On the origin of Artiodactyla 2024

Preface:
I took a vacation from posting for about a month in order to focus study and finalize the mammal subset of the large reptile tree (LRT, 2324 taxa), hoping for complete resolution.

That did not happen, despite great progress and daily insights like this one:

Rose 1996 wrote,
“The first known members of the order Artiodactyla appeared suddenly throughout the Holarctic region at the beginning of the Eocene. They are characterized by distinctive cursorial skeletal specializations.

One artiodactyl overlooked by Rose, Agriochoerus (Figs 3, 4), was a climber with a vestigial opposable thumb (Fig 6), so, not a cursor = runner.

“Owing to their abrupt appearance and the lack of transitional forms, the origin of the order is problematic.

Rose 1996 did not employ phylogenetic analysis nor mention Agriochoerus.

“Descent from a “condylarth,” specifically the arctocyonid Chriacus, has been suggested based on dental resemblances, but until now postcranial anatomy seemed to preclude close relationship between Arctocyonidae and Artiodactyla.”

In the LRT extinct Chriacus (Fig 1) was a water opossum with no cursorial abilities.

In the LRT Arctocyon (Fig 2) was a creodont close to Sinonyx.

Figure 1. Extinct Chriacus and extant Chironectes, a water opposum, compared. ” data-image-caption=”

Figure 1. Extinct Chriacus and extant Chironectes, a water opposum, compared.

” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2022/11/chriacus-chironectes588.jpg?w=157″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2022/11/chriacus-chironectes588.jpg?w=535″ class=”size-full wp-image-73712″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2022/11/chriacus-chironectes588.jpg” alt=”Figure 1. Extinct Chriacus and extant Chironectes, a water opposum, compared.” width=”584″ height=”1117″ />

Figure 1. Extinct Chriacus and extant Chironectes, a water opposum, compared.

Rose 1996 wrote,
“A middle Paleocene specimen of a small arctocyonid (?Chriacus) reported here is much more similar to the oldest artiodactyl, Diacodexis, in the derived condition of the hindlimb, reviving the possibility that Artiodactyla evolved from an arctocyonid.

“Artiodactyla, the even-toed ungulates (pigs, camels, deer, cattle, and their relatives), are the predominant large land mammals of today.”

Absent from this list are hippos, the basal-most living artiodactyls in the LRT (subset Fig 5). Diacodexis was a small gracile taxon close to deer. It is known from a partial skeleton. Rose was apparently looking for a basal pig, camel or deer. The words ‘Hippopotamus‘ ‘hippo’ and ‘oreodont‘ are not found in Rose 1996. That omission turned out to be the Achilles’ heel in Rose’s study.

Once again, taxon exclusion hampered a study by a high-ranking paleontologist.

Figure 1. Sinonyc and Arctocyon shown to the same scale. ” data-image-caption=”

Figure 1. Sinonyc and Arctocyon shown to the same scale.

” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/08/sinonyx-arctocyon.skull588.jpg?w=300″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/08/sinonyx-arctocyon.skull588.jpg?w=584″ class=”size-full wp-image-88425″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/08/sinonyx-arctocyon.skull588.jpg” alt=”Figure 1. Sinonyc and Arctocyon shown to the same scale.” width=”584″ height=”440″ />

Figure 2. Sinonyc and Arctocyon shown to the same scale. These two are creodonts in the LRT.

According to Wikipedia – Agriochoerus,
“Agriochoerus and other agriochoerids possessed claws, which is rare within Artiodactyla, as well as likely being scansorial. Like other agriochoerids, it possessed a body shape unusual for an artiodactyl. The body was rather elongated and supported by more elongated and slender limbs than those of others within the Merycoidodontoidea. The front legs were five-toed, with an atrophied thumb, while the hind legs had only four toes.”

Turns out Agriochoerus (Figs 3, 4, 6) is phylogenetically close to an extant climbing (scansorial) placental, Nasua, the coatimundi (Figs 3, 4 6), in the LRT, subset Fig 5). Apparently this hypothetical interrelationship was overlooked in prior studies.

