Non-technical Summary
Aplodontiid rodents are a species-rich group of rodents from the late Oligocene and Miocene of North America, now only represented by a single living species: the sewellel, sometimes called the mountain beaver (despite no close relationship with beavers). During the late Oligocene, four groups of aplodontiids with distinct morphologies, all of which are represented by multiple species, roamed North America, particularly in the Northern Great Plains, Northern Rockies, and Pacific Northwest regions. One of the more poorly-known aplodontiids of this time period is Rudiomys mcgrewi, known from a single specimen collected in the John Day Formation. Since that first publication, additional material has come to light, both in Oregon and in a similarly aged series of deposits in western Montana: the Cabbage Patch beds. The addition of the new material from Oregon provides a more complete understanding of the tooth morphology of Rudiomys. The Montana occurrences yield two additional species of the genus, Rudiomys drummondensis and Rudiomys intsook, which bear out some tentative ecological inferences from the original description. These additional specimens demonstrate that Rudiomys was very likely a habitat specialist, which explains its limited distribution. Its unusual tooth morphology for an aplodontiid likely indicates an affinity for forested habitats.
Introduction
Late Oligocene rodents worldwide diversified into a wide range of ecological roles as the cooler, drier climates of the mid-Cenozoic encouraged the origin of open habitats on many continents in parallel (Edwards et al., Reference Edwards, Osborne, Strömberg and Smith2010; Strömberg, Reference Strömberg2011; Saarinen et al., Reference Saarinen, Mantzouka, Sakala, Martinetto, Tschopp and Gastaldo2020). In North America in particular, the record of late Oligocene rodents shows a dramatic increase in the range of tooth crown heights and locomotor diversity as more varied habitat types replaced the closed forests that characterized most of the Eocene (Samuels and Hopkins, Reference Samuels and Hopkins2017). During this period of ecological and morphological change, many unusual lineages arose and many taxa appeared that represent transitional stages in the evolution of the hypsodont rodents that persisted into the Miocene and thereafter.
The Oligocene-aged John Day Formation of central Oregon has an excellent record of rodents from middle to late Oligocene strata that show exactly this transition, including multiple lineages of geomyid gophers (Rensberger, Reference Rensberger1971, Reference Rensberger1973), as well as a remarkable diversity of aplodontiid rodents (Rensberger, Reference Rensberger1983; Hopkins, Reference Hopkins2006, Reference Hopkins2008). One enigmatic aplodontiid lineage, the meniscomyine genus Rudiomys Rensberger, Reference Rensberger1983, was described from a single individual represented by a dentary with only two molars and a few associated bones including an otic capsule (Rensberger, Reference Rensberger1983). Although the material was not very complete, Rensberger (Reference Rensberger1983) recognized that the combination of characteristics in this unusual specimen, as well as its occurrence relatively late in the Oligocene, indicated a lineage distinct from Meniscomys Cope, Reference Cope1878, a much more common representative of the Meniscomyinae found high in strata of the Turtle Cove Member of the John Day Formation. The cheek teeth are quite low-crowned but have the distinctive flat wear facets that characterize other meniscomyines, and the otic capsule morphology is intermediate between the that of allomyines and the more derived state found in Meniscomys, with a complex network of reticulating bony septae not unlike those found inside the otic capsules of some heteromyid rodents (Samuels et al., Reference Samuels, Calede and Hunt2023). No additional material of the genus Rudiomys has been formally described since Rensberger’s (Reference Rensberger1983) publication, but several additional specimens have been collected by field crews in the John Day Fossil Beds National Monument, giving a more complete view of the dental morphology in this rare and unusual member of the John Day assemblage.
With more complete knowledge of the morphology of Rudiomys from the John Day, it has become evident that there are at least two undescribed species of this genus in the Oligocene of Montana, where they are locally even more abundant than in the John Day Formation. Rasmussen (Reference Rasmussen1977) recognized the uniqueness of these specimens in his dissertation, and additional material collected during the dissertation research of this paper’s second author (Calede, Reference Calede2016b) reveals two new species of Rudiomys, one of which was assigned a manuscript name in Rasmussen’s dissertation and the other of which comes from a site first mentioned in a paper on faunal turnover in the Cabbage Patch beds of the Renova Formation (Calede, Reference Calede2016a).
In this paper, we provide a description of the new material from the John Day Formation, as well as the new species from the Cabbage Patch beds, comparing the similarities and differences in morphology between Rudiomys taxa. These species provide a key context for the evolution of hypsodonty in aplodontiids at a critical moment in their diversity. The late Oligocene was the point of maximum diversity among aplodontiids, as well as the time of origin of the two most diverse Miocene lineages of aplodontiids, the Aplodontiinae and the Mylagaulidae (Hopkins, Reference Hopkins2008). An understanding of the origins and diversity of these lineages, as well as the factors that led to the extinction of so many aplodontiid lineages at the end of the Oligocene, could offer insights into the processes by which rodent assemblages were reorganized across the Paleogene-Neogene boundary.
Material and methods
The material from the John Day Formation described herein was identified in the course of a complete survey of all fossil aplodontiid specimens found in the collections of the University of California Museum of Paleontology (UCMP), the University of Washington Burke Museum (UWBM), and the John Day Fossil Beds National Monument (JODA). Similarly, the specimens from the Cabbage Patch beds were identified in the course of a study that examined all fossil aplodontiid specimens from Montana in the collections of the University of Kansas Museum of Natural History (KUVP), the University of Montana Paleontology Center (UMPC), and the UWBM. We follow the taxonomy of Hopkins (Reference Hopkins2008). Upper and lower teeth (incisors, premolars, and molars) are denoted by IX/, PX/, MX/ and I/X, P/X, M/X respectively where X represents the numerical tooth position; left and right are indicated by L and R. Other abbreviations are AP = anteroposterior length; frag = fragment; dP = deciduous premolar; and TR = transverse width. The nomenclature for the description of the dentition follows Hopkins (Reference Hopkins2008). All measurements were taken in FIJI (Schindelin et al., Reference Schindelin, Arganda-Carreras, Frise, Kaynig and Longair2012) using a mm ruler to scale images. Photographs were taken using a Dino-Lite Edge Plus microscope camera.
