PALEOECOLOGY OF MIDDLE TRIASSIC FLAT CLAMS

While high-density accumulations of flat clams are well known throughout the Mesozoic fossil record, their origin is still intensely debated. Examples of flat clam facies include Halobia in the Triassic Shublik Formation (Alaska), “Lucina” in the Jurassic Kimmeridge Clay (England), and Bositra in the Jurassic Posidonienschiefer (Germany). These very thin, flat cosmopolitan bivalves are preserved in organic-rich units, interpreted to represent deposition in dysaerobic environments. The units contain a very low diversity fauna. It has been suggested that these bivalves are benthic and highly tolerant of the low oxygen conditions because the thin shells could maximize oxygen exchange. Alternatively, the clams have also been interpreted as planktonic or pseudoplanktonic organisms based on distribution, as well as morphological and experimental observations.

The Middle Triassic Fossil Hill Member, exposed in central Nevada, is a laterally uniform unit composed of interstratified calcareous mudstone, siltstone, and black limestone. It contains a distinctive and exceptionally well-preserved cephalopod and bivalve fauna, including dense accumulations of the flat clam Daonella (Family Halobiidae). The absence of high-energy allochems and bioturbation, along with the finely laminated sediments, suggest that the Fossil Hill Member was deposited in a shelf basin. Daonella bivalves are found in highest abundance and most densely packed in the laminated limestones, and less abundantly in the mudstone and siltstone facies. The shells are found articulated, and preserved with a preferred convex-up orientation. Various specimen sizes have been found on the same bedding plane. These observations suggest the bivalves underwent little to no transport prior to burial. No morphological evidence exists to support a planktonic strategy for Daonella. Combined, these results point to the benthic mode of life for halobiid bivalves.

Because halobiids are among the most short-lived and cosmopolitan bivalves to have ever lived, understanding their life strategy thus provides great insight into an unusual time in Earth’s history. The question remains as to what conditions allowed for the rapid appearance and dominance of halobiids in the dysaerobic environments during the Middle and Late Triassic.