Castorocauda

The holotype specimen, JZMP04117, was discovered in the Daohugou Beds of the Jiulongshan Formation in the Inner Mongolia region of China, which dates to about 159–164 million years ago (mya) in the Middle to Late Jurassic. It comprises a partial skeleton including an incomplete skull but well-preserved lower jaws, most of the ribs, the limbs (save for the right hind leg), the pelvis and the tail. The remains are so well preserved that there are elements of its soft anatomy and hair. The genus name Castorocauda derives from Latin Castor "beaver" and cauda "tail", in reference to its presumed beaver-like tail. The species name lutrasimilis derives from Latin lutra "otter" and similis "similar", because some aspects of its teeth and vertebrae are similar to modern otters. ==Description==

Castorocauda was the largest of known docodonts. The preserved length from head to tail is , but in life it was much larger. Based on the dimensions of the platypus, the lower weight limit was estimated to be in life, and the upper , making it the largest known Jurassic mammaliaform, surpassing the previous record of for Sinoconodon. The first two molars have cusps in a straight row, and interlocked during biting. This feature is similar to the ancestral condition in Mammaliaformes (such as in triconodonts) but is a derived character (it was specially evolved instead of inherited) in Castorocauda. It likely also had claws, and the holotype shows a spur on the hind ankle, which, in male platypuses, is venomous. ==Taxonomy==

}} Castorocauda is a member of the order Docodonta, an extinct group of mammaliaforms. Mammaliaformes includes mammal-like creatures and the crown mammals (all descendants, living or extinct, of the last common ancestor of all living mammals). Docodonts are not crown mammals. When Castorocauda was first described in 2006, it was thought to be most closely related to the European Krusatodon and Simpsonodon. Simpsonodontidae is now considered to be paraphyletic and thus invalid, and Castorocauda appears to have been most closely related to Dsungarodon, Castorocauda is part of a Middle Jurassic mammaliaform diversification event, wherein mammaliaforms radiated into a wide array of niches and evolved several modern traits, such as more modern mammalian teeth and middle ear bones. and evidence for an explosive diversification in the Middle Jurassic – such as the appearance of eutriconodontans, multituberculates, australosphenidans, metatherians and eutherians, among others – disproves this notion. This may have been caused by the breakup of Pangaea, which started in the Early to Middle Jurassic and diversified habitats and niches, or modern traits that had been slowly accumulating since mammaliaforms evolved until reaching a critical point which allowed for a massive expansion into different habitats. ==Paleoecology==

Castorocauda is the earliest known aquatic mammaliaform, pushing back the first appearance of mammaliaform aquatic adaptations by over 100 million years. the rat-like Megaconus and the gliding Arboroharamiya. The plant life of the Tiaojishan Formation was dominated by cycadeoids (mainly Nilssonia and Ctenis), leptosporangiate ferns and ginkgophytes and has pollen remains predominantly from pteridophytes and gymnosperms, which indicate a cool temperate and wet climate with distinct wet and dry seasons, ==See also==