Tooth-digging has convergently evolved multiple times among unrelated clades of fossorial rodents throughout the history of the cenozoic. Contrary to the mole-like digging in which the forelimbs act as shovels, tooth-diggers possess unspecialized, relatively small feet, all the digging is done by the one pair of incisors and the head. One clade of these animals is the Spalacinae.

Based on almost identical shape of the mandible, Rhizospalax was once thought to be a spalacid, but detailed examination of its skull and dentition showed that it was of the sciuromorphous, castoroid type. It possessed a large P4, which when present in muroids, is definitely smaller than M1 (spalacids are muroids). The dentition of Rhizospalax was therefore 1.0.1.2/1.0.1.2 (last premolar substituted the first molar – P4, M1, M2) unlike a typical muroid which has only three true molars. Moreover, its molar pattern is less complex as in muroidea, indicating a different ancestry. Another trait that links this taxon to castorids is the shape and position of the infraorbital canal.

Fossorial beavers are known from Oligocene of North America. As a french find, Rhizospalax is a proof that, tooth-digging castorimorphs likely once inhabited the whole holarctic realm, and occupied ecological niches now filled by the still-living Bathyergidae, arvicoline tribe Ellobuisini and Spalacidae.

Here is shown my life reconstruction of Rhizospalax poirrieri. Missing components are based due to ecomorphological similarities, on Spalax, Tachyoryctes and Capacikala.

Literature:

• Hugueney, M., Mein, P. A comment on the earliest spalacinae (rodentia, Muroidea). J Mammal Evol 1, 215–223 (1993). https://doi.org/10.1007/BF01024708

• A new rodent from the Upper Oligocene of France. Bulletin of the AMNH; v. 41, article 18. Gerrit Smith Miller, James Williams Gidley, B Poirrier, Claude Gaillard

The two extant species of beaver are listed in the genus Castor. Historically this genus had a very wide distribution, in Eurasia ranging from Iberian Peninsula to China and in America from Alaska to northern Mexico.

However, the specimens of Castor from Late Miocene of China exhibit a distinct morphology that falls out of its norms. Therefore, a new genus has been created to list these specimens, called Sinocastor. Some differences between these genera includes: more elongated rostrum and neurocranium with the latter being not as broad, narrow postorbital region, (which are considered to be a more primitive traits) relatively longer incisive foramina and more…

If Sinocastor is a sister to Castor, it would meant that, the common ancestor of these two genera (therefore of a still-living beavers) may come from East Asia. Castor may have dispersed northeastward via Beringian Isthmus to North America and northwestward to Europe.

Here I show my reconstruction of Sinocastor with missing parts filled in by Castor fiber:

Reference:

https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0013990

I will start this article by saying that beavers (castoridae) are fascinating animals. It’s always a worthy moment when you see semiaquatic mammal like this in its environment. The only two extant species, Eurasian beaver (Castor fiber) and American beaver (C. canadensis) are fairly specialized dam constructors. However, in the oligocene and miocene, during the Arikareean North American Stage, when the climate was becoming increasingly drier, compared to the tropical-subtropical conditions of eocene, the situation was a bit different. The Great Plains supported a wide array of not only large game, but also burrowing mammals, including mustelids, moles, gophers and even beavers. Members of the castoridae family were known to occupy different ecological niches than they are today. At that time, they were several fossorial beaver lineages that likely developed burrowing adaptations independently from one another.

One monotypic species that caught my attention specifically is Migmacastor procumbodens. My goal was to reconstruct the species, how it could look like, when it was alive. Just the skull alone posses several indications that this beaver was a digger, more specifically a tooth-digger. Among these characteristics are elongated incisors along longitudinal axis, and the procumbent upper incisors. This means that the incisors have the cutting edge in front of the vertical plane, so they are in a more convex (curved outwards) position.

All this evidence led me to look at other extant fossorial tooth-digging rodents to get an idea of what our beaver’s approximate body proportions, soft tissue and lifestyle could have been like. One family that I referenced is Bathyergidae. The so-called mole-rats or blesmols are almost all tooth-diggers. One species that I took inspirations from, is the Cape mole-rat (Georychus capensis). It’s a solitary tooth-digger. I don’t take the fact that this mole-rat is solitary and modern beavers are social as an obstacle, since sociality largely depends on the environment and lifestyle and even very closely related species can have it differently (for example lion and tiger). In one study, I found the measurements of long bones of G. capensis, so I arranged them relatively to the size of Migmacastor skull, so the proportions are the same. For the spinal cord, rib cage and other parts of the body I used Palaeocastor (another tooth digging beaver) and Bathyergus (mole-rat). Then I drew the muscles, where they would be attached and got this sketch:

Then I had to modify the proportions a bit and reference a bunch of other relevant extant species to get the soft tissue correctly. This is the final product:

References:

• https://www.frontiersin.org/articles/10.3389/fevo.2022.857474/full
• https://www.researchgate.net/profile/William-Korth/publication/249023533_A_new_unusual_castorid_Rodentia_from_the_earliest_Miocene_of_Nebraska/links/5ce8b695a6fdccc9ddccdf4a/A-new-unusual-castorid-Rodentia-from-the-earliest-Miocene-of-Nebraska.pdf
• https://academic.oup.com/mspecies/article/doi/10.1644/799.1/2600528
• https://en.wikipedia.org/wiki/Incisor_procumbency