▁ore od ont ▁of ▁the ▁family ▁M ery co id od ont idae , ▁en demic ▁to ▁North ▁America . ▁They ▁lived ▁during ▁the ▁Early ▁O lig oc ene ▁ 3 3 . 9 — 3 0 . 8 ▁my a , ▁existing ▁for ▁approximately ▁. ▁F oss ils ▁are ▁w ides p read ▁through ▁the ▁western ▁United ▁States . ▁▁▁▁▁▁ ▁M ery co cho er us ▁was ▁a ▁c ud - ch ew ing ▁plant - e ater ▁with ▁a ▁long ▁face , ▁t usk - like ▁can ine ▁teeth , ▁heavy ▁body , ▁long ▁tail , ▁short ▁feet , ▁and ▁four - to ed ▁ho ov es . ▁The ▁ ▁long ▁animals ▁are ▁thought ▁to ▁have ▁been ▁am ph ib ious , ▁as ▁members ▁of ▁the ▁genus ▁possessed ▁an ▁el ong ated , ▁bar rel - sh aped ▁body ▁and ▁short ▁lim bs ▁that ▁are ▁typical ▁adapt ations ▁found ▁in ▁semi - aqu atic ▁m amm als . ▁ ▁Species ▁M . ▁carri ker i ▁( syn . ▁Prom ery co cho er us ▁th om son i ) ▁M . ▁ch ely dra ▁( syn . ▁Prom ery co cho er us ▁bar b ouri ) ▁M . ▁magn us ▁M . ▁m att he wi ▁M . ▁pin ensis ▁M . ▁propri us ▁M . ▁super bus ▁( syn . ▁M . ▁fr icki , ▁M . ▁le id yi , ▁M . ▁macro st eg us , ▁M . ▁mont anus , ▁M . ▁tempor alis , ▁Prom