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Longest crab trackways from the Bay of Bengal Coast, India: Their geological and geotechnical applications

Chirananda De

Plain Language Abstract

Ocypode, a very common variety of coastal amphibious crabs, thrives in thick population in the Bay of Bengal coast of eastern India. They produce different types of traces, such as tracks, trackways, burrows, feeding pellets, chimneys and many other organo - sedimentary features on the soft sandy sediments depending on their physiological requirements. Of these, about 100 m long, cross-coast and parallel trackways constitute the world's longest ever known continuous trackways of decapod. Their long journey from supratidal dwelling place to the then tidal water down the stable beach slope has been attributed to basic physiological need to moisten their gills with seawater containing life-saving dissolved oxygen. These trackways are architecturally different from those produced while searching food, making pellets and runways and escaping from predators. The longer variety of trackways shows subtle morphological changes depending on the moistness and rigidity of the sediments. Although they are vulnerable to erosion by wind and tidal actions, they have a fair chance of preservation as evidenced by the presence of equally delicate decapod trackways in rock records. The discussed trackways, if preserved, can be utilized as a new biological tool in identifying ancient trackways, their producers, ecology and physiological characteristics; beach gradient; tidal range and geomorphic settings. Their occurrence is suggestive of beach stability.

Resumen en Español

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Traducción: Enrique Peñalver

Résumé en Français

Les plus longues pistes de crabe de la baie de la côte du Bengale, Inde: leurs applications géologiques et géotechniques

L'evaluation neoichnologicale du crabe amphibie côté-coureur, Ocypode, produite sur la plage Bakkhali dans l'est de l'Inde, a révélé comment la physiologie de l'animal, les habitudes de vie et le substrat décident l'architecture, l'orientation, la continuité, les variations morphologiques et le potentiel de préservation de leurs pistes généralement asymétriques. Environ 100 m de long, en mouvement vers la mer, parallèles et inter-côte, constituant les plus longues pistes d'invertébrés jamais connues dans le monde parmi les registres anciens et modernes, ces pistes sont générés de manière sélective par des adultes, descendant la pente de la plage en quête d'eau de mer pour humidifier leurs branchies. Plus petit, errant librement et auto-croisant, les pistes dans le plat intertidale humide sont produites par les jeunes lors de la recherche alimentaire et en s'échappant des prédateurs. Les pistes de crustacés asymétriques innomés de l'Éocène de Andaman et du Quaternaire du bassin de Banas en Inde, constituent de anciens analogues aux pistes Ocypode. L'analogie et les pistes d'arthropodes fossiles, aussi délicates que celles présentées, justifient leur potentiel de conservation. Les applications palaéoichnologicales comprennent les révisions ichnotaxonomiques, la création d'un nouveau ichnotaxon comme variantes taphonomiques et éthologiques, et l'identification taxonomique d'anciens responsables de traces, en plus des interprétations paléoécologiques et palaéogéomorphiques. Cette étude fait un lien entre la physiologie animale et l'ichnotaxonomie détaillée, fournit de nouvelles informations à la base de données existante, et porte un potentiel d'application pour les programmes géotechniques côtiers comme un nouvel outil ichnologique.

Mots-clés: Ocypode; les plus longues pistes; applications; géologie; géotechnique

Translator: Kenny J. Travouillon

Deutsche Zusammenfassung

Die längste Krabbenspur der bengalischen Küste, Indien: geologische und geotechnische Anwendungen

