Gorgosaurus lancensis Gilmore, 1946, new species
GORGOSAURUS LANCENSIS, new species Type. C.M.N.H. No. 7541. Skull and lower jaws. Collected by D. H. Dunkle, August 16, 1942. Locality. SE1/4, sec. 11, R. 61 E., T. 4 S., Sand Creek, Carter County, Mont. Horizon. Upper Cretaceous, Lance formation, Hell Creek member. The type and only specimen is a n...
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Zenodo
1946
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| Online Access: | https://dx.doi.org/10.5281/zenodo.3483018 https://zenodo.org/record/3483018 |
| Summary: | GORGOSAURUS LANCENSIS, new species Type. C.M.N.H. No. 7541. Skull and lower jaws. Collected by D. H. Dunkle, August 16, 1942. Locality. SE1/4, sec. 11, R. 61 E., T. 4 S., Sand Creek, Carter County, Mont. Horizon. Upper Cretaceous, Lance formation, Hell Creek member. The type and only specimen is a nearly complete skull with tightly articulated lower jaws. It has suffered the loss of the squamosal, postfrontal, quadratojugal, and the upper half of the quadrate of the right side; there is some disarrangement of the bones of the palate, and many of the teeth are either missing or incomplete. Otherwise the skull is in well-preserved condition. The coalescence of many of the sutures strongly suggests the adult age of the individual. So close is the union between upper and lower jaws that not only is the lower dentition wholly concealed, but the very grave danger of doing permanent injury to the specimen prevents any attempt to separate them. In size the skull is smaller than the cranium of the type of Gorgosaurus sternbergi with which, in several respects, it appears to have the nearest affinities. Unfortunately for the present purposes the type of G. sternbergi has never been adequately described, and its original specific characterization was very meager. Furthermore, a single figure of the mounted skeleton published by Matthew and Brown (1923, fig. 4) constitutes the only published illustration of this type. Since the skeleton is on exhibition and under glass that cannot be readily removed, direct comparison or study of the actual skull is practically out of the question at this time. In plate 4, figure 2, is shown a side view of the skull enlarged from the one and only negative of this specimen and generously placed at my disposal by Dr. E. H. Colbert. Matthew and Brown (1923, p. 7) distinguished Gorgosaurus sternbergi as follows: "It is of smaller size and more slender proi)ortions than the G. libratus. The jaws are much less massive and the muzzle is more slender, the maxilla more elongate and shallow, the orbital fenestra more circular, the tibia is considerably longer than the femur." In most of the skull characters mentioned, the cranium is in agreement with the specimen now before me. In slenderness of the muzzle it is intermediate between G. libratus and G. sternbergi. The dental formula appears to be the same in all three species. It should be pointed out, however, that this apparent slenderness of muzzle in the type of G. sternbergi (see pl. 4, fig. 2) is greatly exaggerated by the downward crushing of the nasal region. Restored to its normal profile this slenderness becomes much less noticeable. In its smaller size, elongate shallow maxillary, and rounder orbital fenestra, G. lancensis appears to have its closest affinities with G. sternbergi. There are some minor differences in skull structure, but none is regarded of specific significance. Such differences as are observed in the lateral temporal fenestral region are clearly due to postmortem distortion. The quadrate having been crushed forward and upward narrows the fenestra and otherwise alters the natural angulation of the elements composing this part of the skull. In the absence of detailed knowledge of the cranial characteristics of the two described species, it is quite impossible at this time, on structural features of the skull alone, properly to characterize the three species now assigned to Gorgosaurus, but in view of the great interval of time that has elapsed between Belly River and Lance, one appears justified in believing that it is too great a span in time for a species to pass unchanged from one to the other, and that with the discovery of additional materials characters will eventually be found that will adequately distinguish them. The genus Gorgosaurus was established on an adequate specimen, consisting of the greater portion of an articulated skeleton, but the skull was badly crushed and broken with much of the top portion missing. Lambe (1917) described in detail such parts of the type skull as were available, and although today there are several well- preserved skulls of this genus known, a description of a complete cranium has not yet appeared. Because of this discrepancy in the literature the present specimen is here described in as much detail as its condition will permit. DESCRIPTION Parietal. The combined parietals are relatively short and unite with the frontals by a nearly straight transverse suture. Their upper surfaces at the center rise into a thin, sharp-edged, antero-posterior sagittal crest and a transverse supraoccipital crest. The latter rises high above the level