[D] Jaxartosaurus aralensis [Su] [T]
Describer
Riabinin, 1939
Time
Cretaceous Late Turonian Coniacian Santonian
Classification
Ornithischia Ornithopoda Hadrosauridae Lambeosaurinae Nomina Dubia
Diet
Herbivore
Fossilsite
Dabrazinskaya Svita, Alim Tau site, Syderinskaya Oblast, Kazakhstan
Fall Under
Jaxartosaurus
Info
Typespecies - Skull
Jaxartosaurus (Riabinin, 1937) > Jaxartosaurus aralensis (Riabinin, 1939) >> Procheneosaurus convincens (Rozhdestvensky, 1968)
Isolated skull roof and braincase.The holotype skull roof and braincase was described in great detail by Rozhdestvensky. A. K. (1968). The limited nature of the type material makes its taxonomic status dubious. Holotype PIN 5009/1
From: Rozhdestvenskiy, A. K. 1968. [In Russian]. Pp. 97-141 in: Tatarinov, L. P., et al. (eds.). [Upper Paleozoic and Mesozoic Amphibians and Reptiles]. Akademia Nauk S.S.S.R., Moscow.
Translated by W. Robert Welsh, copy provided by Kenneth Carpenter and converted by Matthew Carrano.
Genus Jaxartosaurus Riabinin, 1939
Type (unique) species:
Jaxartosaurus aralensis Rjabinin, 1939; Upper Cretaceous (Coniacian–Santonian) of southern Kazakhstan.
Diagnosis
Medium-sized hadrosaur. The skull is wide and low with a helmet-like crest in which the frontals and prefrontals do not participate. The frontals are large and inflated and they are isolated from the edge of the orbit. The supratemporal fenestrae are small, and the supratemporal arches are straight and parallel to the sagittal line of the skull. Comparison. Besides Jaxartosaurus, the subfamily Lambeosaurinae numbers eight genera1 at present (Rozhdestvensky, 1964b). Of these Jaxartosaurus is apparently the farthest removed from the [cheneosaurs] (Procheneosaurus, Cheneosaurus, and Nipponosaurus), having a narrow but higher skull in the occipital region and an underdeveloped crest that looks like a small rise in the prefrontal area of the skull. On the other hand, Jaxartosaurus is no less removed from Parasaurolophus, which has a huge long crest that extends far beyond the occipital edge of the skull and is formed from below by the nasals that are displaced from the roof of the skull, and the frontals that make up the upper edge of the orbit and rise upward from it.
Jaxartosaurus is more similar to the other four genera Lambeosaurus, Corythosaurus, Hypacrosaurus, and Tanius but is notably different. Thus, the frontals and prefrontals of Lambeosaurus participate with the nasals and premaxillae to form the crest, whereas in Jaxartosaurus the crest may be formed by only the premaxillae and nasals. Corythosaurus and Hypacrosaurus are the most similar to Jaxartosaurus.
Corythosaurus is characterized by a skull crest that is more circular and laterally compressed. In some species this crest overhangs the occiput, making contact with the squamosal. Hypacrosaurus (with one species) has a dome-shaped crest that is less laterally compressed, does not overhang the occiput, and does not make contact with the squamosal. Judging by the support areas on the parietal and squamosal, the crest in Jaxartosaurus is probably most similar in shape to that of Corythosaurus, differing only by the slight lateral compression as in Hypacrosaurus and thereby possibly occupying an intermediate position between the crests of these two lambeosaurs. Jaxartosaurus differs from both of these by its rather large frontals, the exterior surface of which reaches the anterior third of the orbit.
According to the shape of the bones in the roof of the skull (especially the frontals), Jaxartosaurus is very similar to the genus Tanius, also known as Tsintaosaurus Young, 1958 (Rozhdestvensky, 1964b), but differs from it in the subquadrate supratemporal fenestrae2 and more massive skull crest (judging by its support area). The latter was destroyed for the most part in the specimens examined by Young (1958), who reckoned this crest to be similar to that of the Saurolophus and assigned Tsintaosaurus to the subfamily [Saurolophinae]. Finally, Jaxartosaurus differs from all four of the genera similar to it by its straight supratemporal arches that are parallel to the sagittal line of the skull.
Jaxartosaurus aralensis Rjabinin, 1939 = (Bactrosaurus prynadai Rjabinin, 1939)
[Tanius sinensis (Wiman, 1929) > Tanius prynadai (Riabinin, 1939) >> Bactrosaurus prynadai (Riabinin, 1939)]
Lectotype3. The Museum of the Central Science and Research Institute for Geological Exploration (Leningrad), No. 1/5009: incomplete skull with no maxillofacial region; Upper Cretaceous (Coniacian–Santonian) of southern Kazakhstan (Kyrk-Kuduk region near the Alym-Tau range).
Diagnosis
The skull narrows drastically behind the orbits and has a short, underdeveloped sagittal crest that is formed by the temporals. The frontals are wedged between the nasals and removed from the edge of the orbit, being isolated from it by the broad band of the prefrontals and the postorbitals. The parietals enter between the frontals with the process that expands anteriorly. The skull is 20% wider in the postorbital region than in the occipital region. The width of the occiput (at the level of the paroccipital process) is greater than its height from the lower point of the occipital foramen by 250%. The orbits are wide in the upper region, the infratemporal fenestrae are narrow and slit-like, and they are almost 25% as wide as the orbits. The supratemporal fenestrae are rhomboidal and more than 150% as long as they are wide. The lower jaw has 34–35 dental rows.
Material
Collection No. 5009—an incomplete skull, a fragment of another skull, and fragments of the postcranial skeleton, bones of several specimens that came from the same deposit and horizon as the holotype.
Skull Roof
Riabinin (1939) noted the character of the exterior sutures of the bones in the skull roof, stressing the surface profile of the frontals and temporals. Overall, his description of the bones was very brief and it can be augmented.
The premaxillae, maxillae, and nasals of skull No. 1/5009 were not preserved. However, judging by the anterior ends of the prefrontals and frontals that underlie them, we might think that the lower rami of the premaxillae are narrow, tapering bands that extended very far posteriorly, lying on the interior surface of the prefrontals and reaching their middle—at the joint with the frontals. The nasals occupy a medial position relative to the lower rami of the premaxillae and a lateral position relative to their upper rami (arranged along the medial line of the skull in hadrosaurs), and must have looked from behind like wide rounded ribbons, bending slightly to the outside, where they occupied a significant area of the frontals.
Between the posterior ends of the premaxillae and nasals, and at the contact point of the prefrontals and frontals that underlie them, are apparently the nasal fenestrae. We can make judgments about this from the natural depression at the boundary of the frontals and prefrontals and by the structure of the interior surface of the latter, which has the explicit nature of a cavity wall. The prefrontals form half of the upper edge of the orbits and their anterior wall. They extend in an anteroposterior direction. Medially the prefrontals have a slightly striated sutural surface for contact with the premaxillae. The posterior end of the prefrontal is wedged between the postorbital and frontal, with which the prefrontal is joined by coarsely serrated sutures.
The frontals are joined together by a coarsely serrated suture and form the roof of the braincase near the forebrain, where they are noticeably swollen. The frontal is separated from the upper edge of the orbit by a wide band from the prefrontal and postorbital, but forms the upper region of the orbits. Distinct, coarsely serrated sutures separate the frontals from the adjacent bones and their external contours are reminiscent of maple leaves. Making contact with the nasals is a broad sutural surface, the medial extent of the frontals make contact with the premaxillae. Medially the frontals penetrate rather far between the nasals and laterally are slightly wedged between the prefrontals and postorbitals. From behind they form a complex suture with the parietals, which in the middle penetrate far between the frontals.
