Methods

Creation of three-dimensional conceptual models of trace fossils differs greatly from the process of direct reconstruction of the three-dimensional morphology of fossil material based on serial grinding and tomography. This paper aims to produce a three-dimensional deterministic model of some phycosiphoniform burrows from turbiditic siltstone of Cretaceous Rosario Formation and compare them to the Phycosiphon model of Bromley (1996). The approach used involves the use of serial grinding and computed tomography as outlined below.

Serial Grinding

Three-dimensional geometry of the studied burrows was systematically exposed through serial grinding of the hand specimen. This approach has been successfully employed for three-dimensional imaging of body fossils (e.g., Baker 1978; Hammer 1999; Sutton et al. 2001), ichnofabric (Wetzel and Uchman 1998, 2001) and trace fossils (Naruse and Nifuku 2008). Serial grinding allowed us to obtain a sequence of regularly spaced images of the resultant vertical cross sections. The photographic dataset thus created is the basis for subsequent computer-based three-dimensional reconstructions.

To aid in creating parallel regularly spaced cross sections, the irregularly shaped sample of turbiditic siltstone was placed in a tight fitting box and set in plaster of Paris. When the plaster was set, and regular 0.5 mm increments inscribed on the outer surface of the rectangular block, it was then ready to be serially ground. The regular outline of the block was essential to create reference points, for alignment of the photographic images to be used in digital analysis. The 0.5 mm spacing of images was chosen to capture a sufficiently large number of data-points to allow gridding of surfaces and reconstruction of the burrows. A total of 60 images were acquired through a 29.5 mm thick slab of the sample. The consecutive series of photographs were taken from parallel surfaces with a digital camera, which was stationed an identical distance above the sample surface, under the same lighting and zoom conditions for every surface. The camera was attached to a photographic stand with height controlling screw feed.

Ichnofabrics have not generally been studied using a serial grinding approach. In contrast to body fossil material, trace fossil fabrics are commonly complex, tortuous, and without sharply defined limits (both morphologically and mineralogically). A particular problem is that burrows may branch and inter-penetrate, making closely spaced slicing essential, and poses particular challenges in image processing (discussed below). The size of the block studied is larger than has typically been studied by palaeontologists, but did not pose any particular methodological problems.

Image Processing

The set of sequential slice images acquired through the serial grinding technique was processed to select the regions to be studied. The phycosiphoniform burrows studied include a dark mud core and a halo of coarser sediment, which in the present material is accentuated through the presence of pyrite (Figure 1.1, Figure 6.1). To obtain adequate contrast, the images were made into gray scales (Figure 6.2). All images were put into a single Photoshop document in consecutive order. Discrete burrow cores were chosen as the objects for tracing the location of the chosen burrow. The burrow core was tracked through each consecutive image and manually selected using layer masking to hide all other burrow cores and halo that might confuse the reconstruction of the chosen burrow (Figure 6.3). A masking layer was used to allow retention of the original, gray scaled images, including location of adjacent burrows, should it become subsequently desirable to study adjacent burrows (Figure 6.4). The layered Photoshop document was then cropped to the smallest size that encompassed the isolated burrow core. Each layer, representing the equidistant ground surfaces, was saved as a JPEG image in the same directory with a numeric name that indicates its position in the sequence. This set of image-processed two-dimensional binary images was used for the subsequent three-dimensional reconstruction.

Three-Dimensional Rendering

The set of the binary images was imported to the commercial edition of VolView 2.0 software. Consecutive, gray scaled intersections of burrow core were converted by the software to the volume shape that represents the three-dimensional geometry of the examined phycosiphoniform burrow. Artificial colors were attributed to the reconstructed burrow and to the halo in order to aid illustration (Figure 6.5, Figure 7, Figure 8, and Figure 9). Three-dimensional reconstruction of the phycosiphoniform burrow from examined rock was additionally saved as a movie file that shows the burrow rotating around the axis that is perpendicular to the bedding plane (see attached animation files, Figure 7.7 and Figure 9.4).

