Fossil of the Week

8/26 – Darwin’s Barnacles

Barnacles – the Cirripedia – are a type of arthropod in the subphylum Crustacea, but until just before Charles Darwin’s travels on the HMS Beagle, they were classified as mollusks (along with clams and snails because of their hard shells). Although there are many different types of barnacles*, what we would most commonly recognize as one – little hard volcano-shapes such as those shown here – is only the calcium carbonate shell with which the organism surrounds its soft body. Inside, the barnacle animal lies on its back and extends specialized legs out of the opening at the top to catch food particles when the tide covers it. This opening can be closed when the water level around the barnacle drops, allowing it to survive until the tide comes up again.

Charles Darwin became interested in barnacles while working his way through material that he’d collected during his travels. He found an unusual specimen that he couldn’t classify. In an attempt to understand it, he examined other barnacles, which led to his eight-year project studying and publishing on the living and fossil barnacles of Great Britain. This work helped him to refine many of his ideas about species and evolution, such as homology (similarity due to common ancestry). It is in large part this work and his geological publications from his travels that established him as a competent and thorough researcher and gained him a receptive audience to the publication of On the Origin of Species in 1859.

Although the Acorn Barnacles shown here aren’t from Great Britain, they are quite similar to those that Darwin would have worked on. This fossil is Balanus concavus (PRI Acc. No. 1459) from the Choptank Formation, a silt and sandstone unit that forms part of the Miocene Chesapeake Group. It outcrops extensively in Maryland – this particular locality at Governor Run is on the western coast of Chesapeake Bay, not far from the famous Calvert Cliffs.

Text by Ursula Smith (reprinted from “Fossil Focus” in American Paleontologist, Spring 2009)

*For more on barnacles, see Fossils of the Week 7/21/10 - Coronula macsotayi, 3/17/10 – Goose Barnacle, and 2/17/10 – Big-Mouthed Barnacle.