The parietals form the roof of the braincase above the mid- and hindbrain area and are at the same time the upper region of the medial walls of the supratemporal. By fusing together without an apparent suture, the parietals are wedged deeply between the frontals that expand ahead of a cuneus that has a depression on the surface. From behind the parietals form a short, but very typical sagittal crest. It is unclear as to how the parietals emerge onto the occipital surface of the skull—by separating the squamosals and lying on the supraoccipital bone: the medial ends of the squamosals that overlap the parietals were not fully preserved. Even less clear is the lower boundary of the parietals. The sutures are barely visible and we can only think it more or less probable that this boundary was horizontal and that the parietals made contact with the laterosphenoid and preotic bones from anterior to posterior, and possibly with the anterior ends of the supraoccipital bone, if we can agree that the latter emerges in the posterior node of the interior wall of the supratemporal fenestrae.
The postorbitals form the posterior half of the upper edge of the orbit and the posterior part of its upper wall, coming together anteriorly with the prefrontals and excluding from the orbital ring the frontals, with which the postorbitals contact the interior lateral side. At the front of the postorbital, directly behind the orbit forming its posterior edge, a process emerged downward—in the direction of the ascending process of the jugals. The second descending, but small, process emerges from the posterior part of the bone and, by lying on a similar process of the squamosal, separates the infratemporal fenestra from the articulation socket for the upper end of the quadrate bone. From behind the postorbital, by merging with the squamosal by means of two large teeth, forms with this bone the upper temporal arch. Beneath, the postorbitals on the entire extent of the lower temporal arch, which the squamosals underlie.
The squamosals are a complex configuration and participate in forming the posterolateral corners of the skull roof. Their anterior growths, like two large teeth, dorsally penetrate the postorbitals and ventrally form the lower surface of the temporal arch, reaching the anterior corner of the supratemporal fenestrae. Two rather long process that bound the socket for entering the quadrate bone emerge downward from the ventrolateral surface of the squamosal. The hindmost of these processes, by lying on the postotic and exoccipital bones, participate slightly with the paroccipital process to form them. The posterior boundaries of the squamosal that exit onto the occipital surface of the skull have the nature of an undulating line that is defined by contact with the supraoccipital and exoccipital bones.The bones of the zygomatic region and palatal surface are unknown.
Braincase
This area of the skull was completely preserved, and most of the sutures between the bones are well differentiated. The occipital area of the braincase includes unpaired supra- and basioccipital bones as well as paired exoccipital bones that fuse together. The otic region includes two bone pairs—the prootic and opisthotic—and is characterized by the least explicit ends of the bones. The base of the skull has been formed by the well-ossified basisphenoid and parasphenoid, the boundaries of which are usually poorly distinguished in most hadrosaurs because of bone fusion. The anterior region of the braincase was formed by a complex of sphenoid elements—an unpaired presphenoid and unpaired latero- and orbitosphenoids, well ossified and distinctly delimited.
The location of the supraoccipital is unclear in most hadrosaurs. Riabinin (1939) makes reference to its being located above the occipital foramen, which indicates relatively low height and the presence of a lengthwise crest on the bone, but does not present precise data about its limits and shape. Rjabinin’s data can be augmented. The supraoccipital contacts the exoccipital from below and from the sides, and contacts the squamosals and parietals from above. The lateral sutures with the exoccipitals are finely serrated. The supraoccipital is isolated from the foramen magnum by a rather wide band of exoccipitals that fuse together. Its boundary with these is distinct and almost horizontal, slightly depressed above the center of the occipital foramen. On the occipital surface of the skull, the supraoccipital has a long and narrow, but massive posteriorly sloped plate. The lateral flanges are pulled into the sides far beyond the limits of the occipital condyle.
A low, smooth medial crest runs along the central region of the supraoccipital. Posteriorly this crest transforms into a round enlargement that hangs slightly above the exoccipitals and the center of the foramen magnum. There are two enlargements that look like low columns in the upper part of the supraoccipital along the sides from the medial crest. These enlargements enter the corresponding depressions in the squamosal. There are openings in the sides of these columns that are not revealed by dissection. They probably opened into the supratemporal fenestrae and may have served as passageways for blood vessels, especially the dorsal head vein.
It is unclear whether these openings are homologous with the post-temporal fenestrae that are typical of many reptiles. The anterior boundaries of the supraoccipital are less certain because the parietals and squamosals overlap them for a considerable extent. We may only assume that the sloping serrated sutures running along the interior surface of the supratemporal fenestrae from the posterior lower node on a diagonal to the center.
The exoccipitals fuse together with no explicit suture between them and embrace the foramen magnum for almost their extent. Riabinin (1939) characterizes the exoccipitals as bones having “unspecified boundaries”; considering the lateral flanges that take part in forming the occipital surface of the paroccipital processes to be postotic bones. There are, however, no sutures on the occipital surface that might be seen as the boundary between the postotic and exoccipitals. As usual, this boundary is hard to see from the outside because of bone fusion. For this reason the question of the extent to which these bones participate in forming the paroccipital processes is debatable.
The visible boundaries of the this boundary on the occipital surface of the skull run: in the center from the supraoccipital, and dorsolaterally from the squamosal. The lower contact—from the basioccipital—was not preserved because this part of the skull was destroyed. The base area of the exoccipitals in the posterior surface of the uncinate paroccipital processes are fairly high proximally but then quickly narrow and drop somewhat below the foramen magnum.
A massive rod-shaped crest, beneath which the surface of the exoccipitals is concave, had developed on the center of the exoccipitals in parallel with the boundary with the supraoccipital. A short, small swelling at the point where the exoccipitals probably fused emerges from the center of the crest downward to the foramen magnum. Each exoccipital forms at a least a third of the occipital condyle, the lower part of which had been destroyed.
With regard to how the exoccipitals participate in forming the lateral surface of the braincase, Ostrom (1961) apparently relates to this negatively, bounding them in his drawings to the occipital surface, whereas most other authors (Romer, 1956; Goodrich, 1958; Langston, 1960) show the exits of the exoccipitals on the lateral wall of the skull where they include openings for nerves IX–XII as in contemporary reptiles.
The boundaries for the exoccipitals were not seen on the lateral surface of the Jaxartosaurus aralensis skull. Bearing in mind the usual location of nerves IX–XII within the occipital, presumably beyond the anterior boundary with the opisthotic, we can raise the crest-overhang that passes obliquely posteriorly from the base of the skull to the paroccipital process, between the fenestra ovalis and the opening for nerves IX–XI. This opening opens outward immediately below this crest. The opening behind is the exit for nerve XII.
Langston (1960) shows several (at least three) exits for nerve XII in Lophorhothon atopus but there are only two openings behind the fenestra ovalis on the lateral wall of the J. aralensis braincase. Therefore, the anterior of these may have been interpreted merely as a common opening for nerves IX–XI, which agrees with the observations of Ostrom (1961), who was studying casts made of the endocranial cavity of various hadrosaurs. The opening for IX–XI splits in two1—the anterior is fairly large and the posterior is small.
From their sizes we may think that nerve X must have passed through the anterior, and nerve XI through the posterior. Nerve IX apparently exited with nerve X, as casts of the brain cavity from various hadrosaurs indicate (Ostrom, 1961), although Langston (1960) linked the IX nerve exit with the otic capsule in Lophorhothon atopus. The opening in J. aralensis, being interpreted here as common for nerves IX and X, actually has a canal into the otic capsule, but it is more likely that this connecting canal was designated not for nerve IX, but for the jugular vein as Ostrom (1961) assumes because this passage is very small for a major vein in the head and was more likely for the perilymphatic duct.