Three-Dimensional Morphology of the Rosario Formation Phycosiphoniform Burrows

By choosing a sparsely bioturbated portion of the ichnofabric, it was possible to identify a single isolated burrow. The burrow consists of a single loop shaped clay-filled tube that is identifiable in the series of ground vertical cross sections. This isolated burrow was subjected to detailed three-dimensional reconstruction of both the mud-filled burrow core (Figure 7.1-7.5) and the burrow halo (Figure 7.6-7.7). The volume of rock subjected to three-dimensional reconstruction, and containing the fossil burrow was 40.9 mm in length (X axis), 21.9 mm in height (Y axis) and 29.5 mm thick (Z axis) (Figure 7, Figure 8, and Figure 9). The two limbs mud-filled burrow core that describe the shape of the lobe are parallel to each other in vertical section and vary in diameter between 3 and 4 mm. Slight thickening in tube width is noted in the distal portion of the loop that cannot be attributed to compaction. Thickening of this part of the tube was described as one of the diagnostic characteristics of Phycosiphon (Wetzel and Bromley, 1994). The paired limbs of the examined form are not in the same horizontal plane, and the terminal portion of the loop is at a steep angle to the limbs.

Nature of the Halo in the Rosario Phycosiphoniform Burrows

Our 3D reconstruction of Phycosiphoniform burrows from the Rosario Formation, Baja California, Mexico, demonstrates that the reworked silt-rich, clay-poor material that forms the halo around the clay-filled burrow core is dominantly present below the level of the clay-filled burrow (Figure 9). This feature is also prevalent in most natural vertical cross sections studied in the field (Figure 1.1). The halo is demonstrably meniscate, as determined from cross-sectional views, but especially through three-dimensional reconstruction (Figure 9). It is also noted that the burrow halos of adjacent burrow limbs are closely juxtaposed with little if any undisturbed host sediment between them (Figure 9). The halo around phycosiphoniform burrow cores has been described from other occurrences (Wetzel and Wijayananda 1990; Ekdale and Lewis 1991), but has not previously been reconstructed in three dimensions.

A similar halo associated with a phycosiphoniform burrow (attributed to Anconichnus) was interpreted as an early diagenetic oxidation halo (Ekdale and Lewis 1991). This feature was subsequently reinterpreted as being due to bioturbation, specifically the formation of spreiten in accord with newer conceptual models (Wetzel and Bromley 1994; Bromley 1996). Three-dimensional reconstruction of the Rosario Formation phycosiphoniform fossil, with its associated coarser-grained structure, demonstrates that the coarser-grained material is indeed asymmetric and lies below the level of each of the two lobe arms (Figure 9). This asymmetry is also visible from vertical surfaces prepared in the laboratory and in natural outcrop (Figure 1). The burrow halo is characteristically pyrite rich (Figure 1.2). Pyritization is interpreted to have been caused by sulphate-reducing bacteria during early diagenesis. The marked color contrast between the pyritized halo and clay-rich burrow cores relative to the surrounding rock matrix allowed us to distinguish the three components of the fabric for the purpose of image analysis.

The presence of the coarser-grained (silt-sized) material, not only between lobe arms, but also external to the marginal tube (Figure 9) precludes the presence of spreite and allows rejection of the possibility that the phycosiphoniform trace fossil reconstructed herein is Phycosiphon. In the accepted conceptual model of Bromley (1996; Figure 3, Figure 4, and Figure 5), spreiten are predicted only between arms of a single lobe and between marginal burrows. The behavioural model proposed for Phycosiphon (Bromley 1996) precludes the possibility of formation of the halo/spreiten below the level of a marginal tube that borders the Phycosiphon structure. Spreiten are demonstrably not present in our material from Rosario Formation. Instead, the phycosiphoniform cross sections are inferred to have been formed by bulk sediment processing at the anterior of the burrow during continuous burrowing rather than successive probing as is proposed for Phycosiphon s.s.