Angelosaurus is an extinct genus of herbivorous caseid synapsids that lived during the late Lower Permian (Kungurian) and early Middle Permian (Roadian) in what is now Texas and Oklahoma. Like other herbivorous caseids, it had a small head, large barrel-shaped body, long tail, and massive limbs. Angelosaurus differs from other caseids by the extreme massiveness of its bones, particularly those of the limbs, which show a strong development of ridges, processes, and rugosities for the attachment of muscles and tendons. Relative to its body size, the limbs of Angelosaurus were shorter and wider than those of other caseids. The ungual phalanges looked more like hooves than claws. The few known cranial elements show that the skull was short and more robust than that of the other representatives of the group. Angelosaurus is also distinguished by its bulbous teeth with shorter and wider crowns than those of other caseids. Their morphology and the high rate of wear they exhibit suggests a diet quite different from that of other large herbivorous caseids, and must have been based on particularly tough plants. A study published in 2022 suggests that the genus may be paraphyletic, with Angelosaurus possibly only represented by its type species A. dolani.
Etymology The name of the genus Angelosaurus refers to the San Angelo Formation in Texas, where this animal was first discovered. It is completed by saurus meaning lizard.
Species Angelosaurus is only known from incomplete skeletons and isolated bones. The genus is represented by three species, the largest, Angelosaurus greeni, reaching a size comparable to that of Cotylorhynchus romeri. Angelosaurus romeri was the smallest species and A. dolani was of intermediate size. However, the difference in size is not a relevant taxonomic criterion because the number of known specimens is too limited to know the maximum size reached by each species. These three species differ essentially in characteristics of the humerus, femur, and pelvis, which are the only bones allowing comparisons. In the type species A. dolani, the humerus is very massive, as long as it is wide with an almost nonexistent shaft. In A. romeri, the humerus is less massive with a very short but distinct shaft. In A. romeri and A. dolani, the internal trochanter is located more or less halfway between the proximal and distal ends of the intertrochanteric fossa, while in A. greeni it is located near the distal end of the fossa. A. romeri is distinguished by the prominence of the fourth trochanter. In A. romeri and A. greeni, the external condyle is the larger of the two and is well separated from the internal condyle. In A. dolani the internal condyle is the most developed but the separation with the external condyle is less clear, probably due to the presence of an extensive cartilage cover. The pelvis of A. dolani is characterized both by the massiveness of the vertebral connections and the strong fusion of the sacral ribs forming a single plate distally, but also by the weak ossification of the dorsal lamina of the ilium and the pubic symphysis. In A. romeri there is practically no fusion of the sacral ribs. The sacrum of A. dolani has three vertebrae of which only the first two are fused. In A. romeri, the pubic symphysis is strongly ossified and the sacrum has four vertebrae. In the holotype, the three preserved vertebrae of the sacrum are fused together. Phylogenetic analysis by Werneburg and colleagues, however, indicates that the species A. greeni and A. romeri may not belong to the genus Angelosaurus, all these taxa requiring a detailed redescription.
Angelosaurus dolani The type species Angelosaurus dolani was erected in 1953 by Everett C. Olson and James R. Beerbower from a partial skeleton (the holotype FMNH UR 149) found in the red mudstones constituting the central part of the San Angelo Formation (Pease River Group), Knox County, Texas. This species was named in honor of Wayne Dolan, whose cooperation made it possible to continue work on the Little Crotton Creek locality, near the town of Benjamin where the holotype came from. The skeleton of A. dolani was largely articulated, with the first three presacral vertebrae, the sacrum and 24 caudal vertebrae being anatomically connected, as were the left pelvis, femurs, left tibia and fibula, and most of the left foot. Four other vertebrae, the right humerus, parts of the right manus, ribs fragments, two poorly preserved fragments of dentary and maxilla with broken teeth, were scattered anterior to the pelvis. A few vertebrae of a second specimen were also reported from the same site, less than a hundred meters of the first site.
