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Interpretation of three-dimensional morphology from
cross sections of phycosiphoniform burrows
Bridging the gap from the two-dimensional cross sections commonly seen in core
and slabbed material to a three-dimensional interpretation of morphology is a
significant challenge for applied ichnologists (McIlroy 2004a,
2008;
Bromley and Pedersen 2008). The starting point for this process has to be
reliable three-dimensional reconstructions of known taxa; preferably type
material. To address the issue of identifying phycosiphoniform burrows from
cross-sectional views, we will review and update the model for Phycosiphon
incertum for comparison with our phycosiphoniform material from Mexico.
Interpreting "Frogspawn Texture" as Phycosiphon-Generated
Ichnofabrics
 Phycosiphon is a morphologically complex trace fossil in three
dimensions; consequently it has a diverse range of expressions in vertical cross
section. These vertical cross sections can closely resemble other
phycosiphoniform burrows (e.g., Helminthoidichnites cf.
Chamberlain 1978;
Nereites cf. Wetzel 2002). The characteristic frogspawn fabric (Bromley
1996) is produced by cross sections of the marginal tube ("embryo") and the spreite or mantle ("jelly"). A number of vertical cross sections of bedding
parallel Phycosiphon have been figured by
Bromley (1996), and are
supplemented by our digitally dissected deterministic model (Figure 5.1-5.3).
Since the emendation of the ichnogeneric diagnosis for Phycosiphon
(Wetzel and Bromley 1994) includes the possibility of non-bedding parallel
lobes, we have created a 3D digital model of Phycosiphon inclined 17o
from the vertical and created virtual vertical cross sections from it (Figure
5.4). The resultant cross- sections include the comma-shaped cross sections so
common in outcrop material but not explained by pre-existing hypothetical models
(Bromley 1996). The vertically stacked, bent paired marginal tubes not linked to
a cross section by Bromley (1996) can also be explained by our model (Figure
5.5).
Comparison of our three-dimensional model, and cross sections obtained from it
(Figure 3 and
Figure 5), with published cross sections of Phycosiphon (Goldring
et al.1991;
Wetzel and Bromley 1994;
Bromley 1996;
Naruse and Nifuku 2008)
allows us to confidently state that the model of
Bromley (1996) has the
potential to produce the full range of Phycosiphon cross sections seen in
vertical sections from core. It is thus entirely possible that the
Phycosiphon trace maker deposit fed in vertical, oblique or bedding parallel
orientations as well as the horizontal orientation seen in the type material.
Not encompassed by our three-dimensional model, are twisted lobes, though it is
inferred that those would produce broadly similar vertical cross sections to
those in Figure 5.4-5.5.
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