Some Salient Points in the Science of the Earth

by Sir John William Dawson

Species of Microsauria. Hylonomus Lyelli.

In the original reptiliferous tree discovered by Sir C. Lyell and the writer, at the Joggins, in 1851, there were, beside the bones of Dendrerpeton Acadianum, some small elongated vertebræ, evidently of a different species. These were first detected by Prof. Wyman, in his examination of these specimens, and were figured, but not named, in the original notice of the specimens. In a subsequent visit to the Joggins I obtained from another erect stump many additional remains of these smaller reptiles, and, on careful comparison of the specimens, was induced to refer them to three species, all apparently generically allied. I proposed for them the generic name Hylonomus, "forest dweller." They were described in the Proceedings of the Geological Society for 1859, with illustrations of the teeth and other characteristic parts.[136] The smaller species first described I named H. Wymani; the next in size, that to which this article refers, and which was represented by a larger number of specimens, I adopted as a type of the genus, and dedicated to Sir Charles Lyell. The third and largest, represented only by a few fragments of a single skeleton, was named H. aciedentatus. This I had subsequently to remove to a new genus, Smilerpeton.

[136] Journal of Geological Society, vol. xvi.

Hylonomus Lyelli was an animal of small size. Its skull is about an inch in length, and its whole body, including the tail, could not have been more than six or seven inches, long. The bones appear to have been thin and easily separable; and even when they remain together, are so much crushed as to render the shape of the skull not easily discernible. They are smooth on the outer surface to the naked eye; and under a lens show only delicate, uneven striæ and minute dots. They are more dense and hard than those of Dendrerpeton, and the bone-cells are more elongated in form. The bones of the snout would seem to have been somewhat elongated and narrow. A specimen in my possession shows the parietal and occipital bones, or the greater part of them, united and retaining their form. We learn from them that the brain case was rounded, and that there was a parietal foramen. There would seem also to have been two occipital condyles, as in modern Batrachians. Several well-preserved specimens of the maxillary and mandibular bones have been obtained. They are smooth, or nearly so, like those of the skull, and are furnished with numerous sharp, conical teeth, anchylosed to the jaw, in a partial groove formed by the outer ridge of the bone. In the anterior part of the lower jaw there is a group of teeth larger than the others. The total number of teeth in each ramus of the lower jaw was about forty, and the number in each maxillary bone about thirty. The teeth are perfectly simple, hollow within, and with very fine radiating tubes of ivory. The vertebras have the bodies cylindrical or hour-glass shaped, covered with a thin, hard, bony plate, and having within a cavity of the form of two cones, attached by the apices. This cavity was completely surrounded by bone, as it is filled with stained calc-spar in the same manner as the cavities of the limb bones. It was probably occupied by cartilage. The vertebræ were apparently bi-concave, and are furnished with upper and lateral processes similar to those of small lacertian animals. The ribs are long, curved, and at the proximal end have a shoulder and neck. They are hollow, with thin hard bony walls. The anterior limb, judging from the fragment procured, seems to have been slender, with long toes, four or possibly five in number. The posterior limb was longer and stronger, and attached to a pelvis so large and broad as to give the impression that the creature enlarged considerably in size toward the posterior extremity of the body, and that it may have been in the habit of sitting erect. The thigh bone is large and well-formed, with a distinct head and trochanter, and the lower extremity flattened and moulded into two articulating surfaces for the tibia and fibula, the fragments of which show that they were much shorter. The toes of the hind feet have been seen only in detached joints. They seem to have been thicker than those of the fore foot. Detached vertebræ, which seem to be caudal, have been found, and show that the tail was long and probably not flattened. The limb bones are usually somewhat crushed and flattened, especially at their articular extremities, and this seems to have led to the error of supposing that this flattened form was their normal condition; there can be no doubt, however, that it is merely an effect of pressure. The limb bones present in cross section a wall of dense bone with elongated bone-cells, surrounding a cavity now filled with brown calc-spar, and originally occupied with cartilage or marrow. I desire to specify the above points because I believe that most of the creatures referred by Fritsch, Credner, and other European naturalists to the Microsauria are of inferior grade to Hylonomus, though admitted to present points of approximation to the true reptiles. Woodward has recently described the remains of a Microsaurian from the English coal formation. Nothing is more remarkable in the skeleton of this creature than the contrast between the perfect and beautiful forms of its bones, and their imperfectly ossified condition, a circumstance which raises the question whether these specimens may not represent the young of some reptile of larger size.

