SVP abstracts – 5 on a new cranium attributed to Chriacus

Holbrook and Chester 2022 wrote:
“The genus Chriacus is an archaic “ungulate” known from the Paleocene and early Eocene of North America. Chriacus has historically been placed in the Arctocyonidae, but other workers have allied it with Pleuraspidotheriidae or considered it to be ancestral to Artiodactyla.”

You can still see older blogposts (based on fewer tested taxa) relating Chriacus to bats here. That was an error based on taxon exclusion (too few taxa back then).

A few months back, during a bout of housekeeping the large reptile tree, Chriacus was removed from the LRT due to loss of resolution. (I was seeking a completely resolved LRT). Adding what little is known of Chriacus back in nested it with or close to the extant water opossum, Chironectes (Fig 2), close to the base of the Placentalia, but an outgroup taxon with a water-tight pouch.

Holbrook and Chester 2022 continue:
“The anatomy of Chriacus is relatively well known, due in large part to postcrania associated with teeth and even partial articulated skeletons. However, the cranial anatomy of Chriacus is not well known.”

See figures 1 and 2 for how much is known.

“We describe here the most complete cranium referred to Chriacus baldwini collected from the Paleocene of the San Juan Basin in New Mexico. While weathering and breakage limit interpretations, the specimen preserves much of the braincase and skull roof, as well as parts of the basicranium, including the petrosals.

Is this the same skull described by Bertand et al 2019? Apparently so. A museum number would have been helpful here.

Recent studies of the endocast and internal structure of the ear of this specimen interpret Chriacus as being more reliant on olfaction than vision and having low to moderate agility. As these studies noted for its brain, the skull of Chriacus exhibits mainly plesiomorphic features for placental mammals. Including information from the new cranium, we added Chriacus to a matrix of 326 characters scored for 77 taxa, including a wide range of placental mammals, emphasizing various ungulates and including two species of Arctocyon and two pleuraspidotheres.”

These authors are cherry picking taxa. Let your wide gamut cladogram tell you which taxa are closer to your taxon under study by minimizing taxon exclusion. Let all potential candidates have a chance to compete for parsimony.

Chironectes is a marsupial close to the origin of placental mammals, which may be why “Chriacus exhibits mainly plesiomorphic features for placentals mammals.”

“Maximum parsimony analysis, both with and without topological constraints reflecting the four major placental clades identified in molecular studies, recovered 464 MPTs with constraints
and 1140 MPTs without constraints.”

The LRT recovered a magnitude fewer MPTs with a magnitude more taxa. Also avoid placental clades identified in molecular studies. They are a waste of your time and ours.

“In both analyses the strict consensus is poorly resolved. While none of the MPTs in the unconstrained analysis unite Chriacus with Arctocyon, 88% of the MPTs in the constrained analysis recovered a Chriacus-Arctocyon clade. The position of Chriacus is still not fully
resolved, but new data from fossils like the cranium described here have the potential to provide new insight into the relationship of Chriacus to other placentals.”

If you know Holbrook and Chester, suggest adding taxa, especially marsupials close to the origin of placentals, which is where their own studies are leading them. Ungulates are not on that list, but test them anyway.

References
Holbrook L and Chester S 2022. A cranium of Chriacus baldwini (Mammalia, Arctocyonidae) from the Paleocene of New Mexico, USA and the phylogenetic position of Chriacus. SVP abstracts 2022: 183.
Bertrand OC et al (7 co-authors) 2019. Virtual endocranial and inner ear endocasts of the Paleocene ‘condylarth’ Chriacus: new insight into the neurosensory system and evolution of early placental mammals. Wiley Online Library.