Taphonomy of Cave Environments

"If you gaze long into an abyss, the abyss will gaze back into you."

Friedrich Nitzsche

Caves always fascinated men, and tales of monstrous bones found in them maybe inspired legends referring to caves as forbidden passages - according to the Maya the karstic caves of the Yucatan peninsula were the gates to Xibalba, the "Place of fear", reign of the Death Gods.
A comprehensibly fear, whoever visit and enters a natural cave will still today be surprised, maybe even concerned, by the darkness and quietness that encompass him. In a reign without daylight or seasons the concept of time becomes meaningless, and it would seem no surprise to discover the bones of a long dead monsters hidden in the shadows of our faint light.

Fig.1. An imaginative drawing of William Buckland transported back to Pleistocene times at Kirkdale Cavern, Yorkshire - a hyaena den full of bones of Pleistocene mammals. (After Buckland, 1823.)

Bone accumulations in caves were known since antiquity - Greek sailors tell that they found the bones of Cyclops in caves on the island of Sicily. Some centuries later, the German Jesuit Athanasius Kircher visited and studied these bones, and in 1678 published an extensive report on this matter, appropriately named "Mundus subterraneus", proclaiming that the remains represent at least the bones of four different kind (and sizes) of prehistoric giants. Some centuries later the fossils were recognised to belonging to Pleistocene elephants. In the 18th and 19th century the bones, even when recognized belonging to animals, were mostly explained as the remains of the victims drown in the biblical flood, and later transported and deposited by the water into the caves.

The first insights that the bones were of animals living in or nearby the cave came in 1794, when the German physician Johann Christian Rosenmüller described the cave bear as species Ursus spelaeus. He rejected the idea of a flood transporting the bones into the cave, and argued

"the amount of bones can not be explained in a simpler way, then assuming that the animals, from which they came, lived, reproduced and died in the caves."

Despite these considerations, until 1970 accumulations of animal remains and broken bones were mostly attributed to human activity, even if earlier observations showed how carnivores can gather and process bones in similar way as expected by human behaviour. Reverend William Buckland, who intensively studied caves and the sediments during the first half of the 19th century, published his results in his masterpiece "Reliquiae diluvianae" (1824):

"I have had an opportunity of seeing a Cape hyaena at Oxford...I was enabled also to observe the animal´s mode of proceeding in the destruction of bones: the shin bone of an ox being presented to this hyaena, he began to bite off with his molar teeth large fragments from its upper extremity, and swallowed them whole as fast as they were broken off. On his reaching the medullary cavity, the bone split into angular fragments...he went on cracking it till he had extracted all the marrow... this done, he left untouched the lower condyle, which contains no marrow, and is very hard. The state and form of this residuary fragment are precisely like those of similar bones at Kirkdale; the marks of teeth on it are very few...these few, however, entirely resemble the impressions we find on the bones of Kirkdale; the small splinters also in form and size, and manner of fracture, are not distinguishable from the fossil ones...there is absolutely no difference between them, except in point of age."
(BUCKLAND 1824 pag. 34)

Fig.2. Section trough Dream Cave near Wirksworth in central England, drawing by Webster based on a sketch by Buckland, published in 1823. A skeleton of a rhinoceros (G) had been found buried in the debris below a fissure (D). Buckland argued that the carcass must have fallen into the cave while being swept along during a geologically recent deluge.

However caves are an environment that in principle is hostile to life. Few mammals, including the hyena and the cave men, regularly use or used deeper areas of the cave, prominent exceptions being bats or the extinct cave bear. Thus the presences of animal remains in caves are the result of complicated and various taphonomic processes.
Taphonomic assemblages in caves often represent a collection of various animals of different geographically locations and time periods. Carcasses of animals from different environments can be washed together, and caves act as sample bag for many centuries, providing a false species assemblage. Accurate interpretation of fossil assemblages can so be challenging.

Fig.3. The "Passalacqua" quarry, situated at the north-western margin of the Gargano promontory, close to the village of Apricena in the italian province Apulia. Here in sediments deposited in a karst network, discovered during quarrying of the limestone, a very diverse and now well-documented fossil vertebrate assemblage was discovered. The sediments returned also lithic artefacts, estimated in an age interval of 1,3 and 1,7 Ma, documenting an early hominid occurrence in the Early Pleistocene, and thus now constitute the oldest record in Europe. The karst network developed during two phases. The first, Miocene in age, extents in the Mesozoic limestone of the "Sannicandro Formation", and consists of red clays ("terra rossa") in part reworked and redeposit by flood events with a huge diversity of vertebrates remains. The second phase of occurred after a tectonic uplift of the Gargano peninsula during the Pliocene. Fissures can so contain in the lower part fossils attributed to the Miocene fauna, and in the upper parts fossils of the Villafranchian mammal age (Upper Pliocene and Lower Pleistocene).

