A COMPLETE MOSASAUR SKELETON,
OSSEOUS AND CARTILAGINOUS.
By Henry Fairfield Osborn.
Copyright ?2008-2009 by Mike Everhart
Page created 04/18/2008 - Last updated 08/02/2009
LEFT: Photo of four-foot long skull of Tylosaurus proriger (AMNH FR221) by Matt Everhart, December 2006. ?Matt Everhart, used with permission. (Click to enlarge)
The following web page is excerpted from an article published by H.F. Osborn in 1899 that provides one of the earliest descriptions of a complete mosasaurs specimen. What makes it truly unique is that it also was the first time that detailed photographs of a complete mosasaur specimen were published (Williston (1898) had just published photographs of individual mosasaur remains as figures in his University Geological Survey (Examples: Plate LXII - Plate LXVIII). While we take such photography pretty much for granted today, it was just beginning to used in paleontology in the late 1890s and required a considerable amount of equipment and effort.
Osborn, H.F. 1899. A complete mosasaur skeleton, osseous and cartilaginous. Memoirs of the American Museum of Natural History 1(4):167-188.
I have included the first four pages of the text and most of the figures, with captions, below. Note that Osborn's mention of a nuchal (dorsal) fringe and the appearance of it on the restoration by Charles Knight was based on an erroneous interpretation of the tracheal cartilages in a specimen by S.W. Williston, which Williston quickly retracted. However, by then it was too late. Once Knight had shown the fringe in his paintings, it became a feature of many mosasaur reconstructions.
NOTE: Osborn's paper describes the specimen as Tylosaurus dyspelor, which is a junior synonym of Tylosaurus proriger.
IV. A COMPLETE MOSASAUR SKELETON,
OSSEOUS AND CARTILAGINOUS.
By Henry Fairfield Osborn
Plates XXI, XXII, and XXIII.
In the spring of 1898, Prof. S. W. Williston's fine memoir upon the Kansas Mosasaurs seemed to cover the subject completely, summing up all the facts derived from the great Kansas University collection, as well as many of the results of the labors of Cuvier, Owen, Marsh, Cope, Dollo, Baur, and others. But it appears impossible to say the last word in palaeontology. Professor Williston himself has recently described a portion of the nuchal fringe of Platecarpus, as well as the epidermal fin contours. The remarkable specimen which forms the subject of the present brief memoir throws new and welcome light not only upon Tylosaurus but upon the anatomy of the Mosasaurs in general.
Together with the practically complete bony skeleton, the chief feature is the unique preservation of the cartilages of the throat and chest, portions of the larynx, trachea, bronchi, the epicoracoids, as well as the suprascapul? the sternum, and sternal ribs. Originally these parts were preserved entire, and we must deeply regret that before this specimen came into possession of the Museum, much damage was done to the relatively inconspicuous cartilages, in course of removal of the bones. Nevertheless 'Mr. Bourne, of Scott City, Kansas, who excavated the fossil, deserves great credit for the skill and care with which the conspicuous parts were removed.1
1 The specimen was examined before its purchase by Dr. W. D. Matthew, and packed for shipment by Mr. Handel T. Martin. At the time no one could judge of the existence of the cartilages or of the exceptionally complete condition of the skeleton.
168 OSBORN, A COMPLETE MOSASAUR SKELETON.
The specimen reached the Museum in a series of large slabs of Kansas chalk, and was worked out under the direction of Mr. Hermann, by Mr. Thompson. With one exception, all the contours of the original slabs were preserved and fitted together by their edges, as in the original bedding; therefore the animal with all its parts, excepting a few minor pieces, lies exactly as it was imbedded in the rocks. The original matrix surrounds practically all the bones, and can be distinguished from the buff-colored outlying plaster, even in the photographs, by its somewhat darker shades. The whole is mounted upon a panel twenty-five feet long and permanently placed in a corridor which is to be devoted to marine reptiles.
Position of the Skeleton. -The animal lies outstretched upon its ventral surface, so that all the bones are exposed upon the dorsal or lateral surfaces, excepting the left humerus and ulna, which are overturned. The skull is crushed to
Fig. I. Complete skeleton of Tylosaurus dyspelor, in frame. 1/54 nat. size.
the left, together with the vertebrae, as far back as the 6th dorsal. From the 7th to the 10th dorsals the vertebrae are confused and displaced. The 11th dorsal to 29th caudal are horizontal with the transverse processes outspread and the spines crushed to the right and left. The remaining caudals, 30th-70th, lie upon the left side apparently in a natural position. The pelvis and hind paddles have evidently shifted backwards in settling, so that the mooted question of the position of the sacral vertebra cannot be positively settled by this specimen.
Examination and Restoration. - The study of the animal has been cooperative. Dr. W.- D. Matthew has carefully examined several regions, and made a number of original suggestions and valuable criticisms in points of interpretation, especially as to the remarkable tail curvature, the atlas complex, and the general structure of the vertebrae. Dr. J. H. McGregor has greatly aided the writer in studying and restoring the sternal region. The photographs are the work of Mr. A. E. Anderson. The drawings are by Mr. Bruce Horsfall.
