Beyond an uncertain and shadowy relationship to the Ostracodermi, and perhaps some points of resemblance to the Arthrodira, the Antiarchi stand alone among Craniates. Nothing is known of their origin; no intermediate forms link them to any other groups, and the high specialisation they have attained is sufficient to negative any idea that they can "be credited with any share in the evolution of the Fishes of more recent periods."
IV. Arthrodira
This group has been instituted for the reception of a number of remarkable armoured Fishes of uncertain relationships which flourished in Europe during the Devonian and Old Red Sandstone periods, and in North America from the Devonian to the Lower Carboniferous. The head (e.g. in Coccosteus)[[633]] is invested dorsally by a series of median and lateral symmetrically-disposed tuberculated plates (Fig. 323). Two of the lateral plates are notched for the orbits, and between them there is an interorbital plate which either has a pit on its inner surface or is perforated by an open funnel, as in Dinichthys, possibly for a parietal or a pineal organ. Some of the bones present some analogy, to say the least, to certain of the dermal bones of a typical Teleostome, apparently representing such elements as paired parietals and frontals, a dermal mesethmoid, and toothless premaxillae and maxillae (Fig. 324, A). As in the Antiarchi, the anterior portion of the trunk is also armoured, above by a dorsal shield, formed by median and lateral plates, and below by a similarly constructed ventral shield (Fig. 324, B). A huge joint connects the head and trunk shields: hence the term Arthrodira or "joint-neck." The rest of the body is naked.
Fig. 323.—Restoration of Coccosteus decipiens. Old Red Sandstone of Scotland. × ¼. A, Articulation of the cephalic and trunk shields; DB and DR, radials of the dorsal fin; H, haemal arches and spines; MC, sensory canals; N, neural arches and spines; NT, notochord; U, median plate; VB, basipterygium; VR, radialia of the pelvic fin. (From Parker and Haswell, after Bashford Dean and Smith Woodward.)
Pectoral fins are unknown, but pelvic fins, each supported by a stout basal plate or basipterygium, and with traces of radials, are present. There is a small dorsal fin. Little is known of the primary cranium, but in the trunk and tail it is evident that there are well-developed and partially calcified neural and haemal arches associated with a persistent notochord. It is possible that the skull is autostylic. Gill-arches are not known. A pair of plates (Fig. 324, A, j) at the postero-lateral angles of the cephalic shield may perhaps be opercula. The teeth are conical. Those in the upper jaw are supported by two pairs of plates, probably vomers and palatines. In the lower jaw there are two series of teeth, one in front near the symphysis, and the other behind, supported by a single bone in each ramus. There is a well-developed lateral line system, indicated by surface markings on the head and trunk shields.
Fig. 324.—Dorsal view of the cephalic and trunk shields of Coccosteus (A); and a view of the ventral part of the trunk armour (B). a.d.l, Anterior dorso-lateral; a.l, antero-lateral; a.m.v, anterior median ventral; a.v.l, anterior ventro-lateral; c, central; e.o, external occipital; i.l, internal lateral; j, jugal; m, marginal; m.d, median dorsal; m.e, dermal mesethmoid; m.o, median occipital; m.v, median ventral; mx, maxilla; n, nasal aperture; o, orbit; p, pineal plate; p.d.l, posterior dorso-lateral; p.mx, premaxilla; p.o, preorbital; pt.o, post-orbital; p.v.l, posterior ventro-lateral. (From Traquair.)
Fam. 1. Coccosteidae.—Coccosteus occurs in the Devonian of Europe and North America, and includes species of relatively small size, not exceeding half a metre in length. C. decipiens, the best known species, is a characteristic fossil in the Old Red Sandstone of Scotland. Phlyctaenaspis[[634]] is found in the Lower Devonian of Canada, England, and Poland. A larger Arthrodiran, with slender toothless jaws, Homosteus,[[635]] is met with in the Lower Old Red Sandstone of the North of Scotland, and in the Devonian of Germany and Russia. The Old World Arthrodira must yield, however, to those of the New World for variety in size and shape, and in the character of their dentition.[[636]] Some of the North American genera (e.g. Dinichthys) probably attained a length of ten feet, or even, as in Titanichthys, a much greater size. Some are fusiform in shape, but Mylostoma is flattened and Ray-like, and, judging from the dentition, their food and habits must have been equally varied. Mylostoma has tritoral plates not unlike those of Neoceratodus or Chimaera. In others the teeth are single, and conical or pointed; in Titanichthys the front teeth in both jaws are beak-shaped structures. It is highly probable that Titanichthys, Mylostoma, and perhaps other genera, are types of distinct families.
The Arthrodira have been regarded as armoured Dipneusti, a view which is mainly based on their supposed autostylism and the nature of the dentition. But this autostylism has yet to be verified, and, if proved, the possibility that it may be a secondary feature, associated with the evolution of a peculiar dentition, must not be forgotten. Much more may be said for their claim to be regarded as a highly specialised race of primitive Teleostomi. Besides a well-developed lower jaw, bones comparable to the elements of a secondary upper jaw are known, and in a general way the disposition of the cranial roofing bones, and the arrangement of the endoskeletal elements of the pelvic fins, tend to conform to the normal Teleostome type. In fact, Dr. Traquair has expressed the opinion that the Arthrodira are Teleostomi and Actinopterygii.[[637]]