None of these Hapalodectes specimens are mesonychids, including the holotype.

It was only
yesterday that we looked at Sinonyx, a putative mesonychid (see Mesonyx) that turned out to nest in the large reptile tree (LRT) with Andrewsarchus and the tenrecs and more distantly with their descendants, the whales.

The much smaller
Hapalodectes compresses  (Matthew 1909; and others; late Paleocene to Early Eocene, 55 mya; ) is another traditional mesonychid, considered ‘otter-like” by the authors of Wikipedia. The original data (Fig. 1) for this taxa came in the form of teeth and mandible scraps, but recently nearly complete skulls have entered the literature (Figs. 2, 3).

Figure 1. Hapalodectes compressus holotype. Even Matthew noted how different these teeth were from mesonychid teeth.

Figure 1. Hapalodectes compressus holotype. Even Matthew noted how different these teeth were from mesonychid teeth. This is not a mesonychid, but a tenrec.

Matthew 1909 reported, “Distinguished from other Mesonychidae by the highly compressed lower teeth, the cusps converted into narrow trenchant blades. This curious little genus is known only from lower jaws, which, while they are evidently referable to the Mesonychidae, differ so widely from any of the described genera as to indicate a phylum of quite different, trend.”

Wikipedia reports, “The genus [Hapalodectes] was once suggested to be related to the Archaeoceti, such as Pakicetus, due to extreme similarities between the skull and teeth anatomies of the two genera. Now, however, Hapalodectes and other mesonychians are thought to be related to basal artiodactyls, while the Archaeoceti are now determined to be descended from more derived artiodactyls, like Indohyus, which are related to hippopotamuses and anthracotheres.” 

See how confusing things can get
without a large gamut phylogenetic analysis?

The LRT does not replicate those results,
but instead nests the IVPP V12385 specimen assigned to Hapalodectes (Fig. 2) as a sister to Solenodon and Zalambdalestes within the clade Glires. Note, the 12385 skull shares no traits at all with the holotype illustrated by Matthew (Fig. 1). One is a skull. The other is a mandible. So we’re starting from scratch here.

The untested holotype mandible (Fig. 1)
does share many traits with Sinonyx, which nests with tenrecs, as mentioned above. So it is not likely a mesonychid either.

Figure 1. Hapalodectes hetangensis (IVPP V 12385) from Ting et al. 2004 with color overlays applied here. This specimen is nmore closely related to Solenodon than to Mesonyx.

Figure 2. Hapalodectes hetangensis (IVPP V 12385) from Ting et al. 2004 with color overlays applied here. This small specimen is more closely related to Solenodon than to Mesonyx.

The other putative Hapalodectes hetangensis specimen
(IVPP V 5253, Ting and Li 1987; Fig. 3) was considered a subadult, somewhat smaller than IVPP V 12385. After testing in the LRT, the 5253 specimen is not conspecific nor is it congenerc with the 12385 specimen or the holotype.

Figure 3. ?Hapoldectes hetangensis (Ting and Li 1987, IVPP V 5235) nests with Tupaia, the tree shrew, not with the IVPP V 12385 specimen nor with Mesonyx.

Figure 3. ?Hapoldectes hetangensis (Ting and Li 1987, IVPP V 5235) nests with Tupaia, the tree shrew, not with the IVPP V 12385 specimen nor with Mesonyx.

In the LRT
the smaller IVPP V 5253 specimen nests with Tupaia also within the Glires, not with Mesonyx or with the IVPP V 12385 specimen (Fig. 2). Like Tupaia, the 5253 specimen has a complete postorbital ring, a low-rounded occiput and a long list of other traits.

Figure 5. Mammal cladogram. This is where this subset of the LRT stands as of today.

Figure 5. Mammal cladogram. This is where this subset of the LRT stands as of today.

References
Matthew WD 1909. The Carnivora and Insectivora of the Bridger Basin, middle Eocene. Memoirs of the American Museum of Natural History 9:289-567. Online here.
Ting SY and Li CK 1987. The skull of Hapalodectes (?Acreodi, Mammalia), with notes on some Chinese Paleocene mesonychid: Vertebrata PalAsiatica 25:161-186.
Ting SY, Wang Y, Schiebout JA, Koch PL, Clyde WC, Bowen GJ and Wang Y 2004. New Early Eocene mammalian fossils from the Hengyang Basin, Hunan China. Bulletin of Carnegie Museum of Natural History 36: 291-301.

wiki/Hapalodectes

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