Is Sinonyx a mesonychid? Or a whale ancestor?

We start with
Mesonyx, our holotype mesonychid (Fig. 1). Add gigantic Andrewsarchus, a taxon traditionally considered a mesonychid, but now nested with tenrecs in the large reptile tree. As we learned earlier, the blue whale is the largest living tenrec. Then add Harpagolestes (Fig. 1) to the LRT and it nests with Mesonyx at the base of the hippos.

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 nested with Andrewsarchus within the Tenreccetacea in the large reptile tree. Mesonychids, as we learned earlier, are basal to two other aquatic mammals, hippos and Paleoparadoxia.

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 tenrecs! The convergence here is incredible 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, tenrecs and mesonychids, not just mesonychids.

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 tenrecs
and whales with tenrecs can be assigned, once again, to taxon exclusion in traditional cladograms, something the LRT attempts to minimize 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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