As to jugular vein, it is likely that it exited the skull area through the cranioquadrate rather than the base of the skull as Ostrom (1961, pg. 138, fig. 5) shows, because in general venous blood was exported through the foramen magnum as in archosaurs (crocodiles) and birds. The basioccipital that forms the base of the occipital condyle was almost completely destroyed, a fact that Riabinin did not note.
It is difficult to make judgments about the boundaries 1 All of these details were revealed after the skull openings were treated with a solution of hydrochloric acid. of this bone. Starting from the area occupied by the exoccipitals, we might think of the basioccipital as a narrow band that formed the ventral region of the occipital condyle, and that the occipital foramen was between the tubera of the exoccipital.
The opisthotics form the lateral walls of the posterior region of the braincase near the medulla oblongata. In most hadrosaurs the opisthotics merge with the neighboring bones without any visible sutures. In J. aralensis the boundaries of the opisthotic are seen as a distinct suture for the lower anterior crest of the bone that is bounded with the basisphenoid. Because of the merging of the bones this contact is usually unknown in hadrosaurs, and a position that clearly belongs to the basisphenoid sometimes rises beyond the surface of the anterior otic bone (Ostrom, 1961).
The boundary suture in J. aralensis in the form of a fine, slightly serrated line descends from the region of the fenestra ovalis located in the anterior node of the opisthotic to the base of the braincase. The boundary between the opisthotic and prootic is seen as a faint crest-like suture directed from the fenestra ovalis dorsally and posteriorly to the joint with the horizontal overhang. The upper boundary of the opisthotic is unclear, but the horizontal overhang above the fenestra ovalis may possibly be the front of it.
Even less clear is the boundary between the postotic and exoccipitals; in this capacity we may conditionally adopt the crest that passes above the opening for nerves IX–XI. The fenestra ovalis, or vestibular region of the otic capsule, is located at the junction of three bones—the preotic, postotic, and basisphenoid—and looks like a subrhomboid-shaped depression split into two halves by a septum. The upper half probably corresponds to the fenestra ovalis, and the lower half includes the fenestra rotunda.
Three openings are visible at the base of the lower half; the anterior opening was possibly the exit for hyomandibular ramus of nerve VII, as it is treated in some hadrosaur species (Langston, 1960; Ostrom, 1961). According to Goodrich (1958), the middle cerebral vein might have passed through this opening, discharging in the otic capsule area into the lateral head vein. As has been mentioned, the posterior opening originates a canal that communicates with the canal for nerves IX and X and probably merges with the perilymphatic duct.
The prootics form the anterior region of the otic capsules, separating drastically into upper and lower parts by a transverse horizontally hanging crest. Riabinin (1939) and then Ostrom (1961) note only the boundary between the prootics and postotics. However, in J. aralensis even the upper boundary of the prootic with the temporal may have followed a long horizontal suture that separates the upper part of the temporal fenestra from the lower.
The anterior boundary is a highly visible vertical suture with the laterosphenoid. The posterior boundary passes through (or at) the origin of the fenestra ovalis, dorsal from which the prootic contacts the postotic and ventrally contacts the basisphenoid into which the prootic is wedged by a shallow “pocket” below the exit opening for nerve V (trigeminal foramen), which lies almost entirely within the prootic.
The prootic is slightly concave at its center, forming the anterolateral wall of the otic capsule. The horizontal overhang mentioned above makes a slight bend in this region. Above the horizontal overhang, the upper portion of the prootic occupies the posteroventral position on the inner surface of the supratemporal fenestra, and is subtriangular in shape. Here the most acute apex of the bone is rotated posteriorly.
The lower portion of the prootic, lower than the horizontal overhang, has an irregular shape. The location of the opening for the nerve V exit in this region looks like a deep (due to the thick-walled prootic) and circular funnel. The boundary of the prootic with the laterosphenoid passes through its anterior wall. Within the prootic is a semicircular canal that is open from above, the anterior wall of which is the anterior boundary of the prootic and is denoted by a distinct suture. The canal is directed ventrally from the trigeminal foramen and apparently closed on the trigeminal nerve (n. trigeminus II), separating further on into maxillary and mandibular branches. It is possible that the inferior orbital artery, being a branch of the stapedial artery, also passed through this canal, as is seen in contemporary reptiles (Romer, 1956; Goodrich, 1958).
Posterior and ventral to this canal is a depression that originates immediately behind the trigeminal foramen and crosses ventrally over onto the lateral surface of the basisphenoid. The pronounced profile provides a basis to presume that this depression has a special purpose, e.g., for the mandibular artery, being the lower branch of the otic artery. True, this structure is seen in lizards, whereas in crocodiles the mandibular artery departs from the ventral aorta (Goodrich, 1958) rather than from the otic artery. Therefore, the interpretation proposed for J. aralensis may be debatable. Below the trigeminal foramen, at the boundary of the prootic and basisphenoid, is another small opening that is probably the exit for nerve VII—more accurately, for the palatine branch.
The basisphenoid is very massive and tall, and is the axial region of the skull between the exits for nerves VIII and VI. Rjabinin did not describe these bones. The posterior and upper boundaries of the basisphenoid diverge at an acute angle: the first ventrally, making contact with the opisthotic, and the second more or less horizontally, descending below the trigeminal foramen where the basisphenoid contacts with the prootic and further on bending the protruding area of the laterosphenoid in order to reach the exit opening for nerve VI. From here the boundary of the basisphenoid, which contacts the parasphenoid as a faint suture, descends ventrally.
The boundary of the basisphenoid with the basioccipital, almost totally lost, remains unknown. The general shape of the basisphenoid, looking from the side, resembles an irregular quadrangle, the centro-posterior angle of which has descended significantly below the anterior. The lateral surfaces of the basisphenoid in the center ahead of the cuneiform, or pterygoid processes1, carry from each side at least two pairs of small mammiform processes that had been directed ventrally.
The entry into the carotid canal (which could not be opened) for the internal carotid artery, initially directed into the hypophyseal cavity and from there into the brain cavity, is at the same level as the posterior of these processes. It is possible that the palatine branch of nerve VII entered the carotid canal together with the internal carotid artery in order to exit through the noted opening at the boundary of the basisphenoid and parasphenoid along with the palatine artery departing from the internal carotid artery, as in modern reptiles (Romer, 1956).
The parasphenoid continues ahead of the basisphenoid, with which it merges with no visible suture in most hadrosaurs. This boundary is unclear in J. aralensis. It most likely ran from the opening for nerve VI ventrally, somewhat ahead of the mammiform processes of the basisphenoid. In this case the parasphenoid looks like a small, thin, flat, subquadrate bone.
The ventral process is directed anteriorly and was not completely preserved. Riabinin mentions the parasphenoid as merely a fragment. It is possible that he had a somewhat different picture of the contours of these bones. More distinct are the contacts between the parasphenoid and the orbitosphenoids that bound it from above, and between the presphenoid that abuts it anteriorly and dorsally and immediately above the anterior process. Along with the bones that have been mentioned, the parasphenoid participates in forming the interorbital septum.
The suture between parasphenoid and orbitosphenoid originates from two openings, the large (lower)2 of these which was probably the exit for nerve VI and possibly for the hypophyseal vein; the smaller (upper) for the exit of nerve III. Ostrom (1961) shows an exit opening for nerves III and VI in Corythosaurus casuarius, which can be explained by the conditions under which it was preserved and prepared. The boundary runs farther horizontally, immediately above two rather large openings in the base of the orbitosphenoid.
