Whales: Are they artiodactyls or tenrecs?

According to Wikipedia
Artiocetus (Gingerich et al. 2001; meaning essentially = artiodactyl + whale) was the first fossil to show that early whales possessed artiodactyl-like ankles (Fig. 1). The sizes, shapes and configurations are indeed similar. And this was a valid conclusion, but it was based on taxon exclusion. Other taxa have such ankles.

Figure 1. A selection of two whale ankles and one artiodactyl (Antilocapra, the pronghorn antelope). Such comparisons are the basis for aligning whales with artiodactyls, but tenrecs were not considered.

Contra the artiodactyl hypothesis
the large reptile tree nests the basal whale, Maiacetus, with a clade of small and large, extinct and extant tenrecs like Hemicentetes, Andrewsarchus and Leptictidium.

So, to shed light on this disparity
here are two whale feet and ankles alongside the foot and ankle of Hemicentetes (Fig. 2), the tenrec without a tail (some other tenrecs have long tails (Figs. 5, 6), but extant examples don’t have online skeletons).

Figure 2. The evolution of the tenrec (Hemicentetes) pes, through the land whale Rhodhocetus and Basilosaurus. Rhodocetus loses pedal digit 1. Basilosaurus loses pedal digit 2.  Note the lack of sharp claws and the lack of artiodactyl hooves in all taxa. Note the plantigrade pes here, not the digitigrade pes of artiodactylus. Note the reduction of distal tarsals (cuneiforms) down to dt3 (lateral cuneiform) in all taxa.

To the credit of the tenrec-whale clade:

1. Hemicentetes has a short pedal digit 1. Rhodhocetus loses pedal digit 1. Basilosaurus loses pedal digit 2 (Fig. 2) where the vestigial pes is no longer in use.
2. Tiny hooves on relatively slender digits
3. Lack of tightly appressed artiodactyl metatarsals in all taxa.
4. Plantigrade pes in all taxa

Figure 3. A basal artiodactyl, Ancodus, pes. As in Rhodhocetus pedal digit 1 is absent and the distal tarsals are reduced to one. This led to the artiodactyl hypothesis, and that is a great first guess! But it is not supported by the LRT.

To the credit of the artiodactyl-whale clade hypothesis:

1. Four pedal digits only, digit 1 is already absent
2. The calcaneal heel is elongate.
3. Metatarsals 2 and 5 axially rotate behind 3 and 4.

Figure 4. Maiacetus is a basal whale with legs and it is also a giant tenrec. Compare Leptictidium (Figs. 5, 6).

Further notes

1. In both artiodactyls and tenrecs: reduction of distal tarsals (cuneiforms, yellow here) down to dt3 (the lateral cuneiform) in all taxa and a long list of other similar traits, both shared with proto-whales.
2. Leptictidium (Fig. nests closer to whales and has a longer calcaneal heel with appressed metatarsals and reduced digits 1 and 5, but all toes are slender as in Rhodhocetus and it has a long muscular tail.

Artiodactyls came to the mind of Gingerich first
because artiodactyls had similar ankles to his land whale discoveries and were of similar size. Few workers both to study tenrecs — but tenrecs should have been included as they have similar pedes and taxon exclusion often arises with enigma taxa, as whales were.

1. Artiodactyls are herbivores. Whales and tenrecs are not.
2. Artiodactyls are digitigrade. Whales and tenrecs are not.
3. Artiodactyls do not echolocate. Some tenrecs and some whales do.
4. Artiodactyls do not have a large muscular tail for aquatic locomotion. Some tenrecs and all whales do.
5. In the large reptile tree Maiacetus nests with a long list of tenrecs, not with artiodactyls, when given the opportunity, not with Ancodus.
6. Several former mesonychids now nest with tenrecs and away from Mesonyx. clearing up that lingering issue.
7. And a long list of traits in the skull and elsewhere…

Figure 5. Elements of Leptictidium from Storch and Lister 1985. Note the long calcaneal heel here.

Added a few days later: here (Fig. 5a) is the double-pulley shape of the Leptictidium astragalus (n yellow).

Figure 5a. The ankle of Leptictidium includes a spool-shaped double-pulley astragalus.

Figure 6. Leptictidium – Often considered a kangaroo like hopper (saltator), the loose sacral connection and phylogenetic bracketing suggest this was a dorsoventral undulatory swimmer instead.

Figure 7. Science magazine cover for Gingerich et al. 2001. Artist: John Klausmeyer. The hands and feet are far from being ungulate hooves, but close to tenrec paws.

Earlier notes
here, here, here and here. Unfortunately, as in several other taxonomic enigmas and mismatches, taxon exclusion prevented Gingerich et al. from making the tenrec connection.

References
Gingerich PD, Haq M, Zalmout I, Khan I and Malkani M 2001. Origin of whales from early artiodactyls: hands and feet of Eocene Protocetidae from Pakistan”. Science. 293 (5538): 2239–42.

Gingerich’s website
has more information. His Elomeryx is very close to Ancodus.

wiki/Artiocetus