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Dickinsonia costata — the first evidence of neoteny in Ediacaran organisms

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  • Paleontological Institute of Russian Academy of Sciences

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Some of the most famous organisms of the Late Precambrian are representatives of the genus Dickinsonia. Four species of Dickinsonia are found throughout the section, characterized by macrofossils, on the southeastern White Sea area. However, their distribution is uneven. Two species, D. costata and D. tenuis are the most common, while the other two occur sporadically. At the lower levels (Verkhovka Formation) thinsegmented D. tenuis is the most frequently encountered. There are few imprints that can be attributed to D. costata, and all of them are of small size. Numerous specimens of D. costata, representing all available for study stages of ontogeny, are present only at the upper stratigraphic levels — in the Zimnie Gory and Erga formations. The statistical analysis of Dickinsonia imprints from a number of fossil assemblages in the southeastern White Sea shows an existence of two distinct groups that could represent different stages of ontogeny of a single species — D. tenuis. All the features of the juvenile D. tenuis — a round body, enlarged head part and a small number of trunk isomers — are distinctive features of D. costata. These features are preserved in mature D. costata. The most likely explanation of this phenomenon is the origin of D. costata from D. tenuis by neoteny.
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© INVERTEBRATE ZOOLOGY, 2017Invertebrate Zoology, 2017, 14(1): 92–98
Dickinsonia costata — the first evidence of neoteny
in Ediacaran organisms
M.A. Zakrevskaya, A.Yu. Ivantsov
A.A. Borissiak Paleontological Institute of RAS, Profsoyuznaya st., 123, Moscow, 117647, Russia.
E-mail: mariazakrevskaya@gmail.com ivancov@paleo.ru
ABSTRACT: Some of the most famous organisms of the Late Precambrian are represen-
tatives of the genus Dickinsonia. Four species of Dickinsonia are found throughout the
section, characterized by macrofossils, on the southeastern White Sea area. However, their
distribution is uneven. Two species, D. costata and D. tenuis are the most common, while
the other two occur sporadically. At the lower levels (Verkhovka Formation) thin-
segmented D. tenuis is the most frequently encountered. There are few imprints that can be
attributed to D. costata, and all of them are of small size. Numerous specimens of D. costata,
representing all available for study stages of ontogeny, are present only at the upper
stratigraphic levels — in the Zimnie Gory and Erga formations. The statistical analysis of
Dickinsonia imprints from a number of fossil assemblages in the southeastern White Sea
shows an existence of two distinct groups that could represent different stages of ontogeny
of a single species — D. tenuis. All the features of the juvenile D. tenuis — a round body,
enlarged head part and a small number of trunk isomers — are distinctive features of D.
costata. These features are preserved in mature D. costata. The most likely explanation of
this phenomenon is the origin of D. costata from D. tenuis by neoteny.
How to cite this article: Zakrevskaya M.A., Ivantsov A.Yu. 2017. Dickinsonia costata
the first evidence of neoteny in Ediacaran organisms // Invert. Zool. Vol.14. No.1. P.92–98.
doi: 10.15298/invertzool.14.1.13
KEY WORDS: Ediacaran biota, Vendian, Metazoa, Invertebrate paleontology, Dickin-
sonia.
Dickinsonia costataпервое свидетельство неотении
у эдиакарских организмов
М.А. Закревская, А.Ю. Иванцов
Палеонтологический институт им. А.А. Борисяка РАН, Профсоюзная ул., 123, Москва,
117647, Россия. E-mail: mariazakrevskaya@gmail.com ivancov@paleo.ru
РЕЗЮМЕ: Одними из самых известных организмов позднего докембрия являются
представители рода Dickinsonia. На территории Юго-Восточного Беломорья 4 вида
дикинсоний встречаются по всему разрезу, охарактеризованному макроископаемы-
ми. Однако распределение их неравномерно. Наибольшее распространение имеют
два вида D. costata и D. tenuis, а два других встречаются спорадически. На нижних
уровнях (верховская свита) наиболее часто встречается тонкосегментная D. tenuis, а
отпечатков, которые можно было бы отнести к D. costata мало и все они мелкого
размера. Многочисленные остатки D. costata, отражающие все доступные для на-
93Dickinsonia costata — the first evidence of neoteny in Ediacaran organisms
Introduction
Dickinsonia Sprigg, 1947 is one of the most
typical and world-known fossil taxa of the Edi-
acaran (Vendian) age. It was described original-
ly from the upper Ediacaran (Pound Quartzite)
of the Flinders Ranges in South Australia. The
genus is common in marine strata of eastern
Europe and South Australia. The flat bilaterally
symmetrical body of Dickinsonia consists of
two parts — head and trunk ones. The trunk part
is divided into numerous transverse elements —
isomers, located on both sides of the body in an
alternating pattern. The head part of mature
individuals is almost indistinguishable from iso-
mers by its overall size and size ratios.
