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Experimental fluid mechanics of an Ediacaran frond

Amy Singer, Roy Plotnick, and Marc Laflamme

Plain Language Abstract

Ediacaran organisms are difficult to place within the tree of life. Interpretations of their ecology are equally contentious. Paleobiomechanics, the application of physics and engineering to the study of fossil organisms, can potentially shed light on these issues. We experimentally examined the interactions of the common Ediacaran morphology, the frond, with water flow. Experiments were carried out in a recirculating flow tank using model reconstructions of two related species of frond. Models were constructed to represent a range of possible physical properties of frond tissues, from quite stiff to highly compliant. Models were also placed at differing angles with respect to flow direction. Qualitative aspects of flow interaction were observed with film and video. Particular attention was paid to flow along the surface of the frond. Forces experienced by model fronds were directly measured and the coefficients of drag calculated. These forces would be those resisted by the attachment of the frond to the substrate. If these forces are limiting factor for these organisms, then it is likely that they assumed a low angle position and were composed of a flexible material. The textured surface of the fronds acted to trap and channel flow. Further, the bent stalk of one species could have made it a self exciting oscillator, which would have served to increase flow along its surface. These findings support a chemo- or osmotrophic lifestyle for these organisms.

Resumen en Español

Mecánica de fluidos experimental aplicada a organismos ediacarienses con forma de hoja

Entre los organismos más representativos de cuerpo blando que componen la biota de Ediacara se encuentran los que tienen forma de hoja, caracterizados por tener morfologías variadas y amplios rangos estratigráficos y ambientales. Como ocurre con casi todas los taxones ediacarienses, existen distintas opiniones sobre su posición filogenética y su ecología, aunque se considera que la mayor parte de esas especies con forma de hoja compartían más un nicho ecológico similar que una historia evolutiva común. La aplicación de la biomecánica experimental puede ayudar a perfilar esas interpretaciones y sugerir nuevos enfoques para la comprensión del modo de vida de estas formas.

Hemos examinado el comportamiento hidrodinámico de dos especies bien conocidas de Charniodiscus de la Formación Mistaken Point de Terranova, Canadá (asociación de Avalon): Charniodiscus spinosus y C. procerus. Modelos correspondientes a interpretaciones alternativas de la morfología superficial y la rigidez estructural fueron sometidos a estudios cualitativos y cuantitativos en un tanque de flujo de circuito cerrado.

A las mismas velocidades y orientaciones, los modelos de C. procerus y C. spinosus experimentan fuerzas de arrastre similares. C. procerus tiene un coeficiente de arrastre menor pero, como su altura es mayor, experimenta mayores velocidades de flujo ambiental. La reorientación a una posición paralela al flujo reduce drásticamente el arrastre en ambas formas. Los modelos demuestran también que C. procerus, y en menor medida C. spinosus, se comportaban como osciladores autoexcitados, lo que incrementaría las tasas de intercambio gaseoso en la superficie y concordaría con un modo osmotrófico de nutrición.

Palabras clave: Ediacariense, biomecánica, morfología funcional

Traducción: Miguel Company

Résumé en Français

Mécanique des fluides expérimetale sur une fronde d'Ediacara

Les frondes d'Ediacara sont des icones de la faune d'organismes à corps mous d'Ediacara. Elles sont caractérisées par des morphologies disparates et une large distribution stratigraphique et environmentale. Comme presque pour toutes les formes d'Ediacara, leur position phylogénétique et leur écologie font l'objet de débats, la plupart des espèces de frondes étant considérées comme partageant une guilde écologique similaire plutôt q'une histoire de vie. La biomécanique expérimentale peut potentiellement contraindre ces interprétations et suggère de nouvelles approches pour comprendre le mode de vie des frondes.

Nous avons examiné le comportement dans un courant de deux espèces bien connues du genre Charniodiscus de la Formation du Mistaken Point, Terre-Neuve, Canada (assemblage Avalon): Charniodiscus spinosus et C. procerus. Les modèles reflétant des interprétations alternatives de la morphologie de surface et de la rigidité de structure ont été testés par des études qualitatives et quantitatives de comportement sous un courant dans un bac permettant la mise en place d'un courant d'eau.

Pour les mêmes vitesses et orientations, les modèles C. procerus et C. spinosus ont montré les même forces de frottement; le coefficient de frottement pour C. procerus étant plus faible, mais sa hauteur plus importante lui impose des vitesses de courant plus élevées. La réorientation dans le sens du courant a réduit de manière drastique le frottement chez les deux formes. Les modèles ont également démontré que C. procerus (et dans une moindre mesure  C. spinosus) se comporte comme un oscillateur propre, ce qui augmenterait les taux d'échanges gazeux à la surface des frondes et témoignerait d'un mode de vie osmotrophique.

