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Arvicoline rodents from Kokoweef Cave, Ivanpah Mountains, San Bernardino County, California.

Abstract.--A late Pleistocene/early Holocene faunal assemblage from Kokoweef Cave (San Bernardino County, California) includes a diverse assemblage of gastropods, reptiles, birds, and mammals. Chronological control for the site consists of a single radiocarbon date of 9830 [+ or -] 150 yr. BP taken from charcoal at approximately the vertical mid-point of the stratigraphic excavation. Microtus and Lemmiscus are extralimital and represent the only arvicoline rodents from the locality. We discovered several distinct morphotypes of the m1 of Lemmiscus from Kokoweef Cave, one of which is not known from any other populations, living or extinct. The recovery of an m1 morphotype with only four closed triangles is notable. This morphology is usually found in middle Pleistocene faunas, but is rare in younger localities. The loss of the 4-triangle morphotype of Lemmiscus represents one of the few documented small mammal morphotype 'extinctions" near the end of the Pleistocene in western North America.

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Kokoweef Cave is situated at an elevation of approximately 1770 m on Kokoweef Peak, along an extension of the Ivanpah Mountains in San Bernardino County, California (Fig. 1; Goodwin and Reynolds 1989; Reynolds et al. 1991a). The site was the center of various mining activities for over 30 years until 1972 when paleontological crews from the San Bernardino County Museum began excavation of an extensive sequence of fossiliferous sediments. Discussion of the history of the cave excavations (mining and paleontological) was provided by Goodwin and Reynolds (1989) and Reynolds et al. (1991a). Published reports of the fauna include preliminary faunal lists (Harris 1985; Reynolds et al. 1991a), preliminary discussion of sigmodontine rodents (Austin 1992), and detailed analyses of the freshwater and terrestrial mollusks (Roth and Reynolds 1990), lizards (Norell 1986), sciurid rodents (Goodwin and Reynolds 1989), and woodrats (Force 1991). In this paper, we discuss the arvicoline rodents recovered from the cave.

[FIGURE 1 OMITTED]

Fossils were collected by previous researchers from a lower chamber of the cave; natural stratigraphy in the fossiliferous sands was not readily discernable and stratigraphic control was maintained by arbitrary 1-foot vertical levels, which may or may not have cross-cut dipping sediments (Goodwin and Reynolds 1989). The possibility that the sediments are temporally mixed by natural means (e.g., small slides from the upper portion of the cave, woodrat midden debris collapsing into the lower chambers) was discussed by Goodwin and Reynolds (1989), but they accepted that the sedimentary sequence preserved a "relatively accurate faunal history" (Goodwin and Reynolds 1989:23). Chronological control for the site is poorly established and there is no consistency in published reports of the radiocarbon date(s) or the level(s) from which they were derived. Two radiocarbon dates were reported by Goodwin and Reynolds (1989). A date of 590 [+ or -] 120 yr. BP from the 31.5-foot level was considered to be derived from reworked charcoal, and was rejected. A more reliable date of 9830 [+ or -] 150 yr. BP was reported by Goodwin and Reynolds (1989) who indicated that it was taken on charcoal recovered from the 20.5 foot level (approximately 6 m above the base of the sequence) and that the date was provided by Beta Analytic (Beta-2155). The same information was provided by Roth and Reynolds (1990), but they attributed the date to a 1985 'in litt.' communication from Andrei Sarna-Wojcicki to R. E. Reynolds. The 9830 [+ or -] 150 yr. BP age also was reported by Reynolds et al. (1991a) but was said to be derived from the 21.5 foot level; in a separate paper in the same volume, Force (1991) gave the age and depth as 8950 [+ or -] 160 yr. BP from a depth of 7 m. An age of 9850 [+ or -] 160 yr. BP was given by Jefferson (1991). Thus, all authors report only one accepted radiocarbon date, and that accepted date is from sediments between 6 and 7 m above the base of the fossiliferous excavation (between 20.5 and 21.5 foot levels; see below).

