The ecology of Dickinsonia on tidal flats
Ecología de Dickinsonia en llanuras mareales

N.G. Sozonov^1,2, N.I. Bobkov^1,2, E.G. Mitchell³, A.V. Kolesnikov^2,4,5, D.V. Grazhdankin^1,2

¹Novosibirsk State University, Pirogova 2, 630090 Novosibirsk, Russia. Email: n.sozonov@g.nsu.ru; ORCID ID: https://orcid.org/0000-0002-4251-2130, https://orcid.org/0000-0003-1876-6445, https://orcid.org/0000-0003-0797-1347

²Trofimuk Institute of Petroleum Geology and Geophysics of Siberian Branch of Russian Academy of Sciences, Koptuga 3, 630090 Novosibirsk, Russia. ORCID ID: https://orcid.org/0000-0003-1028-9082

³Department of Zoology, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK. ORCID ID: https://orcid.org/0000-0001-6517-2231

⁴Geological Institute, Russian Academy of Sciences, Pygevsky 7,119017 Moscow, Russia.

⁵Moscow State Pedagogical University, Faculty of Geography, Kibalchicha str. 16, Moscow 129626, Russia.

IntroductionTOP

Few, if any, Ediacaran organisms have so many alternative interpretations with regards to their position on a phylogenetic tree than does Dickinsonia. Originally described as a jellyfish by Sprigg (1947), it was subsequently regarded as a rhizarian protist (Seilacher et al., 2003), a lichenised fungus (Retallack, 2007), a stem-group placozoan (Sperling & Vinther, 2008), a sister-group eumetazoan (Buss & Seilacher, 1994), a stem-group bilaterian (Gold et al., 2015), and even as a acoelomorph (Fedonkin, 1981), a polychaete (Glaessner & Wade, 1966), and a direct chordate ancestor (Dzik, 2015). In addition to comparison of morphological similarities, different specialists have attempted implementing novel approaches such as studying taphonomic processes, ontogenetic patterns and chemical composition in these organisms. In addition to resolving the course of evolution, the new methods could elucidate so far unnoticed and not fully appreciated processes of modern Metazoan development and ecology. It is well established that Dickinsonia represents a benthic organism with a flattened body, but poor fossil preservation is the crucial problem that paleontologists deal with. The imprints of soft bodies in sedimentary rocks can be subsequently altered by various tectonic processes, and unknown paleobiological information can be lost during excavation, rock splitting, fossil collection and preparation processes. Furthermore, in-situ preservation of large populations of individual specimens from the same bedding surface are rarely found, collected, and studied. Hence, even the assignment of morphologically similar fossils to the same biological species often remains an unsolvable task.

ResultsTOP

Over a period of several years (2016-2018) our team gradually excavated a vertically standing bedding surface dropping off into the rapid mountain river Sylvitsa, western slope of the Central Urals. The bedding surface yielded an exceptionally preserved life association of Dickinsonia. In terms of local stratigraphy, it belongs to the Konovalovka Member of the Cherny Kamen Formation, Sylvitsa Group of supposedly terminal Ediacaran age. The fossils are preserved in negative hyporelief in a package of finely alternating sandstone, siltstones and mudstones. The fine-grained texture of the casting material provided an unparalleled resolution of the fossil preservation. Most important, nine square meters of the surfaces preserved over a hundred of exquisitely preserved Dickinsonia specimens in life association. The sedimentary environment of this fossil assemblage has been reconstructed as a restricted coastal lagoon surrounded by tidal flats (Bobkov et al., 2019), which seems very unusual compared to other fossil localities conventionally regarded as representing shallow marine settings.

Previous studies reported a good correlation between the length and the width of the body in these organisms (Evans et al., 2017; Reid et al., 2018); however, the correlation between these two parameters in our collection showed a weaker trend (R² = 0.58; p<2.2e-16). All of the specimens from the Central Urals appear to be deformed in one direction as a result of tectonic activity (rather than taphonomic or soft sediment deformation processes). This distortion is likely to be the cause of the observed poor correlation between the length and the width of the body in the studied specimens, and it most likely would affect other conclusions. To compensate for this deformation, we developed a method for accurate estimation of direction and degree of surface deformation based on pure geometric calculations depending of orientation of the axis of the specimens. The method has shown consistent results, the constructed hypothetical curve has been confirmed with actual measurement of the specimens. Once we received a coefficient of deformation and determined the angle of deformation, we recalculated the coordinates and the length and width values for each specimen. The length-width correlation in the retrodeformed specimens is stronger (R² = 0.84; p<2.2e-16) compared to undeformed material.

The age structure of the population was estimated based on size frequency distribution. The plot (Fig. 1) shows log-normal distribution for the population (Shapiro-Wilk; W = 0.98, p = 0.27). In contrast, a population of Dickinsonia costata studied from coeval strata in the Crisp Gorge locality, South Australia shows normal distribution (Reid et al., 2018), quite dissimilar from the population studied in the Central Urals. Furthermore, the population of Dickinsonia from the Konovalovka Member can be divided into two cohorts by size (< 20 mm and 21–67 mm); for this purpose we used Bayesian Information Criterion (Fraley & Raftery, 2006, 2007). The analysis of spatial distribution shows different behaviour between the larger and smaller individuals. The larger individuals show spatial segregation (greater distance then expected by random distribution with PCF value is placed under lower bound for random distribution), but smaller individuals tend to form denser aggregations (Fig 2). These spatial patterns are found in extant populations where larger organisms require more food resources than smaller ones, so avoid competing with others by moving away from other individuals.

ConclusionsTOP

This study demonstrates the importance of tectonic and post-depositional processes that can negatively affect morphological analyses of fossil material leading to misidentification of species. Specimens of Dickinsonia from the Central Urals are characterised by clear bilateral symmetry (i.e. modules of the left side extend across the axial structure of the body with no offset). Taking into account observations from Australian fossil localities (Reid et al., 2018; Evans et al., 2017) we consider that the so called ‘glide reflection symmetry’ in these fossils is a taphonomic phenomenon. The size frequency distribution plot shows the predominance of smaller individuals in the studied population of Dickinsonia from the Central Urals (Fig. 1). Assuming that the age of an individual is manifested in the body size, there is a significant predominance of juvenile individuals in the population. Three possible scenarios can be envisaged: (i) the population has a large number of juvenile individuals as the result of high survivorship rate in the intertidal zone; (ii) the population teems with juvenile forms because it is buried immediately after hatching; (iii) assuming that Dickinsonia was an actively motile organisms, that abundance of juvenile individuals could be explained by their inability to escape burial (although it is difficult to imagine that some of the mature individuals are buried with signs of escape behaviour); and (iv) the population could be interpreted as a fossilised ‘nesting ground’ for Dickinsonia in the intertidal zone where juvenile forms underwent maturation before migrating back to the subtidal zone. Our data is most consistent with the latter hypothesis because it explains the high proportion of juveniles in a single population in the intertidal zone, their low proportion in populations preserved in subtidal settings and their spatial behaviour. The studied population can be characterised as expanding or stable; therefore, the intertidal setting can be described as favourable for these organisms.

ACKNOWLEDGEMENTSTOP

The study was supported by the Russian Science Foundation Grant 17-17-01241 (PI: D.V. Grazhdankin). The findings and conclusions have immediate implications for understanding terminal Ediacaran ecology (funded by the Russian Foundation for Basic Research Grant 19-05-00828; PI: A.V. Kolesnikov).

ReferencesTOP