Trilobites Burrowing, Walking, and Feeding in Tidal Flat Facies of the Sequatchie Formation (Late Ordovician), Georgia, USA Restricted; Files Only
Tamez, Amelia (Spring 2024)
Published
Abstract
The Sequatchie Formation (Late Ordovician) in the southern Appalachians is composed of clastic marginal-marine to shallow-marine facies with trace fossils in most beds. The formation is exposed in a roadcut near Ringgold, Georgia, where its lowermost part, the Ringgold Member, consists of interbedded hematitic sandstones and mudstones. Previous researchers who studied abundant burrows in the Ringgold Member attributed most to small-bodied trilobites. In this study, I reexamined these trace fossils with new digital tools to test this hypothesis and clarify how these trace fossils related to their tidal-flat environments. Descriptions and measurements were taken from dislodged boulders of Ringgold Member sandstones at its type section. Digital photos of bedding planes were processed with Reality Capture™ 1.3 photogrammetry software to render digital 3D models of each specimen. ImageJ™ software was then applied to bedding planes to measure and otherwise quantify burrow abundances and burrow cross-sections, while also analyzing the burrows’ proximity to mudcracks. Most trace fossils are Taenidium, with a few associated Rusophycus and Diplichnites, accompanied by mudcracks, wrinkle marks, low-amplitude ripples, and small vertical burrows (Arenicolites). Specimens of Taenidium are meniscate (backfilled) burrows with horizontal and vertical components, commonly intersecting bedding planes with circular to elliptical cross-sections. Backfills include clay clasts, indicating that burrowers dug through fine-grained laminae without digesting them. Meniscate burrow cross-section areas vary, with a mean of 2.2 + 2.0 cm2 (n = 318), but burrow-area distributions suggest multiple growth stages of one trilobite species, with juveniles more abundant than adults. K-means analyses found burrows were close to or frequently cut across mudcracks, indicating that trilobites likely dug into mudcracks for ecologically significant reasons. Thus, I interpret Taenidium as burrows made by small, short-bodied trilobites burrowing for food on and in tidal-flat sediments. Rusophycus and Diplichnites show directions of movement toward or away from burrows, with fine details likely preserved by microbial mats. Hematitic sandstones and mudstones in the Ringgold Member further indicate these tidal flats were well-oxygenated environments. Although trilobites are normally regarded as full-marine fauna, their presence in tidal-flat facies shows adaptations to marginal-marine conditions, where they were actively burrowing and feeding.
Table of Contents
Introduction 1
1.1 Geological Setting 3
1.2 Paleontological Context 4
Material and Methods 5
2.1 Sample Locality 5
Figure 1. General geography of Georgia, USA. The location of study is
in Ringgold, Georgia, denoted by the star label. 6
Figure 2. Geography of US Interstate Highway 75, Exit 348 located in
Ringgold, Georgia, USA. The map includes terrain features (light grey
rectangles = surrounding buildings, dark grey/black = roads/interstates).
The location of study is magnified to illustrate each in-situ slab’s
(Slabs 1-3) location within the site. Slab 1: 34°54’25” N, 85°6’25” W.
Slab 2: 34°54’25” N, 85°6’26” W. Slab 3: 34°54’26” N, 85°6’26” W. 7
2.2 Sample and Data Collection 8
Results 11
3.1 Trace Fossil Description 11
Figure 3. Slab 1 of the Ringgold Member, Sequatchie Formation. A, 3D
photogrammetry model of in-situ slab 1 created using Reality Capture™
1.3, white squares outline study areas 1A and 1B. B, Illustration of traces
on Specimen 1A. C, Illustration of traces on Specimen 1B. B and C
illustrate sedimentary structures and trace fossils (medium grey, unlined
= mudcrack; dark grey, outlined elliptical/circular = Taenidium and
Rusophycus; black, circular = Arenicolites U-shaped burrows; patterned,
unlined = Diplichnites) created using Adobe Illustrator used in
calculations and k-means analysis. Scale = 1 cm. 13
Figure 4. Slab 2 of the Ringgold Member, Sequatchie Formation. A, 3D
photogrammetry model of in-situ slab 2 created using Reality Capture™
1.3, white squares outline study areas 2A and 2B. B, Illustration of traces
on Specimen 2A. C, Illustration of traces on Specimen 2B. B and C
illustrate sedimentary structures and trace fossils (medium grey, unlined
= mudcrack; dark grey, outlined elliptical/circular = Taenidium and
Rusophycus; black, circular = Arenicolites U-shaped burrows; patterned,
unlined = Diplichnites) created using Adobe Illustrator used in
calculations and k-means analysis. Scale = 1 cm. 14
Figure 5. Slab 3 of the Ringgold Member, Sequatchie Formation. A, 3D photogrammetry model of in-situslab 3 created using Reality Capture™
1.3, white squares outline study areas 3A, 3B, and 3C. B, Illustration of
traces on Specimen 3A. C, Illustration of traces on Specimen 3B. D,
Illustration of traces on Specimen 3C. B, C, and D illustrate sedimentary structures and trace fossils (medium grey, unlined = mudcrack; dark grey, outlined elliptical/circular = Taenidium and Rusophycus; black, circular = Arenicolites U-shaped burrows; patterned, unlined = Diplichnites)
created using Adobe Illustrator used in calculations and k-means analysis.