OSBORN, A COMPLETE MOSASAUR SKELETON. 169
Tylosaurus dyspelor Cope. [see note above]
Specific Characters. -This specimen agrees very closely in size with Cope's cotype of T. (Liodon) dyspelor, found in 1871 at Fort Wallace, Kansas, and described by him in the 'Cretaceous Vertebrata' (p. 167). The skull agrees exactly in size with the fine one mounted in the Munich Museum, described by Merriam (1894, Taf. II) as T. proriger. Size is no criterion, or at best an uncertain criterion of a species, but Williston advances (1898, p. 175) no other satisfactory means of separating T. dyspelor from T. proriger. Thirty-five feet is the length assigned by this author to the largest Tylosaurs, a length considerably exceeding that of the present specimen. It is evident that a young T. dyspelor might exhibit exactly the measurements of T. proriger.
We observe, however, in this specimen certain characters which may possibly prove to specifically distinguish this type from T. proriger, as follows:
1. 22 dorsals. Williston assigns 23 dorsals to T. proriger.
2. No rib upon the axis. A rib if present upon C.3 was certainly very small.
Williston figures ribs upon both axis and C.3.
3. A dorsal curvature of the mid-region of the tail, not observed in T. proriger.
4. Phalanges in the manus estimated at 39. In T. proriger, same estimated at 47.
None of these characters, however, are absolutely determined in both types, so as to be clearly distinguishable. A summary of the chief anatomical characters is given in the conclusion.
Measurements and Proportions.
WITH SLIGHT CORRECTIONS FOR CRUSHING.
Eng. Meas. Metres.
Skull, from back of supratemporal arch to rostrum .................... 3 11 1.19
Jaw, angle to rostrum, approximate ................. ..3 10 1.16
Seven cervical vertebrae, actual ..... ............ . . 1 11 .58
Twenty-two dorsal vertebrae,actual ................. . 7 11 2.41
Ten dorsals, with sternal ribs, actual .. ............... 3 7 1.09
Twelve dorsals, with floating ribs, actual ........................ 4 11 1.32
Seventy caudals and pygals, actual length, as mounted (including
spaces left for eight intermediate caudals towards extremity of tail) 13 9 4.20
Total length of tail, estimated ............... 15 4.57
Fore paddle from head of humerus to tip, estimated ................ ... 2 11 .90
Hind paddle from head of femur to tip ................. 3 3 .98
Total length from tip of rostrum to last, or 78th, caudal, as mounted .. 27 4 8.34
According to Williston the tail terminates very abruptly in Tylosaurus proriger, in contrast with its gradual and slender termination in Platecarpus, as described below. If this was the case in this specimen of T. dyspelor, we
170 OSBORN, A COMPLETE MOSASAUR SKELETON
should not allow more than 15 inches or 38 centimetres additional, giving us a total length of about 29 feet or 8.83 metres.
The proportions of different regions of the body, as Williston has shown, are very characteristic of different genera of Mosasaurs. In this individual the total of 29 feet or 9 metres is roughly distributed as follows:
Head and jaw 4 1.22
Neck . 2 .61
Back 8 2.44
Tail . . 15 4.56
Thus the back is four times the length of the neck, twice the length of the head, and about one half the length of the tail. In other words, the tail is longer than the other regions of the body combined. These proportions are carefully observed in Mr. Knight's restoration.
|LEFT: Fig. 7. Anterior chest section of Tylosaurus dyspelor. X 1/10.|
|LEFT: Fig. 8. Epicoracoids and Tracheal Rings of Tylosaurus dyspelor. 2/5 nat. size. [Note that the tracheal rings are composed of cartilage which apparently became calcified sufficiently in this specimen to be preserved]|
|LEFT: Fig. 9. Shoulder Girdle and Paddle of Tylosaurus dyspelor. Restored portions in outlines. Drawn by J. H. McGregor. 1/14 nat. size.|
|LEFT: Fig. 11. Right Shoulder
Girdle and Fore Limb of Tylosaurus dyspelor. 1/6
|LEFT: Fig. 12. Pelvic Girdle and Left Hind Limb of Tylosaurus dyspelor. 1/6I nat. size.|
|LEFT: Fig. 13. Tylosaurus dyspelor. Restoration after drawings by W. D. Matthew and Bruce Horsfall, under direction of the author. 1/40 nat. size.|
|LEFT: Fig. 14. Restoration of the Ram-nosed Tylosaur, by Charles Knight.|
XXI -Upper Figure (see color photo above; note white lettering on bones). Skull, 1/8
natural size. The skull is crushed over to the
left, and thus exposes the right side of the face, the left external nares, the
naso-premaxillaries (pmx.), maxillaries (mx.), lachrymals (la.),
prefrontals (pr. f.), and the complete undistorted upper surface of the frontals
(fr.), perforated posteriorly by the pineal foramen. From this point the
supratemporal arcade (postfronto-orbital, pf. o.), and prosquamosal, (pr. s.)
extends upwards and backwards, making an uplifted acute angle with the squamosals (sq.)
and squamoso-parietal bar. In the space below this angle appear the parietals (pa),
prootics (pro.), exoccipitals (eo.),=exoparoccipitals). The occipital
bones enter to a limited-degree into the condyles.
