Upper Silurian (Ludlovian) limestones from the Heceta Formation of southeastern Alaska (Alexander terrane) contain skeletal stromatolite reefs and stromatolite mud mounds that were colonized by sphinctozoan sponges. Internal growth cavities, synsedimentary marine cements, and stromatolite debris flows in slope deposits indicate that these reefs grew with relief at the seaward margin of the carbonate platform. The biotas under study have evolutionary significance because they contain the earliest widespread reef-building sphinctozoans and represent a previously unrecognized stage in the evolution of Phanerozoic microbial-metazoan (predominantly poriferan) reefs. On the basis of the presence of Silurian stromatolite-sphinctozoan deposits in southeastern Alaska, southwestern and west-central Alaska, and the Urals, this study also establishes a Late Silurian paleobiogeographic connection between the Alexander terrane, cratonic northwestern North America, and the Uralian region.

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