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Zavodsy and Russo 2020 looked at rare deep chevrons in mammals

Friday, March 27, 2026 15:57

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Most mammals lack deep chevrons.
That includes basal mammals.Phylogenetically chevrons last appeared on the early cynodonts Thrinaxodon and Procynosuchus.

That means whenever chevrons do rarely appear in mammals they are either 1) new ossifications or 2) genetic reversals back to those early cynodonts… or both.

Otherwise chevrons are common among chordates.

Several tested mammals in the LRT
have/had deep chevrons = haemal arches = “V-shaped bony structures located on the underside of the tail vertebrae in reptiles, dinosaurs, and certain mammals like kangaroos. These bones protect vital blood vessels and nerves within the tail’s haemal canal and provide muscle attachment sites, especially for locomotion.”

The list of taxa includes:

Big kangaroos (Macropus) – but not Procoptodon

Xenarthrans, including aardvarks (Orycterpus), starting with Pholidocercus
Barylambda and sloths (like Paramylodon) and Glyptodon, anteaters (like Myrmecophagus) and derived pangolins (like Manis) – but not primitive pangolins.

Pakicetids, like Maiacetus, Aetiocetus and Orcinus) – but not Pakicetus.

Mysticetes (like Caperea and Balenoptera).

Beavers (Castor), but not Moeritherum, so then skipping to derived adult sirenians (like Dugong and Trichechus) – but not their calves.

I think there’s a moethere with a long tail and chevrons waiting to be discovered.

Mammals with deep chevrons use them either
1. to support part of their weight in tripod fashion while standing upright (e.g. kangaroos and sloths)
2. to drive ventral flexion in swimming taxa (e.g. the rest of the list).

Other mammals
had either no chevrons or shallow chevrons that were longer than deep.

Let me know if I missed any taxa or clades with deep chevrons.

Zavodszky and Russo 2020 wrote,
“Tail morphology and function vary considerably across mammals. While studies of the mammalian tail have paid increasing attention to the caudal vertebrae, the chevron bones, that serve to protect blood vessels and provide attachment sites for tail flexor musculature, have largely been ignored.

“Here, morphological variation in chevron bones is documented systematically among mammals possessing different tail locomotor functions, including prehensility, terrestrial propulsion (use for pentapedal locomotion), and postural prop, during which chevron bones are presumably under different mechanical stresses or serve different mechanical roles.”

“This study offers information about the functional morphology of mammalian tails and has implications for reconstructing tail function in the fossil record.”

Click on the links above to see the data.

References
Zavodszky AM and Russo GA 2020. Comparative and functional morphology of chevron bones among mammals. Journal of Mammalogy 101(2): 403-416.
https://doi.org/10.1093/jmammal/gyaa010

Source: https://pterosaurheresies.wordpress.com/2026/03/28/zavodsy-and-russo-2020-looked-at-rare-deep-chevrons-in-mammals/

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