The presphenoideum,3 only the posterior region of which was preserved, is bounded posteriorly and ventrally by the parasphenoid and the orbitosphenoids, from which it is separated by a distinct suture, and of which it is the anterior extension. Dorsally the presphenoid abuts the frontal bone and splits into two rami that together with the orbitosphenoids form the anteriormost region of the braincase—near the highly developed (we can judge this from the exit opening from the braincase) olfactory lobes of the forebrain.
The laterosphenoids that merge with adjacent elements without sutures in most hadrosaurs are very distinctly delimited in J. aralensis. The laterosphenoids form the lateral walls of the braincase near the midbrain and the posterior expansion of the large hemispheres. From behind the laterosphenoid borders with the prootic: dorsally it borders with the parietal posteriorly and with the postorbital anteriorly. The anterior boundary of the laterosphenoid does not pass along the crest that separates the orbit from the supratemporal fenestra as Ostrom (1961) showed in Corythosaurus casuarius.
By participating in forming the orbit, the small flange of the laterosphenoid wedges between the frontal bone and the V and VI nerve openings and extends to make contact with the basisphenoid. Because of the horizontal canal, the aforementioned area seems to be separated from the base area of the bone. In general the laterosphenoid looks like an extended rectangle that bends outward in the anterior region (near the forebrain) with a process that emerges ventrally.
Ostrom (1961) proposes that the laterosphenoid area served as the attachment for the supplementary orbital muscle—the levator bulbi muscle(Fig. 6). The canal itself undoubtedly served as the orbital branch (deep ophthalmic branch) of the trigeminal nerve, along with which the superior orbital artery and lateral head vein or its orbital branch (ophthalmic vein) possibly passed, as in modern reptiles (Romer, 1956; Goodrich, 1958).
The laterosphenoid contact with the postorbital does not look like the usual suture, but like the articular fossa on the postorbital into which the laterosphenoid enters with a rounded end. Such a connection between the laterosphenoid and postorbital has not been noted in the literature on hadrosaurs. Riabinin had nothing to say about it. However, a similar connection between the laterosphenoid and postorbital was rather widespread mainly among the hadrosaurs. It can be traced in the skulls of Saurolophus angustirostris and other species of Hadrosaurinae.
Predatory dinosaurs and crocodiles (including modern species) have such a connection and it is more pronounced in the long-snouted animals than in the short-snouted. Because the other sutures of the laterosphenoid and postorbital with the adjacent are serrated, making these bones immobile, it is impossible to assume mobility in the laterosphenoids. There is a gap between the articular processes of the laterosphenoid and the depression in the postorbital that must have been filled with cartilage during the animal’s lifetime.
According to L. I. Khozatskiy (in a verbal communication), who studied reptilian biomechanics, a similar structure might have been an amortizer, although it was not unique. The quadrates enter the deep articulation fossa of the squamosals by their head. The quadrates are in turn supported on the column-like areas of the supraoccipitals. As with the laterosphenoid there are gaps between the contacting bones which in modern crocodiles are filled with cartilage.
The likely purpose of these amortizers is to protect the braincase from the concussion that occurs when the mandibles are suddenly closed. The presence of three amortizers probably reduced the force of the concussion, distributing it in several directions. The analogy in the connection of the skull roof and the braincase of crocodiles and hadrosaurs gives occasion to presume that the latter may feed on not only vegetation, but on small water animals by rapidly (consequently, suddenly) closing the mouth.
The orbitosphenoids, or orbitocuneiform bones are bounded dorsally by the frontals, posteriorly by the laterosphenoids, ventrally by the parasphenoid, and anteriorly by the presphenoid. Riabinin (1939) mentioned in text that upwardly the parasphenoid borders with the orbitosphenoid, but the alisphenoid1 apparently corresponds to the orbitosphenoid in his drawings. Contrary to what is seen in most hadrosaurs, all of the orbitosphenoid sutures in J. aralensis are quite visible. The orbitosphenoid is oval shaped, the lower part of which is vertical and has openings for the exits of the cranial nerves. The upper, larger part turns sharply outward, forming the ventrolateral surface of the braincase and the area in which the forebrain hemisphere begins.
Several openings for the anterior cranial nerves are found on the orbitosphenoid. The interpretation of these openings is complicated by the fact that in modern reptiles the orbitosphenoids do not ossify. For this reason the literature contains many contradictory interpretations. There are three large openings on the lower part of the orbitosphenoid along the flexure line. Above the middle opening is another small opening that extends slightly horizontally.
The posterior opening, isolated form the opening for nerve VI beneath it by a septum, is the exit for nerve III. A larger opening that is somewhat expanded upward lies anterior and dorsal to the exit. This opening leads into the brain cavity and may be the exit for only the optic nerve and the small opening above it (at the most dorsal location of all the cranial nerves) is undoubtedly the exit of nerve IV. The anteriormost opening is located ahead of the exit for nerve II and does not lead to the cranial cavity, but by a narrow canal joins with an analogous opening in the oppositely located orbitosphenoid, significantly below the bottom of the cranial cavity.
This connection is a possible route for the venous anastomosis that connects the left and right orbital sinuses, as is seen in modern reptiles. It is possible that the ophthalmic artery exited along with nerve III. This artery is separated from the internal carotid artery immediately ahead of it by the entry into the cranial cavity from the hypophyseal artery. The joint exit of nerve III and the ophthalmic artery was seen in the skulls (Romer, 1956), whereas in lizards it exits along with nerve II. However, the position of the exit of nerve II significantly anterior to the exit of nerve III in Jaxartosaurus aralensis apparently excludes the possibility that the ophthalmic artery exited with nerve II.
Cranial fenestrae. The largest are the orbits, which are significantly larger than the infraand supratemporal fenestrae. The upper area of the orbits is unusually wide, almost four times that of the infratemporal fenestrae and roughly two and a half times as wide as the supratemporal fenestrae. The lower boundaries of the infratemporal fenestrae are unknown and we can only talk about their width. They are very narrow, almost rimose. In their upper region they are somewhat narrower than the articular fossa in the squamosal (for entry of the quadrate) and much narrower than the supratemporal fenestrae. Looking at the skull from the front, the latter are subrhombic along the upper contours and subtriangular along the lower contours.
Cranial crest. Riabinin (1939) suggested that Jaxartosaurus had a helmet-like crest like Hypacrosaurus, because of the connection by the anterior region of the parietals on which, in Riabinin’s opinion, the crest was supported. The judgment is sufficiently valid because the expanded suture surfaces of the frontals and prefrontals at their boundary with the premaxillae and nasals testify to the substantial weight and large area of the latter. The wide support areas are not intrinsic to the usual sutures in hadrosaur skulls and developed to support the heavy crest. In any event, similar suture surfaces in the prefrontal area were quite undeveloped in the flat-headed dinosaurs (Hadrosaurinae) which lacked the crest.
It is extremely difficult to evaluate the shape of the crest because the crest itself was not preserved. However, we take exception to Rjabinin, who thought this crest was the same type as is seen in Hypacrosaurus. In the latter, by raising the frontals to the front, the crest is not supported on the parietals. At the same time a possibly natural cleavage between the squamosals may indicate the contact of this region with the crest that overhangs the occiput. In this case the crest in J. aralensis was possibly more similar to that of Corythosaurus, but different in its greater thickness, judging from the support areas.
