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.
Name: Tampsia floriformis
Location: San Luis Potosi, Mexico, Cardenas Formation
Age: 66-72 million years ago, Cretaceous Period
Finding this fossilized shell of the species Tampsia floriformis is a sure sign that you're in the Gulf of Mexico sampling a time period a few million years before a meteorite crashed down. It's an example of a species that became ultra-local and sensitive to extinction.
Tampsia floriformis is a species of rudist, an extinct group related to clams and oysters. As early as 113 million years ago, most species of rudists each occupied a narrow region of the ocean floor. Their narrow geographic ranges meant that rudists were highly diverse, but also vulnerable.
There might have been nothing unfit about the rudist way of life, but each species was so localized that one instance of extreme environmental change could have done them in. A widespread species can often survive in one region even if it dies out in another. If times get tough for a species that can only live in a small region, there is no other place for those individuals to go.
Rudists went extinct at about the same time as dinosaurs. Scientists debate whether the meteorite that killed the last dinosaurs was also the extreme environmental change that killed the last rudists, or if rudists were extinct before the meteorite ever touched Earth. It's possible that sea level change or some other oceanic shift could have driven rudists extinct.
Specimen Number: WSA 14925
References:
Coates, A. G. "Cretaceous Tethyan coral-rudist biogeography related to the evolution of the Atlantic Ocean." Special Papers in Paleontology 12(1974):169-174.
Johnson, Claudia. "The rise and fall of rudist reefs." American Scientist 90(2002):148-153.
Myers, R. L. "Biostratigraphy of the Cardenas Formation (Upper Cretaceous) San Luis Potosi, Mexico." Paleontologia Mexicana 24:1-89.
Pons, Jose Maria, Enric Vicens, Angelica Oviedo, Javier Aguilar, Pedro Garcia-Barrera, and Gloria Allencaster. "The rudist fauna of the Cardenas Formation, Maastrichtian, San Luis Potosi State, Mexico." Journal of Paleontology 87(2013):726-754.
Steuber, Thomas, Simon F. Mitchell, Dieter Buhl, Gavin Gunter, and Haino U. Kasper. "Catastrophic extinction of Caribbean rudist bivalves at the Cretaceous-Tertiary boundary." Geology 30(2002):999-1002.
