Monotreme

Physiology

Monotremes' metabolic rate is remarkably low by mammalian standards. The platypus has an average body temperature of about 31 °C (88 °F) rather than the averages of 35 °C (95 °F) for marsupials and 37 °C (99 °F) for placentals. Research suggests this has been a gradual adaptation to the harsh, marginal environmental niches in which the few extant monotreme species have managed to survive, rather than a general characteristic of extinct monotremes.

Monotremes may have less developed thermoregulation than other mammals, but recent research shows that they easily maintain a constant body temperature in a variety of circumstances, such as the platypus in icy mountain streams. Early researchers were misled by two factors: firstly, monotremes maintain a lower average temperature than most mammals; secondly, the short-beaked echidna, much easier to study than the reclusive platypus, maintains normal temperature only when active; during cold weather, it conserves energy by "switching off" its temperature regulation. Understanding of this mechanism came when reduced thermal regulation was observed in the hyraxes, which are placentals.

The echidna was originally thought to experience no rapid eye movement sleep (REM). However, a more recent study showed that REM sleep accounted for about 15% of sleep time observed on subjects at an environmental temperature of 25 °C (77 °F). Surveying a range of environmental temperatures, the study observed very little REM at reduced temperatures of 15 °C (59 °F) and 20 °C (68 °F), and also a substantial reduction at the elevated temperature of 28 °C (82 °F).

Monotreme milk contains a highly expressed antibacterial protein not found in other mammals, perhaps to compensate for the more septic manner of milk intake associated with the absence of teats.

During the course of evolution, the monotremes have lost the gastric glands normally found in mammalian stomachs as an adaptation to their diet. As such, by some definitions, they do not have stomachs as an organ, although the term is widely used in studies of monotreme anatomy. Monotremes synthesize L-ascorbic acid only in the kidneys.

Both the platypus and echidna species have spurs on their hind limbs. The echidna spurs are vestigial and have no known function, while the platypus spurs contain venom. Molecular data show that the main component of platypus venom emerged before the divergence of platypus and echidnas, suggesting that the most recent common ancestor of these taxa was also possibly a venomous monotreme.

Taxonomy

The traditional "Theria hypothesis" states that the divergence of the monotreme lineage from the Metatheria (marsupial) and Eutheria (placental) lineages happened prior to the divergence between marsupials and placentals, and the early divergence explains why monotremes retain a number of primitive traits presumed to have been present in the synapsid ancestors of later mammals, such as egg-laying. Most morphological evidence supports the Theria hypothesis, but one possible exception is a similar pattern of tooth replacement seen in both monotremes and marsupials. This similarity originally provided the basis for the competing "Marsupionta" hypothesis, in which the divergence between monotremes and marsupials occurred later than the divergence between these lineages and the placentals. Van Rheede (2005) concluded that the genetic evidence favors the Theria hypothesis, and this hypothesis continues to be the more widely accepted one.

Monotremes are conventionally treated as comprising a single order Monotremata. The entire grouping is typically placed into a subclass Prototheria, which was extended to include several fossil orders; however, these are no longer seen as constituting a group allied to monotreme ancestry. A controversial hypothesis relates the monotremes to a separate assemblage of fossil mammals in a clade termed Australosphenida, a group of mammals from Jurassic and Cretaceous Madagascar, South America and Australia, that share tribosphenic molars. However, in a 2022 review of monotreme evolution, it was noted that Teinolophos, the oldest (Barremian ~ 125 million years ago) and the most primitive monotreme, differed substantially from non-monotreme australosphenidans in possessing five molars, as opposed to the three present in non-monotreme australosphenidians. Aptian and Cenomanian monotremes of the family Kollikodontidae (113–96.6 ma) have four molars. This suggests that the monotremes are likely to be unrelated to the australosphenidan tribosphenids.

The time when the monotreme line diverged from other mammalian lines is uncertain, but one survey of genetic studies gives an estimate of about 220 million years ago, while others have posited younger estimates of 163 to 186 million years ago (though the already eutherian Juramaia is dated to 161–160 million years ago). Teinolophos like modern monotremes displays adaptations to elongation and increased sensory perception in the jaws, related to mechanoreception or electroreception.

Molecular clock and fossil dating give a wide range of dates for the split between echidnas and platypuses, with one survey putting the split at 19–48 million years ago, but another putting it at 17–89 million years ago. It has been suggested that both the short-beaked and long-beaked echidna species are derived from a platypus-like ancestor.

The precise relationships among extinct groups of mammals and modern groups such as monotremes are uncertain, but cladistic analyses usually put the last common ancestor (LCA) of placentals and monotremes close to the LCA of placentals and multituberculates, whereas some suggest that the LCA of placentals and multituberculates was more recent than the LCA of placentals and monotremes.

