By the Middle Jurassic (about 165 million years ago), mammals were already geographically widespread and lived in ecologically diverse habitats. Recent discoveries reveal that early mammals climbed, dug holes, swam and some mammals even ate dinosaurs. Despite the tremendous increase in anatomical information from fossils, one question still remains unanswered: when did mammals originate? This project will investigate the origin of mammals by incorporating skeletal material of several, until recently, poorly known early mammals into a broad-scale phylogenetic analysis. Detailed morphological descriptions, character matrices, illustrations, and 3D reconstructions will be openly accessible through MorphoBank and MorphoSource. This work has already generated collaborations with high school students and has formed part of a course offered to undergraduate Women in Science and Engineering at Stony Brook University. More specifically, this project centers on an early mammalian grouping called Allotheria (including Gondwanatheria, Haramiyida, Euharamiyida, and Multituberculata). Recently, Allotheria became a focal point in the debate on the origin of mammals as it may include species that are more than 50 million years older than the oldest, previously known mammalian fossils, pushing the origin of mammals back into the Triassic.
Allotheria have specialized dentition which is more similar to that of modern rodents than any other early mammal. Most early mammals either retained primitive reptilian-like teeth or developed a pre-tribosphenic or tribosphenic dentition (a pattern seen in modern placental and marsupials). The clades of Allotheria do not fit this scheme, but whether this is due to a common origin or to convergent evolution is uncertain. Rigorous tests of their phylogenetic relationships have not been possible, because several allotherians were, until recently, represented almost solely by highly specialized teeth and jaws. Fortunately, largely complete skeletons of basal multituberculates, euharamiyids, and haramiyids have been found in China, while a complete and exceptionally well-preserved skeleton of a gondwanatherian-like mammal has been found in Madagascar. Together with a complete skull of a gondwanatherian from Madagascar, these remarkable discoveries provide a tremendous amount of morphological data that was previously unavailable. By including this information into the Tree of Life (AToL) matrix, it will finally be possible to provide more robust tests of the position of Gondwanatheria, Haramiyida, Euharamiyida, and Multituberculata relative to extant mammalian lineages. On a broader scale, scoring previously unsampled specimens for more than 1,500 characters in the AToL Mammal Tree of Life matrix will contribute significantly to the ongoing growth of that project.