The dermal covering of this animal is represented in part by oval bony scales, which are so constantly associated with its bones that I can have no doubt that they belonged to it, being, perhaps, the clothing of its lower or abdominal parts. But the most remarkable and unexpected feature of this little creature was the beautiful and ornate scaly covering of its back and sides. Modern Batrachians are characteristically naked, and though we know that some fossil species had coverings below of bony scales, these seemed rather to ally them with bony fishes. One of the specimens of Hylonomus had associated with it a quantity of crumpled shining skin, black and carbonaceous, and which may perhaps have been tanned and so preserved by the water filling the hollow tree impregnated with solution of tannin from the bark. This skin was covered with minute overlapping scales, which, under the microscope, showed the structure of horn rather than of bone. Besides these ordinary scales there were bony prominences, like those of the horned frog, on the back and shoulders, and a species of epaulettes made of long horny bristles curved downward, and apparently placed at the edges of the shoulders. Besides these there were in front and at the side rows of pendants or lappets, all no doubt ornamented with colouring, though now perfectly black. It may be asked what was the use of the ornate covering, and perhaps the question raises that perplexing problem, of the use of beauty in a world where there were no animals with higher æsthetic faculties than those of Batrachians. Scudder suggests a somewhat prosaic use in supposing them to be an armour against the venomous scorpions which were the contemporaries of these little reptiles, and some of them almost as large in size. But the word "venomous" raises another question, for we only infer that the scorpions were venomous from modern analogy and traces of an inflated joint at the end of the tail in some specimens. We have no absolute certainty that the subtle and complex organic poison of the scorpion, and his beautiful injection syringe for placing it under the skin, were perfected at this early time. Thus we have in the far back Carboniferous age a creature as elaborately ornamented and protected as any of the modern lizards, and this, let it be observed, constitutes another and important departure from that batrachian type to which these animals are supposed to conform. I may add here that subsequently portions of skin were found, which from their size probably belonged to Dendrerpeton, and that these also were scaly and had lappets, though they did not appear to have the horny tubercles and fringes. It may be asked why such advanced characters should be found in Nova Scotia alone. The answer is that the circumstances of preservation in the erect trees were peculiar, and that only animals of purely terrestrial habits could find access to them, whereas the remains of reptiles found in the Carboniferous elsewhere are in aqueous beds in which aquatic forms were more likely to be preserved, and in which all the soft parts were certain to perish.

It is evident from the remains thus described, that we have in Hylonomus Lyelli an animal of lacertian form, with large and stout hind limbs, and somewhat smaller fore limbs, capable of walking and running on land; and though its vertebræ were imperfectly ossified externally, yet the outer walls were sufficiently strong, and their articulation sufficiently firm, to have enabled the creature to erect itself on its hind legs, or to leap. They were certainly proportionately larger and much more firmly knit than those of Dendrerpeton. Further, the ribs were long and much curved, and imply a respiration of a higher character than that of modern Batrachians, and consequently a more highly vitalized muscular system. If to these structural points we add the somewhat rounded skull, indicating a large brain, we have before us a creature which, however puzzling in its affinities when anatomically considered, is clearly not to be ranked as low in the scale of creation as modern tailed Batrachians, or even as the frogs and toads. We must add to these also, as important points of difference, the bony scales with which it was armed below, and the ornate apparatus of horny appendages, with which it was clad above. These last, as described in the last section, show that this little animal was not a squalid, slimy dweller in mud, like Menobranchus and its allies, but rather a beautiful and sprightly tenant of the coal-formation thickets, vying in brilliancy, and perhaps in colouring, with the insects which it pursued and devoured. Remains of as many as eight or ten individuals have been obtained from three erect Sigillariæ, indicating that these creatures were quite abundant, as well as active and terrestrial in their mode of life.