Institutional abbreviations
Types, figured, and other referred specimens included in this study are deposited in the following institutions: University of Kansas Museum of Natural History (KUVP), Lawrence, USA; John Day Fossil Beds National Monument (JODA, JDNM), Dayville, Oregon, USA; University of Washington Burke Museum (UWBM), Seattle, USA; and the University of California Museum of Paleontology (UCMP), Berkeley, USA.
Systematic paleontology
Class Mammalia Linnaeus, Reference Linnaeus1758
Order Rodentia Bowditch, Reference Bowditch1821
Superfamily Aplodontioidea Trouessart, Reference Trouessart1897
Family Aplodontiidae Trouessart, Reference Trouessart1897
Subfamily Meniscomyinae Rensberger, Reference Rensberger1981
Genus Rudiomys Rensberger, Reference Rensberger1983
Type species
Rudiomys mcgrewi Rensberger, Reference Rensberger1983.
Revised diagnosis
Periotic with septa enclosing elongate transverse chambers, septa less closely spaced (0.6 mm) than in Meniscomys; lateral surface of periotic anterior to auditory tube restricted in area, not depressed as in Meniscomys; anteromedial face of periotic less strongly concave than in Meniscomys; bulla less inflated than in Meniscomys.
Teeth lower-crowned than in Meniscomys and Niglarodon Black, Reference Black1961. P/4 shorter anteroposteriorly than in other meniscomyines and with strong anterolabial crest of hypoconid closing a posterolabial fossettid, unlike other meniscomyines. Mesostylid and entoconid of M/1 and M/2 more angular, less rounded than in Meniscomys, but broader and not as bladelike as in Niglarodon; mesostylid crest of M/2 not straight as in Niglarodon, more strongly arched around central fossettid than in Meniscomys. Mesostylid and metaconid more widely separated than in Meniscomys or Niglarodon; mesostylid and entoconid more widely separated in M/1 than M/2, with reentrant between them more open than in Meniscomys or Niglarodon. Metaconid less inflated than in Meniscomys or Niglarodon, with apex less labially positioned; metastylid crest more prominent than in Meniscomys or Niglarodon, with crest extending posterolinguad, not posteriad as in Crucimys Rensberger, Reference Rensberger1980. Anterior fossettid not oval as in Crucimys, traversed and constricted in center by low crest from the metaconid toward the anterolingual wall of the central fossettid. Hypoconid lacking flattening of posterolabial surface characteristic of promylagaulines. Trigonid of M/2 transversely wider relative to talonid than in Meniscomys or Niglarodon. Posterolingual inflection less compressed, with base of inflection in M/2 extending farther toward center of tooth than in Meniscomys. M/3 with posterolingual inflection remaining open with wear longer than in Meniscomys; entoconid shifted strongly anteriorly, yielding a more triangular outline than in M/1 and M/2. P4/ with complete anteroloph connecting anterostyle to protocone and with relatively narrow protocone, not expanded as in other meniscomyines. Upper molars retaining enamel on anterior and posterior faces later in wear and hence shortening less anteroposteriorly with wear than molars of Meniscomys and Niglarodon.
Molars larger than in other meniscomyines of comparable stage of hypsodonty; absolute crown height as low as in Meniscomys uhtoffi Rensberger, Reference Rensberger1983, perhaps slightly lower than in Niglarodon koerneri Black, Reference Black1961. Incisor smaller relative to molar size than in other meniscomyines.
Rudiomys mcgrewi Rensberger, Reference Rensberger1983
Rudiomys mcgrewi Rensberger, Reference Rensberger1983, John Day Formation, late Oligocene, Sheep Rock and Hayes Haven localities: (1, 2) JODA 4514 from JDNM-8B: (1) occlusal view, right maxilla fragment with P4/, M1/; (2) occlusal view, left maxilla fragment with P3/-M1/; (3, 4) RP/4 (JODA 14936 from JDNM-8B): (3) occlusal view; (4) lingual view; (5) occlusal view of RP/4, M/1, and partial M/2 (JODA 14929 from JDNM-8B); (6, 7) LdP/4–M/2 (JODA 2942 from JDNM-8): (6) occlusal view; (7) lingual view. Anterior is to the right for all images. Scale bar = 2 mm.
Figure 1. Long description
A multi-panel photo displaying seven fossil specimens of Rudiomys mcgrewi.
* Panel 1: Occlusal view of a right maxilla fragment containing P 4/ and M 1/ teeth, showing complex enamel folding.
* Panel 2: Occlusal view of a left maxilla fragment containing P 3/, P4/, and M 1/ teeth.
* Panel 3: Occlusal view of an isolated RP/4 tooth.
* Panel 4: Lingual view of the same RP/4 tooth shown in panel 3.
* Panel 5: Occlusal view of a tooth row including RP/4, M/1, and a partial M/2.
* Panel 6: Occlusal view of a lower tooth row LdP/4 through M/2, showing three distinct molariform teeth.
* Panel 7: Lingual view of the same lower tooth row from panel 6, showing the height of the crowns and the jawbone fragment below.
In all views, the anterior side of the teeth is oriented toward the right. A black scale bar at the bottom left represents 2 millimeters.
Rudiomys specimen measurements (in mm). Museum abbreviations and tooth positions as in the text. L = anteroposterior length; TRA = transverse width of anterior portion of tooth (anteroloph on P4/, trigonid of lower cheek teeth); TRP = transverse width of posterior portion of tooth (trigon of upper cheek teeth, talonid of lower cheek teeth)
Table 1. Long description
The table lists measurements in millimeters for specimens of Rudiomys mcgrewi, Rudiomys drummondensis n. sp., and Rudiomys intsook n. sp. Columns include Taxon and specimen, tooth position, L (length), TRA (transverse width of anterior portion), and TRP (transverse width of posterior portion).
* Rudiomys mcgrewi specimens:
- JODA 14929: P/4 (L 3.23, TRA 2.2, TRP 2.33); M / 1 (L 2.14, T R A 1.71, T R P 1.98).
- J O D A 14936: P / 4 (L 2.79, T R A 1.87, T R P 2.12).