At our request, Eric Scott of the San Bernardino County Museum recently located the original documentation for the radiocarbon dates. Beta Analytic sample 2155 yielded a corrected radiocarbon age of 9830 [+ or -] 150 yr. BP. A handwritten note in R. E. Reynolds' handwriting on a photocopy of the Beta report reads "at a depth of 21.5 ft. in sediments within the cave." The cover letter accompanying the report indicates that the sample required "extensive chemistry" for the pretreatment and "multiple burnings" in order to extract sufficient carbon for the analysis. No further information is available pertaining to the radiocarbon date or the charcoal sample from which it was made.

Thus, the total time represented in the sediments is unknown, but the faunal components appear to be late Pleistocene and early Holocene in age. A Pleistocene component is represented by extinct llama, camel, and horse remains recovered from the cave (Reynolds et al. 1991a). Sediments stratigraphically above the radiocarbon date fall within the temporal span of the Holocene as it is generally recognized (Bell et al. in press[b]). We feel there is inadequate documentation to place any confidence in the presumed superpositional relationships of the sediments and their fossils; however, the low taxonomic diversity of arvicolines and their limited stratigraphic range renders stratigraphic analysis of change uninformative in any event. We thus treat the Kokoweef Cave arvicoline assemblage as a composite, or 'bulk' sample, of late Pleistocene to Holocene age.

Methods

All specimens are curated in the SBCM, under collection number L1808. Specimens were identified based on comparisons with dentitions of extant species of arvicolines, and descriptions from the literature. Dental terminology is shown on Figure 2. The number and configuration of triangles on the m1 and M3 of arvicolines are important for species determination and designation of intraspecific morphotype variation. Triangles (and reentrant angles) on lower molars are counted from the posterior end of the tooth, with the first triangle being the first one anterior to the posterior loop (Fig. 2A). Triangles (and reentrant angles) on upper molars are counted from the anterior end of the tooth, with the first triangle being the first one posterior to the anterior loop (Fig. 2B). Relative closure or confluence of triangles was determined in occlusal view and was verified by examination of the base of the molar. We considered a triangle to be closed if there was less than one enamel band width of opening; specimens with greater than three enamel band widths of opening were considered open (confluent). Those with between two and three enamel band widths of opening were scored as "pinched,' and those with between one and two band widths were scored as having 'incipient' closure. Upper molars are designated by upper-case letters (e.g., M3), lower molars by lower-case letters (e.g., m1). On the illustrations, enamel bands are shown in white, dentine in black, and cementum is stippled.

[FIGURE 2 OMITTED]

Original stratigraphic levels were designated in English units (feet). Because all original specimen data labels utilize that system (as do most other published reports on the fauna), we retain its use here for the sake of clarity and consistency in reports on the fauna. Depth measurements were made from an established datum "at the top of the west end of the trestle that ran from the mine portal across the mine opening, approximately at the original level of the fill" (Reynolds et al. 1991a:98). Collection of fossils began at approximately 11 feet below the datum and continued to a depth of 45 feet (Reynolds et al. 1991a). A complete list of diagnostic specimens of Lemmiscus and Microtus is provided in Appendix 1.

Institutional abbreviations: SBCM=Division of Paleontology, San Bernardino County Museum, Redlands, California; MVZ = Museum of Vertebrate Zoology, University of California, Berkeley, California.

Results

Only two arvicoline taxa can be identified from the Kokoweef Cave deposit, Microtus and Lemmiscus. There are many extant North American species of Microtus (Hall 1981) and species identification of isolated teeth is not possible at this time (Bell and Mead 1998; Bell and Repenning 1999; Bell and Barnosky 2000). Only one species of Lemmiscus is currently recognized (L. curtatus), but several different dental morphologies are reported (see below). All arvicoline teeth from Kokoweef Cave are ever-growing, with cementum in the reentrant angles. Taxonomic identifications are based on phenetic similarity and not synapomorphy; the features we used to identify the specimens are discussed below.

