In the first half of the nineteenth century, a marked shift occurred in our understanding and treatment of the chelicerate fossil record, with the differentiation and recognition of entirely extinct genera for the first time. At the heart of this taxonomic revolution were the Eurypterida (sea scorpions) and Xiphosura (horseshoe crabs), although both groups were in fact considered crustaceans until Lankester's (Reference Lankester1881) seminal comparative anatomical study of the extant xiphosuran Limulus Müller, Reference Müller1785 and modern scorpions. The oldest available eurypterid genus is Eurypterus deKay, Reference DeKay1825; the oldest available fossil arachnid genus name is that of the scorpion Cyclophthalmus Corda, Reference Corda1835. However, there has been considerable historical confusion over the oldest available fossil xiphosuran genus name, which has been recognized alternately as Belinurus König (with a publication date of either 1820 or 1851) or the synonymous Bellinurus Pictet, Reference Pictet1846. Most recent treatments (e.g., Selden and Siveter, Reference Selden and Siveter1987; Anderson and Selden, Reference Anderson and Selden1997; Anderson et al., Reference Anderson, Dunlop, Horrocks, Winkelmann and Eager1997; Lamsdell, Reference Lamsdell2016, Reference Lamsdell2021; Bicknell and Pates, Reference Bicknell and Pates2020) have favored Bellinurus Pictet, Reference Pictet1846 as the available name; however, Haug and Haug (Reference Haug and Haug2020) recently argued that Belinurus König, 1820 is valid and has priority, a position then followed by Lamsdell (Reference Lamsdell2020), prompting a reinvestigation of the taxonomic history of the genus. Upon review, it is clear that neither of the previously recognized authorities for Belinurus are accurate and that the two candidate type species for each genus are, in fact, synonyms. Given the convoluted and at times almost illogical history of the competing names, along with the most recent controversy as to which has priority, we present a complete history of the treatment of the genus to resolve the issue.

The issue arises from the partial publication of König's (Reference König1825) Icones Fossilium Sectiles. This volume (Centuria Prima) was bound and published, comprising one hundred figures across eight plates with associated descriptions. Planned further volumes of Icones Fossilium Sectiles beyond the first were never published (Urban, Reference Urban1851); however, it is apparent that a limited number of lithographs for the plates for the second and third volumes were produced and shared by König among interested parties (Woodward, Reference Woodward1830). Among these plates (figure 230, on plate 18) was a single specimen of a small horseshoe crab labeled simply Belinurus bellulus.

The informal proliferation of these plates created a scenario in which the proposed name was widely known among researchers but explicitly recognized as unpublished (see Woodward, Reference Woodward1830) and presumably therefore unavailable. Buckland (Reference Buckland1837), in his contribution to the Bridgewater Treatise series, figured and described a new horseshoe crab species as Limulus trilobitoides, explicitly stating that this new species was the same as König's unpublished Belinurus bellulus and Martin's (Reference Martin1809) Entomolithus Monoculites? Lunatus (a non-binomial name that has been suppressed by the International Commission on Zoological Nomenclature along with all other names within Petrificata Derbiensia). Subsequently, Louis Agassiz, in his German translation of Buckland's (Reference Buckland1839) Bridgewater Treatise, followed Buckland's description of Limulus trilobitoides with a note that the genus Bellinurus König [sic] is deserving of recognition as a distinct genus from Limulus (pl. 46; it is worth noting that this is the first recorded occurrence of the alternative Bellinurus spelling and that it follows an accurate transcription of the spelling of Belinurus from Buckland's text on the same page). In the same year, a summary list of fossil horseshoe crabs appeared in the Neues Jahrbuch für Mineralogie, Geognosie, Geologie und Petrefaktenkunde, which was edited by Karl von Leonhard and Heinrich Bronn. The summary itself is not attributed to either editor; however, the article stated that it was done at the request of van der Hoeven. In turn, van der Hoeven (Reference van Der Hoeven1838) stated that he had been in contact with Bronn regarding the status of the fossil xiphosuran species, and so Bronn (Reference Bronn1839) can be reliably ascribed as the author of the article. In it, Bronn directly assigned the species Entomolithus Monoculites? Lunatus Martin, Reference Martin1809 (as “Entomolites monoculites”) and Limulus trilobitoides Buckland to the genus Belinurus König, with no reference to B. bellulus König. However, the species reappeared in Morris's (Reference Morris1843) A Catalogue of British Fossils, where it was listed as a synonym of Limulus trilobitoides Buckland.

Later, in his Traité élémentaire de paléontologie, Pictet (Reference Pictet1846) included the genus Bellinurus König—once again misspelled—to which he assigned the species Limulus trilobitoides, with the erroneous taxonomic authority of König (notably, there was again no reference to B. bellulus König), although the second edition listed B. bellulus König as a senior synonym of Limulus trilobitoides (Pictet, Reference Pictet1854). Matters were further complicated by Baily (1859a–c), who in a series of papers utilized the name Bellinurus König, recognizing B. bellulus König as a synonym of B. trilobitoides (Buckland) and proposing the genus Steropis to accommodate a variety of species, including B. trilobitoides (Buckland). Baily (Reference Baily1863) later recognized Steropis as a junior synonym of Belinurus König, 1820 (potentially the year that the unpublished lithographs were first made available), stating that Pictet (Reference Pictet1846) had made the genus name available. Baily also here treated B. bellulus König, 1820 as a valid and available species for the first time and considered it to have priority over B. trilobitoides (Buckland). Henry Woodward, in his extensive works on British fossil chelicerates, was inconsistent in his treatment of the genus, at first attributing Baily (Reference Baily1863) as the authority for Belinurus König with B. trilobitoides (Buckland) as the type species (Woodward, Reference Woodward1867) before later following Baily (Reference Baily1863) in recognizing Bellinurus König, 1820 with B. bellulus König, 1820 as the valid type species (Woodward, Reference Woodward1872, Reference Woodward1907).