- J O D A 2942: d P / 4 (L 2.95, T R A 1.65, T R P 1.86); M / 1 (L 2.56, T R A 2.01, T R P 1.94); M / 2 (L 2.48, T R A 1.95, T R P 1.93).
- J O D A 4514: L P 3 / (L 1.27, T R P 1.27); L P 4 / (L 3.1, T R A 1.74, T R P 3.7); L M 1 / (L 2.25, T R P 3.27); R P 4 / (L 3.32, T R A 1.79, T R P 3.39); R M 1 / (L 2.21, T R P 3.29).
* Rudiomys drummondensis n. sp. specimens:
- K U V P 18417: R d P 4 / (L 2.29, T R A 1.19, T R P 2.33).
- K U V P 18430: L d P / 4 (L 2.36, T R A 1.31, T R P 1.66).
- K U V P 18152: R P / 4 (L 2.6, T R A 2.01, T R P 2.29); R M / 1 (L 2.1, T R A 1.72, T R P 1.74); R M / 2 (L 2.28, T R A 1.98, T R P 2.02); R M / 3 (L 2.53, T R A 1.96, T R P 2.38).
- K U V P 18153: L P / 4 (L 2.68, T R P 2.32); L M / 1 (L 2.15, T R A 1.79, T R P 2.03); L M / 2 (L 2.26, T R A 1.97, T R P 1.88); L M / 3 (L 2.47, T R A 2.11, T R P 1.96).
- U W B M 98767: R M / 1 (L 2.2, T R A 1.61, T R P 1.7); R M / 2 (L 2.18, T R A 1.73, T R P 1.73); R M / 3 (L 2.27, T R A 1.74, T R P 1.64).
- U W B M 97362: L P 4 / (L 3.21, T R A 1.63, T R P 3.22).
- K U V P 18412: L P 4 / (L 3.12, T R A 1.71, T R P 3.63).
- K U V P 18898: M 3 / (L 2.13, T R P 2.33).
* Rudiomys intsook n. sp. specimens:
- U W B M 98842: R P 4 / (L 3.38, T R A 1.79, T R P 3.48).
- U W B M 98846: M 1 / or M 2 / (L 2.18, T R P 2.54).
- U W B M 108088: M 1 / or M 2 / (L 1.94, T R P 2.49).
- U W B M 108093: M 3 / (L 1.92, T R P 2.07).
Reference Rensberger1983 Rudiomys mcgrewi Rensberger, p. 27, pl. 5a, c, e.
Holotype
UCMP 105122, dentary with M/1-2, associated I/1 fragment, epiphysis of femur, and otic capsule. From locality V66106, Rudio Creek 3, at base of exposure.
Referred material
From locality JDNM-8B: JODA 14929, R dentary frag w/P/4, M/1, anterior half of M/2; JODA 15571, R dentary frag w/M/2-3; JODA 14936, R dentary frag w/ P/4; JODA 4514, R and L maxilla frags with RP3/-M1/, LP4/-M1/; JDNM-8: JODA 2942, L dentary w/ dP/4, M/1, M/2.
Emended diagnosis
M/2 central fossettid and posterolingual reentrant closed by interior crest connecting metastylid crest to anterolabial crest of entoconid. Anterolabial crest of entoconid on molars relatively straight, running diagonally across tooth toward central fossettid, not bending significantly labially where it forms the posterior wall of the central fossettid. Cusps of upper cheek teeth somewhat angular, neither blade-like nor swollen. P4/ and upper molars with a more prominent mesostyle and more concave labial faces of metacone and paracone than other species. Central transverse valley of P4/ fairly rounded and open, persistent with wear, not united with posterolingual fossette. Metaloph of upper cheek teeth low but distinct, dividing posterolabial fossette from central transverse valley.
Cusps retaining more variation in height with wear than in other meniscomyines, with strongly sloped wear facets. Metaconid and entoconid somewhat tall, with labial cusps a bit lower in unworn specimens. Higher-crowned than other Rudiomys; more crown height remaining when cusps merge with wear.
Occurrence
Rudio Creek 3, ~ 1.5 km south of the Long Creek Highway on northeastern side of Rudio Road east of Kimberly, Grant County, Oregon, USA. JDNM 8B, Hayes Haven, ~ 0.3 km east of the John Day River and 2 km north of Picture Gorge, Dayville, Grant County, Oregon, USA. JDNM 8, Sheep Rock, ~ 1 km east of the John Day River and 2.5 km north of Picture Gorge, Dayville, Grant County, Oregon, USA. 1.5 km east-southeast of the John Day Fossil Beds Thomas Condon Visitor Center. Specimens ranging stratigraphically through units K1–K2 of the Turtle Cove Member of the John Day Formation (Albright et al., Reference Albright, Woodburne, Fremd, Swisher, MacFadden and and Scott2008), late early Arikareean NALMA (Ar2).
Description
P/4 short anteroposteriorly (Table 1) with wear facets on protoconid and metaconid sloping strongly posteriorly and long sloping anterior crests, causing apparent trigonid length to increase with wear. Posterior inflection forming a distinctly triangular fossettid with wear; posterolabial fossettid separated from posterior inflection by a strong crest and forming a small crescentic fossettid anterolingual to the hypoconid. Small but distinct hypoconulid that bounds the posterior inflection with a crest connecting to the entoconid. Protoconid large and triangular, much wider labiolingually than in Meniscomys and Niglarodon, much more like in Sewelleladon Shotwell, Reference Shotwell1958. Strong metastylid crest as in Niglarodon, with a small, narrow crest connecting labially to the mesoconid and dividing the lingual inflection in half into a fossettid and a shorter inflection as in Sewelleladon. Anterolabial inflection deep, extending more than halfway across the width of the tooth anterior to the mesoconid.
Tallest cusps on M/1 are the triangular metaconid, protoconid, and hypoconid, with the last largest and tallest. Mesostylid and entoconid subequal in size, with mesostylid nearly triangular and entoconid narrower but still a strong cusp with a thick crest connected to the mesoconid. Anterior fossettid is triangular, enclosing a basin between the metaconid and protoconid. Anterolingual fossettid also rather triangular, remaining open on lingual side through midwear. Central fossettid not entirely closed as in M/2; opening to the lingual side of the tooth into the posterolingual inflection posterior to the mesostylid, although it is pinched by a pair of incipient crests from the entoconid crest and the metastylid crest. Anterolingual inflection closing much earlier in wear than posterolingual inflection. Posterior fossettid irregular in shape, with anterior wall shaped by the slight kink in the entoconid crest. Hypoconulid small, with only a very subtle anterior crest extending into posterior fossettid. Posterolabial fossettid oval and closing from early wear by anterior crest of hypoconid, which is well joined to mesoconid.