With respect to the affinities of this species, I think it is abundantly manifest that it presents no close relationship with any reptile hitherto discovered in the Carboniferous system, except perhaps some of the smaller forms in the Permian of Europe, with which Credner and Fritsch have compared it. It is scarcely necessary to say that the characters above described entirely remove this animal from the Labyrinthodonts. Equal difficulties attend the attempt to place it in any other group of recent or extinct Batrachians or proper reptiles. The structures of the skull, and of some points in the vertebræ, certainly resemble those of Batrachians; but, on the other hand, the well-developed ribs, evidently adapted to enlarge the chest in respiration, the pelvis, and the cutaneous covering, are unexampled in modern Batrachians, and assimilate the creature to the true lizards. I have already, in my original description of the animal in 1859, expressed my belief that Hylonomus may have had lacertian affinities, but I do not desire to speak too positively in this matter;[137] and shall content myself with stating the following alternatives as to the probable relations of these animals, (1) They may have been true reptiles of low type, and with batrachian tendencies. (2) They may have been representatives of a new family of Batrachians, exhibiting in some points lacertian affinities. (3) They may have been the young of some larger reptile, too large and vigorous to be entrapped in the pitfalls presented by the hollow Sigillaria stumps, and in its adult state losing the batrachian peculiarities apparent in the young. Whichever of these views we may adopt, the fact remains, that in the structure of this curious little creature we have peculiarities both batrachian and lacertian, in so far as our experience of modern animals is concerned. It would, however, accord with observed facts in relation to other groups of extinct animals, that the primitive Batrachians of the coal period should embrace in their structures points in after times restricted to the true reptiles. On the other hand, it would equally accord with such facts that the first-born of Lacertians should lean towards a lower type, by which they may have been preceded. My present impression is, that they may constitute a separate family or order, to which I would give the name of Microsauria, and which may be regarded as allied, on the one hand, to certain of the humbler lizards, as the Gecko or Agama, and, on the other, to the tailed Batrachians.

[137] I am glad to say that Fritsch and Credner now lean to the same view.

It is likely that Hylonomus Lyelli was less aquatic in its habits than Dendrerpeton, Its food consisted, apparently, of insects and similar creatures. The teeth would indicate this, and near its bones there are portions of coprolite, containing remains of insects and myriapods. It probably occasionally fell a prey to Dendrerpeton, as bones, which may have belonged either to young individuals of this species or to its smaller congener H. Wymani, are found in larger coprolites, which may be referred with probability to Dendrerpeton Acadianum. This coprolitic matter, which is somewhat plentiful on some of the surfaces in the erect trees, also informs us that the imprisoned animals may in some cases have continued to live for some time, feeding on such animals as may have fallen into their place of confinement, which was destined also to be their tomb. Some other points of interest appear on the examination of this excrementitious matter. It contains much carbonate of lime, indicating that snails or other mollusks furnished a considerable part of the food of the smaller reptiles. Some portions of it are filled with chitinous fragments, parts of millipedes or insects, but usually so broken up as scarcely to be distinguishable. One curious exception was a part of the head of an insect containing a portion of one of its eyes. The facets of this can be readily seen with the microscope, and are similar to those of modern cockroaches. About 250 of these little eyes are discernible, and they must have been much more numerous. Two points are of interest here: First, the perfection of the compound eye for vision in air. It had long before, in the case of the Trilobites, been used for seeing under water. Secondly, the great age of the still ubiquitous and aggressive family of the cockroaches. In point of fact the oldest known insect, the Protoblattina of the Silurian, is one of these creatures, and they are the most abundant insects in the Carboniferous, so that if they now dispute with us the possession of our food, they may at least put in the claim of prior occupancy of the world. In one mass a quantity of thickish crust or shell appears, which under the microscope presents a minutely tubular and laminated appearance. It may have belonged to some small crustacean or large scorpion on which a Dendrerpeton may have been feeding before it fell into the pit in which it was entombed.