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.

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.

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.

[3][2][1] 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.

[4] 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 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.

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.

[3] 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.

The ilium, short and broad, shows some indication of the dorsal elevation characteristic of caseids, but the apex appears incomplete, possibly cartilaginous.

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.

[3] 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.

[2] 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.

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 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 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.

[9][10][11] However, Michel Laurin and Robert W. Hook argued that the fusuline marine intercalation cited above does not belong to the San Angelo Formation in which it was mistakenly included, and cannot be used to date the latter.

[12] Another possible clue of a Guadalupian age is the presence in the Chickasha fauna of the largest known representative of the varanopid, Watongia, whose large dimensions would be the sign of a late increase in size in the evolution of this clade, which would have taken the place of top predator in the absence of Sphenacodontidae.

[13] However, the presence of the genus Macroleter in both Russia and Oklahoma does not guarantee that the Mezen assemblage and the Chickasha formation are (sub) contemporary because various Permian tetrapods genera had a wide temporal distribution, such Dimetrodon and Diplocaulus.

[15] A long string of microcontinents, grouped under the name of Cimmeria, divided the Tethys in two : the Paleo-Tethys in the north, and the Neo-Tethys in the south.

[16] The San Angelo and Chickasha formations correspond mainly to fluvial and aeolian sediments deposited in a vast deltaic plain dotted with lakes and lagoons.

The rivers ending in the delta came from modest reliefs located further east and corresponding to the ancestral uplifts of the Ouachita, Arbuckle and Wichita mountains.

The central part of the formation consists mainly of red mudstones corresponding to clayey and silty mud deposited on the coastal plains during periodic flooding episodes.

However, in Oklahoma, strata equivalent to the San Angelo Formation, which were also considered fluvio-deltaic and coastal deposits, have been reinterpreted as being of aeolian origin.

[19] The base of the upper San Angelo Formation is characterized by the absence of the genus Angelosaurus and the abundance of Cotylorhynchus hanckoki, which is associated, among others, with the caseid Caseopsis agilis, the sphenacodontid Dimetrodon angelensis, and the captorhinids Rothianiscus multidonta, and Kahneria seltina.

The sediments that compose it are varied and include red shales, sandstones, mudstones, conglomerates, and evaporites, deposited in floodplains and channels bordering the sea and coastal lagoons.

They correspond to deposits of an old channel about 1.50 m thick and 4.6 m wide where the skeletons of Angelosaurus romeri have accumulated, but also those of a second caseid, Cotylorhynchus bransoni, and those of the captorhinid Rothianiscus robustus.

[5][7] Elsewhere in this formation are known the Xenacanthiform Orthacanthus, the Nectridea Diplocaulus,[8] the temnospondyle Dissorophidae Nooxobeia,[20] the pareiasauromorpha Nycteroleteridae Macroleter,[12][21] and the Varanopidae Varanodon and Watongia.