XIPHOSURA, Lamsdell, 2013

Lamsdell, James C., 2013, Revised systematics of Palaeozoic ‘ horseshoe crabs’ and the myth of monophyletic Xiphosura, Zoological Journal of the Linnean Society 167 (1), pp. 1-27 : 4-8

XIPHOSURA

The terminology for aspects of xiphosuran morphology largely follows Siveter & Selden (1987) and Selden & Siveter (1987). The prosoma comprises the entirety of the anterior tagma, including the dorsal carapace and the prosomal appendages. The prosomal appendages are denoted by roman numerals I–VI, appendage I being the chelicerae. In limulids the anterior cephalic region is termed the cephalothorax by Shultz (2001) as a number of opisthosomal somites have been at least partially incorporated into the prosoma (Fig. 1D). The carapace/cephalothorax itself bears a number of structures; the cardiac lobe is located at the posterior of the carapace and is axially inflated, while the ophthalmic ridges extend either side of the cardiac lobe and dorsally shade the lateral compound eyes. Anterior to the cardiac lobe the median ocelli, simple eyes, can be identified in a number of taxa. Some species bear extraophthalmic ridges, transverse ridges that are arrayed on the

· Figure 1. Schematic diagrams of a number of different arthropods showing examples of functional pseudotagmata compared with true tagmata. Somites are labelled 0–XVII; ‘tl’ indicates the telson, ‘m’ the mouth, ‘a’ the anus, and ‘g’ the gonopores. A, the oribatid mite Epilohmannia cylindrica ( Berlese, 1904) . B, the trilobite Olenoides serratus ( Rominger, 1887) . C, the solifuge Galeodes armeniacus Birula, 1929 . Solifuge arachnids possess opercula on the first three opisthosomal segments associated with the genital ducts and repiratory spiracles. These opercula could be considered homologous to the opercula of the thelyphonid Mastigoproctus giganteus ( Lucas, 1835) which Shultz (1993) showed to be opisthosomal appendages that had become completely sutured to the ventral body wall. D, the limulid xiphosurid Limulus polyphemus Linnaeus, 1758 .

surface of the carapace outside of the ophthalmic ridges. Interophthalmic ridges occur within the area demarcated by the ophthalmic ridges, and may correspond to internal apodemes for the attachment of extrinsic limb musculature, indicating arrangement of the prosomal appendages.

The term opisthosoma refers to the post-prosomal segments, and is equivalent to the trunk region of other arthropods (see Hughes, 2003a, b). The opisthosoma itself is not considered a single tagma, contrary to most recent treatments. Lankester (1904) considered chelicerates to possess three tagmata based on appendage differentiation: the prosoma, mesosoma, and metasoma. The prosoma comprises somites I–VI, the mesosoma somites VII–XIII, and the metasoma somites XIV–XVII (in xiphosurans, eurypterids, scorpions, and probably chasmataspidids the metasoma is formed from somites XIV–XIX). van der Hammen (1986a) termed the mesosoma and metasoma pseudotagmata and this has been followed by subsequent authors including Selden & Siveter (1987) and Rudkin et al. (2008), although van der Hammen considered the mesosoma to consist of somites VII–XIV and the metasoma somites XV–XIX (in scorpions). This in fact corresponds to the preabdomen and postabdomen; these are actually non-functional pseudotagmata defined by a dorsal constriction of the tergites ( Fig. 2 ). In xiphosurans the postabdomen comprises somites XV–XVII, while eurypterids frequently correspond to the condition in scorpions but sometimes have a preabdomen and postabdomen that corresponds to the true tagmata of mesosoma and metasoma. In chasmataspidids the situation is somewhat different, having somites VII–X fused together into a buckler and somites XI–XIX forming a freely articulating postabdomen. These are, however, considered to be functional pseudotagmata, as the fusion of the buckler places a functional constraint on the organism.

