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.
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.
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. 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. 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. The holotype (FMNH UR 827) is composed of the pelvis, the right femur, 16 presacral vertebrae, five of which have their associated ribs, 3 sacral, and 4 caudals, two of which are associated with
chevrons. Also known are two sacral vertebrae and 2 presacral vertebrae forming an articulated series (FMNH UR 828), the right side of a pelvis lacking most of the ischium (FMNH UR 844), a right pubis (FMNH UR 845), an interclavicle (FMNH UR 846), two anterior dorsal ribs (FMNH UR 847 and 850), two median dorsal ribs (FMNH UR 848 and 849), a posterior dorsal rib (FMNH UR 851), a cervical rib (FMNH UR 852), four anterior caudal vertebrae (FMNH UR 853), and a fragment of
premaxilla with two teeth (FMNH UR 854). Subsequent excavations in the Omega quarry have uncovered many additional bones, including several previously unknown skeletal elements. This additional material includes a partial skeleton composed of the sacrum, 4 sacral vertebrae, 3 presacral, 18 caudal, and toe bones (FMNH UR 904), a right
scapulocoracoid and a right humerus (FMNH UR 907), 20 presacral vertebrae (FMNH UR 908), a left scapulocoracoid (FMNH UR 909), a poorly preserved tibia (FMNH UR 911, doubtfully attributed to this taxon), 4 bones of the toes (FMNH UR 914), 3 presacral vertebrae with ribs (FMNH UR 916), a large left femur (FMNH UR 917), a fragment of
maxilla with 2 teeth (FMNH UR 926), a
braincase and part of the
palate (FMNH UR 927), a large rib (FMNH UR 928), a
pterygoid (FMNH UR 931), an anterior dorsal rib (FMNH UR 932), a
clavicle (FMNH UR 933), the third presacral rib (FMNH UR 940), an anterior dorsal rib (FMNH UR 941), an isolated tooth (FMNH UR 942), a pair of articulated
lower jaws having retained their tooth rows (FMNH UR 943), 2 median dorsal ribs (FM NH UR 944), several anterior dorsal ribs (FMNH UR 945), 20 caudal vertebrae with ribs on the most anterior (FMNH UR 971), a pubis (FMNH UR 978), ilia and ischia of juvenile individuals (FMNH UR 979), and a left and right pelvis (FMNH UR 980). No complete skull of
Angelosaurus has yet been discovered. The species
A. romeri provides the best picture of the cranial morphology in this genus. The skull is represented by an incomplete premaxilla with two teeth, a fragment of maxilla with two teeth, a braincase, a pterygoid, and a pair of lower jaws bearing an almost complete dentition. All these elements are consistent with each other indicating a fairly short, robust and heavy skull, with strong short teeth. The morphology of the skull in lateral view is not well known, but the premaxilla shows that the
external nares were large, and the fragment of maxilla shows the existence of a large
orbit. These two fragments suggest that the elements of the skull were quite massive compared to those of
Cotylorhynchus of comparable size. The skull of
A. romeri must have looked like a slightly more solid and robust version of the basic pattern of the other derived caseids. The lower jaws show that the dentary is short and heavy with a strong
symphysis. There is a moderately strong and wide shelf at the ventral margin of the symphysis. The teeth of
A. romeri differs very clearly from that of other caseids by their bulbous morphology with very short and wide crowns. The knowledge of the upper dentition is limited to four teeth, two being carried by an incomplete premaxilla and two others coming from a fragment of maxilla. The first premaxillary tooth is very different from that of other caseids genera, in being thick at the base and tapering to a blunt termination of the crown. The second tooth is similar in shape, but much smaller. Neither have cusps. The two maxillary teeth have a general structure similar to that of other derived caseids by having a base with a circular section, followed at the mid-height of the tooth by an enlargement of the crown, which then ends in a
labio-
lingual thinning giving to the upper end of the crown a spatulate morphology. However, they differ from those of other caseids in being shorter and blunter, the crown ending in a flattened edge bearing five small longitudinal cusps. The pair of lower jaws retains a much more complete dentition. The three anterior teeth are very distinctive. They are very short and blunt with a very broad middle part, and lack cusps. The more posterior teeth are short and resemble the upper ones quite closely. The five cusps on the flattened dorsal margin of the crowns are very distinct. There are 11 teeth on the right side and 12 on the left, counting as present those which are being replaced. Apparently 12 is the correct number, the lowest number of teeth among the caseids from
North America. Only the
Russian genus Ennatosaurus has less with only 10 teeth per half jaw. The anterior teeth of the mandibles of
A. romeri are relatively worn. Some of the most posterior teeth are worn and some are not. The cusps disappear quickly with wear and the final stages produce a button-shaped crown whose surface is formed by the bulbous, rounded area of the central part of the tooth. The robustness of these teeth, coupled with the heavy wear they show, indicate that the diet of
A. romeri must have included plants that are tougher than those on which most other herbivorous caseids fed. The postcranial skeleton of
A. romeri differs mainly from that of the other two species of the genus in characteristics of the pelvis, femur, and humerus. The humerus, although large and heavy with strongly developed ridges and processes, differs from that of
A. dolani by its less massive proportions and the presence of a distinct though relatively short shaft. The pelvis is characterized by a pubis and an ischium forming a large, somewhat saddle-shaped plate with a strongly ossified median symphysis whereas it was cartilaginous in
A. dolani. In the holotype, the sacrum has only three vertebrae with three associated sacral ribs. However, the paratype FMNH UR 904 has four sacral ribs indicating the presence of four vertebrae in the sacrum of this specimen. It is likely that four is the normal number of sacral vertebrae in this species, as the sacrum of the holotype is in fact incomplete. In the holotype of
A. romeri, the three sacral vertebrae are fused while in
A. dolani only the first two are fused. The sacral ribs are blade-shaped and are almost completely separated from each other at the joint with the ilium. At most, a slight fusion is observed between the second and the third sacral rib. The femur is relatively small and strongly ossified. It is characterized by the position of the internal trochanter, which is located approximately halfway between the proximal and distal limits of the intertrochanteric fossa. The fourth trochanter is well developed and rises slightly above the ridge of the adductor. The anterior distal condyle is shallow and flat and is well separated from the posterior condyle. The latter is larger and bears a well-defined surface for articulation with the fibula. The
popliteal space is prominent. ==Stratigraphic distribution==