Name: cf. Monopleura
Age: 99-112 million years ago, Cretaceous Period
Location: Texas, USA, Edwards Formation
Multiple generations make up the fossil in the photograph. Fossils formed over several years like this one can preserve extra information about age, growth and survival in extinct animals.
The log-like object on the bottom of the fossil is part of one shell of an adult rudist, an extinct, two-shelled animal related to clams and oysters. The tinier shells encrusting the top are the remains of at least two dozen baby rudists. When living relatives of rudists hatch out of microscopic eggs, they spend at least a day or two floating in the ocean as plankton before settling and cementing themselves to a permanent home. For rudists, sometimes, that home was the shell of another rudist.
Other fossils of accumulated rudists, called bouquets, are even larger than the one in the photograph. One bouquet from Croatia contained 160 individuals. A scientist shaved off such fossils millimetre by millimetre to get inside the fossil look for patterns to the accumulations.
He found that such large bouquets were mainly made up of rudists that died young. Newly-cemented rudists packed in tightly, taking up 85% of the available space, but few grew for long. Of the 160 individuals that settled on top of one another over the years, only one in four made it to an adult size.
Specimen Number: NPL 36298
References:
Gotz, Stefan. "Inside rudist ecosystems: Growth, reproduction, and population dynamics" Cretaceous Rudists and Carbonate Platforms. SEPM Special Publication 87 (2007):97-113.