Somite VII is considered to be opisthosomal in origin, as is the conventional view (see Snodgrass, 1952), and not prosomal as suggested by Stürmer & Bergström (1981) (see also Haug et al., 2012a for discussion on this subject). Evidence for this is shown in the dorsal expression of somite VII as a fully sclerotized tergite in synziphosurines ( Fig. 3A ) and chasmataspidids ( Fig. 3B ) and in the way the tergite remains attached to the opisthosoma after disarticulation of the carapace ( Fig. 4A, B ). This tergite is termed the microtergite in chasmataspidids, but this refers specifically to the heavily reduced condition seen in these taxa and the term pre-opercular tergite is used herein while microtergite is applied as a condition which the pre-opercular tergite can attain. The tergite of somite VIII may be called the opercular tergite and is hypertrophied in some synziphosurines. Embryological studies have shown that in limulids the tergite of somite VII and part of the tergite of somite VIII are incorporated into the prosoma ( Scholl, 1977; Sekiguchi, Yamamichi & Costlow, 1982) with the lateral portions of the opercular tergite distinctly set off from the main body of the tergite, frequently deflected dorsally towards the outer margin, and these regions are termed free lobes. The term thoracetron is used for the fully fused dorsal opisthosomal shield in xiphosurids.

The opisthosomal appendages in aquatic chelicerates are modified into flattened opercula that bear the respiratory organs, termed book gills. In xiphosurans the opercula are not medially fused, although in modern xiphosurids they are connected by a thin membrane which would be unlikely to fossilize, and so its presence in extinct taxa is equivocal. The operculum of somite VIII is the genital operculum and bears the paired gonopores; in modern xiphosurids the genital operculum is devoid of respiratory structures, but they are present on the genital operculum of Offacolus Orr et al., 2000 and the condition is unclear in Weinbergina . The appendages of somite VII are functionally incorporated into the prosoma and may be retained as fully pediform walking limbs or reduced to chilaria, which act in a masticatory function.

The institutional abbreviations for xiphosuran specimens figured in this study are as follows: ELM, Estonian Museum of Natural History , Tallinn, Estonia ; ISEA, Museum of the Institute of Systematics and Evolution of Animals, Kraków, Poland ; MM, Manitoba Museum, Winnipeg, Canada ; PIN, Paleontological Institute , Moscow, Russia ; PWL, Landessammlung für Naturkunde Rheinland-Pfalz, Mainz, Germany ; SMF, Naturmuseum und Forschungsinstitut Senckenberg, Frankfurt am Main , Germany .

Figures

Figure 2. Schematic of a generalized synziphosurine arthropod, showing the distinction between the preabdominal and postabdominal non-functional pseudotagmata and the true tagmata of the prosoma, mesosoma, and metasoma.

Figure 3. A, Bunodes lunula d’Eichwald, 1854 from the late Silurian (Ludlow) of Oesel, Estonia. Specimen ELM G1:262:2, clearly showing the partially reduced pre-opercula tergite of somite VII (arrowed) between the downturned carapace and the hypertrophied tergite of somite VIII. Scale bar = 10 mm. B, ‘Eurypterus’ stoermeri Novojilov, 1959, a chasmataspidid from the early Devonian (Lochkovian) of Siberia, Russia. Specimen PIN 1138-1, exhibiting the microtergite of somite VII (arrowed) positioned between the carapace and the buckler. The microtergite curves anteriorly towards its lateral edges, and is particularly noticeable on the left-hand side. The occurrence of ridges associated with the lateral eye on the carapace is also labelled. Image courtesy of Dave Marshall. Scale bar = 2 mm.

Figure 4. A, Pasternakevia podolica Selden & Drygant, 1987 from the late Silurian (Ludlow) of Zalissia, Ukraine. Specimen ISEA I - F/MP/3/1499/08, isolated opsithosoma with articulated microtergite of somite VII (arrowed). Image courtesy of Ewa Krzemin´ ska. Scale bar = 10 mm. B, undescribed chasmataspidid from the Lower Devonian (Emsian) of Siberia, Russia. Specimen PIN 5116-6, disarticulated buckler with microtergite of somite VII (arrowed) still firmly attached to its anterior margin. Image courtesy of Dave Marshall. Scale bar = 2 mm.