Subclass Synaptosauria

Order Trilophosauria
Large (up to 2.5 m in height) Triassic herbivorous reptiles.
The order includes a single family, Trilophosauridae Gregory, 1945, whose representatives are characterized by the absence of anterior teeth, replaced by a distinctive keratinous beak. Only one coracoid is present. The pelvis is broad, the hindlimbs are longer than the forelimbs, and the foot has five digits.
Seven genera are recognized:
 Trilophosaurus Case, 1928;
Teraterpeton Sues, 2003;
Anisodontosaurus Welles, 1947;
Variodens Robinson,1957;
Coelodontognathus Otchev, 1967;
Doniceps Otschev, 1968;
Vitalia Ivachnenko, 1973.
Representatives of the first two genera are the most typical:

Petrolacosaurus kansensis Lane, 1845 (Araeoscelomorpha)           Pistosaurus grandaevus Meyer,1839 (Sauropterygiomorpha)
The composition of the superorders is as follows:

Superorder Araeoscelomorpha
Small primitive synaptosaurs with a tall, elongated trunk. The skull retains supratemporal, tabular, and postparietal bones. Vertebrae are deeply amphicoelous with intercentra. Two coracoids are sometimes present. Limbs are unspecialized and long.
A characteristic feature of the Order Araeoscelidia is the elongated neck and skull.
Petrolacosaurus and Araeoscelis from the Early Permian and Carboniferous are among the earliest diapsids. These are similar lizard-like forms that feed on insects and have a well-developed dentition.

Petrolacosaurus kansensis Lane, 1945
As an adaptation for a more powerful bite (to crack the hard exoskeletons of insects), Araeoscelis has only posterior tooth rows, which are larger, and the lower temporal fenestra is also closed to strengthen the jaws.

Araeoscelis gracilis  (Williston 1910)
More derived are small gliding species of the family Weigeltisauridae, which have specialized ribs along the sides of the body, most likely serving to support the flight membrane. A similar adaptation is found in modern flying dragons (Draco L., 1758).

The forelimbs and hindlimbs are of equal length, phalanges are long with short, powerful claws. The skull, like that of Araeoscelis, has a single temporal fenestra.
Such forms are characteristic of all genera of the family:

Rautiania Bulanov et Sennikov , 2006 - Late Permian of Eastern Europe;
as well as for Wapitisaurus Brinkmann, 1988 from the Early Triassic of North America.
Additionally, the following are assigned to araeoscelidians: Lower Permian representatives of the genus Zarcazaurus (Brinkman et al., 1984), which possibly secondarily lost the lower temporal fenestra; small long-necked forms of the genera Kadaliosaurus (Kuhn., 1969) and Aphelosaurus (Evans, 1988); and some of the first reptiles to return to water – the swimming Spioequalis (deBraga & Reisz, 1995), as evidenced by anatomical modifications: elongated neural and haemal processes of the caudal vertebrae.

Order Trilophosauria
Large (up to 2.5 m in height) Triassic herbivorous reptiles.
The order includes a single family, Trilophosauridae Gregory, 1945, whose representatives are characterized by the absence of anterior teeth, replaced by a distinctive keratinous beak. Only one coracoid is present. The pelvis is broad, the hindlimbs are longer than the forelimbs, and the foot has five digits.
Seven genera are recognized:
 Trilophosaurus Case, 1928;
Teraterpeton Sues, 2003;
Anisodontosaurus Welles, 1947;
Variodens Robinson,1957;
Coelodontognathus Otchev, 1967;
Doniceps Otschev, 1968;
Vitalia Ivachnenko, 1973.
Representatives of the first two genera are the most typical:

.Order of specialized large aquatic and semi-aquatic (marine) Mesozoic reptiles, with a massive, short, and flattened trunk and an elongated neck. The supratemporal, tabular, and postparietal bones are reduced in the skull, and the palate is closed. Vertebrae are platycoelous or weakly amphicoelous. Intercentra are absent. A single coracoid is developed. Limbs are modified into paddles. Phalanges are elongated, and hyperphalangy is characteristic.

