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INTRODUCTION
THIS PAPER presents sponges from the Late Cretaceous El Rayo Formation, Puerto Rico. Siliceous sponges are common fossils in the Late Cretaceous of Europe (see for example Schrammen, 1910-1912; Moret, 1926; Weidenmayer, 1994; Pisera, 1999). So far only rare siliceous sponges from the Late Cretaceous of the Caribbean region have been reported: lithistid Uerea Lamouroux, 1821, hexactinellids Ventriculites Mantell, 1822, and Plocoscyphia Reuss, 1846 from Trinidad (Thomas, 1935; Trechmann, 1935), and Callopegma Zittel, 1878 from the Cariblanco Formation, Puerto Rico (Howell, 1966). The sponges studied by us are heavily silicifled and thus only their approximate determination was possible, but among bodily preserved sponges they are undoubtedly lithistids with tetraclone and rhizoclone desmas. Among loose spicule material, fragments of hexactinellid skeleton dominate, tetraclone and dicranoclone lithistids desmas are common, and hexactinellid lychniscosid skeletons are very rare.
GEOLOGICAL SETTING
The island of Puerto Rico is the easternmost island of the Caribbean Greater Antilles (Fig. 1.1), a complex island arc with accreted terranes. The island is composed of Lower Jurassic to Lower Cretaceous ophiolites, Lower Cretaceous to Eocene island arc volcanic and sedimentary rocks, Middle Oligocene to Pliocene terrigenous elastics, and limestone unconformably overlying the older rocks along the north and south coasts (Santos, 1999). The lire-Oligocene rocks are divided into the southwest, central, and northeast igneous provinces (Fig. 1.2).
The paleogeographic setting of the earliest island arc volcanism differed in each of the three igneous provinces (Santos. 1999). The southwest igneous province is separated from the rest of the island by the Great Southern Puerto Rico Fault Zone (Fig. 1.2). It contains a basal "Bermeja Complex." which is characterized by linear bodies of serpentinite, blocks of mafic metamorphic rocks, and cherts of Pleinsbachian to Aptian age (Schellekens et al.. 1993; Schellekens. 1998). This basal complex is unconformably overlain by a thick sequence of Santonian to Maastrichtian limestones and mudstones interbedded with volcanic rocks (Santos. 1999: Martínez, 2003).
Cretaceous rocks of southwestern Puerto Rico are dominated by volcanic and volcaniclastic facies, reflecting the back-arc setting of this depositional basin and proximity to active Cretaceous volcanic centers. Large-scale tectonic activity and rapid volcaniclastic sedimentation prohibited stabilization of structural platforms and carbonate sedimentation during much of the island history. But relatively short intervals of carbonate platform and slope development...