A Most Unusual Clam from the Triassic of Israel

September 5th, 2009

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MAKHTESH RAMON, ISRAEL–The Triassic was a time of unusual evolutionary innovations.  The Permian extinctions immediately before the Triassic may have wiped out up to 95% of marine animal species, so the survivors had considerable “empty niche space” to fill as they adapted to new environmental conditions with far fewer competitors.  The strange clam above is now part of this history.  On the left you see a view from above and front of the clam in living position.  It has a flattened base which it used to sit on the seafloor with its two valves extended up into the seawater to filter-feed.  On the right is a view of the base of this clam showing the junction between the two valves.  Note that the valves are not symmetrical as they are in most clams.  This clam was an “edgewise recliner”, meaning it sat on its edge (which it flattened over evolutionary time).  There is no other clam like it in the fossil or living record, and it is found only in one rock unit in the Negev Desert.

Allison Mione (’05) studied this clam and its living environment as part of her Independent Study project, and Tom Yancey (Texas A&M University) and I continued to work on it after she graduated.  The three of us now have a paper in the journal Palaeontology coming out later this year interpreting this clam as a new genus representing a new family of bivalves.  I took the opportunity yesterday to find a few more examples of this unique creature for museum collections.

This fossil was not thoroughly described in the past because finding whole specimens is very difficult.  We found one spot where the clam-bearing rock unit (the flat, tilted bed in the middle ground of the photo below) was dragged along a fault and bent in such a way that large blocks of the rock collapsed downhill upon exposure, releasing whole fossils.  As far as I can tell, this is the only place the whole fossils can be collected (N30.58759°, E34.88685°).

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