[D] Procheneosaurus convincens [Su]
Describer
Rozhdestvensky, 1968
Time
Cretaceous Late Turonian Coniacian Santonian
Classification
Ornithischia Ornithopoda Hadrosauridae Lambeosaurinae Incertae Sedis
Diet
Herbivore
Fossilsite
Dabrazinskaya Svita, Syuk-Syuk wells site, Sydarninskaya Oblast, Kazakhstan
Fall Under
Jaxartosaurus
Info
Skull
Jaxartosaurus (Riabinin, 1937) > Jaxartosaurus aralensis (Riabinin, 1939) >> Procheneosaurus convincens (Rozhdestvensky, 1968)
Nearly complete skeleton lacking only the front of the skull and distal region of the tail. Found in the same region (but in a different strartigraphic region) as Jaxartosaurus.
P. convincens represents a juvenile lambeosaurine hadrosaur.
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.
Procheneosaurus praeceps Matthew, 1920 (= Didanodon altidens Osborn, 1902; = Tetragonosaurus Parks, 1931)
[Lambeosaurus (Parks, 1923) = Didanodon altidens (Osborn, 1902)]
Type species
Tetragonosaurus praeceps Parks, 1931; Belly River Formation, Alberta (Canada).
Diagnosis
Comparatively small (up to 5 m long) crested hadrosaur. The skull is short and tall, with a lower crest that was formed by the premaxillae and nasals. The upper branch of the premaxilla is separated from the lower branch by means of the open nasal fenestra. The nasals are fairly small and outline the nasal fenestrae posteriorly and partially dorsally. The lacrimals are triangular or trapezoidal, ranging in size from moderate to large.
The infratemporal fenestrae range from being moderately wide to narrow, with an increase in its length-to-width ratio of as much as three. There are 40 tooth rows1 in the upper jaw and 33 in the lower. There are 30 presacral vertebrae, of which 13 or 14 are cervical, 16–17 are dorsal, 10 are sacral, and as many as 57 are caudal. Along the last dorsal, sacral, and anterior caudal vertebrae are ossifying tendons. The scapula is slightly curved, with a slightly developed coracoid region; the overall length of the scapula is roughly one and a half times that of the humerus.
The radius is slightly longer than the humerus. The ilium is low, with a long, narrow, anterior lobe (process); overall, the bone is roughly four times as long as it is high. The antitrochanter is asymmetrical with a longer and steeper anterior edge. The ischium has a thin trunk; its iliac and pubic processes are almost the same size and the distal end is moderately widened. The pubis has a short prepubis and a short postpubis that makes up approximately one third the length of the ischium. The knee and talocrural joints are notably extended anteroposteriorly.
The femur is slender, less than twice as long as the humerus and only two and a half times larger than metatarsal III. The tibia is roughly the same length as the femur, or only slightly smaller; the medial condyle of the upper epiphysis is significantly wider than the lateral condyle.
Species composition
Four species: Procheneosaurus convincens sp. nov.; Procheneosaurus praeceps (Parks), 1931; [P. erectofrons] (Parks), 1931; [P. cranibrevis] (Sternberg), 1935.
Distribution
Central Asia (Southern Kazakhstan) and North America (Alberta, Canada and Montana, USA).
Geological Age
Santonian–Campanian (Dabrazinskaya Svita, Belly River Formation, and Two Medicine Formation).
Procheneosaurus convincens sp. nov.
Holotype
PI No. 2230; almost complete skeleton excluding the anterior region of the skull, distal regions of the forelimbs and left hind limb, and last caudal vertebrae; Senonian (Dabrazin Formation - Dabrazinskaya Svita) Southern Kazakhstan, [Shakh-Shakh Formation], 45 km north of Tashkent.
Diagnosis
The skull crest is low and short, its highest point is slightly in front of the orbit. The lacrimal is trapezoidal in shape and fairly large and wide, making up more than half the width of the orbit. The frontal swelling is much closer to the occipital edge than to the highest point on the skull crest. The quadratojugal completely isolates the jugal from the quadrate.
The infratemporal fenestra is narrow, almost two and a half times narrower than the orbit. The neural processes of the dorsal vertebrae are moderately high, whereas for the anterior caudal vertebrae they are long, more than three times the height of the vertebral centrum. The scapula is moderately long, roughly seven times its minimal width. The anterior lobe of the ischium is more than twice as long as the posterior lobe, and the base of the bone has a well-developed pubic process that is almost one and a half times its height.
he length of the anterior process of the pubis is less than twice its maximum width. The minimum diameter of the ischial process is one seventh the height of its proximal region. The distal condyles of the femur, extended anteroposteriorly, are almost one third as long as the entire bone and much less than twice its width. Of the proximal condyles of the tibia the medial is more than twice as wide as the lateral.
The skull, just as with cheneosaurs, is tall and narrow, its height in the orbit area being almost twice its width. Toward the occiput, which is just as tall and comparatively narrow, the skull narrows even more. The anterior region of the skull was destroyed, but judging by the posterior half, we may think that the crest, formed by the premaxillae and nasals, was not tall and slightly elevated above the dorsal surface of the skull, achieving its greatest length somewhat in front of the orbit. It is difficult to judge the shape of the nasal fenestrae because they are known only by their posteriormost region, located at roughly the same level as the upper edge of the orbit.
We may assume that the nasal fenestrae of P. convincens must have been similar to those of the low-crested species of procheneosaurs in its features. The orbits are oval, slightly narrowing ventrally; their length is roughly one and a half times their maximum width. They are oriented at somewhat of an angle, thanks to which the infratemporal fenestrae are isolated from them ventrally by means of an overlapping bar as if they run below the orbits. The infratemporal fenestrae look like long, narrow ellipses, extended slightly ventrally and oriented at an angle, like the orbits. Theinfratemporal fenestrae are three or four times as long they are wide. The supratemporal fenestrae are small, more than twice as short as the infratemporal fenestrae, rhomboid in shape and extended slightly along the medial line of the skull.
Skull Roof
The premaxillae are known only by their lower branches in their posteriormost region where they bound the nasal fenestrae cavity almost vertically. At the upper end, on the same level as the upper edge of the nasal fenestrae, the lower branches of the premaxillae contact the nasals which bound the nasal fenestrae dorsally by means of a short, straight suture. Posteriorly the lower branches of the premaxillae lie on the lower end of the frontals and lacrimals to form a slightly broken line. In those areas that were preserved the premaxillae are almost the same width as the lower end of the frontals and the upper end of the lacrimals, but taper slightly at the apex. The nasal fenestrae look like deep, fairly wide cavities that almost match the width of the skull in the nasal area and are overlaid laterally by the lower branches of the premaxillae. The nasal fenestrae reached the posterior edge of the latter and connected to an outwardly closed cavity, looking like a small rounded pocket within the nasals, where the latter form a ridge.
Excluding the anterior region, the nasals were almost completely preserved to outline the nasal fenestrae dorsally. Above the posterodorsal edge of the nasal fenestrae, the nasals are a dome-shaped swelling that forms a small, low crest (tubercle) within which is a cavity that connects with the nasal fenestrae. According to the shape and location of the crest, one of the most essential features of the cheneosaurs, P. convincens is similar to Procheneosaurus praeceps. The nasals are widest near the crest, making contact with the lower branch of the premaxillae and anterior region of the prefrontals which lay on the nasals along a slightly undulating line.
