Siamoperadectes
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''Siamoperadectes'' is a genus of non-
marsupial Marsupials are any members of the mammalian infraclass Marsupialia. All extant marsupials are endemic to Australasia, Wallacea and the Americas. A distinctive characteristic common to most of these species is that the young are carried in a ...
metatheria Metatheria is a mammalian clade that includes all mammals more closely related to marsupials than to placentals. First proposed by Thomas Henry Huxley in 1880, it is a more inclusive group than the marsupials; it contains all marsupials as wel ...
n from the
Miocene The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...
of
Thailand Thailand ( ), historically known as Siam () and officially the Kingdom of Thailand, is a country in Southeast Asia, located at the centre of the Indochinese Peninsula, spanning , with a population of almost 70 million. The country is b ...
. A member of Peradectidae, it is the first member of its clade known from
South Asia South Asia is the southern subregion of Asia, which is defined in both geographical Geography (from Greek: , ''geographia''. Combination of Greek words ‘Geo’ (The Earth) and ‘Graphien’ (to describe), literally "earth descr ...
, and among the last non-marsupial metatherians.


Description

The type specimen of ''Siamoperadectes'' is a single third upper molar found in the
Li Mae Long The Mae Long Formation in the Li Basin (also referred to as Li Mae Long) is a fossil site in Lamphun Province, Thailand. The fossils found are thought to date to the late Early Miocene, about 18 million years ago, corresponding to the European zon ...
Basin, northern Thailand. It displays a rectilinear predilambdodont centrocrista, lacks an hypocone and has a moderately slender lingual part of the molar, all characteristics that most closely connect it to peradectid metatherians. However, it also displays several unique characteristics: - a deep and narrow protofossa; - very weak conules; - an anteroposteriorly compressed protocone; - a posterior cingulum at the base of the metacone. The molar is quite small, and in life would probably have belonged to a creature about the size of a modern ''
Monodelphis ''Monodelphis'' is a genus of marsupials in the family Didelphidae, commonly referred to as short-tailed opossums. They are found throughout South America. , the most recently described species is ''M. vossi''. Species * Sepia short-tailed ...
'' opossum. Though peradectids have been traditionally considered scansorial, the fact that the relatively closely related herpetotheriids were terrestrial may suggest a similar lifestyle, though the lack of postcranial remains for ''Siamoperadectes'' render this speculation.


Relationships

Currently, ''Siamoperadectes'' is considered to be a peradectid metatherian, and in particular closely related to '' Sinoperadectes'' and '' Junggaroperadectes''. Though described as a didelphid in the original paper, the current general consensus is that peradectids are outside of crown-group Marsupialia, and their appearance in the
Late Cretaceous The Late Cretaceous (100.5–66 Ma) is the younger of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''creta'', ...
greatly predates the estimated initial divergence within marsupials 45 million years ago.


Ecology

''Siamoperadectes'' is known from the Miocene Li Mae Long deposits, which are rich on a variety of other mammal species such as the
eulipotyphla Eulipotyphla (, which means "truly fat and blind") is an order of mammals suggested by molecular methods of phylogenetic reconstruction, which includes the laurasiatherian members of the now-invalid polyphyletic order Lipotyphla, but not the ...
ns '' Thaiagymnura equilateralis'', '' Hylomys engesseri'', '' Neotetracus butleri'' and '' Scapanulus lampounensis'', several rodents such as '' Diatomys liensis'', the treeshrew ''
Tupaia miocenica ''Tupaia miocenica'' is a fossil treeshrew from the Miocene of Thailand. Known only from a single tooth, an upper first or second molar, it is among the few known fossil treeshrews. With a length of 3.57 mm, the tooth is large for a treeshr ...
'' and several bats, ungulates and carnivorans. So far as known, every other mammal in its environment was a
placental Placental mammals (infraclass Placentalia ) are one of the three extant subdivisions of the class Mammalia, the other two being Monotremata and Marsupialia. Placentalia contains the vast majority of extant mammals, which are partly distinguishe ...
eutheria Eutheria (; from Greek , 'good, right' and , 'beast'; ) is the clade consisting of all therian mammals that are more closely related to placentals than to marsupials. Eutherians are distinguished from noneutherians by various phenotypic tra ...
n.


Biogeography

''Siamoperadectes'' is the most southerly known peradectid. The close relations to Chinese peradectids like '' Sinoperadectes'' and '' Junggaroperadectes'' suggest that it had a Laurasian origin rather than having evolved in the Indian subcontinent, and alongside African and Indian herpetotheriids and true marsupials it represents one of several Cenozoic metatherian colonisations of southern landmasses.


Temporal range

Alongside the Chinese '' Sinoperadectes'', ''Siamoperadectes'' is one of the youngest Laurasian metatherians and certainly one of the last non-marsupial metatherians aside from the South American
sparassodonts Sparassodonta (from Greek to tear, rend; and , gen. , ' tooth) is an extinct order of carnivorous metatherian mammals native to South America, related to modern marsupials. They were once considered to be true marsupials, but are now though ...
, dating to the mid-Miocene somewhere between 15 and 11 million years ago. Traditionally, competition with placental mammals has been deemed as a culprit for the ultimate extinction of metatherians outside of
South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere at the northern tip of the continent. It can also be described as the sout ...
and Australia, but this has been placed into question, especially given in light of the coexistence of both clades through most of the Cretaceous and Cenozoic. At least herpetotheriids appear to have been reasonably common until the mid-Miocene, when they suddenly disappear; Asian peradectids followed soon after. After the extinction of ''Siamoperadectes'', Australian-derived bear cuscuses (''Ailurops'') colonised Indonesian islands.


References

{{Taxonbar, from=Q22286304 Miocene mammals of Asia Fossil taxa described in 1992 Prehistoric metatherians