Mots-cléfs: Ediacarien, biomécanique, morphologie fonctionnelle

 Translator: Loïc Costeur

Deutsche Zusammenfassung

Experimentelle Strömungsmechanik von blattartigen Ediacara Fauna-Strukturen

Die blattartigen Strukturen der Ediacara Fauna, gekennzeichnet durch verschiedene Morphologien und eine große stratigraphische und ökologische Reichweite, gehören zu den ikonischen Mitgliedern dieser Biota. Wie bei beinahe allen Ediacara Formen gehen auch hier die Meinungen über ihre phylogenetische Position und Ökologie auseinander. Allerdings besteht die Ansicht, dass die meisten blattartigen Arten eher eine ähnliche ökologische Gilde miteinander teilen als einen gemeinsamen Ursprung. Experimentelle Biomechanik kann diese Interpretationen möglicherweise beschränken und neue Herangehensweisen zum Verständnis der Lebensgewohnheiten dieser blattartigen Strukturen aufzeigen.

Wir untersuchten das Verhalten in Strömung von zwei gut bekannten Arten von Charniodiscus aus der  Mistaken Point Formation von Neufundland, Kanada (Avalon Assemblage): Charniodiscus spinosus und C. procerus.  Alternative Modelle bezüglich Oberflächenmorphologien und struktureller Stabilität unterlagen qualitativen und quantitativen Untersuchungen von Strömungsverhalten in einem zirkulierenden Becken. Eine Neuausrichtung parallel zur Strömung reduzierte den Widerstand bei beiden Formen drastisch. Die Modelle zeigten außerdem, dass sich C. procerus (und in geringerem Ausmaß C. spinosus) wie gegenseitig anregende Oszillatoren verhielten. Dies würde die Gasaustauschraten an der Oberseite der blattartigen Strukturen erhöhen, was mit einer osmotrophischen Lebensweise einher geht.

SCHLÜSSELWÖRTER: Ediacara, Biomechanik, funktionelle Morphologie

 Translators: Eva Gebauer and Anke Konietzka

Arabic

297 arab

Translator: Ashraf M.T. Elewa

Polski Abstrakt

Eksperymentalna mechanika płynów a ediakarskie „pióro morskie"

Ediakarskie "pióra morskie" to ikony miękkociałej fauny Ediacara, cechujące się różnorodnymi morfologiami i szerokimi zasięgami stratygraficznymi i środowiskowymi. Tak jak w przypadku nieomal wszystkich ediakarskich form, poglądy na ich pozycję filogenetyczną i ekologię są równie szerokie; uważa się, że większość gatunków „piór morskich" dzieliła tą samą gildię ekologiczną, ale nie podobny cykl życiowy. Biomechanika eksperymentalna może potencjalnie zawęzić interpretacje i zasugerować nowe podejście dla zrozumienia trybu życia „piór morskich".

Przebadaliśmy zachowanie się w przepływie dwóch dobrze znanych gatunków Charniodiscus z formacji Mistaken Point na Nowej Funlandii, Kanada (zespół awaloński): Charniodiscus spinosus i C. procerus. Modele odzwierciedlające alternatywne interpretacje morfologii powierzchni i sztywności strukturalnej były obiektem badań jakościowych i ilościowych w przepływnie w zbiornikach przepływu recyrkulującego.
Przy tych samych prędkościach i orientacjach, modele C. procerus i C. spinosus doświadczały podobnych oporów; współczynnik oporu C. procerus był niższy, ale ten gatunek jest wyższy i doświadczał wyższych prędkości przepływu otaczającego. Zmiana położenie na równoległe do przepływu diametralnie redukuje opór u obu form. Modele pokazują również, że C. procerus (i w mniejszym stopniu C. spinosus) zachowywały się jak oscylatory samowzbudne, co zwiększałoby tempo wymiany gazowej na powierzchni "pióra morskiego" i jest zgodne z osmotroficznym trybem życia.

Słowa kluczowe: ediakar, biomechanika, morfologia funkcjonalna

Translators: Dawid Mazurek, Robert Bronowicz, and Daniel Madzia

 

Anke Konietzka
 

FIGURE 1. Charniodiscus from the Mistaken Point Formation: (1) Field photograph of Charniodiscus procerus with a prominent stem. (2) Field photograph of C. spinosus with a distal spine and shorter, poorly-preserved stem. Scale bar increments equal 1 cm. Labeled cartoon reconstructions of (3) Charniodiscus procerus and (4) Charniodiscus spinosus. Note presence of spire extending beyond frond in latter form.

figure 1

FIGURE 2. Epoxy models of Charniodiscus spinosus and C. procerus.