Behind the crest-like swelling they narrow radically and look like slender, slightly posteriorly sloped wafers that lie between the prefrontals, but terminate prior to the latter. In the posterior region the nasals again widen slightly and make contact along a slightly serrated suture with the frontals, wedging slightly between them. The prefrontals are wide in the area where they overlay the nasals and form the anterodorsal edge of the orbit. The prefrontals abut the lower edge of the premaxillae anteroventrally and include the apex of the lacrimals ventrally.
The posterior boundary with the postorbitals looks like a slanted, non-uniform suture that runs from the highest point of the orbit’s upper edge in the direction of the anterolateral corner of the supratemporal fenestrae. The second posterior suture between the frontals and prefrontals is directed almost symmetrically to the first as a somewhat non-uniform line from the juncture of the postorbitals and frontals to the posterolateral nodes of the nasals.
The boundaries of the medial surface of the prefrontals are indistinguishable. The frontals are bounded anteriorly by the nasals and posteriorly by the parietals which are slightly wedged between them and isolated from the orbits by the wide band of the prefrontals and postorbitals. Along their periphery the frontals are slightly concave, and medially they form a substantial, round swelling that only slightly exceeds the nasals in size and serves as the roof for the braincase near the large hemispheres.
The parietals form the medial walls of the supratemporal fenestrae and are very short. Excepting the anterior boundaries with the frontals, it is only with some difficulty that we note the posterior boundaries where the squamosals form the sagittal ridge that rises above the parietals. The ventral boundaries of the parietals were quite indistinguishable.
Anteriorly the postorbitals look like a wedge on the dorsal surface of the skull, on the lateral side of which the upper posterior edge of the orbits is formed, and whose medial side bounds the prefrontals and frontals. The posterior portion of the postorbitals wedges into the squamosals by means of two tooth-like processes—the large process is the lower and the small process is the upper.
The lower process is above the middle head of the quadrate and the upper extends to the center of the lateral edge of the supratemporal fenestra. Laterally, at a slight angle to the dorsal surface, the postorbitals yield a thin, but fairly long descending process that descends below the center of the orbit, isolating it and the ascending process of the jugal from the infratemporal fenestra. The boundaries of the postorbitals on the lateral surface are not clear.
The squamosals, which bound the supratemporal fenestra from the outside and behind, have a complex shape as is seen in other hadrosaurs. The postorbitals overlie the squamosals anteriorly, and together with them form the supratemporal arches while one anterior process of the squamosal underlies the postorbital ventromedially, extending to the anterolateral node of the supratemporal fenestra—and the second descends vertically downward, isolating the postorbital and quadrate and bounding the quadrate anteriorly. This second process of the squamosal is parallel to the ventral process of the postorbital, but is roughly twice as short and is far from the jugal and quadratojugal.
The posterior surface of the process bounds the articular fossa in the squamosal for the quadrate, behind which is yet another descending process of the squamosal. This process is longer and includes the quadrate posteriorly, overlaying the paroccipital process. Opposite this process the squamosals turn medially at a right angle, merging medially as the sagittal crest and forming the occipital surface of the skull. At the point of a very short contact between the squamosals and occipitals are corresponding half-moon shaped depressions in the squamosals for the exoccipitals to enter, and the suture surfaces of the squamosals form an overhang above the supraoccipital.
Occipital Surface
The supraoccipital, which underlies almost the extent of the squamosal cross-section, has a small, but very wide, rise, more than three times the height; the lateral flanges of the bone run far laterally beyond the occipital condyle. The dorsomedial region of the bone looks like a column above which the squamosals come together. From laterally and ventrally the supraoccipital is surrounded by the exoccipitals, the lateral sutures with which are finely serrated, and the lower suture is like an almost horizontal straight line.
The exoccipitals in their general features have the typical hadrosaurian two symmetrical ginglymi along the sides of the occipital condyle. By merging at the center without a suture they isolate the foramen magnum from the supraoccipital by a fairly wide band, below which the exoccipitals are notably concave, thanks to which the supraoccipital slightly overhangs them. The boundaries of the exoccipitals with the opisthotics, with which they form the paroccipital process, are not seen even though they are common in other hadrosaurs. The ventral boundary with the basioccipital is not clear owing to the deformation and destruction of the occipital condyle . We may only assume that this boundary ran in the same manner as in other cheneosaurs.
Lateral Surface of the Skull
Only the posterior portions of the maxillae were preserved. They were included with the skull in a monolithic block and are therefore inadequately exposed. Posteriorly they terminate at the level of the coronoid process on the lower jaws, where they join dorsally with the lower processes of the pterygoids. Because the maxillae of hadrosaurs differ insignificantly even within families, we may assume that in P. convincens they are similar to those of the other cheneosaurs.
The lacrimals are subtrapezoidal in shape and wide, similar to the same bones in Procheneosaurus praeceps and [P. erectrofrons]. Anteriorly they are separated by a straight suture that runs dorsally at an angle from the premaxillae. The lower suture—with the jugals—is a slightly undulating line with a depression in the middle for a small growth of the jugals. The dorsal boundary of the lacrimals with the prefrontals is not very clear, but in any case it rises upward at an angle from the premaxillae and then, as if descending to the anterior edge of the orbit, enters the prefrontals, in contrast to other species of Procheneosaurus, in which conversely the prefrontals enter the lacrimals.
Posteriorly the lacrimals form the anterior edge of the orbits in their central part. The jugals have the typical hadrosaurian shape and are especially similar to those of [P. erectrofrons]. These are fairly thin, flat bones that are slightly convex laterally. Anteriorly they widen, forming a rounded cavity that overlaps the maxillae and lacrimals. How they interrelate with the premaxillae is unclear because the anterior portion was destroyed.
Above, the jugals form the ventral boundaries of the orbits and the infratemporal fenestrae, separating them by means of the ascending process which originates at the center of the bone. This process, by meeting the descending process of the postorbitals in the center of the orbits and underlying it posteriorly, reaches almost to the dorsal edge of the infratemporal fenestra. The ventral edge of the jugal forms a notch below the orbit (symmetric with the upper edge) and again widens above the coronoid process, descending to the level of the teeth in the lower jaw. Posterior to the coronoid process, the lower edge of the jugal rises somewhat, reaching the quadrate. The posterior boundary of the jugal with the quadratojugal is unclear.
The quadratojugals, located between the quadrates and jugals, are shaped like a half-moon that mimics the posterior shape of the jugals. Due to the incomplete fossilization or surface loss on the thin and fragile bone, the dorsal and anterior boundaries of the quadratojugals are unclear. If the bones have been reconstructed correctly, then they completely (or almost completely) isolate the quadrates and jugals as in [P. erectrofrons], although Ostrom (1961), for example, did not attempt to carry the boundaries between the jugals and quadratojugals when illustrating specimen No. 5461 of this species.
