From the ‘Proceedings of the Dorset Natural History and Archaeological Society’ Volume 10, 1889, an article written by by John Clavell Mansel-Pleydell, Esq., F.L.S, F.G.S. entitled ‘Cimoliosaurus richarsoni , Lydekker (n. sp.)
Towards the end of the Mesozoic age a remarkable diminution of the huge reptiles which swarmed in the seas of that period commenced, and at the beginningof the succeeding age, Tertiary their annihilation was nearly complete, occasioned by great physical changes, especially affecting the relative positions of land and sea, the sea predominating largely “over the land in Europe. We pass from strata of considerable uniformity and of immense thickness over large and extensive areas to beds of a great variety of structure, from deep to shallow seas, estuaries, and rivers. With one or two doubtful exceptions, not a single Mesozoic species passed up into the Tertiary strata; the numbers of the new genera and species, greatly exceeding those of the previous age. Western Europe at this period had four considerable seas instead of one as now — the Anglo-Parisian, the Pyrennean, the Mediterranean, and one which covered the western parts of France from Normandy to Nantes.
As the chalk rose above the sea and underwent extensive denudation, a material diminution of temperature resulted, mainly through alterations of the ocean currents, which occasioned a disastrous result upon reptile life. During the deposition of the oolitic beds there was a complete uniformity, for, although occasional subsidences occurred, as shewn by the Oxford and Kimmeridge clays, with evidences of tide-level and shore conditions, no great or important break occurred. At the commencement of the Cretaceous age, on the other hand, there was a gradual submergence of land, accompanied by a considerable extension of the sea-area.
The marine beds of Punfield, near Swanage, which rest upon the great fresh-water deposits of the Hastings sands, are a good illustration of this initiatory change. Its effects are remarkably shown in the Yale of Blackmore, where, there is a great overlap or covering over of the upper oolitic beds by the chalk. The Hastings sands, Purbeck beds, and Portland strata are hidden, causing an apparent unconformity of the beds, as if the Lower Greensand had succeeded the Kimmeridge clay directly, without first covering over the intervening beds. Another subsidence and consequent invasion of the sea occurred during the deposition of the Upper Greensand, which spread itself over the oolitic formations as it passed on westwards, finally resting on the Trias of East Devon.
These changes materially affected the climate and temperature of those parts which came under their influence, especially through the alteration of ocean-currents. What would the climate of the greater part of Europe be, if the Gulf Stream was stopped or deflected? The Atlantic would be deprived of one-fifth of the amount of heat it is now receiving in addition to what it has in virtue of the temperature of space. The temperature would be lowered to a condition of climate as severe as that of North Greenland at the present day. If, again, the warm currents of the North Pacific were to be stopped, the northern hemisphere would be subjected to an entire glaciation. The fossils of the Palaeozoic age seem to indicate a uniform mild or temperate condition of climate, but not so in the succeeding Carboniferous age, which shows signs of reaction.
The late Mr. Godwin Austin found large angular blocks in the carboniferous strata of France, which could only be accounted for by referring their inclusion to the agency of ice-carriage, by glacier or iceberg. Large blocks of granite are met with in Scotland in the detrital beds of the coal-basins, which Professor Geikie and other eminent geologists attribute to glacial action. A large block of crystalline rock resembling granite was found embedded in a pit of white chalk near Croydon, and with it were other smaller boulders, all water-worn and composed of a different kind of rock, together with a compact mass of silicious sand derived from the waste of coast line of crystalline rocks, of which there are none in the neighbourhood of Croydon.
All had sunk together without separating, and must have been firmly held together both during the time that they were floating, and whilst sinking to the bottom of the cretaceous sea. Independent of seasonal changes, circulation between the surface and thesea-depths is aided by the co-operation of heat and gravitation. The Gulf of Mexico, which is not exposed to any cold supply of water from the North Pole, is a perfect reservoir of heat; further north, close to the shore of Massachusetts, is a cold current running southwards 60 or 80 miles wide. There are thus two currents of different temperatures running side by side in opposite directions and only mingling, where their edges impinge upon each other. Again, the Gulf Stream divides itself into several channels, the water of which is warm where the channels are deep, and cold in the shallower channels, occasioned by the water, low in temperature, rising from considerable depths over submarine elevated ridges.
We can now see the influence ocean-currents had, as they have now, upon determining the temperature of the globe, and the consequent disastrous effect upon cold-blooded reptiles when suddenly lowered. We have not time to dwell further upon this part of the subject, nor to show that Europe had not at the commencement of the Tertiary age its present continental character) but an insular one, giving free access to the polar currents without the counteracting exchange of warm equatorial currents.
The nearly complete fossil before us belongs to that section of the extinct reptilian class included in the Order Enaliasaurian or sea-lizards, but subsequently divided by Sir Richard Owen, G.C.B., F.R.S., into the Iclitliyopterygia and Sauropterygia; the former represented by the genera Baptanodon, Opthalmosaurus and Ichthyosaurus, the latter by several genera. Until the year 1841 Plesiosaurus was the only representative of this order in Great Britain.
