A characteristic reptile-like feature of monotreme spermatozoa is the circumferential and spiral pattern of condensation of the nuclear contents of spermatozoa that develops in the testis. They are characterised by an elongated, helical and filiform sperm head, consisting of 3–5 turns, about 40 µm in length in the platypus and 50 µm in echidnas and with a small rostrally-positioned acrosome, 5–7 µm in length, which occupies less than a sixth of the length of the sperm head. Platypus and echidna spermatozoa are very similar to typical reptilia and avian spermatozoa ( Fig. 6(a) Carrick and Hughes, 1982). Monotremes diverged from the other two groups of mammals about 166 million years ago (Mya Warren et al., 2008) and display their reptilian/sauropsid ancestry in various characters including oviparity and sperm structure. The living order Monotremata in the infraclass Prototheria – platypus (Ornithorhynchus anatinus) and up to four species of echidnas (Tachyglossus aculeatus and Zaglossus spp.) – were derived from therapsid reptiles and are the earliest branching of the mammalian lineage. Horta Nunez, in Encyclopedia of Reproduction (Second Edition), 2018 Prototheria The short-beaked echidna and platypus are both listed as common, whereas both species of long-beaked echidna are classified as endangered because of land clearance and hunting. It is found only in Tasmania and along the east coast of mainland Australia. The platypus is a streamlined, fur-covered aquatic mammal, with a distinctive bill, webbed feet, and a broad flat tail. Zaglossus bartoni has five claws on the front foot, and Zaglossus bruijnii generally has only three. The two species of long-beaked echidna are larger and have fewer, shorter spines and thicker fur compared with short-beaked echidnas. multiaculeatus from South Australia, and (5) T. aculeatus from eastern New South Wales, Victoria, and southern Queensland, (3) T. acanthion from the Northern Territory, northern Queensland, inland Australia, and Western Australia, (2) T. The short-beaked echidna ( Tachyglossus aculeatus) is divided into five subspecies: (1) T. Fur is present between the spines and over the belly. The spines are firmly attached to the skin and cannot be pulled out as in the case of porcupine spines. The two families in this order are Tachyglossidae, which includes the echidnas, and Ornithorhynchidae, which includes the single species of platypus ( Ornithorhynchus anatinus).Įchidnas are terrestrial mammals with a long, tubular snout, powerful claws, and spines that cover the tail and dorsal surface of the body. The order Monotremata is unique among mammals, as its members lay shell-covered eggs but nurse their young. Peter Holz, in Fowler's Zoo and Wild Animal Medicine, VolBiology Taxonomy However, these teeth are replaced by horny ridged plates that are used by adults to crush food. The teeth are multirooted and have a thin layer of enamel, which shows only traces of prismatic structure but has some incremental markings and some atypical features ( Lester et al., 1987). Adult platypuses lack teeth but juveniles possess a number of vestigial teeth the dental formula is given as I 0 5 C 1 1 P 2 2 M 3 3 = 34 ( Pasitschniak-Arts and Marinelli, 1998). Platypuses are nocturnal and aquatic they are opportunistic predators of bottom-living arthropods together with mollusks, annelids, and fish eggs ( Pasitschniak-Arts and Marinelli, 1998). After loss of the egg tooth, echidnas are toothless. The egg tooth is shed a day or two after hatching. Monotremes are oviparous, and hatchlings, like those of reptiles, are equipped with an egg tooth, which has a basis of dentine and is used to pierce the eggshell ( Green, 1930 Hughes and Hall, 1998). The Monotremata consists of two families: the Tachyglossidae (two genera of echidnas or spiny anteaters) and the Ornithorhynchidae, which contains only one species, the platypus. Barry Berkovitz, Peter Shellis, in The Teeth of Mammalian Vertebrates, 2018 Monotremes
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