M/2 similar in morphology and size to M/1 except that metaconid and protoconid are more widely separated, with metaconid forming a clear 90° corner on the anterolingual end of the tooth. Central fossettid triangular to trapezoidal, with metastylid crest bending around it to the anterior side of the tooth and connected to lingual crest of protoconid rather than mesoconid as in M/1. Posterolingual inflection closed by crests connecting interior crests of entoconid and mesostylid. Hypoconulid crest less distinct than in M/1.
P4/ with prominent mesostyle; labial faces of paracone and metacone concave on either side. Tooth wider relative to anteroposterior length than in other meniscomyines. Labial half of central transverse valley retained with wear to similar depth as other fossettes. Two lingual and posterolabial fossettes semicircular, with concave or flat labial face. Posterolabial crest from anterostyle extending labially into anterolabial fossette, persistent with wear, yielding a wrench-shaped fossette.
M1/ with taller mesostyle than in Meniscomys or Niglarodon, losing anterolabial fossette first with wear as in Meniscomys but not Niglarodon, which loses the anterolingual fossette first. Fossettes oriented at a 45° angle to mesiodistal axis of the toothrow, with a fairly open oval shape, pinched somewhat in the middle unlike the oval fossettes of Meniscomys or the narrow crescent-shaped fossettes of Niglarodon molars. Enamel retained on anterior and posterior faces of tooth with wear; M1/ does not become shorter anteroposteriorly in mid- and late-wear specimens as enamel walls are lost and tooth is crowded by P4/ and M2/.
Remarks
These additional specimens are all from the Sheep Rock area; all but one from the very productive microvertebrate site Hayes Haven (JDNM 8B). Although this is not at all geographically close to the location where the type specimen was found ~ 35 km away, the lower teeth all have morphology consistent with the type specimen. The one specimen of upper dentition is associated based on the morphology of Rudiomys upper molars and premolars as seen in the specimens from the Cabbage Patch beds; it is also consistent in hypsodonty, size, and cusp morphology with the unusual dentition of the Rudiomys type specimen described by Rensberger (Reference Rensberger1983) and quite distinct from Meniscomys and Niglarodon. As explained by Rensberger (Reference Rensberger1983), this relatively late-occurring meniscomyine is nonetheless fairly low-crowned compared with other members of the group.
Rudiomys drummondensis new species
Rudiomys drummondensis n. sp., Cabbage Patch Beds, late Oligocene, Sharp Claw Butte locality: (1) right deciduous P4/ (KUVP 18417 from KU-MT-25), occlusal view; (2) LdP/4 (KUVP 18430 from KU-MT-25), occlusal view; (3, 4) holotype, right dentary with P/4–M/3 (KUVP 18152 from KU-MT-25): (3) occlusal view; (4) lingual view; (5) left dentary with partial P/4, M/1–M/3 (KUVP 18153 from KU-MT-25), occlusal view; (6) left dentary with M/1–M/3 (UWBM 98767 from C1708), occlusal view; (7) left P4/ (UWBM 97362 from C1708), occlusal view; (8) left P4/ (KUVP 18412 from KU-MT-25), occlusal view; (9) left M1/ (KUVP 18896 from KU-MT-25), occlusal view; (10) left M3/ (KUVP 18898 from KU-MT-25), occlusal view. Anterior is to the left for all images. Scale bar = 2 mm.
Figure 2. Long description
A multi-panel photograph of fossilized rodent dentition. A black scale bar at the center-left represents 2 millimeters. All specimens are oriented with the anterior to the left.
* Panel 1: Top-left. Occlusal view of a right deciduous P 4/ (K U V P 18417).
* Panel 2: Below panel 1. Occlusal view of a left deciduous P/4 (K U V P 18430).
* Panel 3: Top-center. Occlusal view of the holotype right dentary showing P/4 through M/3 (K U V P 18152).
* Panel 4: Large central image. Lingual view of the holotype right dentary (K U V P 18152) showing the side profile of the jaw bone and teeth.
* Panel 5: Middle-left. Occlusal view of a left dentary with a partial P/4 and M/1 through M/3 (K U V P 18153).
* Panel 6: Middle-right. Occlusal view of a left dentary with M/1 through M/3 (U W B M 98767).
* Panel 7: Bottom-left. Occlusal view of a left P 4/ (U W B M 97362).
* Panel 8: Bottom-center-left. Occlusal view of a left P 4/ (K U V P 18412).
* Panel 9: Bottom-center-right. Occlusal view of a left M 1/ (K U V P 18896).
* Panel 10: Bottom-right. Occlusal view of a left M 3/ (K U V P 18898).
Reference Rasmussen1977 Fossodontia drummondensis Rasmussen, p. 219, pl. 21 (unpublished).
Reference Calede and Rasmussen2015 Rudiomys? n. sp. (in part); Calede and Rasmussen, fig. 9B.
Reference Calede2020 Rudiomys ‘drummondensis’ (in part); Calede, fig. 8, suppl. info.
Holotype
KUVP 18152, partial right mandible with incisor, P/4-M/3.
Referred material
R dentary w/ I/1, M/1-3: UWBM 98767; L dentary w/ partial P/4, M/1-3: KUVP 18153; L dentary frag w/ M/2: KUVP 18154; RdP4/: KUVP 18417; RP4/: UWBM 98849, KUVP 18407, 18890; LP4/: UWBM 97362, KUVP 18411, 18412, 18891; RM1 or 2/: UWBM 108040, 101387, 101388, KUVP 18892, 18893, 18894, 18895; LM1 or 2/: KUVP 18896; RM3/: KUVP 18185, 18897; LM3/: KUVP 18898; LdP/4: KUVP 18430, 18433, 18587; RP/4: KUVP 18421, 18889; RM/1 or 2: KUVP 18397, 18398, 18399; LM/1 or 2: KUVP 18502, 18520, 18888; RM/3: KUVP 18408.