Die neoichnologische Auswertung von Spuren der amphibischen Geisterkrabbe Ocypode, entstanden am Bakkhali Beach in Ostindien, zeigt, wie sich Physiologie, Lebensgewohnheiten und Substrat auf die Architektur, Orientierung, Kontinuität, morphologische Variationen und das Preservationspotential der typischen asymmetrischen Spuren auswirken. Über 100 Meter lange, seewärts, parallel und quer zur Küste verlaufende Spuren sind die längsten weltweit bekannten Spuren von Invertebraten sowohl unter den heutigen als auch den urzeitlichen Formen. Sie wurden selektiv von Adulten während des Abweidens der Strandböschung erzeugt auf der Suche nach dem lebensrettenden ihre Kiemen befeuchtenden Salzwasser. Kleinere, frei umherziehende und sich kreuzende Spuren in der feuchten Gezeitenzone wurden von Jungtieren erzeugt, die auf Futtersuche waren, Pellets und Laufbahnen produzierten und vor Fressfeinden flohen. Die unbenannten asymmetrischen eozänen Krebsspuren der Andamanen und die des quartären Banas Beckens von Indien stellen urzeitliche Analogien der Ocypode-Spuren dar. Analogie und fossile Arthropoden-Spuren, die ebenso delikat sind wie die gegenwärtigen, rechtfertigen ihr Preservationspotential. Die paläoichnologischen Auswertungen beinhalten neben paläoökologischen und paläogeomorphen Interpretationen ichnotaxonomische Revisionen, Erstellung neuer Ichnotaxa als taphonomische und ethologische Varianten und taxonomische Identifikation von urzeitlichen Spurenverursachern. Sie verknüpfen Tierphysiologie detailliert mit Ichnotaxonomie, liefern neue Information zur bestehenden Datenbank und haben hohes Anwendungspotential für geotechnische Küstenprogramme als neues ichnologisches Hilfsmittel.

Schlüsselwörter: Ocypode; längste Spuren; Anwendungen; Geologie; Geotechnologie

Translator: Eva Gebauer

Arabic

325 arab

Translator: Ashraf M.T. Elewa

 

 

TABLE 1. Bibliographic review of fossil trackways and their interpretations.

Palaeoichnological features

Interpretations (locomotion styles, behavior, taphonomy, physiology etc.)

References

Cruziana of Cambrian-Ordovician trilobite origin

Different swimming-walking styles

Crimes, 1975

Millions of small- to medium-sized reptile trackways from Early to Middle Triassic carbonate tidal flats in the Germanic Basin, central Europe

Collectively attributed to emigration through inter-peninsular bridges exposed during low sea stands

Diedrich, 2008

Late Triassic-Early Jurassic track surface from the lacustrine Santo Domingo Formation (Argentina) containing hundreds of bird tracks and trackways

Behavioural and taphonomic interpretations

Genise et al., 2009

Tetrapod footprints and trackways along with profuse arthropod traces found in the interdune deposits of the Lower Triassic Tumlin Sandstone of Central Poland

Rapid colonization of interdunes

Gradziński and Uchman, 1994

A large variety of Lower Palaeozoic Trilobite trackways

Different types of locomotion such as walking, grazing, galloping, fast and slow sidling, walking - swimming etc.

Seilacher, 2007

Climatichnites

Surface tracks and trails of mollusk having physiological capacity to manipulate sediment to produce the structure

Getty and Hagadorn, 2005

Shoreface crab resting traces

Resting patterns, hydration and respiration

Martin, 2006; Minter et al., 2007

Aestivation burrows of lungfish, earthworm, lysorophid and dicynodont

True physiological need of retaining body water content during extremely dry (draught) situation

Hembree, 2010

 

TABLE 2. Ichnological clues of crab trackways to basinal geomorphology.

Ichnological clues Possible geomorphic implications
Large burrow opening connected with outgoing long trackways Supratidal zone, land - sea (burrow mouth to trackway) directions and shoreline orientation (across the trackway)
Maximum length of individual trackway Minimum beach width up to the then water level
End point of longest trackway Low tide level
Best preserved segment of the long trackway Stretch of moist intertidal flat or biomat substrate
Least preserved segment of the long trackway Dry supratidal flat to coastal dune sand or too moist lower intertidal flat
Larger twists and turns of the long trackway Positions of steep obstructions like dunes that are maneuvered
Overall slope of the long trackway on single surface Local beach slope
Length vs. slope of the best and least preserved segments of the long trackway Approximate tidal range and width of the supratidal and intertidal flats
 

