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Australia: The Land Where Time Began |
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Bilaterian – Oldest Specimen Found in the Ediacaran of South
Australia
It was predicted by analysis of modern animals and trace fossils that
the oldest bilaterians were simple and small. Such animals should have
been part of the
Ediacaran
biota, which is the earliest known macroscopic complex animal
communities that have been preserved, though they would be difficult to
recognise in the fossil record. In this paper Evans et
al. describe
Ikaria wariootia gen.et
sp. nov. from the
Ediacara
Member, South Australia, as a small, simple organism that has
anterior/posterior differentiation. They found that the predictions for
progenitor of the trace fossil
Helminthoidichnites,
which was indicative of soil movement and displacement, was matched by
size and morphology of Ikaria.
Helminthoidichnites
occurs in the Ediacaran Member stratigraphically below classical body
fossils. Together, it is suggested by these that the
Ikaria is one of the
oldest total group bilaterians that have been identified from South
Australia, with little deviation from the characters that were predicted
for their last common ancestor. Also, these trace fossils persist into
the
Phanerozoic, and provides a critical link between the Ediacaran and
the Cambrian. Within the soft bodied biota from the Ediacara the first
macroscopic fossil animals are recognised (Xiao & Laflamme, 2009; Erwin
et al., 2011). Among these
are candidate poriferans (Clites, Droser & Gehling, 2012), cnidarians
(Droser & Gehling, 2008) and ctenophores (Zhu et
al., 2008). Rare taxa of the
Ediacaran biota have been interpreted as bilaterians, namely
Kimberella (Ivantsov,
2013; Gehling, Runnegar & Droser, 2014). Small furrowed trace fossils
have, however, been generally accepted as evidence for total group
bilaterians in the Ediacaran (Jensen, 2003; Buatois & Mángano, 2016;
Parry et al., 2017). It is
suggested by the size and morphology of trace fossils that they were
produced by millimetre-sized organisms that would be difficult to
recognise in the fossil record (Gehling & Droser, 2018).
Helminthoidichnites are horizontal trace fossils that are found in
deposits dating to the Ediacaran and Phanerozoic globally (Buatois et
al., 2014; Wood et
al., 2019). The burrow of
Helminthoidichnites is
curvilinear, and can be preserved on bed tops as well as bottoms and
most commonly occurs on the base of thin (submillimetre to millimetre
scale) discontinuous sand bodies, or shims (Jensen, 2003; Tarhan et
al., 2017). It is indicated
by the preservation of Helminthoidichnites in negative relief that are
flanked by positive levees on the bed bottoms that the progenitor moved
beneath thin sand bodies following deposition and burial, displacing
sediments (Jensen, 2003; Buatois, 2016; Gehling & Droser, 2018; Tarhan
et al., 2017). The ability of
the progenitor to move vertically, though on millimetre scales, is
indicated by relationships that have been observed between intersecting
Helminthoidichnites
(Gehling & Droser, 2018). According to Evans
et al., the rare
Helminthoidichnites that
have been found penetrating body fossils of macroscopic taxa may
represent the oldest evidence of scavenging (Gehling & Droser, 2018).
Helminthoidichnites-type
structures can be produced by a variety of bilaterians in modern
environments (Buatois & Mángano, 2016; Gehling & Droser, 2018). A likely
progenitor for
Helminthoidichnites from
the Ediacaran has not yet been identified, though there have been
suggestions that these were produced by simple “worm-like animals”
(Buatois & Mángano, 2016). Critically, it would have been small, having
a maximum diameter of less than that observed for
Helminthoidichnites,
based on the nature of sediment displacement by a horizontally burrowing
organism. Anterior-posterior differentiation is necessitated by such
behaviour, as well as a coelom, which is consistent with Bilaterian
grade tissue organisation (Jensen, 2003; Buatois & Mángano, 2016;
Gehling & Droser, 2018; Budd & Jensen, 2017). Within the Ediacara
Member, Rawnsley Quartzite, in the Flinders Ranges and the surrounding
regions of South Australia,
Helminthoidichnites have
been abundantly preserved (Jensen, Droser & Gehling, 2006). The
Ediacaran Member consists of shallow marine sandstone event beds 50 to
500 m below a basal disconformity of Cambrian age (Gehling, 2000).
The excavation and reconstruction of 37 m scale fossiliferous bed
surfaces have revealed in situ
communities of the Ediacara Biota, at the National Heritage Nilpena site
(Droser et al., 2019).
