SVP 2022 abstracts – 19: on Wyolestes, an Eocene enigma

This is the nineteenth and last of the SVP abstracts for 2022
reviewed this year. I was not able to get good data on Wylolestes (other than a palate), so a presumed close relative Didymoconous berkeyi (Fig 1) is substituted and added to the LRT.

Zack, Rose and O’Leary 2022 report:
“Wyolestes is a rare mammal from the early Eocene of North America. The relationships of the genus have been unclear, with proposed affinities to mesonychians, didymoconids, and hyaenodontids.

In the large reptile tree (LRT, 2172 taxa) a ?relative of Wyolestes, Didymoconus berkeyi (Figs 1, 2) nests with similar-sized Ocepeia and much larger Harpagolestes uintensis (Fig 2) close to extant hippos and extinct mesonychids all derived from a lineage of oreodonts.

“A partial skeleton of Wyolestes apheles, including a nearly complete skull, was preliminarily reported almost two decades ago, but carbonate matrix obscured many critical aspects of the morphology of the specimen.

This (Fig 1) is the skull of Didymoconus along with the palate of Wyolestes.
For now this will have to do.

Zack, Rose and O’Leary 2022 continue:
“We present an update on the morphology and affinities of Wyolestes based on digital preparation of the skull and parts of the limbs of this specimen using micro-CT data. Two previously unreported specimens, one of W. apheles and one of W. dioctes, provide additional information on the morphology of Wyolestes, as does previously undescribed material of the type specimen of W. iglesius.

The skull of Wyolestes is narrow and elongate, particularly the rostrum, with an elongate but small gauge infraorbital canal and a weak zygomatic arch.

By contrast, Didymoconus is 2x wider than the skull height at the orbit. It has a robust zygomatic arch. The rostrum is not elongate, but it is narrow relative to the rest of the skull.

“Internally, the skull has an ossified tentorium, a distinctive feature shared with pangolins and many carnivorous eutherians.

In the LRT pangolins are xenarthrans close to aardvarks and anteaters. Only genomic tests pull pangolins away from edentates and nest them with the clade Carnivora.

Repairing a myth: mesonychids are traditionally considered meat-eaters. Flattened teeth on Mesonyx indicate a gritty herbivorous diet. Fangs were for defense and show. All relatives were herbivores.

Zack, Rose and O’Leary 2022 continue:
“Basicranial morphology is unspecialized, with the most distinctive feature being an excavation at the anterior tip of the promontorium, likely to accommodate the internal carotid artery as it approached the piriform fenestra. There is no evidence for an expanded, pneumatized ear cavity as occurs in didymoconids.

An ossified auditory bulla is not preserved, nor is there evidence for its attachment.

As in Didymoconus and Harpagolestes (Fig 2).

“The postcranial skeleton of Wyolestes shows some features consistent with digging, including an elevated humeral deltopectoral crest and elongate, deeply fissured unguals, but Wyolestes lacks the more substantial adaptations to a fossorial lifestyle present in Didymoconidae. Fossorial adaptations that are present are concentrated in the forelimbs and may indicate some capacity for scratch digging.

“The totality of the postcranial morphology of Wyolestes suggests a generalized terrestrial locomotor repertoire, broadly comparable to some early hyaenodontids. In fact, the morphology of the tarsus, now nearly completely documented from W. dioctes, shows a striking similarity to contemporary hyaenodontids. Addition of Wyolestes to a phylogenetic analysis that broadly samples living and extinct eutherian mammals recovers the genus close to Hyaenodontidae and extant carnivorans. There is no support for a relationship to either Didymoconidae or Mesonychia, nor are those groups closely related to each other”.

Again, be wary when results are suprageneric. Insist that every genus you test nests with another genus. Then those two can be part of, or transitional to a suprageneric clade. Here the LRT specified two genera and nested them in an established clade.

Hyaenodon (a member of the Hyaenodontidae) is a marsupial carnivore in the LRT.

Carnivorans in the LRT are not related to Hyaenodon. So taxon exclusion might be a factor in the Zack, Rose and O’Leary report.

Harpagolestes is frequently associated with the clade Mesonychia.
Here in the LRT Mesonyx is separated from Harpagolestes + Didymoconus by five nodes. The authors do not mention Harpagolestes nor Ocepeia in the abstract. Were these two taxa part of the Zack, Rose and O’Leary taxon list?

The LRT currently tests no members of the Wyolestidae.
Looking forward to the paper.

As far as the LRT is concerned,
Wyolestes remains an enigma but Didymoconus has been unambiguously nested. Tell Zack, Rose and O’Leary to add Ocepeia and Harpagolestes to their taxon list, if they are missing presently.

And now it appears we have
a new phylogenetically miniaturized clade (or two) with Didymoconus and Ocepeia roaming world-wide like puppy-sized hippos. Giving credit to reader CB, Ocepeia may not be a juvenile, despite appearances. Phylogenetically miniaturized taxa experience neotony, giving the appearance of a juvenile in the adult stage.

This appears to be a novel hypothesis of interrelationships.
If not please send a previous citation so I can promote it here.

References
Wang X, Downs W Xie J and Xie G 2001. Didymoconus (Mammalia: Didymoconidae) from Lanzhou Basin, China and its stratigraphic and ecological significance. Journal of Vertebrate Paleontology 21(3), 555-564. https://doi.org/10.1671/0272-4634(2001)021[0555:DMDFLB]2.0.CO;2
Zack SP, Rose KD and O’Leary M 2022. New information on the enigmatic Wyolestes and the affinities of the genus. SVP abstracts 2022:376.

More on those mysterious wyolestids:
Naish D 2009. https://scienceblogs.com/tetrapodzoology/2009/08/24/didymoconids-and-wyolestids

fossilhunters reported the traditional hypotheses: “Didymoconids (Fig. 7.5) are an enigmatic group of carnivorous Early Tertiary mammals probably endemic to Asia. The phylogenetic position of didymoconids is uncertain. The dentition is consistent with relationship to Cimolesta or Leptictida, but this observation seems to be based primarily on primitive similarity. The primitive anatomy and diverse opinions about didymoconids and wyolestids suggest that they may represent early offshoots of the eutherian stem.”