Desmostylia

Desmostylia is an extinct order of aquatic mammals that lived in the North Pacific Ocean from the early Oligocene to the late Miocene (30.8 to 7.25 million years ago).
They are the only known extinct order of marine mammals.

Desmostylia means "bundle pillar" in Greek, referring to the distinctive shape of their teeth, which were cylindrical and arranged in rows.
These teeth were used for crushing and grinding hard plant material, such as seaweed and kelp.
Desmostylia were herbivorous and probably fed on the sea floor, using their strong limbs and short tails to move around.

Desmostylia were large and bulky animals, ranging from 1.7 to 5 meters in length.
The smallest species was Ashoroa laticosta, while the largest was comparable to Steller's sea cow.
They had elongated and broadened snouts, with the nostrils located on the top.
They also had prominent cheekbones, long downward-pointing processes behind the jaw joints, and large ear holes.

Desmostylia had forward-pointing incisors and canine tusks, followed by a long gap where the premolars were reduced or absent.
The molars were large and complex, with multiple cusps and roots.
The dental formula of Desmostylia was 3.1.2-4.3/3.1.1-2.3, meaning that they had three incisors, one canine, two to four premolars, and three molars on each upper jaw, and three incisors, one canine, one to two premolars, and three molars on each lower jaw.

Desmostylia are classified into two families: Desmostylidae and Paleoparadoxiidae.
Desmostylidae includes four genera: Cornwallius, Behemotops, Desmostylus, and Ounalashkastylus.
Paleoparadoxiidae includes three genera: Ashoroa, Paleoparadoxia, and Neoparadoxia.

Desmostylia are related to other aquatic mammals, such as sirens and proboscideans, and belong to the clade Tethytheria, named after the ancient ocean where they evolved.
However, their exact phylogenetic position is still debated, and some studies suggest that they are closer to perissodactyls, such as horses and rhinos.

Desmostylia are known for their unique morphology, ecology, and evolution, as well as their rarity and mystery.
They are among the most enigmatic and fascinating groups of extinct mammals.

The Desmostylia were an extinct group of aquatic mammals that lived in the coastal waters of the North Pacific Ocean from the Early Oligocene to the Late Miocene (about 30.8 to 7.2 million years ago).
They had a body shape similar to hippopotamuses, with broad feet and cylindrical teeth.
They are the only known extinct order of marine mammals.