Is Sinonyx a mesonychid? Or a whale ancestor?

Updated April 23, 2020 long overdue.
This was originally posted just before the LRT recovered odontocetes nesting apart from mysticetes. Small edits follow.

We start with
Mesonyx, our holotype mesonychid (Fig. 1).

Add
gigantic Andrewsarchus, a taxon traditionally considered a mesonychid, but now nested with leptictids in the large reptile tree. As we learned earlier, the sperm whale is the largest living leptictid and tenrec.

Then add
Harpagolestes (Fig. 1) to the LRT and it nests with Mesonyx at the base of the hippos. Mesonychids, as we learned earlier, are basal to two other aquatic mammals, hippos and Paleoparadoxia... and mysticete whales.

Finally and today we add
Sinonyx jiiashanensis (Zhou et al. 1995; Paleocene, 56 mya; Fig. 1) and, although it sure looks like a mesonychid, it nests with Andrewsarchus as a leptictid within the Tenreccetacea in the large reptile tree.

Figure 1. Andrewsarchus, Sinonyx, Mesonyx and Harpagolestes to scale for direct comparison of these two tenrecs with these two mesonychids.

Figure 1. Andrewsarchus, Sinonyx, Mesonyx and Harpagolestes to scale for direct comparison of these two tenrecs with these two mesonychids.

Shifting Sinonyx over to the mesonychids
adds only 10 steps, despite the distance. Shifting Sinonyx over to either one of the sister taxa to Andrewsarchus, adds 6 steps when Andrewsarchus remains. Delete Andrewsarchus and Sinonyx does not change nodes. So, Sinonyx sure looks like a mesonychid and it was described as a mesonychid, but it nests with the leptictids!

Figure 2. Sinonyx looks like a mesonychid, but nests with elephants shrews and tenrecs in the LRT.

Figure 2. Sinonyx looks like a mesonychid, but nests with leptictid elephants shrews and tenrecs in the LRT.

The convergence here is amazing
and, no doubt, has led to the present and traditional confusion mesonychids and hippos have presented for the origin of whales, which we looked at earlier here. We have two clade here, leptictids and mesonychids, not just mesonychids. Leptictids have been ignored traditionally in discussions of mesonychids and whale origins.

Wikipedia reports, “Sinonyx’ elongated muzzle and tooth shape are atypical compared to other mesonychids, but are features close to those of the cetaceans.” 

The heretical nesting of Sinonyx with leptictids, tenrecs
and odontocetes can be assigned, once again, to taxon inclusion in the LRT, which attempts to minimize taxon exclusion with a large gamut taxon list.


References
Zhou X, Zhai R, Gingerich PD and Chen L 1995. Skull of a new mesonychid (Mammalia, Mesonychia) from the Late Paleocene of China. Journal of Vertebrate Paleontology 15(2):387-400.

wiki/Sinonyx

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