Palæos:		Unit 210: Eureptilia
The Vertebrates		300: Ichthyosauria

Eureptilia: Ichthyosauria

REPTILOMORPHA
|--SYNAPSIDA
`--+--ANAPSIDA
   `--Eureptilia
      |--Protorothyrididae
      `--Diapsida
         |--Araeoscelidans
         `--Neodiapsida
            |--Younginiformes
            `--+--Ichthyosauria 
               |  |--Cymbospondylus 
               |  `--+--Mixosaurus 
               |     `--Merriamosauria
               |        |--Shastasauria
               |        `--Thunnosauria
               `--Sauria
                  |
                  |--LEPIDOSAUROMORPHA
                  `--ARCHOSAUROMORPHA            

Taxa on this Page

1. Cymbospondylus X
2. Ichthyosauria X
3. Merriamosauria X
4. Mixosaurus X
5. Shastasauria X

The Ichthyosaurs

The Ichthyosaurs ("fish lizards") were a successful group of air-breathing Mesozoic marine reptiles that bore an uncanny resemblance to modern day (late Cenozoic) dolphins.  Along with the plesiosaurs, the ichthyosaurs were among the earliest marine reptiles to be discovered, and the two groups had a strong effect on the 19th century Victorian imagination.

Ichthyosaurs were very diverse early in their history (during the Triassic period), where they included some of the largest marine reptiles ever (up to 15 meters - the size of whales).  During the late Triassic they developed their familiar streamlined fish-like form.   The late Triassic and early Jurassic were actually the high points of the Ichthyosaur reign.  The Toarcian turnover affected them heavily and only a single family made it through to the middle Jurassic.  Although the Ichthyosaurs straggled on to the mid Cretaceous (Cenomanian age) they never regained their previous dominance, there place as top marine predator being taken by large pliosaurs.

Despite their dolphin-like form ichthyosaurs lacked the stamina of modern marine mammals.  They were sprinters, not endurance swimmers, with a typically reptilian metabolism.  The reason for their decline was probably tied up a lot with their inability to keep up with the new fast swimming Teleost fish that appeared during the Jurassic.

Being so fish-like in form, ichthyosaurs could not crawl up onto the land to lay their eggs. Instead, the eggs remained in the mother until they hatched.  She then gave birth to live young. Skeletons of baby ichthyosaurs have been found inside the skeletons of adults.  MAK991007.

The most primitive forms, Utatsusaurus and other basal ichthyopterygians, are discussed on a previous page.  Ichthyosauria, strictly speaking, are Cymbospondylus + Ichthyosaurus. Cymbospondylus is traditionally included in the Shastasauria.  However, in the phylogenetic arrangement of  Ryosuke Motani (adopted here) this genus is shown to be much more primitive, even more primitive than the Mixosaurs.  Probably Cymbospondylus belongs in its own family ("Cymbospondylidae").

Cymbospondylus had a very elongate body, and the head, although large taken in isolation, was unusually small relative to the overall body.  As befits its primitive status, Cymbospondylus lacked the familiar dorsal fin on its back and had only a very slight tail kink. It did however have the typical ichthyosaur pointed teeth in the long, beak-like jaws. MAK990620.

Mixosaurus includes several species of small creatures about a meter in length.  They are among the earliest and most primitive of the Ichthyosaurs and were the sister group of the more derived merriamosaurs.  Mixosaurus remained more reptilian than fishlike in appearance, though with flippers and a small fin on the tail.  It also had a small dorsal fin on the back.  Mixosaurus was morphologically intermediate between Cymbospondylus and more advanced Triassic Ichthyosaurs, such as the shastasaurs.  The two lineages, the mixosaurs and Cymbospondylus, actually coexisted for some ten million or so years, along with other, even more basal early ichthyosaurs such as Hupehsuchus and Parvenator.  MAK990620.

The Merriamosauria make up the clade uniting the shastasaurs with the more fish-like, Jurassic ichthyosaurs.  

Shonisaurus popularis a shastasaur.  Image from the popular-science museum in The Hague, The Netherlands

In the shastasaurs we see an example of convergent or parallel evolution with the toothed whales of the Cenozoic oceans; although in the rapid evolutionary tendency to huge size there is a similarity with the baleen whales, which likewise grew from medium-sized to enormous creatures in the space of some five or ten-odd million years (Miocene-Pliocene).

