The best guess as to the affinities of the Ostracophores is perhaps that given by Dr. Ramsey H. Traquair ("Fossil Fishes of the Silurian Rocks of the South of Scotland," 1899). Traquair regards them as highly aberrant sharks, or, more exactly, as being derived, like the Chimæras, from a primitive Elasmobranch stock. In favor of this view is the character of their armature, the bony plates themselves to be regarded as formed by the fusion of shagreen grains or scales. According to Traquair: "Specialization from the most specialized form, Lanarkia, has been accompanied by (1) fusion of the spinelets (Lanarkia) or shagreen grains (Thelodus) into plates, scutes, and rhombic scales, supported by hard matter developed in a deeper layer of skin, and (2) alterations in the pectoral fin-flaps, which, becoming covered up by the postero-lateral plates in Drepanaspis, are finally no longer recognizable in the Pteraspidæ."
Fig. 353.—Odontotodus schrencki (Pander) (Tremataspis), ventral side. Island of Oesel. (After Patten.)
Fig. 354.—Odontotodus schrencki (Pander) (Tremataspis), dorsal side. Island of Oesel. (After Patten.)
Woodward leaves their exact relationship undefined, while others have regarded them as mailed lampreys, at any rate to be excluded from the Gnathostomi, or jaw-bearing series. The apparent absence of true jaws, true limbs, and limb-girdles certainly seems to separate them widely from true fishes, but these characters are negative only, perhaps due to degeneration, and at any rate they are not yet absolutely determined. Certainly they offer no positive proof of affinity with the modern Cyclostomes.
Dr. Traquair regards the Heterostraci or most primitive Ostracophores as most certainly derived from the Elasmobranchs. Other writers have attacked the integrity of the group of Ostracophores, questioning the mutual relationship of its component parts. Reiss, for example, regards the association of the Osteostraci with the Heterostraci as "unbegründet" and "unheilvoll," while Ray Lankester, as quoted by Traquair, affirms that "there is absolutely no reason for regarding Cephalaspis as allied to Pteraspis beyond that the two genera occur in the same rocks, and still less for concluding that either has any connection with Pterichthys." Elsewhere Lankester states that the Heterostraci are associated at present with the Osteostraci, "because they have, like Cephalaspis, a large head-shield, and because there is nothing else with which to associate them." Patten, on the other hand, seems inclined to deny the rank of Heterostraci and Osteostraci as even separate orders, regarding them as very closely related to each other as also to their supposed spider-like ancestors.
Fig. 355.—Head of Odontotodus schrencki Pander, from the side. (After Patten.)