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Title: Three-dimensional microCT analysis of the Ediacarafossil Pteridinium simplex sheds new light on its ecology andphylogenetic affinity
Author: Mike Meyer David Elliott Andrew D. Wood NicholasF. Polys Matthew Colbert Jessica A. Maisano PatriciaVickers-Rich Michael Hall Karl H. Hoffman Gabi SchneiderShuhai Xiao
PII: S0301-9268(14)00142-9DOI: http://dx.doi.org/doi:10.1016/j.precamres.2014.04.013Reference: PRECAM 3971
To appear in: Precambrian Research
Received date: 12-9-2013Revised date: 15-4-2014Accepted date: 21-4-2014
Please cite this article as: Meyer, M., Elliott, D., Wood, A.D., Polys, N.F., Colbert,M., Maisano, J.A., Vickers-Rich, P., Hall, M., Hoffman, K.H., Schneider, G., Xiao,S.,Three-dimensional microCT analysis of the Ediacara fossil Pteridinium simplexsheds new light on its ecology and phylogenetic affinity, Precambrian Research (2014),http://dx.doi.org/10.1016/j.precamres.2014.04.013
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Three-dimensional microCT analysis of the Ediacara fossil Pteridinium
simplex sheds new light on its ecology and phylogenetic affinity
Mike Meyera, 1, *, David Elliottb, Andrew D. Woodc, Nicholas F. Polysc, Matthew Colbertd,
Jessica A. Maisanod, Patricia Vickers-Richb, Michael Hallb, Karl H. Hoffmane, Gabi Schneidere,
Shuhai Xiaoa
a, Department of Geosciences, Virginia Tech, Blacksburg, Virginia, 24061, USA,
<[email protected]>, <[email protected]>; b School of Geosciences, Monash University, Clayton,
Victoria, 3800, Australia, <[email protected]>, <[email protected]>,
<[email protected]>; c Advanced Research Computing, Virginia Tech, Blacksburg,
Virginia, 24061, USA, <[email protected]>, <[email protected]>; d Jackson School of
Geosciences, The University of Texas, Austin, Texas, 78712, USA, <[email protected]>,
<[email protected]>; e Geological Survey of Namibia, Windhoek, Namibia,
<[email protected]>, <[email protected]>; 1 Department of Geosciences and
Natural Resources, Western Carolina University, Cullowhee, NC, 28723
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Abstract
Ediacara fossils often exhibit enigmatic taphonomy that complicates morphological
characterization and ecological and phylogenetic interpretation; such is the case with Pteridinium
simplex from the late Ediacaran Aar Member in southern Namibia. Pteridinium simplex is often
preserved as three-dimensional (3D) casts and molds in coarse-grained quartzites, making
detailed morphological characterization difficult. In addition, P. simplex is often transported,
distorted, and embedded in gutter fills or channel deposits, further obscuring its morphologies.
By utilizing microfocus X-ray computed tomography (microCT) techniques, we are able to trace
individual specimens and their vanes in order to digitally restore the 3D morphology of this
enigmatic fossil. Our analysis shows that P. simplex has a very flexible integument that can be
bent, folded, twisted, stretched, and torn, indicating a certain degree of elasticity. In the analyzed
specimens, we find no evidence for vane identity change or penetrative growth that were
previously used as evidence to support a fully endobenthic lifestyle of P. simplex; instead,
evidence is consistent with the traditional interpretation of a semi-endobenthic or epibenthic
lifestyle. This interpretation needs to be further tested through microCT analysis of P. simplex
specimens preserved in-situ rather than transported ones. The elastic integument of P. simplex is
inconsistent with a phylogenetic affinity with xenophyophore protists; instead, its physical
property is consistent with the presence of collagen, chitin, and cellulose, an inference that would
provide constraints on the phylogenetic affinity of P. simplex.
