Ichthyosauria: their diversity, distribution, and phylogeny

Kurzfassung

Die Ichthyosaurier waren die am stärksten an das Wasserleben angepaßten mesozoischen Reptilien. Die ältesten Ichthyosauier aus der oberen Untertrias (Spath) zeigen schon einen besonderen Satz von Merkmalen (sehr große Augen, verlängerte Schnauze, tief amphicoele Wirbel, zu Flossen umgebildete Beine), die mit ihrer vollaquatischen Lebensweise zusammenhängen, und sie waren vermutlich schon nicht mehr zum Landgang fähig. Die Schlüsselinovation der Ichthyosaurier war das Lebendgebären, das im Fossilbericht seit dem Ende des Anis belegt ist. Wichtige Evolutionstrends im Bewegungsapparat sind die zunehmende Modifikation des Flossenskelettes zu einem Mosaik von gleichartigen, abgeflachten Knochen und der Wechsel vom anguiliformen Schwimmen zum thunniformen Schwimmen bei den jurassischen und späteren Formen. Diese zeichnen sich durch eine Verkürzung des Körpers und die Entwicklung einer halbmondförmigen Schwanzflosse aus.

Fast von Anfang an hatten die Ichthyosaurier eine weltweite Verbreitung, die sie auch bis zu ihrem Aussterben im Cenoman beibehielten. Die Vielfalt der Ichthyosaurier ist in der Mitteltrias am größten, als es sowohl rein piscivore Formen gab, als auch solche mit heterodontem Gebiß und mit Knackgebiß. Im Jura ist die Diversität im Lias am höchsten, in der Kreide reduziert sie sich aber auf eine Gattung (Platypterygius).

Obwohl Schädelmerkmale die Ichthyosaurier als Diapsiden ausweisen, ist ihre genaue Stellung innerhalb dieser Gruppe unklar. Eine computergestützte kladistische Analyse der gut bekannten Gattungen erhellt dagegen die Verwandschaftsbeziehungen innerhalb der Gruppe: Die basalsten Formen sindGrippia undUtatsusaurus, gefolgt von den Mixosauridae (Mixosaurus undPhalarodon). Die Shastasauriden (Cymbospondylus, Shonisaurus, Besanosaurus) sind die fortschrittlichsten Triasformen und stellen die Schwestergruppe aller posttriassischer Ichthyosaurier dar. Letztere sind eindeutig monophyletisch und werden hier mit dem Namen Neoichthyosauria belegt.

Abstract

The Ichthyosauria is the group of Mesozoic marine reptiles that was most highly adapted to the aquatic environment. The first ichthyosaurs from the upper Lower Triassic (Spathian) already show a suite of unique characters (very large eyes, elongate snout, deeply amphicoelous vertebrae, limb modified to fins) correlated with a fully aquatic existence and probably were unable to leave the water. The key evolutionary innovation was vivipary, giving birth to live young, which is documented by the fossil record since the end of the Anisian. Major evolutionary trends in the locomotor apparatus are the increasing modification of the fin skeleton to a mosaic of bones and the change from anguiliform swimming in the earliest forms to thunniform swimming in the Jurassic and later forms, as evidenced by the shortening of the body and the evolution of a semilunate tail fin.

Almost from the beginning, ichthyosaurs had a cosmopolitan distribution which was retained until their extinction in the Cenomanian. Ichthyosaurian diversity is greatest in the Middle Triassic with piscivorous, heterodont, and durophagous forms. Jurassic diversity is greatest in the Liassic, declining to one genus (Platypterygius) in the Cretaceous.

Although skull characters indicate that ichthyosaurs were diapsids, their exact position within Diapsida is unclear. A cladistic analysis of the well known genera clarifies relationships within the Ichthyosauria. Most basal areGrippia andUtatsusaurus, followed by the Mixosauridae (Mixosaurus andPhalarodon). The Shastasauridae (Cymbospondylus, Shonisaurus, Besanosaurus) are the most advanced Triassic forms and represent the sistergroup of all post-Triassic ichthyosaurs. These are clearly monophyletic and are termed here the Neoichthyosauria.

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Sander, P.M. Ichthyosauria: their diversity, distribution, and phylogeny. Paläont. Z. 74, 1–35 (2000). https://doi.org/10.1007/BF02987949

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Keywords

  • Jurassic
  • Middle Triassic
  • Toarcian
  • Pelvic Girdle
  • Neural Spine