AQUATIC TETRAPODS

 

EVOLUTIONARY MODEL

If evolution is correct, a number of terrestrial lineages could develop aquatic species. Transitional states of adaptation should be evident indicating that aquatic adaptation can result from a series of changes.

CREATIONISM MODEL

If the creationism model is correct, all modern aquatic animals have always existed. Since each kind is completely unrelated to all others, there is no need for aquatic vertebrates to display any similarities at all to specific terrestrial lineages.

INTELLIGENT DESIGN

If intelligent design is correct, there is no reason to believe that lineages which were intelligently designed for terrestrial life would need to be redesigned for aquatic life. It is not expected that similar adaptations would need to be created separately in independent lineages.

Do aquatic tetrapods display signs of "intelligent design"? Modern aquatic tetrapods certainly do display a wonderful set of adapatations which contribute to their success in aquatic environments. However, given the range of adaptations evident in animals as diverse as crocodiles, turtles, penguins, whales, seals, otters, beavers, and extinct animals as diverse as plesiosaurs, ichthyosaurs, and demostylans, it is evident that there is no body plan that an aquatic tetrapod needs. There are no adaptations which are required. Apparently, almost any body plan can be modified to adapt an animal for some degree of aquatic living.


Throughout the first half of the Paleozoic, there were no vertebrate predators in aquatic environments other than fish. After amphibians adapted to terrestrial environments, it seems a number of unrelated lineages found that these modifications also increased their risk of success in aquatic environments. It is unknown whether it was their increased size, long muscular tail, strong muscular limbs, or powerful jaws which gave them an adaptation which contemporary fish lacked. Whatever the reason, while most amphibian lineages were becoming better adapted to terrestrial life, some fossil amphibians evolved adaptations for aquatic life.

amphibian
While modern amphibians-frogs, salamanders, and caecilians-are primarily terrestrial, there are a number of species which have increased their dependence on aquatic environments including some which are entirely aquatic. Below are pictures of frogs and salamanders which spend their entire lives in water.

frogs

salamanders
Some caecilians spend much of their time in water.
In general, the success of reptiles compared to amphibians results from their increased adaptation to terrestrial environments. However, species in many separate lineages of reptiles became secondarily adapted to aquatic life include:
1) Primitive anapsid reptiles,

2) Turtles
Not only do many turtles inhabit aquatic environments, some species (fossil and modern) are marine and only come onto land to lay eggs.

turtle
3) Ichthyosaurs
The earliest ichthyosaurs were more crocodile-like in their body form and their fins were closer to the ancestral condition. Later lineages became dolphin-like in their body shape, underwent live birth in water, and developed highly modified fins.

ichthyosaurichthyosaur

fins
4) Lepidosaur Diapsids
a) lizards
Some extinct groups of lizard were marine and could reach considerable sizes.

lizard

mosasaur
b) snakes--
The earliest snakes were marine. After becoming adapted to terrestrial environments, many lineages became secondarily adapted to the water with ecological niches ranging from that of water snakes to those of marine sea snakes.

early snakewater snake

5) Archosaur Diapsids
a) Crocodiles
Although the first crocodiles were terrestrial, modern lineages became adapted for aquatic environments and extinct lineages included some marine forms.

crocodile
b) Other groups of archosauromorphs and archosaurs adapted to aquatic environments with crocodile-like body plans.

parasuchus


c) Plesiosaurs
From small ancestral species with short necks evolved large, long-necked species.

plesiosaur
d) Pliosaurs
From small, less specialized ancestral forms evolved lineages which included the largest carnivore in earth's history with a 25 foot long head.

pliosaur
e) Nothosaurs

nothosaur
5) Birds
Although birds were originally adapted for flight, many have subsequently adapted to an aquatic life including:
a) toothed diving birds of the Cretaceous

bird
b) penguins
The earliest penguins were very similar to cormorants in their body shape but had already adapted their wings for swimming.

penguin
c) modern auks and puffins
d) many lineages of birds which spend much of their time near water including ducks, geese, grebes, cormorants, anhingas, herons, storks, ospreys, and many others
comorants

6) Mammals
Although the earliest mammals were terrestrial, as were the earliest placental mammals, later lineages adapted to aquatic life. This mammalian radiation occurred after the End-Cretaceous extinction had eliminated virtually all competition from aquatic reptiles.
a) Extinct Mesozoic Mammals

mammal
b) Rodents
Some lineages of rodents, such as beavers, muskrats, many rats, and capybaras, have adapted to aquatic environments


c) Whales
While the first whales were still capable of coming onto land, later species became fully adapted to aquatic life.

basilosaurs
d) Elephant relatives
The first elephants were probably largely aquatic, as were their extinct relatives, the demostylans.

early elephant
e) Sirens-dugongs, manatees, sea cows

sea cow
f) Carnivores
While some carnivores (such as otters, sea otters, and polar bears) have become partially adapted to aquatic environments, the pinnipeds (seals, sea lions, and walruses) have become highly modified for aquatic life.

sea lionsseal


Even when diverse lineages share an adaptation, reduction of the hind limbs, for example, molecular analysis demonstrates that similar changes can result through entirely different mutations. In the pufferfish Takifugu rubripes (fugu), a change in the expression of the gene Hoxd9a results in the absence of pelvic fins while in some sticklebacks, the altered expression of Pitx1 results in the absence of fins (Tanaka, 2005). In snakes, it seems that the change in the expression of Hox genes are responsible for the absence of pectoral girdles because they eliminate potential sites for limb initiation. Whales and some snakes begin the embryonic formation of hind limbs but these structures degenerate. Changes in the expression of Sonic Hedgehog in some lizards is responsible for digit reduction (Tanaka, 2005). Hind limb buds are formed in cetacean embryos using the same developmental mechanisms as those of other amniotes but they soon degenerate. Genetic studies implicate the loss of expression of the gene Hand2, a regulator of the important limb signal Sonic Hedgehog, as the reason for this failure to develop (Thewissen, 2006).


No one form of locomotion is required, even within a single group such as whales. While the earliest whales relied on their large feet for swimming (the pakicetids and ambulocetids) and later groups lengthened the tail while reducing their legs (remingtonocetids). Hind limbs had become vestigial in the archaeocetes (dorudontids and basilosaurids) and fossil evidence indicates the presence of a fluke (Thewissen, 2006).


Many of those who oppose evolution will site an animal such as a whale and ask if all of its adaptations could result "by chance" without a purposeful design. Obviously, evolution does not claim that change occurs by chance alone-natural selection (a non-random process) increases the frequency of those variations which increase an organism's ability to survive or reproduce in a specific environment. If only one possible set of body structures are required for a tetrapod to survive in an aquatic environment, then the odds of these adaptations occurring become less likely. In contrast, it is evident that almost any tetrapod body plan can be adapted to aquatic life. Lineages as diverse as whales and ichthyosaurs did not originate with their full suite of traits found in the most advanced lineages. If body plans as diverse as those of frogs, salamanders, lizards, snakes, ichthyosaurs, plesiosaurs, penguins, whales, sirens, and seals can be adapted for aquatic life, then the evolution of at least some aquatic adaptations become a reasonable prediction for any lineage which encounters an opportunity for enhanced survival near water.
The advantages of whales and dolphins over the fish they share the oceans with include a high intelligence which allows both echolocation and the ability to communicate (Sylvestre, 1993).
In dolphins, the respiratory and digestive tracts are completely separate. This removes the possibility of choking, but it does prevent them from breathing through their mouths (Sylvestre, 1993).