Carboniferous Eurypterids of Nebraska
Art. XLV.—Carboniferous Eurypterids of Nebraska; by Erwin H. Barbour.
During the field season of 1912 the Nebraska Geological Survey found a bed of eurypterids about one mile south of Peru. This adds one more to the list of known eurypterid localities, and adds one form believed to be new. Nowhere apparently are eurypterids found in such variety and in such excellent preservation as in New York State, yet the Nebraska beds bid fair to make an unusual showing numerically.
The eurypterid shales of Nebraska, as far as known, outcrop one mile south of Peru, in the Coal Measure bluffs facing the Missouri River. The alternating limestone and shale in the bluffs at the edge of the town change rapidly to shale, which becomes increasingly arenaceous and merges finally into massive sand some forty to fifty feet thick about a mile south of Peru. Here the eunypterid bed is found. The walls and bluffs are vertical, because only recently they were deflection banks of the Missouri River. The tracks of the Burlington Railroad run close to the base of the bluffs. About ten feet above the tracks, and about thirty feet above the river level, may be noticed in the massive sand a distinct shaly band scarcely a foot thick, and two to three hundred feet long. This local band is composed of thin, irregular, shaly layers, seldom a half inch thick, alternating with micaceous sand. The shale seams in the topmost two inches yield the eurypterids. These thin shale seams cleave readily, exposing the eurypterids and numerous associated plants. Because of shrinkage joints, the shales tend to break into rather small blocks, and unfortunately many good specimens are damaged or destroyed.
Since removing a dangerous overhanging ledge the work of collecting has been rendered quite safe and easy, and about forty specimens were found on as many square feet of shale, or approximately one to each square foot. Their chitinous shells are reduced to mere carbonaceous films sometimes scarcely distinguishable from the shaly matrix. Accordingly, some specimens are faint. Others are distinct, however, and well differentiated from the slate-colored background. Some specimens are so well preserved that the details of the gross anatomy may be made out, and even some of the ornamentation and minute sculpturing.
Associated with the eurypterids at Peru are innumerable leaves, stems, and fragments of certain land plants, conspicuously Neuropteris pinnules, stems of Calamites, and leafwhorls of Asterophyllites. The last mentioned add to the beauty of some of the specimens in our collection, because the whorls closely imitate pale yellow flowers pressed in the shale. The association of land plants with eurypterids has been observed before. They suggest the probability that this group, the sea scorpions, which originally were distinctly marine, had undergone adaptive modifications suiting them to a brackish, or even fresh-water habit.
Intimately associated with the eurypterids were considerable amounts of actual plant tissue, preserved as such since Carboniferous times. It still retains its pliability, can be stripped from the shale, floated on glass slips, and made into permanent mounts. There are now about one hundred of these mounts in our collection. The preservation of actual plant tissue in such amounts and in large pieces is quite unique. The tissue is of a bright, transparent orange color. It is capable of close study, and photomicrographic reproductions of the cell structure are readily obtainable. This matter will be made the subject of a special leaflet.
As compared with well-known eurypterids, those of Nebraska are small, the average being a trifle over two inches in length, while the largest, as far as known, do not equal three inches. Whether these represent adult or immature forms, is not apparent. They may be immature forms. However, since no evidences of larger individuals have presented themselves, it may be that this is a group of diminutive eurypterids. Average eurypterids are five to ten times as large. The presumption is that many of the specimens at hand are exuviæ.
The prominent feature of any eurypterid is its scorpioid outline. It has a broad flat head, two broad paddles, and a long segmented abdomen or body tapering towards the tail, which usually ends in a sharp spiniform telson. The head-shield is commonly semi-circular, or, in some instances, somewhat quadrate. It bears two prominent compound eyes, and certain simple eyes, or ocelli, which are generally obscure. The head-shield probably enabled Eurypterus to shovel and burrow as does the horseshoe crab when in pursuit of marine worms. The jawless mouth of Eurypterus is centrally located on the ventral side of the cephalothorax, and is surrounded by six pairs of appendages, some of the bases of which are so serrated, spined, and dentated, as to serve functionally as jaws, or maxillipedes.
The appendages aside from the paddles are rather inconspicuous. The paddles are greatly enlarged for swimming, and perhaps for roiling water to conceal and protect the creature. The paddles are evidence, rather than proof, of active swimming habits. In fact, Eurypterus may have been rather sluggish, content perhaps with grovelling. Its life habits may be inferred from the closely related form, Limulus. Eurypterus probably swam well, but it is not unlikely that it chose to frequent sandy and muddy bottoms.
Undoubtedly the Nebraska eurypterids are closely related to Anthraconectes of the Illinois Coal Measures, described some years ago by Meek and Worthen.
Eurypterus (Anthraconectes) nebraskensis, sp. nov.
The main features of Eurypterus nebraskensis are the vermiform appearance, the long spatulate paddles, and the spinous ridges upon the last five tergites of the postabdomen.
|Fig. 1.||Fig. 2.|
|Fig. 1. Eurypterus nebraskensis. × 3/2.|
Fig. 2. Eurypterus, sp., ventral view, showing a very large metastoma and opercular process. × 3/2.
In Eurypterus nebraskensis the preabdomen is but slightly inflated, and grades insensibly into the postabdomen, giving a vermiform appearance. The telson is long and slender. The scale markings are distinct, and very regular in pattern over the tergites. They are semilunar, and the effect is that of imbricated scales of fishes. The scale-markings continue upon the base of the cephalothorax, but in decreasing numbers anteriorly. At the genal angles, the scale-markings are smaller and closely crowded together. The preabdomen is widest at the second and third somites, and tapers gradually to the telson. In none of the somites are the pleura produced into sharp angles or spines as far as can be observed. The eyes are roundish-reniform, and are set far apart, close to the cephalic rim. Ocelli are not discoverable. The front of the cephalon is ornamented by fine, closely crowded, vermicular lines, which curve forward and outward from the median line until they meet the cephalic rim.
Measurements: The cephalothorax is 9·5mm long by 11mm wide at its base. The preabdomen is 9mm long by 12mm wide at segment 3, its widest point. Segment No. 7 is ·5mm long by 11mm wide. No. 8 is 1·5mm long by 12mm wide.
Each of the others approximate 1·75mm. In the postabdomen segment No. 13 measures 2·5mm long by 8mm wide. No. 18 is 5mm long by 3mm wide. See fig. 1; nat. size.
This form, possibly a different species, has a noticeably inflated body; long narrow paddles; and pleura seemingly produced into spines. See fig. 2.
Segments 11, 12, 13 are short, possibly because telescoped. This may account also for the expanded body. At any rate this specimen was the only one of the kind found. It is pressed as flat as tissue paper, and it seems to be the ventral surface which is exposed.
The elliptical portion of the head, which is unmistakably defined, is taken to be the metastoma, and yet it is altogether too large. It extends well across the head-shield. The median opercular process, assuming it to be such, is broad and well defined proximally, and is fairly distinct along one border. Segment 1, coxa, of the paddles seems plain, while 2 and 3 are reasonably so.
The specimen is small and colored like its background. Accordingly blemishes and irregularities are not always distinguishable from articulations. The greatest width of the preabdomen is between segments 10 and 11. The head-shield is 7mm long by 8mm wide. Figures 1 and 2 ( nat. size) were obtained by tracing around enlarged photographs, and are thought to be reasonably accurate.
Lincoln, Nebraska, July 15, 1914.