Microtus.--Eight Microtus m1s were recovered from Kokoweef Cave (approximately 3% of the total arvicoline m1 sample of 228). All Microtus m1s include a posterior loop, five closed and alternating triangles, and a variable anterior cap (Fig. 2A). The second triangle is distinctly shorter than the first triangle, as is typical of Microtus (Barnosky and Rasmussen 1988); triangles 6 and 7 (the secondary wings of Repenning 1992) are well developed, but are not closed. No fewer than 12 extant North American Microtus species share this general m1 configuration (Bell and Barnosky 2000), and isolated m1s cannot be identified reliably to species.

The M3 of species of Microtus is highly variable, but those recovered from Kokoweef Cave (n = 9) are generally quite complex, with an anterior loop, two or three closed and alternating triangles, and a variably complex posterior portion. We did not quantify the morphological variation in the occlusal morphology of the M3s of Microtus, but an example is shown in Fig. 2B. No North American species of Microtus (extant or extinct) shows the elongate posterior loop seen in the M3 of Lemmiscus (see below).

Lemmiscus.--The m1 of Lemmiscus is superficially similar to that of Microtus, but differs in detail. It includes a posterior loop, and usually five closed and alternating triangles (sometimes as many as six, or as few as four are closed), with a simple and usually somewhat crescent-shaped anterior cap (Fig. 3). The second triangle is as long or longer (across the labial-lingual axis) than the first (Barnosky and Rasmussen 1988). Ninety-seven percent of the arvicoline m1s from the cave are of Lemmiscus (n = 220), and include several noteworthy morphotypes. The most common of these (n = 176) is the morphotype most prevalent in extant populations of Lemmiscus curtatus; in these specimens, there is a simple, crescent-shaped anterior cap, five closed and alternating triangles, and a posterior loop (Fig. 3A, B). A second morphotype in Kokoweef Cave (n = 10) is similar, but only four closed triangles are present (the fifth triangle is not closed; Fig. 3C). A third morphotype (n = 8) also has only four closed triangles, but is characterized by having a modified anterior morphology that previously was unknown for Lemmiscus (living or fossil). In these specimens, the fifth and sixth triangles are confluent, but they are closed or pinched off from a somewhat rounded anterior cap (Fig. 3D). The five specimens with six closed triangles represent a fourth m1 morphotype. The remaining specimens show intermediate morphologies, in which triangle closure is incomplete. In two specimens the fifth triangle is pinched, and in 11 specimens the sixth triangle is pinched. Six specimens show incipient closure of the fifth triangle, and two show incipient closure of the sixth triangle.

[FIGURE3 OMITTED]

A total of 126 Lemmiscus M3s were recovered from Kokoweef Cave. Most of these (n = 94) share a basic morphology (Fig. 4A, labeled as 'Type 1' in Appendix 1) consisting of an anterior loop, at least two triangles (usually closed), an elongate posterior loop, and anteroposteriorly expanded (somewhat 'squared') first lingual and second labial reentrants (Barnosky and Rasmussen 1988; Repenning 1992). Other morphological variants within the M3 sample of Lemmiscus include specimens (n = 18) in which a third triangle is moderately to well developed, but not closed (Fig. 4B). In three specimens, the anterior loop is followed by two triangles that are confluent with one another (Fig. 4C) and in one of these the first triangle is also confluent with the anterior loop. In ten specimens the second triangle is confluent with the posterior loop (and in two of those ten, the first triangle is also confluent with the anterior loop, but the two triangles are closed from one another). In one specimen two triangles are present and closed from one another, but the first is confluent with the anterior loop. The three specimens in which the first triangle is confluent with the anterior loop show the shallow first buccal reentrant angle discussed by Rensberger et al. (1984) and Barnosky and Rasmussen (1988).