of the skull top. Its slightly thickened upper border, viewed from the rear, presents a sinuous outline divided at the center by the truncated end of the longitudinal crest. Whereas the sides of the supraoccipital crest converge slightly from top toward the bottom in G. libratus, as shown by the skull of A.M.N.H. No. 5434, there is a strong overhang of the upper half of the lateral border when viewed from the rear. At midheight the supraoccipital crest has a greatest transverse diameter of ii 6 mm.; at the bottom, which rests upon the exoccipital and paraoccipital bones, a long, tapering process extends outward to increase suddenly the transverse diameter to 144 mm. or more. This process is received in a slotUke depression of the squamosal at its junction with the paraoccipital. Above the top of the supraoccipital the parietal shows a triangular-shaped thickening that articulates directly with the heavy median part of the supraoccipital, probably functioning in the same capacity as the heavy, bluntly pointed posterior projection that stands out so prominently on the Antrodemus skull. In front of the supratemporal crest the parietals are strongly contracted transversely between the supratemporal fossa, again widening at their anterior junction with the frontals, as shown in plate 2. Between the fossa they have a least diameter of 55 mm. The greatest length of the parietals at the center is 68 mm. Frontal. The frontal area in Gorgosaurus lancensis, owing to the coalescence of many of the sutural contacts, can be interpreted only somewhat uncertainly. The union with the parietal and postfrontal is quite clear, but the outline and anterior extent of the frontals where they meet the lacrimal and nasal bones cannot be surely determined. The median suture separating the paired frontals is clearly discernible on the anterior third of their length, but posteriorly the fusion is so complete as to leave no trace of the line of contact. On the right side, between the hornlike projections of the lacrimals, an offset in the bone surface extends backward and outward from the median suture, and is thought to represent the Hne of lateral union between the frontal, nasal, and prefrontal of that side. If this interpretation is correct, the combined frontals end anteriorly in a point deep within a V-shaped notch in the nasals as shown in plate 2. Contributoryevidence as to the correctness of this interpretation is found in a skull of Gorgosaurus libratus, U.S.N.M. No. 12814, which shows a closelysimilar arrangement of these elements. That the frontal is coalesced with the prefrontal latterly appears to be indicated on the left side by a short sutural edge that extends inward and backward from the border of the orbital notch. If this is the posterior end of the prefrontal, it occupies a position similar to that of the prefrontal in Antrodemus, and quite unlike the more central position in the Tyrannosaurus skull. A wedge-shaped process of the frontal extends outward to fill the interspace between the postfrontal and prefrontal bones, but it appears to be excluded from participation in the orbital border by the articulation of these two elements at the bottom of the notch. On the right side of the skull there is no trace of the prefrontal suture. The sagittal crest extends forward from the midline of the parietal on to the frontal, reaching its highest elevation on the posterior fourth of that bone. Anteriorly this crest drops rapidly downward to the level of the skull surface. It is estimated that the nasal bones at the center had a greatest length of about 112 mm. Nasal. The nasal bones, as in other members of the Deinodontidae, are especially long and narrow, with transversely rounded dorsal surfaces anteriorly, but flattening out on the posterior third. The central areas are coalesced into a single rugose element that is suturally separate anteriorly. The contact with the frontals is broad, not narrow as in Tyrannosaurus; the pointed ends of the combined frontals being received in a deep V-shaped notch on the midline, as shown in plate 2. Anteriorly the nasals have bifurcated ends, a heavy upper process that extends forward above the nares to meet an ascending process of the premaxillary which it underlaps, and a much weaker process that extends forward and downward along the upper surface of the maxillary border to meet an ascending process of the premaxillary and thus exclude the maxillary bone from participation in the boundary of the external nares. The lateral contacts of the nasal with the maxillary, lacrimal, and prefrontal bones have the sutures coalesced, and their course can be only approximately determined. A row of small foramina on the side of the left nasal, running posteriorly from the border of the nares, suggests that the roughened areas may have had a chitinous hornHke covering. The greatest length of the nasals is 388 mm. Postorbital.