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Duck bill and echidna – are they our ancestors or relatives?

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25.04.2006

People, as they belong to the mammals, should be particularly interested in the problem of origin of this class in the animal kingdom that has conquered the entire world through evolution: not only the land, but, partly the ocean and the air.

We have already known our ancient ancestors for long – these are medium-sized animals - eutherapsida (which means “real Synapsida”), they appeared at the end of the palaeozoic, approximately 270 million years ago, and continued to develop in the Age of Reptiles – the era of huge dinosaurs’ predominance. Ultimately, dinosaurs became extinct, and eutherapsida, gradually changing, turned into mammals.

However, there are blank spots on this long way, and far from all of its stages are clear once and for all. Thus, investigations by Mikhail Ivakhnenko, Doctor of Biology (Paleontological Institute, Russian Academy of Sciences), make researchers review relations between contemporary mammals and the duck bill and echidna of Australia and the New Guinea long-nosed echidna (Zaglossus). These strangely looking animals belonging to the ovipositor subclass are considered to be primitive mammals (they are also called Prototheria – “primitive animals”).

It is assumed that formerly in the Age of Mammals, they gave rise to contemporary mammals and remained till nowadays. “However, frankly speaking, they are not resembling ordinary mammals that much, says M.F. Ivakhnenko. But, on the other hand, contemporary investigations have revealed suspicious similarity between them and other extinct group of animals – eotherapcida (“ancient Synapsida”). This similarity is particularly connected with specific construction of their auditory apparatus. It looks as if eutherapcida gave origin to only marsupial and placental mammals, and the duck bill, echidna and New Guinea long-nosed echidna (Zaglossus) - are not our ancestors but the top of other branch of evolutionary tree.”

“Ancient Synapsida” had appeared much earlier than “real Synapsida” did, approximately 320 million years ago, and, as paleontologists have proved, the former cannot be the ancestors of the latter. They have a common ancestor group but their ways diverged at once. The two of them differ fundamentally in the cranium structure, or more precisely, they adapted differently the cranial cavities inherited by them from the Crossopterygii fish. “Ancient Synapsida” occupied these cavities with the jaw muscular system – and acquired strong jaws which were certainly useful. But the other way followed by “real Synapsida” turned out to be even more useful and promising. Due to these cavities they extended the cranium and enlarged the size of brain. The difference is really a fundamental one: one got jars, the other – brain.

All the Synapsida acquired hairy “fur coat” (in contrast to reptiles, which protected the body from drying up by scales), the coat preserved moisture and turned out to be useful in the cold spell conditions. By the end of the palaeozoic, they dominated among land quadrupedals and were extremely diverse: some were of a mouse size, gigantic herbivorous – of a rhinoceros size, and predators - of a tiger size. Their reign lasted for more than 150 million years. At the boundary between the palaeozoic and the Age of Reptiles a catastrophe took place, the reasons for which are still being debated by researchers. The diverse paleozoic world disappeared and the dinosaur era began.

It was assumed for a long time that “ancient Synapsida” – eotherapsida, had disappeared completely. But there emerged a new hypothesis. They did not disappear but reached to our time in the form of duck-bills and echidnas, which in that way are not our ancestors, but first cousins.

Sergey Komarov | Source: alphagalileo
Further information: www.informnauka.ru

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