  BY S. W. WILLISTON AND E. C. CASE. PART I, CLIDASTES, WITH PLATES II-VI. The group of extinct Cretaceous reptiles known as the Mosasaurs or Pythonomorpha was defined by Cope, “to whom Science is so largely indebted for its present knowledge of this interesting order of reptiles” (Marsh), in 1869.
The group, whatever may be its rank or position, includes, so far, the following genera: Mosasaurus Conyb., Liodon Owen, Platecarpus Cope, Clidastes Cope, Baptosaurus Marsh, Sironectes Cope, Plioplatecarpus Dollo and Hainosaurus Dollo. Pterycollasaurus Dollo, founded upon Mosasaurus maximilianus Goldf., is omitted as doubtful. All of these genera, save Plioplatecarpus and Hainosaurus, have been recorded from North America, Clidastes, Baptosaurus and Sironectes being peculiar to this country. Of these latter three genera, however, Clidastes alone is well known; but this genus is suspected by Lydekker of being the same as the imperfectly known European Geosaurus Cuvier. Thus it seems that the genera, or at least the most of them, have a wide distribution; Platecarpus, in fact, is said to occur in New Zealand. In America, members of the group have been discovered in the Cretaceous deposits of New Jersey, Alabama, North Carolina, the upper Missouri region, Nebraska, Kansas and New Mexico. Probably nineteen-twentieths of all the known specimens, however, have been obtained in western Kansas. The material now in the University Museum, all from Kansas, comprises several hundred specimens of these animals, including, probably, the best ones known. It is upon this material that the following preliminary studies are chiefly based. The genus Clidastes, as first described by Cope, was based upon two dorsal vertebrae of C. iguanavus, the type species, from New Jersey. Shortly afterward, however, he gave a full and careful generic description, as derived from an unusually good specimen of an allied species, C. propython, from Alabama. Only a little later, Marsh described a genus, which he called Edestosaurus, from Kansas, but without giving any real, distinctive differences from Clidastes, following the very reprehensible practice of naming supposed new forms in the hopes that future distinctive characters might be found. The genus Edestosaurus has been rejected by nearly all save the authors of the American text-books in Geology. It seems hardly necessary to point out the identity. The only distinctive character the author gave for his genus was the insertion of the pterygoid teeth, and even this character he modified later—“Palatine (sic) teeth more or less pleurodont.” This character, even were it real, is of very slight value; indeed it cannot be used to distinguish the species even. Clidastes is, without doubt, one of the most highly specialized genera in the group, and, what is very interesting, is one of the latest. It occurs in Kansas in the uppermost part of the Niobrara beds, in the horizon so markedly characterized by the toothed birds. Both Platecarpus and Liodon occur, though in diminished numbers, almost to the very lowest portion, but Clidastes has never been found except towards the top. From measurements made the past season, the thickness of the beds in which these saurians occur cannot be less than six hundred feet. The following species have been found in Kansas: none of them are known to occur elsewhere. MOSASAURIDAE.Mosasauridae Conybeare, in Cuvier, Ossem. Foss., 2nd ed., p. 338, 1824. Clidastidae Cope, Extinct Batr. Rept. and Aves of N. Amer., Trans. Amer. Phil. Soc. xiv, p. 50, 1870. Edestosauridae Marsh, Amer. Journ. Sci. xxi, p. 59, July 1878. CLIDASTES.?Geosaurus Cuvier, Ossem. Foss. 2nd ed., 328, 1824, (fide Lydekker.) Clidastes Cope, Proc. Acad. Nat. Sci. Phil. 1868, p. 233; Ext. Batr. etc., p. 21, 1870. Edestosaurus Marsh, Amer. Journ. Sci. i, p. 417, June, 1871. C. cineriarum.Clidastes cineriarum Cope, Proc. Amer. Phil. Soc., 1870, p. 583; Cret. Vert. etc. pp. 137, 266, pl. xxi, ff. 14-17; Bullet. U. S. Geol. Surv. Hayden, iii, p. 583. C. dispar.Edestosaurus dispar Marsh, op. cit. i, p. 447, June 1871; iii, pl. xi., June, 1872. C. velox.Edestosaurus velox Marsh, Amer. Journ. Sci. i. p. 450, June, 1871. ?Clidastes affinis Leidy, Proc. Acad. Nat. Sci., 1870, p. 