Keywords; Pteridinium, microCT, Taphonomy, Namibia, Ediacaran
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1. Introduction
Ediacara-type fossils (580–541 Ma) represent the earliest-known complex multicellular
organisms in the fossil record (Laflamme et al., 2013; Narbonne, 2005; Xiao and Laflamme,
2009), but understanding their phylogenetic placement has been impeded by their unusual
taphonomy (Cai et al., 2012; Gehling, 1999; Laflamme et al., 2011; Schiffbauer and Laflamme,
2012) and the lack of morphological analogs among extant taxa (Laflamme et al., 2013;
Seilacher et al., 2003). Among all Ediacara fossils, the genus Pteridinium (Gürich, 1930), first
described from the Nama Group in southern Namibia (Fig. 1A), best encapsulates these two
confounding factors (Fedonkin et al., 2007). Pteridinium has a unique body plan (Fig. 1B),
consisting of three vanes which are connected along an axis known as the ‘seam.’ Each vane is
made of a single row (or possibly two overlapping rows) of segments that are attached to the
seam (Jenkins, 1992). Pteridinium can be preserved two-dimensionally on bedding surfaces or
three-dimensionally within massive quartzite, resulting in multiple taphomorphs. When
preserved in 3D, Pteridinium specimens are often jumbled within mass flow deposits, with their
morphology strongly modified and difficult to interpret (Elliott et al., 2011; Grazhdankin and
Seilacher, 2002).
The unusual morphology and non-actualistic (without modern analog) taphonomy of
Pteridinium have led to various ecological and phylogenetic interpretations. It has been
interpreted as a frondose epibenthic organism similar to Charniodiscus (Glaessner and Daily,
1959), a semi-endobenthic organism similar to Ernietta (Fedonkin et al., 2007), or a fully
endobenthic organism that lived entirely within sediments (Grazhdankin and Seilacher, 2002).
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The phylogenetic affinity of Pteridinium has also been a matter of intense debate; it has been
interpreted as a colonial organism (Pflug, 1994), a giant single-celled protist similar to modern
xenophyophores (Seilacher et al., 2003), a lichen (Retallack, 1994), or an animal (Glaessner,
1984).
Central to the current debate on the ecology and phylogenetic affinity of Pteridinium is a
better understanding of its taphonomy and morphology. For example, the fully endobenthic
interpretation rests on convoluted P. simplex specimens with their vanes growing in sediments,
penetrating each other, and switching identities (e.g., lateral vane changing to medial vane when
organism is twisted) (Grazhdankin and Seilacher, 2002). However, these features are difficult to
resolve without a three-dimensional understanding of the fossils, and it is thus difficult to rule
out the possibility that some of these features may result from taphonomic alteration. Thus, to
better characterize the three-dimensional morphology of P. simplex and to shed light on its
taphonomy, ecology, and phylogenetic affinity, we examined a single hand sample with multiple
P. simplex specimens using microfocus X-ray computed tomography (microCT).
2. Geological and Stratigraphic Background
The analyzed hand sample was collected from the Aar Member of the Dabis Formation,
Kuibis Subgroup, located at Aar Farm in the Aus region of southern Namibia (Fig. 1A) (Hall et
al., submitted; Vickers-Rich et al., 2013). Available radiometric and chemostratigraphic data
constrain the Kuibis Subgroup to be older than 547.32 ± 0.65 Ma but likely younger than 551.09
± 1.02 Ma (Condon et al., 2005; Grotzinger et al., 1995; Narbonne et al., 2012; Schmitz, 2012).
The Aar Member consists mainly of interbedded sandstones (1–2 m thick) and shales with a few
limestone beds. It was likely deposited in an extensive, sandy, braided fluvial to shallow marine
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system, partly reworked into vast inter-tidal sand flats along a low gradient coastal plain (Elliott
et al., 2011; Hall et al., submitted; Saylor et al., 1995). Individual sandstone beds were likely
deposited during sheet flood events, which brought sandy sediments over mud-dominated inter-
tidal to sub-tidal sediments. Pteridinium simplex fossils are found in these sandstone beds,
particularly in the lower part of the Aar Member (Hall et al., submitted). The Aar Member is
directly overlain by bedded limestones of the Mooifontein Member, suggesting a major marine
transgression in the area (Gresse and Germs, 1993).