- The bone here designated postorbital in all probability represents the coalesced postfrontal and postorbital. Since no trace of their sutural union can be detected in this specimen they are described as a single element. The postorbital is a triradiate bone with short, heavy inner-directed process that articulates with the frontal by a strong suture; a second slender, tapering process that extends backward and downward is received in a depressed groove on the outer side of the squamosal, thus forming the upper temporal bar; the third, the longer one of the three, also a tapering process, extends downward and laps along the anterior side of the jugal to form the wide bar separating the orbit and lateral temporal fossa. From the superior border to the end of the inferior branch it measures 114 mm. Its greatest antero-posterior diameter is 145 mm. There is no "postfrontal rugosity" as in Tyrannosaurus and the notch separating this bone from the lacrimal rugosity is shallow. At the bottom of this notch it appears to be in contact with the prefrontal, thus excluding the frontal from the orbital border. Prefrontal. The narrow area on the top of the skull, lateral to the frontal and wedged in between the lacrimal frontal and nasal bones, is here tentatively regarded as pertaining to the prefrontal (see pl. 2). If correctly determined, it has a greater dorsal exposure than in Tyrannosaurus, and it differs further in being in contact with the postfrontal. Lacrimal. The lacrimal of Gorgosaurus lancensis resembles those of Antrodemus and Ceratosaurus in having a stout, blunt-edged elevation rising from its outer superior surface. It may have supported, as first suggested by Osborn (1903), "something in the nature of a low dermal horn." A similar horn-projection is present on the skulls of G. libratus and G. sternbergi, but is absent in Tyrannosaurus rex. On the external side the base of this "hornlike" projection is excavated, and the bOne is still further lightened by internal cavities, as shown by the incomplete right element which lacks its top (see pl. 2), The sutural contacts of the lacrimal with the adjacent, prefrontal (?), frontal, and maxillary bones cannot be accurately traced in this specimen. Posteriorly it is separated from the postorbital by a narrow notch. Ventrally it unites with the jugal, the posterior angle being received in a notch in the top of that bone, but anteriorly these two bones appear to lap one another. Premaxillary. The premaxillary, as in other deinodonts, is reduced in size. It carries either four or five teeth, a point that cannot be certainly determined by this specimen owing to the obHteration of the premaxillo-maxillary suture. In all probability it will be found there are four premaxillary teeth as in Gorgosaurus libratus, so clearly demonstrated by Lambe (1917). The slender superior process rising from the upper anterior border, with its counterpart of the opposite side, meets the nasals above the midlength of the external nares. Maxillary. Roughly triangular in outline, the maxillary is deeply excavated posteriorly by the large antiorbital fenestra. The posterior process that extends backward to meet the jugal is especially long and slender, its tapering termination ending below the center of the orbital opening as it does in G. sternbergi. Above, it unites with the nasal and lacrimal, but in this specimen most of the sutural contacts are obscured by the coalescence of the bones, as is the union with the premaxillary. On the left side of the midlateral surface of the maxilla adjacent to the position of the premaxillo-maxillary suture there is evidence of a small opening, which in Tyrannosaurus Osborn designates the third antiorbital fenestra. It appears to be absent on the right side of this specimen. The maxillary is excluded from participation in the boundary of the nares by a slender downwardly directed process of the nasal. The base of the ascending maxillary process is perforated by the small second antiorbital fenestra. The surface of the bone in front of this fenestra is smooth and depressed, forming a marginal tract at a lower level than the general surface of the skull at this point. Viewed from the side the dental border is sinuous, with a row of dental foramina. The greatest length of the maxillary is about 405 mm., of which 280 mm. is tooth bearing. There are clearly 13 teeth and possibly I more if the premaxillo-maxillary suture cuts between the first and second U-shaped teeth of the front series as Lambe (1917) found it to do in the type of G. libratus. This point cannot be determined in this specimen owing to the coalescence of this suture. Jugal. The jugal has the usual triradiate shape, but is more slender and elongate antero-posteriorly than in Tyrannosaurus. It extends forward as a tapering process to join the maxillary by squamous union. It 'is underlapped for nearly half its length by the slender posterior process of the maxilla that terminates below the center of the orbit. This anterior process of the jugal is perforated by a large oval-shaped jugal fenestra, ventral to its union with the lacrimal. There is a notch in the upper border of the jugal for the reception of the lower posterior end of the lacrimal, but anterior to this notch it overlaps the side of the descending lacrimal. The tapering superior process unites by squamous union with the descending process of the postorbital, the two forming the postorbital bar which separates the orbit and the lateral temporal