At that date Sir Richard Owen removed from it two species, Plesiosaurus grandis and Plesiosaurus troclianterius, under a new genus Pliosaurus. The fossils of this genus were first founded upon two limbs, one of which is preserved in the British Museum, ,the other in our County Museum. It had an enormous head, supported by a short neck, in which it approached the great freshwater Saurians of the present day, with characteristic vertebrae, having a tubercular rising in the centre of the centrum, and resembling Plesiosaurus in its fin-bones and elongated phalanges. Their vertical range was restricted to the middle and upper oolites, whereas Plesiosaurus extended from the Rhætic beds right through to the chalk. Plesiosaurus is characterised by a very long neck and a short tail. The vertebrae are deeper and more solid than those of Ichthyosaurus; the neural arches are ancliylosed with strong outstretched transverse blades to strengthen the spinal column and to sustain the strain upon it in shallow water ; coast-lines, estuaries, and rivers probably being the usual resorts of these monsters.
Their remains have been found in the Wealden freshwater deposits. Ichthyosaurus, on the other hand, lived in. the deep seas, visiting the land only occasionally. It has a weak spinal column : the two faces of the centrum nearly meet in the centre, and the neural arches are unanchy-losed, in which respect it differs from Plesiosaurus. The Immerus and femur of some Plesiosauri — e.g., Plesiosaurus Manselii have a third bone in addition to the ulna and radius, and to the tibia and fibula, which T. W. Hulke, Esq., F.R.S., names the os intermedium and places it between the ulna and radius, tibia and .fibula, the homologue of which is found in the front and hind limbs of some living Saurians. A very interesting morphological question arises as to the possibility of tracing the homology of these bones and their relation to the carpal and tarsal elements of the higher vertebrates.
I have already referred to this splendid Plesiosaurian specimen in my paper on the fossil reptiles of Dorset, and expressed my opinion that it might possibly turn out to be Plesiosaurus plicatus of Phillips. I am now inclined to change my mind and to call it Murcenosaurus Leedsii Seeley, a subgeiius of Plesiosaurus characterised by its shoulder and pelvic girdles having only one coraco-scapula and one obturator foramen, and by a difference in the union of the neural arches, as well as by distinct forms of the ulna and radius, tibia, and fibula. Possibly these differences will not be held sufficient by Mr. Lydekker to justify Professor Seeley’s separation. This palaeontologist is now engaged in tabulating and arranging the fossil reptilian remains in the British Museum ; the result of his labours on the Crocodilia and Deinosauria will soon be before the public, as the volume is now in the printer’s hands, and will be doubtless as invaluable an addition to Paleontological literature as are his five volumes upon the Fossil Mammalia of our National Museum.
The remains of this Plesiosaur were found in a bed of Oxford clay in the neighbourhood of Weymouth last winter, and through the indefatigable and intelligent industry of Mr. and Mrs. Richardson, of ‘Montevideo,’ they have been built up in their present satisfactory condition. The head is missing, which is not surprising, as having only one articulation with the neck, and that an exceedingly small one, it possibly became detached before the carcase settled down in its grave of clay ; that a considerable time elapsed previous to its being finally covered over may be inferred by the aggregations of oyster shells upon the vertebrae and bones, which could only have been attached when the body was uncovered. The spinal column consists of 71 vertebras, of which 31 are cervicals, 19 dorsals, 2 sacrals, and 19 caudals. The shoulder-girdle is nearly complete, consisting of coracoids,- scapulas, and pre-scapulse, two fore and one hind limb (humerus and femur), small portions only of the pubes, the ischia and ilia, radius, ulna, tibia, fibula, carpal, and metacarpal bones, several phalanges, and ribs.
VERTEBRAE. — The dorsal vertebras resemble the last two cervicals, the centrum is rough, its height and length about equal, and both shorter than the breadth. In the fore part of the dorsal region the neural spines are inclined backwards, they then become vertical, and afterwards incline forwards. The neural-arches are not well preserved, only a few retaining their transverse processes. The centra are altered in form to allow the ribs to be raised, on the neural arch; their sides are compressed with a foramen near the middle of some ; the neural spines widen and are extremely compressed from side to side ; the position of the transverse processes remain the same throughout. The cervical and caudal vertebrse are characteristics of this long-necked, short-tailed family, by the non-attachment of the ribs to the shoulder-girdle of the former, and by the long chevron bones of the latter.