Diagnosis
Small (M/1 2.1–2.2 mm in anteroposterior length; Table 1). Cheek tooth cusps rounded, somewhat swollen both in upper and lower teeth. Metaconid shifted strongly labially, nearly twinned with protoconid on M/1. Posterolingual inflection open on all lower cheek teeth, with connection between entoconid crest and mesostylid only a low ridge in the floor of the reentrant, not a fully developed crest as in Rudiomys mcgrewi. Entoconid interior crest oriented nearly straight anteroposteriorly, bending distinctly labially toward the anterior crest of the hypoconid as it intersects the central fossettid. Posterior fossettid essentially a narrow crescent. Anterostyle on P4/ lacks posterolabial crest. Central transverse valley of P4/ highly reduced, with lingual end merging with posterolingual fossette and labial end disappearing rapidly with wear because of convergence between paracone and metacone. Metaloph absent; metacone unconnected to metaconule on upper cheek teeth, although there is a low ridge in the basin showing a degree of separation between posterolabial fossette and central transverse valley. Metaconule connected by strong crest to the protoloph near the posterolingual corner of the paracone on upper cheek teeth. P4/ metacone short posterior to mesostyle, notably shorter than anteroposterior length of paracone. Upper molars transversely narrow, nearly equal in dimension to anteroposterior length in early wear, although transverse width increasing with wear.
Occurrence
Cabbage Patch locality 10 (KU-Mt-25) corresponding to UMPC MV6558-2 (Bert Creek 12-2) and UWBM C1708 (Sharp Claw Butte). Tuffaceous deposit of the lower unit of the Cabbage Patch beds within the Flint Creek basin dated to 29.92 ±0.07 Ma (Calede, Reference Calede2020). A complete description of the lithology and taphonomy of the locality was provided by Calede (Reference Calede2016a). Stratigraphic occurrence in the upper Oligocene (early early Arikareean NALMA; Ar1), Lower Cabbage Patch beds, Powell County, Montana, USA.
Description
Similar in size to Rudiomys mcgrewi, but lower crowned and with more inflated cusps on the cheek teeth. Cusps tall in unworn teeth, but wearing to a relatively flat or low-sloping surface. Wear proceeding from labial side of lower teeth to lingual side such that labial cusps are worn down first.
P/4 often with an interior crest of metaconid running parallel to metastylid crest and posteriorly joining lingual to the mesoconid to make a small but persistent oval fossettid directly posterior to the metaconid. Mesostylid absent or highly reduced. Mesoconid located in the center of the tooth, linguad of its position in the molar teeth, with a labial crest that does not close the posterolingual fossette with wear. Posterolingual inflection ending at the mesostylid crest and is fairly broad as a result of the posterolabial position and triangular shape of the entoconid. Posterolingual fossettid enclosed by entoconid, mesoconid, and hypoconulid to form an oval to triangular shape that is persistent with wear.
M/1 metaconid shifted labially relative to mesostylid, yielding curved lingual margin to tooth. Metastylid crest absent, leaving anterolingual inflection open all the way around metaconid to anterior cingulum, occasionally partially closed (e.g., KUVP 18153) by a low interior crest from the metaconid. This fossettid not very persistent with wear. Mesostylid distinct, inflated, with anterolabial crest joining posterolingual side of protoconid. Entoconid distinctly more labial in position than mesostylid or metaconid. Posterolingual inflection open, with only a low, barely visible ridge in the base of the inflection to distinguish central fossettid from the lingual side of the reentrant. Protoconid small, yielding a relatively labiolingually narrow anterior portion of the tooth. Mesoconid strongly joined to anterior crest of hypoconid to close a posterolabial fossettid early in wear, although this fossettid is not persistent.
M/2 morphology similar to M/1, but distinctly larger (M/2 length 2.3–2.4 mm; Table 1) with more lingually positioned metaconid, so that anterolingual corner of the tooth is roughly a 90° angle. Protoconid is also larger than in M/1 so that anterior width is more similar to posterior width in M/2. Posterolingual reentrant deeper than in M/1 as mesoconid is more labially positioned, and the reentrant narrowing slightly to suggest a central fossettid with a labial bend in the interior crest of the entoconid, although this reentrant is open to its base and does not close to form a fossettid at any wear stage.
M/3 anterolingual inflection closed with wear by a low metastylid crest. Metaconid also has a posterolabial crest that divides the anterolingual fossettid from an anterolabial fossettid that sits just lingual to the protoconid. Protoconid more labially positioned than in M/1 and M/2 to open this fossettid further and give the anterior half of the M/3 a similar width to the posterior half. Mesoconid crest and interior crest of entoconid both kinked labially on their way toward the protoconid and mesoconid, respectively, giving both anterolingual and posterolingual reentrants a labial bend. Low anterolingual crest from the kink in the entoconid crest almost enclosing the central fossettid on this molar. Inflection between entoconid and hypoconulid open much deeper than in more anterior cheek teeth, not closing until late in wear. Hypoconid shifting posterolingually relative to M/1 and M/2, expanding anteroposterior length of the tooth and reducing posterior labiolingual width.
P4/ parastyle with low labial prominence and mesostyle with moderate labial prominence, both less distinct than in Rudiomys mcgrewi. Cusps and conules more inflated than in R. mcgrewi, relatively rounded and distinct. Mesostyle with a groove down its posterolabial surface. Labial side of paracone and metacone slightly convex. Metacone short anteroposteriorly and reduced in size relative to paracone. As in all upper cheek teeth, there is no metaloph connection between metacone and metaconule, so labial half of central transverse valley is united with posterolabial fossette. Anterocone large and similar in size to other major cusps of the tooth. Anterostyle distinct, large, broadening slightly anterolabially, joined to anterocone with a high anterior cingulum that closes the anterolabial fossette anteriorly. Anterostyle not connected directly to protoconule, with anterolingual and anterolabial fossettes joined by a narrow valley anterior to the protoconule. Metaconule and protoconule both connect to paracone, with anterolingual crest of metaconule connecting to protoconule and enclosing lingual half of central transverse valley into a distinct although shallow fossette. Posterolingual fossette crescent-shaped, wrapping around inflated lingual face of metaconule and ending where it intersects the protoconule and the crest connecting the protocone to the protoconule. Posterior cingulum from protocone to posterior side of metaconule complete, but closing later in wear than between metacone and metaconule. Protocone with relatively rounded, narrow lingual surface in comparison with Meniscomys and Niglarodon.
