Stephanosaurus, Lambe, LM, 1914

Lambe, LM, 1914, On a new genus and species of carnivorous dinosaur from the Belly River Formation of Alberta, with a description of the skull of Stephanosaurus marginatus from the same horizon, The Ottawa Naturalist 28, pp. 13-21 : 17-19

Stephanosaurus , gen. nov.

This genus is established for the reception of the species from the Belly River formation of Alberta, originally described, under the name of Trachodon marginatus , by the writer in 1902 * from a ramus of a lower jaw and a maxilla, and from the remains of one individual. With the species were provisionally associated other elements, notably a slender footed-ischium, which associations have since been proved to be correct by further material included in the collection of 1913 from the Belly River formation of Red Deer river. These additional remains, discovered by Mr. Charles H. Sternberg, are of two individuals to which the Writer has lately referred† in describing the integument of the species. With one of these specimens the skull reproduced in plate I is preserved. Part of another skull (collection of 1913), found separately, assists in elucidating the characters displayed by the more perfect skull, and provides additional evidence regarding some of those elements to whose great development is mainly due the surprising shape of the head of this species.

The skull of Stephanosaurus rises to a great height in front of and above the eye opening. In recently describing Gryposaurus , also from the Belly River formation of Alberta, the writer commented on the anterior depth of the skull occasioned by the height to which the nasal rose. In the skull of Stephanosaurus , however, the height attained by the nasals is proportionately twice as great as in Gryposaurus ; the depth of the skull above its midlength is equal to its total length. Viewing the skull from the side, the facial outline is sigmoid, at first concave, ascending rapidly from the front until it is vertical, whence it continues upward and reaches a point directly above by an even convex curve; this, the highest point preserved in the specimen, is vertically above the midlength of the skull. The general slope of the head behind is rapidly downward to the squamosal, but as this part of the specimen is imperfect, the exact outline is unknown. The almost vertical quadrate and the sinuous horizontal contour of the slender mandible below complete the profile of the head.

The orbit is small and its centre is below the midheight of the skull.

The enlargement of the skull in front of and above the orbit is due to the great development mainly of the prefrontal and nasal bones, the latter of which rises upward in front of the prefrontal and passes backward over it and beyond it. This extension of the nasal beyond the upper limit of the prefrontal appears to be supported from below by the frontal, although this last bone has not been satisfactorily recognized. Above the prefrontal and the supposed frontal, the nasal points almost directly upward. In the specimen its upper termination has been broken off, but it probably formed with the other nasal a stout spine somewhat of the shape suggested by the dotted outline in the figure.

The prefrontal is a large triangular bone with its base resting for the most part on the lachrymal, which latter is long and narrow, meets the jugal below, and posteriorly enters largely into the formation of the orbital rim.

i?

By referring to the figure it will be seen that the maxilla, the jugal, the quadrato-jugal, the quadrate, and the mandible have much the same proportions as in Trachodon . The jugal is small, but it has the general shape characteristic of this element in all known members of the Trachodontidae .

Anteriorly, the premaxilla is somewhat depressed, but laterally much expanded. Its upper surface, next to the median line of the head, is continued in a curve outward anteriorly and backward laterally as a marginal area enclosing a wide depression in advance of the long and narrow nasal opening. In the specimen, the outermost portion of the laterally expanded premaxilla is crushed down. The nasal opening is enclosed above by the nasal and below by a backwardly directed extension of the premaxilla. This extension, or lower limb, of the premaxilla passes along the upper front surface of the maxilla and abuts against the prefrontal. Above, posteriorly, it unites with the nasal behind the nasal opening in a short sutural contact. It is not known how far forward the nasal extends, as its suture with the premaxilla in front has not been detected.

The squamosal is preserved in part, as shewn in the figure. The postfrontal is probably represented toward its anterior end, but here its limits are not recognized, and posteriorly the bone is imperfect. As in other members of the Trachodontidae , it no doubt contributed to the formation of the postorbital bar.

The orbital opening is narrowly elliptical, with its longer diameter directed obliquely downward and forward. It is more than twice as long as wide. The lateral temporal fossa is larger than the orbit and is also longer than wide, with a similar obliquity of length.

Detailed descriptions, with illustrations, of the maxilla, the mandible, the teeth, the ischium, the pubis, and the principal bones of the fore- and hind limbs of Stephanosaurus marginatus were published When the writer established the species in 1902. The characters of the integument are known from the writer's recent description (op. cit.).

The nearest approach to Stephanosaurus is Saurolophus of Brown from a higher horizon of the Cretaceous of Alberta (Edmonton formation). In this latter genus the facial slope of the skull is about midway between that of Stephanosaurus and Trachodon . The upwardly directed nasal spine of Stephanosaurus may have heralded the backwardly sloping nasal crest of the later Edmonton dinosaur. The two genera appear to be closely allied and in both the footed form of ischium is present.

Figures

EXPLANATION OF PLATE I Left lateral aspect of skull of Stephanosaurus marginatus; one- fifth the natural size. Abbreviations.-D, lateral temporal fossa; DN, dentary; J, jugal; L, lachrymal; MX, maxilla; N, nasal; NO, nasal opening; OR, orbit; PD, predentary; PF, prefrontal; PM, premaxilla; Q, quadrate; QJ, quadrato-jugal; S, squamosal; SA, surangular.