4; Rep. U. S. Geol. Surv., Hayden, vol. i, p. 283, 1873. ?Edestosaurus dispar Marsh, op. cit. xix, pl. i, f. 1, Jan., 1880. C. Wymani.Clidastes Wymani Marsh, Amer. Journ. Sci. i, p. 451, June, 1871; iii, p. 202, April, 1872. Edestosaurus Wymani Marsh, op. cit. iii, p. 464, June, 1872. C. tortor.Edestosaurus tortor Cope, Proc. Amer. Phil. Soc. Dec., 1871; Marsh, op. cit. iii, p. 464, June, 1872. Clidastes tortor Cope, Cret. Vert. Rep. U. S. Geol. Surv., Hayden, vol. ii, pp. 48, 131, 265, pls. iv, f. i; xiv, f. i; xvi, ff. 2, 3; xvii, f. 1; xix, ff. 1-10; xxxvi, f. 3; xxxvii, f. 2; Bullet. U. S. Geol. Surv. Hayden, vol. iii, p. 583. C. stenops.Edestosaurus stenops Cope, Proc. Amer. Phil. Soc. p. 330, 1871: Marsh, Amer. Journ. Sci. iii, p. 464, June, 1872. Clidastes stenops Cope, Cret. Vert. etc. pp. 133, 266, pls. xiv, ff. 4, 5; xvii, f. 7, 8; xviii, ff. 1-5; xxxvi, f. 4; xxxvii, f. 3; xxxviii, f. 3. C. rex.Edestosaurus rex Marsh, op. cit. iii, p. 462, pl. xxii, f. 1, June, 1872. C. planifrons.Clidastes planifrons Cope, Bullet. U. S. Geol. Surv. No. 2, p. 31, 1874; Cret. Vert. etc. pp. 135, 265, pls. xxii, xxiii. C. Westii.C. Westii Williston, n. sp. infra. CLIDASTES VELOX.A remarkably complete specimen, referred with considerable certainty to this species, was obtained by ourselves in western Kansas, (Butte Creek) in the summer of 1891. A brief preliminary description of the Cervical vertebrae.Atlas. The intercentrum is a small bone with three sides of nearly equal extent. The two upper, articular surfaces are gently concave, and meet in a rounded margin; the inferior surface is convex, both antero-posteriorly and transversely, with a roughened prominence in the middle. The lateral pieces have indistinctly separated facets for articulation with the odontoid, the intercentrum and the occipital condyle. The rather short, flattened lamina extends upward, backward and inward, approaching, but not reaching its fellow of the opposite side; it is somewhat dilated distally. Directed outwards and forwards, there is a stout styliform process. Axis. The neural spine of the axis is elongated antero-posteriorly. It is thin on the anterior portion, but stouter and longer at the posterior part. The large, stout odontoid process is united suturally, as is also the well-developed atlantar hypapophysis, which forms the anterior, inferior portion of the bone. The diapophyses are the smallest of the costiferous series, with only a small articular facet for the rib. The ball is strongly and evenly convex, with its greater diameter transversely. The hypapophysis is the largest of the series; it is suturally united with the stout, exogenous process of the centrum, and projects downward and backward; its distal extremity is roughened for ligamentous attachments. The third cervical vertebra shows a well-developed zygosphenal articulation, and stout articular processes. The transverse process is small, only a little larger than that of the axis, though, unlike that, it is strengthened by a ridge continued from the anterior zygapophyses. The hypapophysis is smaller than that of the axis, but, like that, is directed downward and backward. The spine may be distinguished from that of any other vertebra by its stout, trihedral shape; it is directed rather more obliquely backward than in the following vertebrae. The fourth cervical vertebra differs from the third in having stouter transverse processes; in the hypapophysis being directed more nearly downward, and in its smaller size; and in the spine being flattened antero-posteriorly toward the base. The fifth cervical vertebra differs from the fourth in the broader spine, in the stouter transverse processes, and the smaller hypapophysis. In the sixth cervical vertebra, the hypapophysis is reduced to a small ossification, scarcely longer than broad, directed downward. The spine has reached nearly the full width of those of the following vertebrae, though somewhat stouter above. The transverse processes are yet stouter. In the seventh, or last, cervical vertebra the hypapophysis is wanting, or very rudimentary. The under part of the centrum shows a rounded ridge or carina, with a slight projection corresponding to the hypapophysis. MEASUREMENTS OF THE CERVICAL VERTEBRAE.
