Description of the Remains of a new Enaliosaurian (Eosaurus Acadianus), from the Coal-formation of Nova Scotia
Authors: Marsh O. C. 
Published: October 2009

Introduction
.—The Reptilian remains from the Coal-measures, hitherto described, were stated to be few in number, and mostly of Batrachian or Amphibian affinities; and it was observed that, previous to the year 1844, the evidence of even this low form of reptilian life during the Carboniferous period was unsuspected by most geologists, and its first appearance upon the earth confidently referred to the Permian epoch. The author gave a short résumé of the reptilian remains discovered since that date in rocks of the more ancient period, especially alluding to the joint discoveries of Dr. Dawson and Sir Charles Lyell* and the more recent researches of the former geologist†.

The remains which form the subject of the present communication consist of two centra, or bodies of vertebræ, which were discovered by the author in August 1855. Their resemblance in form and appearance to the vertebræ of
Ichthyosaurus
was considered by him to be so marked that, at the time of the discovery, he referred them to that genus. A subsequent and more careful examination and comparison, though it confirmed his idea of their Enaliosaurian character, showed some important differences; consequently, as every endeavour to procure further remains met with no success, he proposed for the fossil the name of
Eosaurus Acadianus
in a short notice which appeared in the ‘American Journal of Science’ for March 1862, N.S., vol. xxxiii. p. 278. In a subsequent number of the same journal a full description was given, illustrated with figures and microscopic sections of the vertebræ.

Locality whence the fossil was obtained
.—The fossil was found at the South Joggins, in Nova Scotia, on the southern shore of the Chiegnecto Channel, a branch of the Bay of Fundy, in a bed of argillaceous, chocolate-coloured shale, which forms part of group xxvi. in the elaborate section of this formation made in 1852 by Sir Charles Lyell and Dr. J. W. Dawson.

Mr. Marsh then gave some stratigraphical details respecting the coal-measures of this locality, noting the more fossiliferous beds and the principal organic remains found in them.

Osteological description
.—The vertebræ, as already stated, are two in number, and when discovered were attached to each other‡. Their uniformity in size and appearance, as well as their collocation when found, would indicate that they belonged to the same animal, and were contiguous in the vertebral column. They are remarkably well preserved, a result of their complete ossification in their natural state, and of their being imbedded in the peculiar matrix which has since contained them, and furnished the material for their mineralization. The posterior vertebra, in fact, with the exception of a small fracture, seems to be nearly as perfect as in its original condition; and from it the description and measurements given§ were mainly taken.

A detailed account of the osteological characters of the fossil was then given, and the points in which it agrees with, or differs from,
Ichthyosaurus
were particularly described, and contrasted in a table of the measurements of the vertebræ in the two Enaliosaurians.

In the margin of one of the vertebræ there is an angular notch, about a line in depth, which Prof. Agassiz, after a casual examination, considered to be organic rather than accidental, and to indicate an inferiority of structure, as a similar character is common in Fishes. Prof. Jeffries Wyman, on the other hand, regarded the notch as purely accidental, and the result of a fracture, which has also displaced the articular pits of the superior arch. He is also of opinion that the notch would not be sufficiently important, if it were organic, to affect at all the Enaliosaurian character of the remains.

Microscopic structure
.—A microscopic examination of the osseous structure of these vertebræ revealed well-marked reptilian characters, which were fully described and compared with the characters exhibited by microscopic sections of the vertebræ of
Ichthyosaurus
and
Plesiosaurus
.

Comparison with other vertebrœ
.—The author then stated that the vertebrœ of
Eosaurus
, in their biconcave centres, exhibit a structure which prevails in the class of Fishes, in the Labyrinthodonts, as well as in a few genera of extinct Saurians, and which is seen in existing reptiles only in the Geckos and the Perennibranchiate division of Batrachians.

Amongst Fishes, the
Plaqiostomi
were stated to possess vertebræ having more in common with those of
Eosaurus
than others of the class, but that the latter show a much higher degree of ossification in all their parts than the former; also that the osseous structure differs in the two cases, and that these vertebræ do not possess, on their articular faces, any traces of those concentric rings which so generally exist on the vertebræ of fossil and recent Sharks.

