TRACE  FOSSILS

 

Trace fossils are indirect evidence of ancient life.  They are structures representing the behavior of ancient organisms.  Terrestrial traces are moderately common in the Quaternary limestones of San Salvador Island (& elsewhere in the Bahamas).  The marine facies also have trace fossils.

 



 

Vegemorphs below a 9.2 ky calcrete crust at Green Cay, western Graham’s Harbour, offshore from northwestern San Salvador Island.

Vegemorphs are terrestrial fossils consisting of irregularly curvilinear, often downward-branching structures having a subcircular cross-section.  They represent the position of roots of ancient plants.  Vegemorphs are traditionally called rhizoliths or rhizocretions or rhizo-ichnomorphs, or simply “root traces”.  The root word of the 1st three terms, “rhizo-”, literally means “roots”.  It has been demonstrated that these structures sometimes include the stem portions of ancient plants.  In recognition of this, these genetic terms have been replaced by the descriptive term “vegemorph” in the most recent Bahamas geology literature.

On San Salvador, vegemorphs are common below calcrete paleosol horizons and in regressive eolian calcarenite units.

 


 

Vegemorphs below the calcrete paleosol that caps the Grotto Beach Formation (lower Upper Pleistocene, 119-131 k.y.) at Rhizo City, eastern part of southern margin of San Salvador Island.

 



 

Stellate” burrows in back-beach dune facies calcarenites (Hanna Bay Member, upper Rice Bay Formation, middle to upper Holocene) at Hanna Bay, northern part of eastern margin of San Salvador Island.  These terrestrial trace fossils have a clotted stellate pattern when seen in bedding plane view.  They have U-shaped or J-shaped cross-section shapes.  They have been inferred to represent brooding traces of burrowing bees (Curran et al., 1997) or composite trace fossils (Martin, 2006).

 



 

Inferred ant burrows in back-beach dune facies calcarenites (Hanna Bay Member, upper Rice Bay Formation, middle to upper Holocene) at Hanna Bay, northern part of eastern margin of San Salvador Island.  These are apparently the structures identified as Celliforma in Martin (2006), p. 39.  Ant burrow interpretation from Al Curran (pers. comm., 2008).

 



 

Skolithos linearis (Haldeman, 1840) in back-beach dune facies calcarenites (Hanna Bay Member, upper Rice Bay Formation, middle to upper Holocene) at Hanna Bay, northern part of eastern margin of San Salvador Island.  Skolithos is a ~cylindrical, unbranched, vertical burrow.  Traditionally, this trace fossil is considered diagnostic of high-energy, shallow-water conditions (see example) - most examples in the geologic record did occur in such facies.  This Skolithos is a terrestrial trace fossil, representing an insect or arachnid burrow (Curran et al., 1997; Martin, 2006).

 



 

Psilonichnus upsilon Frey et al., 1984 in back-beach facies calcarenites (Hanna Bay Member, upper Rice Bay Formation, middle to upper Holocene) at Hanna Bay, northern part of eastern margin of San Salvador Island.  This large, distinctive, terrestrial burrow was constructed by the ghost crab Ocypode quadrata.

 



 

Cluster” burrows in back-beach dune facies calcarenites (Hanna Bay Member, upper Rice Bay Formation, middle to upper Holocene) at Hanna Bay, northern part of eastern margin of San Salvador Island.  These trace fossils are best seen at Hanna Bay and North Point Peninsula.  Cluster burrows have been interpreted to be brooding/hatching structures constructed by sphecid wasps (digger wasps) (see Curran et al., 1997 and Martin, 2006).

 



 

Ophiomorpha burrows in subtidal fossiliferous limestones (Cockburn Town Member, Grotto Beach Formation, lower Upper Pleistocene, 125-131 k.y.) at “Ophiomorpha Bay”, Cockburn Town Fossil Reef, western margin of San Salvador Island.

Ophiomorpha is a moderately large, curvilinear, subcylindrical burrow having a externally pelleted lining that is constructed on the seafloor by callianassid shrimp.  Differential weathering and erosion often results in empty Ophiomorpha tubes (intraburrow porosity) and significant interburrow porosity (see above photo).  For more info. on this, read over Al Curran’s recent work on Ophiomorpha fabric.

 



 

References cited:

 

Curran, H.A., B. White & M.A. Wilson.  1997.  Guide to Bahamian Ichnology: Pleistocene, Holocene, and Modern Environments.  Bahamian Field Station.  San Salvador Island, Bahamas.  61 pp.

 

Martin, A.J.  2006.  Trace Fossils of San Salvador.  Gerace Research Center.  San Salvador Island, Bahamas.  80 pp.

 


 

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