The Transition To Land

The Transition To Land
1. Transition to land requires significant changes in a number of systems
a. Skeletal support
b. Respiration
c. Cardiovascular
d. Excretory and reproductive
e. Sensory and nervous
2. Skeletal system
a. Craniodental: skull and teeth
i. Feeding mechanism is different on land because food particles no
longer suspended in water.
ii. Terrestrial feeding requires greater head mobility.
1. pectoral girdle is anchored to skull in aquatic verts.
2. aquatic animals have short tongues, while terrestrial forms
have long tongues. This compensates for insufficient head
iii. Hyoid apparatus (gill support in fishes) becomes involved with
tongue in terrestrial verts: tongue is used more extensively to
manipulate food.
iv. Aquatic forms don’t have salivary glands, terrestrial forms do.
v. Branchiomeric muscles of fishes are used in association with the
gills. These are lost in terrestrial verts, except for the cucullaris,
which is homologous to the trapezius.
1. trapezius is innervated by cranial nerves, not spinal nerves,
and thus paraplegics can still shrug their shoulders.
vi. Other branchiomeric muscles become involved in opening and
closing the jaw, and in the ability to swallow food.
b. Axial system (vertebrae, ribs, and muscles)
i. Vertebral column suspends weight of body interrestrial forms.
1. functions like a bridge.
2. in aquatic forms this is not a problem, because water acts
like a skeleton.
ii. Ribs protect internal organs and prevent them from sagging.
iii. Musculature is used in locomotion and postural control.
iv. Problems:
1. vertebral column must tolerate a number of forces.
2. the dynamics of these forces changes with increasing size.
3. because of locomotion on land, vertebral column must
withstand and minimize torsion.
4. zygapophyses (lost or reduced in secondarily aquatic
5. regionalization of vertebral column changes with
locomotory mode.
a. cervical (atlas and axis)
b. thoracic
c. lumbar
d. sacral
e. caudal
6. differentiation of epaxial and hypaxial musculature (e.g.
rectus abdominus)
c. Appendicular system
i. Pelvic girdle
1. dominant propulsive mechanism in tetrapods.
2. hindlimb dominance
3. conservative.
4. comprised of
a. ilium
b. pubis
c. ischium
5. anchored via sacrum.
a. Strength of connection between vertebral column
and girdle increases with increasing locomotor
ii. Pectoral girdle.
1. direct connection between girdle and skull in fishes.
a. Direct conduction of sound.
2. comprised of both endochondral elements (scapula and
coracoid) and dermal elements (cleithrum, clavicle,
3. girdle is relatively free floating, and complexity changes
with mode of locomotion.
a. Turtles
b. Frogs
c. Cats
d. Primates
e. Giraffes.
iii. Locomotion
1. change from lateral undulation to dorsal/ventral undulation
2. typical gaits
a. walk – amble
b. trot
c. bound
d. gallop
e. ricochetal
3. Respiratory
a. Origin of the lungs – some controversy here
i. Modified swim bladder?
ii. Low oxygen saturation only? Or possible in high oxygen
environments? Perhaps as a means to supply heart with increased
b. Lung ventilation is initially through bucal pumping involving the hyoid
c. Amniotes use costal aspiration
i. Intercostals muscles contract thus expanding rib cage and creating
negative pressure in lungs.
ii. Exhalation occurs via elastic return of rib cage, and through
contraction of transverses abdominus.
iii. Carrier’s constraint.
iv. Development of diaphragm in mammals.
v. Lung complexity: amphibians to mammals & birds.
d. Special problems
i. Diving adaptations.
ii. Diving response.
4. Cardiovascular system
a. Lymphatics
i. Change in structure mirrors locomotor mode.
ii. Reduction in distribution and number of lymph hearts.
b. Plumbing of heart changes
i. Sinuous venosus reduced/disappears.
ii. Conus arteriosus reduced/disappears.
iii. Change from 2 chambers, to 3, to 3 ½, to 4.
iv. Change from single circuit to double circuit.
c. Reduction in number of portal systems.
5. Excretory and reproductive systems
a. Kidneys
i. In fish the gills are involved in regulation of nitrogen and some
ions, while kidneys regulate water and some ions.
ii. In amniotes, kidneys regulate water and nitrogen as well as ions.
iii. Holonephric – Opisthonephric (embryonic mesonephric plus
metanephric) – Metanephric kidneys.
iv. Change in osmotic environment.
b. Reproduction
i. Change from external fertilization to internal fertilization
1. intromittent organs.
2. change from cloaca to separate urinogenital ducts.
ii. Development of amniotic egg.
iii. Change in structure of placenta
iv. Structure of genitalia
1. Spotted hyaenas
2. Spider monkeys
3. Gorillas vs humans
6. Comments about scaling.