The quadrates enter the depression in the squamosals via their dorsal end and enter a notch in the lower jaws via the ventral end. The base trunk of the bone, which outlines the posterior profile of the skull, is slightly anteriorly concave. The lateral surface of the bone, widening from the epiphyses to the center, forms a deep, round notch for the quadratojugals here, dorsally and ventrally from which the quadrate acquires the nature of two small round, anterior-leaning lobes. The medial surface, forming an angle with the lateral surface, transforms anteriorly into a broad triangular, thin, process flatly adjoining the oncoming process of the pterygoid.
The rounded shape of the dorsal and ventral quadrate condyles confirm its mobility, although Ostrom (1961) contradicts this because there is a descending process on the squamosal ahead of the dorsal quadrate condyle and a close union with the pterygoid. However, the dentary suture between the pterygoid process and quadrate processes that Ostrom found in Corythosaurus casuarius was not seen in P. convincens. Also, the descending process of the squamosal anterior to the quadrate condyle in P. convincens is very small, and the articular fossa of the lower jaw is rather large, explicitly allowing mobility.
Palatal Surface
The pterygoids consist of three regions, or processes: dorsal, ventral, and anterior. The dorsal occupies the greatest area—a thin quadrate process that medially underlies the anterior quadrate process and traverses ventrally into a ridge-like ski-abutment that runs along the ventral edge of the anterior process of the quadrate. The mammiform pterygoid process of the basisphenoid descends anteriorly from each side into the depression formed by the vertical portion of the pterygoid quadrate process and its transverse ski.
The articulation of the basisphenoid with the pterygoids apparently allowed motion in this region, thereby elevating the streptostyly near the quadrates. Anterior to the ridge-like ski-abutment, a rather thin, subtriangular process departs ventrally. This process is supported on the dorsal surface of the posterior end of the maxillae. The anterior pterygoid process contact with the palatine is poorly traced because of inadequate stripping of the skull in this area. For the same reason it is difficult to characterize the other bones on the palatal surface.
The palatines are the most accessible bones. The impression is created that the contact between the pterygoids and palatines was sliding and that the pterygoids probably underlie the palatines laterally and somewhat ventrally. The anterior ends of the pterygoids and palatines rise acutely, extending beyond the center of the orbit elevation and meeting at the anterior end of the parasphenoid.
Braincase
We have previously described the bones of the skull roof that form the roof of the braincase, and the bones of the occipital surface that form its posterior wall. The lateral walls and base of the braincase, not distinguished because of poor preservation, are partially covered laterally by other bones. On the lateral surface that was accessible for observation, the sutures between the bones were quite invisible. This makes it extremely difficult to describe these bones, and little can be used to characterize the species.
We will only note that the union of the laterosphenoids with the skull roof by means of an articular process that is characteristic of other hadrosaurs was observed here to be only slight. This fact gives reason to assume that, analogous to crocodiles, the described articulation between the laterosphenoids and postorbitals is more highly developed in the longsnouted hadrosaurs, in which the force of collision arising when the jaws closed must have been greater, and correspondingly there is a more developed amortization. It is therefore possible that the primitive lambeosaur Jaxartosaurus aralensis, in which the previously mentioned type of articulation is well developed, maintained a fairly long snout in contrast to its descendants which became short-snouted, which may be associated with other adaptations to feeding and breathing.
Lower Jaws
Only the posterior ends of the lower jaws were preserved. They are characterized by very high coronoid processes that reach the foundation of the orbital fenestrae and exceed the height of the dentaries by a factor of almost two. The lower jaw rami are so close to each other that there is practically no interstice between the dental batteries on both sides. Such closeness of the mandibles leaves a very narrow passageway for food, and a movable joint between the quadrates and the axial skull compels us to assume the possibility of motion within the skull itself. The sliding overhang of the jugal and quadrates and the sliding contacts of the pterygoids testify to this.
Teeth
The crowns of the teeth are barely visible and, what is more, are present as vertically long rhomboids with a single medial ridge and lacking supplementary ridges on the sides only at the end of the mandibles.
Hyoid Apparatus
Parallel to the lower jaw rami, almost abutting the mandibles, are the hyoids—apparently ceratobranchials I (Romer, 1956; Ostrom, 1961). They look like narrow, slightly curved wafers that contact each other in their broad anterior regions, extending beyond the anterior edge of the coronoid processes.
Vertebral Column
The structure of the vertebral column in P. convincens is rather typical for hadrosaurs in general and for lambeosaurs in particular. The number of bones is apparently the same as in Procheneosaurus praeceps although the behavior of the boundaries between the vertebrae is arguable. Thus, the following formula is adopted for P. praeceps: 14 cervical, 16 dorsal, 10 sacral, and as many as 57 caudal vertebrae. But Parks (1931) noted that there may have been one fewer cervical and one more dorsal vertebra.
Such a correction may be appropriate for P. convincens because the 14th vertebra, located behind the anterior edge of the scapula, had a tubercular neural process (the neural process is barely evident in the first 13 vertebrae, excluding the axis) and a rather long rib. In general, the number of presacral vertebrae in P. convincens is the same as in Procheneosaurus praeceps, 30. The substantial height of the neurapophyses on the dorsal vertebrae of P. praeceps, more than two to three times the height of the vertebral centra (Lull, Wright, 1942), was not seen in P. convincens, which may in fact be explained by the incomplete preservation of the neurapophyses. There may be as many as ten sacral vertebrae, as in P. praeceps, but the first two have no direct contact with the ilia, even though they are located between them: the connections to the costal vertebrae, as is visible at least on the second of these, runs along the ilia, not touching the latter. It would probably be more correct to name these vertebrae false sacral vertebrae.
Approximately 40 vertebrae from the caudal region were preserved, and the first of these have very long neurapophyses, more than three times the height of the vertebral centrum. On the very first caudal vertebra, the wide-based, triangular processes grew abundantly and tend to approach the ilia. Highly developed ossified tendons that are intrinsic to Procheneosaurus praeceps are seen along the dorsal, sacral, and caudal vertebrae of P. convincens.
Pectoral Girdle and Forelimb
The sternum has the typical hadrosaur shape—its proximal area looks like a wide lobe that tapers notably upward, and ventrally ends as an uncinate process; distally this lobe transitions into the body of the bone, gradually widening at the end where the cross-section is almost twice the greatest minimum diameter of the body. The length of the bone body (to the uncinate process) is roughly equal to the lobe height.
The scapula looks like a fairly thin wafer, moderately concave along the ventral edge and almost straight along the dorsal edge. Above the dorsal edge the coracoid region the scapula projects slightly ventrally, without making an abrupt transition anteriorly, as though “truncated”, and does not extend beyond the glenoid region. The distal end of the scapula widens ventrally and is somewhat wider than the proximal end; and the minimum diameter of the scapular blade is approximately one seventh its overall length. The distal ends of both scapulae were somewhat destroyed (as a consequence of their extreme thinness and brittleness, or incomplete fossilization), but there is no reason to presume that the destroyed area was significant and may have significantly altered the ratios presented here.
