Ischyrotherium antiquum, Leidy, 1856

Leidy, J., 1860, Extinct vertebrata from the Judith River and Great Lignite formations of Nebraska., Transactions of the American Philosophical Society 11, pp. 139-154 : 150-151

ISCHYROTHERIUM ANTIQUUM .

Among the most enigmatic fossil remains of vertebrata collected by Dr. Hayden, in Nebraska Territory, are a number of fragments of bones , obtained from an out-lyer of the Great Lignite Tertiary Formation, between the Moreau and Grand Rivers.

The specimens consist of two vertebral bodies, the half of a third one, two apparent transverse processes, and numerous fragments of ribs. We cannot positively determine the affinities of the animal represented by these bones, but from their solidity of structure and the cylindroid form of the ribs, we suspect Ischyrotherium to be more nearly allied to the Manatee than to any other known animal.

The vertebral bodies, ( figs. 8—11, plate 10 ,) apparently posterior dorsal, are segments of a cylinder compressed from above downward, so that their articular faces are transversely oval in outline. They are comparatively slightly constricted at the middle; and in this position present a number of orifices of large vascular canals, which converge to the centre of the bodies. Both articular faces are slightly concave, with obtuse margins. The dorsal surface, ( figure 8, plate 10 ,) exhibits a narrow tract corresponding with the spinal canal, and on each side, a broad, concave, porous articular surface for conjunction with the sides of the vertebral arch.

The broken vertebral body, ( figure 11, plate 10 ,) presents an equally dense structure throughout, except at the articular surfaces, which are finely porous. The large vascular canals are seen in this specimen converging from the middle circumference to the centre of the bone, and smaller ones are observed pursuing a like course from the borders of the articular surfaces.

The specimens of transverse processes, ( figs. 12, 13, 14, plate 10 ,) are remarkable for their robust character and cylindroid form. The outer extremity of the longer specimen, though abruptly truncated, appears nevertheless to be entire. The inner extremity of the specimens, inferiorly, presents a broad, convex, porous, articular surface, for conjunction with the corresponding surfaces of the vertebral bodies. Above this surface, there is a smooth arching one forming the side of the vertebral canal and overhung by the abutment for the articular and spinous processes.

The numerous fragments of ribs, generally indicate these bones to have a curved fusiform shape, as seen in fig. 15, plate 10 , representing one of the more perfect specimens. In structure they exhibit the same remarkable solidity noticed in the corresponding bones of the Manatee.

Though I have supposed the remains above described to indicate the former existence of a mammal allied to the Manatee, they yet appear to me of such a singular character, that I have suspected they may have belonged to an aquatic reptile, unlike any known, and perhaps foreshadowing in its constitution the Sea Cows, just as Iguanodon appears to have foreshadowed the herbivorous pachyderms of the Eocene Tertiary Period.

Explanation of Figures, Plate 10.

Figures 8—15.—Vertebrae and rib of Ischyrotherium antiquum ; two-thirds the diameter of nature.

Figure 8. Dorsal view of vertebral body. Articular surface on each side.

Figure 9. Anterior view of the same vertebral body.

Figure 10. Ventral view of a second and similar specimen.

Figure 11. Broken surface of a third specimen, exhibiting its dense structure and converging nutritious canals.

Figure 12. Inferior view of a vertebral half arch and transverse process, exhibiting the articular surface, adapted to a corresponding one of figure 8.

Figure 13. Anterior view of same specimen as the last.

Figure 14. Anterior view of another specimen like that indicated in figures 12, 13.

Figure 15. Fragment of a rib, with outline sections (16, 17,) of the size of nature, from the upper end and middle SAURIA.

Figures

1—7. Thespesrus occidentalis. 8-17 Ischyrotherium antiquum.