Mr. Marsh also alluded to the persistence of the notochord in the Palmozoic Fishes, and the incomplete ossification of the vertebræ in Mesozoic species; also to the general difference in the osseous structure of these vertebræ and those of Fishes. He therefore rejects that class, and places the
Eosaurus
among the Reptiles.

The differences between the fossil in question and reptilian biconcave vertebræ, other than those of the
Ichthyopterygia
, were then pointed out, the author remarking that in the orders
Ganocephala
and
Labyrinthodontia
of Owen, either the notochord was persistent or the neurapophyses were anchylosed to the centrum, neither of which characters is to be observed in
Eosaurus
. The
Crocodilia
of the Secondary formations differ from it in having the superior arch united to the centrum by suture; and in the only genus of
Sauropterygia
whose vertebræ resemble it in their proportions (cervical vertebræ of
Pliosaurus
), the articular faces were flat, or but very slightly concave.

The characters of the vertebræ of
Eosaurus
were contrasted at length with those of
Ichthyosaurus
, and a very close resemblance was found to exist between them, especially in their flattened and subhexagonal form, in their deep and regular terminal cavities, and in the separate state of the neural arch. The most marked differences are seen in the absence of costal articular surfaces from the sides of the centrum, in the deeper concavities at the vertebral extremities, and in the form and dimensions of the superior arch in this fossil. It was therefore concluded that the points of similarity between it and the
Ichthyosauri
, which it most resembles, clearly indicate that they belong to the same natural group of marine reptiles, and to the same order; while the differences which exist between them seem to be sufficiently numerous and important to authorize the conclusion that they are generically distinct, as might naturally be expected from the vast periods of time that separated their existence.

Habits of the Eosaurus
.—These vertebræ of the
Eosaurus
, although the only remains of the genus at present known, are so characteristic and well preserved that Mr. Marsh considers them sufficient to indicate that the animal, like the later Enaliosaurians, was of great size, air-breathing, cold-blooded, and carnivorous; that it was aquatic, and probably marine, inhabiting the sea or estuaries, or, possibly, as might be inferred from the place of its entombment, the mouths of rivers flowing into the sea. The flattened form of the vertebræ, the great depth of their terminal concavities, the separate condition of the neural arch, and its short longitudinal extent at the base, were stated to be all consistent with the conclusion that the
Eosaurus
was capable of rapid progress through the water in pursuit of its prey, which was probably fishes; and since it had then, according to our present knowledge, no superior in point of size, it probably reigned supreme in the waters of the Carboniferous era.

Chemical examination
.—Mr. Marsh observed that a comparison of the composition of recent and fossil bone is interesting, both in a chemical and a physiological point of view, and has already attracted some attention. As he considered it desirable to add to the limited data on this subject hitherto collected, he analysed a portion of a vertebra of the
Eosaurus
, and the results were given in full.

A preliminary qualitative examination showed the presence of iron, manganese, copper, alumina, lime, magnesia, potash, soda, organic matter, and water, as well as fluorine, chlorine, and sulphur, with silicic, carbonic, phosphoric, and sulphuric acids. The complete separation of these various constituents was necessarily attended with some difficulty, and the methods resorted to were therefore fully described, and a table showing the percentage of each constituent substance was given.

A nitrogen-determination was made with a portion of the fossil of a rather different character, which gave 776 per cent. for the amount of that substance in it. This corresponds essentially with the results obtained by M. Delesse, who has made some extensive researches on this point, and who considers that the quantity of nitrogen in fossil bones is, within certain limits, a reliable indication of their age. The above-mentioned percentage of nitrogen is much greater than that indicated by the analysis, and was probably due in part to the substance used in the two cases being somewhat different; but part of the nitrogen in the determination may have been derived from ammonia, which is sometimes introduced into fossils by infiltration. A want of sufficient material prevented fuller investigations of the organic elements in these remains.

The fossil bones hitherto analysed appear to have been all from the more recent formations; but it was remarked that the results of this analysis of a Palæozoic fossil did not differ materially, in most respects, from those previously obtained by analysing fossil bones from much newer deposits.