Periptychus: a big Paleocene monodelphid with cylinder teeth
According to Wikipedia – Periptychius
“Periptychus is an extinct genus of mammal belonging to the family Periptychidae. It lived from the Early to Late Paleocene and its fossil remains have been found in North America”.
Note the lack of the terms ‘placental’ or ‘marsupial’ applied here. And what are Periptychidae related to? Academics can’t say with certainty due to taxon exclusion.
“This animal was of medium size and could exceed one meter in overall length; Periptychus is supposed to have weighed about 23 kilograms. Periptychus was an unusual mammal that combined a number of rather specialized dental, cranial, and postcranial features with a relatively generalized skeletal structure.”
This ‘generalized structure’ makes more sense
with the new nesting in the large reptile tree (LRT, 2338 taxa). Periptychus (Figs 1–3) now nests as a much larger, more herbivorous version of the tiny extant monodelphid marsupial without a pouch, Monodelphis domestica (Figs 1, 4). These taxa nest at the base of the clade that includes Hyopsodus, Orthaspidotherium and Amphicyon major in the LRT. These, in turn, are derived from the Virginia opossum, Didelphis (with a pouch).
Related marsupial didelphids
include extant dasyurids, Early Cretaceous Ambolestes and Vincelestes, the giant Andrewsarchus, and the recently extinct Tasmanian wolf, Thylacinus. So this is a clade that extended into antiquity and displays variation in size, dentition, diet, niche, reproduction and geography.
Figure 1. Skulls to scale of Periptychus, Monodelphis domestica and Orthaspidotherium.
” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/04/periptychus-skull3views588.jpg?w=155″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/04/periptychus-skull3views588.jpg?w=528″ class=”size-full wp-image-92333″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/04/periptychus-skull3views588.jpg” alt=”Figure 1. Skulls to scale of Periptychus, Monodelphis domestica and Orthaspidotherium. ” width=”584″ height=”1133″ srcset=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/04/periptychus-skull3views588.jpg?w=584&h=1133 584w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/04/periptychus-skull3views588.jpg?w=77&h=150 77w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/04/periptychus-skull3views588.jpg?w=155&h=300 155w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/04/periptychus-skull3views588.jpg 588w” sizes=”(max-width: 584px) 100vw, 584px” />
Figure 1. Skulls to scale of Periptychus (new restoration), Monodelphis domestica and Orthaspidotherium. Note the flat molars and premolars.
Periptychus carinidens
(Cope 1881; Shelley et al. 2018) was originally and later unable to determine closest relatives. Here it nests with tiny Monodelphis (Fig 4). The manus and pes are primitive in having no reduced fingers or toes, yet the unguals are stub-like, sometimes round hooves. The teeth are all cylindrical, having lost their traditional cusps.
That’s why the study of mammalian teeth can only take one so far. Evolution can change tooth shapes while leaving the rest of the body largely unchanged.
On that same note, if you want to nest Periptychus with artiodactyls due to the presence of round unguals, you are ‘pulling a Larry Martin‘. Avoid that method. Instead use the last common ancestor method, which requires a wide-gamut, trait-based phylogenetic analysis, like the LRT, as a first step.
Figure 4. Manus and pes of Periptychus with some bones restored.
” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2018/07/periptychus_pes588.jpg?w=300″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2018/07/periptychus_pes588.jpg?w=584″ class=”size-full wp-image-32150″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2018/07/periptychus_pes588.jpg” alt=”Figure 4. Manus and pes of Periptychus with some bones restored.” width=”584″ height=”498″ srcset=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2018/07/periptychus_pes588.jpg?w=584&h=498 584w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2018/07/periptychus_pes588.jpg?w=150&h=128 150w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2018/07/periptychus_pes588.jpg?w=300&h=256 300w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2018/07/periptychus_pes588.jpg 588w” sizes=”(max-width: 584px) 100vw, 584px” />
Figure 2. Manus and pes of Periptychus with some bones restored. Note some broad unguals, none like claws.
Halliday, Upchurch and Goswami 2017
in their attempt to ‘resolve relationships of Paleocene placental mammals’ nested Periptychus with Arctocyon and Ectoconus. These widely scattershot taxa are not related to one another in the LRT.