Dorsal vertebrae.There are thirty-five vertebrae between the cervicals and the first non-rib-bearing vertebra, to which the pelvis was, evidently, attached. The distinction between the cervicals and thoracics cannot be made from any characters they possess, as the seventh vertebra does not bear a distinct hypapophysis. Neither can it be said with certainty from this specimen which is the first thoracic vertebra, as the cervical ribs had, unfortunately, been displaced in the collection and preparation of the specimen. In another specimen, referred to C. pumilus, and which, as will be seen later, cannot be specifically distinguished from the present species, short cervical ribs were found attached to six vertebrae posterior to the atlas. That the eighth vertebra is a thoracic one is shown by the relation of the ribs in this specimen. Posteriorly there is no distinction, also, between the true thoracic vertebrae and those of the lumbar region. All the vertebrae anterior to the pelvis bear ribs, and will all be considered as dorsal vertebrae, the true thoracic vertebrae being restricted to those of which the ribs are elongated, and, probably, connected with the sternum. In the anterior vertebrae of the series, the centra are subcarinate below, the obtuse, rounded ridge becoming less and less apparent until no indications of the keel can be seen, before the middle of the series. The transverse processes are stoutest, with a more elongated, sigmoid articular surface, with little or no constriction, and projecting only slightly beyond the stout articulating processes, in the anterior vertebrae. In the tenth or eleventh, the articular surface has become markedly smaller, more vertical, and less sigmoid in outline. Thence to the last, the articular surface for the ribs remains nearly the same. The process itself, however, becomes gradually more prominent and constricted, as the zygapophyses becomes smaller. The spinous processes increase slightly in length and breadth, and are only slightly oblique throughout. In length, the centra increase gradually. The vertical diameter of the ball increases gradually, while the transverse diameter remains more nearly the same. MEASUREMENTS OF THE DORSAL VERTEBRAE.
Caudal vertebrae.Immediately following the thirty-fifth rib-bearing vertebra there is an abrupt change, the tubercular process for the rib giving place to an elongated transverse process. From the position of the pelvis, it is evident that the ilia were attached to the first pair of these. Precisely this relation of pelvis to the vertebrae is found in such lizards as the Monitor and Iguana, and it is probable that such is the relation in all the Pythonomorpha. It will thus be seen that there are no distinctively lumbar vertebrae, if by such are meant free, MEASUREMENTS OF THE PYGIAL CAUDAL VERTEBRAE.
The centra of those caudal vertebrae which have chevrons do not differ much in shape. They become less constricted, and, back of the middle of the series, are smoothly cylindrical in shape. The transverse processes decrease gradually in length, disappearing entirely in the twenty-fifth or twenty-sixth. The spinous processes are more or less incompletely preserved in the anterior vertebrae. They increase only gradually in length for the first twenty of the series, and are markedly oblique, with the posterior border stout, and the anterior border alate. With the twenty-sixth they begin to increase more rapidly in length, and have become more nearly vertical in position, and are thinner at each margin. In the thirty-fifth or thirty-sixth they attain their greatest length, and are here directed slightly forwards. Thence to the end of the tail, the length decreases gradually, and, in position, they are directed more and more obliquely backward. The chevrons are strongly oblique throughout the series and are firmly co-ossified with the centrum. The tail, it is thus seen, has a broad, vertical, fin-like extremity, which, doubtless, aided much in the propulsion of the animal through the water. There are sixty-seven vertebrae with chevrons present in the specimen, all continuous, except in one place. The last one is less than one-fourth of an inch in diameter, and shows that there had been yet another, possibly several more. Toward the base of the series the tail has been bent forwards over the back, and it is possible that, where the break occurs, there has been a vertebra lost. The measurements, however, do not seem to indicate any loss. The entire series of vertebrae was not less than sixty-eight, and probably not more than seventy, making for the entire vertebral series one hundred and seventeen to twenty. MEASUREMENTS OF THE CHEVRON-BEARING CAUDAL VERTEBRAE.