The coracoid, like the sternum and scapula, is a fairly flat, light bone. Its ventral process does not look uncinate as is often seen in hadrosaurs, but subtriangular with a straight proximal side. This process is highly developed, descending significantly below the glenoid cavity, thanks to which the length (height) of the coracoid is more than one and a half times its width. The humerus has the typical hadrosaur shape but is not massive, and has a moderately developed deltopectoral ridge. The dimension of the humerus is three quarters the length of the scapula.
The forearm bones, ulna and radius, also have the typical hadrosaur structure and they are both insignificantly longer than the humerus. The carpus is unknown, but we may assume that it too had the typical hadrosaur form because the free-hanging forelimb apparently had a less functional load than the hind limbs, and its structure is therefore extremely unique in all hadrosaurs.
Pelvic Girdle and Hind Limbs
The ilium is long, slightly curved, and low, such that it is more than four times as long as it is tall, and the fraction of the anterior process (lobe) is almost half the entire bone. The posterior lobe is subtriangular, slightly more than twice as short as the anterior process. The pubic process is broad, only slightly wider than the ischial, thanks to which the bone base is almost one and a half times its height. The antitrochanter projection is asymmetrical—its posterior boundary is perpendicular to the dorsal surface of the ilium, and the anterior boundary is steeply inclined toward it.
The pubis has a short but wide anterior lobe (prepubis), so that its length is less than twice its maximum width. The dorsal edge of the prepubis looks like a broken line—initially a smooth curve and then straight—which bounds the widest portion of the lobe. The postpubis is small, the portion that was preserved being less than one-third the length of the ischium, although it was probably somewhat larger.
The ischium is characterized by a highly developed pubic process that is almost as wide as the ilium. The dorsal boundary of the bone shaft is almost straight, and the ventral is slightly concave. The bone shaft is thin, its minimum diameter being one-seventh the height of the proximal region. The bone widens notably toward the distal end—no less than two times in comparison with the minimum width of the shaft.
The femur is characterized by substantial anteroposterior extension of the distal condyles, reaching almost one-third the length of the bone. The lengthwise measurement of these condyles is almost twice that of their cross-section. The medial condyle is one and a half times wider than the lateral condyle. The condyles merge together, bounding an oval foramen, which is circular in most hadrosaurs. Here the oval must extend the condyles in not only posteriorly, but the anteriorly as well. Because of this the anterior surface of the femur is noticeably curved both proximally and distally. The fourth trochanter looks like a scalene triangle in which the upper of the two outer sides is smaller than the lower.
The tibia is as long as the femur or slightly shorter (the left leg is not the same measure as the right leg). The upper shaft is much wider in cross-section, so that the lengthwise diameter is greater than the cross-sectional diameter by a factor of less than two. There is a deep, narrow notch between the condyles.
The upper shaft of the fibula is extended anteroposteriorly, just like the shafts of the femur and tibia, comprising two-thirds the lengthwise diameter of the upper shaft of the tibia. The tarsals are represented by the astragalus and calcaneum, but were badly damaged in the holotype.
The metatarsals also have shafts that have been anteroposteriorly extended. Here the measurements of the upper shafts are on average twice that of their cross-sections. The difference between the lengthwise and cross-sectional measurements of the lower shafts are even less. The upper shafts of all three metatarsals very tightly abut one another, and are pear-shaped in plan view, widening inward from metatarsal IV to metatarsal II. The shaft of metatarsal II is very wide anteriorly and convex medially as well as on side facing the shaft of metatarsal III, which has a corresponding notch where the bones make contact.
The metatarsal III and IV shafts are half-moon shaped in plan view as is metatarsal II in general, but more symmetrical. The lower shafts do not tightly abut one another. The metatarsal II shaft is fairly narrow and the smallest of the three, having a bean-like shape, whereas metatarsal III shaft is the largest of the three and has a subquadrate shape in plan view, and the metatarsal IV shaft is of intermediate size and has a trapezoidal shape. In contrast to the lower shafts, the area of the upper shafts proceed in reverse order: the largest belongs to metatarsal III, next is metatarsal II, and the last is metatarsal IV. Overall, the metatarsus is fairly long and metatarsal III is two and a half times shorter than the femur. The lateral metatarsals are notably smaller than the middle, and metatarsal IV is shorter than metatarsal II.
All but the distal (ungual) phalanges are known. The first row of phalanges on all three digits is fairly long, their lengths substantially exceeding their widths, even including the central digit. The first phalanges of the lateral digits (II and IV) are the same size. The second phalanx of the digit II is subtriangular in shape, but with a very tapered anterior facet and interior boundary. The other phalanges of the III and IV digits are wide and short as in most hadrosaurs.
Analyzing the measurements, we can see asymmetry between the left and right sides, and this affects the postcranial skeleton as well as the skull. Especially noteworthy are the differences in the hind limbs: the bones of the right leg—femur + tibia is 4.5 cm longer than the same combination of the left leg.
Because the left pes was not preserved, it is not clear whether this difference was compensated to some extent by the pes or, on the contrary, was enhanced. In any event, the differences in the framework of a single specimen compel us to bear in mind the more significant magnitudes of individual and age variability commonly accepted as taxonomic features (Rozhdestvensky, 1965).
It is therefore possible, for example, that the small specimens from the Two Medicine Formation (Montana) were assigned to [P. erectofrons] on the basis of its greatest similarity with it in the skull. These specimens were in fact members of [P. cranibrevis] that proceeded from the higher horizon of the Belly River Formation (Alberta) than the first species.
The similarity with the earlier (judging by the stratigraphic level in the Belly River Formation) species P. praeceps in the postcranial skeletal structure is fully understandable in terms of evolutionary changes that presented the more conservative region with respect to the skull. The longer snout region of the young Montana specimen in comparison with P. praeceps and [P. erectofrons] also does not contradict assigning it to the fairly long-snouted [P. cranibrevis]. Here a situation may occur, analogous with Saurolophus, in which the young specimens of a later species are closer in terms of skull structure to the adult specimens of the ancestor than to its own species.
Comparison
The fundamental in the skull of Procheneosaurus convincens with other species of the same species can be seen in Table 2. It follows from the table and the description that P. convincens is closer to Procheneosaurus praeceps and [P. erectofrons], with which it is similar by way of the lower skull crest and overall configuration of the skull.
P. convincens differs from P. praeceps in its large, broad lacrimal, substantially more posterior location of the frontal distension, separation of the quadrate from the jugal, and narrow infratemporal fenestrae. It differs from [P. erectofrons] in the trapezoidal shape of its lacrimal, more posterior location of the frontal distension, and substantially narrower infratemporal fenestra. [P. cranibrevis] is the species most remote from P. convincens, as well as from P. praeceps and [P. erectofrons], and differs from all of them by its highly developed skull crest which extends to the level of the posterior edge of the orbit with an overhang above the frontal distension. Also, differences between P. convincens and [P. cranibrevis] are seen in the lacrimal (trapezoidal and triangular) and in the infratemporal fenestra—it is very narrow in P. convincens and broad in [P. cranibrevis]; the quadratojugal of P. convincens completely isolates the quadrate and jugal, but only partially separates them in [P. cranibrevis].
