Morphology of an Early Oligocene beaver Propalaeocastor irtyshensis and the status of the genus Propalaeocastor

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.

Keywords: Beavers; Early Oligocene; Irtysh River Formation; Propalaeocastor; Xinjiang.

Figure 1. Jeminay and Burqin Propalaeocastor irtyshensis fossil localities in the Irtysh River drainage area in northwestern Xinjiang, China (modified from Stidham & Ni, 2014).

(A) Map showing the location of the P. irtyshensis localities in the Irtysh River region within China adjacent to several other countries. (B) Detailed map showing the border region between Xinjiang and Kazakhstan and the localities of P. irtyshensis. (C) Panoramic view of the fossiliferous profile of the Irtysh River Formation that produced the additional material of P. irtyshensis.

Figure 2. The upper dental structure of the Castoridae after the example of the moderately worn fourth premolars of Propalaeocastor irtyshensis.

(A) Propalaeocastor; (B) Steneofiber; (C) Castor; (D) Dipoides. From left to right: lingual view, occlusal view, buccal view. Enamel = white, dentine = dark grey, cement = light grey, -fossette = -flexus = -stria. Modified from Stirton (1935), Hugueney (1975), Hugueney (1999), Lopatin (2003) and Wu et al. (2004).

Figure 3. The lower dental structure of the Castoridae after the example of the moderately worn fourth premolars of Propalaeocastor irtyshensis.

(A) Propalaeocastor; (B) Steneofiber; (C) Castor; (D) Dipoides. From left to right: lingual view, occlusal view, buccal view. Enamel = white, dentine = dark grey, cement = light grey, -fossettid = -flexid = -striid. Modified from Stirton (1935), Hugueney (1975), Hugueney (1999), Lopatin (2003) and Wu et al. (2004).

Figure 4. Maxilla and isolated upper cheek teeth referred to Propalaeocastor irtyshensis from Jeminay area, northwestern Xinjiang, China.

Yellow shadow showing the divergence of palatine nerve. (A1-3) broken maxilla with right P4-M1 (IVPP V 23138.1); (B1-3) left P4 (IVPP V 23138.2); (C1-3) left M1 (IVPP V 23138.3). (A1), (B1), (C1) lingual; (A2), (B2), (C2) buccal; (A3), (B3), (C3) occlusal. All in same scale.

Figure 5. 3D virtual reconstruction of the maxillae of Propalaeocastor irtyshensis by the X-ray computed tomography.

Red shadow showing a residual P3 alveolus mesial to the mesial roots of P4; dashed cycle displaying a relative large and round infraorbital foramen dorsal to the zygomatic arch root preserved in both holotype of Burqin (A1-3: IVPP V 13690) and additional specimen of Jeminay (B1-2: IVPP V 23138.1).

Figure 6. Fragmentary dentaries of Propalaeocastor irtyshensis from Jeminay, Xinjiang.

Red shadow displaying articular facet of mandibular symphysis. (A–C) fragmentary right dentary with broken p4-m3 (IVPP V 23139); (D–F) broken right dentary with p4-m1 (IVPP V 23140); (G–I) broken right dentary with p4 (IVPP V 23141). (A), (D), (G) lingual; (B), (E), (H) occlusal; (C), (F), (I) buccal. All in same scale.

Figure 7. Transverse and sagittal sections of dentaries and transverse section of lower incisor of Propalaeocastor irtyshensis of Jeminay, Xinjiang.

Showing convex enamel surface of lower incisor, permanent fourth premolar and root number of lower cheek teeth (p4:m1:m2:m3 = 2:3:3:3). (A–B) fragmentary right dentary with p4 (IVPP V 23141); (C–D) broken right mandible with p4-m1 (IVPP V 23140); (E–F) fragmentary right dentary with p4-m3 (IVPP V 23139). (G) lower incisor (IVPP V 231411). (A), (C), (E) sagittal section; (B), (D), (F), (G) transverse section.

Figure 8. Majority-rule consensus of six most parsimonious trees.

Parsimony analysis is based on a data matrix including 145 characters scored for 42 taxa. Marmota monax, Keramidomys fahlbuschi and Eutypomys inexpectatus were selected as outgroup taxa. Numbers before the slashes at the internodes are the absolute Bremer Support values; numbers after the slashes are Relative Bremer Support values; numbers after the comma are percentage of consensus. Internodes without the percentage of consensus show the topologies that are present in all the six most parsimonious trees. The geographic distribution of all the taxa was mapped on the majority-rule consensus tree and the ancestral states were reconstructed using the parsimony criterion in Mesquite 3.2 (Maddison & Maddison, 2017). Red clades represent Asian origin, blue clades represent European origin, and black clades represent North American origin. Clades in dashed line indicate equally-parsimonious or ambiguous Asian, European or North American origins. Scale bar equals 20 character changes.

Figure 9. Chronologic and geographic distribution of Propalaeocastor and Agnotocastor, and comparisons of dentary and dental patterns.

Displaying the developments of digastric eminence and angular process of the mandible extending orientations of their mandibles and similarities of dental constructions. Asterisk showing the type species of Agnotocastor and Propalaeocastor. Except for the figures of Propalaeocastor irtyshensis (dentary, IVPP V 23141; lower dentition, IVPP V 23139, upper dentition, IVPP V 13690), the illustrations in the Dentaries, Lower Dentitions and Upper Dentitions columns are facsimiles of their original figures (Stirton, 1935; Wilson, 1949b; Borisoglebskaya, 1967; Lytschev, 1970; Emry, 1972; Hugueney, 1975; Lytschev, 1978; Korth, 1998; Korth, 1996a; Korth, 1998; Lytschev & Shevyreva, 1994). Abbreviations used in left column are biochrons of North American Land-Mammal Ages (NALMA): Ch-1 = Early Chadronian; Ch2-3 = Middle Chadronian; Ch4 = Late Chadronian; Or1–Or4 = Orellan; Wh1–Wh2 = Whitneyan (see Flynn & Jacobs, 2008). Dentaries and dentitions are in same scales, respectively.