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Unit 210: Eureptilia |
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The Vertebrates |
200: Neodiapsida |
Abbreviated CladogramREPTILOMORPHA |--SYNAPSIDA `--+--ANAPSIDA `--Eureptilia |--Protorothyrididae `--Diapsida |--Araeoscelidans `--Neodiapsida |--Claudiosaurus `--+--Coelurosauravidae `--+--Younginiformes `--+--Ichthyopterygia | |--Utatsusaurus | `--+--Grippidia | `--Ichthyosauria | |--Shastasauria | `--Thunnosauria `--Sauria |--LEPIDOSAUROMORPHA `--ARCHOSAUROMORPHA |
Contents210.000 Overview |
Claudiosaurus was a medium-sized (60 cm) low-slung reptile with a small head, fairly long neck, and a long tail. It lived by the shores of lakes or sheltered seas along a pair of rift valleys in what is now Madagascar in the Late Permian -- just before whatever event(s) it was that killed off most life forms on Earth at the end of the Paleozoic. As discussed elsewhere, many of the basal diapsids are rather scrappy; and this has led to some rather odd phylogenies. Claudiosaurus is an exception. It is well known from a number of good specimens. Even better, it seems rather clear that, as Carroll (1981) pointed out in his original description, Claudiosaurus is closely related to a group of neodiapsids now known as the Younginiforms.
Claudiosaurus
was almost certainly amphibious. Caldwell
(1994). This is not really obvious from a first glance at the skeleton, but
has been quite convincingly shown by Carroll and Caldwell. Its most enigmatic
feature is the skull. As in snakes, there seems almost nothing holding the skull
together. The lower temporal bar is completely absent, the post-orbital skull is
full of large holes, and the pre-orbital skull was weakly bound and may not even
be completely ossified. It is hard to make sense of all the data, but at least
there is no lack of data to make sense of.
In spite of this wealth of information, it has proven no easier to get an exact placement for Claudiosaurus than it has been for Paliguana. Most of the confusion may be due, in addition to the inclusion of taphonomic detritus, to inconsistencies in nomenclature. The most logical definitional benchmark in this area is clearly that of Jacques Gauthier. The benchmark clade is the crown group Sauria = the last common ancestor of birds and snakes. Then Lepidosauromorpha = all Saurians closer to snakes than birds; and Archosauromorpha = all Saurians closer to birds than snakes. "Closer" in this parlance means "having a more recent common ancestor with." We can't use a "diapsid" benchmark because the diapsid condition is a physical character. It has no phylogenetic meaning. We can't really use Reptilia (= turtles + turtledoves) because of the continuing concern that turtles might turn out to be inside Sauria after all. Thus, if our discussions are to be tied to reality at all, it is quite important that the Saurian anchor be firmly in place. In spite of this logical necessity, many phylogenies -- including, until recently, these Notes -- have allowed each member of this crown-stem triad (see Sereno (1998)) to live a wild, free life of its own. See, e.g. Caldwell (1996); Motani et al. (1998) and (to a far lesser extent) deBraga & Rieppel (1997).
Interestingly, when we apply a consistent nomenclature and simply remove the uncertain bits and pieces, everyone's cladogram looks like this:
"Diapsida" (any arbitrary definition)
|--Araeoscelidia
`--+--Claudiosaurus
`--+--Younginiformes
`--+--Ichthyosauria
`--SAURIA
|--Archosauromorpha
`--Lepidosauromorpha
|--Sauropterygia
Lepidosauriformes
The only genuinely unsettled points are (a) whether turtles are in this mix somewhere and (b) the position of the Coelurosauravidae. The latter have variously been placed between Claudiosaurus and the Younginiformes, or as a sister to Ichthyosauria. (I have chosen the former alternative for no particular reason.) In truth, the original results of Caldwell (1994) are not quite consistent with this scheme, but Motani et al. (1998) do obtain this pattern with the same data set when they recode Ichthyosauria to incorporate more recent information. ATW010415
Neodiapsida: defined: younginiforms + crown diapsids [C97a], or Sauria > Araeoscelidia ToL
Range: from the Middle Permian.
