Interrelationships of the elasmobranchs with a key to the order Galea. Bulletin of the AMNH ; v. 74, article 2.
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Date
1937
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New York : The American Museum of Natural History
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Abstract
"The results of this investigation tend to show that the characters of phylogenetic value are those not easily affected by the immediate environment or by the habits of a species. These characters have determined the main lines of development and are of value in determining groups of the grade of class, order, or suborder. They are deep-seated characters and include the vertebral structure, the basals of the paired fins and of the myxopterygia, the jaw attachment, and the rostral cartilages. The physiological characters are those which appear to have direct application to the environment or habits of the species. They cause wide variation within the groups determined by the phylogenetic characters and may be used in determining family, genus, or species relationships. These characters include the radials of the pectoral fin, the axial cartilages of the myxopterygia, the position of the dorsal fins, the teeth and denticles, the mouth and nostrils, the general body form, and all modifications of the skin such as folds, keels, pits, etc. The phylogenetic characters determine the lines of adaptive radiation and the physiological characters determine the adaptive radiation within the subgroups. The phylogenetic characters were laid down during the geological age of origin and when present today have been retained. The physiological characters may continue to vary at the present time. Certain physiological characters have become linked to the phylogenetic so consistently that linkage groups are suggested. Slight variations from the linkage may mean crossing over. The modern groups which were separated from the main stock more recently have proved to be more stable than those originating at an earlier time. Variation is, therefore, found in the more primitive groups and these may be supposed to be still in a varying or mutating condition. Certain species have been found to vary in characters otherwise typical of a group and these may represent mutating species. Feeding habits seem to have determined the direction of tooth development; temperature and water pressure, denticle development; depth and water pressure, the general body form, position of the dorsal fins, and numerous dermal specializations. Bottom and shallow water habits have determined the movement of the nostrils toward the mouth, the width of the mouth gape, the size of the spiracles, and the spreading of the pectoral fins. Restricted environments have tended to lengthen the body and free environments to deepen the body and increase bulk, leading in either direction to over-specialization in some cases. The intermediate grovelling type has been primitive in any group because of the fundamental need for securing food, which would have been primarily more abundant close to the shores. The Hexanchea and Holocephali branched from the ancestral stock during the Paleozoic; the Hybodonts, in the Triassic; the Squalea, Platosomeae, Orectoloboidea and Catuloidea, in the Jurassic; the Odontaspoidea and Isuroidea, in the Cretaceous; the Carcharinoidea, in the Eocene. The groups established in the Jurassic have been less stable than those of later origin and appear to be still in mutating condition. An hypothetical ancestral shark may be constructed on the basis of characters found to be primitive: A grovelling shore type depressed anteriorly, tapering toward the tail; mouth wide, nearly terminal and ventral; nostrils very near the mouth with a nasoral groove; eyes large, lateral, with large spiracle behind them and without a nictitating fold or membrane; gill slits five in number, lateral and wide, all open to the exterior, all anterior to the pectoral fins and at equal distances apart; two spiny dorsal fins of equal size with ridges and ornamentations on the spines, the first over or just back of the pelvic fins; a large anal fin; a caudal fin with axis not raised and without notches, keels, or pits; denticles as flat plates with bases fitted together; teeth small, multicuspid, with a broad base deeply embedded in the jaw; coloration uniform, lighter below; the rostral support absent; the jaw suspension amphyhyostylic and the pterygoquadrate articulation extensive; the basals of the pectoral fins of equal size and expansion, the radials unsegmented rods about equally distributed on the basals; the notochord unconstricted, the vertebrae with no secondary calcifications; a condition of diplospondyly in the tail; the spiral valves of the spiral type with from five to ten valves; the heart valves in three series arranged in two rows; the myxopterygia absent indicating an oviparous habit with external fertilization; the fish probably lived in the early Devonian and followed the armored Stegoselachians. The results of the present investigation show the need of reorganization in the order Galea. Previous classifications have either included the Orectolobidae with the Catulidae or given them parallel standing. There are two types of vertebral structure in the Galea: the Orectoloboidea have one type, and the Catuloidea have the transitional stages approaching the other. I believe the two types of vertebrae to have had a separate origin in the Jurassic, and the two groups to represent, therefore, fundamentally distinct groups. These I have called suborders, within which the Orectoloboidea form the stem group of one, and the Catuloidea the stem group of the other. Therefore superfamilies have been established and the members of the order Galea have been distributed according to the foregoing key (p. 109)"--P. 130-132.
Description
p. 25-138, 51 p. of plates : ill. ; 25 cm.
Includes bibliographical references (p. 132-138).
Includes bibliographical references (p. 132-138).
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