Dickinsonia costata Sprigg, 1947 is the type
species of the genus. It is characterized by a
round shape with the length/width ratio ap-
proaching 1:1, by a small number of isomers
with relatively large width, and by the enlarged
head part.
During 70 years of the study of Dickinsonia,
understanding of this genus was changed radi-
cally. At the beginning, Dickinsonia was inter-
preted as coelenterates of uncertain origin
(Sprigg, 1949) and as annelids (Glaessner, Wade,
1966). D. Seilacher combined Dickinsonia along
with the rest of the Vendian biota into the special
extinct kingdom of Vendobionta on the basis of
their specific features, such as the chambered
construction, the symmetry of the sliding reflec-
tion, a presumable absence of the oral opening
and distinct inner structures (Seilacher, 1989,
1992). He suggested that Dickinsonia was feed-
ing by absorbing dissolved organic matter by
the entire body surface using chemo- or photo-
symbionts. Later on, Seilacher reduced the com-
position of the Vendobionta and lowered the
rank of this group to a class level within rhizo-
pod protozoans (Seilacher et al., 2003). G. Re-
tallack suggested that Dickinsonia was remains
of fungi or lichens on the basis of its leaf shape
and unlimited growth (Retallack, 1994). E. Sper-
ling and J. Vinther proposed a hypothesis on the
placozoan level of organization of alike Vendi-
an organisms, based on the assumption that
Dickinsonia fed by external digestion (Sper-
ling, Vinther, 2010). Dickinsonia was also in-
terpreted as ancestral chordates (Dzik, 2003).
According to the most compelling suggestions,
Dickinsonia is classified as the Proarticulata, an
extinct phylum of benthic mobile multicellular
animals (Fedonkin, 1990). This statement was
confirmed by the finds of Dickinsonia trace
fossils in association with the body imprints of
the trace maker (Ivantsov, Malakhovskaya, 2002;
Gehling et al., 2005; Evans et al., 2015). The
pattern of Dickinsonia trace fossils revealed
that this animal fed on upper surface of micro-
bial mats using its ventral side (Ivantsov, 2011,
2013). Distinct internal structures, interpreted
as digestive-distributive systems, were also
found in some specimens of Dickinsonia (Ivan-
блюдения стадии онтогенеза этого вида, присутствуют только на верхних уровнях
в зимнегорской и ергинской свитах. Проведенный статистический анализ совокуп-
ности отпечатков дикинсоний из ряда захоронений Юго-Восточного Беломорья
показал существование 2 четких групп, которые могут представлять собой различ-
ные стадии онтогенеза одного видаD. tenuis. Все признаки ювенильных D. tenuis
округлое тело, широкий головной отдел и небольшое количество туловищных
изомеровявляются характерными признаками D. costata и сохраняются у нее во
взрослом состоянии. Наиболее вероятным объяснением выявленного феномена
является происхождение D. costata от D. tenuis путем неотении.
Как цитировать эту статью: Zakrevskaya M.A., Ivantsov A.Yu. 2017. Dickinsonia
costata — the first evidence of neoteny in Ediacaran organisms // Invert. Zool. Vol.14.
No.1. P.92–98. doi: 10.15298/invertzool.14.1.13
КЛЮЧЕВЫЕ СЛОВА: эдиакарская биота, вендский период, Metazoa, палеонтология
беспозвоночных, Dickinsonia.
94 M.A. Zakrevskaya, A.Yu. Ivantsov
tsov, 2004). An indirect evidence of this organ-
ism mobility can also be derived from the study
of paleocommunities. In paleocommunities con-
sisting of juveniles only and almost lacking
mature individuals, very large single individu-
als of Dickinsonia costata occur. Such individ-
uals, representing the previous generation, prob-
ably migrated from nearby areas and gave the
birth to juveniles that formed a new paleocom-
munity (Zakrevskaya, 2014).
