possession of an anterior zone with broad tentacle-like structures. This new discovery is consistent with the vetulicystids representing stem-group ambulacrarians.

Observed effects of mechanical grain-size reduction on the domain structure of pyrrhotite

Abstract The magnetic mineral grains that carry natural remanent magnetization (NRM) in rocks can undergo changes in volume, chemical composition, or both. Because domain structure is a function of particle size, composition, and the dominant anisotropy, a change in these properties could trigger changes in the number of domains, drive domain walls to new positions, and thus reset natural remanence. The manner in which active grain-size reduction alone affects domain state has not been studied, however. To address this problem, we have observed Bitter patterns on two mutually perpendicular surfaces of natural pyrrhotite grains, whose sizes were reduced mechanically in the laboratory. Both one-dimensional and two-dimensional thinning experiments were performed. Each grain was thinned until at least one dimension was approximately 20–50% of its original size. Domain widths, the overall positions of surviving walls, and even many small-scale details in the shapes of curved walls were remarkably insensitive to thinning. These results lead to three conclusions. First, local energy minimum (LEM) domain states in pyrrhotite could be stable across a wide range of grain sizes and shapes. Second, defects that pin domain walls could inhibit LEM–LEM transitions by impeding the wall motions required for such transitions. Third, in pyrrhotite the defects which prevent walls from adjusting to thinning-induced changes in demagnetizing field appear to be densely and rather homogeneously distributed across a wall’s surface, rather than being volumetrically rare. When viewed together, these results strongly suggest that an NRM direction, originally carried by two-domain grains, would survive grain-size attrition as the two-domain parents were reduced to stable single-domain daughters.