3. Materials and Methods
The analyzed hand sample (V-8-2009; Fig. 2) contains multiple Pteridinium simplex
specimens. The fossils are preserved as casts or molds in massive quartzite, with their vanes
probably replicated by pyrite but represented by stylolitic surfaces, void spaces, or secondary
calcite formed during diagenesis or weathering (Meyer et al., 2014). The segments in each vane
are seen as parallel lines or ridges emanating perpendicularly from the seam. These ridges fade
away and grade into a smooth surface farther away from the seam (Crimes and Fedonkin, 1996;
Elliott et al., 2011; Grazhdankin and Seilacher, 2002; Jenkins, 1992). The hand sample was cut
into three slabs perpendicular to the main axis/seam of the largest and most prominent P. simplex
specimen. The slabs are labeled 1–3 and cut lines are marked by i–ii in Fig. 3A. Slab 1 was used
for petrographic analyses (Meyer et al., 2014) while slabs 2 and 3 were used in this study. Slabs
2 and 3 were cut so that they can fit in the microCT scanner.
The two slabs were scanned separately using the ultra-high-resolution subsystem of the
ACTIS scanner at the University of Texas High-Resolution X-ray CT Facility. A FeinFocus
microfocal X-ray source operating at 200 kV and 0.24 mA with no X-ray prefilter was
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employed. An empty container wedge was used. Slice thickness corresponds to one line in a
CCD image intensifier imaging system, with a source-to-object distance of 275 mm for slab 2
and 330 mm for slab 3, resulting in 0.096 mm and 0.115 mm interslice spacing, respectively. For
each 1024×1024 pixel slice, 1400 views were acquired with three samples per view over 360
degrees of rotation. The field of reconstruction was 91 mm for slab 2 and 107 mm for slab 3,
resulting in 0.089 mm and 0.104 mm in-plane resolution, respectively.
The scan data were processed to reduce ring and beam-hardening artifacts, and 16 bit
TIFF image stacks were reconstructed (1312 and 1083 slices for slabs 2 and 3, respectively).
While useful on their own, these image stacks did not allow for easy 3D visualization of the
fossils within the slabs. Thus, the data were further processed using open-source image
processing, segmentation, and volume rendering tools to create 3D reconstructions of the
features of interest. Because of the large size of the original data volumes (2.6 and 2.2 Gb for
slabs 2 and 3, respectively), the images were reduced to 512 × 512 pixels and 8 bit depth using
ImageJ (Schneider et al., 2012) to facilitate rendering. The resulting stack was saved as a RAW
file and then converted to NRRD format using the command line utility UNU (2013) for initial
rendering and import into segmentation software
The volumes were then segmented into individuals and vanes using a combination of
manual mask painting and semi-automated tools available in Seg3D (CIBC, 2013), an image
processing and segmentation application built on ITK (Yoo et al., 2002). The segment masks of
interest (individual specimens, vanes) were then exported as image stacks, resized using ImageJ
to a useable 258 × 258 × 258 voxels for real-time rendering, and converted to NRRD using
UNU. Finally, the NRRD segments were composited and rendered using H3D (Sensegraphics,
2013), an X3D browser with strong support for NRRD and the volume rendering styles provided
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by the X3D 3.3 standard (Brutzman and Daly, 2007; Consortium, 2012). Each segment was
represented as an isosurface and uniquely false-colored for easy visualization. Using H3D, the
two slabs were then scaled and “stitched” to restore their original pre-cut configuration. The
original microCT data, reslicings, and derivative animations are available at
www.digimorph.org/specimens/Pteridinium_simplex.
4. Results
A comparison of light photographs and corresponding microCT renderings, shown at two
different perspectives, is presented in Fig. 3. MicroCT reveals five Pteridinium simplex
specimens embedded in the scanned blocks (labeled 1–5 in Fig. 4). These specimens do not have
a consistent orientation; for example, specimens 1 and 3 are oriented at 90° from each other.
Four of the five specimens are partially exposed, with the exposed vanes marked by colored
arrows matching the color of segmented specimens in Fig. 3. Three specimens (1, 2 and 4) have
some part of all three vanes present while the other two specimens have at least one preserved
vane (Figs. 3–5).