Helminthoidichnites is
found more than 100 m beneath the first appearance of Kimberella at
Nilpena and the Flinders Ranges (Gehling & Droser, 2012; Gehling et
al., 2019). Currently there
are no radiometric dates that could constrain the absolute age of the
Ediacaran Member; there are, however, significant overlap of taxa with
deposits that are well established from the White Sea Region of Russia
that indicates that these are likely to be between 560 and 551 million
years old (Martin et al.,
2000; Waggoner, 2003; Grazhdankin, 2014; Boag, Darroch & Laflamme,
2016). The early appearance of trace fossils in South Australia may be
corroborated by a similar pattern of leveed, horizontal trace fossils
(though in this case assigned to ichnogenus
Archaeonassa) occurring
stratigraphically below White Sea assemblage body fossils in Russia
(Buatois & Mángano, 2018; Grazhdankin, 2014).
Systematic Description
Ikaria Wariootia
gen. et sp. nov.
Etymology
The generic Name is after the word “Ikara,” Adnyamathanha name for
Wilpena Pound, and
means “meeting place” in the Adnyamathanha language. In the view from
Nilpena Ikara is the major landmark, and the fossil has been named to
acknowledge the original custodians of the land; species are named for
Warioota Creek, which runs from the Flinders Ranges to Nilpena Station.
Holotype.
P57685 (South Australian Museum)
Paratype.
P57686 (South Australian Museum)
Field Paratypes.
1T-A bed 001to 007; Nilpena.
Horizon and Locality.
Ediacaran Member, Rawnsley Quartzite at the National Heritage Nilpena
field site and Bathtub Creek.
Diagnosis.
Irregular millimetre-scale ovoid preserved in negative hyporelief. The
major axis length averages 2.3 times the minor axis. There is a distinct
asymmetry along the major axis with one end wider and more broadly
curved. In profile the broader end is preserved in more significant
negative relief and with a steeper curvature. Rare specimens are bent
about the long axis and/or exhibit potential evidence of modularity,
with 2 to 5 body divisions.
Description.
I. wariootia are
well-defined elongate ovals, fusiform in shape. Three-Dimensional (3-D)
laser scans demonstrate clear anterior/posterior differentiation, with
one end distinctly smaller and more tightly curved. Length of the major
axis ranges from 1.9 to 6.7 mm and the minor axis from 1.1 to 2.4.
Preserved depth ranges from 0.6 to 1.6 mm. There is a consistent linear
relationship between total length and total width (SI Appendix, Fig. S1A)
The relationships between total length and depth (SI
Appendix, Fig. S1B)
as well as a width and depth (SI
Appendix, Fig. S2). Depth is always less than width, suggesting that
the fossil of
Ikaria is compressed.
This confirms previous interpretations that the preserved depth of
specimens from the Ediacaran Member is strongly influenced by taphonomic
processes (e.g. ref. Evans, Droser & Gehling, 2017).
The morphology of
Ikaria is consistent
across specimens and is unambiguously distinct from other structures,
though it is very simple. For rip-up clasts of organic mats, which are
irregular, the consistent shape and length-to-width rations are not what
is observed (Tarhan et al.,
2017). Rip-ups do not occur in the same lithologies and facies as
Ikaria (Tarhan et
al., 2017; Gehling & Droser,
2013), which represent deposition in an environment of lower energy,
though mat rip-ups are present within the Ediacara Member. Also, the
biostratinomic and diagenetic history of rip-up clasts are different
those of
Ikaria and all other body
fossils (Tarhan et al.,
2017).
Ikaria have a sharp outer
margin and they are preserved with considerable relief, distinct from
the surrounding matrix and organic mat textures. This is consistent with
other taxa from the Ediacara Member that are nonsessile (Droser et
al., 2018), which suggests
that
Ikaria represent the body
of a free-living organism.
Ikaria
can be easily differentiated from other taxa that are preserved on the
same bed surface and of similar size and scale
(SI Appendix Fig. S3).Therefore,
it is not likely to be a juvenile form of a previously described taxon.
A maximum size of ~7 mm is suggested by the lack of larger specimens
with comparable morphology. The biologic, body fossil origin of
Ikaria is corroborated by
the recognition of other taxa on the same scale and with similarly
well-defined out margins that are distinct from the organic mat.
There have been rare finds of specimens of
Ikaria and
Helminthoidichnites
together. The range of widths of
Ikaria plots entirely
within those measured for Helminthoidichnites with the maximum size of
body fossils no exceeding that of trace fossils (SI
Appendix, Fig. S4).
Also, it is indicated by the Anderson-Darling test that the
size-frequency distributions do not differ significantly (P
value 0.448). And, this combined with clear anterior-posterior
differentiation, suggests that
Ikaria is the only known
contemporaneous body fossil that has the suite of body characters that
have been predicted for the progenitor of
Helminthoidichnites.
