Anatomical sciuromorphy in "protrogomorph" rodents

Abstract

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.

Mandibular characteristics of early Glires (Mammalia) reveal mixed rodent and lagomorph morphotypes

“…Furthermore, the high and flat lateral aspect of the muzzle in Heomys with a marked horizontal ridge could point to the possibility of the sciuromorphous type of the anterior deep masseter attachment [ 52 ]. The sciuromorphous masseteric condition has been noted to be more efficient in incisor biting than the supposedly ancestral protrogomorphous condition [ 3 , 53 ], and has recently been suggested to have evolved much earlier within Rodentia than previously suspected [ 54 ]. The presence of a truly sciuromorphous masseter type in the stem Glires would cast a new light on the rodent ancestral morphotype and the extent of convergent masseter morphology among Glires taxa [ 43 ].…”

Total evidence tip-dating phylogeny of platyrrhine primates and 27 well-justified fossil calibrations for primate divergences

“…Nevertheless, Mimotona does share one distinctive derived dental synapomorphy with lagomorphs that is not seen in rodents or other simplicidentates, namely a longitudinal groove on the labial surface of the anteriormost upper incisor (Li and Ting, 1993;. Mimotona was also placed as a stemlagomorph in the recent phylogenetic analyses of Asher et al (2019) and Rankin et al (2020).…”

Twenty-five well-justified fossil calibrations for primate divergences

“…Phylogenetic justification. Recent phylogenetic analyses (Asher et al, 2019;Rankin et al, 2020) support Heomys sp. as the earliest known member of Simplicidentata, which includes Rodentia.…”

“…Nevertheless, Mimotona does share one distinctive derived dental synapomorphy with lagomorphs that is not seen in rodents or other simplicidentates, namely a longitudinal groove on the labial surface of the anteriormost upper incisor (Li and Ting, 1993;. Mimotona was also placed as a stem lagomorph in the recent phylogenetic analyses of Asher et al (2019) and Rankin et al (2020). Based on this collective evidence, we consider Mimotona wana and M. lii to be the oldest known stem representatives of Lagomorpha, at 66.0-62.278 Ma, and so provide the second CladeAge calibration for this node.…”