Abstract
back to Publications Krautter, M. (1997): Aspekte zur Paläökologie postpaläozoischer Kieselschwämme. - Profil 11: 199-324, 79 figs.; Stuttgart.
Aspekte zur Paläökologie postpaläozoischer Kieselschwämme
Abstract: Sedimentary rocks, characterized by siliceous sponges occur several times in earth history. The spongiolithic facies culminates in Late Jurassic. On the northern shelf of the Tethys and in the adjacent northatlantic basins, siliceous sponges formed a reef belt extending over more than 7000 km.
Aim and scope of this study is the palaeoecology of postpaleozoic siliceous sponges in order to get a valuable means for the interpretation of siliceous sponge-bearing rocks. To reach this aim, the actualistic or uniformitarianistic concept was used the first time for the group of sponges. The knowledge of the biology and the ecology of recent siliceous sponges is an important basic prerequisite to transform the results to the fossil record.
The heterogeneous group of siliceous sponges belong either to the desma-bearing demosponges ("Lithistida") or to the hexactinellida. Both siliceous sponge groups differ fundamentally in their biology (e.g., anatomy, reproduction, nutrition). Consequently, they also differ remarkably in their ecologicals demands.
Ecological effective factors are bathymetry, substrate, water energy, temperature, quantity and quality of food, sedimentation rate, settling density, available space, space competition, water chemistry and reproduction rate. Usually, these factors do not effect separate. Their interaction decides about the spatial distribution and the dominance of siliceous sponge groups.
Most demosponges are active filter feeders. Due to the size of their ostia, bacteria are their main food source. Demosponges are able to host bacteria to a great amount in their mesohyl. Most of these sponges can live either on the hosted bacteria itself or on the metabolic products of the bacteria.
In the water column, the vertical distribution of bacteria differs considerable. Towards greater depths, they decrease gradually. Therefore the bathymetric distribution of filter feeding demosponges is strongly influenced by the bathymetric distribution of bacteria and these sponges also decrease towards greater depth.
At low nutrition rates demosponges enlarge their mesohyl so that more bacteria can be stored. In morphoconstant demosponges, this strategy results in a thickening of the sponge walls in order to increase the available volume. Morphovariable sponges are able to completely change their form and will preferably develop thick knob-like or sturdy tube-like morphologies. Due to this ability, morphovariable demosponges reach a certain independancy of exteral food supply and are able to settle in greater depths, in which they normally are not to be found. Morphoconstant taxa cannot react in this way and they are directly linked to the vertical distribution of the bacteria.
Hexactinellid sponges are poor filter feeders. This is caused by the organisation of the sponge tissue. On the other hand, this organisation allows them to absorb colloidal organic matter and/or dissolved amino acids, which represents their predominant feeding strategy. This substances are enriched in quiet waters below the zone of the highest biomass productivity. Therefore, the deeper parts of the oceans are the main area of spatial distribution of osmotrophic sponges.
If this type of food is scarce, hexactinellids tend to reduce wall thickness which improves the intense overall contact wih sea water. Moreover, morphovariable taxa enlarge their surface by developing thin plate- or dish-shaped forms. Among morphoconstant taxa dish-shaped forms have an adaptional advantage in these setting and will outcompete other forms.
Reduced sedimentation rate is a general precondition for the establishment of sponge communities, although sponges can adapt to a certain degree of sedimentation. Elevated sedimentation favours tube-shaped sponges, since a narrowed osculum produces a bundled exhalant water current which shelters the animal from the settlement of sediment particles. As only morphovariable sponge are able to this reaction, morphoconstant sponges cannot survive in such environments. Due to the spatial separation of the different kind of food, there is also a spatial separation of hexactinellids and demosponges.
Basin architecture and sediment input may allow that hexactinellids and demosponges occur together. Nutrition for both siliceous sponge groups has to be sufficiently available. This is only the case in areas with very restricted water exchange and depths not below 200 m.
The correctness of the results is checked at the spongiolithic Yátova Formation (Oxfordian, Late Jurassic) of eastern Spain.
Richness of hardgrounds, automicrites framboidal pyrite and glauconite, very reduced sediment thicknesses as well as overall facial character, abundance, low diversity and uniformity of benthic fauna over a minimum area of 75000 qkm suggest a moderately deep, uniform low-energy ramp setting with extremely reduced carbonate and terrigeneous background sedimentation. Reduced carbonate input is due to the lack of a carbonate factory in more proximal areas of the Iberian basin. Marine surface currents, parallel to the shoreline catch the terrigeneous siliciclastics, transport them in southward direction and keeps the Iberian Basin free from higher sedimentation rates.
The very reduced influx is interpreted to have resulted in a very low nutrient level, which is reflected by a strong reduction in filter-feeding benthic organisms such as bivalves, crinoids, brachiopods or serpulids frequent in other Late Jurassic sponge settings. The sponge fauna itself is characterized by the almost exclusive occurrence of a uniform low-diversity, specimen rich fauna of hexactinosan dish-shaped sponges, uncommon in most other Late Jurassic sponge faunas.
These observations indicate that the major factors controlling the morphological and taxonomic composition of siliceous sponge faunas are sedimentation rate and food supply