Discussion
It is rare, in general, to find trace fossils as well as the organism
that produced them preserved together, in particular with respect to the
mobile metazoans. According to Evans et
al. this can be attributed to
both different pathways of preservation between the body and trace
fossils and the ability of the animal to move away from the area that
contains evidence of its activity (Bromley, ed. 1996; Seilacher, ed.,
2007). The morphological characteristics of body fossils from the same
deposit can be used, in certain cases, to determine reliably the
progenitors of particular trace fossils (e.g., Gibb, Pemberton &
Chatterton, 2016).
In the Ediacara Member body fossils include
Ikaria, are well
preserved on the bottoms of centimetre-scale sandstone beds with the
tops of these organisms being overlain with early mineralisation of sand
casting following burial (Jensen, 2003; Tarhan et
al., 2017; Jensen, Droser &
Gehling, 2006). Though in rare cases counterparts are identified on bed
tops, these are poorly preserved and at a resolution that is not likely
to produce identifiable features at the same scale as
Ikaria.
Helminthoidichnites is
found, in contrast, on both bed tops and bottoms, though most commonly
on the base of millimetre-thick shims, where it is rare to find fossils
that are well preserved (Jensen, 2003; Tarhan et
al., 2017). It is indicated
by negative hyporelief preservation that Helminthoidichnites formed
after the deposition of overlying sand, with the organism that produced
it capable of moving into and out of thin layers of sand (Gehling &
Droser, 2018). It is predicted by this that
Helminthoidichnites
should only be found and its progenitor on the same bed bottom in rare
instances when it died while it was burrowing underneath thin bodies of
sand. Even if
Ikaria was preserved at
the end of a trail, it is not likely that it would be possible to
identify as distinct from that trace, given the simple morphology and
preservation of both body and trace fossil in negative relief. Evans et
al. interpreted the
surprising discovery of
Helminthoidichnites with
nearby
Ikaria as the result of
vertical movement from the bedding plane in the region between the end
and its trace fossil and its final resting place. While it is likely
that this scenario was exceedingly rare, it may represent the only
situation when it would be possible to distinguish associated body and
trace fossils, and corroborate further interpretations of
Ikaria as the progenitor
of
Helminthoidichnites.
Evans et al. proposed that
Ikaria is the trace
marker of
Helminthoidichnites and
potentially the oldest, definitive bilaterian, at least as represented
in the fossil record of South Australia. The only other bilaterian from
the Ediacaran Member that is consistently reconstructed as a bilaterian,
Kimberella, is found
significantly higher stratigraphically than the earliest appearance of
Helminthoidichnites
(Ivantsov, 2013; Gehling, Runnegar & Droser, 2014; Gehling & Droser,
2012; Gehling; Gehling et al.,
2019). Similarities between taxonomic assemblages have been consistently
cited as evidence that White Sea assemblage fossils from the Ediacaran
Member,
Kimberella, are
conservatively 560-551 Ma (Martin et
al., 2000; Boag, Darroch &
Laflamme, 2016). It is suggested by the stratigraphic position of
Helminthoidichnites that
the first appearance of
Ikaria, was likely to be
from within this age range, or even possibly earlier. In Uruguay,
burrows that were initially interpreted to be from Ediacara age rocks
that were much older have age constraints that are uncertain (Pecoits et
al., 2012; Gaucher et
al., 2013). Trace fossils
from Brazil representing the activity of meiofaunal bilaterians, are
present about 30-40 m above a tuff that has been dated to 555 Ma and it
is in close association with
Cloudina, which indicates
that they are likely to be younger than
Ikaria (Parry et
al., 2017; Boag, Darroch &
Laflamme, 2016). Recently, a segmented bilaterian from South China has
been described, that was associated with trace fossils, was interpreted
to be younger, larger and more complex than
Ikaria (Chen, 2019).
According to Evans et al.,
the ability to move and produce trace fossils that are recognisable is
not unique to bilaterians. It is probable that complex and trace fossils
from older Ediacaran deposits produced by muscular eumetazoans that were
interpreted to be cnidarians (Liu, 2014). It is not likely that the
crown group bilaterians are represented by
Dickinsonia and other
similar fossils from the Ediacaran though were mobile and left trace
fossils (Sperling & Vinther, 2010; Evans, Gehling & Droser, 2019).
Simple burrows are generated by modern protists, though typically they
are smaller than
Ikaria (Serevin & Culver,
1982; Matz et al., 2008). It
has been demonstrated in lab experiments that mobile foraminifera form
burrows in clay and silt; they do not produce burrows, however, in
fine-grained or coarse-grained sand (Serevin, Culvin & Blanpied, 1982).