Order Sauropterygia
Piscivorous reptiles inhabiting lagoons and coastal marine zones, known from the Lower Triassic of Western Europe and China, and surviving until the terminal Cretaceous.
Among the most unique apomorphies are:
-Scapulae in the pectoral girdle are positioned superficially relative to the clavicles (the opposite of other tetrapods), with their lower processes forming a complete arch in front of the coracoids, which can connect to them via the median process of the scapulae. The massive coracoids are joined by an extensive symphysis (Carroll, Gaskill, 1991; Rieppel, 1987). This complex of modifications is associated with the loss of the function of supporting the head and anterior body in space during aquatic life.
-Complex joints form between platycoelous vertebrae: zygosphenes and zygantra, which provide sauropterygians with greater mobility and body flexibility.

Pectoral girdle of the primitive sauropterygian Pachypleurosaurus edwardsi (Middle Triassic of Switzerland, after Carrol, Gaskill, 1985).
The suborders Sauropterygia – primitive Eosauropterygia and more specialized Plesiosauria – are well distinguished by the degree of their adaptation to aquatic life.

Phylogeny of Sauropterygia after Rieppel, 1997
Eosauropterygia includes weakly specialized pistosaurs, pachypleurosaurs with strongly expressed pachyostosis of the ribs and vertebrae, as well as highly specialized nothosaurs (Rieppel, 1994).
Pistosaurs have an elongated snout, very narrow parietal bones, and frontal bones that form posterolateral processes extending to the edge of the temporal fenestra. The interpterygoid fossa is also retained, and the posterior temporal fenestrae are well developed. Teeth are sparsely spaced, and the anterior lower jaw teeth are modified into canines. Vertebrae are amphicoelous, with three sacral vertebrae, and the sacral ribs are expanded at the ends.
Typical representative: Pistosaurus longaevus (Huene, 1948)

Among pachypleurosaurs, the small Middle Triassic Keichousaurus hui Young, 1958 is best known, in which significant growth of the humerus is noted, and sexual dimorphism is also expressed in its structure. This modification of the pectoral girdle indicates a more primitive swimming style – rowing type, rather than "flight".

Keichousaurus hui Young, 1958
Nothosaurs are larger reptiles than pachypleurosaurs. They have large teeth (caniniform teeth may develop). Vertebrae are platycoelous, and the neck is elongated. The postorbital region of the skull is somewhat elongated, the jugal bone enters the anterior base of the temporal arch, and the parietal bones are sharply narrowed, which brings nothosaurs closer to higher sauropterygians. The superfamily is characterized by the presence of various morphological forms.
Simosaurus from the Middle Triassic of Western Europe occupies the most isolated position, with the following unique features:
-T-shaped interclavicle;
-snout is relatively short and wide, with no prenasal constriction;
-parietal bones are relatively wide but fuse with age;
-teeth are closely spaced, no caniniform forms are observed.

Simosaurus gaillardoti Meyer, 1842
Advanced Plesiosauria appear in the Jurassic and reach their greatest diversity in the Jurassic and Cretaceous of Western Europe and North America. They are usually inhabitants of marine water bodies, but are also found in freshwater bodies. Length exceeds 3 m, in some cases reaching 15–16 m. The tail is shortened, and due to the development of powerful abdominal ribs occupying all the space between the pectoral and pelvic girdles, the trunk is relatively immobile.
This unusual structure led D. Sily (Seeley, 1892) to compare these marine lizards to "a snake threaded through a turtle shell", as is clearly seen in Rhomaleosaurus cramptoni.

Rhomaleosaurus cramptoni (Tate and Blake, 1863)
Plesiosaurs swam using both limbs, without lateral body bends, via simultaneous vertical strokes, although the hindlimbs could also act as rudders; this type of swimming is called "underwater flight". Morphologically, this is associated with the expansion and flattening of the ventral parts of the limb girdles.

Swimming type "underwater flight" in Plesiosauria.
They include the superfamilies Pilosauroidea and Plesiosauroidea, which have opposite morphological characteristics, indicating different pathways of adaptation to the same conditions.

