Question: How old is the fossil record of the order that contains the only strictly marine herbivorous mammal?

full_context: Document 1: Platybelodon: Platybelodon ("flat-spear tusk") was a genus of large herbivorous mammal related to the elephant (order Proboscidea). It lived during the late Miocene Epoch in Asia and the Caucasus.
Document 2: Torynobelodon: Torynobelodon was a genus of large herbivorous mammal related to the elephant (order Proboscidea). It lived during the late Miocene Epoch in Asia and North America.
Document 3: Sea mink: The sea mink ("Neovison macrodon") is a recently extinct species of carnivore from the eastern coast of North America in the family Mustelidae, the largest family in the order Carnivora. It was most closely related to the American mink ("Neovison vison"), with debate about whether or not the sea mink should be considered a subspecies of the American mink (making it "Neovison vison macrodon") or a species of its own. The main justification for a separate-species designation is the size difference between the two minks, but other distinctions have been made, such as its redder fur. The only known remains are fragments unearthed in Native American shell middens. Its actual size is speculative, based largely on tooth remains.
Document 4: Piauhytherium: Piauhytherium is an extinct genus of herbivorous mammal, pertaining to the order of the notoungulates and the family of the toxodontids. It lived during the Late Pleistocene (about 10.000 years ago; fossils have been found in Brazil. The only known species is "Piauhytherium capivarae".
Document 5: Sea lion: Sea lions are sea mammals characterized by external ear flaps, long foreflippers, the ability to walk on all fours, and short, thick hair, big chest and belly. Together with the fur seals, they comprise the family Otariidae, eared seals, which contains six extant and one extinct species (the Japanese sea lion) in five genera. Their range extends from the subarctic to tropical waters of the global ocean in both the Northern and Southern Hemispheres, with the notable exception of the northern Atlantic Ocean. They have an average lifespan of 20–30 years. A male California sea lion weighs on average about 300 kg and is about 8 ft long, while the female sea lion weighs 100 kg and is 6 ft long. The largest sea lion is Steller's sea lion, which can weigh 1000 kg and grow to a length of 10 ft . Sea lions consume large quantities of food at a time and are known to eat about 5–8% of their body weight (about 15 – ) at a single feeding.
Document 6: Dugong: The dugong ( , ; "Dugong dugon") is a medium-sized marine mammal. It is one of four living species of the order Sirenia, which also includes three species of manatees. It is the only living representative of the once-diverse family Dugongidae; its closest modern relative, Steller's sea cow ("Hydrodamalis gigas"), was hunted to extinction in the 18th century. The dugong is the only strictly marine herbivorous mammal.
Document 7: Giraffe: The giraffe ("Giraffa") is a genus of African even-toed ungulate mammals, the tallest living terrestrial animals and the largest ruminants. The genus currently consists of one species, "Giraffa camelopardalis", the type species. Seven other species are extinct, prehistoric species known from fossils. Taxonomic classifications of one to eight extant giraffe species have been described, based upon research into the mitochondrial and nuclear DNA, as well as morphological measurements of "Giraffa," but the IUCN currently recognizes only one species with nine subspecies.
Document 8: Threatened mammals of Europe: The list below contains threatened mammals that dwell in or migrate to any region in Europe, the East Atlantic Ocean, and any nearby islands of the Atlantic Ocean. This includes mammals that are found in the East Atlantic Ocean (Azores), Iceland, the Adriatic Sea, the Sea of Azov, the Black and Caspian Sea, Corsica, Cyprus, Palearctic, Russia, Eurasia, North African Coast, the Mediterranean Sea and islands located in the Mediterranean Sea, and the islands of Spain (Canary, Balearic). The list below was compiled from data on the IUCN Red List of Endangered Species (IUCN). The International Union for Conservation of Nature identifies species in need of attention before approaching extinction and works to increase prevention of extinction. The list below includes vulnerable (VU), endangered (EN), critically endangered (CR), and recently extinct (EX) species.
Document 9: Evolution of sirenians: Sirenia is the order of placental mammals which comprises modern "sea cows" (manatees and the Dugong) and their extinct relatives. They are the only extant herbivorous marine mammals and the only group of herbivorous mammals to have become completely aquatic. Sirenians are thought to have a 50-million-year-old fossil record (early Eocene-recent). They attained modest diversity during the Oligocene and Miocene, but have since declined as a result of climatic cooling, oceanographic changes, and human interference. Two genera and four species are extant: "Trichechus", which includes the three species of manatee that live along the Atlantic coasts and in rivers and coastlines of the Americas and western Africa, and "Dugong", which is found in the Indian and Pacific oceans.
Document 10: Protungulatum donnae: Protungulatum donnae is the type species for the "Protungulatum" genus, an extinct early form of eutherian mammal. Though it is by no means the earliest mammal in the fossil record, a 2013 study considers "P. donnae" to be the oldest undisputed placental mammal fossil, though more recent examinations conclude that it was a more basal eutherian and that no placental predates the Paleocene.