Source: Polyglot Paleontologist
Rozhdestvensky, 1968
Time
Cretaceous Late Turonian Coniacian Santonian
Classification
Ornithischia Ornithopoda Hadrosauridae Lambeosaurinae Incertae Sedis
Diet
Herbivore
Fossilsite
Dabrazinskaya Svita, Syuk-Syuk wells site, Sydarninskaya Oblast, Kazakhstan
Fall Under
Jaxartosaurus
Info
Skull
Jaxartosaurus (Riabinin, 1937) > Jaxartosaurus aralensis (Riabinin, 1939) >> Procheneosaurus convincens (Rozhdestvensky, 1968)
Nearly complete skeleton lacking only the front of the skull and distal region of the tail. Found in the same region (but in a different strartigraphic region) as Jaxartosaurus.
P. convincens represents a juvenile lambeosaurine hadrosaur.
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.
Procheneosaurus praeceps Matthew, 1920 (= Didanodon altidens Osborn, 1902; = Tetragonosaurus Parks, 1931)
[Lambeosaurus (Parks, 1923) = Didanodon altidens (Osborn, 1902)]
Type species
Tetragonosaurus praeceps Parks, 1931; Belly River Formation, Alberta (Canada).
Diagnosis
Comparatively small (up to 5 m long) crested hadrosaur. The skull is short and tall, with a lower crest that was formed by the premaxillae and nasals. The upper branch of the premaxilla is separated from the lower branch by means of the open nasal fenestra. The nasals are fairly small and outline the nasal fenestrae posteriorly and partially dorsally. The lacrimals are triangular or trapezoidal, ranging in size from moderate to large.
The infratemporal fenestrae range from being moderately wide to narrow, with an increase in its length-to-width ratio of as much as three. There are 40 tooth rows1 in the upper jaw and 33 in the lower. There are 30 presacral vertebrae, of which 13 or 14 are cervical, 16–17 are dorsal, 10 are sacral, and as many as 57 are caudal. Along the last dorsal, sacral, and anterior caudal vertebrae are ossifying tendons. The scapula is slightly curved, with a slightly developed coracoid region; the overall length of the scapula is roughly one and a half times that of the humerus.
The radius is slightly longer than the humerus. The ilium is low, with a long, narrow, anterior lobe (process); overall, the bone is roughly four times as long as it is high. The antitrochanter is asymmetrical with a longer and steeper anterior edge. The ischium has a thin trunk; its iliac and pubic processes are almost the same size and the distal end is moderately widened. The pubis has a short prepubis and a short postpubis that makes up approximately one third the length of the ischium. The knee and talocrural joints are notably extended anteroposteriorly.
The femur is slender, less than twice as long as the humerus and only two and a half times larger than metatarsal III. The tibia is roughly the same length as the femur, or only slightly smaller; the medial condyle of the upper epiphysis is significantly wider than the lateral condyle.
Species composition
Four species: Procheneosaurus convincens sp. nov.; Procheneosaurus praeceps (Parks), 1931; [P. erectofrons] (Parks), 1931; [P. cranibrevis] (Sternberg), 1935.
Distribution
Central Asia (Southern Kazakhstan) and North America (Alberta, Canada and Montana, USA).
Geological Age
Santonian–Campanian (Dabrazinskaya Svita, Belly River Formation, and Two Medicine Formation).
Procheneosaurus convincens sp. nov.
Holotype
PI No. 2230; almost complete skeleton excluding the anterior region of the skull, distal regions of the forelimbs and left hind limb, and last caudal vertebrae; Senonian (Dabrazin Formation - Dabrazinskaya Svita) Southern Kazakhstan, [Shakh-Shakh Formation], 45 km north of Tashkent.
Diagnosis
The skull crest is low and short, its highest point is slightly in front of the orbit. The lacrimal is trapezoidal in shape and fairly large and wide, making up more than half the width of the orbit. The frontal swelling is much closer to the occipital edge than to the highest point on the skull crest. The quadratojugal completely isolates the jugal from the quadrate.
The infratemporal fenestra is narrow, almost two and a half times narrower than the orbit. The neural processes of the dorsal vertebrae are moderately high, whereas for the anterior caudal vertebrae they are long, more than three times the height of the vertebral centrum. The scapula is moderately long, roughly seven times its minimal width. The anterior lobe of the ischium is more than twice as long as the posterior lobe, and the base of the bone has a well-developed pubic process that is almost one and a half times its height.
he length of the anterior process of the pubis is less than twice its maximum width. The minimum diameter of the ischial process is one seventh the height of its proximal region. The distal condyles of the femur, extended anteroposteriorly, are almost one third as long as the entire bone and much less than twice its width. Of the proximal condyles of the tibia the medial is more than twice as wide as the lateral.
The skull, just as with cheneosaurs, is tall and narrow, its height in the orbit area being almost twice its width. Toward the occiput, which is just as tall and comparatively narrow, the skull narrows even more. The anterior region of the skull was destroyed, but judging by the posterior half, we may think that the crest, formed by the premaxillae and nasals, was not tall and slightly elevated above the dorsal surface of the skull, achieving its greatest length somewhat in front of the orbit. It is difficult to judge the shape of the nasal fenestrae because they are known only by their posteriormost region, located at roughly the same level as the upper edge of the orbit.
We may assume that the nasal fenestrae of P. convincens must have been similar to those of the low-crested species of procheneosaurs in its features. The orbits are oval, slightly narrowing ventrally; their length is roughly one and a half times their maximum width. They are oriented at somewhat of an angle, thanks to which the infratemporal fenestrae are isolated from them ventrally by means of an overlapping bar as if they run below the orbits. The infratemporal fenestrae look like long, narrow ellipses, extended slightly ventrally and oriented at an angle, like the orbits. Theinfratemporal fenestrae are three or four times as long they are wide. The supratemporal fenestrae are small, more than twice as short as the infratemporal fenestrae, rhomboid in shape and extended slightly along the medial line of the skull.
Skull Roof
The premaxillae are known only by their lower branches in their posteriormost region where they bound the nasal fenestrae cavity almost vertically. At the upper end, on the same level as the upper edge of the nasal fenestrae, the lower branches of the premaxillae contact the nasals which bound the nasal fenestrae dorsally by means of a short, straight suture. Posteriorly the lower branches of the premaxillae lie on the lower end of the frontals and lacrimals to form a slightly broken line. In those areas that were preserved the premaxillae are almost the same width as the lower end of the frontals and the upper end of the lacrimals, but taper slightly at the apex. The nasal fenestrae look like deep, fairly wide cavities that almost match the width of the skull in the nasal area and are overlaid laterally by the lower branches of the premaxillae. The nasal fenestrae reached the posterior edge of the latter and connected to an outwardly closed cavity, looking like a small rounded pocket within the nasals, where the latter form a ridge.
Excluding the anterior region, the nasals were almost completely preserved to outline the nasal fenestrae dorsally. Above the posterodorsal edge of the nasal fenestrae, the nasals are a dome-shaped swelling that forms a small, low crest (tubercle) within which is a cavity that connects with the nasal fenestrae. According to the shape and location of the crest, one of the most essential features of the cheneosaurs, P. convincens is similar to Procheneosaurus praeceps. The nasals are widest near the crest, making contact with the lower branch of the premaxillae and anterior region of the prefrontals which lay on the nasals along a slightly undulating line.