Ribs.As has already been stated, the cervical ribs were displaced in the present specimen, and measurements of them cannot be given. In a smaller specimen, specifically indistinguishable from the present one, the entire cervical series is preserved with the ribs attached. The first, that articulating with the axis, is very short. The following ones are stouter, but increase only moderately in length, that of the sixth measuring only thirty-five millimeters, while that of the seventh is but a little longer. In the specimen of C. velox described, there is a detached cervical rib sixty-five millimeters in length; it probably belongs with the seventh. The thoracic ribs are simple, somewhat flattened rods, moderately expanded at the proximal end. The greatest convexity is shown about the middle of the series, where the versedsine of the curvature is forty millimeters, the chord being one hundred and sixty. Posteriorly, the short ribs are only gently curved. Lying by the side of the vertebral column, and between the ribs, as they have been pressed down, are a number of flattened, soft, punctulate bones, which are evidently the costal cartilages. Posteriorly four rows of them are seen, lying closely side by side, some of them eight or ten inches in length. The sternum, composed of the same material, has been so crushed and crumpled that its shape cannot be made out. The whole structure here, whether of ribs, cartilages or sternum, reminds one very strongly of such lizards as the Iguana or Monitor. There is no indication, however, in any specimen, of an episternum. MEASUREMENTS OF RIBS.
The lengths of the different regions, as they lie in their natural relations, are as follows:
The measurements of an excellent specimen of C. tortor are as follows:
A very complete specimen of a Liodon in the Museum, in which the complete vertebral column is present, numbering one hundred and seventeen vertebrae, gives the following measurements. The skull is complete, save the most anterior portion.
The vertebral series in this specimen is composed of seven cervicals, twenty-three dorsals, seven pygials, and eighty chevron-caudals. The relative proportions of the different regions in the two genera, as shown by the two specimens of Clidastes and Liodon, may be represented as follows. The first column is for Clidastes.
Limbs.The figures in plates II and III will give a sufficiently good idea of the limbs in this specimen. They are figured as they were lying, showing the outer sides of the coracoid, scapula and pelvic bones, and the palmar or plantar surface of the remaining bones. Coracoid.It will be observed in plates II and IV that there are two very different types of coracoid, one with a deep emargination, the other without the slightest indication of such. The same non-emarginate form occurs in C. tortor, as specimens in our Museum show, in C. propython Cope (Ext. Batr. etc. pl. xii, f. 16,) and in C. dispar, as figured by Marsh That the emargination was overlooked by the author seems strange, as in the same paper in which this figure is given occurs the description of Holosaurus, founded upon that very character. If the emargination is sufficiently important to base a genus in the one case, then it should be in the other, and the character could not be applied to Edestosaurus, based upon characters which it hardly seems possible that the author himself could seriously consider, for E. dispar was the type of Edestosaurus. It will be observed, further, that the figured coracoids differ very materially in size, those with the emargination pertaining to a small species, while C. dispar is one of the largest. In our Museum there are three specimens with the emarginate coracoid, all of them small or very small, the described specimen of C. velox being the largest. The point of chief interest in this relation is the value that can be given to this character. Is it individual, specific or generic? Marsh has called it generic, but we think an examination of the two very complete specimens of C. tortor and C. velox in our Museum will convince any unprejudiced student that he is in error. A comparison of the figures herewith given of the paddles will show their great resemblance, and these two forms of paddles have been figured because the species are the most unlike of any that we know in the genus. As all the small specimens seem to possess this character, and as they cannot be called immature specimens, we believe the character is a specific one. As Marsh says, typically both Clidastes and Edestosaurus have a non-emarginate coracoid, so that neither name could apply to the emarginate form, were it generically distinct. Our Museum also contains both forms of the coracoid pertaining to the genus Platecarpus, of which Holosaurus is a synonym. While studying the specimen above described, a striking similarity was observed to several other specimens already determined with confidence as C. pumilus Marsh. A more careful comparison failed to bring out any real differences beyond size, and even this was shown to be very inconstant. The following comparison of the descriptions given by Marsh will be of interest.