Measurements of Jaxartosaurus aralensis skull, Rjab., No. 1/5009 (centimeters)
Width in the orbital region 25.0 - Width, orbits 11.0
Width in the occipital region 20.0 - Width, infratemporal fenestra 3.0
Occiput height from the lower point of foramen magnum 10.0 - Width, supratemporal fenestra 4.5 - Length, supratemporal fenestra 6.5
Source: Polyglot Paleontologist
Riabinin, 1939
Time
Cretaceous Late Turonian Coniacian Santonian
Classification
Ornithischia Ornithopoda Hadrosauridae Lambeosaurinae Nomina Dubia
Diet
Herbivore
Fossilsite
Dabrazinskaya Svita, Alim Tau site, Syderinskaya Oblast, Kazakhstan
Fall Under
Jaxartosaurus
Info
Typespecies - Skull
Jaxartosaurus (Riabinin, 1937) > Jaxartosaurus aralensis (Riabinin, 1939) >> Procheneosaurus convincens (Rozhdestvensky, 1968)
Isolated skull roof and braincase.The holotype skull roof and braincase was described in great detail by Rozhdestvensky. A. K. (1968). The limited nature of the type material makes its taxonomic status dubious. Holotype PIN 5009/1
From: Rozhdestvenskiy, A. K. 1968. [In Russian]. Pp. 97-141 in: Tatarinov, L. P., et al. (eds.). [Upper Paleozoic and Mesozoic Amphibians and Reptiles]. Akademia Nauk S.S.S.R., Moscow.
Translated by W. Robert Welsh, copy provided by Kenneth Carpenter and converted by Matthew Carrano.
Genus Jaxartosaurus Riabinin, 1939
Type (unique) species:
Jaxartosaurus aralensis Rjabinin, 1939; Upper Cretaceous (Coniacian–Santonian) of southern Kazakhstan.
Diagnosis
Medium-sized hadrosaur. The skull is wide and low with a helmet-like crest in which the frontals and prefrontals do not participate. The frontals are large and inflated and they are isolated from the edge of the orbit. The supratemporal fenestrae are small, and the supratemporal arches are straight and parallel to the sagittal line of the skull. Comparison. Besides Jaxartosaurus, the subfamily Lambeosaurinae numbers eight genera1 at present (Rozhdestvensky, 1964b). Of these Jaxartosaurus is apparently the farthest removed from the [cheneosaurs] (Procheneosaurus, Cheneosaurus, and Nipponosaurus), having a narrow but higher skull in the occipital region and an underdeveloped crest that looks like a small rise in the prefrontal area of the skull. On the other hand, Jaxartosaurus is no less removed from Parasaurolophus, which has a huge long crest that extends far beyond the occipital edge of the skull and is formed from below by the nasals that are displaced from the roof of the skull, and the frontals that make up the upper edge of the orbit and rise upward from it.
Jaxartosaurus is more similar to the other four genera Lambeosaurus, Corythosaurus, Hypacrosaurus, and Tanius but is notably different. Thus, the frontals and prefrontals of Lambeosaurus participate with the nasals and premaxillae to form the crest, whereas in Jaxartosaurus the crest may be formed by only the premaxillae and nasals. Corythosaurus and Hypacrosaurus are the most similar to Jaxartosaurus.
Corythosaurus is characterized by a skull crest that is more circular and laterally compressed. In some species this crest overhangs the occiput, making contact with the squamosal. Hypacrosaurus (with one species) has a dome-shaped crest that is less laterally compressed, does not overhang the occiput, and does not make contact with the squamosal. Judging by the support areas on the parietal and squamosal, the crest in Jaxartosaurus is probably most similar in shape to that of Corythosaurus, differing only by the slight lateral compression as in Hypacrosaurus and thereby possibly occupying an intermediate position between the crests of these two lambeosaurs. Jaxartosaurus differs from both of these by its rather large frontals, the exterior surface of which reaches the anterior third of the orbit.
According to the shape of the bones in the roof of the skull (especially the frontals), Jaxartosaurus is very similar to the genus Tanius, also known as Tsintaosaurus Young, 1958 (Rozhdestvensky, 1964b), but differs from it in the subquadrate supratemporal fenestrae2 and more massive skull crest (judging by its support area). The latter was destroyed for the most part in the specimens examined by Young (1958), who reckoned this crest to be similar to that of the Saurolophus and assigned Tsintaosaurus to the subfamily [Saurolophinae]. Finally, Jaxartosaurus differs from all four of the genera similar to it by its straight supratemporal arches that are parallel to the sagittal line of the skull.
Jaxartosaurus aralensis Rjabinin, 1939 = (Bactrosaurus prynadai Rjabinin, 1939)
[Tanius sinensis (Wiman, 1929) > Tanius prynadai (Riabinin, 1939) >> Bactrosaurus prynadai (Riabinin, 1939)]
Lectotype3. The Museum of the Central Science and Research Institute for Geological Exploration (Leningrad), No. 1/5009: incomplete skull with no maxillofacial region; Upper Cretaceous (Coniacian–Santonian) of southern Kazakhstan (Kyrk-Kuduk region near the Alym-Tau range).
Diagnosis
The skull narrows drastically behind the orbits and has a short, underdeveloped sagittal crest that is formed by the temporals. The frontals are wedged between the nasals and removed from the edge of the orbit, being isolated from it by the broad band of the prefrontals and the postorbitals. The parietals enter between the frontals with the process that expands anteriorly. The skull is 20% wider in the postorbital region than in the occipital region. The width of the occiput (at the level of the paroccipital process) is greater than its height from the lower point of the occipital foramen by 250%. The orbits are wide in the upper region, the infratemporal fenestrae are narrow and slit-like, and they are almost 25% as wide as the orbits. The supratemporal fenestrae are rhomboidal and more than 150% as long as they are wide. The lower jaw has 34–35 dental rows.
Material
Collection No. 5009—an incomplete skull, a fragment of another skull, and fragments of the postcranial skeleton, bones of several specimens that came from the same deposit and horizon as the holotype.
Skull Roof
Riabinin (1939) noted the character of the exterior sutures of the bones in the skull roof, stressing the surface profile of the frontals and temporals. Overall, his description of the bones was very brief and it can be augmented.
The premaxillae, maxillae, and nasals of skull No. 1/5009 were not preserved. However, judging by the anterior ends of the prefrontals and frontals that underlie them, we might think that the lower rami of the premaxillae are narrow, tapering bands that extended very far posteriorly, lying on the interior surface of the prefrontals and reaching their middle—at the joint with the frontals. The nasals occupy a medial position relative to the lower rami of the premaxillae and a lateral position relative to their upper rami (arranged along the medial line of the skull in hadrosaurs), and must have looked from behind like wide rounded ribbons, bending slightly to the outside, where they occupied a significant area of the frontals.
Between the posterior ends of the premaxillae and nasals, and at the contact point of the prefrontals and frontals that underlie them, are apparently the nasal fenestrae. We can make judgments about this from the natural depression at the boundary of the frontals and prefrontals and by the structure of the interior surface of the latter, which has the explicit nature of a cavity wall. The prefrontals form half of the upper edge of the orbits and their anterior wall. They extend in an anteroposterior direction. Medially the prefrontals have a slightly striated sutural surface for contact with the premaxillae. The posterior end of the prefrontal is wedged between the postorbital and frontal, with which the prefrontal is joined by coarsely serrated sutures.
The frontals are joined together by a coarsely serrated suture and form the roof of the braincase near the forebrain, where they are noticeably swollen. The frontal is separated from the upper edge of the orbit by a wide band from the prefrontal and postorbital, but forms the upper region of the orbits. Distinct, coarsely serrated sutures separate the frontals from the adjacent bones and their external contours are reminiscent of maple leaves. Making contact with the nasals is a broad sutural surface, the medial extent of the frontals make contact with the premaxillae. Medially the frontals penetrate rather far between the nasals and laterally are slightly wedged between the prefrontals and postorbitals. From behind they form a complex suture with the parietals, which in the middle penetrate far between the frontals.
