Rucervus schomburgki was endemic to Thailand and is thought to have become extinct when the last captive individual was killed in 1938; the last known wild animals were killed in 1932 near Sai Yoke and Kwae Yai, although this date was not universally accepted at the time (Harper 1945; Lekagul and McNeely 1988). The recent range in Thailand seems to have lain within 13°30′–18°N, 98°30′–102°E, comprising the central plain of Thailand (Giles 1937; Harper 1945; G.J. Galbreath in litt. 2008). Attempts to circumscribe the range have, however, been bedevilled by a surprisingly large number of statements of occurrence elsewhere.
Statements of occurrence in the Sanda Valley, Yunnan, China by Sclater (1891: 180) and Bentham (1908) led to this locality being included without any caveat by Grubb (2005), and relate to both a frontlet and a skin. However, Sclater (1891) listed the set of antlers as of unknown locality, and the skin has a "?" mark that makes its Sanda Valley locality no more than tentative (G.J. Galbreath in litt. 2008). Then, Bentham (1908) indicated that the antlers had been collected in the Sanda Valley by John Anderson in 1878, without giving explicit source for this statement. In fact, Anderson was in the Sanda Valley in 1868, not 1878, and moreover recorded no such antlers in his thorough write-up of zoological results of his Yunnan expeditions. This locality assignment for the antlers should be seen as an error of Bentham's, presumably stemming from the earlier, tentative, assignment of Sanda Valley to the deer skin Sclater also reported (G. J. Galbreath in litt. 2008). Pocock (1943) included under distribution of the species “N. Siam [Thailand] and, it has been alleged, Yunnan and Lao PDR”, indicating his own concern with the Sanda valley ‘evidence’. The Shan states (Myanmar) have also figured as part of the species’s range, e.g. by Blanford (1891: 540). Reference to historical occurrence in these areas seems to stem from Brooke (1876), who wrote, based on written communication from a Dr Campbell of the Bangkok British Consulate) in reference to new specimens sent to him that ". . . all specimens were procured in northern Siam, probably even in the tributary states named Lao PDR and Shan". Campbell seems to have based his opinion at least partly on what professional indigenous hunters told him or others. Brooke gave no definition of "northern Siam" (conceivably it simply meant some way north of Bangkok), and Lao PDR in this context could well have included part or all of the Korat Plateau and other areas in present-day Thailand which are ethnically Lao and were often referred to in the past by terms such as ‘Siamese Lao PDR’. It is not even clear that Lao PDR and Shan were not just speculation on the part of Campbell and/or Brooke (G.J. Galbreath in litt. 2008).
Not surprisingly, Harper (1945) considered the Shan states report to be “highly indefinite” and occurrence in “Indo-China” [presumably = Lao PDR, Cambodia and Viet Nam] to be “in error”, although he took the ‘Sanda valley’ frontlet as “apparently authentic”, perhaps being focussed on the identity (which is not in doubt) rather than the provenance (presumably assuming that the association with the highly-respected Anderson’s name was enough). A more modern reanalysis concluded that "the northern Thailand, Myanmar, Yunnan, and Lao PDR range extensions suggested in the literature are based on erroneous or quite inconclusive items of evidence", sometimes perhaps influenced by a switch of Thai names between Eld’s and Schomburgk’s deer by Flower (1900) (G.J. Galbreath in litt. 2008).
Two loose antlers, presumed to be a pair, of Schomburgk’s deer photographed in Phongsali province, far northern Lao PDR, in 1991 were a most surprising find. This has been taken to suggest the species might survive, at least into the 1980s, and that it did indeed occur in Lao PDR (Schroering 1995) and it was even taken as sufficient evidence that the species was not extinct by MacPhee and Flemming (1999). However, while these are indeed Schomburgk’s deer antlers, it has proven impossible to recover a consistent story from their owner of the antlers’ origin (e.g. Duckworth et al. 1999) and there is no compelling evidence that they had come from a recently killed animal, rather than being decades-old stock still in trade; Williams (1941) noted that even then that Schomburgk’s deer antlers "keep coming down to Bangkok from Paknampo and Korat by railway amongst collections of ordinary deer-horns consigned to Bangkok Chinamen . . . the horns are probably quite old . . ." and Schomburgk’s deer antlers are still in trade in Thailand (Srikosmatara et al. 1992). The owner of the antlers was, according to his daughter, a middleman trader in wildlife parts sourced from all over Indochina (W.G. Robichaud pers. comm. 2008, based on visit in 1996). Consequently, while remains of the species have been seen in Lao PDR in recent times, they cannot be taken as evidence that the species survives today, or ever occurred in Lao PDR.