Fossil Ark Clam
Arca zebra abisiniana Weisbord, 1964
Family Arcidae (Ark Clams)
This is an ark shell, a member of the bivalve family Arcidae, which has members still living today. If you are a shell collector, this one probably looks very familiar. It is Arca zebra abisiniana, described by Weisbord in 1964 in Bulletins of American Paleontology, no. 204. It is the holotype, collected from the Abisinia formation of Playa Grande village in northern Venezuela. It is a very young fossil, Pleistocene in age or less than 2 million years old.
The species: This specimen was described as a subspecies of Arca zebra, the modern “Turkey Wing” shell found today from North Carolina through the Caribbean. The fossil subspecies is said to differ from the living species by lacking the broad radial depression so characteristic of the modern form (although in our experience, this is a highly variable feature even today). The living species is striped brown and white, as reflected by the species name “zebra.” The fossil subspecies is named after its fossil formation.
Evolution on the Half Shell...
The Assembling the Tree of Life: Bivalvia project (BivAToL) is a part of the Assembling the Tree of Life initiative, a large research effort sponsored by the National Science Foundation. Its goal is to reconstruct the evolutionary origins of all living things.
Jetsam & Flotsam
Some of the BivATOL team met in early May at the Mote Marine Laboratory’s Tropical Research Station at Summerland Key, FL for a combined collecting trip and coding workshop. Both activities are essential to our project’s goal of determining the phylogenetic relationships among the bivalve families.
After collection, many of the species’ visible and molecular characteristics must be compared and “coded,” after which the phylogenetic computer analyses will be run to produce the final “tree” from which a hypothesis of relationships can be made. Below is an example of a portion of such a phylogenetic tree. Families that are on nearby branches are more closely related to each other than those further away.