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

Context paragraphs:
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.

Antilohyrax: Antilohyrax was a genus of herbivorous mammal belonging to the order Hyracoidea.  Fossils were found in 1983 in Egypt, 46 m above the bottom of the Jebel Qatrani Formation.  The species "Antilohyrax pectidens" had an approximate weight of 33–35 kg.  It had features not seen in other Hyrax including a "broad hyper-pectinate comb-like first incisor" on its lower jaw, selenodont molars and a rostrum similar to that seen in even-toed ungulates.

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.

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.

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.

Dinosaur biostratigraphy: Dinosaur biostratigraphy studies the distribution of dinosaur taxa through rock layers.  It can be useful for dating and correlating rock units and reconstructiong ancient ecosystems.  Most dinosaur-bearing rock formations do not contain multiple distinct stratigraphically separated faunas.  Typically dinosaur faunas are static throughout a formation or change piecemeal over time.  Faunal turnover usually occurs between formations.  The fossil record can give an appearance of faunal turnover due to multiple causes including evolution, migration, or changing preservational biases.  Turnover events can have extremely minor causes like the migration of a taxon to a new area or extremely conspicuous ones like an ecosystem destroying catastrophe.  Since the fossil record is incomplete assessing the nature and causes of faunal turnovers is fraught with difficulty, except in cases where the fossil record is "unusually complete."

Xenastrapotherium: Xenastrapotherium is an extinct genus of astrapothere, a type of hoofed herbivorous mammal, native to South America, which lived in the Middle to Late Miocene period, typically during the Laventan stage.  It is a member of the family Astrapotheriidae in the subfamily Uruguaytheriinae, large astrapotheres, equipped with a trunk-like nose and protruding teeth, similar to the elephants, but their tusks were the canine teeth, not the incisors.  "Xenastrapotherium" (named after the Greek word "xenos" "strange" add to the genus "Astrapotherium", "lightning beast") was a genus widely distributed in northern South America, in contrast to other species of astrapotheres which lived in the area of the Southern Cone of the continent.  It differed from other astrapotheres by having two lower incisors on each side of the jaw and the tusks have a pronounced longitudinal curvature, although their general shape and size are probably very similar to "Astrapotherium", whose weight would be 900 to 1,500 kilograms, comparable to the current black rhino.

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".

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.

Kharmerungulatum: Kharmerungulatum is an extinct genus of herbivorous mammal from the Late Cretaceous (Maastrichtian) Intertrappean Beds of Andhra Pradesh, India.  Its specific epithet honors Leigh Van Valen.  It was originally considered to be one of the earliest known condylarths., but more recent studies find it to be a zhelestid, part of an assemblage of herbivorous non-placental eutherian mammals also present in Europe, Asia and possibly Africa.

Supporting paragraphs: Evolution of sirenians Dugong
Answer: 50-million-year-old fossil record