• ORDER MONOTREMATA
  • Superfamily Ornithorhynchoidea
    • Family Ornithorhynchidae: platypus
      • Genus Ornithorhynchus
        • Platypus, O. anatinus
    • Family Tachyglossidae: echidnas
      • Genus Tachyglossus
        • Short-beaked echidna, T. aculeatus
          • T. a. aculeatus (Common short-beaked echidna)
          • T. a. acanthion (Northern short-beaked echidna)
          • T. a. lawesii (New Guinea short-beaked echidna)
          • T. a. multiaculeatus (Kangaroo Island short-beaked echidna)
          • T. a. setosus (Tasmanian short-beaked echidna)
      • Genus Zaglossus
        • Sir David's long-beaked echidna, Z. attenboroughi
        • Eastern long-beaked echidna, Z. bartoni
          • Z. b. bartoni
          • Z. b. clunius
          • Z. b. diamondi
          • Z. b. smeenki
        • Western long-beaked echidna, Z. bruijni

Summary of extant species

Fossil monotremes

The first Mesozoic monotreme to be discovered was the Cenomanian (100–96.6 Ma) Steropodon galmani from Lightning Ridge, New South Wales. Biochemical and anatomical evidence suggests that the monotremes diverged from the mammalian lineage before the marsupials and placentals arose. The only Mesozoic monotremes are Teinolophos (Barremian, 126 Ma), Sundrius and Kryoryctes (Albian, 113–108 Ma), and Dharragarra, Kollikodon, Opalios, Parvopalus, Steropodon, and Stirtodon (all Cenomanian, 100.2–96.6 Ma) from Australian deposits, and Patagorhynchus (Maastrichtian) from Patagonian deposits in the Cretaceous, indicating that monotremes were diversifiying by the early Late Cretaceous. Monotremes have been found in the latest Cretaceous and Paleocene of southern South America, so one hypothesis is that monotremes arose in Australia in the Late Jurassic or Early Cretaceous, and that some migrated across the Antarctic land bridge to South America, both of which were still united with Australia at that time. This direction of migration is the opposite of that hypothesized for Australia's other dominant mammal group, the marsupials, which likely migrated across Antarctica to Australia from South America.

In 2024, a prominent assemblage of early monotremes was described from the Cenomanian deposits (100–96.6 Ma) of the Griman Creek Formation in Lightning Ridge, New South Wales. One of these, the fossil jaw fragment of Dharragarra, is the oldest known platypus-like fossil. The durophagous Kollikodon, the pseudotribosphenic Steropodon, and Stirtodon, Dharragarra, Opalios, and Parvopalus occur in the same Cenomanian deposits. Oligo-Miocene fossils of the toothed platypus Obdurodon have also been recovered from Australia, and fossils of a 63 million-year old platypus relative occur in southern Argentina (Monotrematum), see fossil monotremes below. The extant platypus genus Ornithorhynchus in also known from Pliocene deposits, and the oldest fossil tachyglossids are Pleistocene (1.7 Ma) in age.

Fossil species

Excepting Ornithorhynchus anatinus, all the animals listed in this section are known only from fossils. Some family designations are hesitant, given the fragmentary nature of the specimens.

• Family †Kollikodontidae
  • Genus †Kollikodon
    • Species †Kollikodon ritchiei
  • Genus †Kryoryctes
    • Species †Kryoryctes cadburyi
  • Genus †Sundrius
    • Species †Sundrius ziegleri
• Family †Steropodontidae
  • Genus †Parvopalus
    • Species †Parvopalus clytiei
  • Genus †Steropodon
    • Species †Steropodon galmani
• Family †Teinolophidae
  • Genus †Stirtodon
    • Species †Stirtodon elizabethae
  • Genus †Teinolophos
    • Species †Teinolophos trusleri – 123 Ma, oldest monotreme specimen
• Superfamily Ornithorhynchoidea
  • Family †Opalionidae
    • Genus †Opalios
      • Species †Opalios splendens
  • Family Ornithorhynchidae
    • Genus †Dharragarra
      • Species †Dharragarra aurora
    • Genus †Monotrematum
      • Species †Monotrematum sudamericanum – 61 Ma, southern South America
    • Genus Ornithorhynchus – oldest Ornithorhynchus specimen 9 Ma
      • Species Ornithorhynchus anatinus (platypus) – oldest specimen 10,000 years old
    • Genus †Obdurodon – includes a number of Miocene (24–5 Ma) Riversleigh platypuses
      • Species †Obdurodon dicksoni
      • Species †Obdurodon insignis
      • Species †Obdurodon tharalkooschild – Middle Miocene and Upper Miocene (15–5 Ma)
    • Genus †Patagorhynchus
      • Species †Patagorhynchus pascuali - Maastrichtian, earliest known South American monotreme
  • Family Tachyglossidae
    • Genus Zaglossus – Upper Pleistocene (1.8–0.1 Ma)
      • Species†Zaglossus robustus
    • Genus †Murrayglossus
      • Species †Murrayglossus hacketti
    • Genus †Megalibgwilia
      • Species †Megalibgwilia ramsayi – Late Pleistocene
      • Species †Megalibgwilia robusta – Miocene