Taxon exclusion again.
I know you’re getting tired of this, but Siamotherium (Fig. 1) is an anagalid, a sister to Anagale (Fig. 1) in the large reptile tree (LRT, 1378 taxa), basal to the leptictid-tenrec-odontocete clade. These are taxa ignored in prior studies.
There is no guesswork here.
The LRT considers a wider gamut of mammal candidates and lets the taxa nest wherever they want to without restriction. Sometimes taxa nest in places that paleontologists had not yet imagined.
For those wondering what a helohyid is…
… and count me among that curious group, Helohyus (sorry, no Wikipedia entry), is the namesake for this primitive pig-like artiodactyl clade. More on this taxon in later posts.
The resemblance of Siamotherium to Anagale is strong.
(Fig. 1) One wonders why this similarity went unnoticed before.
Figure 1. Siamotherium to scale with Anagale. Both nest basal to the tenrec-odontocete clade. The mandible of Anagale fits well on the maxilla of Siamotherium when enlarged.
Anagale gobiensis (Simpson 1931; early Oligocene; 30cm in length; AMNH 26079) was originally considered an insectivore, close to the tree shrew Tupaia and tending to link to lemurs like Notharctus. Thirty years later McKenna 1963 argued against tupaioid affinities, but could not provide a more suitable nesting.
Here rabbit-sized Anagale nests with Siamotherium and the IVPP V2385 specimen of Hapalodectes. The closest living relative to these taxa is Rhynchcyon, the golden rumped elephant shrew (Fig. 3).
The teeth of Anagale were typically worn and the claws were shovel-shaped, suggesting a diet of subterranean worms and beetles. The peculiar combination of large fissured claws of the manus and distally spatulate unguals of the pes is uncommon in mammals. The ectotympanic bulla protecting the middle ear bones is quite large, and so is the eardrum that it framed.
Siamotherium pondaungensis (Suteethorn et al. 1988; Soe et al. 2017; Eocene) was originally considered a small anthracothere close to Hippopotamus, but here nests with Anagale. Siamotherium is larger, has a straight jugal and only three molars.
Sisters of these taxa
had a Mesozoic genesis. Distinct from other clades is the posterior rise of the post-parietal creating a nuchal crest. This crest is still present in the giant elephant shrew, Andrewsarchus, and odontocete whales (Fig. 2). Also of interest, the digitigrade hands and feet with fewer digits converge on those of basal artiodactyls (Fig. 3). No wonder these get confused and ignored in traditional paleontology.
Figure 2. Odontoceti (toothed whale) origin and evolution. Here Anagale, Andrewsarchus, Sinonyx, Hemicentetes, Tenrec Indohyus and Leptictidium precede Pakicetus. Maiacetus and Orcinus are aquatic odontocetes.
Like basal artiodactyls,
elephant shrews, like Rhynchocyon (Fig. 3), also have digitrade hands and feet. This has led to all sorts of confusion with regard to whales and their putative, but invalidated ancestors among the artiodactyls.
Figure 3. Skeleton of the elephant shrew, Rhynchocyon. Note the digitigrade manus and pes, like those of basal artiodactyls.
Simpson GG 1931. A new insectivore from the Oligocene, Ulan Gochu horizon, of Mongolia. American Museum Novitates 505:1-22.
Soe AN t 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 Pal€aontologie, Monatshefte 9:563–570.
wiki/Siamotherium (no Wikipedia entry yet)