進化：「狂獣」の発見によって充実したゴンドワナ大陸の化石記録

The discovery of the first near-complete skeleton of a gondwanatherian - a mammal that lived on the ancient southern supercontinent Gondwana - from the Cretaceous period of Madagascar, is reported in Nature this week. The finding sheds light on early mammal evolution.

The fossil record of mammals of the Mesozoic era (252 to 65 million years ago) from the southern supercontinent Gondwana is much smaller than that of the northern supercontinent Laurasia. One group of mammals known from Gondwana is Gondwanatheria, but these animals were previously represented only by isolated jaws and teeth, and a single skull.

David Krause and colleagues describe the most complete skeleton known for a Gondwanan Mesozoic mammal. It represents a new species, which the authors have named Adalatherium hui, from a Malagasy word meaning ‘crazy’ and the Greek word for ‘beast’. The skeleton includes a large number of trunk vertebrae and a short, broad tail. Small bones and cartilaginous tissue have also been preserved. Although it represents an immature individual, at an estimated body mass of 3.1 kg, this Adalatherium specimen is among the largest of the mammaliaforms known from Mesozoic Gondwana. This may reflect gigantism as a result of the species’ evolution in isolation, as the most obvious and quantifiable influences of evolving on islands have been found to be related to body size.

Through analysis of evolutionary relationships with other species, the authors have placed the new species close to the multituberculates, a group of rodent-like mammals largely known from the northern continents. The completeness of the skeleton and its existence in the insular environment of Madagascar provide an opportunity to study how Mesozoic mammaliaforms evolved in isolated environments.