BSCI392
11-16-2007
Marine Reptiles:

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We now switch to the sauropsid side of the amniote tree. Sauropsida is, for all intents and purposes, the same is Reptilia used in a phylogenetic sense.

Synapomorphies of Sauropsida mostly involve soft tissue. One example will suffice here:

The living diversity of Sauropsida breaks down into:

Sauria is characterized by Two pairs of temporal fenestrae:


Sphenodon

We began our survey with an interesting diversion: Reptiles of the Sea.

Organizing the information

I. Reptiles have successfully invaded the oceans at least seven times (ten if you count birds). At least four of those invasions led to long-term radiations of marine saurians encompassing a bewildering range of ecolgies. For them not to seem confusing and random we need organizational schemes.

Evolutionary Context

  • "Marine reptiles" are not monophyletic. There have been several successful invasions of the shallow oceans and of the open oceans.
  • Some groups (E. G. squamates, crocodylians, and birds) have done it more than once.
  • Odd considering that amniotes had spent the last half of the Paleozoic evolving adaptations for life on land, foremost among them, the amniotic egg that would drown if laid in water.

    Chronological Context

    Here there are interesting patterns:

    A Rogue's Gallery of Marine Reptiles

    Major groups taken roughly in their order of appearance in the fossil record:

    Euryapsida:

    The euryapsids encompass a great range of morphological diversity in three major groups:

  • Eosauropterygia: (Middle Triassic - end of Cretaceous) The plesiosaurs of your childhood prehistoric animal books are best known examples. Marine predators.

    Characteristic features:
    • Limb-propelled swimmers
    • Long necks
    • Trend toward heads with longish snouts and long temporal regions.

    Greatest range of morphological disparity in the Late Triassic.


  • Placodontia: (Middle - Late Triassic) including Placodus pictured here was restricted to shallow marine environments of the middle and late Triassic. They probably swam clumsily or walked on the bottom in the manner of a snapping turtle. In contrast to their locomotor apparatus, their skulls were intensely modified for withstanding terrific occlusal forces.

    As this palatal view shows, their teeth were transformed into a dental pavement with which they presumably crushed hard-bodied invertebrates. Some placodonts evolved extensive armor that superficially resembled that of turtles.


  • Ichthyosauria: (Early Triassic - beginning of Late Cretaceous)

    Included many ecologically shark or dolphin-like pelagic predators. In fact, ichthyosaurs were the first marine reptiles to invade the open oceans.

    Ichthyosaurs appear in the fossil record highly modified for marine life, with:

    The first ichthyosaurs were rather different from the familiar forms in being:

    By the Late Triassic, there were whale-sized ichthyosaurs like Cymbospondylus. See also.

    The early Jurassic was the peak of ichthyosaur diversity and the time in which they assumed their familiar form. Most preyed on small fish and cephalopods, but some were orca-sized macropredators.

    Indeed, the eel-like near shore forms became extinct in the Late Triassic. Pelagic ichthyosaurs diminished through the Late Jurassic and Cretaceous, becoming extinct at the beginning of the Late Cretaceous.


    Protorosauria: (Late Permian - Late Triassic)

    Small to moderately large saurians close to the ancestry of archosaurs.