Brachiopods: The Lamp Shells of the Paleozoic Seas

Brachiopod fossils in the Supai Group, Grand Canyon National Park. NPS photo by Michael Quinn

Brachiopods are among the commonest fossils of marine invertebrates in the Paleozoic rocks of the national parks. They were a dominant group in Paleozoic seas, filling many of the ecological roles that bivalves took over in the Mesozoic and Cenozoic — after about 95% of brachiopod species died out at the end of the Paleozoic.

Arm-foot, or lamp shell

The phylum is Brachiopoda, from the Greek brachion, "arm," and podos, "foot." The name reflects an old mistake: brachiopods were once taken for mollusks, which have a muscular internal foot. What a brachiopod actually has is a feeding organ, the lophophore, which grows as two "arms." Their other name, lamp shells, comes from the way some of their shells look like ancient oil lamps.

They are still alive today, but their shells rarely wash up on beaches, because most living brachiopods are found in deep, cold sea water. Their great age was the Paleozoic. They first appear in the Cambrian Period, and their fossil record is one of the best of any invertebrate group.

Not a clam

Both have two shells, called valves, so a brachiopod looks superficially like a bivalve mollusk. The resemblance stops there.

The lophophore itself rarely fossilizes, but some brachiopods have a mineral internal frame that held it up, the brachidium, which can survive in well-preserved fossils. The shells vary a great deal — ribs, concentric ridges, growth lines, fine spines.

Two kinds

Brachiopods are usually sorted into two informal groups:

Because of that difference in material, articulate shells are more likely to be dissolved and replaced over geological time.

In the national parks

A Permian productid brachiopod from Lake Mead National Recreation Area. NPS photo by Andy Cattoir

Brachiopod fossils are known from at least 73 parks, usually alongside other common Paleozoic sea animals such as bryozoans and crinoids. Almost every record is Paleozoic.

Many brachiopod species have been named from Grand Canyon National Park (Arizona), Guadalupe Mountains National Park (Texas), Yellowstone National Park (Idaho, Montana and Wyoming) and Mississippi National River and Recreation Area (Minnesota).

Molds and casts of brachiopods, Mississippi National River and Recreation Area. NPS photo by Justin Tweet

Early Ordovician inarticulate brachiopods, Great Basin National Park. NPS photo by Gordon Bell

Park geology to explore

Each park's Geodiversity Atlas page describes its rocks and fossils:

• Carlsbad Caverns · Chesapeake and Ohio Canal · Death Valley · Delaware Water Gap · Grand Canyon · Great Basin
• Guadalupe Mountains · Lake Mead · Mississippi National River · Noatak · Yellowstone · Yukon-Charley Rivers

Sources

Based on "Fossil Brachiopods," in the series Invertebrate Fossils in National Parks, National Park Service; a work of the United States government in the public domain. Two photos on the original page are left out: one credited to the Identification Guide to the Fossils of the Guadalupe Mountains by Mary Carol Coleman and Cameron Coleman, and one without a credit.