Occurrence of Arthrodires.—These fishes occur in abundance from the Silurian times to the Mesozoic. In the Devonian their gigantic size and thick armor gave them the leading position among the hosts of the sea. Among the genera there occurred "series of forms most interesting as to their evolution." "It is found more and more evident," says Dr. Dean ("Fishes, Living and Fossil," pp. 135, 136) "that the Arthrodirans may have represented the dominant group in the Devonian period, as were the sharks in the Carboniferous, or as are the Teleosts in modern times. There were forms which, like Coccosteus, had eyes at the notches of the head-buckler; others, like Macropetalichthys, in which orbits were well centralized; some, like Dinichthys and Titanichthys, with the pineal foramen present; some with pectoral spines(?); some with elaborately sculptured dermal plates. Among their forms appear to have been those whose shape was apparently subcylindrical, adapted for swift swimming; others (Mylostoma) whose trunk was depressed to almost ray-like proportions. In size they varied from that of the perch to that of a basking shark. In dentition they presented the widest range in variation, from the formidable shear-like jaws of Dinichthys to the lip-like mandibles of Titanichthys, the tearing teeth of Trachosteus, the wonderfully forked tooth-bearing jaw-tips of Diplognathus, to the Cestraciont type, Mylostoma. The latter form has hitherto been known only from its dentition, but now proves to be, as Newberry and Smith Woodward suggested, a typical Arthrodiran."
Classification of Arthrodira.—Our knowledge of the systematic relations of the Arthrodira is mostly of recent origin. Woodward refers most of the remains to the best known genus Coccosteus, and recognizes as families the Coccosteidæ, Mylostomidæ, Asterosteidæ, and Phyllolepidæ.
Fig. 368.—An Arthrodire, Dinichthys intermedius Newberry, restored. Devonian, Ohio. (After Dean.)
Dr. Bashford Dean in different papers has treated these fishes in great detail. In a recent paper on the "Relationships of the Arthrognathi"[158] he recognizes the group as a class coordinate with Cyclostomi and Elasmobranchii. This class, which he calls Arthrognathi, is first divided into two suborders, Anarthrodira, without joint at the neck, and Arthrodira, with such a joint. The former comprises one order, Stegothalami, and the latter two orders, Temnothoraci and Arthrothoraci. The following is Dr. Dean's definition of these orders and their component families:
Arthrognathi.—"Chordates whose anterior body region is encased in dermal elements, and divisible by a more or less definite partition into head and trunk. Dermal plates which surround the mouth function as jaws. No evidence of branchial arches. Column notochordal, showing no traces of centra; well-marked neural and hæmal elements. Paired limbs [absent or uncertain]. Dermal plates consisting typically of two layers, the superficial tuberculate, the inner bony with radiating lamellæ. Orbits situated near or at the margin of the head-shield and separated from one another by fixed integumental plates. A pineal funnel present situated in a fixed plate. A mucous system whose canals radiate from the preoccipital region."
Anarthrodira.—"Arthrognaths in which the cranial and dorsal regions are separated by a fixed partition whose dorsal rim is overlapped and concealed by superficial plates. Of these a large median dorsal element is present which extends backward superficially from the region near the pineal funnel. Also a pair of elements which overlie the position of the external occipital joint. Suborbital plates apparently absent. Jaw elements undescribed."
Stegothalami (στέγος, roof; θάλαμος, chamber).—"Anarthrodires in which the cranio-dorsal septum is vertical and deep, its height equal apparently to that of the arch of the head-shield. By this deep partition the latter appears to inclose two chambers (whence the ordinal name). Orbits inclosed by pre- and postorbital plates. Mucous system lacks a postorbital canal."
One family, the Macropetalichthyidæ, thus defined:
"Stegothalami with large orbits and well-arched cranio-dorsal shield. Dorso-central shield long, wide, gomphoidal, extending backward to the hinder margin of the shield and bordered by all plates save the postorbitals and marginals. Pineal funnel small and obscure." Macropetalichthys sullivanti from Ohio Devonian rocks, and Macropetalichthys agassizi from the Devonian of Germany, are important species of this group.