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A recent genomic study at chromosome scale by Zhou et. al suggests a divergence of monotremes (prototheria) from other therian mammals (eutherians/placentals and metatherians/marsupials) at ~187Ma. -
The “establishment of a marked climatic gradient restricting dispersal from South America to Australia/East Antarctica across the West Antarctica archipelago” may have restricted the distribution of other mammals to support the development of a unique Australian mammalian fauna.
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Additionally, T. trusleri exhibits morphology resembling a mechano/electroreceptive “bill”/”beak” present in all modern monotremes. This is believed to have been a feeding adaptation to a densely forested environment with months of darkness and periodic snow cover, when Australia lay within the polar circle throughout the Early Cretaceous (Flannery et al. 2022).
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The oldest recognized family of monotremes, Teinolophidae, represented by Teinolophos trusleri, was identified in late Barremian deposits of the Strzelecki Group in Victoria, Australia from dentaries (depicted in figure). It is distinct in having five molars. Its body size is estimated to be ~40g, making it the smallest known monotreme. This time period coincides with the Gondwana supercontinent breaking away from Laurasia (Flannery et. al 2022). -
(113-108Ma) Emergence of large-bodied (~4-8kg) monotremes Kryoryctes cadburyi and Sundrius ziegleri, correlated to the lower Albian Otway Group of western Victoria, Australia. The former was based on a humerus (depicted in figure) that implied "heavily built forelimbs and a fossorial (burrowing) lifestyle similar to modern echidnas" (Pridmore et al. 2005). Tooth morphology suggests Sundrius may not have been durophagous. -
(100-96Ma) Contemporaneous monotreme species Kollikodon ritchiei, Steropodon galmani, and Stirtodon elizabethae were all recovered from the Finch Clay Facies of the Wallangulla Sandstone. Image shows artistic rendering of Steropodon -
A pattern of reduction in molar number starts to emerge as evolution progresses with Kollikodon possessing four molars and Steropodon only three. Extremely thick enamel with “basined dents exposing dentine on the cusp apices” on Kollikodon implies this species consumed hard-shelled prey, such as molluscs (Flannery et. al 1995). -
Only aquatic stem monotremes survived the K-Pg mass extinction (Flannery et al. 2022). This is likely due to terrestrial and oceanic photosynthesis being severely impacted while bacterial decomposition of plant remains in freshwater habitats supported the monotremes’ residual food chains (Robertson et. al 2013). -
(63-61Ma) Monotremata sudamericanum (holotype RM2 depicted in image) stem ornithorhynchid, is discovered based on upper right second molar recovered from sediments of early Paleocene age in Patagonia, southern Argentina. Though more fossils have been uncovered since then, very little about this species is known. It is nonetheless important as it is the only monotreme found outside the Australian continent (Pascual et. al 1992). -
This monotreme’s distribution in South America suggests a similar paleobiogeographic history to that of Australia, with both continents sharing a regional biota under Gondwana before drifting apart northwards. It subsequently went extinct (Pascual et. al 1992).
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A recent phylogenomic reconstruction at chromosome scale (Zhou et al. 2021) revealed Tachyglossidae (echidnas) diverged from Ornithorhynchidae (platypus) ~55Ma. -
Flannery proposes that if Obdurodon tharalkooschild lived contemporaneously to the oldest toothless ornithorhynchid, Obdurodon may be actually be a paraphyletic relative to Ornithorhynchus and additional geological/molecular dating is needed.
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(26-13Ma) The Obdurodon genus is believed to be a close ancestor to the modern Ornithorhynchus. It encompasses three species - O. insigni, O. dicksoni, O. tharalkooschild. The holotype (depicted in image, from various angles) of the latter is based on a tooth found in a cave deposit of uncertain age. -
(5.5Ma) Estimate for the divergence of the two echidna genera - Zaglossus (long-beaked) and Tachyglossus (short-beaked) (Phillips et. al 2009). -
(5.3 Ma) Tachyglossidae migrate to Australia via landbridge known as the Sahul shelf. Flannery hypothesizes that Tachyglossidae originated in Melanesia. This theory is supported by the sudden appearance of tachyglossidae in the Australian fossil record from the late Miocene to Pleistocene. -
(3.8Ma) First appearance of the Ornithorhynchus genus, which includes the world's only extant modern platypus, Ornithorhynchus anatinus (Flannery et. al 2022). -
The success and distribution of the latter was helped by extinction of Pleistocene megafauna, which endorsed termites as its principal food to a major ‘grazing’ influence on the Australian grasslands (Flannery et. al 2022).
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(1.8 Ma) First appearance of extant echidnas: Zaglossus, the long beaked echidna, now endemic to New Guinea, including three living species - Z. bruijnii, Z. bartoni, Z. attenboroughi, and Tachyglossus, the short beaked Australian echidna represented only by T. aculeatus. Time based on upper estimate for Zaglossus/Tachyglossus divergence (Philips et. al 2009). -
(1.78Ma) Tachyglossid genus Megalibgwilia was described from a partial associated skeleton attributed to M. owenii and a humerus belonging to M. robusta. Published measurements of the linear limb suggest body mass was similar to Zaglossus, (Murray 1978b). Its short, broad skull suggests Megalibgwilia fed primarily on large soil invertebrates, particularly insect larvae. This specialization is thus used as evidence for evolution towards a more terrestrial lifestyle.
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Its shortened tibia and relatively long femur is an adaptation to its body’s center of gravity shifting backwards. ‘This may have allowed mobility of the forelimbs for digging or tearing and may have permitted the animal to easily assume an assisted bipedal stance while feeding on ants or termites nests, a posture sometimes used by both living genera’ (Murray 1978b). -
This semi-vertical posture also suggests an arboreal lifestyle including a scansorial (climbing) feeding pattern on arboreal colonial invertebrates. -
(0.1Ma) Murrayglossus, a new genus proposed by Flannery, is the largest known monotreme discovered from a partial skeleton in Mammoth Cave, Western Australia. It had an estimated body mass 20-30kg and a max body length ~1m - a case of ‘insular’ gigantism owing to prolonged isolation. Its unusual size and distinct morphology suggest that Quaternary monotremes occupied a broader range of ecologies than is currently suspected (Flannery et. al 2022).
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(~0.05Ma) Megalibwilia and Zaglossus genera become extinct in Australia. This is likely due to a degradation of mesic (abundant in moisture) habitats during the Late Pleistocene-Holocene that could have depleted soft-bodied prey, as well as the advent of human hunting (Flannery et. al 2022).