Discussion and Conclusion

Microtus is widely distributed throughout California today and populations occur to the north, northwest, west, and southwest of Kokoweef Cave (Hall 1981), but neither Lemmiscus curtatus nor Microtus are found in the vicinity of Kokoweef Cave today (Ingles 1947; Johnson et al. 1948; Hall 1981). Marginal records reported by Hall (1981) indicate that the nearest extant populations of Lemmiscus are in the Inyo Mountains, approximately 265 km to the northwest. The only other fossil record of Lemmiscus in California was reported from Antelope Cave, approximately 4.8 km west-northwest of Kokoweef Cave (Jefferson 1991; Reynolds et al. 1991b). We examined this material and found 11 m1s. Seven show a typical morphology with five closed triangles, two have six closed triangles, one has a pinched sixth triangle, and one specimen has only four closed triangles. The Antelope Cave fossils were recovered from unstratified back-dirt derived from within the cave (Reynolds et al. 1991b). A single radiocarbon date of 11,080 [+ or -] 160 yr. BP (taken from an ulnar shaft of a condor) was reported by Emslie (1990).

There are five other fossil localities that produced 4-triangle morphotypes of Lemmiscus. Three of these are cave localities of middle Pleistocene (Irvingtonian) age and include the earliest known records of Lemmiscus. The oldest known record is from the lower levels of the Pit excavation in Porcupine Cave, dating to between 0.85 and 1 Ma (Bell and Barnosky 2000; Barnosky and Bell in press). Approximately contemporaneous material was recovered from the oldest deposits in SAM Cave, New Mexico (Rogers et al. 2000). The SAM Cave deposits have both 4- and 5-triangle morphotypes present, but there are inconsistencies between the stratigraphic distributions plotted on table 2 of Rogers et al. (2000:96) and the faunal list and catalogue in their appendix (Rogers et al. 2000:115). Both morphotypes are also present in the Cathedral Cave fauna in Nevada (Bell 1995; Bell and Barnosky 2000) and at least four other excavations in Porcupine Cave in Colorado (the Gypsum Room, Kate's Cupola/Mark's Sink locality, Carnegie Museum Velvet Room excavation, and Denver Museum of Natural History Velvet Room excavation; Bell et al. in press[a]). The Ferret Room fauna in Porcupine Cave contains a single specimen of a 4-triangle morphotype of Lemmiscus, but it is one of only three arvicoline specimens recovered (Bell et al. in press[a]).

The Kennewick Roadcut locality in Washington was the first locality from which 4-triangle m1 morphotypes of Lemmiscus were reported (Rensberger et al. 1984). 4- and 5-triangle m1 morphotypes were recovered throughout the section, but their relative abundance changes, with 5-triangle forms gaining in importance upsection (Rensberger et al. 1984; Rensberger and Barnosky 1993). In the upper units of the Kennewick Roadcut, 6-triangle morphotypes occur in low abundance (Rensberger and Barnosky 1993), and they are found in low abundance in several modern populations, and as a regular feature in at least some populations from northern Nevada (Barnosky and Bell, from unpublished data based on examination of the collections in the MVZ). The age of the Kennewick sequence is not well established, especially for the lower portions of the section, which may be entirely within the Rancholabrean mammal age (Rensberger and Barnosky 1993) or may include an Irvingtonian mammal age faunal component (Bell et al. in press[b]).

The youngest known occurrence of a 4-triangle morphotype of Lemmiscus is from Unit III in Snake Creek Burial Cave in Nevada (Bell and Mead 1998), with a radiocarbon date at the top of the unit of 9460 [+ or -] 160 yr. B.P. (Mead and Mead 1989). The numerous other Pleistocene and Holocene faunas that include Lemmiscus are not reported to contain any 4-triangle morphotypes (Kurten and Anderson 1980; Harris 1993; FAUNMAP Working Group 1994; Bell and Glennon in press; Mead et al. in press), and this morphology is unknown from extant populations of Lemmiscus (Bell and Barnosky 2000).