Specimen 1 is bent tightly and twisted slightly, but its three vanes are easily identifiable
(colored individually in Fig. 5). It can be seen in Fig. 5D that the blue medial vane is
incompletely preserved where the specimen is bent. As the fossil is bent over itself, the outer
lateral vane is stretched (yellow vane in Fig. 5) and the inner lateral vane is compressed (green
vane in Fig. 5). Interestingly, the green lateral vane is also ruptured or torn along segment
boundaries, although the proximal part of the vane (the part closer to the central seam) is still
intact (Fig. 5A). The two torn pieces are oriented at ~90º to each other; because of this, they
could be easily mistaken for two different vanes, with the dangling piece in Fig. 5C–D
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misidentified as a medial vane, particularly when the real medial vane (blue color in Fig. 5B and
5D) is not well preserved. Thus, taphonomic alteration could lead to false interpretation of vane
identity change.
Specimen 3 abuts against specimen 1, with their exposed vanes oriented at a right angle
(Figs. 3A, C, 4A. 6A), giving an impression of possible penetrative growth. However, rock
material at the exposed contact between the two specimens has been removed by weathering,
making it impossible to determine the exact nature of the contact based on visual examination of
the hand sample. MicroCT allows a closer look at the nature of the contact inside the hand
sample, and the slice images show that (1) the vanes of these two specimens do not crosscut each
other, and (2) a vane of specimen 3 is bent slightly at the distal edge when it comes in close
contact with one of the lateral vanes of specimen 1 (Fig. 6). The latter relationship is best seen in
unexposed vanes where the effect of weathering is minimal (e.g., compare the exposed and
unexposed parts of the same vane marked by green arrows in Fig. 6B and 6C, respectively).
Thus, we have found no evidence for penetrative growth, even though the jumbled specimens
seem to give the impression of a penetrative relationship.
The other three specimens also interact with specimen 1. One lateral vane of specimen 2
is also in contact with the distal ends of the two lateral vanes of specimen 1 (Figs. 3D, 4B).
Again, there is no evidence for penetrative growth. Specimen 4 can be clearly seen as a curved
space on the surface of the hand sample (Fig. 2B). However, only a small part of this specimen is
preserved and one of its vanes is in contact with the seam of specimen 1 (Fig. 4B). Specimen 5
sits against one of the lateral vanes of specimen 1 (Fig. 3D, 4B), but this specimen has the least
preservational fidelity and is difficult to reconstruct. Despite the interactions among the five
specimens in the analyzed sample, there is no evidence for penetrative growth.
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5. Taphonomic and Ecological Interpretations
Our microCT analysis confirms the basic morphological architecture of Pteridinium
simplex, with three vanes attached to a single central seam. The nature of vane preservation as
void space or secondary calcite, however, does not allow us to resolve whether each vane is
composed of a single row or two overlapping rows of segments (Jenkins, 1992). Nonetheless, the
microCT data do reveal several important aspects of P. simplex morphology, taphonomy, and
paleoecology.
First, the folding, bending, and twisting of Pteridinium simplex specimens, as well as the
stretching, warping, and breakage of P. simplex vanes, are interpreted as taphonomic in origin.
Such taphonomic alteration is expected given the sedimentological inference that these
specimens were entrained and transported in mass flow deposits (Elliott et al., 2011).
Entrainment in high-density mass flow deposits also better explains the lack of preferred
orientation among tightly packed specimens. Indeed, different parts of a single specimen can
assume different orientations; for example, the bending and twisting of specimen 1 resulted in
different orientations of the two ends of this fossil (Figs. 5C–D, 7).
Second, although Pteridinium simplex vanes are stretchable, they can be torn, apparently
along segment boundaries which may represent mechanical weakness. That the distal part of the
vane is torn whereas the proximal part is intact suggests that the former may have been
mechanically weaker than the latter. This inference is consistent with previous interpretations
that the seam of P. simplex seems to be the most rigid part of the organism, with the vanes and
segments quickly losing definition farther away from the seam and grading into a smooth surface
(Elliott et al., 2011; Grazhdankin and Seilacher, 2002; Jenkins, 1992). The taphonomic
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reorientation of broken vanes would also make it difficult to determine vane identity, particularly
when the medial vane is poorly preserved (as commonly is the case), and when three-
dimensional morphological reconstructions are not available and thus the determination of vane
identity has to be based on vane orientation.
Third, detailed microCT imaging of the interrelationship between tightly packed
Pteridinium simplex specimens with sharply abutting vanes shows no evidence for penetrative
growth; instead, the distal edges of vanes are bent when compressed against other vanes.