The basicranial axis is beautifully shown: Basioccipitals, bo., with prominent basioccipital processes (which are lacking in Varanus). Basisphenoid, bs., with two pterygoid processes directed downwards and forwards (as in Varanus). Presphenoid, ps., a long splintered bar extending over the displaced left pterygoids and sclerotics, scl. In the centre of the skull mass lie the left jugals, ju., beneath which are the left pterygoids, pt., and the right pterygoids. The right jugal lies below the jaw. The ectopterygoids are possibly represented by a small bone lying just above the prootics. The exposure of the left pterygoid is interesting because it displays a large fossa for the epipterygoid. This element itself, ? ep., is probably represented by a large rod-like bone' lying beneath the basisphenoid. At the inferior extremity of this bone appears a slender rod which possibly represents the columella auris, or stapes.
Supraciliare. Below the jaw is a small element which can only be identified as a portion of the supraciliare.
|LEFT: PLATE XXI - Lower Figure - Chest and neck section, and shoulder girdle, 1/14 natural size. The axis of Tylosaurus has been figured by Cope (Cretaceous Vertebrata, P1. 29) with a narrow spine; in this specimen the axis is vertically crushed, and the form of the spine cannot be positively determined, but the base indicates that it was broad like that of Platecarpus; the diapophysis (transverse process) is a broad flattened lamella which bears no sign of a distinct rib facet; such a facet is present in Platecarpus, and is figured by Williston in T. proriger (op. cit., P1. 72). This vertebra undoubtedly bore an intercentrum. Cervicals 2-6 are distinguished by the following characters: 1. Broad diapophysial plates (broader than those of Platecarpus, Fig. 2), which bear rib facets posteriorly. The rib upon C. 3, if present, was very small. 2. Neural spines, increasing in antero-posterior diameter. 3. Stout zygapophysial processes and facets. 4. The intercentra are figured upon C. 2-C. 6 (as in Platecarpus) by Williston, but cannot be observed in this specimen, owing to the horizontal flattening of the vertebrae. The cervical ribs are thus .less strongly developed than in Platecarpus, in which they are present from C. 2 to C. 7. Cervical 7, according to Williston, in T. proriger has no intercentrum (hypapophyseal process). Cervicals 6 and 7 bear large ribs to support the muscles of the scapula which directly overlaps these ribs. (See Restoration, p. 186.|
|LEFT: PLATE XXII - Upper Figure -Sacral and pygal section of pelvic girdle, 1/14 natural size. In this specimen, as in the living Monitor lizards, the 30th vertebra behind the head is distinguished by the absence of a rib, and by the sudden expansion of the diapophysis. This first expanded vertebra, as determined by Williston, must be considered the sacral, analogous with the most anterior of the two sacrals in Varanus. There is a discrepancy between Williston's table (I898, p. 143) in which the sacral is said to be the 31st vertebra of the spine, and his plate 72, in which it is represented as the 32d. This vertebra is not perceptibly different in size from the pygals behind it. Unfortunately the tips of the diapophyses are not preserved, and there is no means of demonstrating positively that the ilium was attached by joint or ligament. The crest of the ilium is actually removed 28 centimetres from the tip of the diapophysis.|
|LEFT: Plate XXII- Lower Figure - Caudal vertebrae,
showing natural curvature, 1/14 natural size. The postsacrals 1-18 continue backwards,
lying in a horizontal plane and covering the line of demarcation or passage to the
chevron-bearing caudals. They exhibit a gradual diminution in all their dimensions.
Beneath postsacral 11 is a small fragment of bone which represents part of a chevron.
Beneath postsacral 19 is an unmistakable chevron, and these elements are beautifully shown
behind this point, especially upon caudals 29 to 63; they are deep with a wide canal for
the caudal aorta. The diapophyses steadily diminish in size as we pass from postsacral 1
to caudal 38. In caudal 30 they begin to ascend upon the sides of the centrum, and in C 38
they rise to a point just below the neural spine. There is no sign of a diapophysis upon
BELOW: PLATE XXIII - Tylosaurus dyspelor Cope
Cope, E. D., 1875. The Vertebrata of the Cretaceous formations of the West. Report, U. S. Geological Survey Territories (Hayden). 2:302 p, 57 pls.
Merriam, J. C. 1894. Ueber die Pythonomorphen der Kansas-Kreide. Palaeontographica, 41:1- 39, 4 pls. (in German)
Parris, D. C., B. S. Grandstaff, R. K. Denton and S. G. Lucas, 1997. Type locality of Liodon dyspelor (Reptilia: Mosasauridae), Proceedings Academy Natural Sciences of Philadelphia, 147:193-203.
Williston, S. W. 1898. Mosasaurs. The University Geological Survey of Kansas, Part V. 4:81-347, pls. 10-72.