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Swimming through the substrate: the neoichnology of Chalcides ocellatus and biogenic structures of sand-swimming vertebrates

Angeline M. Catena and Daniel I. Hembree

Plain Language Abstract

Although terrestrial burrowing organisms are common in arid environments such as deserts and dune fields, trace fossils are rarely documented from ancient examples of these environments. Skinks (Squamata: Scincidae) are the most diverse group of lizards and are present in the fossil record as early as the Cretaceous. Most skink species are either ground-dwellers or burrowers, and are common in a wide range of habitats, including arid environments. This study documents the traces produced by Chalcides ocellatus, a generalist, sand-swimming skink common in deserts of Europe, Asia, and Africa in order to record the diverse suite of biogenic structures that can be produced by small, desert-dwelling vertebrates. Individual specimens of C. ocellatus were placed in sand-filled terrariums for periods of one to two weeks. The grain size and percent moisture of the sand was altered in order to evaluate the variations in biogenic structures produced by the skinks over a range of environmental conditions. Traces produced by C. ocellatus included layer truncations, sinuous, bilobate trails on and below the surface, flame structures, and isolated-to-connected divots in subsurface sand layers. The traces varied in sharpness, size, and type in response to the alterations in environmental conditions. The results of this study can be used to help increase the awareness of the diverse number of traces produced by modern desert-dwelling animals and, through their recognition in the fossil record, will allow for more complete interpretations of the ecology of ancient arid environments.

Resumen en Español

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Traducción: Enrique Peñalver

Résumé en Français

Nager à travers le substrat: la néo-ichnologie de Chalcides ocellatus et les structures biogéniques des vertébrés nageurs de sable

Des expériences néo-ichnologiques impliquant une espèce de scinque nageur de sable, Chalcides ocellatus, démontrent la diversité des structures biogéniques produites par ces lézards de désert. Les scinques ont été placés dans des terrariums avec dix couches, avec 1 cm d'épaisseur de sable fin, moyen ou à gros grains pour des périodes de 7 à 14 jours. La teneur en humidité des sédiments a été maintenue constante à 0% ou 20%. Les scinques de sable ont démontré des comportements de locomotion, de fuite, et de repos. Ils se sont enfouies jusqu'à une profondeur maximale de 4,2 cm avec une activité maximale limitée au 2 cm supérieure de sédiment. Les structures biogéniques produites inclus des monticules de surface et des dépressions; des couches troncatures; des sentiers sinueux bilobé en surface et sous-surface; des mottes en forme de U à V, et isolé à connectée; des structures de flamme; et des terriers ouverts. La taille et la diversité des structures biogéniques produites par C. ocellatus ont changé avec des variations dans la taille des grains des sédiments et la teneur en humidité. Les structures biogéniques ont tendance à être mieux définies en sable fin. La largeur des mottes isolées en forme de U à V a été plus importante dans les sables moyens et à gros grains. Les mottes connectées en forme de U à V ont seulement eu lieu dans les sables sec, fin et à grains moyens alors que les terriers ouverts ne sont produits que dans du sable humide. Pendant la nage de sable, les scinques ont modifié le tri granulométrique, desserré le tissu des sédiments, augmenté la porosité et la perméabilité, et ajouté de la matière organique sous forme de restes de nourriture, de matières fécales, et de peaux mortes. Les données recueillies dans cette étude peuvent être utilisés pour mieux interpréter les assemblages à trace fossiles terrestres dans des paléo-environnements arides et éoliens afin de mieux évaluer la paléoécologie de ces paramètres anciens.

Mots-clés: trace fossile; ichnofossile; bioturbation; éolienne; reptile; continental

Translator: Kenny J. Travouillon

Deutsche Zusammenfassung

Schwimmen durch Substrat: die Neoichnologie von Chalcides ocellatus und biogene Strukturen sandschwimmender Vertrebtaten

