- It is widely accepted that the order Lithistida is polyphyletic. The group as a whole is defined by the possession of fused and complexly interwoven spicules called desmas that are formed by successive deposition of silica around a spicular core. This additional silicification also fuses the desma to its neighbors. Depending on the group, the desmas can be highly irregular or follow a basic plan, i.e., being monocrepid, tetracrepid, etc. Desmas apparently evolved independently on two or more occasions, but all lithistids have traditionally been grouped together. This has been more a matter of practicality than anything else, because our knowledge of lithistid anatomy, ontogeny, and phylogeny has been too poor to allow any meaningful separation between the groups.
- Demospongiae of diverse origin which have developed a spicule type known as a desma. This can be either monaxonid (monocrepid) or tetraxonid (tetracrepid) in derivation, but always has complex branching pattern and these branches interlock to form an extremely hard stony skeleton. Other megascleres, and various microscleres, accompany the desmas and it is largely on the basis of accessory spicule composition, and disposition, that attempts have been made to assign lithistid genera to other orders of the Demospongiae. No complete re-study of this group has been made, and attempts to divide the group are premature at this time. For the most recent subdivision of the Lithistida the reader is referred to Levi (1973). Because of interlocked desma skeleton, lithistid sponges are very common as fossils and frequently preserve their overall shape.
Treatise on Invertebrate Paleontology, Part E (1955):
- Sponges characterized by lumpy spicules called desmas, and these generally so interlocked or cemented that rigid frameworks result. In nearly all lithistid sponges some spicules are comparatively simple and regular in form; typical microscleres also occur, but these are almost invariably lost from fossils. Lithistids are represented in the Cambrian but are more abundant in the Ordovician and subsequent systems; they are most abundant in Cretaceous rocks. Doubtless in the past, as now, other orders were much more common, but lithistids are most apt to be recognizable fossils.
- Original Text: Die Trennung dieser Gruppe von den Hexactinelliden, mit denen sie bisher vereinigt waren (als Coralliospongia Gray oder Vitrea W. Thomson) ist oben vorbereitet worden und sich nun durch die nahere Darstellung vollends als unabweisbar rechtfertigen. Wir verstehen darunter die Spongien mit zusammenhangendem Kieselgewebe, dessen Fasern nicht nach dem dreiaxigen Typus wuchsen, sondern ein scheinbar ganz regelloses Gewirr bilden. In diesem ist in der Regel eine centrifugate und eine concenirische Hauptrichtung nicht zu verkennen, worin sich jedoch, wie aus dem 1. Abschnitte hervorgeht, nicht der Einfluss eines dominirenden Nadeltypus ausspricht, sondern die Anpassung an die allgemein gultigen Stroemungsverhaltnisse. Obschon auch ihre Sarcode Eigenschaften hat, welche sie einigermassen den Hexactinelliden und mit ihnen wahrscheinlich den fossilen Spongien an. In der ausseren Korperform ist innerhalb der Familie keine Uebereinstinnung, doch sind schussel und loffelforanige Arten haeufig.
- Translation: The separation of this Hexactinellid group with which they have been previously united (as Coralliospongia, Gray, or Vitrea, W. Thomson) has been prepared above (under Lithistidae) and now justified by the representation as completely irrefutable. We understand the sponges with pebble fabric whose spicules did not grow with the usual 6 rays, but instead form a seemingly disorderly tangle. They usually grow in a concentric main direction, and do not have a dominant spicule type. Although their features have properties of the Hexactinellid and the group of fossil sponges, the Lithistidae body shape is only found within this family, as they have a unique character.