The parietals form the roof of the braincase above the mid- and hindbrain area and are at the same time the upper region of the medial walls of the supratemporal. By fusing together without an apparent suture, the parietals are wedged deeply between the frontals that expand ahead of a cuneus that has a depression on the surface. From behind the parietals form a short, but very typical sagittal crest. It is unclear as to how the parietals emerge onto the occipital surface of the skull—by separating the squamosals and lying on the supraoccipital bone: the medial ends of the squamosals that overlap the parietals were not fully preserved. Even less clear is the lower boundary of the parietals. The sutures are barely visible and we can only think it more or less probable that this boundary was horizontal and that the parietals made contact with the laterosphenoid and preotic bones from anterior to posterior, and possibly with the anterior ends of the supraoccipital bone, if we can agree that the latter emerges in the posterior node of the interior wall of the supratemporal fenestrae.
The postorbitals form the posterior half of the upper edge of the orbit and the posterior part of its upper wall, coming together anteriorly with the prefrontals and excluding from the orbital ring the frontals, with which the postorbitals contact the interior lateral side. At the front of the postorbital, directly behind the orbit forming its posterior edge, a process emerged downward—in the direction of the ascending process of the jugals. The second descending, but small, process emerges from the posterior part of the bone and, by lying on a similar process of the squamosal, separates the infratemporal fenestra from the articulation socket for the upper end of the quadrate bone. From behind the postorbital, by merging with the squamosal by means of two large teeth, forms with this bone the upper temporal arch. Beneath, the postorbitals on the entire extent of the lower temporal arch, which the squamosals underlie.
The squamosals are a complex configuration and participate in forming the posterolateral corners of the skull roof. Their anterior growths, like two large teeth, dorsally penetrate the postorbitals and ventrally form the lower surface of the temporal arch, reaching the anterior corner of the supratemporal fenestrae. Two rather long process that bound the socket for entering the quadrate bone emerge downward from the ventrolateral surface of the squamosal. The hindmost of these processes, by lying on the postotic and exoccipital bones, participate slightly with the paroccipital process to form them. The posterior boundaries of the squamosal that exit onto the occipital surface of the skull have the nature of an undulating line that is defined by contact with the supraoccipital and exoccipital bones.The bones of the zygomatic region and palatal surface are unknown.
Braincase
This area of the skull was completely preserved, and most of the sutures between the bones are well differentiated. The occipital area of the braincase includes unpaired supra- and basioccipital bones as well as paired exoccipital bones that fuse together. The otic region includes two bone pairs—the prootic and opisthotic—and is characterized by the least explicit ends of the bones. The base of the skull has been formed by the well-ossified basisphenoid and parasphenoid, the boundaries of which are usually poorly distinguished in most hadrosaurs because of bone fusion. The anterior region of the braincase was formed by a complex of sphenoid elements—an unpaired presphenoid and unpaired latero- and orbitosphenoids, well ossified and distinctly delimited.
The location of the supraoccipital is unclear in most hadrosaurs. Riabinin (1939) makes reference to its being located above the occipital foramen, which indicates relatively low height and the presence of a lengthwise crest on the bone, but does not present precise data about its limits and shape. Rjabinin’s data can be augmented. The supraoccipital contacts the exoccipital from below and from the sides, and contacts the squamosals and parietals from above. The lateral sutures with the exoccipitals are finely serrated. The supraoccipital is isolated from the foramen magnum by a rather wide band of exoccipitals that fuse together. Its boundary with these is distinct and almost horizontal, slightly depressed above the center of the occipital foramen. On the occipital surface of the skull, the supraoccipital has a long and narrow, but massive posteriorly sloped plate. The lateral flanges are pulled into the sides far beyond the limits of the occipital condyle.
A low, smooth medial crest runs along the central region of the supraoccipital. Posteriorly this crest transforms into a round enlargement that hangs slightly above the exoccipitals and the center of the foramen magnum. There are two enlargements that look like low columns in the upper part of the supraoccipital along the sides from the medial crest. These enlargements enter the corresponding depressions in the squamosal. There are openings in the sides of these columns that are not revealed by dissection. They probably opened into the supratemporal fenestrae and may have served as passageways for blood vessels, especially the dorsal head vein.
It is unclear whether these openings are homologous with the post-temporal fenestrae that are typical of many reptiles. The anterior boundaries of the supraoccipital are less certain because the parietals and squamosals overlap them for a considerable extent. We may only assume that the sloping serrated sutures running along the interior surface of the supratemporal fenestrae from the posterior lower node on a diagonal to the center.
The exoccipitals fuse together with no explicit suture between them and embrace the foramen magnum for almost their extent. Riabinin (1939) characterizes the exoccipitals as bones having “unspecified boundaries”; considering the lateral flanges that take part in forming the occipital surface of the paroccipital processes to be postotic bones. There are, however, no sutures on the occipital surface that might be seen as the boundary between the postotic and exoccipitals. As usual, this boundary is hard to see from the outside because of bone fusion. For this reason the question of the extent to which these bones participate in forming the paroccipital processes is debatable.
The visible boundaries of the this boundary on the occipital surface of the skull run: in the center from the supraoccipital, and dorsolaterally from the squamosal. The lower contact—from the basioccipital—was not preserved because this part of the skull was destroyed. The base area of the exoccipitals in the posterior surface of the uncinate paroccipital processes are fairly high proximally but then quickly narrow and drop somewhat below the foramen magnum.
A massive rod-shaped crest, beneath which the surface of the exoccipitals is concave, had developed on the center of the exoccipitals in parallel with the boundary with the supraoccipital. A short, small swelling at the point where the exoccipitals probably fused emerges from the center of the crest downward to the foramen magnum. Each exoccipital forms at a least a third of the occipital condyle, the lower part of which had been destroyed.
With regard to how the exoccipitals participate in forming the lateral surface of the braincase, Ostrom (1961) apparently relates to this negatively, bounding them in his drawings to the occipital surface, whereas most other authors (Romer, 1956; Goodrich, 1958; Langston, 1960) show the exits of the exoccipitals on the lateral wall of the skull where they include openings for nerves IX–XII as in contemporary reptiles.
The boundaries for the exoccipitals were not seen on the lateral surface of the Jaxartosaurus aralensis skull. Bearing in mind the usual location of nerves IX–XII within the occipital, presumably beyond the anterior boundary with the opisthotic, we can raise the crest-overhang that passes obliquely posteriorly from the base of the skull to the paroccipital process, between the fenestra ovalis and the opening for nerves IX–XI. This opening opens outward immediately below this crest. The opening behind is the exit for nerve XII.
Langston (1960) shows several (at least three) exits for nerve XII in Lophorhothon atopus but there are only two openings behind the fenestra ovalis on the lateral wall of the J. aralensis braincase. Therefore, the anterior of these may have been interpreted merely as a common opening for nerves IX–XI, which agrees with the observations of Ostrom (1961), who was studying casts made of the endocranial cavity of various hadrosaurs. The opening for IX–XI splits in two1—the anterior is fairly large and the posterior is small.
From their sizes we may think that nerve X must have passed through the anterior, and nerve XI through the posterior. Nerve IX apparently exited with nerve X, as casts of the brain cavity from various hadrosaurs indicate (Ostrom, 1961), although Langston (1960) linked the IX nerve exit with the otic capsule in Lophorhothon atopus. The opening in J. aralensis, being interpreted here as common for nerves IX and X, actually has a canal into the otic capsule, but it is more likely that this connecting canal was designated not for nerve IX, but for the jugular vein as Ostrom (1961) assumes because this passage is very small for a major vein in the head and was more likely for the perilymphatic duct.
