FOSSIL SHARK. Fossilized remains of sharks occur from the Lower Devonian upward, and even in the Upper Silurian detached tin-spines, teeth, and dermal dentieles resembling those of elasmobranchs are found, being thus among the earliest known remains of vertebrates. The re lationships of these Silurian forms are doubtful, however, and some of them (the Ccelolepidce, in cluding Lanarkia and Thelodns) possibly have closer affinities with the remarkable group of ostracoderms than with elas.umbranchs. From the Devonian upward undoubted sharks are met with, many known only from fragments of the dermal structures—teeth, shagreen denticles, and fin-spines. These spines. when not definitely as signable to any genera, are termed `ichthyodorn lites.' In a few cases the cartilaginous endo skeleton is hardened by deposition of phosphate of lime—calcified—so that jaws, vertebra, fin structure, etc., are readily fossilized. Elasmo branch paleontology, which may be said to have originated in the work of Louis Agassiz, has demonstrated that the sharks and rays of the present time represent hut an insignificant rem nant of a group which attained its maximum degree of differentiation and specialization as early as the Carboniferous. The characteristic forms of the Paleozoic, however, the primitive as well as the highly specialized, died out in the Permian, and their descendants of the Mesozoic have persisted to the present with little change.
The most primitive of fossil elasmobranchs are included in the order Pleuropterygii (side fin) of which the most typical genus is Cladose lache from the Upper Devonian or Lower Carbon iferotts of Ohio. In this form the paired fins arc mere horizontal lappet-like folds along the sides of the body, supported by two rows of cartila ginous rods, the 'basals,' imbedded within the body, and the 'radials' within the tin-lappet and extending outward In its edge. According to the commonly accepted fin-fold theory of paired limbs, this is the most primitive known type of paired fin, and the lappets are to be regarded as persistent portions of a former continuous lateral fold, possessed by some unknown ancestor. Since these lappet-fins, or 'plenropterygia,' were capa ble of but very slight motion. their function was chiefly that of balancing-organs, while the power ful turned-up or 'heteroeerear tail served as the organ of propulsion. Other primitive characters of this fish are the terminally placed mouth, the unconstricted notochord, and simple dermal skeleton. Clodoselache, judging from its many primitive characters and lack of specialization, probably stands structurally very near the an cestral form which gave rise to the more spe cialized sharks, to the bony fishes, and through these to the higher vertebrates. Several eladose laehids are known, and the most generalized of these may he regarded as the most primitive true fish. None of them exceeds six feet in length. The spiny sharks (commonly ranked as an order Acantliodii) comprise a number of Paleo zoic forms which resemble the cladoselaehids in ninny respects, but differ from them in that the blade of the fins, except the caudal. is almost en
tirely dermal, the skeletal fin support being re duced to a stiff spine at the anterior border; genera Acanthodes and Mesaeauthus. In one family, represented by Climatius, a series of spines along the side of the body suggests the continuous lateral fin-fold. The aeanthodians have the dermal skeleton highly developed, espe cially in the region of the skull and shoulder girdle. Some ichthyologists place the group among the pleuropterygians. A widely different order of Paleozoic sharks is that termed Ichthyo tomi or Pleuraeanthea, represented by Pleura canthus of the Carboniferous and Permian of Europe. Of the many distinguishing features of this group, the most noteworthy is the possession of pectoral fins of the `archipterygium' type, which many morphologists (Gegenbaur and his maintain to be the fin-form from which are evolved all other types of paired fins, and even the five-toed limbs of higher vertebrates. In the perfect archipterygium the basals form an axis projecting from the body, while the radials are ranged along this axis in two rows, like the veins of a leaf along the midrib. This type of fin is also common to the lung-fishes and some of the most primitive bony fishes. There is strong reason to believe that it is derived from the lap pet-like type of the Pleuropterygii. (See FIN.) The elasmobranchs thus far mentioned did not survive beyond the Paleozoic. but it is these early types only which are sufficiently primitive to be of importance in tracing time ancestry of higher vertebrates.
The order Seinchii, comprising all the modern sharks and rays, appeared in the Lias, though one family, the cestracionts, may be trace able to the Permian. The basals of the pectoral fin are reduced to two or three pieces, and the blade of all the fins is chiefly dermal. The males are provided with claspers on the pelvic fins. The vertebral centra, with few exceptions, are well developed, and the form of calcification of the vertebra., i.e. Whether radial, in a single ring, or several concentric rings, has been made by Hasse (1879) a criterion for subdivision of Scbiehii into Asterospondyli, Cyclospondyli, and Tcetospandyli, but, like most systems based upon a single character, it is not very satisfactory. A more prac•tic•al division into suborders is the following: (1) l'rotus,/achii, sharks with more than Live (6 o• 7) gilharehes, and a number of primitive skeletal characters—extending from Lipper Jurassic to recent, and including Ilep tanchus and Chlamydoselaehe; (2) Squalida, all live-gilled true sharks; most families appear in the Mesozoic, but the Port Jackson sharks (cestracionts), which have large crushing teeth, possibly originate in the Carboniferous; (3) Rajida, the rays and skates—Mesozoic to recent.
BIBLIOGRAPHY. Dean, Fishes, Living and FosBibliography. Dean, Fishes, Living and Fos- sil (New• Yo•k, 1895) ; Woodward, Vertebrate Palrt•uriloIqgy (London, 1898) ; Von Zittel, Text book of Pabrontology (Eng. trans., London, 1902).