Neoichnologische Experimente mit dem sandschwimmenden Skink Chalcides ocellatus zeigen die von sandlebenden Echsen produzierte Diversität biogener Strukturen. Die Skinke wurden für Perioden von sieben bis vierzehn Tagen in Terrarien mit zehn 1cm starken Schichten von fein -, mittel-oder grobkörnigem Sand gesetzt. Der Feuchtigkeitsgehalt des Sediments wurde konstant bei 0% oder 20% gehalten. Die Sandskinke zeigten Bewegungs-, Flucht – und Ruheverhalten. Sie tauchten bis zu einer Maximaltiefe von 4,2cm ein, die meiste Aktivität trat jedoch in den obersten 2cm auf. Die erzeugten biogenen Strukturen beinhalteten Oberflächenhügel und Depressionen; Schichttrunkierungen; gewundene oberflächennahe und unterirdische Gänge; bilobate Spuren; isolierte bis verbundene U-bis V-förmige Divote; Flammenstrukturen und offene Bauten. Die Größe und Vielfalt der von C. ocellatus produzierten biogenen Strukturen änderte sich mit den Korngrößen des Sediments und dem Feuchtigkeitsgehalt. Die biogenen Strukturen waren im feinkörnigen Sand besser definiert. Die Breite der U-bis V-förmigen Divote war im mittel-feinkörnigen Sand größer. Verbundene U-bis V-förmige Divote gab es nur in trockenem fein-mittelkörnigem Sand, während offene Bauten nur in feuchtem Sand produziert wurden. Durch die Aktivitäten im Sand veränderten die Skinke die Körnersortierung, lösten das Sedimentgefüge, erhöhten die Porosität und Durchlässigkeit und fügten organisches Material in Form von Nahrungsresten, Kot und Hautresten hinzu. Die Daten aus dieser Studie können zur besseren Interpretation von terrestrischen Spurenfossilien-Assemblagen in ariden äolischen Paläoumgebungen genutzt werden um die Paläoökologie dieser vergangenen Landschaften besser zu verstehen.

Schlüsselwörter: Spurenfossil; Ichnofossil; Bioturbation; äolisch; Reptil; kontinental

Translator: Eva Gebauer

Arabic

463 arab

Translator: Ashraf M.T. Elewa

 

 

FIGURE 1. 1, The sand skink Chalcides ocellatus. 2, Sand skink engaged in sand swimming near the sediment surface.

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FIGURE 2. 1, Initial stratification of a terrarium filled with ten, 1-cm-thick layers of fine-grained sand. 2, Vertical cut through a core of undisturbed 1-cm-thick layers of coarse-grained sand.

figure 2

FIGURE 3. Videos of the burrowing techniques of Chalcides ocellatus. 1, Burrowing by sand swimming or intrusion. 2, Burrowing by excavation. Click on image to activate animation.

 

figure 3 

figure 3 

FIGURE 4. 1, A sequence of surficial mounds and depressions in an experiment with alternating sand grain sizes. 2, A terrarium showing the homogenization of the upper 2 cm of fine-grained sand.

figure 4

FIGURE 5. 1, Surficial sinuous bilobate trail (SSBT) in medium-grained sand with an enlarged view (insertion). 2, SSBT in coarse-grained sand. 3, A subsurficial sinuous trail from the lower portion of a core from fine-grained sand. 4, SST from the upper portion of a core from fine-grained sand.

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FIGURE 6. 1, Truncation of the upper 2 cm of medium-grained sand (at arrow) and an infilled surficial depression. 2, An isolated U-shaped divot with a sharp boundary in fine-grained sand. 3, An isolated U-shaped divot with a gradational boundary in medium-grained sand. 4, Connected U- to V-shaped divots with gradational boundaries in medium-grained sand. 5, A sharply defined flame structure (arrow) and an accompanying U-shaped divot in fine-grained sand. 6, A small flame structure (arrow) in coarse-grained sand.

figure 6

FIGURE 7. 1, Open burrow produced by Chalcides ocellatus in a terrarium with 20% sediment moisture. 2, The open burrow at day 12 of the trial. 3, The resulting biogenic structure from the collapse of the open burrow on day 14 of the trial.

figure 7

 

catena photoAngeline M. Catena
Department of Biology
Case Western Reserve University
10900 Euclid Avenue
Cleveland, Ohio, 44106
USA
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Angeline Catena received her Master’s of Science in Geology from Ohio University and is currently a Ph.D. student at Case Western Reserve University. Her interests involve reconstructing the paleoenvironments, paleoclimates and paleocologies of diverse terrestrial environments through the use of ichnology, paleopedology, and mammalian ecological diversity analyses.

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hembree photoDaniel I. Hembree (corresponding author)
Department of Geological Sciences
Ohio University
316 Clippinger Laboratories
Athens, Ohio, 45701
USA
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Daniel Hembree is an Associate Professor in the Department of Geological Sciences at Ohio University. His research interests primarily lie with animal-substrate interactions in ancient and modern continental environments. These interactions are preserved in the fossil record as trace fossils. Trace fossils provide an in situ record of ancient biodiversity, ecology, and environment. The study of trace fossils, therefore, provides vital information for accurate paleoenvironmental reconstructions. This involves not only the study of paleosols and continental trace fossils throughout geologic time, but also the experimental study of burrowing behaviors of extant terrestrial annelids, arthropods, amphibians, and reptiles. Current research projects involve the study of the influence of climate changes on ancient soils and soil ecosystems including those of the Pennsylvanian and Permian of southeast Ohio, Permian of eastern Kansas, and the Eocene to Miocene of Colorado and Wyoming.

