Title: The structure of Saccospongia laxata Bassler (Ordovician) and the phylogeny of the Demospongia
Abstract: The demosponge, Saccospongia laxata Bassler, from the Ordovician (Trentonian) of Tennessee, is redescribed. It has a spiculation of styles arranged in plumose fashion in spicule tracts that form a reticulate net. Further, each tract is coated with a layer of tan- gentially arranged heloclonid desmas. This structure has some parallels in living desmacidon- tids. The complexity of its structure is unparalleled in any other Ordovician demosponge. Since all known pre-Carboniferous demosponges have only monaxon spicules, it is suggested that the primitive spicules of the Class Demospongea were monaxonic and that tetraxons arose subsequently in certain lines. The suggestion is made, based on this hypothesis and on the fossil record, that a line, or lines, ancestral to the living Sigmatosclerophora, Keratosa, Axinellida, Clavulina, and possibly Helomorina, extends back through, or close to, Sacco- spongia; that the Orchocladina and Rhizomorlna form separate, exclusively monaxonic lines; and that the Paleozoic Eutaxicladina and Sphaerocladina are ancestral to most of the later lithistids with monocrepid desmas and tetraxial dermalia, as well as being close to the an- cestry of most other sponges with tetraxon megascleres and astrose microscleres.
Publication Year: 1967
Publication Date: 1967-09-01
Language: en
Type: article
Access and Citation
Cited By Count: 22
AI Researcher Chatbot
Get quick answers to your questions about the article from our AI researcher chatbot