Document 11: Caribbean monk seal: The Caribbean monk seal, West Indian seal or sea wolf (as early explorers referred to it), "Neomonachus tropicalis", was a species of seal native to the Caribbean and is now believed to be extinct. The Caribbean monk seals' main predators were sharks and humans. Overhunting of the seals for oil, and overfishing of their food sources, are the established reasons for the seals' extinction. The last confirmed sighting of the Caribbean Monk Seal was in 1952 at Serranilla Bank, between Jamaica and Nicaragua. In 1994 the species was officially declared extinct in the United States of America after an exhaustive search for the seals which lasted for about five years. This analysis was conducted by the National Oceanic and Atmospheric Administration and the National Marine Fisheries Service. Caribbean monk seals were closely related to the Hawaiian monk seals, which live around the Hawaiian Islands and are now endangered, and Mediterranean monk seals, another endangered species. An estimated 600 Mediterranean monk seals and 1,100 Hawaiian monk seals are alive in the wild.
Document 12: List of South American mammals: This is a list of the native wild mammal species recorded in South America. South America's terrestrial mammals fall into three distinct groups. The marsupials and xenarthrans are 'old-timers', their ancestors having been present on the continent since at least the very early Cenozoic Era. During the early Cenozoic, South America's only land connection was to Antarctica, so it was effectively cut off from most of the world; as the fragments of Gondwana continued to separate, this connection was lost, leaving South America an island continent. Caviomorph rodents and monkeys arrived as 'waif dispersers' by rafting across the Atlantic from Africa in the Eocene epoch, 35 million or more years ago. All the remaining nonflying mammals of South America are recent arrivals, having migrated from North America via Central America during the past seven million years as part of the Great American Interchange; this invasion, which peaked around three million years ago, was made possible when the formation of the volcanic Isthmus of Panama bridged North and South America. The newcomers out-competed and drove to extinction many unique mammals that had evolved during South America's long period of isolation, as well as some species from other classes (e.g., terror birds).
Document 13: Stegotetrabelodon: Stegotetrabelodon is an extinct genus of primitive elephant with gomphothere-like anatomical features from the Late Miocene to Early Pliocene of Africa and Eurasia. The type species is "S. syrticus" of late Miocene Africa, which reached roughly 4 m (13.1 ft) in shoulder height. The other unequivocally recognized species is "S. orbus", also of late Miocene Africa. Other species outside of Africa are questionably placed in this genus, including teeth from Late Miocene Hungary and Iran originally described as being of the "Mastodon" subgenus "Bunolophodon", Chinese specimens originally described as being also of "Mastodon", as well as of "Tetralophodon" and "Stegodon", and a species from the late Miocene-aged Dokh Pathan Formation in Pakistan, "S. maluvalensis".
Document 14: Pinniped: Pinnipeds ( ) (from Latin "pinna" "fin" and "pes, pedis" "foot"), commonly known as seals, are a widely distributed and diverse clade of carnivorous, fin-footed, semiaquatic marine mammals. They comprise the extant families Odobenidae (whose only living member is the walrus), Otariidae (the eared seals: sea lions and fur seals), and Phocidae (the earless seals, or true seals). There are 33 extant species of pinnipeds, and more than 50 extinct species have been described from fossils. While seals were historically thought to have descended from two ancestral lines, molecular evidence supports them as a monophyletic lineage (descended from one ancestral line). Pinnipeds belong to the order Carnivora and their closest living relatives are bears and musteloids (weasels, raccoons, skunks, and red pandas), having diverged about 50 million years ago.
Document 15: Hydrodamalis: Hydrodamalis is a genus of extinct herbivorous sirenian marine mammals, and included the Steller's sea cow ("Hydrodamalis gigas"), the Cuesta sea cow ("Hydrodamalis cuestae"), and the Takikawa sea cow ("Hydrodamalis spissa"). The fossil genus "Dusisiren" is regarded as the sister taxon of "Hydrodamalis": together, the two genera form the dugong subfamily Hydrodamalinae. They were the largest member of the order Sirenia, whose only extant members are the dugong ("Dugong dugon") and the manatees ("Trichechus" spp.). They reached up to 9 m in length, making the Steller's sea cow among the largest mammals other than whales to have existed in the Holocene epoch. Steller's sea cow was first described by Georg Wilhelm Steller, Cuesta by Daryl Domning, and Takikawa by Hitoshi Furusawa. The Steller's sea cow was the only member of the genus to survive into modern times, and, although had formerly been abundant throughout the North Pacific, by the mid 1700s, its range had been limited to a single, isolated population surrounding the uninhabited Commander Islands. It was hunted for its meat, skin, and fat by fur traders, and was also hunted by aboriginals of the North Pacific coast, leading to its and the genera's extinction 27 years after discovery. The Cuesta sea cow along with the Takikawa sea cow were probably extinct at the end of the Pliocene due to the onset of the Ice Ages and the subsequent recession of seagrasses—their main food source.
ground_truth: 50-million-year-old fossil record