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 In recognition of this, these genetic terms have been replaced by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.