Subterranean passages and chambers within the crust of the Earth form unique environments, often carved out over thousands of years, which create homes for rare and bizarre groups of organisms and provide valuable fossil resources. This episode, we discuss the geology, biology, and paleontology of Caves.
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Holes in the Earth
The exact definition of a cave varies, but everyone generally agrees that a cave is a natural subterranean passage. They occur all over the planet, on every continent, many islands, and even under the sea. The smallest caves can be shallow rock shelters or short shafts, while the largest can be staggeringly immense, like Mammoth Cave in Kentucky, with around 660 km (415 miles) of mapped passages, or Veryovkina Cave in the country of Georgia, whose tunnels extend over 2 km (1.2 miles) below the surface.
Most caves are formed by the dissolution of carbonate rock, often limestone. As water flows through its cycle, it picks up just enough carbon dioxide to form an acid that will carve through carbonate rock, slowly opening fissures, tunnels, and chambers, and occasionally re-depositing carbonate minerals into cave formations like stalagmites and stalactites. Many such caves are still active, with streams flowing through them and ponds pooled inside. Other caves can be formed by the pounding of waves on shore, the abrasion of sand in desert winds, or the flowing of lava out of a volcano.
If you’ve ever been inside a cave, you might also be familiar with the unique conditions within. The farther you get from the entrance to the surface, the less the cave interior is affected by outside light and weather. Deep inside a cave, there’s no light, temperature and humidity are relatively stable, and there’s very little for life to feed upon.
Life in the Holes in the Earth
The conditions within a cave create an unusual habitat for hardy life, cavernicolous life. Many microbes make their living in caves, some feeding off the rocks inside, as well as microscopic and macroscopic cave fungi. Plants typically prefer cave entrances to cave interiors, though some species – like certain Chinese nettles – can survive in surprisingly little light, and some plants – a category called Lampenflora – have managed to invade caves by thriving on the artificial lights inserted by humans.
Animal life in caves is often split into a few categories: trogloxenes, or cave guests, are animals that use caves sometimes but can’t live their whole lives there (such as bears and bats); troglophiles are animals that can live all or most of their lives in caves or in other habitats such as surface soils (including many insects); accidentals are animals that have fallen, washed, or wandered into the cave, and often don’t last long before leaving or dying; and troglobites are true cave-dwellers, found only in the subterranean homes they’re adapted for. (Note that these definitions vary a bit. For a deep dive into this terminology, read here.)
Troglobites (or troglobionts) often share a set of convergently-evolved cave features, including reduced or lost eyes, reduced or lost pigment, shrunken or absent wings (in the case of insects), and enhanced non-sight senses, including lengthened antennae (for arthropods), strengthened lateral line systems (for cavefish), or improved hearing and smell. These features are found in cave insects, cavefish, cave salamanders, cave worms, and many more. The evolution of cave creatures has been the subject of intense research, as they make great case studies for rapid and repeated evolution.
Remains of the Life in the Holes in the Earth
True troglobites are exceedingly rare in the fossil record. A 2020 study identified the oldest cave-adapted animal, a Cretaceous cockroach, though other suspected fossilized troglobites have proven difficult to identify for sure. On the other hand, troglophiles and trogloxenes are extremely common as fossils, and they’re often found still inside the caves wherein they fossilized.
Cave paleontology is a big deal. A huge portion of our understanding of Earth history, particularly in the past few million years, comes from excavations in caves. Fossils in caves often provide an excellent sampling of the surrounding ancient environments, including organisms that were washed in by water, trapped within by gravity, dragged in by predators, carried in by nest-builders, or came in intentionally seeking shelter. And the layered sediments and carbonate structures inside a cave can preserve great records of changing climates over thousands of years.
Most cave fossils are young because most caves are young, susceptible to becoming collapsed or filled-in before a few millions years have passed. However, there are filled-in caves and cave-related structures that preserve older fossils. A cavern or fissure that filled in with sediments during the Permian or Cretaceous might preserve fossils of those ages. And, of course, our favorite fossil site, the Gray Fossil Site, is a sinkhole (the collapsed top of a cave) that filled in with Pliocene-aged sediments and fossils.
If you want to delve deeper into cave geology:
Caves and Karstic Phenomena by the Italian Ministry of the Environment and Territory Protection
Geology of Caves by the United States Geological Survey
Encyclopedia of Caves and Karst Science, edited by John Gunn
Biospeleology, Illinois Natural History Survey
Importance of the Discovery of the First Cave Beetle (technical)
Evolution and development in cave animals: from fish to crustaceans (technical)
The importance of selection in the evolution of blindness in cavefish (technical)
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