Siamoperadectes

Siamoperadectes is a genus of non-marsupial metatherian from the Miocene of Thailand. A member of Peradectidae, it is the first member of its clade known from South Asia, and among the last non-marsupial metatherians.

Siamoperadectes Temporal range: 15–11 Ma PreꞒ Ꞓ O S D C P T J K Pg N ↓ Miocene
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Infraclass:	Metatheria
Family:	†Peradectidae
Genus:	†Siamoperadectes
Type species
Siamoperadectes minutus Ducrocq et al. 1992
Species
Siamoperadectes minutus

Description

The type specimen of Siamoperadectes is a single third upper molar found in the Li Mae Long Basin, northern Thailand.[1] It displays a rectilenear predilambdodont centrocrista, lacks an hypocone and has a moderately slender lingual part of the molar, all charactertistics 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 opossum. Though peradectids have been traditionally considered scansorial, the fact that the relatively closely related herpetotheriids were terrestrial[2] 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.[3] Though described as a didelphid in the original paper,[1] the current general consensus is that peradectids are outside of crown-group Marsupialia,[4][5] and their appearance in the Late Cretaceous greatly predates the estimated initial divergence within marsupials 45 million years ago.[6]

Ecology

Siamoperadectes is known from the Miocene Li Mae Long deposits, which are rich on a variety of other mammal species such as the eulipotyphlans Thaiagymnura equilateralis, Hylomys engesseri, Neotetracus butleri and Scapanulus lampounensis, several rodents such as Diatomys liensis, the treeshrew Tupaia miocenica and several bats, ungulates and carnivorans.[7] So far as known, every other mammal in its environment was a placental eutherian.

Biogeography

Siamoperadectes is the most southerly known peradectid.[1] 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, dating to the mid-Miocene somewhere between 15 and 11 million years ago.[1] Traditionally, competition with placental mammals has been deemed as a culprit for the ultimate extinction of metatherians outside of South America 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.[8][9] At least herpetotheriids appear to have been reasonably common if unspeciose until the mid-Miocene,[10] when they suddenly disappear; Asian peradectids followed soon after.

After the extinction of Siamoperadectes, Australian-derived bear cuscuses (Ailurops) colonised Indonesian islands.

References

1. Ducrocq, Stephane; Buffetaut, Eric; Buffetaut-Tong, Haiyan; Jaeger, Jean-Jacques; Jongkanjanasoontorn, Yaowa-Lak; Suteethorn, Varavudh (1992). "First fossil marsupial from South Asia". Journal of Vertebrate Paleontology. 12 (3): 395–399. doi:10.1080/02724634.1992.10011468.
2. Sánchez-Villagra, Marcelo; Ladevèze, Sandrine; Horovitz, Inés; Argot, Christine; Hooker, Jeremy J.; Macrini, Thomas E.; Martin, Thomas; Moore-Fay, Scott; de Muizon, Christian; Schmelzle, Thomas; Asher, Robert J. (2007). "Exceptionally preserved North American Paleogene metatherians: adaptations and discovery of a major gap in the opossum fossil record". Biology Letters. 3 (3): 318–322. doi:10.1098/rsbl.2007.0090. PMC 2390683. PMID 17426007.
3. Ni, Xijun; Meng, Jin; Wu, Wenyu; Ye, Jie (2006). "A new Early Oligocene peradectine marsupial (Mammalia)from the Burqin region of Xinjiang, China". Naturwissenschaften. 94 (3): 237–241. doi:10.1007/s00114-006-0182-2. PMID 17136514.
4. Kielan-Jaworowska, Zofia; Cifelli, Richard L.; Luo, Zhe-Xi (2004). Mammals from the Age of Dinosaurs: Origins, Evolution, and Structure. New York: Columbia University. ISBN 0231509278.
5. Williamson, Thomas E.; Brusatte, Stephen L.; Carr, Thomas D.; Weil, Anne; Standhardt, Barbara R. (2012). "The phylogeny and evolution of Cretaceous–Palaeogene metatherians: cladistic analysis and description of new early Palaeocene specimens from the Nacimiento Formation, New Mexico". Journal of Systematic Palaeontology. 10 (4): 625–651. doi:10.1080/14772019.2011.631592.
6. Hubrecht, edited by Robert; Kirkwood, James (2010). The UFAW handbook on the care and management of laboratory and other research animals (8th ed.). Chichester, West Sussex: Blackwell Publishers. ISBN 978-1444318784.CS1 maint: extra text: authors list (link)
7. Mein, P. and Ginsburg, L. 1997. Les mammifères du gisement miocène inférieur de Li Mae Long, Thaïlande : systématique, biostratigraphie et paléoenvironnement. Geodiversitas 19(4):783-844.
8. Sánchez-Villagra, Marcelo R. (2013). "Why are There Fewer Marsupials than Placentals? On the Relevance of Geography and Physiology to Evolutionary Patterns of Mammalian Diversity and Disparity" (PDF). Journal of Mammalian Evolution. 20 (4): 279–290. doi:10.1007/s10914-012-9220-3.
9. Carter, Anthony Michael; Mess, Andrea Maria (2013). "Conservation of placentation during the tertiary radiation of mammals in South America". Journal of Morphology. 274 (5): 557–569. doi:10.1002/jmor.20120. PMID 23355381.
10. Klietmann, J.; Nagel, D.; Rummel, M.; Hoek Ostende, L.W. van den (2013). "Amphiperatherium and Erinaceidae of Petersbuch 28". Bulletin of Geosciences. doi:10.3140/bull.geosci.1454.