Dickinsonia is the richest genus of the late
Precambrian macroorganisms. At present, nine
species of this genus were described: D. costa-
ta, D. tenuis, D. lissa, D. menneri, D. rex, D.
brachina, D. minima, D. spriggi, and D. elon-
gata. However, the majority of these species
probably are synonyms. In the White Sea sec-
tions, four species only are found, namely D.
costata, D. tenuis, D. lissa, D. menneri. They
differ primarily by ratios of body dimensions
and by the width of transverse elements (iso-
mers) and the head part. Of them, D. costata and
D. tenuis only are present in a significant num-
ber to use statistical analysis.
Materials and methods
Dickinsonia specimens are collected in the
southeastern White Sea area (Arkhangelsk re-
gion) by the Precambrian team of the A.A.
Borissiak Paleontological Institute of RAS and
housed in the Laboratory of Precambrian Or-
ganisms of this institute.
The fossils were preserved in situ as im-
prints on sandstone bottom surfaces (Flinders-
type preservation; Narbonne, 2005), as a result
of burial of benthic paleocommunities by storm
sediments. In the White Sea section, a series of
successive burials is observed at different strati-
graphic levels embracing an interval of over 3
million years (Martin et al., 2000; Llanos et al.,
2005). Dickinsonia is found throughout the sec-
tion, characterized by macrofossils. However,
generally only thin-segmented D. tenuis occurs
at the lower levels (the Verkhovka Formation)
while imprints that can be attributed to D. cos-
tata are few and small. Numerous specimens of
D. costata preserving all the stages of ontogeny
available for study are restricted to the upper
levels, in the Zimnie Gory and Erga formations.
For the statistical analysis of the fossil im-
prints, over 500 specimens, presumably belong-
ing to different species of Dickinsonia, were
measured. The length and the width of each
imprint (with 0.1 mm precision), the number of
its isomers, and the width of the first isomer at its
widest dimension are measured in 220 speci-
mens of Dickinsonia. Fragmentary preservation
of other specimens does not allow us to include
them in our analysis. However, an excellent
preservation of the majority of the White Sea
specimens bearing the finest details provides an
opportunity to make necessary measurements
even in the smallest individuals.
Results and discussion
A statistical analysis has been applied to the
imprints of both D. tenuis and D. costata from
a number of burials in the Verkhovka and Erga
formations, respectively, of the Karakhta and
Zimnie Gory localities (Zakrevskaya, Ivantsov,
2015). Due to certain peculiarities of the forma-
tion of Flinders-type localities, each fossil as-
semblages of a burial reflects directly the pre-
burial state of populations and is not enriched or
depleted during posthumous processes. The re-
sulting plots show a variation in the number of
isomers and the width of the first isomer of
Dickinsonia in relation to the body size (length,
width). The body length of D. tenuis being
figured against the width of its first isomer
reveals a bimodal distribution of these dimen-
sions (Fig. 1). The first group is represented by
small-sized specimens with a wide head part,
and the second group — by larger specimens
with a narrow head part. These two sets of
individuals are distinct and their ratio variables
do not overlap.
The ratio of the length to the number of
isomers also demonstrates a similar division
into two groups for D. tenuis (Fig. 2), where the
first group is characterized by a small number of
isomers and a small body size, and the second
one — by a large number of isomers and a bigger
95Dickinsonia costata — the first evidence of neoteny in Ediacaran organisms
Fig. 1. Ratio of length to width of the head part of Dickinsonia tenuis from Karakhta locality.
Рис. 1. Соотношение длины и ширины головного отдела Dickinsonia tenuis из Карахтинского
местонахождения.
Fig. 2. Ratio of length to number of isomers in Dickinsonia tenuis from Karakhta locality and D. costata from
Zimnie Gory locality.
Рис. 2. Соотношение длины и количества изомеров у Dickinsonia tenuis из Карахтинского и Dickinsonia
costata из Зимнегорского местонахождения.
96 M.A. Zakrevskaya, A.Yu. Ivantsov
size. The plot shows that the size ratios of the
first group correlate with same ratios of D.
costata from the Zimnie Gory locality (marked
by black triangles) and together these data com-
pile a single linear trend. Such a pattern keeps
pass in D. costata throughout the entire length
range up to very large specimens (up to 140 mm
in length). In D. tenuis, as the size increases, the
linear trend abruptly changes its direction, and
the growth in the number of isomers overcomes
the increase in size of larger individuals com-
pared to smaller ones.