Previous interpretation of penetrative growth in P. simplex was based on hand sample
observation of specimens with vanes abruptly abutting against each other (Grazhdankin and
Seilacher, 2002). Such relationship can be alternatively interpreted as taphonomically broken
vanes jammed against other specimens. A positive identification of penetrative growth requires
microCT data to confirm that vanes crosscut each other rather than just abut against each other.
Thus, some of the evidence previously used to infer a fully endobenthic lifestyle of
Pteridinium simplex—convoluted specimens with penetrative growth and vane identity change
(Grazhdankin and Seilacher, 2002)—needs to be reconsidered in light of the new data presented
here. Because the analyzed specimens have been transported, it is important to carry out
microCT analysis of specimens preserved in life position to further test the endobenthic
hypothesis. Nonetheless, our data are consistent with an epibenthic model, and they do not
falsify a semi-endobenthic life mode of P. simplex (Fig. 1B) with its central seam buried in the
sediments but the distal part of the vanes remaining free in the water column (Elliott et al., 2011;
Grazhdankin and Seilacher, 2002; Meyer et al., 2014). The epibenthic or semi-endobenthic
ecological interpretations can also account for Pteridinium specimens that are preserved more or
less two-dimensionally on the bedding surface (Narbonne et al., 1997).
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6. Discussion on biomaterial composition and phylogenetic affinity
A major taphonomic problem for many Ediacara fossils (including Pteridinium simplex)
is that, although they are ‘soft-bodied’, their integument seems to be rigid enough to survive
transport, but also pliable enough to account for bent and twisted specimens (Dzik, 1999, 2003;
Fedonkin et al., 2007; Gehling, 1999). The absence of preserved organic material in many
Ediacara fossils, particularly those hosted in sandstones, has led some investigators to pursue
biomechanical studies using flume or flow tank experiments, in order to infer the biomechanical
properties of their original integument (Schopf and Baumiller, 1998; Singer et al., 2012). While
this study was not set up to examine the biomechanics of P. simplex, biomechanical information
can be indirectly inferred from the three-dimensional scan data. The degree of stiffness and
elasticity exhibited by the vanes of P. simplex may help to constrain the biomechanical
properties and possible biomaterial makeup of its integument. This information can then be used
to indirectly infer the possible phylogenetic affinity of P. simplex. However, in order to constrain
the biomaterial makeup of P. simplex integument, we must first understand what stiff yet elastic
biomaterials are present in extant (and extinct) organisms and how they may have affected the
taphonomic behavior of P. simplex vanes.
6.1. Extinct biomaterials
Ediacara fossils were once placed in the extinct clade Vendobionta and were noted for
their distinct morphology, preservation, and biocoenosis (Grazhdankin and Seilacher, 2002;
Seilacher, 1989, 1992, 2007). The placement of Pteridinium simplex within the Vendobionta
opens up the possibility that its integument may have been composed of a distinct biomaterial
that has since been lost along with the extinction of Ediacara organisms. While we cannot
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exclude this possibility, it is very difficult to constrain, without the aid of modern analogs, the
range of materials that accord with the inferred biomechanical properties of P. simplex
integument. Even though the vendobionts may represent an extinct clade, they must be
evolutionarily related to extant organisms. Thus, our search for the possible biomaterial makeup
of P. simplex integument is necessarily based on extant analogs.
6.2. Extant biomaterials
Collagen is exclusively found in animals and is most commonly present in their
integuments, muscles, and connective tissues (Müller, 2003; Vogel, 2003). It is also present (in a
more loosely packed form) in the mesoglea of diploblastic animals such as cnidarians and
ctenophores (Hernandez-Nicaise and Amsellem, 1982; Tucker et al., 2011). Collagen fibers
(both bound, as in muscles, and unbound, as in mesoglea) are structurally elastic, meaning that it
can be stretched or compressed and still return to its original form. Other potentially elastic
biomaterials in extant organisms are mostly structural polysaccharides, including chitin and
cellulose. Chitin is a nitrogen bearing polysaccharide that has variable biomechanical properties
depending on the inclusion of additional materials such as sclerotin or calcium carbonate
(Bengtson and Conway Morris, 1992; Varki, 2009). Various forms of chitin are present in the
cell walls of fungal hyphae, the proteinaceous matrix of arthropods (with high levels of
sclerotin), and the exoskeletons of crustaceans and calcitic shells molluscs (Marin and Luquet,
2005; Schönitzer et al., 2011). Cellulose is the main structural component in the cell walls of
algae and plants (Vogel, 2003), but it is also present in the tunic of tunicate animals (Endean,
1961; Vogel, 2003). Cellulose fibers are highly flexible, as can be observed in some algal fronds
(Koehl, 1999; Martone, 2007; Martone and Denny, 2008), but highly packed cellulose fibers can
be brittle due to the crystalline nature of cellulose (Vogel, 2003).
