Kitabı oku: «Remarks on some fossil impressions in the sandstone rocks of Connecticut River», sayfa 3
FOSSIL IMPRESSIONS.—II
Since writing the preceding article, I have been able to obtain, through the kindness of President Hitchcock, a number of additional specimens of fossil impressions. By the aid of these, I may hope to give an idea of the system of impressions, so far as it has been discovered, without, however, attempting to enter into minute details. For these, I would refer to the account of the "Geology of Massachusetts," by President Hitchcock; to his valuable article published in the "Memoirs of the American Academy;" and to his geological works generally.
The numerous tracks which have been assembled together in the neighborhood of Connecticut River have afforded an opportunity of prosecuting these studies to an extent unusual in the primitive rocky soil of New England. These appearances are not, indeed, wholly new. Such traces had been previously met with in other countries; but, in their number and variety, the valley of the Connecticut abounds above all places hitherto investigated.
Twenty years have elapsed since the study of Ichnology has been prosecuted in this country; and, in this period of time, about forty-nine species of animal tracks have been distinguished in the locality mentioned, according to President Hitchcock; which have been regularly arranged by him in groups, genera, and species.
I propose now to lay the specimens, recently obtained, before the Society, as a slight preparation for the more numerous and more valuable articles which they are soon to receive.
The traces found on ancient rocks, as has been shown in the previous article, are those of animals, vegetables, and unorganized substances. The traces of animals are produced by quadrupeds, birds, lizards, turtles, frogs, mollusca, worms, crustacea, and zoophytes. These impressions are of various forms: some of them simple excavations; some lines, either straight or curved, and others complicated into various figures.
President Hitchcock has based his distinctions of fossil animal impressions on the following characters, viz.:—
1. Toes thick, pachydactylous; or thin, leptodactylous.
2. Feet winged.
3. Number of toes from two to five, inclusive.
4. Absolute and relative length of the toes.
5. Divarication of the lateral toes.
6. Angle made by the inner and middle, outer and middle toes.
7. Projection of the middle beyond the lateral toes.
8. Distance between tips of lateral toes.
9. Distance between tips of middle and inner and outer toes.
10. Position and direction of hind toe.
11. Character of claw.
12. Width of toes.
13. Number and length of phalangeal expansions.
14. Character of the heel.
15. Irregularities of under side of foot.
16. Versed sine of curvature of toes.
17. Angle of axis of foot with line of direction.
18. Distance of posterior part of the foot from line of direction.
19. Length of step.
20. Size of foot.
21. Character of the integuments of the foot.
22. Coprolites.
23. Means of distinguishing bipedal from quadrupedal tracks.
By these characters, President Hitchcock has distinguished physiological tracks, or those made by animated beings, into ten groups provisionally. To these may be added, "organic impressions," made by organized bodies; and the impressions made by inanimate bodies, called "physical impressions."
The specimens under our hands enable us to give some notion of the distinctions which characterize the greater part of these groups.
GROUP FIRST—STRUTHIONES
The ostrich-tracks present a numerous natural and most remarkable group; remarkable from the great size of some species,—all of them tridactylous and pachydactylous. The ostrich of the Old World has only two toes, but this family exists in South America at the present time under the name of Rhea Americana; and tracks of an animal, probably of the same family, are found in the numerous impressions near Connecticut River,—all of them having three toes in front, and the rudiment of a fourth behind.
This group contains a number of genera. The First Genus, denominated Brontozoum, presents the tracks of a most extraordinary bird. These tracks appear less questionable since the discovery in Madagascar of the eggs of the Epyornis.
The tracks of the largest species, the Brontozoum Giganteum, are four times the magnitude of those made by the existing ostrich of Africa. They are very numerous, and congregated together. The foot of the Brontozoum Giganteum, including the inferior extremity of the tarso-metatarsal bone, which makes a part of the foot, measures in our specimen twenty inches; in the Mastodon Giganteus, the foot measures twenty-seven inches; the width also is less, being ten inches across the metacarpals, while that of the Mastodon is twenty-two: but the one is a bird, the other a quadruped. The toes are three in number, and present the same divisions with existing birds; the inner toe having three, the middle four, the outer five phalanges. Some of the articulations of the toes of this noble specimen are remarkable for the manner in which they illustrate the mode of formation of the tracks. These phalanges have become separated from the solid rock in which they were encased, so as to be removable at pleasure; and they thus show that the whole foot is not a simple impression in the rock which contains it, but a depression filled by foreign materials, i.e. by sand, clay, and other relics of pre-existing rocks. These materials had been gradually deposited in the mould formed by the bird's foot, and are therefore independent of this rock, in the same way as the plaster-of-Paris cast of a tooth, or any other body, is independent of the mould to which it owes its form. The impressions are in gray sandstone.