PECTORAL GIRDLE. — The coracoids have a short median symphysis five inches long; and diverge from their posterior border, taking an outward diagonal direction, and terminating hy a convex sweep outwards into an extremely thin dilated plate. The bones are thickest where the scapula and humerus articulate, forming a transverse ridge or keel. This ridge is equally marked on the dorsal as well as the ventral surface. Their width immediately behind the articulation is 15 inches, the least width across is 20 inches. The length of the scapular-articulation is three inches, looking obliquely and forward, and lies in front of the ridge. The scapula consists of a plate which is anchylosed to the coracoid, and from which a bone rises and ascends towards the dorsal surface, making an angle of about 50° with the central plate. This plate is 6in. long and 4in. broad. The inner margin, which is thin and concave at the base, is a continuation of the curve of the front border of the coracoid bone. There is 110 indication of clavicle or inter-clavicle bones. The inner margin of the ascending plate is concave, the outer straight. The coraco-scapular foramen,(It appears from a complete restoration now made by Mr. Richardson of the pectoral girdle that the coraco-scapular foramen was divided by a median bony bar as is now known to be the case in C. plicatus (Leedsii), of which the original restoration was erroneous.) one of the differences upon which Professor Seeley forms his genus Murcenosaurus, is not subdivided into two foramina, as is the case with many of this family. This continuous foramen is bounded laterally by the concave inner border of the scapula and posteriorly by the anterior margin of the coracoid. It is 14in. wide from side to side and 4in. from the anterior to the posterior margins.
PELVIC BONES. — The pubes are thin, a small portion only of them is preserved, and there is no indication of the syinphysis, this part of the bone being unfortunately lost. The outer margins are compressed from side to side, and are not so deep as those of the coracoid. The length is 18¾ in. Both the ischia are well preserved. Their length from the median line to the femoral margin is 8in. ; breadth at distal end, 5⅛in.; at proximal end, 8¼in.; at the narrowest part, 2¼|in. The iliac bones are expanded at both extremities, so as to extend over the upper part of the head of the femur.
HUMERUS. — The third part of the proximal end of the humerus is cylyndrical and thick ; it then widens into a broad distal end, shewing an articulate surface.
Cimoliosaurus richardsoni. — Ventral aspect of part of the right pectoral limb ⅛ nat. size; h humerus ; tr, trochanter of ditto; r, radius; u, ulna ; r’, radiale; i, intermedium; u’, ulnare.
The ulna and radius are short, the radial portion concave ; two of the carpal bones are trigonal, the rest are polygonal.
FEMUR. — The articular surface of the femur is deeply pitted and tuberculate. The proximal end is constricted below the head before it begins to expand. Both margins are nearly straight and gradually flatten out into a broad distal end. Length 1⅝in., breadth 8in., 3¾in. at the narrowest part of the shaft. The tibia and fibula, and several of the carpal and phalangal bones, are well preserved.
Since this paper was read last autumn before the members of the Club, the Plesiosauridœ have undergone a complete revision under the experienced and critical eye of Mr. Lydekker, F.G.S., to whom I am indebted for his valuable assistance in the classification of this saurian. He refers Mr. Richardson’s saurian to the genus Cimoliosaurus which he distinguishes from Plesiosaurus on account of structural differences, especially in the shoulder-girdle, which are of so marked a character as to require a generic distinction.
He restricts Plesiosaurus proper to those whose scapulae do not meet in the median line throughout their whole extent from the upper to the lower margin, hut diverge anteriorly about half-way down. The scapulae are rod-like, small, and narrow, and widely separated from each other, resting diagonally upon a long plate (omosternum) which is wedged into the coracoid at its summit, taking the place of the clavicle of mammalia and of some reptiles. The anterior portion of each scapula lies at right angles to the dorsal portion, which has a long projection. Cimoliosaurus, on the other hand, has large, broad scapulae, which meet at the median line throughout, and are in the same plane with the coracoids, forming with these one shield-like plate. The size and strength of the scapulae do not require the supporting bone omosternum of Plesiosaurus. The dorsal plates, as with the Plesiosaurus are at right angles to the ventral, but differ in being short and narrow. Mr. Lydekker, finding the fossil possesses all the characters referable to Cimoliosaurus, gives it a place in that genus.
It is, however, specifically distinct from C. phcatus, Phil., the only other known Oxford clay member of the family, and to which I referred it in vol. ix. of “The Proceedings.” Among the other distinctive characters already described, the cervical vertebrae are shorter with flatter, terminal faces, and about 31 in number instead of 44 as in plicatus. Mr. Lydekker names it Cimoliosaurus richardsoni after its fortunate discoverer. Plesiosaurus proper is restricted to the Rhæustic and Liassic beds, while Cimoliosaurus extends vertically from the Inferior Oolite to the Upper Chalk inclusive.
- The Internet Archive: Proceedings of the Dorset Natural History and Archaeological Society’ Volume 10, 1889
- Jurassic Coast Fossil Finder
- Fossil Works Gateway to the Paleobiology Database: Cryptoclidus richardsoni
- From Mud and Bones to a Museum Specimen
- Monster fossils from Dorset and Wiltshire declared as new species