Desmostylia represents an extinct order of marine mammals, a unique group that once roamed the ancient oceans. These intriguing creatures existed during the Cenozoic Era. Their fossil remains have been discovered along the northern Pacific Rim, stretching from southern Japan across Russia and the Aleutian Islands to the Pacific coast of North America, extending as far south as Baja California. This distribution indicates their presence in a specific geographic range during their existence.
Unveiling Desmostylians
Desmostylians possessed distinctive physical characteristics that set them apart from other mammals. Their name, derived from Greek words meaning “bundle” and “pillar,” refers to their unique columnar teeth. The cusps of their teeth, particularly in the genus Desmostylus, were notably large, high, and featured thick enamel.
Beyond their peculiar dentition, desmostylians had a robust skeletal structure, suggesting a powerful build. They were generally large, often described as hippo-sized animals, with some species like Desmostylus being more active swimmers. Early classifications, based on limited fossil evidence, initially linked them to sirenians due to some shared features. However, more complete fossil discoveries, particularly in Japan, revealed their distinctiveness.
Life in Ancient Seas
Evidence from their bone microanatomy suggests that all desmostylians lived an aquatic or semi-aquatic lifestyle. Studies of their ribs and long bones show a lack of a medullary cavity, a feature interpreted as an aquatic adaptation in other amniotes. Different genera within Desmostylia exhibited varied adaptations for buoyancy and movement. For instance, Behemotops, Paleoparadoxia, and Ashoroa likely inhabited shallow waters, possibly hovering or walking along the bottom.
The diet of desmostylians is believed to have consisted of aquatic plants, such as marine algae and sea grass, which they rooted up from coastlines. Their unique tooth and jaw structure, along with their suspected feeding behavior, indicates they may have used a specialized method, perhaps akin to suction feeding or “vacuuming” vegetation from the shorelines. They were well-suited to coastal, shallow-water environments.
A Unique Branch on the Evolutionary Tree
Desmostylians occupy a distinct and isolated position within the mammalian evolutionary tree, having no living descendants. They represent the only known order of marine mammals to have gone extinct. Their geological timeline spans from the early Oligocene epoch, approximately 30.8 million years ago, to the late Miocene epoch, around 7.25 million years ago.
The eventual disappearance of desmostylians remains a subject of scientific inquiry, with various theories proposed for their extinction. Possible factors include significant climate changes during the Oligocene-Miocene transition, which saw global cooling and changes in ocean conditions. Competition with other emerging marine mammal groups, such as early baleen and toothed whales, might also have contributed to their decline. The unique combination of their specialized feeding and habitat preferences might have made them particularly vulnerable to environmental shifts or increased competition.