To fully understand the taphonomy of caves it is necessary to consider the depositional environment and sediments found in caves: Sediments can be of very different origins, clastic sediments derived from collapse of cavern roofs, sediments transported into the cave by superficial and underground streams or accumulating under sinkholes, chemical deposition of minerals, like speleotherms, and sediments resulting from the accumulation of organic matter, like guano.

Fig.4. Examples of Speleothems or flowstones - deposition of minerals like calcite from aqueous solutions - found in the Conturines Cave in the Dolomites (Alps), the highest situated locality were fossils of the Cave bear (Ursus spelaeus) were found. The floor of the cave is covered by a 2m thick flowstone that shows a fine lamination, maybe representing annual growth layers. Such deposits are well suited for radiometric dating method because their calcite crystals are usually large and have little tendency to recrystallize and incorporate radioactive elements only in the period of crystal growth. However in this case the fossils were found in the sand overlying the flowstones (older than 350kyr), implying that the fossils are younger as this unit.

In contrast to the surface, caves are however very stable environments - the transport of sediments in conduit systems is episodic with abrupt storm flow and little or no movement during low flow conditions. Carcasses and bones can so be transported into the cave by sudden flood events or mudflows, and become accumulated during phases of decreasing flow.

Fig.5. Example of clastic sediments as infill's of the Mammut-Höhle in the Dachstein-massif (Austria). Rounded pebbles, consisting mainly of siliciclastic and metamorphic rocks, form a massive conglomerate (Augenstein-Formation) that became cemented and later partially eroded. The source and age of this unit is unclear, probably transported from the Central-Alpine to the north during the Late Eocene to Early Oligocene.

For example the two skeletons of Australopithecus sediba, described in April 2010, were discovered in a massive, up to 1.5-m-thick stratigraphic unit filling a cave in the karst landscape of South Africa, The heterogeneity in the sediment-grains, ranging from sand to pebbles to larger boulders, and lacking sedimentary structures (like stratification) suggested the deposition of this unit as a single event, like a debris flow, maybe caused by a flood or a storm. The superb preservation and state of articulation of fossil material also indicate rapid deposition, limited transport distance, and laminar flow conditions consistent with a debris flows.

A peculiar bone bed formation found in caves is under sinkholes. Fissures, hidden under vegetation, snow cover or a thin soil layer can act like a pitfall trap - animals fell trough them and die on the impact or later by starvation. Under these natural traps a talus of rubble accumulates that contains a chaotic assemblage of bones from animals died at different times.

In parts of the cave accessible by animals, and used as shelter or resting place, animals that die of natural causes, get lost or become entrapped can became accumulated. Many carnivorous animals, mammals and birds, carry their prey or parts of it in their shelter, were the bones later are found (a prominent recent example is the den of the man-eaters of Tsavo).
Buckland notes about hyenas that:

"Their habit of digging human bodies from the grave, and dragging them to their den, and accumulating around it the bones of all kinds of animals,...[] "
(BUCKLAND 1824 pag.22)

Fig.6. Fissures that act as sediment and also bone accumulation traps can develop not only in carbonates, but also in evaporitic sediments. Near the small village of Westeregeln (Thuringia, Germany) past quarrying activity for clay has exposed underlying Mesozoic gypsum and limestone formations, which in the upper part show an intensive "karst" network, refilled with Pleistocene sediments and fossils. The uppermost part of the stratigraphic column of the infilling sediments is represented by a postglacial soil, developed on Loess - aeolian sediment deposited during the last great glacial period. These sediments cover ancient matrix supported breccias, presumably generated by partial collapse of former caves or fissures.

Fig.7. Between these breccias, mostly on the top of the deposits, lithic artefacts (marked by the plastic bags), bones of ice age mammals and hyena coprolites are found exclusively in pebbly horizons.

The amount of bone present and the species representation in prey assemblages vary from predator to predator but are always biased with respect to the community from which they came. Predator size, for example, is related to prey size, but it varies with seasonal and annual cycles of climate, habitat, and prey populations.
Even without this disadvantage, predators and other animals tend to have the nasty habit to break and destroy bones, by nibbling, gnawing, digesting or trampling on it, or by removing or mixing the sediment that should protect the bone.

Caves tend to have a very restricted catchment area of their "victims", delimited for example by the range of the predators living in them, or the area where entrances to the cave form pitfall-traps - that makes them suitable to reconstruct the conditions and species assemblage of a spatial delimitated ecosystems.
Caves and fissures can also gather animals' remains over a certain time interval, until they became filled or obstructed, under optimal conditions this enable a temporal reconstruction of the changes experienced by the ecosystem. Unfortunately homogenous sediments, produced by roof collapse or debris flows, tend to mix bones and remains of different time periods together, enabling a general but not time specific reconstruction of an ecosystem. Despite this inconvenient, caves are still important for palaeontology by their effect to gather, accumulate and perfectly preserve bones.