Agriochoerus was first described by Leidy 1850, 1869.
Wortman 1895 reported on Agriochoerus latifrons (Figs 3, 4 6) noting, “The trapezium [a medial wrist bone] of Agriocherus, at least in the species under consideration, is not only remarkable for its connections, but what is still more surprising, it gives evidence of having supported a more or less opposable pollex… Is it possible that the remote ancestors of the Artiodactyla had opposable thumbs, and that they were more or less arboreal in habit?”

The answer to Wortman’s prescient question is: yes.

Please note in the LRT subset (Fig 5) the position of the basal primate Notharctus with an opposable thumb) , close to Nasua on the way to Agriochoerus latifrons.

Note also that Nasua does not have an opposable thumb (Fig 6).
Nasua is also basal to the clade Carnivora in the LRT.

Figure 1. Nasua, the coatimundi, compared to Notharctus, the basal primate and Agriochoerus latifrons, the basal artiodactyl in the LRT. ” data-image-caption=”

Figure 1. Nasua, the coatimundi, compared to Notharctus, the basal primate and Agriochoerus latifrons, the basal artiodactyl in the LRT.

” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/10/notharctus.agriochoerus.latifrons588.jpg?w=184″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/10/notharctus.agriochoerus.latifrons588.jpg?w=584″ class=”size-full wp-image-89292″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/10/notharctus.agriochoerus.latifrons588.jpg” alt=”Figure 1. Nasua, the coatimundi, compared to Notharctus, the basal primate and Agriochoerus latifrons, the basal artiodactyl in the LRT.” width=”584″ height=”952″ />

Figure 3. Nasua, the coatimundi, compared to Notharctus, the basal primate and Agriochoerus latifrons, the basal artiodactyl in the LRT.

According to the LRT
(Fig 1–3) both aquatic and cursorial artiodactyls (e.g. hippos and deer) are derived from scansorial (= climbing) Agriochoerus latidens (Fig 1). In turn, Agriochoerus is derived from scansorial Notharctus and Nasua.

This transition had been overlooked by prior studies due to taxon exclusion.

Figure 1. Nasua, the coatimundi, compared to Agriochoerus latifrons, the basal artiodactyl in the LRT. ” data-image-caption=”

Figure 1. Nasua, the coatimundi, compared to Notharctus, the basal primate and Agriochoerus latifrons, the basal artiodactyl in the LRT.

” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/10/notharctus-agriochoerus.latifrons588.jpg?w=161″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/10/notharctus-agriochoerus.latifrons588.jpg?w=549″ class=”size-full wp-image-89294″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/10/notharctus-agriochoerus.latifrons588.jpg” alt=”Figure 1. Nasua, the coatimundi, compared to Agriochoerus latifrons, the basal artiodactyl in the LRT.” width=”584″ height=”1089″ />

Figure 4. Nasua, the coatimundi, compared to Agriochoerus latifrons, the basal artiodactyl in the LRT. Note the large claws in both and the opposable thumb in Agriochoerus.

Janis 1988 wrote,
“All hoofed mammals, with the few exceptions discussed later, are now considered to represent a monophyletic clade (i.e. to possess a single common ancestor, although some controversy still exists.”

That single common ancestor was not reported by Janis in that pre-cladistic, pre-software 1988 paper that reviewed several hypotheses circulating at the time. She also omitted oredonts and Agriochoerus.

Figure 3. Subset of the LRT focusing on basal placental 1 taxa between Nasua and Agriochoerus latifrons. ” data-image-caption=”

Figure 3. Subset of the LRT focusing on basal placental 1 taxa between Nasua and Agriochoerus latifrons.

” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/10/lrtnasua.agriochoerus588.jpg?w=81″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/10/lrtnasua.agriochoerus588.jpg?w=276″ class=”size-full wp-image-89296″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/10/lrtnasua.agriochoerus588.jpg” alt=”Figure 3. Subset of the LRT focusing on basal placental 1 taxa between Nasua and Agriochoerus latifrons.” width=”584″ height=”2170″ />

Figure 5. Subset of the LRT focusing on basal placental 1 taxa between Nasua and Agriochoerus latifrons. Note the proximity of Agriochoerus latifrons to Merycoidodon and the other oreodonts, basal to hippos.