As to jugular vein, it is likely that it exited the skull area through the cranioquadrate rather than the base of the skull as Ostrom (1961, pg. 138, fig. 5) shows, because in general venous blood was exported through the foramen magnum as in archosaurs (crocodiles) and birds. The basioccipital that forms the base of the occipital condyle was almost completely destroyed, a fact that Riabinin did not note.
It is difficult to make judgments about the boundaries 1 All of these details were revealed after the skull openings were treated with a solution of hydrochloric acid. of this bone. Starting from the area occupied by the exoccipitals, we might think of the basioccipital as a narrow band that formed the ventral region of the occipital condyle, and that the occipital foramen was between the tubera of the exoccipital.
The opisthotics form the lateral walls of the posterior region of the braincase near the medulla oblongata. In most hadrosaurs the opisthotics merge with the neighboring bones without any visible sutures. In J. aralensis the boundaries of the opisthotic are seen as a distinct suture for the lower anterior crest of the bone that is bounded with the basisphenoid. Because of the merging of the bones this contact is usually unknown in hadrosaurs, and a position that clearly belongs to the basisphenoid sometimes rises beyond the surface of the anterior otic bone (Ostrom, 1961).
The boundary suture in J. aralensis in the form of a fine, slightly serrated line descends from the region of the fenestra ovalis located in the anterior node of the opisthotic to the base of the braincase. The boundary between the opisthotic and prootic is seen as a faint crest-like suture directed from the fenestra ovalis dorsally and posteriorly to the joint with the horizontal overhang. The upper boundary of the opisthotic is unclear, but the horizontal overhang above the fenestra ovalis may possibly be the front of it.
Even less clear is the boundary between the postotic and exoccipitals; in this capacity we may conditionally adopt the crest that passes above the opening for nerves IX–XI. The fenestra ovalis, or vestibular region of the otic capsule, is located at the junction of three bones—the preotic, postotic, and basisphenoid—and looks like a subrhomboid-shaped depression split into two halves by a septum. The upper half probably corresponds to the fenestra ovalis, and the lower half includes the fenestra rotunda.
Three openings are visible at the base of the lower half; the anterior opening was possibly the exit for hyomandibular ramus of nerve VII, as it is treated in some hadrosaur species (Langston, 1960; Ostrom, 1961). According to Goodrich (1958), the middle cerebral vein might have passed through this opening, discharging in the otic capsule area into the lateral head vein. As has been mentioned, the posterior opening originates a canal that communicates with the canal for nerves IX and X and probably merges with the perilymphatic duct.
The prootics form the anterior region of the otic capsules, separating drastically into upper and lower parts by a transverse horizontally hanging crest. Riabinin (1939) and then Ostrom (1961) note only the boundary between the prootics and postotics. However, in J. aralensis even the upper boundary of the prootic with the temporal may have followed a long horizontal suture that separates the upper part of the temporal fenestra from the lower.
The anterior boundary is a highly visible vertical suture with the laterosphenoid. The posterior boundary passes through (or at) the origin of the fenestra ovalis, dorsal from which the prootic contacts the postotic and ventrally contacts the basisphenoid into which the prootic is wedged by a shallow “pocket” below the exit opening for nerve V (trigeminal foramen), which lies almost entirely within the prootic.
The prootic is slightly concave at its center, forming the anterolateral wall of the otic capsule. The horizontal overhang mentioned above makes a slight bend in this region. Above the horizontal overhang, the upper portion of the prootic occupies the posteroventral position on the inner surface of the supratemporal fenestra, and is subtriangular in shape. Here the most acute apex of the bone is rotated posteriorly.
The lower portion of the prootic, lower than the horizontal overhang, has an irregular shape. The location of the opening for the nerve V exit in this region looks like a deep (due to the thick-walled prootic) and circular funnel. The boundary of the prootic with the laterosphenoid passes through its anterior wall. Within the prootic is a semicircular canal that is open from above, the anterior wall of which is the anterior boundary of the prootic and is denoted by a distinct suture. The canal is directed ventrally from the trigeminal foramen and apparently closed on the trigeminal nerve (n. trigeminus II), separating further on into maxillary and mandibular branches. It is possible that the inferior orbital artery, being a branch of the stapedial artery, also passed through this canal, as is seen in contemporary reptiles (Romer, 1956; Goodrich, 1958).
Posterior and ventral to this canal is a depression that originates immediately behind the trigeminal foramen and crosses ventrally over onto the lateral surface of the basisphenoid. The pronounced profile provides a basis to presume that this depression has a special purpose, e.g., for the mandibular artery, being the lower branch of the otic artery. True, this structure is seen in lizards, whereas in crocodiles the mandibular artery departs from the ventral aorta (Goodrich, 1958) rather than from the otic artery. Therefore, the interpretation proposed for J. aralensis may be debatable. Below the trigeminal foramen, at the boundary of the prootic and basisphenoid, is another small opening that is probably the exit for nerve VII—more accurately, for the palatine branch.
The basisphenoid is very massive and tall, and is the axial region of the skull between the exits for nerves VIII and VI. Rjabinin did not describe these bones. The posterior and upper boundaries of the basisphenoid diverge at an acute angle: the first ventrally, making contact with the opisthotic, and the second more or less horizontally, descending below the trigeminal foramen where the basisphenoid contacts with the prootic and further on bending the protruding area of the laterosphenoid in order to reach the exit opening for nerve VI. From here the boundary of the basisphenoid, which contacts the parasphenoid as a faint suture, descends ventrally.
The boundary of the basisphenoid with the basioccipital, almost totally lost, remains unknown. The general shape of the basisphenoid, looking from the side, resembles an irregular quadrangle, the centro-posterior angle of which has descended significantly below the anterior. The lateral surfaces of the basisphenoid in the center ahead of the cuneiform, or pterygoid processes1, carry from each side at least two pairs of small mammiform processes that had been directed ventrally.
The entry into the carotid canal (which could not be opened) for the internal carotid artery, initially directed into the hypophyseal cavity and from there into the brain cavity, is at the same level as the posterior of these processes. It is possible that the palatine branch of nerve VII entered the carotid canal together with the internal carotid artery in order to exit through the noted opening at the boundary of the basisphenoid and parasphenoid along with the palatine artery departing from the internal carotid artery, as in modern reptiles (Romer, 1956).
The parasphenoid continues ahead of the basisphenoid, with which it merges with no visible suture in most hadrosaurs. This boundary is unclear in J. aralensis. It most likely ran from the opening for nerve VI ventrally, somewhat ahead of the mammiform processes of the basisphenoid. In this case the parasphenoid looks like a small, thin, flat, subquadrate bone.
The ventral process is directed anteriorly and was not completely preserved. Riabinin mentions the parasphenoid as merely a fragment. It is possible that he had a somewhat different picture of the contours of these bones. More distinct are the contacts between the parasphenoid and the orbitosphenoids that bound it from above, and between the presphenoid that abuts it anteriorly and dorsally and immediately above the anterior process. Along with the bones that have been mentioned, the parasphenoid participates in forming the interorbital septum.
The suture between parasphenoid and orbitosphenoid originates from two openings, the large (lower)2 of these which was probably the exit for nerve VI and possibly for the hypophyseal vein; the smaller (upper) for the exit of nerve III. Ostrom (1961) shows an exit opening for nerves III and VI in Corythosaurus casuarius, which can be explained by the conditions under which it was preserved and prepared. The boundary runs farther horizontally, immediately above two rather large openings in the base of the orbitosphenoid.