Moving into the twentieth century, there appeared to be a consensus that Belinurus König, 1820 was the accurate name and authority, with B. trilobitoides (Buckland) being a junior synonym of the type species B. bellulus König (Dix and Pringle, Reference Dix and Pringle1929; Eller, Reference Eller1938; Størmer, Reference Størmer1952). Uncertainties regarding the taxonomic history of the species continued, however, with Størmer (Reference Størmer1952) erroneously listing Woodward (Reference Woodward1866–1878) as the taxonomic authority for B. bellulus. Størmer (Reference Størmer and Moore1955) later compounded this error in the Treatise on Invertebrate Paleontology, which continued to recognize Belinurus König, 1820, but incorrectly listing Meek and Worthen (Reference Meek and Worthen1865) as the authority of the junior synonym Bellinurus.

This situation changed dramatically with the publication of Morris's (Reference Morris1980) Catalogue of the type and figured specimens of fossil Crustacea (excl. Ostracoda), Chelicerata, Myriapoda and Pycnogonida in the British Museum (Natural History). In it, Morris reported that the second part of König's work had become available only after his death and that the earliest the genus and species could date to is 1851. This would render Bellinurus, made available by Pictet in 1846, as having priority over Belinurus König, c. Reference König1851. Similarly, B. trilobitoides (Buckland, Reference Buckland1837) would have priority over B. bellulus König, c. Reference König1851. The next treatment of the group, by Selden and Siveter (Reference Selden and Siveter1987), followed Morris in recognizing Bellinurus Pictet as having priority but considered B. bellulus König, c. Reference König1851 the valid type species. Subsequent workers all recognized Bellinurus Pictet, Reference Pictet1846 as the appropriate taxon name and authority (Schultka, Reference Schultka1994; Anderson and Selden, Reference Anderson and Selden1997; Anderson et al., Reference Anderson, Dunlop, Horrocks, Winkelmann and Eager1997; Lamsdell, Reference Lamsdell2016, Reference Lamsdell2021; Bicknell and Pates, Reference Bicknell and Pates2020). Very few of these treatments considered the issue of the appropriate type species, although Bicknell and Pates (Reference Bicknell and Pates2020) listed both B. bellulus (ascribed to Pictet, Reference Pictet1846) and B. trilobitoides (Buckland) as distinct, valid species. Then Haug and Haug (Reference Haug and Haug2020) argued on the basis of an available scan of Icones Fossilium Sectiles from the Biodiversity Heritage Library that König's plates were published in 1820 and that Belinurus König had priority over Bellinurus Pictet, which Lamsdell (Reference Lamsdell2020) then followed in a comprehensive revision of xiphosurid taxonomy.

With such a turbulent taxonomic history, the question remains: what is the correct formulation of the genus name, and what is the correct taxonomic authority? To determine this, it must first be ascertained whether the plates for König's second volume were published before 1851, or if not, which publication first made Belinurus available and whether it pre- or post-dated Pictet's making Bellinurus available in 1846. With regard to the publication of König's plates, all contemporary sources (Woodward, Reference Woodward1830; Urban, Reference Urban1851) are clear in stating that the later volumes of Icones Fossilium Sectiles were not published and the plates were not publicly available. The fact that the plates for the second and third volumes were distributed and appended to bindings of the first volume after König's death, as stated by Sherborn (Reference Sherborn1902) and Lang et al. (Reference Lang, Smith and Thomas1940), explains the undated binding from the Natural History Museum, London, that led Haug and Haug (Reference Haug and Haug2020) to believe that these plates were published before 1851. The matter then becomes when Belinurus and Bellinurus each first became available. Although Buckland (Reference Buckland1837) stated his Limulus trilobitoides is the same as König's Belinurus bellulus, he did so with reference to the unpublished plates and so did not make the genus or species available as per Article 12.3 of the International Code of Zoological Nomenclature (International Commission on Zoological Nomenclature, 1999). Similarly, Agassiz's note in his translation of Buckland (Reference Buckland1839) was the first reference to Bellinurus in the literature but did not include an assignation of an available species name and so did not make that spelling of the genus available. However, Bronn's (Reference Bronn1839) summary of fossil horseshoe crabs explicitly listed Belinurus König as a valid genus including the valid species Limulus trilobitoides Buckland by indication. This satisfies Article 12.2.5 of the International Code of Zoological Nomenclature (International Commission on Zoological Nomenclature, 1999) and is sufficient for the genus name to become available, whereby its taxonomic authority is Bronn (Reference Bronn1839) following Article 50.1 (International Commission on Zoological Nomenclature, 1999). Bellinurus was first made available by Pictet, and so Belinurus Bronn, Reference Bronn1839 is the valid senior synonym of Bellinurus Pictet, Reference Pictet1846. Pictet, however, made no reference to B. bellulus, which Morris (Reference Morris1980) stated was first made available by the distribution of König's plates in or after 1851. If the plates were distributed after 1854, B. bellulus would be attributable to Pictet (Reference Pictet1854), who figured a specimen alongside a diagnosis of the species. Irrespective as to whether the species was made available in 1851 or 1854, as B. trilobitoides (Buckland, Reference Buckland1839) and B. bellulus König, c. Reference König1851/Pictet, Reference Pictet1854 are subjective synonyms, B. trilobitoides has seniority and is the valid name.

To clarify these issues for future paleontologists, we present a revised systematic paleontology for Belinurus Bronn, Reference Bronn1839, including all currently valid species assignable to the genus.