Aublysodon mirandus (Leidy, 1868)

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

The present opportunity is an appropriate one to make a few remarks on the American allies of Megalosaurus . Since I have had the opportunity of inspecting the remains of the remarkable reptile from the green sand of New Jersey, described by Prof. Cope ( Proc. 1866, 275 ) under the name of Laelaps aquilunguis , in observing the comparative uniformity of the teeth, identical in character with those of Megalosaurus , I am more strongly impressed with the idea that the teeth of like shape forming part of those referred by me to Dinodon , alone belong to this genus. The others, of which no representatives have been discovered or recognized as belonging to Megalosaurus or Laelaps , most probably indicate a distinct genus and species,

for which I propose the name of Aublysodon mirandus .

Future discovery may prove Laelaps and Dinodon identical, and, judging from the comparison of corresponding parts of the jaws and the teeth, will be found to be more closely allied to Megalosaurus than was suspected, even should they not prove to be generically the same.

It is clear, from an examination of the anterior portion of the mandible of Megalosaurus described and figured by Buckland, Cuvier, Owen, etc., that no such teeth as those now referred to Aublysodon occupied the forepart of the jaw. It is also probable that the upper teeth of Megalosaurus and of its allies differ in no important point from those below. It follows, therefore, that the teeth now referred to Aublysodon , if they belong to the maxillary or mandibular series of Megalosaurus or its allies, could only pertain to the back part. The variation in form of the teeth in question appears too great for such a position.

The teeth now viewed as characteristic of Aublysodon are represented in figs. 36—45, pl. ix of vol. xi of the Transactions of the American Philosophical Society. The specimens consist of parts of three teeth, which differ much in size and other important points. In general the crowns are laterally compressed conical, with the anterior part thick and convex transversely as well as longitudinally, and with the sides nearly parallel. The posterior part forms a surface nearly as wide as the thickness of any part of the crown, and is defined from the lateral surfaces at right angles. In the two larger teeth these angles or borders are denticulated, like the trenchant borders of the teeth of Megalosaurus and its American allies. In the longest tooth ( fig. 35, 36, op. cit. ) the posterior surface forms an even plane; in the second sized tooth ( figs. 37—40 ) the posterior surface presents a median elevation. In the smallest tooth ( figs. 41—45 ), which indeed may belong to a different animal from the preceding, the borders defining the posterior surface are somewhat prominent backward, non-denticulate, and subside approaching the base of the crown so as to make a transverse section in this position oval ( fig. 45 ).

Hadrosaurus Foulkii , the bulky vegetable feeder, and cotemporary of the rapacious Laelaps aquilunguis , was at most probably only specifically distinct from Trachodon mirabilis , the teeth of which were found in association with those of Dinodon , so that, according to the laws of nomenclature, as Trachodon has priority of name, I suppose the first mentioned animal must be called Trachodon Foulkii , though the names of Hadrosaurus Foulkii and H. mirabilis would appear more appropriate for these powerful dinosaurs.

The best preserved tooth of those originally referred to Trachodon , represented in figs. 1—6 of the plate above cited, is identical in form with those referred to Hadrosaurus , and differs only in the absence of the rugulations of the lateral borders of the crown, and in some less important points.

The remaining specimens of teeth referred with the former to Trachodon , are represented in figs. 7—20 of the plate cited. Most of them are so worn and probably altered from their original form, that it is rendered uncertain whether they belong to the same animal as the preceding tooth, and one unworn ( figs. 18—20 ) has a very different shape from this. Perhaps these specimens belonged to another Dinosaur, for which the name Trachodon might be reserved, while that of Hadrosaurus might include the first mentioned and more characteristic tooth.

As Iguanodon had its enemy in a species of Megalosaurus , Trachodon , the representative of the former both in the western and eastern portions of the North American continent, was accompanied by an equally bloodthirsty enemy, which may, perhaps, on nearer comparison of corresponding parts, prove to be another species of the same genus, until now supposed to be different, under the names of Dinodon and Laelaps .

Prof. Cope remarks of Laelaps ( Pr. A. N. S. 1866, 276 ), that “in its dentition and huge prehensile claws it resembled closely Megalosaurus , but the femur, resembling in its proximal regions more nearly the lguanodon, indicated the probable existence of other equally important differences, and its pertinence to another genus.” Thus the genus is especially distinguished by the apparent peculiarity of the femur, but in my estimation even this disappears if the bone referred to Laelaps be viewed in the corresponding position to that of M. Bucklandi , represented in pl. vii, pt. iii, of Prof. Owen’s Monograph of the Fossil Reptiles of the Wealden, which appears to me to be the reversed one to that in which Prof Cope has described it in Pr. A. N. S. 1866, 276 .

The teeth of Bathygnathus , a huge carnivorous reptile, whose remains have been found in the triassic red sandstone of Prince Edward’s Island, have the same form as those of Megalosaurus , Dinodon and Laelaps . But here, so far as we have the corresponding parts for comparison, the resemblance ceases. The remarkable depth of the dentary bone in relation with its length in Bathygnathus , indicates a form of head very different from that of Megalosaurus and its American representatives. It was this unusual relation of depth to breadth which led me to suspect a form of head more in accordance with that of the skeleton of an upright animal, and led me to ask the question, “was this animal probably not one of the bipeds which made the so-called bird tracks of the New Red Sandstone of the valley of the Connecticut?” (See Jour. Ac. Nat. Sc. 1854, 329)

Subsequently, in examining the remains of Hadrosaurus , the American representative of Iguanodon , from the great disproportion between the fore and hind parts of the body, I was led “to suspect that this great herbivorous lizard sustained itself in a semi-erect position on the huge hinder extremities and tail, while it browsed on plants growing upon the shores of the ocean.” (Cret. Rept, of the U. S. 1865, 97.)

The remains referred to Laelaps exhibit even a far greater disproportion between the fore and hind limbs than in Hadrosaurus , which, together with its long bird-like claws, etc., suggested to Prof. Cope a similar position of body to that of Hadrosaurus , and a use of the hind limbs in attack upon the prey of the animal analagous with that in the eagle ( Pr. A. N. S. 1866, 279 ). The extraordinary disproportion between the fore and hind limbs of Laelaps , which appears to me so closely related with Megalosaurus , leads me to suspect that the remains described by Buckland, Cuvier, Owen and others, and attributed to the shoulder of M. Bucklandi , perhaps, at least in part, belong to the pelvis, if they in whole or part do not belong to other animals. Had the humerus of Laelaps been found isolated, I never would have thought of associating it in the same skeleton with the huge bones of the hinder extremity of that animal. Perhaps, when this great disproportion comes to be known, it may be discovered that there exist specimens of remains of the fore limbs of Megalosaurus , from the Wealden, in the British or other museums of England, which heretofore have excited no suspicion as to their true relations.

Teratosaurus , from the upper Keuper, in the vicinity of Stuttgart, described by Meyer (Palaeontographica, 1859-61, 258), approached Bathygnathus most in the proportions of its face, as well as resembled it in the form of the teeth, but the fossil dentarybone of the latter is even still shorter and deeper than would relate to the fossil maxillary of the former.

Figures

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