On the reversed surface of the slab is seen a small piece of broken quartz, about half an inch square. This piece forms a beautiful illustration of a part of the process by which the sandstone rocks are formed.
The second species of the same genus is the Brontozoum Sillimanium. Of this we have three specimens; the tracks have the same general character with the preceding, but are smaller.
The third species of this genus is styled the Brontozoum Loxonyx, from λοξὸς, a bow, and ὄνυξ, a nail,—a curved nail. It is smaller than the Sillimanium, and has the nail set to one side.
The fourth species, still smaller, is the Brontozoum Gracillimum. On this slab the impressions are in relief; viz.: 1st, of Brontozoum Gracillimum; 2d, of Brontozoum Parallelum; 3d, of the track of a tortoise, fourteen inches long, and two wide. Other extensive eminences and depressions, with rain-drops, may be observed on the same surface.
The fifth species is called Brontozoum Parallelum, from the tracks being on a line with each other. Of this there are two specimens, one of them, however, being a single track. On the surface of the other slab there are at least five distinct tracks, one of them being a small new and undescribed species,—thus making the whole number of species of Brontozoum which we possess to be at least six.
The Second Genus of Struthiones is called Æthyopus, from αἴθυια, a gull, and ποὺς, a foot,—gull-footed. This genus is smaller than the Brontozoum Giganteum; and we have two species, viz. the Æthyopus Lyellianus, which is the larger, and two specimens of Æthyopus Minor. All of these are distinguished from the preceding genus by the winged foot, and in the Lyellianus by the shallowness of the impression. The Æthyopus Minor is not always distinguished by the superficiality of its impression. This is sometimes deep. Therefore this character may not be considered a distinctive one, or the Æthyopus Minor might be referred to another genus. Of the two specimens of this latter species, the first is in depression, tridactylous. The depressions are deep with rain-drops, marks of quadrupeds and zoophytes over the whole surface. The ornithichnic impressions are two in number; one superficial, the other very deep. The reversed surface of this slab contains one tridactylous impression in relief. The second specimen has three depressions; two of which are superficial, and the third is quite deep, displaying, by a depressed surface, the webbed character of the foot.
GROUP SECOND
We shall take, to characterize this group, the Argozoum, from ἀργὴς, swift, winged.
Of this genus there are two species, the larger of which is the Argozoum Disparidigitatum. It is leptodactylous, and remarkable for the length of the middle toe. We have another species, which is smaller than the last named, and in which the toes are nearly of equal length; hence called Argozoum Paridigitatum.
The other genus of this group is the Platypterna, and our specimen is named Deaniana. This genus is remarkable for the width of the heel; hence the name, from πλατὺς, broad, and πτέρνη, a heel. It has three toes like the other genera of this group.
GROUP THIRD
This and the succeeding group are tetradactylous; having one toe behind, three forwards.
The third group is leptodactylous; foot usually small, but sometimes of medium size. Of it we have two specimens, viz.: Ornithopus Gallinaceus, and Ornithopus Gracilis. The former is so called from the resemblance to the domestic fowl: for convenience sake, in this and other instances, we use the whole for a part. It is about three inches in length, and the Ornithopus Gracilis about two.
This latter specimen is particularly interesting. It consists of two parts, which open like the covers of a book. These covers present four impressions: first, the superficial, which is distinct, slender, and beautiful—the heel is broad; second, corresponding with this depression and on the inside, is a figure in relief as distinct as the depression; third, on the inside of the second cover is a depression corresponding with the relief last mentioned; fourth, on the outer side is a second relief corresponding with the second depression, but less distinct than either of the other three, still, however, exhibiting three toes pointing anteriorly, but the hind toe is wanting. The whole of this double slab forms a series of cameos and intaglios, measuring four inches by three, and in thickness an inch and a quarter.
GROUP FOURTH
Of the fourth group we have five specimens. The Triænopus, so called from its resemblance to a trident, has besides three leptodactylous toes pointing forwards, a fourth extending backwards in a remarkable way, like the handle of a trident; the impression, however, being expanded so as to show an extensive displacement of the mud. All the specimens of Triænopus are in a beautiful red shale, very thin and fragile, but presenting well-defined impressions, generally about three inches long.