Fig.8. Generalized section of a cave as a sedimentary system. The numbers indicate different forms of accumulation of bones and sediment:
1. Breccias formation under a pitfall trap
2. and 3.Waterlain silts;
4. Den accumulation of bones;
5. Accumulation of bat remains beneath roosting area in the cave roof and accumulation of small mammals beneath owl roosting/nesting area in the cave roof;
6. Water transported mud from further inside the cave;
7. Speleothems;
8. Roof fall; Accumulation of bones of cave bear, died during hibernation in the deeper regions of the cave;
9. Former lower chamber of the cave that acted as a natural trap/ accumulation room of amalgamated debris flows.

The sedimentation rate in caves is irregular and very low, however bones, even when exposed, can survive in caves for much longer time destruction or degradation as compared to the surface, until they become finally embedded.

"In all these cases, the bones found in caverns are never mineralised, but simply in the state of grave bones more or less decayed or incrusted by stalagmite"
(BUCKLAND 1824 pag.10)

The preservation of bones is helped by the stable conditions found in caves - bones are sheltered from atmospheric processes, and there are no or only minor changes of temperature, humidity or chemical parameters in the environment of the cave. The high content of dissolved minerals in the percolating water also helps to conserve skeletal remains by limiting chemical alteration. The oversaturated water is unable to dissolve more carbonate, and is so harmless to the bone-structure.

An inaccessible cave also protects bones and a carcass from large scavengers, decomposition occurs mainly by bacteria and smaller invertebrates - organism unable to disarticulate bones. Under optimal conditions, like no subsequent water transport, the result is a fully articulated skeleton, however such findings are still exceptional events.
In caves with speleotherms formation bones can become entrapped, fixed and in a certain manner "mummified" in their position by the new formed rock.

"The effect of the loam and stalagmite in preserving the bones from decomposition, by protecting them from all access of atmospheric air, has been very remarkable; some that had lain uncovered in the cave for a long time before the introduction of the loam were in various stages of decomposition; but even in these the further progress of decay appears to have been arrested as soon as they became covered with it; and in the greater number, little or no destruction of their form, and scarcely any of their substance, has taken place"
(BUCKLAND 1824, pag.12-13)

Fig.9. Bones of Cave bear (Ursus spelaeus) found in the Conturines Cave. The bones in this cave were scattered around on the floor or embedded in loose dolostone sand with pebbles. Some of the fossils became incorporated or covered by concretions and flowstones. Because of the inaccessibility of the cave and an observed bias in the age structure of the remains, they came from either young or very old individuals, it is thought that this cave served for hibernation, and eventually weak animals died during their winter sleep and their bones becoming incorporated in the cave sediments.

In even rarer circumstances the air circulation inside a cave can completely dry out a body, preventing even the decomposition work of bacteria - the result is a true mummified body.

Bone beds in caves were found and described all over the world, from the first scientific research in Victorian England to the studies of the chronology of extinction deduced from the remains of the Australian Megafauna conserved in caves, from the presumed bones of Cyclopes found in caves of the Mediterranean Sea to bones in mountain caves attributed to mythical dragons.
Beyond their mystical significance, caves have proven in the past, and also in the present, to be a stroke of luck for geologists and palaeontologists in the attempt to reconstruct past ecosystems and their inhabitants.

Bibliography:

BUCKLAND, W. (1823): Reliquiae Diluvianae; or Observations on the Organic Remains Contained in caves, Fissures, and Diluvial Gravel, and on Other geological Phenomena, Attesting the Action of an Universal Deluge. John Murray. London: 303
GUNN. J. (ed.)(2006): Encyclopedia of Caves and Karst Science. Taylor and Francis Group, New-York, London: 1940
KOE, A.M. (2003): Pre-burial taphonomic characterisation of a vertebrate assemblage from a pitfall cave fossil deposit in southeastern Asutralia. Journal of Archaeological Science 30: 769-779
ROSENMÜLLER, J.C. (1795): Beiträge zur Geschichte und nähern Kenntniß fossiler Knochen. Georg Emil Beer, Leipzig.
RUDWICK, M.J.S. (2008): Worlds before Adam - The Reconstruction of Geohistory in the Age of Reform. The University of Chicago Press: 614
SASOWSKY, I.D. & MYLROIE, J. (ed.) (2007): Studies of Cave Sediments. Physical and Chemical Records of Paleoclimate. Springer: 329
PAVIA & ZUNINO (2009): Giornate di paleontologia IX edizione Apricena (FG), 28-31 Maggio 2009 - Guida alle escursioni 30 e 31 maggio 2009.
OSTBYE, E.; LAURIRTZEN, S.E.; MOE, D. & OSTBYE, K. (2006): Vertebrate remains in Holocene limestone cave sediments: faunal succession in the Sirijorda Cave, northern Norway. Boreas, 35: 142-158