Class notes Unit 3 - Understand Animal Anatomy and Physiology
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The Pentadactyl Limb
“Pentadactyl is the condition of having five digits on each limb. It is believed that all living
four limbed vertebrates (tetrapods) are descended from an ancestor with a pentadactyl
limb (fifth digit/thumb), although many species have now lost or transformed some or all of
their digits by the process of evolution.”
Dolphins
The pentadactyl limb in dolphins has evolved to fit within large fins at the front of their
body. These pentadactyl limbs are formed of certain bones i.e humerus, radius and ulna,
carpals, metacarpals and phalanges. The metacarpals and phalanges are usually 5 separate
digits that are fixed and formed to the carpels in a crescent like shape. All of these bones are
found within a dolphin’s flippers.
Compared to a human where the humerus, radius and ulna bones are longer in length to
make two areas of an arm which can move as a hinge joint (upper arm and forearm) and
allow the arm to bend at the middle point (elbow joint). The upper and forearm in a dolphin
flipper are compressed in length and do not have a hinge joint. The smaller size of these
bones allows the 5 digits of the metacarpals and phalanges to fit which is extremely
important due to the fact that the second and third digits are much longer than the rest of
them as this makes the base of the flipper extremely lightweight to allow free movement,
however these digits are fused in place by
cartilage.
In today's day and age, dolphins have
forelimbs (flippers) which are primarily used
for movement and steering within the water.
They do not have back limbs at all due to
there being no physical use or need for them
nowadays, but early evolutions of the dolphin
show evidence of 4 pentadactyl limbs being
present rather than just two, showing the existence of hind limb bones (femur, tibia and
fibula, tarsels, metatarsals and phalanges) within the skeleton of the early adaptations.
Around 50 million years ago, one of the first dolphins was discovered and it is known as the
Pakicetus. It was a typical land mammal with all four dog-like limbs. In early years these
limbs were useful for walking across the land, however, they mainly lived in freshwater
semi-aquatic habitats, so they had many interactions with water extremely early on.
As time went on, the next known past relatives of the dolphin became more adapted to
their aquatic lifestyle as their limbs became shorter and the surface area of their limbs
became larger to replicate a paddle-like shape and function to allow movement throughout
the water, however, they could not steer themselves through the water just yet. Their tails
also became longer and more muscular to help their movement in the water as their hind
limbs eventually became smaller until they disappeared. When prehistoric dolphins began
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