Affinities
of prototheria
The subclass
Prototheria includes egg-laying mammals, considered the most ancestral forms
within the class Mammalia.
There are only three
surviving species of Prototheria, which are quite rare. These species are the
platypus and the echidna (spiny anteater), found in Australia and New Guinea.
It has affinities with reptelia, birds and mammals.
Characteristic
Features
a) Aquatic, terrestrial, nocturnal,
primarily insectivorous.
b)
Quadrupedal, walking on four legs, and
oviparous (egg-laying).
c)
They nourish their newborns with mammary
gland secretions like all other mammals.
d)
Small bodies covered with hair and
spines.
e)
No external ears.
f)
The muzzle or snout forms a beak.
g)
Develop a temporary mammary pouch during
the breeding season.
h)
The skull lacks an alisphenoid bone but
includes a bone homologous with the reptilian ectopterygoid.
i)
Absence of vertebral epiphyses (except
in the caudal vertebrae of the platypus), and no zygapophyses on the cervical
vertebrae.
j)
A T-shaped interclavicle similar to
those found in reptiles is present.
k)
Teeth develop embryonically but are lost
early, with molars replaced by horny pads used for grinding food.
l)
Known as monotremes, meaning they have a
single opening for excretion and reproduction, called a cloaca, similar to
birds and reptiles.
m)
Pulmonary respiration with a
four-chambered heart.
n)
Metanephric kidneys.
o) A
small, simple brain without a corpus callosum.
Reptilian
Affinities
1. Prototheria
exhibit similarities to reptiles and birds while showcasing some advanced
features, indicating their mammalian ancestry.
2. Presence
of a cloaca.
3. Presence
of an ectopterygoid bone in the skull.
4. Vertebrae
lack epiphyses and have cervical ribs.
5. Ribs
are single-headed.
6. A
median T-shaped interclavicle is present.
7. The
acetabulum in echidnas is perforated.
8. Body
temperature is not constant.
9. The
cochlea of the inner ear includes a lagena.
10. Ureters
lead into a urogenital sinus.
11. Absence
of the corpus callosum with a well-developed anterior commissure.
12. Testes
are abdominal.
13. Oviparous
with meroblastic segmentation.
Avian
Affinities
1. The
beak of the platypus resembles that of birds.
2. Teeth
are absent in adults.
3. Presence
of webbed feet.
4. An
oil gland is present.
The shared
characteristics between Prototheria and birds/reptiles are primarily due to
their common reptilian ancestry.
Mammalian
Affinities
1. Presence
of hair, mammary glands, oil glands, and sweat glands.
2. Presence
of a palate.
3. A
typical mammalian diaphragm is present in the body cavity.
4. The
skull is dicondylic.
5. The
sternum is segmented.
6. Heart
is four-chambered.
7. Only
the left aortic arch is present.
8. The
circulatory system is typically mammalian.
9. Presence
of large ear ossicles.
10. The
cochlea is slightly coiled.
11. Fertilization
is internal.
12. A
slender caecum demarcates the two intestines.
13. Red
blood cells are small, circular, and non-nucleated.
14. Presence
of four optic lobes (corpora quadrigemina).
15. Presence
of milk glands secreting milk.
Conclusion
The presence of both reptilian and mammalian characteristics indicates an intermediate evolutionary stage. The possession of primitive and specialized features represents an early divergence from the main evolutionary group, leading to a separate development pathway. Reptiles, birds, and mammals form a homogenous group with some common ancestry.
