Smithsonian miscellaneous collections - Smithsonian Institution

SUGGESTIONS ON THE POSSIBLE MECHANICAL ORIGIN. a) That the extension of the skull was already advanced at the earliest. In most modern cetaceans it is a prominent feature of the skull (see especially pi.

NO. 5 TELESCOPING OF THE CETACEAN SKULL 9

Putter has recorded various features of the eye and its accessory structures which seem to have this meaning (Zool. The most it can be said that these conditions are suggested by the structure of the jaw in Sinopa and others.

12 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 76 tion of the zygoma accompanying the loss of the posterior teeth and

THE THREE CETACEAN PHYLA OR SERIES

NO. 5 TELESCOPING OF THE CETACEAN SKULL 1

KEY TO THE SUBORDERS OF CETACE-\

14 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 76 Bones of both rostral and cranial portions of the skull departing con-

THE DETAILS OF TELESCOPING AND THEIR RELATION TO CLASSIFICATION

According to one of these trends, the forward movement of the posterior elements of the skull appears to have taken precedence over the backward movement of the anterior elements; according to the others, the opposite seems to have been true.

DETAILS IN THE BALEEN WHALES

Such a relation of the parts in the base of the rostrum presents no very unusual features when compared with the conditions existing in ordinary land mammals. Intermaxillae expanded posteriorly to the same level as the base of the mandible;

NO. 5 TELESCOPING OF THE CETACEAN SKULL I7 flattening the posterior border of the latter bone until in the region of

It lies entirely behind the nostrils and nasal branches of the intermaxillae and jaws. It is everywhere posterior to the frontal, except in the region where the supraorbital part of the frontal joins the part which is.

KEY TO THE FAMILIES AND GENERA OF BALEEN WHALES

While the jaw is not conspicuously larger in proportion to the size of the skull, the parts of the skull which serve as its suspensorium are enlarged and specialized to a greater extent than in RacJiiancctes. Within the limits of the group, the humpbacks, Mcgaptera, are sharply contrasted with the finbacks, Balcenoptera and Sibhaldus, by the unusual structure of the scapula and by the great elongation of the manus.

22 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 76

DETAILS IN THE TOOTHED CETACEA

24 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 76 of which, in Agorophius at least (the skulls of the two other animals

NO. 5 TELESCOPING OF THE CETACEAN SKULL 2^

26 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 76 supraorbital process of the frontal; but its position in the region

NO. 5 TELESCOPING OF THE CETACEAN SKULL 27 on side of braincase, but area of temporal fossa considerably reduced

28 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 7O a strong tendency to unusually close contact of the two bones, would

5 TELESCOPE OF CETACEAN POO 29transported by transverse processes which usually, in Hving are formed in.

NO. 5 TELESCOPING OF THE CETACEAN SKULL 29 ported by transverse processes which ordinarily, in Hving forms at

76orbital breadth, the type representing the extreme breadth of orbital structure, the type representing the most extreme structural departure from normal mammals, is realized among those toothed cetaceans in which the jawbone passes faintly upward and backward from the anterior border of the orbit (pi. 3, the genus in which these features are more conspicuous in lateral view, with that of a normal mammal, pi. Whatever its origin, this second type of telescope seems not to be derived mechanically from a condition in which the orbit was originally covered by two plates in a horizontal position (compare Kogia,.

By collapsing this system, the maxillae are brought up and back from the anterior edge of the orbit, at an angle of 50 to 70 degrees above the line representing the posterior extension of the therostral axis, and the frontal width is maintained visible in the area above and behind the eye. ; the track is immediately covered only by a thick mass of the front, and not, as in the lake. So when the maxillary motion reached the mid-orbital level in its posterior motion, it would begin to incline. 5 TELESCOPEING THE WHALEAN SKULL 3The upper edge was at a level slightly or not at all higher than that of the.

NO. 5 TELESCOPING OF THE CETACEAN SKULL 3I upper margin lay at a level slightly if at all superior to that of the

32 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 76 to a small element not visible in adults but appearing in very young

NO. 5 TELESCOPING OF THE CETACEAN SKULL 33 KEY TO THE FAMILIES AND SUBFAMILIES OF

TOOTHED CETACEA

NO. 5 TELESCOPING OF THE CETACEAN SKULL 35 BEHAVIOR OF THE MODERN CETACEAN SKULL

36 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 76 in the main, as if it had consisted of three loosely joined masses of

NO. 5 TELESCOPING OF THE CETACEAN SKULL 37 ceeded as though influenced by forces which were relatively simple

5 TELESCOPING THE SKULL OF THE WHALFOOD 37walked as if influenced by forces that were relatively simple. 76parietals of the rodent." The fact that the modification went parietals of the rodent." The fact that the modification has gone further in whales than in rodents may not be entirely separate from the circumstance that in the one case the muscle activity is needed only during the digging action, while in the other case it is needed at its maximum when the animal feed; and the part of it required to oppose gravitation must be continuous from birth to death. Among the latter there is no enlargement of the mouth to swallow large quantities of water; the combined weight of the rostrum and jaws is almost invariably much less, relative to the weight of the skull, than it is in the mysticetes; and.

