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:

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:

    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.


    Thalattosuchia: (Early Jurassic - Early Cretaceous) Sea-crocs!

    We will take these up in detail in a later lecture. For now, be aware that during the Mesozoic and Cenozoic, crocs have invaded the oceans several times.


    Chelonioidea: (Late Jurassic - Recent) Sea turtles

    The first turtles are land or freshwater animals from the Triassic. Sea turtles were diverse and common by Late Cretaceous. Living sea turtles and their fossil relatives like Toxochelys, Protostega, and Archelon are monophyletic with respect to other turtles. Derived characters they share include

    Living sea turtles primarily eat jellyfish and seaweed and so, presumably did ancient ones. Unlike other marine reptiles, these creatures sailed through the turnover of the mid-Cretaceous and the Cretaceous-Tertiary extinction event suffering little damage.


    Mosasauroidea: (Late Cretaceous) Giant sea lizards

    Mosasaurs are scleroglossan lizards closely related to monitors, snakes, Gila monsters, and anguids. As squamates their skulls are highly kinetic.

    This flexibility enabled mosasaurs to swallow very large prey items whole. Indeed, some researchers posit a very close relationship with snakes, with whom they shared derived characters like.

  • Mosasaurs were Komodo dragon-sized to sperm whale-sized predators. This, coupled with their ability to engulf large prey made them the top marine predators of their age. Mosasaur stomach contents typically include other marine reptiles.
  • Probably incapable of emerging onto land. One Plotosaurus has been found with embryos inside.
  • Worldwide distributions, so probably at home in open oceans.
  • And yet as far as locomation is concerned, they were big eel-like ambush predators, like Late Triassic ichthyosaurs.

  • The notion that mosasaurs and snakes are sister-taxa is strengthened by a series of recent discoveries of fossil snakes of Late Cretaceous age with:

    Consider the distribution of ecological types through time.

    Some odd patterns:

    The first pattern might reflect the effects fo the Permo-Triassic extinction. Maybe so many lineages of marine reptiles became established in the Early Triassic because there was suddenly

    In the Cretaceous, reptiles mostly remained ambush predators. This may reflect the changing roster of fishy predators. Starting in the Late Jurassic, ichthyosaurs began to wane as modern sharks and large predatory bony fish proliferated. By the Late Cretaceous, modern lamniform sharks like Cretoxyrhina were common.