Behind the crest-like swelling they narrow radically and look like slender, slightly posteriorly sloped wafers that lie between the prefrontals, but terminate prior to the latter. In the posterior region the nasals again widen slightly and make contact along a slightly serrated suture with the frontals, wedging slightly between them. The prefrontals are wide in the area where they overlay the nasals and form the anterodorsal edge of the orbit. The prefrontals abut the lower edge of the premaxillae anteroventrally and include the apex of the lacrimals ventrally.
The posterior boundary with the postorbitals looks like a slanted, non-uniform suture that runs from the highest point of the orbit’s upper edge in the direction of the anterolateral corner of the supratemporal fenestrae. The second posterior suture between the frontals and prefrontals is directed almost symmetrically to the first as a somewhat non-uniform line from the juncture of the postorbitals and frontals to the posterolateral nodes of the nasals.
The boundaries of the medial surface of the prefrontals are indistinguishable. The frontals are bounded anteriorly by the nasals and posteriorly by the parietals which are slightly wedged between them and isolated from the orbits by the wide band of the prefrontals and postorbitals. Along their periphery the frontals are slightly concave, and medially they form a substantial, round swelling that only slightly exceeds the nasals in size and serves as the roof for the braincase near the large hemispheres.
The parietals form the medial walls of the supratemporal fenestrae and are very short. Excepting the anterior boundaries with the frontals, it is only with some difficulty that we note the posterior boundaries where the squamosals form the sagittal ridge that rises above the parietals. The ventral boundaries of the parietals were quite indistinguishable.
Anteriorly the postorbitals look like a wedge on the dorsal surface of the skull, on the lateral side of which the upper posterior edge of the orbits is formed, and whose medial side bounds the prefrontals and frontals. The posterior portion of the postorbitals wedges into the squamosals by means of two tooth-like processes—the large process is the lower and the small process is the upper.
The lower process is above the middle head of the quadrate and the upper extends to the center of the lateral edge of the supratemporal fenestra. Laterally, at a slight angle to the dorsal surface, the postorbitals yield a thin, but fairly long descending process that descends below the center of the orbit, isolating it and the ascending process of the jugal from the infratemporal fenestra. The boundaries of the postorbitals on the lateral surface are not clear.
The squamosals, which bound the supratemporal fenestra from the outside and behind, have a complex shape as is seen in other hadrosaurs. The postorbitals overlie the squamosals anteriorly, and together with them form the supratemporal arches while one anterior process of the squamosal underlies the postorbital ventromedially, extending to the anterolateral node of the supratemporal fenestra—and the second descends vertically downward, isolating the postorbital and quadrate and bounding the quadrate anteriorly. This second process of the squamosal is parallel to the ventral process of the postorbital, but is roughly twice as short and is far from the jugal and quadratojugal.
The posterior surface of the process bounds the articular fossa in the squamosal for the quadrate, behind which is yet another descending process of the squamosal. This process is longer and includes the quadrate posteriorly, overlaying the paroccipital process. Opposite this process the squamosals turn medially at a right angle, merging medially as the sagittal crest and forming the occipital surface of the skull. At the point of a very short contact between the squamosals and occipitals are corresponding half-moon shaped depressions in the squamosals for the exoccipitals to enter, and the suture surfaces of the squamosals form an overhang above the supraoccipital.
Occipital Surface
The supraoccipital, which underlies almost the extent of the squamosal cross-section, has a small, but very wide, rise, more than three times the height; the lateral flanges of the bone run far laterally beyond the occipital condyle. The dorsomedial region of the bone looks like a column above which the squamosals come together. From laterally and ventrally the supraoccipital is surrounded by the exoccipitals, the lateral sutures with which are finely serrated, and the lower suture is like an almost horizontal straight line.
The exoccipitals in their general features have the typical hadrosaurian two symmetrical ginglymi along the sides of the occipital condyle. By merging at the center without a suture they isolate the foramen magnum from the supraoccipital by a fairly wide band, below which the exoccipitals are notably concave, thanks to which the supraoccipital slightly overhangs them. The boundaries of the exoccipitals with the opisthotics, with which they form the paroccipital process, are not seen even though they are common in other hadrosaurs. The ventral boundary with the basioccipital is not clear owing to the deformation and destruction of the occipital condyle . We may only assume that this boundary ran in the same manner as in other cheneosaurs.
Lateral Surface of the Skull
Only the posterior portions of the maxillae were preserved. They were included with the skull in a monolithic block and are therefore inadequately exposed. Posteriorly they terminate at the level of the coronoid process on the lower jaws, where they join dorsally with the lower processes of the pterygoids. Because the maxillae of hadrosaurs differ insignificantly even within families, we may assume that in P. convincens they are similar to those of the other cheneosaurs.
The lacrimals are subtrapezoidal in shape and wide, similar to the same bones in Procheneosaurus praeceps and [P. erectrofrons]. Anteriorly they are separated by a straight suture that runs dorsally at an angle from the premaxillae. The lower suture—with the jugals—is a slightly undulating line with a depression in the middle for a small growth of the jugals. The dorsal boundary of the lacrimals with the prefrontals is not very clear, but in any case it rises upward at an angle from the premaxillae and then, as if descending to the anterior edge of the orbit, enters the prefrontals, in contrast to other species of Procheneosaurus, in which conversely the prefrontals enter the lacrimals.
Posteriorly the lacrimals form the anterior edge of the orbits in their central part. The jugals have the typical hadrosaurian shape and are especially similar to those of [P. erectrofrons]. These are fairly thin, flat bones that are slightly convex laterally. Anteriorly they widen, forming a rounded cavity that overlaps the maxillae and lacrimals. How they interrelate with the premaxillae is unclear because the anterior portion was destroyed.
Above, the jugals form the ventral boundaries of the orbits and the infratemporal fenestrae, separating them by means of the ascending process which originates at the center of the bone. This process, by meeting the descending process of the postorbitals in the center of the orbits and underlying it posteriorly, reaches almost to the dorsal edge of the infratemporal fenestra. The ventral edge of the jugal forms a notch below the orbit (symmetric with the upper edge) and again widens above the coronoid process, descending to the level of the teeth in the lower jaw. Posterior to the coronoid process, the lower edge of the jugal rises somewhat, reaching the quadrate. The posterior boundary of the jugal with the quadratojugal is unclear.
The quadratojugals, located between the quadrates and jugals, are shaped like a half-moon that mimics the posterior shape of the jugals. Due to the incomplete fossilization or surface loss on the thin and fragile bone, the dorsal and anterior boundaries of the quadratojugals are unclear. If the bones have been reconstructed correctly, then they completely (or almost completely) isolate the quadrates and jugals as in [P. erectrofrons], although Ostrom (1961), for example, did not attempt to carry the boundaries between the jugals and quadratojugals when illustrating specimen No. 5461 of this species.