Reaching lengths of 10 to 15 meters, and with bodies that were quite deep and sturdy, the shastasaurs were not only the largest ichthyosaurs, but also among the largest of the marine animals of the Mesozoic.   These giant ichthyosaurs, which were far larger than any of the contemporary Triassic land-living archosauromorphs and early dinosaurs, would have lived a lifestyle much like that of the present day toothed whales, feeding primarily on fish and other marine reptiles.  They might, however, have been slow swimmers, rowing themselves slowly along with their long paddle-like limbs.

Ichthyosauria:  

Range: Early Triassic to Late Cretaceous

Phylogeny: Ichthyopterygia:: Grippidia + *. Cymbospondylus + (Mixosaurus + Merriamosauria)

Characters:  Spindle-shaped body, 0.7-15.0 m; lacrimal participates in nares in most forms, but perhaps not primitive for group [C97]; largest orbit of any animal (up to 26 cm); often substantial sclerotic rings [C97]; postorbital does not participate in upper temporal fenestra [C97]; reduced cheek; strong upper temporal fenestra, but lower fenestra lost except in most basal species; supratemporal probably present primitively, but lost in many derived forms [C97a]; quadratojugal present primitively [C97]; teeth in long groove without separate alveoli in later forms; vertebral centra very short & deeply amphicoelous; neural arches separated from centra and without transverse processes(?); caudal fin in J-K species; manus 1 absent, but 2 neomorphic digits in some advanced forms; polydactyly in some, polyphalangy common and very well-developed; length of phalanges reduced; forelimbs used for steering, not propulsion; pelvic girdle not attached to spine (except Shonisaurus of the Middle Triassic); limbs reduced to steering fins; viviparous; diet varies, but squids probably common. 

Links: Ichthyosaur Page Home (English & Japanese); Ichthyopterygia -- The Dinosauricon; Introduction to the Ichthyosauria; ucmp berkeley museum ab24; Ichthyosaur- Enchanted Learning Software; order61.htm; Ichthyosaur fossil discovery at Charmouth Dorset UK. (after Ryosuke Motani's page, perhaps the most useful site); Kleiner Museumsrundgang (German); Faktasider om marine øgler (Norwegian); Senckenberg Fossile Wirbeltiere der Meere (German); Marine Reptiles (NOT Dinosaurs); ICHTHYOSAURIA; Rolex Awards for Enterprise - Laureates; Water Reptiles of the Past and Present; The Fernleaf: Karen Carr; The Fernleaf: Karen Carr. 

Note: Phylogenetic position still controversial. There has been a good deal of activity recently, both generally at the base of the diapsids and, specifically, in Ichthyosaur phylogeny. In particular the "Ichthyopterygia" name has been dusted off and retooled to cover Hupehsuchus, Grippia, and a few others.

References: Callaway (1997) [C97]; Callaway (1997a) [C97a] . ATW020515

Cymbospondylus: 

Range: Middle Triassic to Late Triassic of North & South America, China & Europe.

Phylogeny: Ichthyosauria: (Mixosaurus + Merriamosauria) + *.

Characters: ~10 m; small head; elongate, longirostrine jaws; relatively small orbits [C97]; tooth implantation thecodont, but with ankylosis at bottom of socket [M97]; replacement teeth lingual of functional teeth [M97]; tooth roots not expanded [M97]; rib articulations as in shastasaurs [S97]; diapophysis makes contact with the anterior margin of centrum in dorsal vertebrae [S97]; caudal "fins" absent; paddles short. 

Links: Cymbospondylus; Dinosaurios; Kleiner Museumsrundgang; Dinolist; Buena Vista Museum. 

References: Callaway (1997) [C97], Motani (1997) [M97]; Sander (1997) [S97]. ATW020515

Mixosaurus: Mixosaurus (= Phalarodon, Sangiorgiosaurus, Contectopalatus all per [C97]) 

Range: Middle Triassic of Europe & China (Tethys Sea), North America, New Zealand. [C97].

Phylogeny: Ichthyosauria:: Merriamosauria + *.

Characters: ~1 m; maxilla large (primitive) [C97]; elongated preorbital skull [C97]; lacrimal excluded from nares [C97]; frontals & nasal roughly equal size (primitive) [C97]; upper temporal fenestra very small [C97]; orbit large [C97]; postorbital cheek region very short [C97]; supratemporal present [C97a]; quadratojugal retained (primitive) [C97]; pterygoid transverse process absent in some specimens [C97]; dentition ankylosed thecodont [C97], but subthecodont anteriorly [M97]; tooth roots not expanded [M97]; rib articulations uncertain [S97]; gastralia present & well-developed, forming tightly-knit plastron  [C97]; dorsal fin present; caudal "fin" little developed; scapula & coracoid very similar, coracoid less symmetrical and having sickle-shaped extension [C97]; short forefin; 4+ (5?) digits on all extremities [S97]; primitive broad, plate-like pubis & ischium [C97]; ilium short & distally broad [C97]; found in deeper waters than, e.g. nothosaurs.

