Validating Dipodomys Data from Premolar Measurements
by Tabbatha Ostlie (Pimi)

Kangaroo rats are one of the many unique species that populate Anza-Borrego Desert State Park. Our paleontological collection comes mostly from Vallecito-Fish Creek Basin and is one of the largest from any single local fauna. This is considered the oldest record of Dipodomys within southern California, with the lowest age being ~3.6 million years. With over 350 specimens, ranging throughout the Palm Spring Formation and spanning from the late Pliocene to the late Pleistocene, our Dipodomys provide important knowledge about the taxon's ecology, environment, and evolutionary history.
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​Extant species still roam Anza-Borrego, but our primary focus resides in 4 species whose fossils were found in ABDSP:
1. Dipodomys compactus (extant; living forms not in ABDSP)
2. Dipodomys hibbardi​ (extinct; previously described by Richard Zakrzewski (1981))
3. Dipodomys n. sp. A (extinct; newly described by Gregory Cunningham (1984))
4. Dipodomys n. sp. B (extinct; newly described by Gregory Cunningham (1984))
In the summer of 2023, an organization project was started to sort all described Dipodomys type specimens (ie. holotype, paratype) within the collection. Some previously completed projects included type specimens of horse and turtle. This marks the 4th project in the series and will be another milestone for future organization.

​With the measurements by Gregory Cunningham in his 1984 M.S. thesis (Idaho State University), we can validate our data by comparing measurements of the specimens he studied.
​Premolars of Dipodomys are the most diagnostic tooth due to root shape, root size, and dentine tract heights (how much wear is on the occlusal surface of the tooth). By measuring their occlusal surface and comparing to Cunningham's data, we can validate his taxonomic identifications and correct our labels on each specimen. ​
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Dipodomys ingens (C) California Conservation Genomics Project

How to tell the difference between an upper and lower premolar?
(Hover over images to see captions)

Photos by Blois Lab Image Gallery

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​​Upper premolars look like a funny little heart
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​​Lower premolars look like a loaf of bread... mmm...
Overall shape and size of Dipodomys teeth vary between individuals. Some premolars do not follow the same drastic variation shown above and may be extremely worn down due to age differences. ​While age does not generally affect the identification of this taxon, it is important to note that most specimens examined in this study were distinct enough to determine upper/lower. However, some were fractured or roots were not visible enough to confidently verify previous taxon assignment.

Materials and Methods

The number of specimens varies greatly between species, with Dipodomys n. sp. A having the most amount of specimens while Dipodomys n. sp. B only has 8 lower premolar specimens.
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​This is due to the limitation of what is available in our collection.
Picture
Photography with a DinoLite microscope (C) Gabriel Vogeli
Despite vast differences in technology between our study and Cunningham's, the variation of measurements was not drastic at all. In fact, our measurements fell within ~0.05mm of Cunningham's observed ranges!
APL = Antero-posterior length
​TW = Transverse width
Each specimen was photographed and measured using a DinoLite digital microscope and DinoCapture 2.0 software on a laptop.
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Specimens were positioned with occlusal surface facing the microscope's lens, which was placed directly above.
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​The way each tooth was angled was all by hand (or rather, fingers) so that there was a consistent point of view. 
Some photographs were taken laterally (sideways showing the entire length of the tooth) in an attempt to see dentine tracts, but determination of where a tract started/ended was inconsistent. 
​Instead, measuring the length (APL) and width (TW) of each tooth's occlusal surface gave us enough information to compare to Cunningham's own observed ranges.
​"Edges" of occlusal surface were determined by the blue box surrounding the tooth.

Results

Scrolling through the graphs, we see that... it's a huge mess. Variation between species is too clustered to be able to determine any differences. In upper premolars, all trend lines show very little variation at all. However, in lower premolars, there is a noticeable difference in trend line slopes for Dipodomys compactus​. This may be due to personal measurement errors or less confidence in taxon assignment. The averages of each type of measurement also shows very little distinction between species. 


So what does this tell us?

It's possible that we're not looking at 4 species as examined by Cunningham, but perhaps only 1 or 2 instead. The small variation between species is odd because there's no visual distinction between measurements. Now, if we looked at modern Dipodomys and saw a similar statistic, then it's possible this is "just a kangaroo rat thing" and there's other distinguishing features we may have missed.
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​For instance, if we focused primarily on Dipodomys compactus with both fossil and living forms, would this trend be similar? Would it show even more variation since this species currently lives in a different area vs. its prehistoric ancestors? These are some of the questions that can spark future research.
Picture
Dipodomys compactus (C) iNaturalistUK
Dipodomys are territorial animals and like to live a solitary life. Displayed above are examples of how extant species distribute across California. Some overlap may occur, but typically there are only 1-2 species per habitat range and rarely overlap with an additional species. 

​​... What now?
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If we also looked at where they were discovered (locality), more information may be revealed. If certain specimens were all found in the same locality (so 4 species in 1 locality), that would also be strange since kangaroo rats have low interspecies tolerance. Even in modern day, there are only 2 extant species living in Anza-Borrego. (See map distribution to the left)

If the specimens were found throughout different localities, then it would make sense IF there was a distinction in time period as well.
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​For example, if Dipodomys compactus and Dipodomys hibbardi were in localities from 3 mya - 2 mya while Dipodomys n. sp. A and Dipodomys n. sp. B were in localities from 2 mya - 1 mya, then it would suggest ecological changes.
​​However, if compactus and hibbardi were found between 3 mya - 1 mya alongside Dipodomys n. sp. A, then that would indicate a possible error in taxon assignment or a possible transition between species' ecological niches.

In Conclusion...

Dipodomys are small and fascinating creatures that tell us a lot about the environment they live in. Learning about their evolutionary history, ecology, and environmental interactions in ABDSP through fossils are some of the many fun aspects of our paleontological program.
​By measuring their premolars, we can validate our data with Gregory Cunningham's and apply his taxonomic identifications to our specimens. Narrowing down our search to the species level (or even the genus level) of these specimens allows our collection to expand into a reliable resource for future researchers.​






​​"Thus concludes our talk about my teeth! Hope you learned a lot along the way... I sure did! Who knew my teeth could be so informative?
Thank you for listening to me! Make sure to check out the fun little extras down below if you want to explore more!" 
​- Pimi the Dipodomys​
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Photo (C) Richard Hansen

Fun Extras
References
(all images and videos can be clicked on to give a direct link to where they are from)
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Dipodomys deserti (C) Animalia.bio
Pimi the Dipodomys says, "Phew, that sure tuckered me out! Thank you again for reading!... Time for a nap."

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