The quadrates enter the depression in the squamosals via their dorsal end and enter a notch in the lower jaws via the ventral end. The base trunk of the bone, which outlines the posterior profile of the skull, is slightly anteriorly concave. The lateral surface of the bone, widening from the epiphyses to the center, forms a deep, round notch for the quadratojugals here, dorsally and ventrally from which the quadrate acquires the nature of two small round, anterior-leaning lobes. The medial surface, forming an angle with the lateral surface, transforms anteriorly into a broad triangular, thin, process flatly adjoining the oncoming process of the pterygoid.
The rounded shape of the dorsal and ventral quadrate condyles confirm its mobility, although Ostrom (1961) contradicts this because there is a descending process on the squamosal ahead of the dorsal quadrate condyle and a close union with the pterygoid. However, the dentary suture between the pterygoid process and quadrate processes that Ostrom found in Corythosaurus casuarius was not seen in P. convincens. Also, the descending process of the squamosal anterior to the quadrate condyle in P. convincens is very small, and the articular fossa of the lower jaw is rather large, explicitly allowing mobility.
Palatal Surface
The pterygoids consist of three regions, or processes: dorsal, ventral, and anterior. The dorsal occupies the greatest area—a thin quadrate process that medially underlies the anterior quadrate process and traverses ventrally into a ridge-like ski-abutment that runs along the ventral edge of the anterior process of the quadrate. The mammiform pterygoid process of the basisphenoid descends anteriorly from each side into the depression formed by the vertical portion of the pterygoid quadrate process and its transverse ski.
The articulation of the basisphenoid with the pterygoids apparently allowed motion in this region, thereby elevating the streptostyly near the quadrates. Anterior to the ridge-like ski-abutment, a rather thin, subtriangular process departs ventrally. This process is supported on the dorsal surface of the posterior end of the maxillae. The anterior pterygoid process contact with the palatine is poorly traced because of inadequate stripping of the skull in this area. For the same reason it is difficult to characterize the other bones on the palatal surface.
The palatines are the most accessible bones. The impression is created that the contact between the pterygoids and palatines was sliding and that the pterygoids probably underlie the palatines laterally and somewhat ventrally. The anterior ends of the pterygoids and palatines rise acutely, extending beyond the center of the orbit elevation and meeting at the anterior end of the parasphenoid.
Braincase
We have previously described the bones of the skull roof that form the roof of the braincase, and the bones of the occipital surface that form its posterior wall. The lateral walls and base of the braincase, not distinguished because of poor preservation, are partially covered laterally by other bones. On the lateral surface that was accessible for observation, the sutures between the bones were quite invisible. This makes it extremely difficult to describe these bones, and little can be used to characterize the species.
We will only note that the union of the laterosphenoids with the skull roof by means of an articular process that is characteristic of other hadrosaurs was observed here to be only slight. This fact gives reason to assume that, analogous to crocodiles, the described articulation between the laterosphenoids and postorbitals is more highly developed in the longsnouted hadrosaurs, in which the force of collision arising when the jaws closed must have been greater, and correspondingly there is a more developed amortization. It is therefore possible that the primitive lambeosaur Jaxartosaurus aralensis, in which the previously mentioned type of articulation is well developed, maintained a fairly long snout in contrast to its descendants which became short-snouted, which may be associated with other adaptations to feeding and breathing.
Lower Jaws
Only the posterior ends of the lower jaws were preserved. They are characterized by very high coronoid processes that reach the foundation of the orbital fenestrae and exceed the height of the dentaries by a factor of almost two. The lower jaw rami are so close to each other that there is practically no interstice between the dental batteries on both sides. Such closeness of the mandibles leaves a very narrow passageway for food, and a movable joint between the quadrates and the axial skull compels us to assume the possibility of motion within the skull itself. The sliding overhang of the jugal and quadrates and the sliding contacts of the pterygoids testify to this.
Teeth
The crowns of the teeth are barely visible and, what is more, are present as vertically long rhomboids with a single medial ridge and lacking supplementary ridges on the sides only at the end of the mandibles.
Hyoid Apparatus
Parallel to the lower jaw rami, almost abutting the mandibles, are the hyoids—apparently ceratobranchials I (Romer, 1956; Ostrom, 1961). They look like narrow, slightly curved wafers that contact each other in their broad anterior regions, extending beyond the anterior edge of the coronoid processes.
Vertebral Column
The structure of the vertebral column in P. convincens is rather typical for hadrosaurs in general and for lambeosaurs in particular. The number of bones is apparently the same as in Procheneosaurus praeceps although the behavior of the boundaries between the vertebrae is arguable. Thus, the following formula is adopted for P. praeceps: 14 cervical, 16 dorsal, 10 sacral, and as many as 57 caudal vertebrae. But Parks (1931) noted that there may have been one fewer cervical and one more dorsal vertebra.
Such a correction may be appropriate for P. convincens because the 14th vertebra, located behind the anterior edge of the scapula, had a tubercular neural process (the neural process is barely evident in the first 13 vertebrae, excluding the axis) and a rather long rib. In general, the number of presacral vertebrae in P. convincens is the same as in Procheneosaurus praeceps, 30. The substantial height of the neurapophyses on the dorsal vertebrae of P. praeceps, more than two to three times the height of the vertebral centra (Lull, Wright, 1942), was not seen in P. convincens, which may in fact be explained by the incomplete preservation of the neurapophyses. There may be as many as ten sacral vertebrae, as in P. praeceps, but the first two have no direct contact with the ilia, even though they are located between them: the connections to the costal vertebrae, as is visible at least on the second of these, runs along the ilia, not touching the latter. It would probably be more correct to name these vertebrae false sacral vertebrae.
Approximately 40 vertebrae from the caudal region were preserved, and the first of these have very long neurapophyses, more than three times the height of the vertebral centrum. On the very first caudal vertebra, the wide-based, triangular processes grew abundantly and tend to approach the ilia. Highly developed ossified tendons that are intrinsic to Procheneosaurus praeceps are seen along the dorsal, sacral, and caudal vertebrae of P. convincens.
Pectoral Girdle and Forelimb
The sternum has the typical hadrosaur shape—its proximal area looks like a wide lobe that tapers notably upward, and ventrally ends as an uncinate process; distally this lobe transitions into the body of the bone, gradually widening at the end where the cross-section is almost twice the greatest minimum diameter of the body. The length of the bone body (to the uncinate process) is roughly equal to the lobe height.
The scapula looks like a fairly thin wafer, moderately concave along the ventral edge and almost straight along the dorsal edge. Above the dorsal edge the coracoid region the scapula projects slightly ventrally, without making an abrupt transition anteriorly, as though “truncated”, and does not extend beyond the glenoid region. The distal end of the scapula widens ventrally and is somewhat wider than the proximal end; and the minimum diameter of the scapular blade is approximately one seventh its overall length. The distal ends of both scapulae were somewhat destroyed (as a consequence of their extreme thinness and brittleness, or incomplete fossilization), but there is no reason to presume that the destroyed area was significant and may have significantly altered the ratios presented here.