Links: Treasures of the Earth Ltd. Mixosaurus Page.; Buena Vista Museum Of Natural History - Ichthyosaurs In Kern County; Fossilien aus dem Muschelkalk des Kraichgaus (German. Nice life paining & photos of vertebrae); Paleontology and Geology Glossary: Mi; Dinosaurios: Mixosaurus (Spanish); Marine Reptiles: Ichthyosaur: Mixosaurus; Kleiner Museumsrundgang (German, with photo of fossil); DinoNet  - Senhores dos Mares - ICTIOSSAUROS (Portuguese); ??2 - Fishes2 (detailed pictures of poorly prepared specimens); Saurier aus der Mitteltrias des Monte San Giorgio (German, with very good picture of beautifully prepared specimen); forelasning7.pdf (Swedish -- I think);

References: Callaway (1997) [C97]; Callaway (1997a) [C97a]; Motani (1997) [M97]; Sander (1997) [S97]. ATW020516.

Merriamosauria: 

Range: Middle Triassic to Late Cretaceous. Mostly Late Triassic and Jurassic forms.

Characters: tooth implantation aulacodont except in Shonisaurus (Shastasauria), which has unique implantation like Cymbospondylus [M97]; tooth roots not expanded [M97].

Phylogeny: Ichthyosauria:: Mixosaurus + *: Shastasauria + Euichthyosauria.

Links: Eyes. ATW020516. 

References: Motani (1997) [M97].

Shastasauria: (= Shastasauridae) Besanosaurus, Himalayasaurus, Metashastasaurus, Shastasaurus, Shonisaurus.

Range: Early? Triassic to Late Triassic of Japan, China and North America

Phylogeny: Merriamosauria: Euichthyosauria + *.

Characters: medium to quite large in size; teeth isodont [S97]; implantation probably aulacodont in some species [Mo97]; $ parapophysis diminishes and disappears along cervical series [S97]; $ diapophysis elongates along cervical series [S97]; in Shonisaurus, diapophysis may split briefly into two dorsal & ventral, separate articulations over dorsal series [C80], [S97]; diapophysis separates from neural arch midway through dorsal series & moves down centra, becoming rounded instead of elongate [S97] (see also image at Californosaurus); no dorsal fin or semilunate caudal "fin"; less than 4 complete digits on fins [S97] [M97]; humerus very robust & usually as wide as long [M99]; humerus anterior flange reduced, thickened, and may be emarginated in the middle [M99]; humerus anterior flange tilted slightly dorsally [M99]; reduction of posterior humeral shaft proximally and distally [M99]; humerus with articular facet for radius enlarged, twice as wide as ulnar facet [M99]; deltopectoral ridge may run anterodistally or distally from deltopectoral crest, or posterodistally (plesiomorphic state) [M99]; humerus posterodistal tuberosity pronounced [M99]; humeral head may be directed proximodorsally (not proximally) &, if so, scapula glenoid facet tilted ventrally [M99]; movement still basically anguilliform.  

Links: Eyes; New Shastasaurid from Canada; Besanosaurus (Italian); modello del besanosauro; Paleontology and Geology Glossary: Be; EXN Science Wire: Daily news from the world of science; SVPCA 2000 -Abstracts L-M; NRC Research Press: Canadian Journal of Earth Sciences; Darren Naish. A new species of shastasaurus ...; Welcome to Prehistorics Illustrated; Dinosaurier-Interesse - Saurier- Ordnung der Ichthyosauria (German); DinoData Marine Reptiles Ichtyo; ON THE EVOLUTION AND HOMOLOGIES OF ICHTHYOPTERYGIAN FOREFINS; TERRITÓRIO DOS DINOSSAUROS (Portuguese: extensive discussion of Shonisaurus); Re- NEW RHYNCHOSAUR & TEMNOSPONDYLS (summarizes some new phylogenies); Scopriamo insieme.    

References: Camp (1980) [C80], McGowan (1997) [M97]; Motani (1997) [Mo97]; Motani (1999) [M99]; Sander (1997) [S97]. ATW031003

checked ATW031003