The same pattern is observed on the plot
showing the ratio of the number of isomers to the
width of the first isomer for the same specimens
of D. tenuis and D. costata (Fig. 3). An overlap
of dimensions of the smallest D. tenuis with
those of D. costata is observed, where the latter
is distinguished by the presence of a wide head
part and a small total number of isomers. Over
specimens of D. tenuis form a separate set that
does not overlap with the previous one and is
characterized by a gradually increasing total
number of isomers and a constant very small
relative width of the head part, which is much
inferior to the width of the head part in speci-
mens from the first set.
In fossil assemblages of the Karakhta local-
ity, all small individuals of Dickinsonia are
characterized by body dimensions of D. costa-
Fig. 3. Ratio of number of isomers to width of the head part in the widest section of Dickinsonia tenuis from
Karakhta and D. costata from Zimnie Gory locality.
Рис. 3. Соотношение количества изомеров и ширины головного отдела в самой широкой его части
у Dickinsonia tenuis из Карахтинского и Dickinsonia costata из Зимнегорского местонахождения.
97Dickinsonia costata — the first evidence of neoteny in Ediacaran organisms
ment can be indicative of the fact that Dickinso-
nia had a distinct juvenile stage and this stage
was characterized by an ecology different from
that of mature individuals.
It can be seen that all the features of juvenile
D. tenuis — the round body, the wide first
isomer and a small number of isomers – are
characteristics of D. costata and remain so at
mature stages (Fig. 3, 4). These observations
can indicate a probable origin of D. costata
from D. tenuis by the neoteny. This is the first
case of neoteny being observed in Precambrian
organisms.
ta, and all larger individuals have features typ-
ical of D. tenuis (Fig. 4). The transition from the
first set to the second one is abrupt but no gap in
the sequence is observed. In this regard, we
suggest that both sets represent different stages
of ontogeny of the same species — D. tenuis.
The transition is accompanied by a discontinu-
ous growth and by a sharp, almost double in-
crease in the number of trunk isomers, as well as
a reduction in the width of the head part. Thus,
the body assumes the appearance of a typical D.
tenuis. The well-marked morphological differ-
ences between early and later stages of develop-
Fig. 4. Individual development of Dickinsonia. 1–8 — a part of the ontogeny series of D. tenuis based on
specimens from Karakhta locality (1–3 — juveniles); 9, 10 — D. costata from Zimnie Gory locality; latex
casts of the specimens from the PIN RAS collection, nos.: 1 — specimen PIN, No 4852/38, 2 — 4852/39,
3 — 4852/33, 4 — 4852/79, 5 — 4852/60, 6 — 4852/30, 7 — 4852/44, 8 — 4852/17, 9 — 3993/5107, 10
— 3993/5243. Scale bar 1 cm.
Рис. 4. Возрастные преобразования тела дикинсоний. 1–8 — фрагмент онтогенетического ряда
Dickinsonia tenuis по материалам Карахтинского местонахождения (1–3 — ювенильная стадия); 9,
10 — Dickinsonia costata из Зимнегорского местонахождения; латексные слепки с экземпляров
коллекции ПИН РАН, №№: 1 — 4852/38, 2 — 4852/39, 3 — 4852/33, 4 — 4852/79, 5 Ї 4852/60, 6 —
4852/30, 7 — 4852/44, 8 — 4852/17, 9 — 3993/5107, 10 — 3993/5243. Длина масштабного отрезка
1 см.
98 M.A. Zakrevskaya, A.Yu. Ivantsov
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... Even basic symmetry-whether modules were offset or meet precisely at the midline-remains the subject of considerable debate (e.g., Gold et al. 2015;Ivantsov et al. 2019). Many studies have focused on growth patterns, dating back to original descriptions by Sprigg (1949) and including several recent analyses (Gold et al. 2015;Evans et al. 2017;Hoekzema et al. 2017;Zakrevskaya and Ivantsov 2017;Ivantsov et al. 2019). These support either the traditional view that modules were added at the region where they are smallest, considered the posterior (Gold et al. 2015;Evans et al. 2017;Ivantsov et al. 2019), or alternatively, proposals that they were instead added at the opposite end (Hoekzema et al. 2017). ...