In the absence of any associated upper dentitions, it is not possible to describe differences between M1/ and M2/; hence, this description focuses on the features present in isolated upper molars. Anterior and posterior cingula relatively low, closing fossettes with wear. Cusps high, thin, and sharp in unworn specimens, but inflated at base, becoming more rounded in cross section with wear. Mesostyle extending only approximately half the height of the crown between the metacone and paracone, with low to moderate labial prominence on the tooth, less than in P4/. Protocone narrow, with anterior cingulum extending anterolabiad and connecting to paracone. Protoconule not joining anterior cingulum; anterolingual and anterolabial fossettes connected anteriorly around protoconule. Posterolingual fossette crescent-shaped, joining or not joining with lingual end of central transverse valley depending on variable presence of lingual crest from metaconule connecting to center of protoconule. Metaconule connecting labially either with posterior side of paracone or directly with mesostyle, with posterolabial fossette wrapping around lingual and anterior sides of metacone.
M3/ protocone a bit wider than in M1/ and M2/, with anterior and posterior crests trending less labiad. Labiolingual width slightly less than in other molars. Paracone a distinct cusp, round, and with convex labial face. Metacone reduced to a crest at the posterolabial corner of the tooth but connected by a distinct metaloph to the metaconule, isolating a central transverse valley between metaloph and protoloph. Posterolabial fossette small, round. Mesostyle reduced or absent. Protoconule not connected to anterior cingulum but strongly connected by a labiolingually oriented anterior crest to the protocone, isolating a labiolingually oriented anterolingual fossette. Anterolabial fossette bending around protoconule to form right-angled shape. Posterolingual fossette deeper than other fossettes, wide, and crescent-shaped. Posterior cingulum high, but slightly lower than in M1/ and M2/.
Etymology
Named for the town of Drummond, Montana, near which the species was found.
Remarks
This species was first described in Rasmussen’s (Reference Rasmussen1977) dissertation, but never formally published. At the time, he recognized it as a genus distinct from anything else in the literature at the time, but Rensberger’s (Reference Rensberger1983) description of a lower jaw of Rudiomys mcgrewi pre-empted Rasmussen’s suggested genus. The two are quite clearly representatives of the same genus, sharing the same low-crowned, bulbous-cusped form at a time when other meniscomyines are significantly more hypsodont. The timing of publication was a bit unfortunate, because the sample from the Cabbage Patch beds is more extensive, with several complete jaws and representative specimens of the entire upper cheek tooth dentition, although Rensberger’s (Reference Rensberger1983) type also includes incisors and an otic capsule in association with the lower jaw. However, this well-known species makes it possible to confidently recognize the upper dentition from R. mcgrewi found in the John Day region, because the similarity in occurrence patterns, morphology, and abundance suggest the same sort of association of upper and lower dentition in both regions.
This species was identified as Rudiomys ‘drummondensis’ in Calede’s work on the biostratigraphy of the Cabbage Patch beds (Calede and Rasmussen, Reference Calede and Rasmussen2015; Calede, Reference Calede2016b, Reference Calede2020), but it was not actually described in those publications.
Rudiomys intsook new species
Rudiomys intsook n. sp., Cabbage Patch beds, late Oligocene, Hops Garden locality: (1, 2) holotype, right P4/ (UWBM 98842 from C1721): (1) occlusal view; (2) labial view; (3, 4) UWBM 108088 from C1721: (3) occlusal view, left M1/; (4) labial view, left M1/ or M2/; (5) right M1/ or M2/ (UWBM 98846 from C1721), occlusal view; (6) left M3/ (UWBM 108193 from C1721), occlusal view. Anterior is to the right for all images. Scale bar = 2 mm.
Figure 3. Long description
A multi-panel photo containing six numbered views of fossilized upper teeth.
* Panel 1: Occlusal view of a right P 4/ holotype, showing a dark, complex grinding surface with multiple ridges and basins.
* Panel 2: Labial view of the same right P 4/ holotype, showing two distinct roots extending upward from the dark crown.
* Panel 3: Occlusal view of a left M1/, showing a triangular shape with deep enamel folds.
* Panel 4: Labial view of a left M 1/ or M2/, showing a vertical cross-section of the crown and root structure.
* Panel 5: Occlusal view of a right M 1/ or M2/, displaying a worn surface with lighter sediment trapped in the crevices.
* Panel 6: Occlusal view of a left M3/, showing a more rounded and compact molar structure.
In all views, the anterior side is oriented to the right. A black scale bar at the bottom right represents 2 mm.
Reference Calede and Rasmussen2015 Rudiomys? n. sp. (in part); Calede and Rasmussen, p. 168, fig. 9B.
Reference Calede2020 Rudiomys ‘drummondensis’ (in part); Calede, fig. 8, suppl. info.
Holotype
UWBM 98842, a right P4/.
Referred specimens
UWBM 108088, LM1/ or LM2/; UWBM 98846, RM3/; UWBM 108093, LM3/.
Diagnosis
Teeth relatively high-crowned for Rudiomys, near the hypsodonty of a low-crowned Meniscomys or Niglarodon. Mesostyle and parastyle reduced on the labial side of P4/. Conules of upper cheek teeth crescentic with wear as a result of distinct anterior and posterior crests. Fossettes of upper teeth roughly diagonally oriented across tooth, not orthogonal to long axis of the toothrow. Protoconule of P4/ not connecting to paracone via a protoloph; protoloph ending at protoconule. Anterior crest of protoconule on P4/ connecting to anterostyle relatively early in wear, not to lingual side of anterior cingulum as in R. mcgrewi. Metaconule of P4/ shifted slightly labially relative to other Rudiomys, connecting to posterolabial corner of protoconule in center of tooth. Labial crest of metaconule on P4/ connected to paracone, obliterating central transverse valley. Mesostyle on molars less distinct than in R. mcgrewi. Metaconule on M1 and M2/ lacks an anterior crest connecting to center of protoconule; posterolingual fossette extending between conules to end at the ectoloph between the paracone and the anterolabial crest of the metacone. Anterolabial crest of metaconule not forming a true metaloph by joining the metacone, but extending to the ectoloph just posterior to the mesostyle. Posterior cingulum of molars high, closing the posterior fossettes relatively early in wear. Molars smaller relative to P4/ than in R. mcgrewi. M3/ more reduced in size relative to molars (Table 1) than in other Rudiomys. Protoconule with anterior crest connected to anterior cingulum, rather than to anterior crest of protocone as in R. drummondensis. Metaconule of M3/ short anteroposteriorly, somewhat round.
