The Eurypterid Endostoma and Its Homology with Other Chelicerate Structures

The endostoma of eurypterids is a small ventral structure previously known from only 11 specimens and in detail from only a single species, Eurypterus tetraganophthalmus. Four previously undescribed eurypterid specimens from the Samuel J. Ciurca Jr. Collection, Yale Peabody Museum of Natural History, likely belonging to Eurypterus, and a previously documented specimen of the pterygotid Acutiramus cummingsi in the New York State Museum, are considered here to reinterpret the morphology and homologies of the endostoma. We demonstrate that the endostoma is a bilateral, flaplike appendage, with distal margins bearing anteriorly facing setae. An anterior embayment and posterior medial notch are noted. The latter represents the point where the endostoma articulates with an oval sclerite. Here we propose that, contrary to previous interpretations, the endostoma is an appendage, likely of the seventh body segment, and is probably homologous to the xiphosuran chilaria. This has direct implications for the hypotheses of homology for the metastoma, a ventral structure common to eurypterids and chasmataspids.