The presphenoideum,3 only the posterior region of which was preserved, is bounded posteriorly and ventrally by the parasphenoid and the orbitosphenoids, from which it is separated by a distinct suture, and of which it is the anterior extension. Dorsally the presphenoid abuts the frontal bone and splits into two rami that together with the orbitosphenoids form the anteriormost region of the braincase—near the highly developed (we can judge this from the exit opening from the braincase) olfactory lobes of the forebrain.
The laterosphenoids that merge with adjacent elements without sutures in most hadrosaurs are very distinctly delimited in J. aralensis. The laterosphenoids form the lateral walls of the braincase near the midbrain and the posterior expansion of the large hemispheres. From behind the laterosphenoid borders with the prootic: dorsally it borders with the parietal posteriorly and with the postorbital anteriorly. The anterior boundary of the laterosphenoid does not pass along the crest that separates the orbit from the supratemporal fenestra as Ostrom (1961) showed in Corythosaurus casuarius.
By participating in forming the orbit, the small flange of the laterosphenoid wedges between the frontal bone and the V and VI nerve openings and extends to make contact with the basisphenoid. Because of the horizontal canal, the aforementioned area seems to be separated from the base area of the bone. In general the laterosphenoid looks like an extended rectangle that bends outward in the anterior region (near the forebrain) with a process that emerges ventrally.
Ostrom (1961) proposes that the laterosphenoid area served as the attachment for the supplementary orbital muscle—the levator bulbi muscle(Fig. 6). The canal itself undoubtedly served as the orbital branch (deep ophthalmic branch) of the trigeminal nerve, along with which the superior orbital artery and lateral head vein or its orbital branch (ophthalmic vein) possibly passed, as in modern reptiles (Romer, 1956; Goodrich, 1958).
The laterosphenoid contact with the postorbital does not look like the usual suture, but like the articular fossa on the postorbital into which the laterosphenoid enters with a rounded end. Such a connection between the laterosphenoid and postorbital has not been noted in the literature on hadrosaurs. Riabinin had nothing to say about it. However, a similar connection between the laterosphenoid and postorbital was rather widespread mainly among the hadrosaurs. It can be traced in the skulls of Saurolophus angustirostris and other species of Hadrosaurinae.
Predatory dinosaurs and crocodiles (including modern species) have such a connection and it is more pronounced in the long-snouted animals than in the short-snouted. Because the other sutures of the laterosphenoid and postorbital with the adjacent are serrated, making these bones immobile, it is impossible to assume mobility in the laterosphenoids. There is a gap between the articular processes of the laterosphenoid and the depression in the postorbital that must have been filled with cartilage during the animal’s lifetime.
According to L. I. Khozatskiy (in a verbal communication), who studied reptilian biomechanics, a similar structure might have been an amortizer, although it was not unique. The quadrates enter the deep articulation fossa of the squamosals by their head. The quadrates are in turn supported on the column-like areas of the supraoccipitals. As with the laterosphenoid there are gaps between the contacting bones which in modern crocodiles are filled with cartilage.
The likely purpose of these amortizers is to protect the braincase from the concussion that occurs when the mandibles are suddenly closed. The presence of three amortizers probably reduced the force of the concussion, distributing it in several directions. The analogy in the connection of the skull roof and the braincase of crocodiles and hadrosaurs gives occasion to presume that the latter may feed on not only vegetation, but on small water animals by rapidly (consequently, suddenly) closing the mouth.
The orbitosphenoids, or orbitocuneiform bones are bounded dorsally by the frontals, posteriorly by the laterosphenoids, ventrally by the parasphenoid, and anteriorly by the presphenoid. Riabinin (1939) mentioned in text that upwardly the parasphenoid borders with the orbitosphenoid, but the alisphenoid1 apparently corresponds to the orbitosphenoid in his drawings. Contrary to what is seen in most hadrosaurs, all of the orbitosphenoid sutures in J. aralensis are quite visible. The orbitosphenoid is oval shaped, the lower part of which is vertical and has openings for the exits of the cranial nerves. The upper, larger part turns sharply outward, forming the ventrolateral surface of the braincase and the area in which the forebrain hemisphere begins.
Several openings for the anterior cranial nerves are found on the orbitosphenoid. The interpretation of these openings is complicated by the fact that in modern reptiles the orbitosphenoids do not ossify. For this reason the literature contains many contradictory interpretations. There are three large openings on the lower part of the orbitosphenoid along the flexure line. Above the middle opening is another small opening that extends slightly horizontally.
The posterior opening, isolated form the opening for nerve VI beneath it by a septum, is the exit for nerve III. A larger opening that is somewhat expanded upward lies anterior and dorsal to the exit. This opening leads into the brain cavity and may be the exit for only the optic nerve and the small opening above it (at the most dorsal location of all the cranial nerves) is undoubtedly the exit of nerve IV. The anteriormost opening is located ahead of the exit for nerve II and does not lead to the cranial cavity, but by a narrow canal joins with an analogous opening in the oppositely located orbitosphenoid, significantly below the bottom of the cranial cavity.
This connection is a possible route for the venous anastomosis that connects the left and right orbital sinuses, as is seen in modern reptiles. It is possible that the ophthalmic artery exited along with nerve III. This artery is separated from the internal carotid artery immediately ahead of it by the entry into the cranial cavity from the hypophyseal artery. The joint exit of nerve III and the ophthalmic artery was seen in the skulls (Romer, 1956), whereas in lizards it exits along with nerve II. However, the position of the exit of nerve II significantly anterior to the exit of nerve III in Jaxartosaurus aralensis apparently excludes the possibility that the ophthalmic artery exited with nerve II.
Cranial fenestrae. The largest are the orbits, which are significantly larger than the infraand supratemporal fenestrae. The upper area of the orbits is unusually wide, almost four times that of the infratemporal fenestrae and roughly two and a half times as wide as the supratemporal fenestrae. The lower boundaries of the infratemporal fenestrae are unknown and we can only talk about their width. They are very narrow, almost rimose. In their upper region they are somewhat narrower than the articular fossa in the squamosal (for entry of the quadrate) and much narrower than the supratemporal fenestrae. Looking at the skull from the front, the latter are subrhombic along the upper contours and subtriangular along the lower contours.
Cranial crest. Riabinin (1939) suggested that Jaxartosaurus had a helmet-like crest like Hypacrosaurus, because of the connection by the anterior region of the parietals on which, in Riabinin’s opinion, the crest was supported. The judgment is sufficiently valid because the expanded suture surfaces of the frontals and prefrontals at their boundary with the premaxillae and nasals testify to the substantial weight and large area of the latter. The wide support areas are not intrinsic to the usual sutures in hadrosaur skulls and developed to support the heavy crest. In any event, similar suture surfaces in the prefrontal area were quite undeveloped in the flat-headed dinosaurs (Hadrosaurinae) which lacked the crest.
It is extremely difficult to evaluate the shape of the crest because the crest itself was not preserved. However, we take exception to Rjabinin, who thought this crest was the same type as is seen in Hypacrosaurus. In the latter, by raising the frontals to the front, the crest is not supported on the parietals. At the same time a possibly natural cleavage between the squamosals may indicate the contact of this region with the crest that overhangs the occiput. In this case the crest in J. aralensis was possibly more similar to that of Corythosaurus, but different in its greater thickness, judging from the support areas.
Measurements of Jaxartosaurus aralensis skull, Rjab., No. 1/5009 (centimeters)
Width in the orbital region 25.0 - Width, orbits 11.0
Width in the occipital region 20.0 - Width, infratemporal fenestra 3.0
Occiput height from the lower point of foramen magnum 10.0 - Width, supratemporal fenestra 4.5 - Length, supratemporal fenestra 6.5
Source: Polyglot Paleontologist