Echidna and Platypus May Have Common Ancestor
30 million-year old fossils of monotremes, previously believed to be a Cretaceous platypus, have been shown to be closely related to present day ground-dwelling echidnas
Echidnas (Tachyglossus and Zachyglossus species) are the Australian animals most similar to an ant eater. Although they eat mainly ants and termites, they are not related to the true anteaters of the Americas.
Common in Australia, New Zealand and New Guinea, echidnas are monotremes – egg laying mammals. Platypus are the only other existing monotremes.
Both have long beak-like snouts – the platypus’ ‘duckbill’ and the enchidna’s snout, which it uses to dig up ants and termites in Australia, and worms and insect larvae in New Zealand and New Guinea.
Their name refers to a mythological Greek monster, Echidna, the ‘Mother of All Monsters’ who lived in an underground cave.
Echidnas are so called as they dig burrows under the earth, and because of their hairy spiny coats, rather like hedgehogs and porcupines.
Despite their fearsome name, echidnas are generally shy creatures, and most Australians view them with affection.
Platypus and Echidnas Closely Related
Biologist Dr Matt Phillips, Postdoctoral Fellow at the Australian National University Research School of Biology and colleagues from the University of Adelaide and the South Australian Museum considered that that modern platypus and echidnas shared a common ancestor as recently as 30 million years ago, contrary to the common palaeontological belief.
Their conclusions, were published in Proceedings of the National Academy of Sciences.
"Recent reassignment of the Early Cretaceous Teinolophos and Steropodon to the platypus lineage implies that platypuses and echidnas diverged >112.5 million years ago, reinforcing the notion of monotremes as living fossils,” Dr Phillips said in the abstract to his paper.
However, his team’s research showed that modern platypus and echidnas diverged only about 13 million years ago from an amphibious platypus-like ancestor.
Comparing Morphology Proved Link Between Platypus and Echidnas
The classification of the Teinolophos and Steropodon fossils to the platypus lineage “was based primarily on characters related to a single feature, the enlarged mandibular canal, which supplies blood vessels and dense electrosensory receptors to the platypus bill,” he said in the paper.
The fossils actually looked more like a modern shrew than either a platypus or an echidna, Dr Phillips said, whereas another fossil species, the Obdurodon was clearly related to platypus.
To clarify which fossils were related to the modern platypus and echidna, the team compared 440 different morphological features of the skull and forelimbs.
“Our reevaluation of the morphological data instead groups platypus and echidnas to the exclusion of Teinolophos and Steropodon and suggests that an enlarged mandibular canal is ancestral for monotremes (partly reversed in echidnas, in association with general mandibular reduction).
“We confirmed this with an examination of DNA sequences. These revealed that platypus and echidnas shared a common ancestor as recently as 30 million years ago," he said.
“We know from the fossil record that 61 million year old fossil monotremes already looked and would have acted much like platypuses. This tells us that the approximately 30 million year old common ancestor of platypuses and echidnas must also have been an amphibious platypus-like creature, whose echidna descendants some time later made a transition from feeding in water to feeding on land.”
Echidnas and Platypus Not ‘Living Fossils’
Dr Phillips said the research disproved the common belief that echidnas and platypus were “evolutionary dead ends”.
Because they display “supposedly primitive characteristics of monotremes – such as egg-laying”, he said, they were considered to be primitive species that evolved long before marsupials and mammals.
“The expansion onto land of the ecological territory that monotremes hold occurred long after marsupials had arrived in Australia,” he said.
“This contradicts the common assumption that monotremes are ‘living fossils’, just treading water in an evolutionary sense, waiting to go extinct in the face of competition with ‘superior’ mammals like marsupials.
“The finding that echidnas evolved relatively recently also helps to explain the mystery of their absence from well sampled fossil sites prior to 13 million years ago,” he added.
Monotremes Occupy Ecological Niches
The echidna’s shift from water to land allowed monotremes to occupy ecological niches despite potential competition from marsupials. These niches included rocky and sandy areas where termites and ants thrived, where the echidna's strong front legs and sharp digging claws would be be an advantage
“Monotremes might have survived the invasion of marsupials into Australasia by exploiting ecological niches in which marsupials are restricted by their reproductive mode,” Dr Phillips said.
Monotremes lay eggs in a burrow (underwater in the case of the platypus). Once the eggs hatch, the mother moves the foetus to her pouch where milk patches secrete milk.
“Ironically, it is some of the supposedly primitive characteristics of monotremes – such as egg-laying – that probably helped them compete against marsupials," Dr Phillips said.
Marsupial young are born alive and the newborn attaches to the nipple with its mouth. “This makes feeding in water like a platypus a problem for marsupials and probably prevents marsupials from being able to evolve an echidna-like beak,” he said.
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