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Pectin and pellicle are present in the cell walls of some eukaryotes (Kirby, 1941). Pectins
are complex polysaccharides in the primary cell walls of non-woody terrestrial plants and bind
the cells together (Braconnot, 1825; Buchanan et al., 2000). Pellicle is a thin layer of
proteinaceous strips supporting the cell membrane in various protozoa (Kirby, 1941). Both pectin
and pellicle can be flexible, elastic, or rigid, but these variable biomechanical characteristics are
only expressed on small scales (Kirby, 1941). Additionally, bacterial cell walls have a mesh-like
layer of peptidoglycan (murein) that serves a structural role (increasing the strength of the cell
wall) and controls osmotic pressures (Demchick and Koch, 1996; van den Ent et al., 2001).
However, because we do not have any biomechanical information of Pteridinium simplex at the
cellular scale, the presence/absence of pectin, pellicle, or peptidoglycan in P. simplex integument
cannot be tested using the currently available data.
Finally, some living organisms form an agglutinated test as their “integument”. For
example, some single-celled foraminifers are well known for their agglutinated tests (Murray,
2006). Agglutinated walls consist of sediment particles, sometimes bound with organic material
(Murray, 2006). Xenophyophoran foraminifers can form agglutinated tests of macroscopic size,
consisting of fine sediment bound with fecal pellets (Murray, 2006; Seilacher et al., 2003;
Tendal, 1972, 1979). Structurally, however, their tests are very brittle and do not deform easily
(Seilacher et al., 2003; Tendal, 1972).
6.3. Taphonomic constraint on phylogenetic affinity
Any phylogenetic interpretations for Pteridinium simplex must also consider its
taphonomy. Since Pteridinium fossils are preserved as casts or molds in massive quartzite, there
is no original organic material or any carbonaceous material associated with the fossils.
Preservation in coarse-grained siliciclastic rocks is common for Ediacara fossils (Gehling, 1999;
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Schiffbauer and Laflamme, 2012), with few of them being preserved through carbonaceous
compression, the main taphonomic mode of Burgess Shale-type fossils. One of the exceptions is
the Ediacara fossil Eoandromeda, which is preserved both as casts/molds in sandstone and as
carbonaceous compressions in black shale (Zhu et al., 2008). It is thus inferred that P. simplex
must have had an organic integument and the lack of carbonaceous preservation is due to
secondary loss. Indeed, many Phanerozoic plant fossils, when preserved in coarse-grained
sandstones, lack carbonaceous material as well (Driese et al., 1997; Sharon, 2010).
The minor clay minerals found in association with P. simplex vanes (Meyer et al., 2014)
may be important, because detrital and authigenic clay minerals are thought to play a
constructive role in the preservation of soft tissues (Anderson et al., 2011; Butterfield, 1990; Cai
et al., 2012; Laflamme et al., 2011; Meyer et al., 2012; Orr et al., 1998). Clay minerals have been
known from Burgess Shale-type biotas where carbonaceous material is preserved (Butterfield,
1990; Gaines et al., 2008; Orr et al., 1998; Xiao et al., 2002). Although detrital or authigenic
clays may have played a constructive role in the preservation of Ediacaran fossils such as
Aspidella (Laflamme et al., 2011), Conotubus (Cai et al., 2012), and Shaanxilithes (Meyer et al.,
2012), the origin of clays in association with P. simplex vanes cannot yet be resolved with
available data and they have been tentatively interpreted as weathering products (Meyer et al.,
2014). With the lack of carbonaceous material and the problematic origin of minor clay minerals
in association with P. simplex (Meyer et al., 2014), our phylogenetic interpretation has to be
indirectly based on the integument biomaterials, which are in turn inferred from the taphonomic
behavior and the elastic biomechanical property of the integument.
