Loxolophus: ancestor to? or young of? Arctocyon
Early Palaeocene mammals
started off small, like Loxolophus, then grew larger, like Arctocyon, which also developed a large cranial crest, a possible gender trait, a possible maturity signal or just necessary for larger jaw muscles. Here they are to scale and side-by-side for ready comparison (Fig 1) for the first time.
Sometimes ontogeny recapitulates phylogeny.
Figure 1. Large Arctocyon to scale with small Loxolophodon, a possible ancestor or a possible juvenile. The anterior premolar is unerupted within the empty alveolus.
” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/06/arctocyon-skull588.jpg?w=286″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/06/arctocyon-skull588.jpg?w=584″ class=”wp-image-93146 size-full” src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/06/arctocyon-skull588.jpg” alt=”Figure 1. Large Arctocyon to scale with small Loxolophus, a possible ancestor or a possible juvenile. The anterior premolar is unerupted within the empty alveolus.” width=”584″ height=”612″ srcset=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/06/arctocyon-skull588.jpg?w=584&h=612 584w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/06/arctocyon-skull588.jpg?w=143&h=150 143w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/06/arctocyon-skull588.jpg?w=286&h=300 286w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/06/arctocyon-skull588.jpg 588w” sizes=”(max-width: 584px) 100vw, 584px” />
Figure 1. Large Arctocyon to scale with small Loxolophus, a possible ancestor or a possible juvenile. The anterior premolar is unerupted within the empty alveolus. Note the posterior premolars are deeper than the molars, a trait not scored by the LRT.
Earlier a similar issue was documented
involving one smaller and one larger extinct mammal taxon.
According to Wikipedia – Loxolophus:
“A phylogenetic analysis by Kondrashov and Spencer 2015 recovered Loxolophus a sister taxon to Arctocyon.”
The LRT supports this interrelationship.
“Halliday, Upchurch and Goswami 2015 recovered Loxolophus as part of a clade also including Anacodon, Oxyclaenus, and several other traditional arctocyonid genera, distantly related to pangolins.”
The LRT supports the arctocyonid interrelationship, but pangolins nest elsewhere.
Loxolophus hyattianus
(Cope 1885; Early Paleocene, 62mya) was originally considered a raccoon-size condylarth and an ungulate when Arctocyon was included in these clades. Here Loxolophus is a small arctocyonid.
Arctocyon primaevus
(Blainville 1841, Gould and Rose 2014; YPM VP 021233; Paleocene, 60 mya) was long and widely considered a primitive plantigrade ungulate condylarth procreodi placental. In the LRT that is confirmed.
Arctocyonids and mesonychids share this clade
with other fanged taxa thought to be predators. On the other hand, several are like hippos and a living representative, the elephant, is not predatory.
Let’s rethink this clade.
References
Blainville HM 1841. Osteographie et description iconographique des Mammiferes récentes et fossiles (Carnivores) 1, 2 Paris.
Cope EC 1885. The oldest Tertiary Mammalia. American Naturalist 19:385-387.
Gould FDH and Rose KD 2014. Gnathic and postcranial skeleton of the largest known arctocyonid ‘condylarth’ Arctocyon mumak (Mammalia, Procreodi) and ecomorphological diversity in Procreodi. Journal of Vertebrate Paleontology 34(5):1180-1202.
Source: https://pterosaurheresies.wordpress.com/2025/06/07/loxolophus-ancestor-to-or-young-of-arctocyon/
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