| Title Information | |
 |
MEHL-JANUSSEN, Dorte:Die frühe Evolution der Porifera: Phylogenie und Evolutionsökolgie der Poriferen im Paläozoikum mit Schwerpunkt der desmentragenden Demospongiae (Lithitide)1999. [in German] – 72 pp., 9 figures, 1 tables, 13 plates. ISBN 978-3-931516-61-1 Euro 50.00 |
The taxon Porifera as the earliest definite Metazoan phylum is documented from ca. 545-543 Mio. Y. old sediments, e.g. of Ediacara: Palaeophragmodictya, a hexactinellide-like early sponge from these strata may not have possessed any mineralized spicules at all. Tiny fossils from the ca. 580 Mio. Y. old Duoshantuo-phosphorite have been interpreted as juvenile sponges, in which case they belong to the Demospongiae. However, the clade Porifera most probably separated from the Eumetazoa at least 200 Mio. Y. earlier. During the earliest Cambrian the Hexactinellida were predominant, but already from the Atdabanian, the taxon Pinacophora has become highly diverse, e.g. the demospongide group Astrophora, including the ecologically flexible Geodiidae. During the Early Cambrian, the Archaeocyatha flourished; they are here regarded as stem lineage representative of the Pinacophora. The first major radiation of the Demospongiae took place during Middle Cambrian. At this time the earliest desma bearing demosponges appeared: the Anthaspidellidae, which reached their maximal diversity during the Ordovician and became definitely extinct by the end of the Permian. From the Anthaspidellidae the group Chiastoclonellidae evolved and from this taxon probably the Hindiidae and the Sphaerocladina derived. On the other hand, the Tetracladina derived independently from the Palaeozoic »Lithistida« during the Late Permian or Early Triassic. Most probably no Palaeozoic »lithistid« group, except maybe the Sphaerocladina, continued into the Mesozoic, but they all became extinct at latest by the end of the Permian. The Sphaerocladina, which recently have been documented also from the Late Palaeozoic, may have continued in the shape of the Jurassic Neosphaerocladina, which had become extinct by the end of the Mesozoic era. From the Early Carboniferous the Plakinidae are documented, which lead to the modern Homoscleromorpha, probably sister taxon of the Spirasterophora. All modern representatives of the desma bearing Demospongiae are relatively young taxa, known not until the Triassic when spirasters and phyllotriaenes appeared. The group of heloclone or rhizoclone bearing Saccospongiidae/Haplistiidae most probably has no phyletic connection to the modern Megamorina or Rhizomorina, respectively. It can also be assumed that no such connection existed between the Palaeozoic Tricranocladina and the modern Corallistidae eigther. The Calcarea are documented from the Early Cambrian, including both exclusively Palaeozoic groups, such as the Polyactinellidae and Heteractinellidae, and modern appearing genera, such as the pharetronide Gravestockia. Massive calcitic spicules, probably of three rayed symmetry, belong to the basic pattern of the Calcarea. As the Early Cambrian Polyactinellidae already showed triradiate spicular symmetry, this group may have been closer to the Calcarea stem lineage than the Heteractinellidae, whose polyactine spicules are here considered to be a derived character. The monophylum Calcaronea/»Calcinea« is definitely documented since the Devonian, but the exact age and phylogenetic origin of this group are still unknown. Like today, the Palaeozoic calcareous sponges, especially the Heteractinellidae and »Inozoa«, inhabited mainly warm waters of shallow to mid-shelf areas. The Polyactinellidae preferred the pelagic environments in interreef basins and on distal ridges and plateaus. The Cambrian Chancelloriidae remain a controversal group with respect to their systematic position; according to the author, this group does not belong to the Porifera. Chancelloriids had their niche in areas of the non-turbulent shallow subtidal, commonly associated with archaeocyathids. During the Cambrian and Early Ordovician, the »soft demosponges« were hardly ecologically specialized, however, since the Late Ordovician their evolution was confined predominantly to the outer shelf and the slope. Also the desma bearing Demospongiae appear to have preferred this environment, whereas the Hexactinellida thrived mainly at the outer slope and in the basin. Since the Late Palaeozoic, the Plakinidae, precursors of the modern Homoscleromorpha, also evolved in deep water environments. However, during the entire Palaeozoic, generalists repeatedly occurred within all major groups of siliceous sponges. These generalists were comparably flexible, and e.g. during the Permian, they were able to replace the Calcarea as inhabitants of shallow shelf environments.
| Copyright © 2012 Verlag Dr. Friedrich Pfeil |