6.4. Phylogenetic discussion
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Collagen, cellulose, and chitin have been the most commonly hypothesized biomaterials
for Pteridinium simplex integument (Buss and Seilacher, 1994; Dzik, 1999; Elliott et al., 2011;
Retallack, 1994, 2007; Seilacher, 1992), although agglutinated tests similar to extant
xenophyophores have also been suggested as possible constructional biomaterials (Seilacher et
al., 2003). The elastic behavior of P. simplex vanes is consistent with the presence of collagen or
chitin in its integument. Both of these materials can add to the elasticity and flexibility of the
integument. However, the sharply torn edge of P. simplex vanes also suggests a stiffness or
rigidity that could be attributed to the presence of cellulose fibers within the integument.
Agglutinated tests, however, can be excluded as they do not have the degree of flexibility as
observed in the taphonomic analysis of P. simplex. Thus, a phylogenetic affinity with
xenophyophores can also be excluded. As stated above, the presence/absence of pectin, pellicle,
or peptidoglycan in P. simplex integument cannot be tested because of the lack of cellular scale
behavior of P. simplex integument.
The possible presence of collagen, cellulose, and chitin in the integument of Pteridinium
simplex can be used to constrain the phylogenetic affinity of this enigmatic Ediacara fossil.
Collagen is found exclusively in animals. Extant animals use collagen in a diverse array of
functions (e.g., collagen in muscles and connective tissues can assist locomotion), but these
functions may have been derived characteristics that evolved after the origin of collagen. With a
few exceptions (Ivanstov, 2009, 2011; Ivanstov and Malakhovskaya, 2002), most Ediacara
fossils including P. simplex were probably immobile organisms. Thus, the presence of collagen
P. simplex integument may not be related to mobility. Chitin and cellulose have broader
phylogenetic distribution than collagen, but both are present in modern animals (e.g., chitin in
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arthropods and cellulose in tunicates). Thus, the presence of chitin and cellulose in P. simplex
does not discriminate against a phylogenetic relationship with animals.
If collagen was indeed present in Pteridinium simplex integument, then this enigmatic
Ediacara fossil may be more closely related to living animals than any other living clades
because collagen is exclusively present in the animal kingdom. On the other hand, because P.
simplex does not seem to share its body plan with any other living animals, we surmise that it is
likely a stem-group animal. Because the animal kingdom and its living sister group—the
choanoflagellates—are separated by significant morphological gaps, it is expected that stem-
group animals share one or a few features (collagen in the case of P. simplex) collectively
defining the living animal clade, and it is also expected that these extinct stem-group animals
may have evolved their own autapomorphies (three-vaned body architecture in the case of P.
simplex) that make them drastically different from crown-group animals.
7. Conclusions
MicroCT offers an effective approach to study Pteridinium simplex specimens preserved
three-dimensionally within mass flow deposits. Our investigation confirms the basic morphology
of P. simplex: three vanes attached at the seam. The vanes can be bent, folded, twisted, stretched,
and torn, through taphonomic processes. Such taphonomic alterations complicate morphological
reconstruction of P. simplex, and can potentially lead to false interpretation of vane identity
change and penetrative growth, particularly when observation is limited to the external surface of
fossiliferous blocks. Our microCT analysis reveals no evidence for vane identity change or
penetrative growth. Thus, this important basis for postulating an entirely endobenthic P. simplex
should be treated with caution and should be further tested with microCT analysis of specimens
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preserved in life position. Nonetheless, the data presented here are consistent with the traditional
ecological interpretation of P. simplex as a semi-endobenthic or epibenthic organism because this
life style also accounts for specimens preserved two dimensionally on bedding surfaces. The
taphonomic deformation of P. simplex indicates that it had an elastic integument, which is
inconsistent with a phylogenetic affinity with xenophyophores. Its integument may have been
made of a collagenous, chitinous, and cellulose biomaterial. Because collagen synthesis is a
synapomorphy of the animal kingdom, P. simplex may be phylogenetically related to animals,
likely representing a stem-group animal considering its unique body plan.
