The Affinities of the Monotremes

Griffiths, Mervyn

doi:10.1016/b978-0-12-303850-0.50013-6

Cited by 37 publications

(30 citation statements)

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“…Stages for platypus are after Ashwell [26]. Staging of echidna H.SP EC5 is estimated by cross-referencing previous studies [19,27]. CT cans of adult platypus were a gift of Anjali Goswami, the Natural History Museum, London.…”

Section: Methodsmentioning

confidence: 99%

“…Monotremes and therian mammals (marsupials and placentals) are evolutionary distant, with the common ancestor of the two subclasses being a mammal like reptile form around 160 million years ago [18]. Monotremes have a number of “reptile” like anatomical features such as a cloaca, external embryonic development in an egg, a straight cochlear in the inner ear and laterally protruding legs [19]. The absence of a disc in both echidna and platypus suggests that the disc evolved after the split between monotremes and therian mammals, and is therefore a therian novelty.…”

Section: Introductionmentioning

confidence: 99%

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The TMJ disc is a common ancestral feature in all mammals, as evidenced by the presence of a rudimentary disc during monotreme development

Anthwal

Tucker

2020

Preprint

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The novel mammalian jaw joint, known in humans as the temporomandibular joint or TMJ, is cushioned by a fibrocartilage disc. This disc is secondarily absent in therian mammals that have lost their dentition, such as giant anteaters and some baleen whales. The disc is also absent in all monotremes. However, it is not known if the absence in monotremes is secondary to the loss of dentition, or if it is an ancestral absence. We use museum held platypus and echidna histological sections to demonstrate that the developing monotreme jaw joint forms a disc primordium that fails to mature and become separated from the mandibular condyle. We then show that monotreme developmental anatomy is similar to that observed in transgenic mouse mutants with reduced musculature. We therefore suggest that the absence of the disc on monotremes is a consequence of the changes in jaw musculature associated with the loss of adult teeth. Taken together, these data indicate that the ancestors of extant monotremes likely had a jaw joint disc, and that the disc evolved in the last common ancestor or all mammals.

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Section: Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

The TMJ disc is a common ancestral feature in all mammals, as evidenced by the presence of a rudimentary disc during monotreme development

Anthwal

Tucker

2020

Preprint

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“…Retraction of the tongue is the result of contraction of the sternoglossus muscles that extend from the tongue tip to the xiphoid process of the sternum. Protrusion is the result of contraction of circular lingual muscles and two genioglossus muscles (Griffiths, 1968, 1978; Doran, 1973). The tongue is stiffened by an engorgement of vascular sinusoids within the tongue that make the tongue firm enough to penetrate termite‐ridden wood and soil (Doran and Baggett, 1970).…”

Section: Introductionmentioning

confidence: 99%

Microscopy of the Echidna Sublingual Glands

Krause

2011

Anatomia, Histologia, Embryologia

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The secretory units and duct system of the echidna sublingual glands exhibit subtle architectural modifications to accommodate the viscous secretion produced by these glands. The glands are compound tubular glands, the secretory units of which are elongate with open lumina and consist only of mucous cells. Closely packed spindle-shaped myoepithelial cells invest the secretory units, but are absent around the ducts. The branched secretory tubules open into an abbreviated duct system characterized by wide lumina. Striated ducts normally associated with the second portion of the intralobular duct system are absent. The duct system shows the most obvious modification of general salivary gland architecture presumably to accommodate the viscous secretion propelled from the secretory units by surrounding myoepithelial cells.

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“…Weaning occurs around postnatal day 94 60 (Keyte and Smith, 2008). 95 In contrast to therian mammals, young extant monotremes do not obtain milk in quite the 96 same way as therian mammals due to the absence of teats in the mother (Griffiths, 1978). 97…”

mentioning

confidence: 99%

“…Instead young monotremes suck up milk vigorously from the flattened but protuberant 98 nipple-like areola on the mother's abdomen (Griffiths, 1978). In the case of echidnas, these 99 areolae are within the pouch.…”

mentioning

confidence: 99%

A cranio-incudo joint as the solution to early birth in marsupials and monotremes

Anthwal

Fenelon

Johnston

et al. 2020

Preprint

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Mammals articulate their jaws using a novel joint between the dentary and squamosal bones. In eutherian mammals, this joint forms in the embryo, supporting feeding and vocalisation from birth. In contrast, marsupials and monotremes exhibit extreme altriciality and are born before the bones of the novel mammalian jaw joint form. These mammals need to rely on other mechanisms to allow them to feed. Here we show that this vital function is carried out by the earlier developing, cartilaginous incus of the middle ear, abutting the cranial base to form a cranio-mandibular articulation. The nature of this articulation varies between monotremes and marsupials, with monotremes retaining a double articulation, similar to that described in the fossil mammaliaform, Morganucodon, while marsupials use a versican rich matrix to stabilise the jaw against the cranial base. These findings provide novel insight into the evolution of mammals and the relationship between the jaw and ear.

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The Affinities of the Monotremes

Cited by 37 publications

References 0 publications

The TMJ disc is a common ancestral feature in all mammals, as evidenced by the presence of a rudimentary disc during monotreme development

The TMJ disc is a common ancestral feature in all mammals, as evidenced by the presence of a rudimentary disc during monotreme development

Microscopy of the Echidna Sublingual Glands

A cranio-incudo joint as the solution to early birth in marsupials and monotremes

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