Phylogeny: Diapsida: Araeoscelidans + *: Claudiosaurus + (Coelurosauravidae + (Younginiformes + (Ichthyopterygia + Sauria))).
Characters: lacrimal reduced or absent [R89]; descending flange of parietal participates in UTF [R89]; quadrate laterally exposed (not covered by squamosal in lateral view) [R89]; quadrate embayed posteriorly [R89]; retroarticular process present [R89]; caniniform teeth absent [R89]; parasphenoid without teeth [R89, dubitante]; olecranon absent [R89]; femur slender & sigmoid [R89]; femoral distal condyles not projecting markedly beyond shaft [R89]; $ femoral ventral ridge system reduced (without prominent ventral adductor crest for, e.g. m. caudofemoralis); $ proximal carpals and tarsals small.
Links: Neodiapsida; Autapomorphies of Diapsid Clades.
References: Callaway (1997a) [C97a]; Rieppel (1989) [R89]. ATW070113.
Range: Late Permian of Madagascar.
Phylogeny: Neodiapsida: (Coelurosauravidae + (Younginiformes + (Ichthyopterygia + Sauria))) + *.
Characters: About 60cm. Head small; ~50 small, sharp marginal teeth (none procumbent); shagreen of denticles on palatal bones; Meckelian canal partly open; transverse pterygoid flange undifferentiated and weak; interpterygoid vacuities reduced or absent; small suborbital fenestra present; anterior skull poorly known; premaxillae with long, thin nasal process; nasals large; prefrontal (?) large; lacrimal is an odd, oval element probably excluded from orbit; orbit large; large upper temporal fenestra; lower temporal bar and posterior process of jugal absent; quadrate strongly supported by pterygoid (unlike squamates); stapes unknown (?); no stapedial foramen on quadrate; posterior margin of parietal sculpted and may have supported postparietals, tabulars, etc.; long neck (from posterior displacement of pectoral girdle?), with 8 cervical vertebrae; 25 presacral vertebrae, all except atlas with ribs; dorsal centra ~twice as long as wide; intercentra present; neural arches in trunk well-developed and articulate; rib cage probably complete (in cartilage); ribs not pachyostotic; 2 sacral vertebrae with partial incorporation of a 3rd; tail slender & not specialized for aquatic locomotion; caudal vertebrae not specialized; sternum not ossified but clearly present; scapulocoracoid ossified as single plate-like units, forming complete plate with large interclavicle; glenoid quite far posterior and angled posteriorly; digit 3 is longest on manus; ilium elongated; no thyroid fenestra; calcaneum with distinct lateral flange (gastrocnemius?); some indication that proximal and distal tarsals were partially locked together; lateral centrale reaching fourth distal tarsal. Amphibious?
Links: Icarito Interactivo - dinosaurios (Spanish); Autapomorphies of diapsid clades.
References: Carroll (1981).
Note: determinate growth of long bones, but ossification of distal limb and some cranial bones continued throughout life. Carroll (1981) makes a fairly good case that Claudiosaurus was a derived relative of Thadeosaurus, now classified as a younginiform.
Coelurosauravidae: Drepanosaurus, Coelurosauravis, Megalancosaurus. Garbage taxon (?) of stem Diapsida including various, possibly related, arboreal or gliding forms.
Range: Late Permian to Early Triassic
Phylogeny: Neodiapsida:: (Younginiformes + (Ichthyopterygia + Sauria)) + *.
Characters: Coelurosauravis: elongate, horizontal ribs – gliding membrane like Draco? Sues (1997) says not ribs: different support for gliding membrane. Loss of lower temporal bar, elaborate squamosal frill. Megalancosaurus had (1) osteological correlates for the existence of a birdlike prepatagial membrane; (2) elbow joints that, similar to many birds, "locked" at about 165 degrees extension (i.e., about 25 degree short of full extension); (3) fusion of dorsal vertebrae into a somewhat birdlike/pterosaur-like notarium; (4) lightened, externally "hollowed-out" long bones; (5) extremely long forelimbs, and possibly other flight or gliding adaptations. (Ruben, J. (2000) pers. comm on dinosaur listserver 4/6/00).
