Around 180 million years ago, the ancient supercontinent of Gondwana slowly began to break up, a process that would lead to the individual formations of modern continents and regions in the Southern Hemisphere. This fractionalizing of regions would eventually lead to the unique evolution of ancestors of modern mammals.
Named for the Malagasy meaning of “crazy” and the Greek word for “beast,” the newly titled Adalatherium hui represents the most complete skeleton of an ancient animal that lived on Gondwana, an organism also known as a gondwanatherian. It is the unique location of A. hui that likely resulted in its peculiar evolution. Gondwana once incorporated South America, Africa, Arabia, India, Australia, Antarctica, and – until around 88 million years ago – Madagascar, where the skeletal remains were found. A. hui was previously only known by a set of jaws, teeth, and an individual skull so its life history, biology, anatomy, and phylogenetic relationships were poorly understood.
“This skeleton reveals an array of unusual and even unique adaptations that we hypothesize are due to evolution in an insular environment,” write the study authors in Nature, adding that A. hui also had a large number of trunk vertebrae, a short and broad tail, and is so well-preserved that researchers were even able to detect cartilaginous tissue. Though the specimen is believed to be an immature animal weighing just 31 kilograms, researchers note that it is among the largest specimens found from the Mesozoic era of Gondwana spanning between 65 and 252 million years ago, and may suggest gigantism as a result of evolving on an island habitat.
“Among mammals, the most obvious and quantifiable influences of evolving on islands are those related to body size. This observation has led to the articulation of the ‘island rule’, which states that—evolutionarily—small mammals on islands increase, and large mammals decrease, in size,” note the authors, adding that the “island rule” is somewhat controversial and “clearly not ubiquitous.”
“In addition, evolution in insular environments is thought to result in changes in anatomy, physiology, behavior and life-history strategies, and relatively low species richness, taxonomic imbalance, high endemism and a general level of ‘primitiveness,’” they add.
As Madagascar separated from the Indian subcontinent and the Seychelles, animals located on the island evolved in “complete isolation” unique to island environments that promote unique evolutionary paths in animals, largely due to “limited resources, reduced interspecific competition and a paucity of predators and parasites,” note the researchers. Organisms that were able to access the island either had to fly, swim, or raft to it and, because it does not appear that A. hui could fly or was particularly suited for swimming, the mammal likely evolved on the island.
At least two other known gondwanatherians are recorded in the scientific literature, Adalatherium Lavanify and Vintana. The new species is placed next to multituberculates on the phylogenetic tree, a group of rodent-like mammals from the northern continents.