EUREPTILIA |--ARCHOSAUROMORPHA | Lepidosauromorpha |--Sauropterygia | |--Placodontia | `--+--Pachypleurosauridae | `--+--Nothosauridae | `--Plesiosauria | |--Pliosauroidea | `--Plesiosauroidea | |--Elasmosauridae | `--Cryptocleidoidea `--Lepidosauriformes |--RHYNCHOCEPHALIA `--SQUAMATA
The Sauropterygia or "lizard flippers" are an important group which includes a wide range of aquatic lizard-like, crocodile-like, long-necked "Loch Ness monster"-like, and large-headed killer whale-like aquatic reptiles. Among the great prehistoric success stories, they first appeared at (or just before) the very start of the Mesozoic and flourished right until the end of the era. Yet as with the Ichthyosauria, they are so specialized their evolutionary relationships are still somewhat obscure.
Earlier forms (pachypleurosaurs) were mostly small amphibious lizard-like animals with slender, elongated limbs and small heads, that lived in estuarine and near-shore environments. They quickly increased to several meters in size and ventured further out to sea, although still retaining amphibious traits and sinuous lizard-like form (nothosaurs). By the Middle Triassic there were a number of divergent lineages. In addition to the line leading to the successful Jurassic plesiosaurs, there were several divergent branches of early sauropterygians, including pachypleurosaurs, several lines of nothosaurs, and the transitional pistosaurs. These early divergent branches show successive stages in adaptation to oceanic life.
The Carnian extinction took out most of these early Sauropterygia, but the one surviving group, the plesiosaurs, underwent a tremendous evolutionary radiation around the very end of the Triassic period and the very start of the Jurassic, diverging rapidly into variously lineages in a typical example of a macroevolutionary adaptive radiation. It is not yet clear whether this event coincided with and was caused by the full development of the plesiosaur locomotor plan, or whether it was simply opportunistic factors, such as the mass extinction of potential competitors in the Rhaetian turnover, that allowed the plesiosaurs to exploit the potential of a previously evolved locomotor plan (see below). In any case by the early Jurassic there were a diverse range of long- medium- and short-necked forms, some with small heads others with large. These were all open ocean types, completely adapted for marine life, and usually 3 metres or more in length. The two principal morphs were the plesiosaurs (long necked "Loch Ness monster" types) and pliosaurs (reptilian killer whales). However, the evolutionary relationships of the Plesiosauria are complex, and it appears that long-necked and short-necked types occurred in a number of lineages.
The Sauropterygia are united by unique derived features features of the skull and shoulder girdle. As Carroll & Gaskill (1985) note, the fundamental relations of the shoulder girdle elements are altered in the Sauropterygia. The figure illustrates the differences in shoulder structure between a basal amniote (adapted from Carroll, 1988) and the basal sauropterygian, Pachypleurosaurus (adapted from Carroll & Gaskill 1985). In the terrestrial amniote, force is directed by the humerus against the plate-like scapulocoracoid, which dissipates the force over a wide area and is, in turn, supported by extensive contact with the dermal bones of the clavicle and interclavicle. In the sauropterygian shoulder, a direct, medially directed force would probably cause the shoulder girdle to collapse. However, the anterior and posterior strains caused by paddling and steering simply bind the girdle more strongly together by forcing the interclavicles and coracoids against their opposite numbers. The fundamental re-engineering of the shoulder girdles suggests that the strains involved were considerable, i.e., that the sauropterygians were fast, agile swimmers.
A remarkably similar arrangement was evolved in the pelvic girdle of derived sauropterygians, e.g. plesiosaurs, and both girdles were connected by a net of gastralia (stomach ribs) probably reinforced by stout ligaments. This arrangement and the problems of plesiosaur locomotion, will be covered on a subsequent page. MAK991206, ATW020627.
Lepidosauromorpha: definition: lizards > buzzards
Range: from the Late Permian.
Phylogeny: Sauria: Archosauromorpha + *: Sauropterygia + Lepidosauriformes.
Characters: retroarticular process large; teeth absent on lateral pterygoid flanges; parasphenoid teeth absent; postfrontal borders upper fenestra; supratemporal absent; quadrate wide in posterior view, forming conch; quadrate bowed in lateral view; quadrate foramen absent; intercentra absent from trunk vertebrae; no bipedal specializations; rigid pectoral girdle; large sternum; overlapping scales.
Links: Lepidosauromorpha -- (lizards > birds); Reptilian Systematics; S. Karger AG, Basel - Brain, Behavior and Evolution Abstract 198, Vol. 50, No. 4, 1997; Reptilia.html; Lepidosauromorpha -- The Dinosauricon; PhyloCode Article 13; Diapsida.htm; serp_origem.pdf (Portuguese, English); Acrocoracoids & reptile phylogeny; Lepidosauromorpha (ToL: still nothing here); Untitled (Spanish: short, but useful discussion of anatomical changes in locomotion). ATW030715.
Sauropterygia: definition variable in the literature. Used here as placodonts + plesiosaurs.
Range: Late Permian to Late Cretaceous.
Phylogeny: Lepidosauromorpha: Lepidosauriformes + *: Placodontia + (Corosaurus + (Pachypleurosauridae + Eusauropterygia)).
Characters: Postorbital skull elongated; upper fenestra enlarged; "euryapsid" (lower temporal arch lost); elongate neck (>7 cervicals); curved humerus; radius equal length to ulna.
Links: Sauropterygia -- The Dinosauricon; Sauropterygia; Turtle Origins; Literature - Sauropterygia; muschelkalk lipperland (German: never saw this site before. Very nice.); ???????? (Japanese); Untitled Document; classification; Literature - Sauropterygia; Sauropterygia after Rieppel, 1997, 1998ab and Rieppel & Hagdorn, 1997; Fossil Room; some additions to Re: MOSASAURS AND PLIOSAURS; MOSASAURS AND PLIOSAURS; muschelkalk lipperland (German). ATW021013.
Placodontia: Placodus, Henodus.
Range: Early Triassic to Late Triassic of Europe, North Africa & the Middle East.
Phylogeny: Sauropterygia: (Corosaurus + (Pachypleurosauridae + Eusauropterygia)) + *.
Characters: Marine or amphibious, short (1-3m), very-wide walrus-like bodies, relatively unspecialized for aquatic life. Massive consolidated skull; large flat teeth or plates; buck-toothed (procumbent) premaxillary teeth; neck short; pachyostosis; centra deeply amphiocoelous; long neural & transverse processes; ribs massive, giving body rather square outline; tail short; limbs and pectoral girdles poorly ossified; pelvis plate-like; dermal plate armor common, some with epidermal scutes; molluscivores.
Links: ichthyosaur.html; Lecture 9 - Triassic: Moenkopi, Karroo, Ischigualasto; ; Paleontology and Geology Glossary: Pl to Po; Senckenberg Fossile Wirbeltiere der Meere; PLACODONTS; 1- Microstructures; Saurier von Lörrach (German).
Note: related to turtles, according to some. ATW010311.