FIGURE 1. General geological and geomorphological map of Sundarban Delta Complex showing present extent of mangrove forest, regional stratigraphy, geomorphic units and the location of the studied Bakkhali beach sector (1). A schematic coastal profile (2) is drawn to show distribution of different coastal geomorphic units from land to sea.

 figure1

FIGURE 2.  Geological and geomorphological map of the Bakkhali beach showing different ichnological subzones, location of long trackways, and stable and unstable beach segments.

 figure2

FIGURE 3. Distant view of the longest observed Ocypode trackway (shown by arrow). Note the positions of land and sea, trackway continuity, trackway maneuvering, and trackway morphology in different geomorphic units (1), Ocypode trackways on dry supratidal sand (2), best engraved trackway in moist sand of middle intertidal flat (3), trackway in very moist sands of lower intertidal flat (4). Note substrate specific morphological variations of the trackways. The scale measures 16 cm.

 figure3

FIGURE 4. Close views and line drawings of best produced trackways for morphological details. Arrow heads indicate the direction of movement: moving upward trackway (1) and its line drawing (2), moving downward crab trackway (3) overprinted by bird trackway and its line drawing (4), trackway moving zigzag maneuvering mangrove bushes on the left (5) and fossilized decapods trackways from Eocene Mithakhari Group of Andaman Islands (6), the closest possible ancient analogue of the describe modern longest crab trackways from Bay of Bengal coast. The scale measures 16 cm.

 figure4

FIGURE 5. Photographs show variations in trackway morphology. Trackways produced on very moist substrate of lower intertidal flat - note morphological indistinctiveness (1-2); trackway made on dune slope - note morphological changes due to sliding effect (3); trackways produced on dry and flat sand surface - note their different appearance (4); shorter, narrower, freely wandering, and self-crossing young Ocypode trackways produced around a burrow opening in the moist middle intertidal flat as a result of searching food and feeding pellet making activity (5); long trackway produced on extra rigid and sticky biomat substrate in the supratidal depression - note different morphological appearance relative to the others (6). The scale measures 16 cm.

figure5

 

authorChirananda De
Geological Survey of India
Palaeontology Division
Western Region, 15 – Jhalana Dungri
Jaipur 302015
Rajasthan, India
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Chirananda De, an Indian Palaeontologist who served Geological Survey of India for about 32 years (1980-2012), graduated and defended PhD thesis at the University of Calcutta. He earned a place in the Guinness World Record for his discovery of oldest fossil rain imprints from Vindhyans of India. His research expertise covers classical ichnology, ichnostratigraphy, Quaternary ichnology, environmental ichnology and applied ichnology. His research contributions (>100 papers, books, memoirs, catalogues and reports) that are highly commended and widely cited by world experts possess tremendous significance and universal application potential and incorporate several sensational discoveries, conceptual breakthroughs, promotion of fundamental principles and novel applications of ichnological tools for the benefit of mankind. His discovery of Ediacaran faunal assemblages from Vindhyans of central India and Marwars of western India has been rated as ever most significant palaeontological breakthrough in the century long history of palaeontological research in India. His discoveries of several unique ichnological tools (e.g. Diopatra cuprea burrow tubes for instantaneous measurement of current annual rates of coastal erosion and deposition, Uca mud volcanoes for delineating areas of coastal instability, oriented crustacean burrows as palaeoindicator of buried channels and aquifers, burrow population and architecture as a measure of beach erosion, algal microboring on ooids and continental mayfly burrow as indicators of rising sea level) bear universal and innovative geotechnical application potential in human service and promote new concepts of “Applied Ichnology” and “Ichnological exploration of groundwater”. First ever introduction of biophysical model of burrowing has revolutionized all standing ideas and introduced several new principles and concepts for detailed ichnological characterization of coastal environments, dynamics and stability; sea level changes; ecosystem degradation and ecospace shrinking: all serious concerns of modern world. He co-authored Elsevier-published book “Trace Fossils as Indicators of Sedimentary Environments” (Edited by Dr. Dirk Knaust and Prof. Dr. Richard G. Bromley). He is now available to join any India centric international ichnological research programme.