Reappearance of the procoracoid + coracoid in two extinct rodents
Reversals can be tricky. Many remain unrecognized.
Earlier we looked at simple-cone odontocete teeth representing a reversal hearkening back to basal pelycosaur teeth.
Earlier we also looked at the primitive jaw-ear bone structures in multituberculates, identified in the LRT as reversals within Glires rather than primitive traits around the origin of mammals.
Today we look at yet another reversal, this time in the shoulder joint.
Figure 1. Little Jurassic Maiopatagium compared to scale with extant Sciurus and Eocene Paramys.
” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/05/maiopatagium-skeleton-diagram588.jpg?w=240″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/05/maiopatagium-skeleton-diagram588.jpg?w=584″ class=”size-full wp-image-86302″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/05/maiopatagium-skeleton-diagram588.jpg” alt=”Figure 1. Little Jurassic Maiopatagium compared to scale with extant Sciurus and Eocene Paramys.” width=”584″ height=”730″ />
Figure 1. Little Jurassic Maiopatagium compared to scale with extant Sciurus and Eocene Paramys.
When Meng et al 2017 described Middle Jurassic Maiopatagium
they reported, “The scapula–coracoid consists of a coracoid, a separate procoracoid perforated by a foramen, and a scapula, all of which are similar to those of non-mammalian cynodonts, Morganucodon and Sinoconodon.”
The coracoid and procoracoid are absent in most mammals.
Exceptionally and by reversal,
these two shoulder joint bones also appear in the basal rodent – squirrel from the Miocene, Paramys (Figs 1, 2) and closely related Maiopatagium (Figs 1–3).
Figure 1. Top: Paramys forelimb with two otherwise unidentified round bones illustrated at the shoulder joint. Bottom: Jurassic Maiopataigum procoracoid and coracoid, a reversal for this gliding rodent.
” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/05/paramys-procoracoid588.jpg?w=167″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/05/paramys-procoracoid588.jpg?w=570″ class=”size-full wp-image-86293″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/05/paramys-procoracoid588.jpg” alt=”Figure 1. Top: Paramys forelimb with two otherwise unidentified round bones illustrated at the shoulder joint. Bottom: Jurassic Maiopataigum procoracoid and coracoid, a reversal for this gliding rodent.” width=”584″ height=”1049″ />
Figure 2. Top: Paramys forelimb with two otherwise unidentified round bones illustrated at the shoulder joint. Bottom: Jurassic Maiopataigum procoracoid and coracoid, a reversal for this gliding rodent.
The reappearance of the procoracoid and coracoid
are reversals in these two LRT rodents. They don’t appear elsewhere in Placentalia, so far as I am aware. This insight is the result of recent housekeeping in the LRT.
Figure 1. Maiopatagium in situ in white and UV light. The X marks an area surrounded by fur lacking proptagial data. Is the propatagium wishful thinking?
” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2017/08/maiopatagium_insitu588.gif?w=221″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2017/08/maiopatagium_insitu588.gif?w=584″ class=”size-full wp-image-27779″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2017/08/maiopatagium_insitu588.gif” alt=”Figure 1. Maiopatagium in situ in white and UV light. The X marks an area surrounded by fur lacking proptagial data. Is the propatagium wishful thinking?” width=”584″ height=”792″ />
Figure 3. Maiopatagium in situ in white and UV light. The X marks an area surrounded by fur lacking proptagial data. Is the propatagium wishful thinking? This image is 1.1x life size @72dpi.
Maiopatagium furculiferum
(Luo et al. 2017; Meng et al. 2017; Late Jurassic (Callovian); 160 mya) was originally described as a pre-mammal haramiyid close to Sinoconodon. Here it nests with the Eocene sqirrel, Paramys (Fig 1) and both are derived from the extant porcupine, Coendou.
Paramys compressidens
(Leidy 1871; Matthew 1910; Prufrock et al 2021, middle Eocene, 45 mya, AMNH FM 12506) is one of the earliest known squirrel-like rodents. Manual digit 1 (the thumb) is absent. Prufrock et al 2021 reported, “Paramys delicatus was found to have generally greater mechanical advantages and limb robusticity than most living squirrels, including semifossorial taxa.”
References
Leidy J 1871. Remains of extinct mammals from Wyoming. Proceedings of the Academy of Natural Sciences 23: 113–116.
Luo Z-X, (6-co-authors) 2017. New evidence for mammaliaform ear evolution and feeding adaptation in a Jurassic ecosystem. Nature. in press (7667): 326–329. doi:10.1038/nature23483
Matthew WD 1910. On the osteology and relationships of Paramys and the affinities of the Ischyromyidae. Bulletin of the American Museum of Natural History, 28:43–72.
Meng Q-J, Grossnickle DM, Liu D, Zhang Y-G, Neander AI, Ji Q and Luo Z-X 2017.
New gliding mammaliaforms from the Jurassic. Nature (advance online publication)
doi:10.1038/nature23476
Prufrock KA Ruff CB and Rose KD 2021. Locomotor Behavior and Body Mass of Paramys delicatus (Ischyromyidae, Rodentia) and Commentary on Other Early North American Paramyines. Journal of Mammalian Evolution, 28(2), 435-456. https://doi.org/10.1007/s10914-020-09523-8
Source: https://pterosaurheresies.wordpress.com/2024/05/29/reappearance-of-the-procoracoid-coracoid-in-two-extinct-rodents/
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