fenestra. Posteriorly the jugal is overlapped by a fingerlike process from the quadratojugal, as shown in plate 1. In this specimen crushing has broken this process in two so that its full posterior extent is not to be observed (see pl. 1). The lower border, anterior to the forward termination of the quadratojugal, has a flattened projection that extends prominently downward as a blunt process (see pl. 1). That this is a normal development is demonstrated by its presence on the opposite side. This process appears to be absent in G. sternbergi but is present in G. libratus. The jugal forms all of the lower boundary of the orbit and little, if any, of the postero-ventral boundary of the large antiorbital fenestra. The greatest height of the jugal is 170 mm. Squamosal. The right squamosal is missing but the left is almost complete, and the open sutures give a clear picture of its extent and relationship to the surrounding elements (see pls. 2 and 3). Externally the forwardly directed bar is overlapped on the outside by the posterior process of the postorbital, the two forming the upper supratemporal bar; an inwardly directed process passes in front of the high vertical supraoccipital crest to lap along the parietal on the posteromedial side of the supratemporal fossa; posteriorly the squamosal rests against the front of the paraoccipital process; on the external ventral side a slender process curves forward and downward to cap the top of the quadratojugal. Ventrally it articulates with the quadrate, but this side is hidden by enveloping matrix. Quadratojugal. The lateral view of the skull (see pl. 1) shows clearly how the quadratojugal and quadrate bones have been crushed forward, thus closing the lower part of the lateral temporal fenestra to a narrow slotlike opening. As shown by the overlapping broken ends of the lower anterior process, this fossa should be at the least 26 mm. wider at the bottom than as shown in plate 1. The anterior border of the quadratojugal within the fossa is incomplete, so that its full extent and proper outline cannot be determined. Immediately below midlength, on the outer side, the surface is indented by a shallow rounded depression. Internally it unites by suture with the quadrate except at the point of the large quadrate foramen. The proximal end is capped by a descending process of the squamosal. The greatest length of the inferior border is estimated to be about 95 mm.; the greatest height no mm. The sutures on the inner side with the quadrate are coalesced and cannot be traced. Quadrate. Both quadrate bones are so much hidden by matrix, or closely enveloped by the squamosal above and the mandible below, that little is to be seen of these elements except from the rear. The large vertical ovate quadrate foramen lies between the quadratojugal externally and the quadrate internally. Below this foramen the distal portion of the quadrate rapidly widens toward the outside where it is coalesced with the quadratojugal. The left quadrate at the distal end has a greatest transverse diameter of 67 mm. The bilobed distal extremity fits closely into the cotyloid depression of the mandible. On the inner side slightly above the distal end a process extending inward and forward articulates with the pterygoid, but its full forward development is hidden in the adhering matrix. Occipital region. The coalescence of most of the bones of the occipital region of the present skull makes a detailed study of its several elements very unsatisfactory. Viewed from the rear the median upper part of the occiput is dominated by the vertically and horizontally expanded supraoccipital crest formed wholly by the parietal. Medially it envelopes the stout supraoccipital; its lower outer border is continued outward as a slender, pointed process that rests on the upper border of the paraoccipital process in a transverse depression crossing the back of the squamosal. The upper end of the supraoccipital is squarely truncate, with its surface rugosely roughened. This surface, in conjunction with the lateral pits, probably gave attachment for the ligamentum nuchae. Laterally and ventrally its sutural limits cannot be traced. For that reason it is not known whether the supraoccipital participated in the boundary of the foramen magnum or not. The exoccipitals contribute extensively to the formation of the occipital condyle as in Antrodemus valens. Their dorsal connection with the supraoccipital can no longer be traced. The surfaces lateral to the foramen magnum are perforated by two foramina, the larger and most posterior one being for the passage of the twelft : Published as part of Gilmore, Charles W., 1946, A new carnivorous dinosaur from the Lance Formation of Montana, pp. 1-19 in Smithsonian Miscellaneous Collections 106 on pages 2-15, DOI: 10.5281/zenodo.3239146 : {"references": ["OSBORN, H. F. 1903. The skull of Creosaurus. Bull. Amer. Mus. Nat. Hist., vol. 19, art. 31, pp. 697 - 701.", "Lambe, Lawrence M. 1904. On Dryptosaurns mcrassatiis (Cope) from the Edmonton series of the Northwest Territories. Contr. Can. Paleont. Geol. Surv. Canada, vol. 3, pt. 3, pp. 5 - 25, pi. 8."]} |
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