In deep sea environments large testate amoeba are associated with trails
similar to those that have been observed in the Ediacara, though these
are surficial and represent movement by rolling (Matz et
al., 2008). Flatworms are
mobile, though they do not burrow below the sediment-water interface,
and only rarely leave fossil traces (Valentine, Erwin & Jablonski,
1996). Expression on bed bottoms with furrows is unique among these
examples to
Helminthoidichnites,
which suggests movement associated with significant displacement of
medium sand grains. This is consistent with reconstructions of
Ikaria which has
musculature and a coelom (Gehling & Droser, 2012; Clark, 1981). These
characteristics are unique to bilaterians when combined with the
relative size of the body and trace fossils.
Anterior/posterior differentiation in
Ikaria is characterised
by polarity and relief, which is supported by movement that is directed
in trace fossils. Within
Helminthoidichnites
preservation of V-shaped transverse ridges suggests peristaltic mobility
(Gehling & Droser, 2018). A potentially modular body construction of
Ikaria is implied by its
morphology, would have aided in the organisation of muscles that would
be required for peristalsis (Carroll, Grenier & Weatherbee, 2001). It is
revealed by sediment displacement and scavenging that
Ikaria is likely to have
had a coelom, mouth, anus, and through-gut (Gehling & Droser, 2018; Budd
& Jensen, 2017; Clark, 1981), though it is not likely that these would
have been preserved in the fossil record. It is indicated by
preferential preservation of
Helminthoidichnites
beneath thin sand bodies that
Ikaria sought out these
environments, possibly as a result of increased availability of oxygen.
Ultimately, due to decomposition of organic matter, which is supported
by the restriction of
Helminthoidichnites to
beds <15 mm thick, as the depth of the overlying sand increases,
oxygenated environments are gradually replaced by sulphidic,
inhospitable settings (Gehling & Droser, 2018; Tarhan, 2017; Gehling,
2019). It is likely that
Ikaria had the ability to
detect organic matter that was buried in environments that were well
oxygenated as well as conditions that were potentially toxic, which
suggests rudimentary sensory abilities. These features combined suggest
that this organism was markedly complex, compared with contemporaneous
Ediacaran biota, in spite of the simple morphology that can be observed
directly on fossil specimens of
Ikaria. It has been
demonstrated by molecular phylogenetic analysis of modern metazoans that
developmental programming is highly conserved between disparate groups.
This led initially to the hypothesis that the last common ancestor (LCA)
of bilaterians (animals with an opening at each end of a trough-gut) was
relatively complex, having many of the features that are common to a
variety of such groups, which include eyes, segmentation, appendages,
and a heart (Carroll, Grenier & Weatherbee, 2001; Knoll & Carroll, 1999;
Robertis, 2008). It is indicated, instead, by expansion of this analysis
to nonbilaterian animals and their closest single-celled ancestors that
components of these conserved developmental pathways have deep ancestry
(see Erwin, 2015 for discussion). This suggests, when combined with
recent evidence for a sister group relationship between Xenacoelamorpha
and Bilateria, that the bilaterian LCA was a simple, small, mobile
organism with anterior/posterior differentiation and limited sensory
abilities (Erwin, 2015; Erwin & Davidson, 2002; Struck et
al., 2014; Hartenstein &
Stollewerk, 2015; Cannon et al.,
2016; Cunningham et al.,
2017). According to Evans et al.
it is remarkable that these predictions agree closely with characters
identified by this study for
Ikaria.
Body and associated trace fossils are relied upon for recognition of the
totality of traits of
Ikaria. It is not likely
that given the simple morphology of
Ikaria it would be
possible to assign it confidently to the Bilateria, or even the Metazoa,
without the relationship between
Ikaria and
Helminthoidichnites. This
is consistent with hypotheses that predictions of the apparent gap
between molecular clock predictions for the early divergence of
bilaterians and their later appearance in the fossil record is a result
of their predicted simple morphology (Erwin, 2015; Cunningham et
al., 2017). Therefore,
similar prephylum, total group bilaterians may be found in other
deposits of the Precambrian fossil record;
Ikaria provides a search
image for the identification of such forms in the future.
It is suggested by the stratigraphic position of
Helminthoidichnites that
Ikaria is the oldest
total group bilaterian from the fossil record of South Australia. Ikaria
represents an example in early animal evolution where phylogenetic
predictions correspond directly with the fossil record. Also, the global
distribution and recognition of
Helminthoidichnites in
strata of Cambrian age (Buatois et
al., 2014; Wood et
al., 2019) is distinct from
the overwhelming majority of the Ediacaran Biota. It is likely that
Ikaria and/or related
taxa existed across the Ediacaran-Cambrian boundary, though
Ikaria is not necessarily
responsible for the production of all
Helminthoidichnites.
Evans, S. D., et al. (2020). "Discovery of the oldest bilaterian from
the Ediacaran of South Australia." Proceedings of the National
Academy of Sciences: 202001045.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |