Dinodon horridus (Leidy, 1856)

Leidy, J., 1868, Remarks on a Jaw Fragment of Megalosaurus, Proceedings of the Academy of Natural Sciences of Philadelphia 20, pp. 197-200 : 197-198

A fossil worthy of notice in the Museum of the Academy consists of the fragment of a jaw, apparently of the Megalosaurus , which, if it does not belong to a different species from M. Bucklandi , indicates an individual larger than any one of those referred to by Buckland, Cuvier, Owen, etc. The fossil was purchased in England, and was presented to the Academy by Dr. Thomas B. Wilson. It is labelled, “ Fragment d’une machoire de Megalosaurus trouvé dans le lias a Boué (or Boues). L’animal est extremement rare ici. Il avait 45 pied de longeur.” In another hand it is marked “ Jura Mts.”

The fragment contains two mutilated teeth, visible throughout their length from the inner part of the jaw being broken away. The matrix adhering to the fossil consists of an oolite composed of a homogeneous clay-colored basis, with imbedded granules, of a rounded form, brown and shining.

The teeth are inserted into the jaw about two-thirds their length, and more than three-fourths the depth of the bone. They have measured 5 1/2 and 6 inches in length. The breadth at the base of the enamelled crown of the best preserved tooth is 14 3/4 lines, which is nearly the fourth of an inch greater than in the largest tooth represented in any of Prof. Owen’s figures in his Monograph of the Fossil Reptiles of the Wealden Formation. A tooth apparently nearly as large in an American ally, is one referred to Dinodon horridus , and represented in

fig. 21, pl. 9 , of my memoir on the Extinct Vertebrata of the Judith River, published in the eleventh volume of the Transactions of the American Philosophical Society . The reconstructed outline of this figure is, however, too large, rendered so by the too distant removal of the apex of the tooth from the other fragment. The breadth of this specimen really did not exceed an inch.

The longest tooth of the fossil under inspection, for the most part broken away, exhibits a mould of the large interior pulp cavity. This mould, from the bottom of the latter to its broken end in the position of the crown, is 5 1/4 inches long. The broken end is 8 lines wide and 1 3/4 lines thick; the widest and thickest part of the mould near the middle of the length of the tooth is 11 1/2 lines wide and 5 lines thick.

The fangs of the teeth do not continue of the same width to the bottom, as in the teeth of crocodiles, and, as I believe, is considered to be the case in Megalosaurus , but from about their middle they contract, or become narrower, as is ordinarily the case in mammals. Indeed, one of these teeth isolated might be taken for the canine of a Drepanodon , or sabre-toothed tiger. In the fossil the bottoms of the fangs narrow antero-posteriorly, and become thinner from without inwardly, and they also curve somewhat in the latter direction.

The long fangs of the teeth in the fossil, and their becoming narrowed at bottom, at first led me to suspect the specimen belonged to a different genus from Megalosaurus , but a view of fig. 1, plate xii, of Prof. Owen’s monograph above mentioned, seems to prove by the appearance of the successional teeth within the jaw, that the fangs actually become narrowed towards the bottom in that genus.

In the best preserved tooth of the fossil, the enamelled crown exhibits the same shape, familiar as the characteristic form of that of Megalosaurus . The trenchant borders of the crown are denticulate, and the enamel is comparatively smooth, or only very feebly striate.

The contracted condition of the bottom of the fangs of the teeth would leave more space than there otherwise would be for the development of successional teeth within the jaw. In the fossil the remains of one of the latter is seen at the lower part internally of one of the functional teeth, and an impression in a corresponding position of the other functional tooth indicates a similar occupant.

In the progress of the successional teeth of Megalosaurus , their summit first appeared at the margin of the jaw internally to the teeth in functional position. In the course of growth and protrusion they excited absorption in the contiguous bone and fang of their predecessors, and continuing to advance from within and beneath (in the lower jaw), as it were, shouldered the latter from the jaw. A third tooth in Megalosaurus appears to have occupied a position internal to the second one, before the protrusion of this from the jaw.

The outer portion of the jaw bone retained in the specimen has an average depth from the alveolar border of 5 inches. Its outer surface is a vertical plane, rounding only near the base.

Figures

1-20. Trachodon mirabilis 21-48. Deinodon horridus 49-52. Palaeoscincus costatus 53-55. Troodon formosus.