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There are thousands of problematic organisms — organisms unassigned to a specific phyletic group, or ones that were assigned to different groups by different workers. We have simply picked a few that are particularly distinctive and/or that are important in rocks of hydrocarbon exploration interest. We list prior phyletic assignments and age ranges below and provide descriptions and keys to recognition in the figure captions.

Receptaculitids - grouped with sponges, corals, dasycladacean green algae, or problematica — common from Early Ordovician to Late Devonian, with smaller, more globular forms extending into the Permian

Nuia - grouped with problematic codiacean algae or as an unassigned organism — Late Cambrian-Ordovician

Palaeoaplysina - grouped with sponges, phylloid algae, or hydrozoans — Mid. Pennsylvanian-Early Permian

Tubiphytes - variously grouped with cyanobacteria/blue-green algae, red algae, calcareous sponges, foraminifers, hydrozoans — at least Late Carboniferous to Late Jurassic

Lithocodium - grouped as codiacean algae or loftusiid foraminifers — Late Triassic to Early Cretaceous (Albian)

Hensonella - grouped as mollusks (scaphopods), coralline red algae, or dasycladacean green algae — Cretaceous (Hauterivian-Albian)

A cross section of the wall structure of Calathium sp., with its central cavity and moderately well-preserved radiating wall structure. Calathids are the earliest receptaculitids — they had ovoid or tubular skeletons that strongly resemble sponges (one of the groups in which receptaculitids commonly are classed). The sparry calcite-filled areas (and micrite-filled circles) are recrystallized, originally aragonitic, elongate pillars that constituted the skeletal wall (see Nitecki et al., 1999); the rest of the micritic sediment has filled areas of former void spaces or sites of later-decomposed organic tissues.

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