Morphology of an Early Oligocene beaverPropalaeocastor irtyshensisand the status of the genusPropalaeocastor

Abstract

The Early to Late Oligocene Propalaeocastor is the earliest known beaver genus from Eurasia. Although many species of this genus have been described, these species are defined based on very fragmentary specimens. Propalaeocastor irtyshensis from the Early Oligocene Irtysh River Formation in northwestern Xinjiang, China is one of the earliest-known members of Propalaeocastor. This species is defined on a single maxillary fragment. We revise the diagnosis of P. irtyshensis and the genus Propalaeocastor, based on newly discovered specimens from the Irtysh River Formation. The dental morphology of P. irtyshensis is very similar to other early castorids. The caudal palatine foramen of P. irtyshensis is situated in the maxillary-palatine suture. This is a feature generally accept as diagnostic character for the castorids. On the other hand, P. irtyshensis has two upper premolars, a rudimentarily developed sciuromorph-like zygomatic plate, and a relatively large protrogomorph-like infraorbital foramen. Some previous researchers suggested that Propalaeocastor is a junior synonym of Steneofiber, while other took it as a valid genus. Our morphological comparison and phylogenetic analysis suggest that Propalaeocastor differs from Steneofiber and is a valid genus. We also suggest that Agnotocastor aubekerovi, A. coloradensis, A. galushai, A. readingi, Oligotheriomys primus, and “Steneofiber aff. dehmi” should be referred to Propalaeocastor. Propalaeocastor is the earliest and most basal beaver. The origin place of Propalaeocastor and castorids is uncertain. The Early Oligocene radiation of castorids probably is propelled by the global climate change during the Eocene-Oligocene transition.

Rodents (Mammalia)from Fitterer Ranch, Brule Formation (Oligocene), North Dakota

“…6). The lower incisor of O. magnus is much narrower (transversely) and is not nearly as flattened anteriorly as in Propa laeocastor (Li et al, 2017: fig. 7C).…”

“…Li et al's (2017:8) diagnosis of Propalaeocastor included mainly primitive characters but also the unilateral mesodonty of the upper cheek teeth and complex occlusal pattern of all the cheek teeth. Li et al (2017) separated Agnotocastor from Propalaeocastor by the simpler occlusal pattern of the cheek teeth of the former. However, Agnotocastor was separated from Propa laeocastor on the basis of a comparison with only the holotype of A. praetereadens, a partial cranium that has heavily worn cheek teeth (Stirton, 1935: fig.…”

“…Recently, Li et al (2017) included most of the North American species previously referred to Agnotocas tor in the Asian genus Propalaeocastor Borisoglebskaya, 1967, retaining only the type species of Agnotocastor, A. praetereadens Stirton, 1935, in the latter genus. They included only the Eurasian A. devius Lytschev and Shevyreva, 1994 in the genus along with the type species.…”

“…No specimens of the Eurasian species cited by Li et al (2017) have been directly examined as part of this study. However, on the basis of the available figures, A. devius appears to have a much simpler occlusal morphology than any other species of either Agnotocastor or Propalaeocastor (Li et al, 2017: fig. 9) and is not likely referable to either genus.…”

“…The similarity of NDGS 2660 to the m3 of the holotype of "E." magnus suggests that the upper dentition originally referred to O. primus is also referable to "E." magnus, making the former a junior synonym of the latter but maintaining the generic identification as Oligotheriomys. Li et al (2017) included "O. primus" in Eurasian Propalaeo castor. Oligotheriomys does possess the unilateral mesodonty of the upper molars and complex occlusal pattern observed in Pro palaeocastor but lacks the diagnostic character of "metaflexus buccally open" of the latter (Li et al, 2017:8) and has an overall greater complexity.…”