The same study
nested Orthaspidotherium basal to Rhynchocyon, Procavia, Eohippus and Pakicetus. Hee only Rhynchocyon and Pakicetus are related to one another in the LRT. The authors did not realize Periptychus and Orthspidotherium were marsupials = outgroups to the placental mammals they intended to focus on.
Sometimes taxon inclusion can be a problem when cherry-picked taxa from outside the focus of the study are inappropriately included. The LRT solves that problem by including a wide-angle view of chordate taxa. No more cherry-picking taxa. The LRT tells you which taxa to include and exclude in your more focused study.
Nice to see confirmation of the Rhynchocyon – Pakicetus interrelationship.
Figure 3. Periptychus skeleton restored.
” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2018/07/periptychus-carinidens588.jpg?w=300″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2018/07/periptychus-carinidens588.jpg?w=584″ class=”size-full wp-image-32148″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2018/07/periptychus-carinidens588.jpg” alt=”Figure 3. Periptychus skeleton restored.” width=”584″ height=”221″ srcset=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2018/07/periptychus-carinidens588.jpg?w=584&h=221 584w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2018/07/periptychus-carinidens588.jpg?w=150&h=57 150w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2018/07/periptychus-carinidens588.jpg?w=300&h=114 300w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2018/07/periptychus-carinidens588.jpg 588w” sizes=”(max-width: 584px) 100vw, 584px” />
Figure 3. Periptychus skeleton restored.
Likewise, the most recent study by Shelley, Williamson and Brusatte 2018
considered Periptychius a ‘placental‘ and ‘ungulate-like‘, but did not mention Monodelphis. While the team chronicled the history of this genus and every bone in detail, in the end they were unable to ascertain its phylogeny and did not provide an analysis.
That’s where the LRT comes in handy. Key to any and every paleo study is a phylogenetic analysis. Don’t write a paper if you don’t know what your taxon is. As a scientist, you are supposed to provide answers, not extend myths and traditions.
Figure 6. Monodelphls and pups exposed as no pouch is present in this basal placental taxon. Note the tail is not bushy.
” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2016/10/monodelphis_and_pups588.jpg?w=300″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2016/10/monodelphis_and_pups588.jpg?w=584″ class=”size-full wp-image-24463″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2016/10/monodelphis_and_pups588.jpg” alt=”Figure 6. Monodelphls and pups exposed as no pouch is present in this basal placental taxon. Note the tail is not bushy.” width=”584″ height=”554″ srcset=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2016/10/monodelphis_and_pups588.jpg?w=584&h=554 584w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2016/10/monodelphis_and_pups588.jpg?w=150&h=142 150w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2016/10/monodelphis_and_pups588.jpg?w=300&h=285 300w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2016/10/monodelphis_and_pups588.jpg 588w” sizes=”(max-width: 584px) 100vw, 584px” />
Figure 4. Monodelphls and pups exposed as no pouch is present in this basal placental taxon. Note the tail is not bushy.
Implications
Since North American Periptychius fossils are found in the Paleocene, that implies the more primitive – but still extant South American Monodelphis was native to Cretaceous South and North America. Furthermore, related taxa, like Chinese Ambolestes and South American Vincelestes, are Early Cretaceous – which puts the more primitive Monodelphis and Didelphis back even further – to the Jurassic.
Opossums are ancient survivors. Let’s take a moment to appreciate that fact.
References
Cope ED 1881. The Condylarthra (Continued). American Naturalist 84;18: 892–906.
Halliday TJD, Upchurch P nd Goswami A 2017. Resolving the relationships of Paleocene placental mammals. Biological Reviews. 92 (1): 521–550.
Shelley SL, Williamson TE and Brusatte SL 2018. The osteology of Periptychus carinidens: A robust, ungulate-like placental mammal (Mammalia: Periptychidae) from the Paleocene of North America. PlosOne July 18, 2018. https://doi.org/10.1371/journal.pone.0200132
wiki/Monodelphis
wiki/Hyopsodus
wiki/Amphicyon
wiki/Periptychus
wiki/Periptychidae
Source: https://pterosaurheresies.wordpress.com/2025/04/08/periptychus-a-big-paleocene-monodelphid-with-cylinder-teeth/
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