The description, otherwise, shows no discrepancies of importance. The chief difference given by the author is the size, and this character we think our specimens show to be of little specific value. “It is a question of some importance how far difference in size among the Mosasauroids may be a test of difference in species. Among the numerous remains of these animals which have been discovered I have never yet observed any which presented any evidence relative to age. * * * In this view of the case, some of the many described species of Mosasauroids may have been founded on different sizes of the same.” The length of the cervical vertebrae in the specimen above described is thirty-seven or thirty-eight millimeters. The cervical vertebrae in two specimens referred to C. pumilus have lengths respectively of twenty-two and thirty millimeters. In the type specimen of C. velox they must have had a length of at least forty-two millimeters. It thus appears that, between the smallest specimen, which, in life, could have hardly exceeded eight feet in length, our specimens, indistinguishable anatomically, represent forms of ten and twelve feet, while the type itself was about fifteen feet in length. Of the material originally referred to C. pumilus, there are in the collection five or more specimens, which, altogether, furnish nearly every part of the skeleton. They present no tangible differences from the skeleton of C. velox described above. There can be, hence, little or no doubt but that the name C. pumilus is a synonym. It is hardly possible to say with certainty that C. affinis Leidy is or is not the same as C. velox, but, so far as the description goes, we can find few differences. The type is of about the same size as the type of C. velox, and the figures agree well with the bones of the skeleton described. Although the description was not published till 1873, the author makes no mention of the species of Marsh’s. Leidy describes the back teeth as having the enamel strongly striated, with the surface presenting evidences of subdivision into narrow planes. In this respect, only, it disagrees with the specimen. Plioplatecarpus Dollo is described by its author as having a sacrum of two conjoined vertebrae, CLIDASTES WESTII, N. SP.A specimen of much interest in the University collection differs so markedly from the other forms represented by specimens, as also from the descriptions of the known species, that we are constrained to regard it as new. It was collected by Mr. C. H. Sternberg from the uppermost of the Niobrara beds, in the vicinity of the old town of Sheridan. The character of the associated invertebrate fossils seems to indicate a different geological horizon, either the Fox Hills group, or transition beds to that group. The specimen consists of a complete lower jaw, quadrate, portions of the skull, the larger part of the vertebral column, and the incomplete hind and fore paddles. The vertebrae preserved are in two series, the one, numbering thirty-three, continuous with the skull; the other, sixty-three in number, all chevron caudals. The terminal caudals preserved indicate that there were several more in life, perhaps five or ten; the first of the series was evidently among the first of those which bore chevrons. Altogether the tail may have had seventy-five chevron caudals. The lengths of the two series are respectively seventy-one and seventy-two inches. Assuming that there was the same number of precaudal vertebrae as in C. velox, the entire vertebral column would have measured in life fifteen feet and four inches. The lower jaw shows the skull to have been very nearly twenty-four inches in length, making, for the animal when alive, a length of seventeen and one-half feet. This is one of the largest species, and it is interesting to observe that the real size here, as usually elsewhere among fossil vertebrates, is less than supposed. It is doubtful whether there is a Clidastes known that exceeded twenty feet in length. While the skeleton was only about one half longer than the specimen of C. velox described in the foregoing pages, or of about the same length as a very complete specimen of C. tortor in the museum, the proportions of the animal were very much stouter. The figures given in plate VI of the twenty-fifth, or eighteenth dorsal, vertebra will show the relations between length and breadth: it is upon these remarkably stout proportions, and the shape of the articular faces, as indicated by the figures and by the measurements appended, that the species is chiefly based. The articular surfaces of the basal caudal vertebrae are remarkably triangular in shape, with the angles rounded, and the sides of nearly equal length. This triangular shape is persistent for the first twenty of the series as they are preserved. The paddles, as shown in plates IV and V, show much stouter proportions than in either C. velox or C. tortor. The species comes nearest to C. stenops Cope, but it seems hardly the same. It is, also, evidently allied to C. dispar Marsh. From these and other described species, the following, extracted from the original descriptions, will serve to show the differences, in comparison with the specimen of C. Westii. C. dispar.The articular faces in the cervicals are a broad transverse oval, faintly emarginated above for the neural canal. In the dorsals and lumbars the cup continues transverse, and the emargination is deeper, but in the anterior caudals the outline becomes a vertical oval. There appears to have been thirteen mandibular teeth.
C. Wymani.In the cervical vertebrae, the outline of the articular faces is transversely cordate. The centra of the anterior dorsals are elongate, and much constricted behind the diapophyses. In the anterior caudals, the articular faces are a broad vertical oval.
C. rex.The cervical vertebrae have very broad, transversely oval faces, with indications of emargination. The dorsals are elongated, with transverse faces, and a distinct superior excavation for neural canal. The articular ends of the anterior caudals are vertically oval.
C. stenops.The anterior caudals possess wide diapophyses. Their articular faces are a vertical oval, a little contracted above, sometimes a straight outline. They present a peculiarly elongate form.
MEASUREMENTS OF CLIDASTES WESTII.
This species is named in memory of Judge E. P. West, lately deceased, to whom our Museum owes so much for his long, diligent and faithful labors in the collection and preparation of the geological material. Erratum: P. 17, line 15, for “Edestosaurus,” read Clidastes, and in next line, strike out “Proc. Acad.” etc. |
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