Occurrence
UWBM locality C1721, Hops Garden. Deposit of moderately sorted mudstone from the lower Cabbage Patch beds in the Flint Creek basin with multiple lenses of reworked ashes. A complete description of the lithology and taphonomy of the locality was provided by Calede (Reference Calede2016a). Stratigraphic occurrence in the upper Oligocene (early early Arikareean NALMA; Ar1); Lower Cabbage Patch beds, Granite County, Montana, USA.
Etymology
Based on the Kootenai word for ‘mouse’ spelled ʾinȼuk̓ and pronounced ‘in-ts-ook.’
Remarks
Although these specimens still retain the cusp morphology distinctive of Rudiomys, they are higher crowned than other Rudiomys spp., including the later-occurring R. mcgrewi in the John Day Formation. They are morphologically distinct from R. drummondensis, which lies only slightly lower in the stratigraphic section in the Flint Creek Basin of western Montana. This clade is unfortunately represented only by isolated upper cheek teeth; however, the clear similarities to and differences from the upper cheek teeth of R. drummondensis allow placement within the genus Rudiomys, but as a clearly distinct species.
This species, like Rudiomys drummondensis, occurs in a single locality and is morphologically coherent within that locality, further supporting assignment to a distinct species. Rasmussen’s (Reference Rasmussen1977) dissertation did not include specimens from this locality, but subsequent work by Calede (Reference Calede2016b, Reference Calede2022) collected a number of specimens from this site. A preliminary paper on Cabbage Patch stratigraphy (Calede, Reference Calede2020) did not recognize the existence of this species, although it did offer preliminary identifications of some aplodontiids from the Cabbage Patch, including these specimens within R. ‘drummondensis.’
?Rudiomys sp. indet.
Referred specimens
KUVP 18855, RM/1 or RM/2 fragment.
Occurrence
KU-Mt-7, Helmville loc. 2. Lone mammal fossil bearing mudstone outcrop of the Lower Cabbage Patch beds from the Blackfoot Basin, described by Rasmussen (Reference Rasmussen1977).
Description
KUVP 18855 is a fragmentary lower molar specimen. It is reduced to little more than the tooth roots, which are consistent in size and morphology with Rudiomys, but not diagnostic to any deeper taxonomic level.
Remarks
This specimen was listed among the specimens referred to ‘Fossodontia drummondensis’ by Rasmussen (Reference Rasmussen1977). As a broken specimen, it is too damaged to be definitively referred at the genus level in spite of the morphological similarity to Rudiomys. It is not clear from the description of R. mcgrewi in Rasmussen’s (Reference Rasmussen1977) thesis whether there was any more to this specimen that has been lost in the intervening decades or if it was already broken to the degree visible today.
Discussion
The additional material of Rudiomys from both the John Day Formation and from the Cabbage Patch beds offers much greater insight into the distinctive identity of this genus relative to other meniscomyines, as well as their potential ecological differences. The additional material from John Day reinforces speculation about its ecological distinctiveness, and the occurrences from the Cabbage Patch not only show similar evidence of ecological selectivity, but also help demonstrate biogeographic and environmental connections between the John Day region and western Montana during the Arikareean.
Rensberger (Reference Rensberger1983) recognized in his original description that Rudiomys mcgrewi was ecologically distinct from the other meniscomyines of similar age given its combination of large body size, less derived periotic morphology, gracile incisors, and low degree of hypsodonty. As a low-crowned meniscomyine, it occurs high in the John Day Formation, co-occurring with Meniscomys specimens that are significantly higher crowned. The gross cusp morphology is also quite distinct from Meniscomys; tooth cusps are much more gracile and generally the enamel morphology is more complex on the occlusal surfaces, with deeper fossettes and fossettids on molar teeth retained much later in wear. Rensberger (Reference Rensberger1983) also highlighted the sedimentological distinctiveness (with thinner and more oxidized sediments) of the Rudio Creek 3 locality where the type specimen was found, which he asserted represented a very different environment from many of the other sites in which aplodontiids occur in the John Day Formation.
All of the new material of Rudiomys mcgrewi described here comes not from the type locality, but from Sheep Rock to the south; all of the specimens are from a narrow stratigraphic range in units K1 and K2 of the Turtle Cove Member of the John Day (Albright et al., Reference Albright, Woodburne, Fremd, Swisher, MacFadden and and Scott2008). This corresponds well with Rensberger’s (Reference Rensberger1983) assignment of Rudio Creek 3 to his unit E1, which is roughly equivalent to unit K2 of Albright et al. (Reference Albright, Woodburne, Fremd, Swisher, MacFadden and and Scott2008). All but one of these specimens comes from JODA locality 8B, known as Hayes Haven. Given the hundreds of specimens of aplodontiids known from dozens of sites in the John Day Formation (including arguably more fossiliferous sites preserved in unit K2 than almost any other level in the John Day stratigraphy), the fact that all five specimens definitively referable to R. mcgrewi come from two localities (and four of the five from a single site) suggests some level of ecological specialization. At a time when many rodent species showed increasing hypsodonty, finding a form with lower-crowned teeth that is limited to a small number of sites suggests that it might have occurred in a pocket of more closed habitat in the late Oligocene landscape in the Pacific Northwest, a scenario consistent with the greater oxidation of the sediments in the type locality, which has the most complete known specimen of the species.
