General organization and affinities of Monotremata and Marsupials 2nd semester
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Prototheria resemble the reptiles and birds with some advanced characters over them, establishing mammalian ancestry.
Reptilian affinity:
Presence of cloaca.
Presence of ectopterygoid in skull.
Vertebrae without epiphysis and with cervical ribs.
Ribs are single headed.
Cervical ribs are present.
Thoracic ribs are single headed.
A median T-shaped interclavicle present.
Acetabulum in echidna is perforated.
Body temperature is not constant.
Cochlea of internal ear with lagina.
Ureters lead into a urinogenital sinus.
Corpus callosum is absent and anterior commissure is well developed.
Testis abdominal.
Oviparous and meroblastic segmentation.
Presence of strong reptilian features in Monotremata speaks of its primitiveness. These primitive mammals have failed to cope up with many of the evolutionary transformations which culminated in the establishment of better characteristics in higher mammals.