Acknowledgments
Financial support for this study was provided by the National Science Foundation (EAR-
0844235, EAR-0948842, EAR‐1124062, EAR‐1250800), International Geosciences Program
Projects 493 and 587, and National Geographic Society, and both the international and national
committees of the International Geoscience Programme (IGCP). We would like to thank the
anonymous reviewers for constructive comments and the owners of Farm Aar, B. Boehm-Erni
and the late B. Boehm, for granting access to the fossils site and their hospitality.
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Figure Legends
Fig. 1. Geological map and stratigraphic column and construction of Pteridinium simplex. (A)
Geological and stratigraphic column showing the location and horizon from which the hand
sample in this study was collected (in the Aar Member, Dabis Formation, Kuibis subgroup).
Radiometric dates from Schmitz (2012). (B) Morphological and ecological reconstruction and
descriptive terms of P. simplex.
Fig. 2. Light photographs of hand sample V-8-2009, shown in different views. Colored arrows
point to correspondingly colored P. simplex specimens in Figs. 3-4. Asterisks, triangles, and stars
mark landmark locations in all views.
Fig. 3. Light photography and 3D reconstruction of Pteridinium simplex specimens within hand
sample block V-8-2009. (A, C) Light photographs from two different perspectives, with (C)
showing a view from the yellow arrow in (A). (B, D) MicroCT segmentations of P. simplex
specimens, oriented at the same perspectives as (A) and (C), respectively, with dashed lines
representing the outline of the hand sample. Colored arrows point to specimens segmented in
corresponding colors. Yellow arrow points to main specimen (specimen 1) in this study.
Numbers 1–3 denote the three slabs resulting from cuts represented by lines i and ii. Slabs 2 and
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3 were microCT scanned. Asterisks mark the same location on slab 2 and can be used for
orientation. Scale bar = 1 cm.
Fig. 4. Two different views of the five segmented specimens of Pteridinium simplex. Specimens
are colored and numbered. Asterisks mark the same location locations on slab 2. Scale bar = 1
cm.
Fig. 5. Four different views of Pteridinium simplex specimen 1. Lateral vanes are yellow and
green, and medial vane is blue. (A–C) Views including slabs 2 and 3. (D) View of only slab 3.
Asterisks and triangles mark the same two locations on slab 3 in all views. Scale bar = 1 cm.
Fig. 6. Three-dimensional reconstruction and microCT slice images of Pteridinium simplex
specimens 1 (yellow) and 3 (green). (A) View showing that the vanes of the two specimens
abruptly abut against each other. (B–C) MicroCT slice images, showing the relationship between
the two specimens. Colored arrows point to vanes of specimens segmented in corresponding
colors. Note that the apparent vane termination of specimen 3 at specimen 1 is due to weathering
on exposed surface (green arrows and green dashed line in B) and that, when unexposed, the
same vane is distally bent where it is compressed against specimen 1 (yellow arrows and yellow
dashed line in C). Scale bar = 1 cm.
Fig. 7. Simplified diagram showing bending, twisting, and tearing in specimen 1 (vane colors
correspond to those in Figure 5). (A) Pre-deformation morphology. (B–C) Bending. (D) Twisting
and tearing along segment boundaries.
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Three-dimensional microCT analysis of the Ediacara fossil Pteridinium simplex
sheds new light on its ecology and phylogenetic affinity
Mike Meyer*, David Elliott, Andrew D. Wood, Nicholas F. Polys, Matthew Colbert, Jessica A.
Maisano, Patricia Vickers-Rich, Michael Hall, Karl H. Hoffman, Gabi Schneider, Shuhai Xiao
Highlights:
We utilize microCT techniques to reconstruct Pteridinium simplex in 3D
In situ microCT analysis allows fossil examination with minimal destruction
P. simplex is found to be very flexible and somewhat elastic
P simplex’s integument can be bent, folded, twisted, stretched, and torn
This physical property is consistent with the presence of collagen, chitin, and cellulose
*Highlights (for review)
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Graphical Abstract (for review)
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Figure 1
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Figure 2
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Figure 3
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Figure 4
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Figure 5
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Figure 6
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Figure 7
