Name: Paucicrura corpulenta
Age: 444-450 million years ago, Ordovician Period
Location: Minnesota, USA, Maquoketa Formation
The early life of extinct brachiopods is preserved in a part of the shell so small that it’s studied with a microscope (blue arrow). In the case of the specimens of Paucicrura in the photograph, the young left those parts behind in spectacularly preserved fossils.
Brachiopods are more closely related to tiny, colonial moss animals, called bryozoans, than they are to clams they might be confused for. When they first hatch they are tiny ocean plankton, floating and growing a special part of their shell called the protegulum.
Some grow the protegulum part of the shell only when they are in their egg, some grow it while they are floating plankton, and some do both. Some spend weeks in their floating larval stage and some float only days before they settle and metamorphose into adults. At metamorphosis, the protegulum stops growing.
In each scenario, the protegulum grows for different lengths of time and ends up a different size. In the specimens in the photograph, the protegulum is about 1/3-½ mm wide, or just barely visible as a speck to the naked eye. It has no special marks that come from growing in-egg. From that information researchers think that the young of Paucicrura started growing their shell as larvae after hatching and then spent weeks floating before settling into adulthood on the ocean floor.
Photo credit: Angie Thompson
Specimen Number: UT 0613
References:
Freeman, Gary, and Judy W. Lundelius. “The transition from planktotrophy to lecithotrophy in larvae of the Lower Paleozoic Rhynchonelliform brachiopods.” Lethaia 38(2005):219-254.
