MARINE REPTILES

 

EVOLUTIONARY MODEL

If evolution has occurred, it is not unlikely that a number of separate lineages of reptile would have adapted to marine life independently. One could observe a variety of degrees of adaptation and significant variations within a group.

CREATIONISM MODEL

If the creationism model is correct, all groups of marine reptiles existed since the first week of life on earth and extinct groups died in the same global cataclysm. SIgnificant variations within a group would be inconsistent with the conclusion that adaptations for aquatic life could not evolve.

INTELLIGENT DESIGN

If intelligent design is correct, then it is not expected that a reptilian body plan designed for terrestrial life would subsequently need to be redesigned for aquatic life. It is ceratinly not expected that such a reworiking of ancestral design would occur multiple times to achieve similar ends. If irreducible complexity applies to aquatic adaptation, then a gradual modification of a body plan would be of no use to an organism.



Many marine reptiles are among the most specialized tetrapods to evolve. Although the many special adaptations which contributed to the success of each lineage, two arguments could be made against invoking design:
1) Given that there are apparently a diversity of ways in which a reptile can adapt to aquatic environments, no one adaptation is absolutely required.
2) Given the diversity of structures and forms which can exist within the groups which contain marine reptiles, there is no reason to doubt that ancestral reptiles could have evolved into these aquatic lineages if they produced similar degrees of diversity.


The mesosaurs became adapted to aquatic habitats. They possessed an elongated snout with many teeth for grabbing fish and a large paddle-shaped rear foot. They were the first amniotes to become adapted to aquatic life.

mesosuarus

The thalattosaurs were a marine group of primitive diapsids which evolved elongated bodies with long necks and tails.

askeptoaurus
Some Teiid lizards are semi-aquatic and the extinct mosasaurs were highly specialized for marine life. Fossil lizards of the family Aigialosauridae were semiaquatic and were transitional between varanoids and mosasaurs.

skulls

aigialosaur

aigialosaur

early mosasuar
mosasuar

mosasaur

The first snakes were marine and many modern snakes have adapted to aquatic environments (such as water snakes, anacondas, and sea snakes).

snake


Although most sphenodontids were lizard-like in their overall appearance (as is the only modern surviving member), many species (known as pleurosaurs) became aquatic with elongated bodies (including some with 57 trunk vertebrae), elongated skulls and reduced limbs (especially the forelimbs in some groups).

pleurosaurs


Although the first crocodilians were terrestrial, many lineages were modified for life in aquatic environments. Different groups of crocodiles became marine including the teleosaurs of the Late Triassic, thalattosuchians of the Early Jurassic, pholidosaurs of the Late Jurassic, and the dryosaurs of the Late Cretaceous.

crocodiles

Other reptile groups were superficially similar to crocodiles including champosaurs and phytosaurs.

champosaurus

The phytosaurs were abundant in the Upper Triassic. They were very similar to crocodiles overall but were a distinct group (for example, their nostrils were far back on the top of the head and they had a primitive pelvis). Phytosaur skulls were variable.

Nanchangosaurus either represents a primitive ichthyosaur or a separate lineage which also became inchtyosaur-like.

nanchangosaurus

Ichthyosaurs were the most specialized marine reptiles. The first bones of ichthyosaurs were found in 1705 and were interpreted to be Pre-Flood monsters. Cranial anatomy suggest that the ichthyosaurs evolved from non-diapsid reptiles. Although they do possess a temporal fenestra, it seems to have evolved separately from those of diapsids. A number of amphibian and reptile groups have evolved different fenestrations of the skull (Maisch, 1998a). They first appeared in the Early Triassic, were most numerous in Jurassic, and died out before the Early Cretaceous. The earliest forms (such as Chensaurus and Utatsusaurus) were long, slender, and probably swam with an eel-like motion. These Late Triassic forms had both primitive characteristics and traits of later ichthyosaurs (Montani, 1996).

. ichthyosaur

 

Ichthyosaurs



The basal ichthyosaur Choahusaurus was more derived than Utatsusaurus but possessed a more primitive fin in which the radius and ulna were longer and not fused to each other, unlike the condition of later forms. The ichthyosaurs which survived to the Jurassic lacked digit I and others lacked digit II as well. Additional digits evolved in some, such as Ichthyosaurus (Montani, 1999). Early ichthyosaurs possess a tarsal bone (the centrale) which is lost in later ichthyosaurs (Brinkman, 1992).

utatsusaurus


Later ichthyosaurs resembled dolphins in body shape, possessed a large orbit, and swam differently due to a stiffened vertebral column. Some specimens show young in utero or in the process of being born. They declined in the late Mesozoic, perhaps because sharks were becoming dominant. Shonisaurus approached 50 feet in length, had a skull of 3 m, and weighed 30 tons. One 9 meter ichthyosaur appeared to have fed on large quantities of small marine life, as do many modern whales (Grady, 2001).

shonisaurus
In the middle Triassic, the primitive ichthyosaur family Mixosauridae was widespread. The largest known mixosaurids are estimated to have reached 5 meters in length. This group possessed a sagittal crest formed by the nasal, frontal, and parietal bones (Maisch. 2001).

skull

Ichthyosaurs varied in the girdle, fin, and skull bones.

pelvic bones

shoulders

skullskull


Sauropterygians were a group of marine reptiles that include nothosaurs and plesiosaurs.

Sauropterygians

Sauropterygians

nothosaur

Nothosaurs were similar to crocodiles. Although they were aquatic, their limbs were reduced but not highly adapted for aquatic life. Some were under 20 cm; others reached 4 meters. They were intermediate between terrestrial animals and plesiosaurs. Pistosaurus is a nothosaur intermediate between other nothosaurs and plesiosaurs.

Plesiosaurswere large reptiles whose long necks possessed up to 76 vertebrae. Plesiosaurs seem to have evolved from nothosaur-like ancestors. Plesiopterys was a small plesiosaur with fewer than 40 neck vertebrae and which retained a number of primitive characteristics not found in more advanced plesiosaurs (O’Keefe, 2004).

plesiosaur

plesiosaur

A second group, the pliosaurs, had larger skulls and shorter necks. Liopleurodon was the largest predator that has ever lived. Its skull could reach 5 meters in length and its body 25 meters, approaching the size of the largest modern whales. It might have weighed about 150 tons.

.

pliosaur

Other aquatic reptiles include some younginoids of the Permian (which developed tail modifications for swimming).

 

Several dinosaurs were adapted for a lifestyle in aquatic environments. Baryonyx, “heavy claws”, resembled a large crocodile. One fossil specimen was found with a partially digested one meter long fish in its abdomen. Baryonyx is unlike most other theropods in its long snout, its long and low nostril, its large hand claw, its large number of teeth, and a longer, straighter neck than most theropods. Its large thumb claws would have been useful in grabbing fish. It may have been capable of both bipedal and quadrupedal locomotion (Charig, 1997). (Charig, 1986).

.baryonyx


Spinosaurus ,“thorn lizard”, possessed a spinal frill, possibly for thermoregulation or display. It was a fish eating theropod. Suchomimus, an early spinosaurid, has a lower sail than Spinosaurus and has a thumb claw and other features which link it to Baryonx.

spinosaurus

suchomimus

Many turtles, fossil and modern have adapted to freshwater and marine environments.

turtle

Other aquatic reptiles were also turtle-like.

reptile reptile