Hexasterophora are a subclass of glass

sponges Sponges or sea sponges are primarily marine invertebrates of the animal phylum Porifera (; meaning 'pore bearer'), a basal clade and a sister taxon of the diploblasts. They are sessile filter feeders that are bound to the seabed, and ar ...

in the class Hexactinellida. Most living hexasterophorans can be divided into three orders: Lyssacinosida,

Lychniscosida Lychniscosida (sometimes spelled Lychniscosa) is an order of sponges belonging to the class Hexactinellida and subclass Hexasterophora. They are dictyonal sponges (with parenchymal spicules fully fused into a 3D framework) characterized by t ...

, and Sceptrulophora. Like other glass sponges, hexasterophorans have skeletons composed of overlapping six-rayed spicules. In addition, they can be characterized by the presence of hexasters, a type of microsclere (microscopic spicules) with six rays unfurling into multi-branched structures. A living sponge is commonly firmly attached by its base to a hard

substratum Substrata, plural of substratum, may refer to: *Earth's substrata, the geologic layering of the Earth *''Hypokeimenon'', sometimes translated as ''substratum'', a concept in metaphysics *Substrata (album), a 1997 ambient music album by Biosphere * ...

; less often rooted by the anchoring spicules and rarely inserted directly into the loose bottom sediments. The three orders are differentiated by the extent to which adjacent spicules have fused. The oldest order, without fused spicules, is Lyssacinosida (or Lyssacinosa), which appears in the

Ordovician The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era, and the second of twelve periods of the Phanerozoic Eon (geology), Eon. The Ordovician spans 41.6 million years f ...

. The other two orders have fully fused their spicules into a framework of dictyonal strands. The most fused order is Lychniscosida (or Lychniscosa), which develops octahedral frames (lychniscs) around the nodes between their dictyonal strands. Lychniscosa fossils first appeared in the

Middle Jurassic The Middle Jurassic is the second Epoch (geology), epoch of the Jurassic Period (geology), Period. It lasted from about 174.1 to 161.5 million years ago. Fossils of land-dwelling animals, such as dinosaurs, from the Middle Jurassic are relativel ...

. Sponges intermediate between these two extremes were historically placed into the order Hexactinosa (or Hexactinosida). Genetic sequencing has determined that this proposed order is

polyphyletic A polyphyletic group is an assemblage that includes organisms with mixed evolutionary origin but does not include their most recent common ancestor. The term is often applied to groups that share similar features known as Homoplasy, homoplasies ...

, with its constituents split between the valid order Sceptrulophora (bearing sceptrules) and the family Dactylocalycidae (lacking sceptrules). The oldest "hexactinosan" fossils are from the

Devonian The Devonian ( ) is a period (geology), geologic period and system (stratigraphy), system of the Paleozoic era (geology), era during the Phanerozoic eon (geology), eon, spanning 60.3 million years from the end of the preceding Silurian per ...

.''Treatise on Invertebrate Paleontology'' Part E, Revised. Porifera, Volume 3: Classes Demospongea, Hexactinellida, Heteractinida & Calcarea, xxxi + 872 p., 506 fig., 1 table, 2004, availabl
here
.

References

Hexactinellida Sponge subclasses {{Sponge-stub