Anatomical sciuromorphy in "protrogomorph" rodents
· Managed by: Robert Asher
Historically, high-level rodent taxonomy has been intertwined with the anatomical condition of the masseter muscle, leading to the widespread use of grades based on four anatomical conditions: protrogomorphy, sciuromorphy, hystricomorphy, and myomorphy. Although many previous studies have since shown these grades to be paraphyletic, the idea of a “protrogomorph” rodent has remained popular for extinct species. We examined the oldest and most complete articulated skeleton yet known of Ischyromys (USNM 617532) from the late Duchesnean of West Canyon Creek, Wyoming. Ischyromys species are usually treated as anatomically protrogomorphous, but USNM 617532 shows attachment of the deep masseter anterior and dorsal to the infraorbital foramen on the rostrum, creating a zygomatic plate, and therefore exhibits anatomical sciuromorphy. A geometric morphometric analysis of cranial landmarks suggests that I. typus resembles extant, anatomically protrogomorphous rodents, whereas USNM 617532 falls within the range of non-protrogomorphous rodents. USNM 617532 differs from Ischyromys typus in terms of incisor procumbency, infraorbital foramen size and zygomatic arch shape. In addition to its distinctive rostrum and zygomatic plate, USNM 617532 possesses a sagittal crest, accessory cusps on the lower molars and large metaconules on the upper molars, features that help to diagnose this specimen as a member of Ischyromys douglassi. Masseteric patterns mapped onto a recent phylogenetic estimate suggest that none of the three main rodent clades, or even crown Rodentia itself, was characterized by an anatomically protrogomorphous common ancestor, although protrogomorphy did characterize many simplicidentate taxa on the stem leading to Rodentia.
See Rankin, Aime H., Emry, Robert J., and Asher, Robert J. 2020. Anatomical sciuromorphy in "protrogomorph" rodents. Palaeontologia Electronica, 23(2):a25. https://doi.org/10.26879/1049
also Asher RJ, Smith MR, Rankin AH, Emry, RJ. 2019. Congruence, fossils, and the evolutionary tree of rodents and lagomorphs. Royal Society Open Science 6: 190387.