The probability that water pressure was one of the main factors in determining the behavior of the modern whale skull seems to be greatly strengthened by an examination of the characters of the two other groups of mammals whose members adopted an exclusively aquatic mode of existence. A definite overlap of the posterior edge of the parietal by the occipital occurs in the pig and peccary, both of which use the snout for "rooting". This appears to be associated with a forward thrust of the base of the brain case indicating the operation of some modeling force which is not at work in the cetaceans and rodents.

5 TELESCOPING THE WHALE SKULL 39 differed widely from this primitive type; but in neither of them is there any.

NO. 5 TELESCOPING OF THE CETACEAN SKULL 39 departed widely from this primitive type ; but in neither is there any

40 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 76 intervening between the formation of the chondrocranium and the

REMARKS ON THE CLASSIFICATION HERE ADOPTED

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Most of the supergeneric groups recognized here have already been discussed sufficiently for the purposes of this paper. This may be the first step towards the eventual elevation of all three of the currently recognized major groups to the rank of order. According to Pompeckj (Senckenbergiana, vol. 4, p. 43. 100, October 20, 1922), the structure of the periotic bone in the Zeuglodonts resembles that now found in the baleen whales.

Apparently the maxilla may have formed part of the anterior orbital wall as in Arehceodelphis. The maxillar}- is represented as broadly united to the frontal by a straight suture extending directly inward to the intermaxilla from a point situated on the side of the rostrum in front of the anterior edge of the orbit. No part of the jaw extends beyond this level either above or below the front; however the intermaxillary runs far behind both maxilla and orbit, its edge touching the parietal.

In theseuglodons, the relations of these two bones have remained normal, and the posterior border of the jaw lies behind the extremity of the intermaxillary (pi.

44 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 76 second specimen), or in Van Beneden's drawing of the original type

NO. 5 TELESCOPING OF THE CETACEAN SKULL 45

In this position they may be allowed to remain, though it must be observed that their separation from the living iniidce (Inia and Lipotes) does not at present rest on characters very satisfactory or very well understood. However, the peculiarities of the squalodont teeth, especially the tendency of the posterior teeth to assume a conspicuously sharp character, seem to indicate a line of development which did not lead directly towards any of the extant groups of porpoises. The genus Prosqualodon as described and figured by Lydekker appears to be a member of this family.

Abel describes the parietal bones as forming a broad band across the vertex between the frontal and occipital shields, a condition noted by Lydekker and not shown on his type figure or the London specimen later studied by Abel. Such a character, if verified, would indicate the presence of a degree of telescoping distinctly anterior to that present in Squalodon, and would place the genus Prosqualodon in its own family.

NO. 5 TELESCOPING OF THE CETACEAN SKULL 47

Various extinct European genera, such as Champsodelphis, CyrtodelphU, and Acro-delphishash, were supposed to be allied to Inia, chiefly on account of the presence of a deep longitudinal groove on each side of the lower surface of the mandible. In Lipotes and Inia very little is shown, although these genera are most closely allied to Stenodclphisthanania of the three is to Platanista. The presence of Lipotesin in the Old World makes the eventual discovery of extinct members of the subfamily seem certain; but their positive identification will depend on finding material sufficiently well preserved to show the true structure of the thepterygoid and maxilla, as well as the size of the temporal fossa and the extent to which the frontal participates in the formation of the lateral wall of the skull. the brain.

DelphinincB. – After removing Inia and Lipotes as families and Stcnodelphis, Delphinapteriis, Monodon and, provisionally, Eurhinodelphis as representatives of the four subfamilies, dolphins become a rather compact group characterized above all by a high degree of telescoping. of the braincase, the small temporal fossa, the small pterygoid, duplicated only on its inner side and not extending over the alisphenoid; normally growing teeth (never canines); The orbits are located so posteriorly that they lie in the same transverse plane as the nasal passages. As the nasal passages were pushed back as far as the brain would allow, it would seem at first sight that the equally posterior position of the orbits might be regarded as a feature of high specialization in comparison with the more anterior position they occupy.

Monodon, Delphinapterus, the iniids, and even more strikingly in Platanista, a position from which there seems to be less deviation.

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The structure of the rostrum does not differ greatly from that of the rostrum of Stcnodclphis. Diagonally across the side of the rostrum, from the main lateral sulcus to the alveolar level, in the position marked by the white suture line in Abel's plates, there is a faint groove of about 3 mm. This seems to indicate the course of some nerve or blood vessel which ran forwards and downwards along the surface of the maxillary from the lateral sulcus to the anterior part of the roof of the mouth.

If the characters of the European specimens are not more unusual than those of this Maryland skull, there seems no reason to regard Eurhinodclphis as the representative of a distinct group. The characters of the group are found in the structure of the braincase, the temporal fossa. The external reduplication of the pterygoid is like that of Dclphinaptcrn, but still better developed; the internal reduplication.

Orbits relatively smaller than in Delphinhicc, their position immediately opposite the transverse plane occupied by the narial passages; these features agreeing with the condi-.

52 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 76 posterior orifice to the infraorbital canal, and by the situation of the

54 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 76 rostrum until the infraorbital foramen is brought to a position behind

EXPLANATION OF PLATES

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The large orbital plate in the baleen whale is a structure absent in both the zeuglodont and the toothed whale. The missing tear lay in the depression of the jaw in front of the jugal, as can be seen in fig. 3 of plate 6.).