412 FOSSIL INVERTEBRATES/Porifera sponges are exclusively marine and are most common in shallow tropical environments Those included in the Orders Murrayonida and Lithonida are the traditional ‘pharetronids’ and the other orders are the traditional ‘inozoids.’ The class has a geological record ranging questionably from the Late Precambrian, but certainly from the Lower Cambrian to the Recent or Holocene Class Calcarea Bowerbank 1864, Cambrian– Holocene Subclass Calcinea Bidder, 1898,?Precambrian, Cambrian–Holocene Order Clathrinida Hartman, 1958, Holocene Order Murrayonida Vacelet, 1981,?Precambrian, Cambrian–Holocene Subclass Calcaronea Bidder, 1898, ?Cambrian,? Triassic, Jurassic–Holocene Order Leucosoleniida Hartman, 1958, Holocene Order Sycettida Bidder, 1898, Holocene Order Sphaerocoeliida Vacelet, 1977, Cretaceous Order Lithonida Doederlein, 1892, Jurassic– Holocene Figure Representative fossil demosponges (A) Phymatella, Cretaceous, Germany (B) Aulocopoides, Devonian, Western Aus tralia (C) Camellaspongia, Ordovician, Minnesota, USA (D) Car yospongia, Silurian, Tennessee, USA All approximately normal size Adapted with permission from Rigby JK (1987) Phylum Porifera In: Boardman RS, Cheetham AH, and Rowell AJ (eds.) Fossil Invertebrates Palo Alto, Oxford, London: Blackwell Scientific Publications Hexactinellid sponges are exclusively marine forms and, in modern seas, they are most common on seafloors from 200 to 2000 m deep, although many species have been reported from lower bathyal depths They also occur, but are less abundant, in hadal depths over 6000 m deep, where they may form dense reeflike clusters On the other hand, they are known to range up into water as shallow as 25–30 m off the south-western Canadian Pacific Coast, where they also form distinct reef-like structures Calcarea The Class Calcarea includes sponges with calcareous skeletal elements that range from those with distinct three-rayed spicules of calcite or aragonite (Figure 2), to those with rigid skeletons of fused polygonal elements or imbricate calcitic plates Living calcareous Many genera of chambered fossil sponges with calcareous skeletons (Figure 7) had been included in the Calcarea, in the Order Sphinctozoa, until recently, when the polyphyletic origins of the sphinctozoans and their development principally as a structural grade were recognized They are now treated as a group of ‘hypercalcified’ sponges drawn from other classes Sphinctozoan grade sponges appeared in the Middle Cambrian, but played a minor role until the Carboniferous and Permian, when they helped to produce massive reefs, such as those in the Guadalupe Mountains in western Texas and south-eastern New Mexico, and in Tunisia and China, and during the Triassic, when they were similarly involved in the construction of reefs in what is now the Alpine region of southern Europe They were thought to have become extinct by the end of the Cretaceous, but a living form was discovered in 1977 Their chambered construction, in which ‘living space’ was limited by an outer rigid skeleton, required that new chambers be added for growth of the sponge Such chambered skeletons developed at various times in several groups, so that a few calcareous and hexactinellid genera and many demosponge genera all had ‘sphinctozoan’ skeletal patterns Chambered hexactinellid ‘sphinctozoans’ have been recognized, for example, because their chambered skeletons contain reticulate hexactines Chambered demosponge sphinctozoans with rigid skeletons of aragonite or high-magnesium calcite are similarly included here Skeletons of these latter