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
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.
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,
Not only do many turtles inhabit aquatic environments, some species (fossil
and modern) are marine and only come onto land to lay eggs.
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.
4) Lepidosaur Diapsids
Some extinct groups of lizard were marine and could reach considerable
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
5) Archosaur Diapsids
Although the first crocodiles were terrestrial, modern lineages became
adapted for aquatic environments and extinct lineages included some marine
b) Other groups of archosauromorphs and archosaurs adapted to aquatic
environments with crocodile-like body plans.
From small ancestral species with short necks evolved large, long-necked
From small, less specialized ancestral forms evolved lineages which included
the largest carnivore in earth's history with a 25 foot long head.
Although birds were originally adapted for flight, many have subsequently
adapted to an aquatic life including:
a) toothed diving birds of the Cretaceous
The earliest penguins were very similar to cormorants in their body shape
but had already adapted their wings for swimming.
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
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
Some lineages of rodents, such as beavers, muskrats, many rats, and capybaras,
have adapted to aquatic environments
While the first whales were still capable of coming onto land, later species
became fully adapted to aquatic life.
d) Elephant relatives
The first elephants were probably largely aquatic, as were their extinct
relatives, the demostylans.
e) Sirens-dugongs, manatees, sea cows
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.
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).