Janis 1988 wrote,
“The ‘basal order’ of ungulates is traditionally seen as the Condylarthra, a mainly Paleogene radiation that is held as being ‘primitive’ and generally ancestral to all later ungulate lineages.”

In the LRT the condylarths are not related to, nor give rise to ungulates.

“Traditionally seen” = generally agreed upon = consensus prior to testing in the LRT, a cladogram with a large taxon list in order to minimize taxon exclusion.

This is why testing always trumps opinion, tradition, textbooks or consensus. Testing can be confirmed, modified or refuted with more testing. That is the scientific method.

Here again, when a phylogenetic vacuum exists
and professional paleontologists do not fill it, the LRT provides a hypothesis of interrelationships that can be falsified with a similar taxon list and an independent character list.

Figure 6. Manus and pes of Nasua, Notharctus and Agriochoerus latifrons compared. ” data-image-caption=”

Figure 6. Manus and pes of Nasua, Notharctus and Agriochoerus latifrons compared.

” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/10/notharctus-agriochoerus-manus-pes588.jpg?w=253″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/10/notharctus-agriochoerus-manus-pes588.jpg?w=584″ class=”size-full wp-image-89308″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/10/notharctus-agriochoerus-manus-pes588.jpg” alt=”Figure 6. Manus and pes of Nasua, Notharctus and Agriochoerus latifrons compared.” width=”584″ height=”693″ />

Figure 6. Manus and pes of Nasua, Notharctus and Agriochoerus latifrons compared. Apparently the ancestors of Agriochoerus arose between Nasua and Notharctus. These are all North American post-Cretaceous taxa.

Janis concluded,
“Recent advances in ungulate systematics have altered our old notions about the role of the ‘order Condylarthra’ as the basal group of ungulates, and have destroyed taxonomic unity of the endemic South American ungulates and the African ‘subungulates’.”

36 years after Janis 1988 the LRT continues to alter “our old notions” about the role of overlooked taxa “as the basal group of ungulates” and other clades.

Figure 1. The coatimundi (Nasua) compared to the ring-tailed lemur (Lemur). ” data-image-caption=”

Figure 1. The coatimundi (Nasua) compared to the ring-tailed lemur (Lemur).

” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2021/02/coatimundi_lemur_invivo588.jpg?w=300″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2021/02/coatimundi_lemur_invivo588.jpg?w=584″ class=”size-full wp-image-53307″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2021/02/coatimundi_lemur_invivo588.jpg” alt=”Figure 1. The coatimundi (Nasua) compared to the ring-tailed lemur (Lemur).” width=”584″ height=”418″ />

Figure 7. The coatimundi (Nasua) compared to the ring-tailed lemur (Lemur). Did Agriochoerus latifrons have an elevated striped tail? Other traits link the three taxa together.

Agriochoerus latifrons
(Leidy 1850, 1869; Wortman 1895, O’Hara 1920, Late Eocene to Oligocene, 38–16mya, skeleton) has a distinctly different tooth pattern with large lateral incisors and a diastema. In the LRT, this taxon is the basalmost artiodactyl and shares an ancestry with Nasua, the extant coatimundi, and Notharctus, a basal adapid lemur.

Don’t omit
hippos and oreodonts, including agriochoerids, from your ungulate analyses. That initial rapid radiation of placentals that split carnivores from primates and artiodactyls is now better understood by including traditionally omitted taxa.

The LRT tells us that hippos and blue whales had lemur-like ancestors,
perhaps with thumbs.

Otherwise, the LRT tells us something about the origin of primate thumbs.

References
Janis CM 1988. New ideas in ungulate phylogeny and evolution. Trends in Ecology & Evolution = TREE 2(11):291–297. DOI: 10.1016/0169-5347(88)90104-8
Leidy J 1869. Extinct Mammals of Dakota and Nebraska:Including an account of some allied forms from other localities, together with a synopsis of the mammalian remains of North America., Journal of the Academy of Natural Sciences, Philadelphia 2(7): 135–141.
Rose KD 1996. On the origin of Artiodactyla. Proceedings of the National Academy of Science USA 93:1705–1709.
Wortman JL 1895. On the osteology of Agriochoerus. Bulletin of the American Museum of Natural History 7(4):145–179.

wiki/Agriochoerus