The horseshoe crab Limulus polyphemus (Linnaeus, Reference Linnaeus1758) is a famous species, renowned as a ‘living fossil’ (Owen, Reference Owen1873; Barthel, Reference Barthel1974; Kin and Błażejowski, Reference Kin and Błażejowski2014) for its apparently little-changed morphology for many millions of years. The genus Limulus Müller, Reference Müller1785 was used by Leach (Reference Leach1819, p. 536) as the basis of a new family Limulidae and synonymized it with Polyphemus Lamarck, Reference Lamarck1801 (Lamarck's proposed but later unaccepted replacement for Limulus, as discussed by Van der Hoeven, Reference Van der Hoeven1838, p. 8) and Xyphotheca Gronovius, Reference Gronovius1764 (later changed to Xiphosura Gronovius, Reference Gronovius1764, another junior synonym of Limulus). He also included the valid modern genus Tachypleus Leach, Reference Leach1819 in the family. The primary authority of Leach (Reference Leach1819) is widely recognized in the neontological literature (e.g., Dunlop et al., Reference Dunlop, Compton and Friederichs2012; Smith et al., Reference Smith, Brockmann, Beekey, King, Millard and Zaldívar-Rae2017). It is also the authority recognized in the World Register of Marine Species (WoRMS Editorial Board, 2021).

The family Limulidae was listed by Zittel in the 1881−1885 part of his multivolume Handbuch der Palaeontologie (Zittel, Reference Zittel1885), but without including an authority. He listed the family name, again without attribution, in his recapitulated 1895 Grundzüge der Paläeontologie and 1903 revision (Zittel, Reference Zittel1895, rev. Reference Zittel1903). In both publications, Zittel diagnosed the family using only the genus Limulus. It is unclear whether Zittel was aware of Leach's earlier usage or whether he intended this as a new family name. Whether considered a homonym—in which case the Rule of Priority defers to Leach, Reference Leach1819—or an assumed attribution to Leach's original name, the use of Limulidae Zittel, Reference Zittel1885 is unambiguously invalid.

The influential English translation of Zittel's Grundzüge was published in 1900 by Charles R. Eastman of Harvard University (Zittel, Reference Zittel1900). This translation included “such far-ranging modifications as to render the volume for practical purposes an almost entirely new work” (Kitchen, Reference Kitchen1904, p. 93), albeit “with [Zittel's] consent” (Eastman's preface to Zittel, Reference Zittel1900, p. v). The section on the now-disused Acerata (including horseshoe crabs, eurypterids, and arachnids) was co-authored by John M. Clarke, New York State Paleontologist. For reasons not stated, the English translation placed Zittel's name to the right of the family name: “Family 3: Limulidae. Zittel. (King or Horseshoe Crabs)” (Zittel, Reference Zittel1900, p. 672). This is not the only instance in which the family was explicitly attributed to Zittel. Based apparently on personal correspondence at the time of publication, Packard (Reference Packard1885, p. 150, 152) also attributed the family Limulidae to Zittel's Reference Zittel1885 Handbuch.

This incorrect attribution appears to have persisted in the paleontological literature for over a century to the present. For example, the section on Limulidae in the Treatise on Invertebrate Paleontology (Størmer et al., Reference Størmer, Petrunkevitch, Hedgepeth and Moore1955, p. P22) lists Zittel, Reference Zittel1885 as the authority. (Several paleontological publications alternatively use Limulidae Zittel, 1881, the year that Zittel started the relevant mollusk and arthropod volume, although its publication did not occur until 1885.) Aside from these 1885, 1900, and 1955 publications, the other oldest publications attributing the family name to Zittel we can locate include Eller (Reference Eller1940), Størmer (Reference Størmer1952), and Boada and de Villalta Comella (Reference Boada and de Villalta Comella1966), all of which are paleontological publications, the last one citing the Treatise volume as their source reference for the family. Størmer (Reference Størmer1952, p. 636), co-author of the 1955 Treatise volume, explicitly wrote that “[t]he family Limulidae, [was] erected by Zittel (Reference Zittel1885).” Dozens of paleontological publications henceforth (e.g., Allen and Feldmann, Reference Allen and Feldmann2005; Błażejowski et al., Reference Błażejowski, Niedźwiedzki, Boukhalfa and Soussi2017; Lamsdell, Reference Lamsdell2020; Bicknell et al., Reference Bicknell, Błażejowski, Hitij, Wings and Botton2021) have continued to cite Zittel (Reference Zittel1885) as the original authority for Limulidae.

Given the wide early-twentieth century influence of Zittel's translated text among English-speaking paleontologists, at the least, and its later codification in the even-more-influential Treatise on Invertebrate Paleontology, it appears these texts are the reason that essentially all paleontological publications have misattributed family Limulidae to Zittel, Reference Zittel1885 instead of Leach, Reference Leach1819. To clarify the proper authority for this well-known family, we encourage paleontologists to recognize Limulidae Leach, Reference Leach1819 as the proper authority, and that the attributions Limulidae Zittel, Reference Zittel1885 and Limulidae Zittel, 1881 no longer be used.