Previous Work
In 1847, C. G. Giebel reported the finding of some enigmatic fossil molds in the Cretaceous sandstones around Salzburg. He described the genus Sidetes Giebel, 1849, two years later, concluding that these structures were the aptychi of Sepia Linnaeus, 1758 (Giebel, 1849). The specimens he observed were semicircular, and marked with fine concentric lines.
Later, John M. Clarke (1882) described several odd fossils from Naples, New York, found in Givetian and Frasnian age shales of the Hamilton and Genesee formations. He collected thirty specimens over the course of several years, but remained doubtful as to their biological affinities. The fossils ranged in size from a few millimeters in breadth to as much as ninety millimeters, were flattened elliptical bodies marked with concentric lines or ridges, and bore a wedge-shaped cleft. He concluded they were not brachiopods as they were too large and did not display any trace of a corresponding ventral valve.
Clarke's descriptions were based on what he thought were incomplete specimens, consisting of isolated carapaces. He noted their general similarity to the Silurian arthropod genus Discinocaris Woodford 1856, and suggested these new specimens belonged properly with the phyllopod crustaceans, and that the discovery of a complete specimen would come with time. He assigned them to two crustacean genera, Spathiocaris Clarke 1882, and Lisgocaris Clarke 1882.
At the same time, Whitfield (1882) found similar fossils, which he referred to the crustacean genus Plumulites Barrande 1872. These specimens were all recovered from the Cleveland Shale in Erie County, Ohio.
Over the course of several decades of work with the British Museum, Henry Woodward had occasion to describe many small fossils of similar type. He proposed (Woodward, 1865; and see Woodward, 1885a) that some of these "ink-flecks" were chiton plates, while others were isolated barnacle plates, for which he proposed the genus Turrilepas Woodward, 1865. He subsequently (Woodward, 1882) referred similar specimens from the Devonian of Büdesheim, Germany to the new phyllopod genera Cardiocaris Woodward, 1882, and Pholadocaris Woodward, 1882, and a specimen from the Silurian of Wales to the phyllopod genus Aptychopsis Barrande, 1872. Later, he (Woodward, 1885b) agreed that some such fossils may be cephalopod aptychi, but that others were certainly phyllopod carapaces.
Ruedemann's discovery, in 1901, of a very large brachiopod in the "Hudson river shales" [sic] of New York prompted Clarke to summarize 25 years of collecting Spathiocaris and similar fossils (Clarke, 1902). He observed that none had yet been discovered with abdominal fragments, and a specimen of Spathiocaris had been found in the body chamber of the Devonian goniatite Manticoceras intumescens in Germany (Kayser, 1882), and he had earlier illustrated a similar occurrence of a "phyllocarid" (Dipterocaris Clarke 1883) within a goniatite from the Naples (Portage) shales of New York (Hall & Clarke, 1888, pl.35). His conclusions at this time were that Spathiocaris and similar forms were probably cephalopod aptychi, but that Discinocaris was perhaps a brachiopod. He gave this interpretation for the latter as its occurrence in the Silurian preceded the appearance of the ammonites.
At about the same time, other dark shales were being studied. Girty's monograph (1909) on the Caney Shale (Devonian-Mississippian) of Oklahoma included a new genus, Idiotheca, which Girty hesitantly described as a conulariid. He briefly stated other possibilities, including its interpretation as a cephalopod aptychus. He ruled out the possibility of its being an inarticulate brachiopod.
Later, Ruedemann (1916) described four new species of Spathiocaris from New York and northeastern Ohio. He suggested several reasons to consider these fossils as belonging to the Cephalopoda, observing that the method of growth seen in Spathiocaris and related genera is similar to that seen in aptychi and not like that of brachiopods or arthropods. He supposed that the horny anaptychus would logically precede calcareous diaptychi in the evolution of these structures, so the presumed lack of diaptychi from the Paleozoic does not mean that spathiocarids could not have been anaptychi. Finally, he suggested that similar structures would, "also have existed in the Ordovician and Silurian cephalopods and thus account for those earlier anaptychi considered as Discinocarina [sic]" (Ruedemann, 1916, p. 102).
Six new species of Spathiocaris were described by C. L. Cooper (1932) from the Woodford Formation of Oklahoma, including the redescription of Girty's Idiotheca specimen as the new species Spathiocaris woodfordi. The Woodford is an interbedded black shale/chert unit of Late Devonian to Early Mississippian age. Many of Cooper's descriptions are similar to species from the east. He described the fauna strictly as crustacean.
Ruedemann (1934) expanded upon the idea of Spathiocaris as a cephalopod aptychus, citing as evidence Matern's (1931) discovery of Spathiocaris koeneni Clarke, 1884 within the body chamber of Crickites holzapfeli Wedekind, 1913, a European Devonian goniatite. It is unlikely that this represents the preservation of a phyllocarid preying upon a goniatite. The suggestion was made by Matern, and echoed by Ruedemann, that the anaptychi were separated from the conchs as the cephalopods decomposed while still buoyed by gases in the conch.
In his description of the New Albany Shale of Indiana, Campbell (1946) mentions several thin beds within this Devonian black shale as "Spathiocaris beds." He suggested these horizons, where these fossils were locally abundant, might be useful in stratigraphic correlation. Unfortunately, such occurrences are too rare to be useful (Lineback, 1970; Hasenmueller and Leininger, 1987).
Spathiocaris has also been identified in drill cores from western Canada (Copeland and Boulton, 1960), along with a phyllopod (crustacean) telson. This last has been removed from association with Spathiocaris and redescribed as Montecaris (Pratt, 1987).