Scale = 1 cm. 15
Figure 6. Slab 4 of the Ringgold Member, Sequatchie Formation. A, 3D photogrammetry model of ex-situslab 4 created using Reality Capture™
1.3. B, Illustration of traces on Specimen 4A. B illustrates sedimentary
structures and trace fossils (medium grey, unlined = mudcrack; dark grey, outlined elliptical/circular = Taenidium and Rusophycus; black, circular = Arenicolites U-shaped burrows; patterned, unlined = Diplichnites)
created using Adobe Illustrator used in calculations and k-means analysis.
Scale = 1 cm. 16
Figure 7. Slab 5 of the Ringgold Member, Sequatchie Formation. A, 3D photogrammetry model of ex-situslab 5 created using Reality Capture™
1.3. B, Illustration of traces on Specimen 5A. B illustrates sedimentary
structures and trace fossils (medium grey, unlined = mudcrack; dark grey, outlined elliptical/circular = Taenidium and Rusophycus; black, circular = Arenicolites U-shaped burrows; patterned, unlined = Diplichnites)
created using Adobe Illustrator used in calculations and k-means analysis.
Scale = 1 cm. 17
Figure 8. Arenicolites (simple U-shaped burrows) of the Ringgold
Member, Sequatchie Formation. Specimen 5A, black circles show
Location of burrows. Scale = 1 cm. 18
Figure 9. Diplichnites walking traces of the Ringgold Member,
Sequatchie Formation. Specimen 5A, black arrows represent the
tracemaker’s direction of movement. Scale = 1 cm. 19
Figure 10. Diplichnites walking traces of the Ringgold Member,
Sequatchie Formation. Specimen 3B, black arrows represent the
tracemaker’s direction of movement. Scale = 1 cm. 20
Table 1. Ringgold Member rock slab ichnofossils and sedimentary
structures. 22
Figure 11. Histogram of the Taenidium burrow cross-section area (cm2)
in one-centimeter intervals from the Ringgold Member, Sequatchie
Formation. 22
3.2 Taenidium and Rusophycus distances from mudcracks 23
Figure 12. Histogram of the distance (cm) of trilobite burrows
(Taenidium and Rusophycus) from their nearest mudcrack in one-
centimeter intervals from the Ringgold Member, Sequatchie Formation. 24
3.3 Burrow statistical distribution in relation to adjacent burrows 24
Figure 13. K-means clustering analysis representing Taenidium and
Rusophycus trilobite burrow clusters (K = number of clusters) spatially
analyzing trace fossils located at the Ringgold Member, Sequatchie
Formation. Specimens 1A, 1B, 2A, and 2B are represented by XY
plots (left) locating each burrow and sectioning each cluster into colors
(black, red, green, and blue) created using RStudio™. Spatial maps
(right) plot each cluster mean with matching colors to each clustering
for each specimen, indicating their proximity to surrounding mudcracks,
created using Adobe Illustrator. 26
Figure 14. K-means clustering analysis representing Taenidium and
Rusophycus trilobite burrow clusters (K = number of clusters) spatially
analyzing trace fossils located at the Ringgold Member, Sequatchie
Formation. Specimens 3A, 3B, and 3C are represented by XY plots
(left) locating each burrow and sectioning each cluster into colors
(black, red, green, and blue) created using RStudio™. Spatial maps
(right) plot each cluster mean with matching colors to each clustering
for each specimen, indicating their proximity to surrounding mudcracks,
created using Adobe Illustrator. 27
Figure 15. K-means clustering analysis representing Taenidium and
Rusophycus trilobite burrow clusters (K = number of clusters) spatially
analyzing trace fossils located at the Ringgold Member, Sequatchie
Formation. Specimens 4A and 5A are represented by XY plots (left)
locating each burrow and sectioning each cluster into colors (black, red,
green, and blue) created using RStudio™. Spatial maps (right) plot each
cluster mean with matching colors to each clustering for each specimen, indicating their proximity to surrounding mudcracks, created using
Adobe Illustrator. 28
Table 2. K-means cluster distance to nearest mudcrack by specimen. 29
3.4 Diplichnites Trilobite Tracks 29
3.5 “Dimples” or U-shaped Arenicolites burrows 30
Figure 16. Histogram of the U-shaped Arenicolites burrow/“dimple”
cross-section areas (cm2) in one-centimeter intervals from the Ringgold
Member, Sequatchie Formation. 32
3.6 Clay Clasts 32
Discussion 33
4.1 Trilobite burrow-desiccation crack intersection and spatial analysis 34
4.2 Arthropod burrow-U-shaped burrow association 35
Figure 17. Diagram representing trace fossils present at the Ringgold
Member of the Sequatchie Formation and possible feeding methods. A,
trilobite trace fossil ichnogenera Diplichnites, Taenidium, and
Rusophycus (left); and, Arenicolites simple U-shaped burrow (right). B,
Taenidium feeding burrow, trilobite burrows into the sediment to feed on
organic material trapped in mudcrack crevice. C, Taenidium hunting
burrow, trilobite burrows into the sediment to hunt the infaunal species responsible for Arenicolites traces. 36
References 38
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