An imaginary mandible for Andrewsarchus

All I did
was take the mandible from sister Sinonyx and scale it to Andrewsarchus (Fig. 1; Osborn 1924). I also added a patch to extend the apparently broken and missing posterior nasals over the fontanelle between the frontals because that’s how far the nasal extends in Sinonyx.

See how sometimes
you don’t ‘see’ something until after you see it in a sister?

Figure 1. Andrewsarchus with Sinonyx mandible. The lower canine helps constrain the shape of the missing upper canine. 

Figure 1. Andrewsarchus with Sinonyx mandible. The lower canine helps constrain the shape of the missing upper canine. Note the transparent extension of the posterior nasals to cover up the fontanelle between the frontals, as in Sinonyx.

BTW
it bothered me that Sinonyx and Andrewsarchus were so much larger than their sisters, especially their closest sister, a type of elephant shrew, Rhynchocyon. Moreover, several traits appear to be homologous. So I retested the relationship of Sinonyx and Andrewsarchus with mesonychids and I retested them with prejudice. Any traits that could relate Sinonyx and Andrewsarchus with mesonychids I scored that way.

In the end,
I was not able to nest Sinonyx and Andrewsarchus with mesonychids.

Furthermore
when I removed all tenrec and odontocete sisters from the tenrec clade (see Fig. 2), leaving only Sinonyx and Andrewsarchus alone they still did not nest with mesonychids, but kept their node unchanged between the Ptilocercus clade and Onychodectes.

Figure 3. Tenrec-Odontocete clade with Leptictis now nesting with the elephant shrew Rhynchcyon and the long-tailed tenrecs nesting with the short tailed tenrecs, basal to Pakicetus.

Figure 2. Tenrec-Odontocete clade with Leptictis now nesting with the elephant shrew Rhynchcyon and the long-tailed tenrecs nesting with the short tailed tenrecs, basal to Pakicetus. This tree moves Sinonyx closer to Pakicetus. Indohyus has already been associated with pakicetids.

Testing like this
brought certain problems to the surface. The current tree has been improved over earlier versions.

Here’s how the tenrec clade now stands:
(Fig. 2) Leptictis and the elephant shrew Rhynchocyon now nest together. They are both similar in size and build.

Giant Andrewsarchus and smaller Sinonyx still nest together. Would still like to see some post-crania for  these two.

The two living short-tailed terrestrial tenrecs, Hemicentetes and Tenrec now nest with two extinct long-tailed aquatic tenrecs, Lepticitidium and Indohyus. The latter has already been associated with pakicetids in the literature  (Rao 1971, Thewissen et al. 2007.)

Likewise Sinonyx and Andrewsarchus have already been associated with the origin of whales in the literature. The new tree topology brings them closer to Pakicetus.

Early members of the tenrec clade
were insectivore speedsters with long slender legs, based on the habits of Rhynchocyon. More derived tenrecs like Tenrec, are not speedy and Hemicentetes is protected with spinesLeptictidium had much longer hind limbs than fore limbs and was likely bipedal. Indohyus had subequal limbs so likely remained a quadruped. Tradtionally Indohyus has been considered an artiodactyl, but given the opportunity to nest with artiodactyls in the LRT, it does not do so.

Perhaps the most convergent clade
By all the present evidence, some tenrecs converged with rabbits and elephant shrews, some with mesonychids, others with artiodactyls and still others with mysticete whales. It’s a pretty amazing and woefully under appreciated clade.

It is interesting to consider the possibility
that since both elephant shrews and tenrecs have a proboscis that extends beyond the jaw line, it is possible that early land whales, Andrewsarchus and Sinonyx, might have had a similar long nose. Some of these taxa might have used such a snorkel to breathe while underwater, just below the surface — or — the long nose was the first soft tissue to disappear during the transition, because whales have no such nose.

References
Osborn HF 1924. Andrewsarchus, giant mesonychid of Mongolia. American Museum Noviattes 146: 1-5.
Rao AR 1971. 
New mammals from Murree (Kalakot Zone) of the Himalayan foot hills near Kalakot, Jammu and Kashmir state, India. Journal of the Geological Society of India. 12 (2): 124–34.
Thewissen JGM, Cooper LN, Clementz MT, Bajpai S and Tiwari BN 2007. Whales originated from aquatic artiodactyls in the Eocene epoch of India. Nature 450:1190-1195.

wiki/Leptictidium
wiki/Indohyus

 

 

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