Younginiformes: Acerosodontosaurus, Youngina, Hovasaurus, Tangasaurus. Medium-sized lizard-like, some forms aquatic. Possibly paraphyletic as ancestral to lepidosaurs.
Range: Late Permian to Early Triassic.
Phylogeny: Neodiapsida::: (Ichthyopterygia + Sauria) + *.
Characters: Aquatic forms quite similar to terrestrial with laterally flattened tails. Single coracoid; tabular absent; quadrate not embayed; vertebrae with relatively tall neural spines; stapes large; sternum and vertebral transverse processes present; cleithrum present; olecranon process of ulna absent; primitive tarsus, pes; $ lateral centrale not in contact with 4th distal carpal; gastroliths.
Links: Diapsids Phylogeny; Autapomorphies of diapsid clades. ATW010403.
Range: Early Triassic to Late Cretaceous
Phylogeny: Neodiapsida:::: Sauria + *: Utatsusaurus + (Grippidia + Ichthyosauria)
Characters: euryapsid skull; primitive tooth implantation probably subthecodont [M97]; replacement teeth outside pulp cavity [M97]; uniquely down-turned vertebral column that enters the lower lobe of its caudal fin; flippers for limb; live birth.
Links: Dinosauricon; JVP; Notes; CVA.
Image: Utatsusaurus from [M+98].
References: Motani (1997) [M97]; Motani et al. (1998) (M+98) (image). ATW020518.
Range: Early Triassic of Japan & Canada
Phylogeny: Ichthyopterygia: (Grippidia + Ichthyosauria) + *.
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| Utatsusaurus from Fossil Gallery. Note that the forelimbs show a characteristic ichthyosaur pattern of polyphalangy, but are very small. The dorsal vertebrae are narrow and relatively long. |
Characters: 1.4 - 3.0 m "lizard with flippers"; lower temporal fenestra absent [M+98]; tooth implantation pleurodont anteriorly, shifting to subthecodont [M97]; replacement teeth are lingodistal to functional tooth [M97]; tooth roots expanded [M97]; body relatively long and thin; about 40 small diameter, cylindrical vertebrae (undulatory swimming) [M+98]; 2 well-defined sacral (but transversely thin and weakly articulated with ilium) ribs with distal expansion [M+98]; sacral ribs not fused to vertebrae [M+98]; small flippers; humerus & femur of equal length [M+98]; hindlimb larger than forelimb [M+98]; restricted to continental shelf
Links: Fish-lizards; Fossil Gallery; JVP; Utatsusaurus; Utatsusaurus (Japanese); Ordnung der Ichthyosauria (German -- probably an excellent page); Tohoku MNH; Yamagata (Japanese: excellent photo); Utatsusaurus ??????? ???? (Japanese models); ??????? (Korean); 05.20.98 - UC Berkeley study establishes that extinct ... (press release); The Journal of Vertebrate Paleontology (abstract); Scientific American- Feature Article- Rulers of the Jurassic Seas ... (also in Polish); 07.15.98 - "Fish-Lizards" Turn Out to Be More Lizard than Fish (press release); The Journal of Vertebrate Paleontology (another abstract); ?? (fossil); Dan Varner Paleo-Life Art (Dan's image of Utatsusaurus); ???.
References: Motani (1997) [M97]; Motani et al. (1998) (M+98) ATW030203.
Grippidia: Chaohusaurus (= Chensaurus), Grippia
Range: Early Triassic of Greenland, China, etc.
Phylogeny: Ichthyopterygia:: Ichthyosauria + *.
Characters: <1 m?; lacrimal excluded from nares [C97]; relatively large upper temporal fenestra [C97]; postorbital participates in upper temporal fenestra (primitive) [C97]; tooth implantation subthecodont [M97]; replacement teeth lingodistal [M97]; tooth roots expanded [M97]; dentition, vertebral number & form similar to Utatsusaurus; humerus longer than femur (as in all other ichthyopterygians) [M+98].
References: Callaway (1997) [C97]; Motani (1997) [M97]; Motani et al. (1998) [M+98]. ATW021024
checked ATW031014