The two Rudiomys species from the late Oligocene of Montana also show evidence of this ecological distinctiveness. Rudiomys are locally abundant in the two sites in which they occur, but there are roughly 13 other rodent-bearing sites from the Lower Cabbage Patch beds without Rudiomys. Furthermore, each of the two sites containing Rudiomys contains the only occurrence of a morphologically distinct species, suggesting further ecological differences between the two. Based on lithology, taphonomy, and paleontology, both sites are interpreted as low-energy fluvial environments that were likely periodically ponded in a forested environment based on lithology, taphonomy, and paleontology (Calede, Reference Calede2016a, Reference Calede2022), a paleoenvironment consistent with ecological interpretations for the John Day Formation.
The stratigraphic relationship between the two Cabbage Patch species with Rudiomys drummondensis occurring lower in the Cabbage Patch section than R. intsook suggests increasing crown height through time in the Montana lineage. This finding is consistent with Rensberger’s (Reference Rensberger1981, Reference Rensberger1983) description of within-lineage trends toward increasing hypsodonty in meniscomyines over time, despite the lower-crowned starting point of this genus relative to Niglarodon and Meniscomys.
The fact that Rudiomys, as an ecological specialist, occurs only in the Pacific Northwest and the Northern Rocky Mountains at this time might partially reflect the limited distribution of Arikareean localities across North America. The units containing R. mcgrewi are dated between 28.35 and 26.60 Ma (Mohr et al., Reference Mohr, Famoso, Samuels, Laib and Schmitz2025) and were placed by Albright et al. (Reference Albright, Woodburne, Fremd, Swisher, MacFadden and and Scott2008) in the late early Arikareean (Ar2). Rudiomys drummondensis occurs at a single site directly dated to 29.92 Ma. Rudiomys intsook occurs below a tuff dated to 27.78 Ma, which is located within the middle unit of the Cabbage Patch beds. The lower unit of the Cabbage Patch beds is interpreted to be no younger than 28.13 Ma (Calede, Reference Calede2020). Both species of Rudiomys from the Renova Formation are therefore limited to localities placed in the Lower Cabbage Patch, which are interpreted to be early early Arikareean (Ar1) in age (Calede, Reference Calede2020). Screenwashed and well-studied early Arikareean sites are known from a handful of locations elsewhere in the USA including southern California, South Dakota, Mississippi, Florida, Montana east of the continental divide, and even Idaho. Although many of these sites contain aplodontiids, including Allomys Marsh, Reference Marsh1877, Meniscomys, and Niglarodon, all of which co-occur with Rudiomys in the John Day Formation (Rensberger, Reference Rensberger1983) and/or the Cabbage Patch beds (Calede, Reference Calede2020), none have any published specimens attributable to Rudiomys. Given that these sites include the Peterson Creek Beds of Idaho (Nichols, Reference Nichols1976, Reference Nichols1979), which lies geographically between the John Day and Cabbage Patch assemblages and is similar in age, it seems that Rudiomys was a habitat specialist even within the limited time span between 30 and 26.5 Ma when it was in existence. The earlier record of Rudiomys in the Cabbage Patch beds suggests that the origins of the genus might have been in the Rocky Mountain region with R. mcgrewi (which bears more resemblance to R. intsook in size and morphology than it does to the earlier R. drummondensis) representing an immigrant into the John Day region during late early Arikareean time. It has been suggested previously that habitat similarities across the northern Rockies during the Arikareean were critical to the biogeography of small ruminants (Calede et al., Reference Calede, Constenius, Famoso and Kehl2024); the same could be true across the northwestern USA more broadly, as Rudiomys suggests.
Finally, it is worth considering the phylogenetic implications of the more extensive sample of Rudiomys. In the phylogeny of aplodontiids published by Hopkins (Reference Hopkins2008, p. 772), R. mcgrewi and R. drummondensis (referred to in that analysis as “Drummond, MT meniscomyine”) are placed in the tree bracketing Meniscomys and Niglarodon with R. drummondensis as the basalmost member of the meniscomyines and R. mcgrewi as one of the most derived members of this grade, just outside the aplodontiine subfamily. This is likely a result of the limited set of characters that could be coded from the two somewhat fractured molars of the type of R. mcgrewi. Only approximately half of meniscomyines are known from upper dentitions, even though the upper teeth are often more morphologically distinct between taxa. The new information that can be coded from these additional specimens will be informative for a revised phylogeny of the paraphyletic Meniscomyinae, which are ancestral to both the aplodontiine and mylagauline radiations that occur near the Oligo-Miocene boundary and could explain the large diversity of meniscomyine taxa. Although an updated phylogenetic analysis is beyond the scope of this paper, the additional material published here will be key to disentangling the origins of these unusual rodents.
Acknowledgments
The authors would like to thank A. Barnosky and G.P. Wilson Mantilla (respectively) for mentoring during the parts of this work that took place during our graduate studies. A. Barnosky, J. Rensberger, D. Rasmussen, T. Fremd, and J. Samuels advanced the work by encouraging study of these specimens and helping our understanding of the geologic context of their occurrence. S. Foss, P. Holroyd, K. Moore, G. Stanley, K. Anderson, R. Eng, M. Rivin, C. Sidor, R. Dunn, C. Schierup, M. Sims, D. Burnham, D. Rasmussen, and D. Miao provided specimen access at various collections studied for this work. R. Hunt, D. Whistler, E. Davis, P. Holroyd, and N. Famoso engaged in research discussions that advanced our understanding of the problems presented by Rudiomys. V. Finley, Kootenai Culture Committee, provided the name for one of the new species described; we are grateful to him for sharing the culture of the tribes of the Flathead Reservation from whose ancestral land this animal was recovered. Many landowners, in particular the Radtke, Dingwall, Dutton, and Morse ranches, allowed access to their land. Many students helped recover, identify, and curate specimens. J. Storer, L. Flynn, L.B. Albright, and an anonymous reviewer provided helpful reviews of the manuscript. This work was funded in part by the National Science Foundation (award EAR-2322803 to Hopkins) and received extensive logistical support from the John Day Fossil Beds National Monument.
Competing interests
Jonathan Calede is Editor-in-chief of the Journal of Paleontology; he had no role in the editing and publication of this manuscript. The senior author has no competing interests to declare.