figure 2  

FIGURE 3. Reconstructions of Charniodiscus spinosus and the surface textures tested. (1) smooth model with no interpreted structure, (2) quilted model based on reconstruction of Charnia quilts by Seilacher(1992), (3) membrane model with two orders of branching connected by a membrane, and (4) 3D with perpendicular rods to represent three orders of branching with a membrane. The same textures were evaluated in C. procerus as well. Scale bar equals 1 cm increments.

figure 3

FIGURE 4. Charniodiscus spinosus model oriented for determination of drag coefficients. Flow is from right to left.

figure 4

FIGURE 5. Drag coefficients of Charniodiscus spinosus epoxy models as a function of angle of attack relative to ambient flow and surface texture. Flow velocity 0.095 m/s.

figure 5

FIGURE 6. Drag coefficients of Charniodiscus procerus epoxy models as a function of angle of attack relative to ambient flow and surface texture. Flow velocity 0.095 m/s.

figure 6

FIGURE 7. Drag coefficients as a function of angle of attack relative to ambient flow for Charniodiscus spinosus and C. procerus. Flow velocity 0.095 m/s. Fitted lines generated by locally weighted scatterplot smoothing (LOWESS).

figure 7

FIGURE 8. Silicon model of Charniodiscus spinosus bending in flow. Flow velocity 0.2 m/s, from right to left. Base of model mounted at bottom of tank.

figure 8

FIGURE 9. Drag on epoxy (stiff) and silicon (compliant) models as a function of velocity squared. Velocity squared is used since drag is linear with respect to velocity squared. Base of stem mounted near bottom of tank; stalk perpendicular to flow at zero velocity. Linear regression lines for the two species and materials shown.

figure 9

FIGURE 10. Video of an agar model of Charniodiscus spinosus with quilted surface showing overall movements in frond and movement of flow around and along surface of frond. Observed motions include vertical vibrations limited to the tip of the frond; motion laterally across the frond typically observed at the margins of the petalodium, and slow sinusoidal vibrations along the entire length of the frond. The dye streams show backflow along the surface of the frond, concentrated in the "canals" produced by the texture. Velocity is approximately 0.1 m/s.

movie_leadin

FIGURE 11. Video of periodic vibrations in silicon model of Charniodiscus procerus. Flow is away from the viewer, at approximately 0.10 m/s.

movie_leadin

 

TABLE 1. Dimensions of models used in this study.

 

 

Frond length
(m)

Frond width
(m)

Stem length
(m)

Spire length
(mm)

Frontal surface area
(m2)

C. procerus

0.096

0.046

0.060

0

0.00419

C. spinosus

0.010

0.056

0.020

0.018

0.00472

 

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singerGeosciences Department
The University of Montana
32 Campus Drive #1296
Missoula, Montana 59812-1296
USA

Amy Singer received her MS in Earth and Environmental Sciences from the University of Illinois at Chicago for her work on the paleobiomechanics of Ediacaran fronds. She is currently pursuing her PhD at the University of Montana working on the paleoecology of the macroinvertebrates of the Bear Gulch Limestone, a late Mississippian Konservat-Lagerstätten. Her research interests continue to focus on the relationships between invertebrates and their environment at different scales.

 

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This email address is being protected from spambots. You need JavaScript enabled to view it. (correspondence author)
plotnickDepartment of Earth and Environmental Sciences
University of Illinois at Chicago
845 W. Taylor St.
Chicago, Illinois 60607
USA

Roy Plotnick is a professor in the Department of Earth and Environmental Sciences at the University of Illinois at Chicago, where he has been for the more than twenty-five years since he received his doctorate at the University of Chicago. The inertia of his affiliation is in total contrast to the unpredictability of his scientific interests, which can be best be characterized as eclectic (some may some unfocused!). He has published on eurypterids, arthropod taphonomy, functional morphology, the nature of wastebasket taxa, disparity, quantitative stratigraphy, trace fossils, and the applications of fractal and related methods in paleontology, stratigraphy, and landscape ecology. He is currently working on a remarkable Pennsylvanian paleokarst and cave fill, which includes extraordinarily preserved plants and scorpions. Roy has been associated with both PBDB and CHRONOS and hserved as Treasurer of the Paleontological Society. Other interests include amateur astronomy, toy trains, and The Little Engine that Could.

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la flammeDepartment of Paleobiology
Smithsonian Institution
PO Box 37012, MRC 121,
Washington, DC 20013-7012
USA

Marc Laflamme received his PhD at Queen’s University in Kingston, Ontario, Canada, and was a postdoctoral fellow at Virginia Tech, Yale University, and the Smithsonian National Museum of Natural History. His research interests lie in the classification and preservation of the Ediacara biota, and in using decay-based experiments to study taphonomic pathways responsible for the preservation of soft-tissue in the fossil record.