Siamotherium joins tiny Dromiciops in the LRT
Long time readers may realize
Eocene Siamotherium (Fig 1) has been difficult to nest in the LRT. It is represented by a 16cm skull without a mandible.
Today Siamotherium finds a new home
in the LRT as a large relative of the tiny extant marsupial from South America, Dromiciops, commonly known as the monito del monte or colocolo opossum (Fig 1).
These taxa all lack a canine, but often have a canine-like anterior premolar with two roots or a mediolaterally narrow alveolus in this case.
Figure 1. Siamotherium compared to Dromiciops and other canine-less relatives, including Docofossor and Glironia.
” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/03/siamotherium-pondaungensis588.gif?w=290″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/03/siamotherium-pondaungensis588.gif?w=584″ class=”size-full wp-image-92122″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/03/siamotherium-pondaungensis588.gif” alt=”Figure 1. Siamotherium compared to Dromiciops and other canine-less relatives, including Docofossor and Glironia.” width=”584″ height=”604″ srcset=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/03/siamotherium-pondaungensis588.gif?w=584&h=604 584w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/03/siamotherium-pondaungensis588.gif?w=145&h=150 145w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/03/siamotherium-pondaungensis588.gif?w=290&h=300 290w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/03/siamotherium-pondaungensis588.gif 588w” sizes=”(max-width: 584px) 100vw, 584px” />
Figure 1. Siamotherium compared to Dromiciops and other canine-less relatives, including Docofossor and Glironia. Tabulars (light red) are here added to the Siamotherium skull. The premaxillary alveoli lean posteriorly. The one procumbent tooth is assumed here to be dis-aligned during taphonomy.
Siamotherium pondaungensis
(Suteethorn et al. 1988; Soe et al. 2017; Eocene) was originally considered a small anthracothere close to Hippopotamus, but here nests with the tiny marsupial, Dromiciops.
Figure 2. Above: Dromiciops skull and tooth row (right rotated). Below: Glironia skull and tooth row (right, rotated).
” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/08/glironia_venusta588-2.jpg?w=163″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/08/glironia_venusta588-2.jpg?w=558″ class=”size-full wp-image-88629″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/08/glironia_venusta588-2.jpg” alt=”Figure 2. Above: Dromiciops skull and tooth row (right rotated). Below: Glironia skull and tooth row (right, rotated).” width=”584″ height=”1073″ srcset=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/08/glironia_venusta588-2.jpg?w=584&h=1073 584w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/08/glironia_venusta588-2.jpg?w=82&h=150 82w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/08/glironia_venusta588-2.jpg?w=163&h=300 163w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/08/glironia_venusta588-2.jpg 588w” sizes=”(max-width: 584px) 100vw, 584px” />
Figure 2. Above: Dromiciops skull and tooth row (right rotated). Below: Glironia skull and tooth row (right, rotated).
If Siamotherium nests elsewhere in the future
I will let you know. At present this appears to be a novel hypothesis of interrelationships. Some of the problems in scoring mammals for the LRT are illustrated above.
References
Gray JE 1821. On the natural arrangement of vertebrose animals. London Medical Repository 15(1):296–310.
Marshall LG 1978. Mammalian species, Dromiciops australis, American Society of Mammalogists, pp. 1-5. pdf
Soe AN et al. (6 co-authors) 2017. New remains of Siamotherium pondaungensis (Cetartiodactyla, Hippopotamoidea) from the Eocene of Pondaung, Myanmar: Paleoecologic and phylogenetic implications. Journal of Vertebrate Paleontology 37(1):e1270290https://doi.org/10.1080/02724634.2017.1270290
Suteethorn V, Buffetaut E Helmcke-Ingava Rt JaegerJ-J and Jongkanjanasoontorn Y 1988. Oldest known Tertiary mammals from South-East Asia: Middle Eocene primate and anthracotheres from Thailand. Neues Jahrbuch f€ur Geologie und Palaontologie, Monatshefte 9:563–570.
Thomas O 1894. On Micoureus griseus, Desm., with the description of a new genus and species of Didelphyidae. Ann. Mag. Natural History Ser 9, 3:199-212.
wiki/Dromiciops
wiki/Siamotherium – not yet posted
Source: https://pterosaurheresies.wordpress.com/2025/03/25/siamotherium-joins-tiny-dromiciops-in-the-lrt/
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