Notes on Ceraurus pleurexanthemus, Green
The writer has had the opportunity, by his residence at the type-locality of the Trenton Limestone, to make some investigations upon the structure and habits of the trilobites of that interesting horizon. The results of these observations and studies, he hopes to present from time to time, as they shall become sufficiently definite to call for permanent record.
In the present article, it is proposed to consider certain facts of occurrence, which seem to bear upon the habits and mode of life of one of the principal species of the Trenton rocks, Ceraurus pleurexanthemus. Asaphus and other genera are referred to here, only as giving additional evidence on the points involved.
Ceraurus pleurexanthemus is one of the most characteristic trilobites of the Trenton Limestone, in numbers and distribution exceeded only by Asaphus gigas, A. megistos, and Calymene senaria. It has a wide geographical, as well as vertical, range. Entire specimens, however, are rare in most localities, the head and the hypostoma being the parts usually found. At Trenton Falls, N. Y., in the upper third of the limestone, the separated heads are found in immense numbers; in many places, the surface of the rock is nearly covered with them, while only an occasional pygidium or portion of the thorax is seen.
About twenty-seven feet below the coarse crystalline limestone that caps the upper portion of the ravine at Trenton Falls, there is a thick layer of uneven gray limestone; upon this rests a thin layer of shale and clay, varying from a quarter of an inch to an inch in thickness. This was the sea-bed where the remains of trilobites, crinoids, and other forms of animal life lay when buried by the overlying deposit, which now is a thin layer of bluish-gray limestone, one to two inches thick. Attached to the under surface of this layer, the following species of fossils have been found:
Stenopora fibrosa, S. lycoperdon, Ptilodictya acuta, P. recta, Aulopora arachnoidea, Stellipora antheloidea, Stictopora elegantula, Alecto inflata, Intricaria reticulata, three species of Heterocrinus, two of Glyptocrinus, one each of Anomalocystites and Glyptocystites, one of Stenaster, Lingula quadrata, Trematis filosa, Trematis n. s., Leptœna sericea, Strophomena alternata, Orthis testudinaria, O. lynx, Rhynchonella recurvirostra, Crania Trentonensis, Conularia Trentonensis, Asaphus gigas, Calymene senaria, Ceraurus pleurexanthemus, Acidaspis Trentonensis, Acidaspis n. s., Proetus parviusculus, Phacops callicephalus, Dalmanites.
These fossils are generally found in groups of associated species, but often commingled, so that trilobites, crinoids, cystids, brachiopods, and bryozoans occur on the same slab of stone. The prevailing and characteristic fossil is Ceraurus pleurexanthemus. Individuals from three-sixteenths of an inch to two inches in length, are scattered over the surface, often to the exclusion of all other fossils. In a space thirty by forty feet, 326 entire specimens were seen. Of this number, and of many seen before, a record was kept; eight lay with the dorsal surface up; the remainder were on their backs, attached by the ventral surface of the dorsal shell to the under side of the layer. The view that this was the natural position of the trilobite is sustained by the following considerations:—
1. Individuals of all stages of growth are preserved entire; which would not have been the case, had they been subjected to the action of the water in drifting them into the position found. The thorax is easily dismembered and broken, and could not have withstood such transportation.
2. Very few fragments are found, and when consisting of the head or pygidium, they have the dorsal surface down.
3. Upon all uneven layers, and those showing the action of strong currents, and holding coarser fossils, the trilobites are distorted and broken up.
4. When found upon smooth layers above the Ceraurus layer, they are invariably back-down. Fine specimens showing the interior of the shell, are obtained from the upper surface of several layers.
5. The drifting of the shell into the position found, would not probably have taken place, as the shell is nearly flat. Asaphus might, from its boat-like shape, assume such a position; but a force sufficient to place a trilobite like Ceraurus upon its back, if the natural position when living was the reverse, would not have left the bryozoans and crinoids as they grew, without breaking the more delicate portions, which are often like fine hairs of stone, lying loosely in the imbedding clay, and breaking at the slightest touch.
6. The trilobites never have shells or corals drifted into them, or lying on them, when upon the upper surface of the layers. Occasionally a coral encrusts the upper surface, and frequently a coral (Stenopora lycoperdon) has taken the interior or ventral surface as a base for its growth, showing clearly that the shell had assumed the inverted position prior to the growth of the coral.
Forty specimens of Acidaspis Trentonensis were associated with the Ceraurus,—every individual upon its back. Calymene senaria, when not coiled (numerous), Proetus parviusculus, Asaphus gigas (one only), and Dalmanites, were uniformly back down.
Upon the upper surface of the Ceraurus layer, a layer of clayey shale was deposited, giving the same conditions as below the Ceraurus layer. Attached to the under surface of the succeeding layer, or imbedded in the clayey shale, were bryozoans, cystids, crinoids, brachiopods, and trilobites of the following genera:—Ceraurus, Acidaspis, Calymene, Phaeops, and Spherocoryphe. The trilobites, without exception, were back-down. In the succeeding layer, which is six inches thick, many of Asaphus gigas and A. megistos have been found, scattered through the lower three inches of its thickness. Of seventy-five noted, thirty were back-down, twenty-nine presented the dorsal surface up, sixteen were in various positions, coiled, perpendicular to the layer, and edgeways. The succeeding five feet of the stratum is of the same nature as that below. Fossils are rare, especially trilobites. Above this, the coarse earthy limestone extends to the thick crystalline strata.
Prof. Burmeister, in his "Organization of Trilobites,"[2] gives the following among other conclusions, as the result of comparison of the trilobites with the recent crustacea.
1. That these animals moved only by swimming; that they remained close beneath the surface of the water; and that they certainly did not creep about at the bottom.
2. That they swam in an inverted position, the belly upwards, the back downwards, and that they made use of their power of rolling themselves into a ball, as a defence against attacks from above.
4. That they most probably did not inhabit the open seas, but the vicinities of coasts, in shallow water; and that they here lived gregariously in vast numbers, chiefly of one species.
If the first and second conclusions are correct, we should look in a quiet, undisturbed deposit for evidence as to the position of the trilobites while living, by their position when buried in the sediment after death. As before stated, the conditions are such in the species mentioned, as to preclude the idea of their arrangement by other causes than the natural position of the living animal, which must, therefore, have been with the back downward.
The Asaphus is more frequently broken; but the finest and most perfectly preserved specimens, with but few exceptions, are found on their backs.
That portion of the fourth conclusion in reference to trilobites living gregariously in vast numbers, is true of Ceraurus pleurexanthemus, Asaphus gigas, and A. megistos, as found in the stratum mentioned.
Note. To October 16th, 1875, 1160 specimens of Ceraurus pleurexanthemus have been noted on the under surface of the thin layer ("Ceraurus layer"). Of these 1110 lay on their backs; while but fifty presented the dorsal surface up. Forty-five of these fifty were very small, the remaining five of medium size.
- ↑ The genus Ceraurus (Green, 1832, Monograph, p. 84) was founded upon specimens not clearly showing all the characteristics of the genus, as subsequently known. The description, however, was sufficiently accurate for the ready identification of the genus, and of the species, C. pleurexanthemus. The name should therefore stand; and Cheirurus of Beyrich (1845), must be regarded as a synonym; since the objection raised to Green's figure, on the ground of its indistinctness, is not tenable. The use of Cheirurus by authors is not allowable, under the rule as to priority of date adopted by the British Association for the Advancement of Science, twelfth meeting, 1842.
- ↑ Page 52, conclusions 1, 2, 4.
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