... Analysis of growth has been conducted on other species (Hoekzema et al. 2017;Zarkrevskaya and Ivantsov 2017;Ivantsov et al. 2019), but D. costata has been the subject of considerable research (Sprigg 1949;Runnegar 1982;Gold et al. 2015;Evans et al. 2017;Hoekzema et al. 2017). A recent study suggests that differences between D. costata and Dickinsonia tenuis Glaessner and Wade, 1966 are the result of neoteny (Zakrevskaya and Ivantsov 2017). All specimens on TB-ARB appear to represent one stage of that potential ontogeny (i.e., D. costata). ...
... These would indicate the addition of multiple modules simultaneously at various locations. We consider this unlikely, especially given the relatively large size of these specimens, interpreted to represent a later growth stage wherein module addition slowed relative to inflation (Evans et al. 2017;Hoekzema et al. 2017;Zakrevskaya and Ivantsov 2017;Fig. 4B). ...
Article
Constraining patterns of growth using directly observable and quantifiable characteristics can reveal a wealth of information regarding the biology of the Ediacara biota—the oldest macroscopic, complex community-forming organisms in the fossil record. However, these rely on individuals captured at an instant in time at various growth stages, and so different interpretations can be derived from the same material. Here we leverage newly discovered and well-preserved Dickinsonia costata Sprigg, 1947 from South Australia, combined with hundreds of previously described specimens, to test competing hypotheses for the location of module addition. We find considerable variation in the relationship between the total number of modules and body size that cannot be explained solely by expansion and contraction of individuals. Patterns derived assuming new modules differentiated at the anterior result in numerous examples in which the oldest module(s) must decrease in size with overall growth, potentially falsifying this hypothesis. Observed polarity as well as the consistent posterior location of defects and indentations support module formation at this end in D. costata . Regardless, changes in repeated units with growth share similarities with those regulated by morphogen gradients in metazoans today, suggesting that these genetic pathways were operating in Ediacaran animals.
... Dickinsonia is inferred to have been a flattened, sheet-like organism (though see [11]) with a broadly ovate outline and a bilaterally symmetrical body constructed of multiple elongate units serially arranged along a central growth axis running down the longitudinal midline [5,20,22,25] ( figure 1). There is a strong linear relationship between overall length and width of D. costata specimens, and a positive correlation between the overall length of the organism and the number of units within local populations [22,26]. Previous assessments of Dickinsonia have disagreed about whether the organism grew isometrically [22,25] or allometrically [21], and whether individual units initially expanded in volume before halting their growth after certain dimensions were reached [27], or inflated continuously throughout life [4]. ...
... This change in gradient is observed in other Dickinsonia studies (e.g. [22], figure 3b; [26], figure 2), but is here interpreted as an ontogenetic shift that may reflect a change in life history, for example, as part of a switch to a reproductive phase. However, without knowing the rate of unit insertion, it is not possible to derive an absolute sense of time from these data. ...
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The late Ediacaran soft-bodied macroorganism Dickinsonia (age range approx. 560–550 Ma) has often been interpreted as an early animal, and is increasingly invoked in debate on the evolutionary assembly of eumetazoan body plans. However, conclusive positive evidence in support of such a phylogenetic affinity has not been forthcoming. Here we subject a collection of Dickinsonia specimens interpreted to represent multiple ontogenetic stages to a novel, quantitative method for studying growth and development in organisms with an iterative body plan. Our study demonstrates that Dickinsonia grew via pre-terminal ‘deltoidal’ insertion and inflation of constructional units, followed by a later inflation-dominated phase of growth. This growth model is contrary to the widely held assumption that Dickinsonia grew via terminal addition of units at the end of the organism bearing the smallest units. When considered alongside morphological and behavioural attributes, our developmental data phylogenetically constrain Dickinsonia to the Metazoa, specifically the Eumetazoa plus Placozoa total group. Our findings have implications for the use of Dickinsonia in developmental debates surrounding the metazoan acquisition of axis specification and metamerism.
... The remaining number of species may still be inflated (Evans et al., 2019). For example, D. tenuis and D. costata could be synonymous and represent different ontogenetic stages (Zakrevskaya and Ivantsov, 2017). D. tenuis, D. brachina, and D. lissa are also highly similar and difficult to distinguish from each another practically. ...
