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Home » Paleontology » Fossils of Arkansas » Crusty Crustaceans, Mystery Monday Revealed

Crusty Crustaceans, Mystery Monday Revealed

I posted a new fossil last Monday. Were you able to figure it out?

1 brachylepas

You can find live versions of these animals covering rocks on most shores, such as these I found on the Pacific coast of Washington.

2002-03-07 15.36.45

They will attach themselves onto anything, including other animals.

Encrusted North Pacific right whale

Encrusted North Pacific right whale

All of these pictures show barnacles, which will attach themselves to rocks, whales, boats, piers, and anything else they come into contact with during their free-swimming larval stage. The two most common barnacles one tends to find are either goose barnacles, like the ones shown on the rock, or acorn barnacles, like those shown on the whale. Goose barnacles are in the group called Pedunculata, so named because they have a peduncle, the stalk that attaches the shell to the underlying substrate (what they’re attached to, i.e. the rock, boat, whale, etc.). Acorn barnacles, on the other hand, are in the group called Sessilia, barnacles without stalks that attache their shell directly onto the substrate.

Barnacles are crustaceans, which are within the group Arthropoda. There seems to be some confusion about this on various websites, so I will explain a bit further. Arthropods include all segmented, invertebrate animals with an exoskeleton (hard exterior; literally, skeleton on the outside),  and jointed legs. It is important to note here that while these are all characteristics shared by arthropods, they do not define Arthropoda. The group itself is defined by all of them sharing a common ancestor. The shared characteristics are simply clues to their evolutionary relationship. Arthropods include insects, arachnids (spiders, scorpions, and related animals), myriapods (millipedes and centipedes) and crustaceans. Arthropods also include trilobites.

Biramous vs. uniramous appendages. cnx.org

Biramous vs. uniramous appendages. cnx.org

Crustaceans are arthropods in which, among other things, the legs attached to each segment are biramous, meaning they split into two. Barnacles are, more specifically, crustaceans comprise the group Cirripeda, which means “curled foot” (while there is much argument about whether Cirripedia is a suborder, infraclass, or some other level of phylogenetic classification, these terms are are essentially meaningless and are really just holdovers of a time in which classifications were not built on evolutionary relationships, so I don’t use them; a proper term would be clade, but most people would not understand what that meant, so “group” it is). Most crustaceans are dioecious, meaning they have both males and females. Most barnacles though, are hermaphrodites, meaning that each individual is both male AND female at the same time. Much is often made of the fact that they have possibly the longest penis for their body size of any animal. This is necessitated by the fact that they are sessile, permanently attached. They can’t go walking around looking for a mate, so unless they are going to just release their sperm into the water and hope for the best (not normally very effective for animals using internal fertilization, although there are exceptions), they have to compensate. Since they are hermaphroditic, they could simply fertilize themselves, which occasionally happens, but not usually. Self-fertilization is the ultimate in being inbred, which is commonly known to have its problems (thus the reason inbred is often used as an insult).

Fossils of barnacles have been found in rocks dating back to the Cambrian Period over 500 million years ago, although they are not common until about 20 million years ago.Since that time, they have become very widespread and found throughout the world. Their first appearance is in the Burgess Shale, one of the best known fossil sites in the world. In Arkansas, they can be found in many of the Carboniferous aged limestones in the Ozark Mountains. Their shells are made of calcium carbonate, just like the limestone they are found in, as well as clams, with which barnacles are sometimes confused. The shells of barnacles are not hinged like clams, though. The shells of barnacles are also usually surrounded by additional material that anchors them to the rock, forming a roughly circular cone around the barnacle, which is not found in clams. It is not uncommon to find barnacles on clams, which shows a nice comparison of the two.

Barnacles on a clam. Natural History Museum, Humboldt University. http://www2.humboldt.edu/

Barnacles on a clam. Natural History Museum, Humboldt University. http://www2.humboldt.edu/


1 Comment

  1. […] if Figure 5 from the only real publication on Arkansas fossil barnacles. I posted an articles on barnacles once before, but time grew short and I neglected to mention specifically the Arkansas ones, an […]

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