The coracoid, like the sternum and scapula, is a fairly flat, light bone. Its ventral process does not look uncinate as is often seen in hadrosaurs, but subtriangular with a straight proximal side. This process is highly developed, descending significantly below the glenoid cavity, thanks to which the length (height) of the coracoid is more than one and a half times its width. The humerus has the typical hadrosaur shape but is not massive, and has a moderately developed deltopectoral ridge. The dimension of the humerus is three quarters the length of the scapula.
The forearm bones, ulna and radius, also have the typical hadrosaur structure and they are both insignificantly longer than the humerus. The carpus is unknown, but we may assume that it too had the typical hadrosaur form because the free-hanging forelimb apparently had a less functional load than the hind limbs, and its structure is therefore extremely unique in all hadrosaurs.
Pelvic Girdle and Hind Limbs
The ilium is long, slightly curved, and low, such that it is more than four times as long as it is tall, and the fraction of the anterior process (lobe) is almost half the entire bone. The posterior lobe is subtriangular, slightly more than twice as short as the anterior process. The pubic process is broad, only slightly wider than the ischial, thanks to which the bone base is almost one and a half times its height. The antitrochanter projection is asymmetrical—its posterior boundary is perpendicular to the dorsal surface of the ilium, and the anterior boundary is steeply inclined toward it.
The pubis has a short but wide anterior lobe (prepubis), so that its length is less than twice its maximum width. The dorsal edge of the prepubis looks like a broken line—initially a smooth curve and then straight—which bounds the widest portion of the lobe. The postpubis is small, the portion that was preserved being less than one-third the length of the ischium, although it was probably somewhat larger.
The ischium is characterized by a highly developed pubic process that is almost as wide as the ilium. The dorsal boundary of the bone shaft is almost straight, and the ventral is slightly concave. The bone shaft is thin, its minimum diameter being one-seventh the height of the proximal region. The bone widens notably toward the distal end—no less than two times in comparison with the minimum width of the shaft.
The femur is characterized by substantial anteroposterior extension of the distal condyles, reaching almost one-third the length of the bone. The lengthwise measurement of these condyles is almost twice that of their cross-section. The medial condyle is one and a half times wider than the lateral condyle. The condyles merge together, bounding an oval foramen, which is circular in most hadrosaurs. Here the oval must extend the condyles in not only posteriorly, but the anteriorly as well. Because of this the anterior surface of the femur is noticeably curved both proximally and distally. The fourth trochanter looks like a scalene triangle in which the upper of the two outer sides is smaller than the lower.
The tibia is as long as the femur or slightly shorter (the left leg is not the same measure as the right leg). The upper shaft is much wider in cross-section, so that the lengthwise diameter is greater than the cross-sectional diameter by a factor of less than two. There is a deep, narrow notch between the condyles.
The upper shaft of the fibula is extended anteroposteriorly, just like the shafts of the femur and tibia, comprising two-thirds the lengthwise diameter of the upper shaft of the tibia. The tarsals are represented by the astragalus and calcaneum, but were badly damaged in the holotype.
The metatarsals also have shafts that have been anteroposteriorly extended. Here the measurements of the upper shafts are on average twice that of their cross-sections. The difference between the lengthwise and cross-sectional measurements of the lower shafts are even less. The upper shafts of all three metatarsals very tightly abut one another, and are pear-shaped in plan view, widening inward from metatarsal IV to metatarsal II. The shaft of metatarsal II is very wide anteriorly and convex medially as well as on side facing the shaft of metatarsal III, which has a corresponding notch where the bones make contact.
The metatarsal III and IV shafts are half-moon shaped in plan view as is metatarsal II in general, but more symmetrical. The lower shafts do not tightly abut one another. The metatarsal II shaft is fairly narrow and the smallest of the three, having a bean-like shape, whereas metatarsal III shaft is the largest of the three and has a subquadrate shape in plan view, and the metatarsal IV shaft is of intermediate size and has a trapezoidal shape. In contrast to the lower shafts, the area of the upper shafts proceed in reverse order: the largest belongs to metatarsal III, next is metatarsal II, and the last is metatarsal IV. Overall, the metatarsus is fairly long and metatarsal III is two and a half times shorter than the femur. The lateral metatarsals are notably smaller than the middle, and metatarsal IV is shorter than metatarsal II.
All but the distal (ungual) phalanges are known. The first row of phalanges on all three digits is fairly long, their lengths substantially exceeding their widths, even including the central digit. The first phalanges of the lateral digits (II and IV) are the same size. The second phalanx of the digit II is subtriangular in shape, but with a very tapered anterior facet and interior boundary. The other phalanges of the III and IV digits are wide and short as in most hadrosaurs.
Analyzing the measurements, we can see asymmetry between the left and right sides, and this affects the postcranial skeleton as well as the skull. Especially noteworthy are the differences in the hind limbs: the bones of the right leg—femur + tibia is 4.5 cm longer than the same combination of the left leg.
Because the left pes was not preserved, it is not clear whether this difference was compensated to some extent by the pes or, on the contrary, was enhanced. In any event, the differences in the framework of a single specimen compel us to bear in mind the more significant magnitudes of individual and age variability commonly accepted as taxonomic features (Rozhdestvensky, 1965).
It is therefore possible, for example, that the small specimens from the Two Medicine Formation (Montana) were assigned to [P. erectofrons] on the basis of its greatest similarity with it in the skull. These specimens were in fact members of [P. cranibrevis] that proceeded from the higher horizon of the Belly River Formation (Alberta) than the first species.
The similarity with the earlier (judging by the stratigraphic level in the Belly River Formation) species P. praeceps in the postcranial skeletal structure is fully understandable in terms of evolutionary changes that presented the more conservative region with respect to the skull. The longer snout region of the young Montana specimen in comparison with P. praeceps and [P. erectofrons] also does not contradict assigning it to the fairly long-snouted [P. cranibrevis]. Here a situation may occur, analogous with Saurolophus, in which the young specimens of a later species are closer in terms of skull structure to the adult specimens of the ancestor than to its own species.
Comparison
The fundamental in the skull of Procheneosaurus convincens with other species of the same species can be seen in Table 2. It follows from the table and the description that P. convincens is closer to Procheneosaurus praeceps and [P. erectofrons], with which it is similar by way of the lower skull crest and overall configuration of the skull.
P. convincens differs from P. praeceps in its large, broad lacrimal, substantially more posterior location of the frontal distension, separation of the quadrate from the jugal, and narrow infratemporal fenestrae. It differs from [P. erectofrons] in the trapezoidal shape of its lacrimal, more posterior location of the frontal distension, and substantially narrower infratemporal fenestra. [P. cranibrevis] is the species most remote from P. convincens, as well as from P. praeceps and [P. erectofrons], and differs from all of them by its highly developed skull crest which extends to the level of the posterior edge of the orbit with an overhang above the frontal distension. Also, differences between P. convincens and [P. cranibrevis] are seen in the lacrimal (trapezoidal and triangular) and in the infratemporal fenestra—it is very narrow in P. convincens and broad in [P. cranibrevis]; the quadratojugal of P. convincens completely isolates the quadrate and jugal, but only partially separates them in [P. cranibrevis].
Source: Polyglot Paleontologist