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Dickinsonia is an iconic fossil of the Ediacara biota (∼575–539 Ma). It was previously known from siliciclastic successions of the White Sea assemblage in Australia, Baltica, and possibly India. Here we describe Dickinsonia sp. from the terminal Ediacaran Shibantan Member limestone (ca. 551–543 Ma) of the Dengying Formation in the Yangtze Gorges area of South China. The stratigraphic distribution of Ediacara-type fossils in the Shibantan Member indicates that this biota uniquely preserves both the White Sea and Nama assemblages in stratigraphic succession. The new data presented here suggests that Dickinsonia had wider paleogeographic and paleoenvironmental distributions, implying its strong dispersal capability and environmental tolerance.
... The Indian specimens have narrow segments and high length to width ratio like Dickinsonia tenuis from South Australia, and distinct from D. costata (Fig. 3). Russian Dickinsonia ovata has been proposed to be a juvenile form of D. tenuis (Zakrevskaya and Ivantsov, 2017), but large D. ovata missing in Russia are present in Australia (Fig. 3). Dickinsonia lissa is much more elongate than the present material and also has distinctive lateral bulges to the undulating midline, and D. menneri is a small form with proportionally larger and flaring terminal deltoid (Ivantsov, 2007). ...
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The discovery of Dickinsonia in India allows assessment of biogeographic provinces and plate tectonic reconstructions for the late Ediacaran. The fossils were found in the roof of Auditorium Cave at Bhimbetka Rock Shelters, a UNESCO World Heritage Site for Paleolithic and Mesolithic cave art, near Bhopal, Madhya Pradesh. The fossils are identical with Dickinsonia tenuis from the Ediacara Member of the Rawnsley Quartzite in South Australia, and like them also show deformation due to lateral impingement, arcuate pieces missing, and alignment. They are within the late Ediacaran, Maihar Sandstone of the Bhander Group, in red sandstones formed in coastal plain paleoenvironments, including eolian, tsunamite, and intertidal bedforms. This new occurrence confirms assembly of Gondwanaland by 550 Ma, but not reconstructions adjusted for true polar wander. Cloudina and other small shelly marine fossils were low latitude, but vendobionts such as Dickinsonia were at temperate to subtropical latitudes.
... The appearance of the deformed White Sea specimens of Dickinsonia with a posterior region distinctly Ivantsov et al.-Intravital damage to the late Ediacaran metazoan, Dickinsonia separated from the rest of the body along the break line and resembling, for example, the trilobite pygidium suggests the possibility of a third hypothetical variant of the formation of new isomers: insertion of new isomers somewhere within the middle of the organism, in front of some primary group of isomers ( Fig. 12.3), formed behind the triangular lobe at the earliest stages of individual development. This variant can be supported by our conclusion concerning the existence of a dissected larval stage in the ontogenesis of Dickinsonia (Zakrevskaya and Ivantsov, 2017). ...
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Several specimens of Dickinsonia cf. D. menneri, originating from a single burial event at the Lyamtsa locality of the late Ediacaran (Vendian) in the southeastern White Sea area, Russia, represent deviations from normal morphology: a reduction in the total length of the body; the loss of portions of the body; various deformations of the transverse elements, called isomers; and splitting of the longitudinal axis with the formation of two posterior ends. It is assumed that these deformations were formed as a result of non-lethal damage, which occurred long before the burial event, and the response of Dickinsonia to them. The progress of the regeneration process at the damaged areas, and especially its deviations, indicates that the growth zone was located at the posterior end of the Dickinsonia body. The cause of non-lethal damage to Dickinsonia could not be established, but the local distribution of deformed specimens preserved in the same burial event alongside cyanobacterial colonies, and the presence of weak deformations, expressed only in shortening of the length of some isomers, lead to the conclusion that damage resulted from short episodes of physicochemical impact, rather than occasional attacks by a hypothetical macrophage.
... The head section of small and juvenile proarticulates is relatively large in size and can cover the body from the anterior side and laterally (Ivantsov, 2008). The growth and transition to more adult forms was accompanied by an abrupt increase in a number of isomers, which may indicate the existence of a particular larval stage in ontogenesis (Zakrevskaya and Ivantsov, 2017). ...
