Erakina

Prototheria: The Mammals which lay Eggs -Erakina

Published Date : February 6, 2022

The term ‘Prototheria’ is the term given to the subclass of the class Mammalia in the Animal Kingdom. This subclass is the oldest form of the Mammalian class. The animals in this group are mostly extinct. However, only three existing species of two families and single order, the Monotremata are there. The Monotremes are represented in today’s period by the platypus and other species of echidna.

A Platypus

The controversy of the relationship of the Monotremes to the other lineages of the mammals, marsupials (Metatheria) and placentals (Eutheria), is not clear. Some evidence supports the Prototherians to be with Marsupials, and others suggest them to be a part of both, marsupials and placentals. Monotremes, however, have many characteristics that prove them to be a mammal like fur, a four chambered heart, a single dentary bone, three middle ear bones, and their lactating ability.

Home and Habitat:

Monotremes are natives of Australia. Their existence is stricted to the regions of Australia and New Guinea. According to the fossils found, their distribution in the early ages was wide. Their fossils found in Argentina, suggests the distribution of this subclass widely in earlier years.

Short-beaked Echidna

The habitats of the subclass Prototheria are either terrestrial or aquatic. Some Prototherians prefer the terrestrial region such as deserts, plains that are sandy, rocky areas, and forests of the lowland as well as the mountains. Platypuses that belong to this class prefer aquatic habitat. They mainly live in terrestrial freshwater like lakes, ponds and streams. They live in burrows alongside river banks and spend most of their time in water.

Physical Appearance:

  • The skeletons of Prototherians are unique among mammals.
  • Most of their skeleton is inherited from therapsids and is not found in any other living animal.
  • The modern day Prototherians have a modified skeleton because of their burrowing habits.
  • Their shoulder girdle is complex. It is more rigidly attached to the axillary skeleton as compared to other mammals.
  • Their femur and humerus are roughly parallel to the ground, which is more like the therapsids.
  • They have ribs present in the neck and chest.
  • The Prototherians have an absence of teeth and lacrimals and sutures.
  • The jugals in their skulls are reduced or absent. 
  • Prototherians also have Epipubic bones present, which was found in therapsids for having pouches. Though not all mammals have pouches, the presence of Epipubic bones have become vestiges of therapsids’ skeleton.
  • However, the presence of Epipubic bones in the Prototherians works to provide extra support to the weight of their abdominal muscles.
  • Prototherians are endothermic in nature. They have unusually low body temperature.
  • Metabolic rate is also low in them.
  • Male Prototherians are known to have spurs in their ankles that work for fighting and also as defensive in their fights.

Reproduction:

The Prototherians live alone for most of the year. The knowledge about their reproduction is very little but the observations show that they are not monogamous and neither do the males provide parental care. The males do not associate with females post- copulation. Prototherians are found in pairs during their mating season. Their mating season lasts for 1to 3 months which is generally between July to October. 

A baby Echidna

Prototherians are the only mammals that lay eggs. Their eggs are very small about 13mm to 15mm in diameter and their egg shells are somewhat leathery in texture. Females lay about 1 to 3 eggs and keep them in their pouches or burrows. Their eggs resemble very much like bird’s eggs, that have large yellow yolk concentrated at one end. The incubation period of the egg is about 12 days and the young monotremes hatch out of the egg using their ‘milk tooth’. They are fed milk by the mammary glands . The young monotremes are weaned upto 3 months by their mothers.

Facts about Prototherians:

  • Prototherians natural lifespan is not known but they are known to live for around 50 years in captivity.
  • The female Echidna are known to have pouches. The pouches are absent in Platypuses.
  • Echidnas have mammary glands in their pouches where their young ones are kept and fed.
  • They are diurnal, nocturnal or even crepuscular.
  • They are excellent diggers. They dig to make their shelters and escape their predators.
  • The sense of touch is most important for the Prototherians. This helps them to search for food.
  • All the Prototherians are carnivores.
  • The Echidnas roll up and erect their spine to escape the predators.
  • The Platypuses dig burrows to escape.
  • Two out of three species of Prototherians are listed as endangered in the IUCN list.

References:

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