A. dolani is characterized by a heavy and massive skeleton, with shorter and wider limbs than those of other caseids. The humerus is one of the most distinctive elements of the skeleton by its extremely massive proportions. There is virtually no shaft, and the width of the proximal end is almost equal to the total length of the bone. The delto-pectoral crest is highly developed and bears a prominent, knotty and rough process for the insertion of part of the pectoral musculature. Along the anterior margin of the proximal part of the humerus there is a strong ridge for the insertion of the deltoid muscle. The radial condyle of the humerus lies far anteriorly on the ventral and distal surface, and its proportions suggest that the head of the radius was massive. The femur is very massive with a deep and heavy proximal end. The internal trochanter is exceptionally strong and has a rough surface. Characteristically, it extends very close to the proximal end of the intertrochanteric fossa (a large concave surface located on the ventral side of the proximal end of the femur and where the puboischiofemoralis externus muscle inserted). The fourth trochanter, on the other hand, is weak. The distal condyles of the femur are large but not strongly differentiated, probably due to the presence of a cartilaginous cap. A strong ridge passes proximally from the intercondylar fossa along the dorsal margin of the femur and continues over the area where the femoro-tibial portion of the triceps surae muscle likely originated. Like the femur, the tibia and fibula are characterized by their ends formed of unfinished bone, suggesting the presence of a substantial cartilage cover. This peculiarity, which is usually a sign of skeletal immaturity, is surprising because the holotype of A. dolani belongs to an adult animal as indicated by the complete ossification of various other parts of the skeleton. On the left foot, relatively large compared to the size of the body, the tarsal bones were found articulated but the metatarsals and phalanges were somewhat disarticulated. So Olson and Beerbower had some difficulty in determining the phalangeal formula. After re-examining the material discovered, Olson later assessed the phalangeal formula of the foot to be 2-2-2-3-3. The ungual phalanges are distinguished from those of other caseids by being short, broad, and smooth, and resemble hooves rather than claws. Some elements of the right manus show that the phalanges were wide and short. The pelvis is characterized by a pubis and ischium forming a broad flattened plate, with a weak symphysis for most of its length. The ilium, short and broad, shows some indication of the dorsal elevation characteristic of caseids, but the apex appears incomplete, possibly cartilaginous. The acetabulum is proportionately very wide and deep. The sacrum is composed of three vertebrae, the first and the second being fused together. The distal ends of the three sacral ribs are fused to form a broad plate that fits over the ilium. The thoracic vertebrae have relatively long transverse processes and there appears to have been a small ventral facet, indicating that the ribs were double-headed. The neural spines, although poorly preserved, appear to have been rather short. Two fragments of the upper and lower jaws indicate that the skull was rather small in relation to the size of the body, as is the case in other derived caseids. The two fragments of dentary and maxilla bear a few teeth which, although broken, had an ovoid section and were quite spaced, indicating that their total number should be low.
Angelosaurus greeni Angelosaurus greeni was named in 1962 by Everett C. Olson from a very fragmentary skeleton and various isolated elements found in red mudstones at the top of the San Angelo Formation in Knox County, Texas. The specific epithet honors J. Green of the Alexander Ranch, whose cooperation and assistance during many years has made it possible to explore and collect the fossiliferous beds outcropping on the lands of the Alexander Ranch. It is the largest species of the genus with dimensions comparable to that of the caseid Cotylorhynchus romeri. It is also the most poorly known species. The remains found were fragmentary, in large part due to an extensive chemical decomposition after burial. In the holotype, this decomposition has altered and destroyed many elements of an originally more complete skeleton, as indicated by the presence of many fragments that are difficult to identify, but also the presence on the ground of the indistinct outline of some fully dissolved bones. The holotype (FMNH UR 257) is composed of a femur, some vertebrae, and fragments of ribs. Paratypes include a distal end fragment of a humerus (FMNH UR 258), pelvic fragments, distal limb elements, and indeterminate fragments (FMNH UR 264), and a flattened humerus with poorly preserved distal end (FMNH UR 259), all coming from the same general horizon and the same locality as the type.
This species is mainly characterized by the morphology of its femur. The location of the internal trochanter is very characteristic in being located near the distal end of the intertrochanteric fossa. While it is located near the middle or proximal part of this fossa in the other two species. This internal trochanter is apparently fused with the fourth trochanter. There is no adductor ridge. The anterior (internal) distal condyle is small and is sharply separated from the posterior by a deep groove. Finally, the shaft of the femur is very short.
Angelosaurus romeri Angelosaurus romeri was named in 1962 by Everett C. Olson and Herbert Barghusen from a partial skeleton and many other isolated elements found in the Omega Quarry in Kingfisher County, Oklahoma. Its remains were originally described as coming from the central part of the Flowerpot Formation. Olson later corrected this attribution by specifying that these remains belong to a tongue of the Chickasha Formation (El Reno Group) whose deposits interfinger in places with those of the Flowerpot, Blaine, and Dog Creek formations. Although Olson and Barghusen give no information on the name of the species, it probably refers to the American paleontologist Alfred Sherwood Romer. Another species of caseid already bore his name, Cotylorhynchus romeri. A. romeri is the better known species of the genus, and the only one for which various parts of the skull are available. It is the smallest species of the genus and differs from its cousins by a moderate development of the massiveness of the limbs and a weak increase in the robustness of the vertebrae, which can only be distinguished with difficulty from those of Cotylorhynchus.