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Due to homonymy, a new name Cephalonega Fedonkin, nom. nov. was proposed for the genus of Vendian macroorganisms, Onega Fedonkin. The improved diagnosis of this genus and evidence that this genus belongs to Proarticulata, an extinct phylum of Metazoa, are given. A detailed characterization of the phylum and all Proarticulata classes is given for the first time.
... In this scenario, which we support, the generative zone of Dickinsonia may be considered pre-terminal (Fig. 4F). Further recent work has considered Dickinsonia costata to represent a paedomorphic variant of Dickinsonia tenuis (which possesses a greater unit count than D. costata; Zakrevskaya & Ivantsov, 2017). ...
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Rocks of the Ediacaran System (635-541 Ma) preserve fossil evidence of some of the earliest complex macroscopic organisms, many of which have been interpreted as animals. However, the unusual morphologies of some of these organisms have made it difficult to resolve their biological relationships to modern metazoan groups. Alternative competing phylogenetic interpretations have been proposed for Ediacaran taxa, including algae, fungi, lichens, rhizoid protists, and even an extinct higher-order group (Vendobionta). If a metazoan affinity can be demonstrated for these organisms, as advocated by many researchers, they could prove informative in debates concerning the evolution of the metazoan body axis, the making and breaking of axial symmetries, and the appearance of a metameric body plan. Attempts to decipher members of the enigmatic Ediacaran macrobiota have largely involved study of morphology: comparative analysis of their developmental phases has received little attention. Here we present what is known of ontogeny across the three iconic Ediacaran taxa Charnia masoni, Dickinsonia costata and Pteridinium simplex, together with new ontogenetic data and insights. We use these data and interpretations to re-evaluate the phylogenetic position of the broader Ediacaran morphogroups to which these taxa are considered to belong (rangeomorphs, dickinsoniomorphs and erniettomorphs). We conclude, based on the available evidence, that the affinities of the rangeomorphs and the dickinsoniomorphs lie within Metazoa.
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Recently reported specimens of the enigmatic Ediacaran fossil Dickinsonia from Russia show damage and repair that provides evidence of how they grew, and of their biological affinities. Marginal and terminal areas of wilting deformation are necrotic zones separating regenerated growth, sometimes on two divergent axes, rather than a single axis. Necrotic zones of damage to Dickinsonia are not a thick scar or callus, like a wound or amputation. Nor are they smooth transitions to a regenerated tail or arm. The wilted necrotic zone is most like damage by freezing, salt, or sunburn of leaves and lichens, compatible with evidence of terrestrial habitat from associated frigid and gypsic paleosols. Dickinsonia did not regrow by postembryonic addition of modules from a subterminal or patterned growth zone as in earthworms, myriapods, trilobites, crustaceans, and lizards. Rather Dickinsonia postembryonic regrowth from sublethal damage was from microscopic apical and lateral meristems, as in plants and lichens. Considered as fungal, Dickinsonia , and perhaps others of Class Vendobionta, were more likely Glomeromycota or Mucoromycotina, rather than Ascomycota or Basidiomycota.
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The new taxa Vendia rachiata sp. nov. and Cyanorus singularis gen. et sp. nov. are described from the Upper Vendian of the Onega Peninsula, Arkhangel'sk region. These taxa belong to the invertebrate phylum Proarticulata, which became extinct in the Precambrian. The species Vendia janae Ivantsov, 2001 is assigned to the new genus Paravendia based on new data and the results of restudying the holotype of the type species of the genus Vendia Keller. The axial structure of Proarticulata is interpreted as a digestive-distributive system.
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Functional, constructional, and preservational criteria led to a reinterpretation of seemingly complex trace fossils and the majority of assumed metazoan body fossils from Vendian lagerstatten. In the new scenario, Ediacaran biota were dominated by procaryote biomats and giant protozoa (Xenophyophoria and Vendobionta), which developed a great variety of shapes and lifestyles in the climatically controlled "golden age" that followed the Marinoan snowball earth. Contemporary metazoans (sponges; polyps; soft-bodied mollusks; possible echinoderms; worm-like burrowers) were adapted to this non-uniformitarian environment, but they remained scarce and relatively small. Some phyla (arthropods, brachiopods) appear to have still been absent. Our study also accentuates the Cambrian Explosion, which put an end to the peaceful "Garden of Ediacara". Not only did the former rulers become extinct or restricted to less favorable environments, but the radiation of metazoan phyla was also accompanied by an ecological revolution that established a new and more dangerous world, which persists to the present day.