Pliosauroidea are characterized by a large head (up to 3 m) and a relatively short neck (11–30 vertebrae). Pilosaurus irgisensis (Novozhilov, 1948) is one of the most well-known Jurassic representatives found in Russia.

Pilosaurus irgisensis (Novozhilov, 1948)
The longest-necked among plesiosaurs are representatives of the genus Elasmosaurus from the Upper Campanian and Lower Maastrichtian of North America, which occurred not only in seas but also in freshwater lakes. The neck was 14 times longer than the head, and the number of vertebrae reached 76. In the jaws, some teeth are modified into canines.

Elasmosaurus (Coup, 1868)
It is believed that plesiosaurs became the prototype for the Loch Ness Monster myth.

Order Placodontia
Placodontia are usually classified as part of sauropterygians at the rank of order. They differ radically from typical sauropterygians in cheek structure – with an unusually massive quadratojugal bone that may extend to the edge of the temporal fenestra.
Coastal Triassic reptiles inhabiting primarily the region of the Tethys Sea (the precursor to the Mediterranean Sea). They get their name from their extremely flattened and large teeth, especially the large ones on the palate. Body length without tail is up to 1–1.5 m (sometimes up to 2.5 m). The skull is massive, akinetic, with a strengthened connection between the skull roof and the palate. Like sauropterygians, placodonts have a single temporal fenestra, but the lower temporal region, which in sauropterygians usually has a more or less extensive notch, is closed in placodonts (as in primitive ichthyosaurs) by large, massive quadratojugal bones that cover the entire cheek region. The snout is short, but in some armored forms it is elongated into a short proboscis. Vertebrae are deeply amphicoelous, trunk ribs are unicephalous, the clavicle is massive, the ilium is shortened, digits show no signs of hyperphalangy, and some representatives have claws, indicating retained ability to move on land.
There are few deep adaptations to swimming, and those that exist perform this function insufficiently effectively.
Placodonts are usually divided into two suborders: Placodontoidea (dugong-like) and Cyamodontoidea (armored "turtle-like").
Unarmored Placodontoidea (more primitive) have a heavy build with an elongated tail (40–50 vertebrae) and well-developed pentadactyl limbs. Long, forward-projecting massive incisors indicate adaptation to tearing off attached mollusks and brachiopods. The order is represented by two families: Paraplacodus (7 teeth in the maxilla) and Placodus (up to 5).

Placodus gigas (Agasis, 1833)
Armored Cyamodontoidea have greater morphological diversity than placodontoids. The body is sharply flattened and covered with a carapace formed from expanded ribs and osteodermal plates, which does not cover the pectoral girdle as in turtles. The pelvic part of the dorsal carapace is usually separated from the main anterior part, except for Placochelys with a solid carapace. The carapace is covered with horny scales forming longitudinal keels, which apparently increase the hydrodynamic qualities of these placodonts. The snout is shortened, and incisors are retained in some representatives. The snout forms a proboscis-like structure: most elongated in the Upper Triassic Placochelys and Psephoderma, absent in Henodus.

Henodus helyops (von Huene, 1936)
About five families are recognized within the suborder: Cyamodontidae, Macroplacidae, Protenodontosuridae, Placochelyidae, Henodontidae.
Most researchers believe that Cyamodontidae are the most primitive, with the maxillary tooth row reduced to two teeth, and in the Chinese Sinocyamodus, the dorsal carapace does not cover the limb girdles. In all others, except Protenodontosuridae, incisors are absent. Macroplacidae and Protenodontosuridae are considered close to Placochelyidae and distant from Cyamodontidae. Placochelyidae have very narrow palatine processes, while Macroplata has expanded posterior palatine teeth. Henodus also lacks teeth and does not form a proboscis, and the snout is strongly transversely expanded.

Cladogram of placodont phylogeny (after [Rieppel, 2000b])
Despite their relatively low diversity, placodonts vividly illustrate the abundance of parallelisms in the evolution of their skull and teeth.