I am holding in my hand a cluster of fossil barnacles, Balanus concavus1, from the Miocene (5-23 million years ago). I found this specimen at a location along the Calvert Cliffs, Maryland, an area famous for its fossils.
Compiled from Lippson & Lippson2.
Barnacles (Cirripedia) are marine invertebrates. A barnacle starts out as a free-swimming larva, called a nauplius, (plural nauplii) . The nauplius grows a pair of shells around its body. At this stage it is called a cypris larva. The cypris larva attaches itself permanently to a rock, a mollusk shell, an adult barnacle or any other hard structure. The larva, now sessile, starts to develop into an adult. It casts off its shells and begins to secrete several calcareous plates that end up completely surrounding its body. The fossil in the picture above consists of the shells of adult barnacles.
A barnacle feeding. From Lippson & Lippson2.
A barnacle feeds by opening its shell and unfolding its appendages that are covered with fine hairs. The appendages sweep tiny particles of food suspended in the water into the barnacle's mouth.
Because of their calcareous shells, until the early 19th century people thought barnacles were mollusks. It was finally determined around 1819 that barnacles were actually crustaceans.
During the 19th century, the world's leading authority on barnacles was none other than Charles Darwin. Darwin spent 8 years studying, dissecting and classifying barnacles. The outcome of his efforts was a set of authoritative books. (Links to complete texts and scanned plates of Darwin's barnacle books and other publications are available on this page.)
His work on barnacles exposed Darwin to the extent of variation that exists in nature within and between species. Later, in On the Origin of Species, Darwin developed the idea that one species gradually evolves into another one. According to Ernst Mayr3, Darwin's studies of barnacles may have influenced the development of his ideas on speciation. Darwin himself noted this in his autobiography:
"The Cirripedes form a highly varying and difficult group of species to class; and my work was of considerable use to me, when I had to discuss in the 'Origin of Species' the principles of a natural classification."
1. Harold E. Vokes, John D. Glaser and Robert D. Conkwright. Bulletin 20: Miocene fossils of Maryland. Second Edition January, 2000. Electronic Publication No. 00-1. Available as a CD-ROM from the Maryland Geological Survey.
2. Lippson, A.J. & Lippson, R.L. 1984. Life in the Chesapeake Bay. Johns Hopkins University Press.
3. Mayr, E. 1991. One long argument. Harvard University Press.
Also posted at Transitions.