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TABLE 1. Experimental parameters. 1, Basic morphology experiments. 2, Increased grain size experiments. 3, Increased sediment moisture experiments. C.O: Chalcides ocellatus specimen number. Grain size: FS: fine-grained sand, MS: medium-grained sand, CS: coarse-grained sand, AS: alternating fine-, medium-, and coarse-grained sand.

1. Experiment 1: Basic Morphology

Specimen

Terrarium

Grain Size

Temperature

% Moisture

Time (days)

C.O 7 38 L FS 32⁰C 0 7
C.O 2 38 L FS 32⁰C 0 7
C.O 4 114 L FS 32⁰C 0 7
C.O 6 114 L FS 32⁰C 0 7
C.O 2 38 L FS 32⁰C 0 14
C.O 6 38 L FS 32⁰C 0 14
C.O 4 114 L FS 32⁰C 0 14
C.O 7 114 L FS 32⁰C 0 14
           

2. Experiment 2: Grain Size

Specimen

Terrarium

Grain Size

Temperature

% Moisture

Time (days)

C.O 7 38 L MS 32⁰C 0 14
C.O 8 38 L MS 32⁰C 0 14
C.O 4 114 L MS 32⁰C 0 14
C.O 2 114 L CS 32⁰C 0 14
C.O 7 38 L CS 32⁰C 0 14
C.O 2 38 L CS 32⁰C 0 14
C.O 8 114 L CS 32⁰C 0 14
C.O 6 114 L AS 32⁰C 0 14
C.O 6 38 L AS 32⁰C 0 14
C.O 4 38 L AS 32⁰C 0 14
C.O 2 114 L AS 32⁰C 0 14
C.O 7 114 L AS 32⁰C 0 14

3. Experiment 3: Sediment Moisture

Specimen

Terrarium

Grain Size

Temperature

% Moisture

Time (days)

C.O 4 38 L FS 32⁰C 20 14
C.O 2 38 L FS 32⁰C 20 14
C.O 6 114 L FS 32⁰C 20 14
C.O 7 114 L FS 32⁰C 20 14

TABLE 2. Quantitative properties of sand skink structures in fine- (FS), medium- (MS), and coarse-grained (CS) sand. 1, Depth of homogenization. 2, Width of U- to V-shaped divots. 3, Height of U- to V-shaped divots. All values are in cm. Bold values are the calculated mean for the structures produced in each grain size. Italicized values are the p values from a two sample Mann-Whitney test; values <0.05 indicate a significant difference in the median rank value.

1. Homogenization Depth

 

2. Divot Width

 

3. Divot Height

FS

MS

CS

 

FS

MS

CS

 

FS

MS

CS

0.8 0.9 0.7   0.6 1.6 1.8   0.2 0.6 0.4
1.1 1 1   0.7 1.6 2.0   0.2 0.6 0.4
1.2 1.4 1.1   0.8 2.1 2.1   0.2 0.7 0.6
1.3 1.6 1.1   0.8 2.3 2.5   0.2 0.7 0.7
1.3 1.6 1.2   0.9 3.1 3.6   0.3 0.8 0.8
1.5 1.9 1.3   1.0 3.5 6.0   0.3 0.8 1.0
1.6 2.1 2.1   1.1 3.6     0.3 0.9  
1.7 2.4 2.4   1.2 4.1     0.3 1.0  
1.8 2.6 4   1.3 4.2     0.4 1.1  
3.3       1.5 4.4     0.4 1.4  
        1.5 4.6     0.4 1.4  
1.6 1.7 1.7   1.5 5.2     0.4 1.9  
        1.5       0.5    
FS/MS p=0.390     1.6       0.5    
FS/CS p=0.623     1.6       0.5    
MS/CS p=0.507     1.8       0.5    
        1.8       0.5    
        1.8       0.5    
        1.8       0.5    
        1.8       0.6    
        1.9       0.6    
        2.1       0.6    
        2.1       0.6    
        2.1       0.6    
        2.1       0.7    
        2.3       0.7    
        2.4       0.8    
        2.4       0.9    
        2.6       0.9    
        2.8       1.0    
        3.3       1.0    
        3.7       1.2    
        4.2       1.3    
        4.7       2.4    
                     
        1.9 3.3 3.0   0.6 1.0 0.7
                     
        FS/MS p=0.001     FS/MS p=0.001  
        FS/CS p=0.048     FS/CS p=0.434  
        MS/CS p=0.452     MS/CS p=0.073