The timing of the invertebrate invasion into continental settings is
problematic, with the Ordovician advent mired in controversy. Few
unequivocal Silurian examples of continental ichnofossils have been
reported. In eastern Pennsylvania, the Silurian Bloomsburg Formation
consists of fluvial deposits that have yielded a low-diversity
ichnofauna described here. The rarity of reported Silurian
continental ichnofauna makes this discovery significant by expanding
our understanding of continental ichnodiversity.
The Bloomsburg Formation in eastern Pennsylvanian is characterized
by erosion based, fining-up sequences from medium-grained sandstone
to mudstones. Channel fills are dominated by ripple strata and
thinner graded beds. Overbank deposits are characterized by thinly
laminated graded beds of fine-grained siltstones to mudstones.
Thicker, laminated mudstones contain reduction halos around ribosome
traces, mudcracks and some well-developed Bk horizons. The
Bloomsburg fluvial systems are low gradient and low sinuosity
systems. The strata contain a low-diversity ichnofauna that
includes: 1) sinuous traces, nonmeniscate (?Planolites) and
meniscate(Taenidium), 2) bilobate, oval traces (Rusophycus),
3) slightly sinuous traces with a medial ridge and scratches (Cruziana),
4) two rows of parallel circular depressions (Diplichnites),
and 5) low, slightly sinuous ridges (Diplopodichnus). This
ichnoassemblage has been recognized by the authors, both vertically
in stratigraphic sections as well as at different localities within
the Bloomsburg. The ichnofossils are not penetrative into the
underlying strata. Planolites/Taenidium is the dominant ichnofossil
in the Bloomsburg Formation and appears to be an addition to the
previously recognized epifaunal arthropod ichnofauna of the
Dilichnites ichnoguild developed on floodplain environments during
the Silurian to Devonian.
Preliminary report on continental
ichnofossil diversity from the Silurian Bloomsburg Formation,
Eastern Pennsylvania