Plant and vertebrate remains from the sites of Detan, Dvérce, Valec, Vrbice, and others are interbedded in the lowermost layers of the Doupov Mountains stratovolcano, which were originally exposed at Altes Heu (Laube 1899). The locality of Altes Heu, famous as the site near Valec originally considered to be Miocene, is close to a drainage cutting through strata of the same subaerial volcanic tuffs exposed in 1970 at the Detan clay pit, ≈300 m to the west. At Dvérce-Wärzen (Wenz 1917), mammal fossils (most importantly, the small anthracothere Elomeryx crispus crispus) are found in irregular lenses of tuffaceous limestones rich in lacustrine molluscs and with rare freshwater fishes, imprints of leaves, calcified wood, and casts of fruits and rare molds of hickory (Carya) nuts ( Figure 2).

The most productive site for mammalian remains is situated in a large clay pit south of the village Detan. The layer of white sandy kaoline clay is covered by 45 to 50 m of basaltic tuff ( Figure 3 ). The tuff-clay contact is sharp in most places. The basal beds of the tuff sequence exhibit both subaerial and lacustrine facies along the flat southern slopes of the volcano. All tuff layers are altered and slightly calcareous. Montmorillonite is the most prevalent diagenetic clay mineral. Altered books of biotite and drops of volcanic glass are locally present in the fossiliferous beds.

Aquatic molluscs are missing from the fossiliferous level in the Detan clay pit, although they are abundant in the nearby fine-grained lacustrine facies found at Valec and Dvérce. Leaf imprints are rare, but parts of calcified woody roots, some in situ, are common in some parts of the outcrop. Small pieces of wood charcoal are present but very rarely. Aragonite pseudomorphs, apparently of wood branches and stems (diameter = 100 to 150 mm) are occasionally found.

Mammal bones at Detan are extremely fragile, fragmented but not abraded, and widely scattered through the basal ash layers. The only example of articulation is a partial vertebral column of a snake in situ. Non-mammalian remains are rare. In the upper part of the ash sequence several large, thick, plastron plates of the giant turtle Geochelone were preserved. A small crocodilian is represented by a dermal plate. Bones and dentine of mammals are white, but enamel is light brown with black dendrites.

Age and Correlation

The biochronologic age of the fauna is determined by the presence of significant mammalian taxa (Table 1, Figure 4,Figure 5), especially the rodents Eucricetodon, Paracricetodon, Pseudocricetodon, Eomys, Plesispermophilus, Palaeosciurus, Suevosciurus, and the artiodactyls Gelocus, Entelodon, Lophiomeryx, Anthracotherium, and Elomeryx. This assemblage indicates an age in the mammalian Paleogene zone MP 21. Arguing for placement in the older portion of MP 21 are the index species Entelodon antiquum and the general evolutionary level of some rodent species. MP 21 falls within the early Oligocene, in the Stampian (Suevian) stage, and after the Grand Coupure. A K/Ar date of 37.5 Ma was determined for Biozone MP 21 from unweathered biotite from the mammal-bearing horizon at Detan (Fejfar 1987, Fejfar and Storch 1994). Correlated European localities include Hoogbutsel (Belgium), Ronzon, Aubrelong 1, Villebramar, and Soumaille (France), and the Bavarian karst fissures of Möhren 13 and 19 (Fejfar 1987).

Insect Activity at Detan

In the main fossiliferous layer of Detan, in the lower part of the ash sequence, local concentrations of peculiar drop-like natural casts (diameter = 5 to 8 mm) are found. These trace fossils are apparently brood cells or pupal cocoons of solitary burrowing hymenopteran insects (cf. Sphecoidea or Halictini; Figure 6). Similar trace fossils were recognized at Laetoli, Tanzania (Ritchie 1987).

A unique black lens (about 1.5 m wide, 0.6 m high) was discovered in the lower part of the ash sequence during fieldwork in 2000. The shape of the lens is oval, with a distinct reddish zone on its surface (Figure 7). The lens matrix is composed of hard, calcareous, black Mn-Fe oxides, with less resistant pockets of sediment. The lens contains shiny black shells of terrestrial molluscs, which are otherwise completely unknown in the Detan site, along with a concentration of crushed small bones and rare rodent molars, both light gray in color. Within the black lens, the most prevalent mammal is the small dormouse Gliravus and Bransatoglis. Paracricetodon, Pseudocricetodon, Eomys, and indeterminate insectivore molars are rare and partly crushed. The lens is interpreted as the remnant of an insect colony, possibly that of termites, because within the hard matrix of the lens, passages reminiscent of those in termitaries occur.

Bone and dentine surfaces from Detan show peculiar traces on unweathered surfaces (Figure 8, Figure 9, Figure 10, Figure 11). These traces are unlike rodent gnawing or corrosion from plant roots. They appear most similar to marks seen on bones found in the Pliocene tuffaceous Upper Laetolil beds (3.46 Ma) of Tanzania (Sands 1987, Watson and Abbey 1986). For comparison, Figure 12 shows a specimen in the Natural History Museum (Berlin) collected by the 1934-1936 Kohl-Larsen expedition from the Pliocene tuffaceous beds in the southern Serengeti.