The Castlecomer Fauna, a Konservat-Lagerstätte from the Upper Carboniferous of Ireland, is dominated by two species of exceptionally preserved spinicaudatan conchostracan branchiopod crustaceans; over 300 articulated specimens are known from short lengths of core from two boreholes. The spinicaudatans were entombed at various angles to bedding in thin event beds deposited in a paralic environment. Cuticle is the only tissue represented. The fossils are essentially two-dimensional as specimens collapsed following decay-induced loss of the structural strength of the cuticle. Some three-dimensionality was retained, however, as clay-grade material, probably sediment, infiltrated the carapace during or after burial, preventing the valves from being compacted to the same plane as the body of the specimen. Cuticle-lined voids created by decay of the labile tissues (notably within the tubelike antennal rami) were lined or infilled by authigenic clay minerals that were subsequently metamorphosed. The antennae, mandibles, distal parts of the claspers, and the telson are preserved in excellent detail, often retaining all or most of their original relief. Other anatomical features (antennal protopods, head, proximal part of claspers, trunk appendages) are usually preserved in two dimensions, either as a film of carbon or a dark-colored stain. The antennules, maxillules, and maxillae are rarely preserved. Decay experiments confirm that this variation reflects differences in the recalcitrance (thickness and degree of sclerotization) of the cuticle that covered different parts of the body. Complete decay of certain appendages precedes appreciable deterioration in the morphology of others. Some of the disparity exhibited by exceptionally preserved fossil arthropods is likely to be taphonomic rather than biological in origin.