Available evidence thus suggests that throughout their evolutionary history, Lemmiscus populations were characterized by evolution of increasing morphological complexity of the m1, initially by closing off the fifth triangle, and eventually by development and closure of a sixth triangle. This general pattern was observed in stratified deposits in Washington (Kennewick) and Colorado (Porcupine Cave), and is supported by the morphotypes found in other Pleistocene localities. The Kokoweef Cave assemblage records one of the youngest occurrences of the 4-triangle morphotype, and is one of only three localities containing this morphotype that fall within the range of radiocarbon dating techniques (Antelope Cave and Snake Creek Burial Cave are the others; see above). Loss of this morphology in Lemmiscus populations represents one of the few readily discernable small mammal morphotype 'extinctions' near the end of the Pleistocene in North America.

Acknowledgments

Robert E. Reynolds originally suggested this study and was responsible for the excavations that resulted in collection of the specimens reported here. We appreciate the patience and understanding of Kathleen Springer and Betsy Slemmer at SBCM during what turned out to be a long-term loan of the specimens. Eric Scott was especially helpful to us in facilitating curation of specimens and for his extensive help in tracking down original documentation pertaining to the collection and the radiocarbon date. Jeff Horowitz prepared Figure 1, and Kate Reynolds prepared figures 2, 3, and 4. We benefited from years of discussion about Lemmiscus with Tony Barnosky and Charles Repenning, and their contributions to our thinking are gratefully acknowledged. Comments on an earlier draft of this paper were provided by Amy Balanoff, Gabe Bever, Eric Ekdale, Christian George, Ted Macrini, Jim Mead, Holly Nance, Dennis R. Ruez, Jr., Geraldine Swartz, Patrick Wheatley, and Richard Zakrzewski.

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Accepted for publication 27 March 2003.

Appendix 1

Diagnostic specimens of Lemmiscus and Microtus from Kokoweef Cave. Specimens are arranged by taxon, element, morphotype, and stratigraphic depth. I = incipient closure (see text for explanation); P = pinched closure (see text for explanation); T# = triangle number (see Fig. 2 for dental terminology); Type 1 = 'typical' morphotype of Lemmiscus M3, with an anterior loop, two closed triangles, and an elongated posterior loop; #T = number of closed triangles present. Stratigraphic designations are in feet below datum (see Reynolds et al. 1991a). Specimen numbers (provided parenthetically) are numbered within SBCM collection number L1808.

Lemmiscus m1, 4T morphotype (n = 10): Level 22/23 NW (2502, 2526); Level 23/24 (2557); Level 23/24 NW (2541); Level 25/26 NW (2599); Level 27/28 SW (2612); Level 28/29 SW (2615, 2621); Level 28/29 NW (2629); Level 29/30 SW (2636). Lemmiscus m1, 4T morphotype with T5-T6 confluent and pinched from cap (n = 8): Level 22/23 NW (2529); Level 23/24 (2709); Level 23/24 NW (1212, 2548); Level 28/29 SW (2618, 2625); Level 29/30 (2650); Level 38/39 SW (2668). Lemmiscus m1, P5 morphotype (n = 2): Level 22/23 W (2498); Level 23/24 (2688). Lemmiscus m1, I5 morphotype (n = 6): Level 22/23 NW (2511); Level 23/24 (2684, 2694); Level 23/24 NW (2550); Level 27/28 SW (2614); Level 28/29 SW (2617). Lemmiscus m1, 5T morphotype (n= 165): Level 21 (2491-2493); Level 21 NW (2494-2496); Level 22/23 NW (2503-2510, 2512-2521, 2523-2525, 2527-2528, 2531-2534); Level 23-24 (2556, 2673-2680, 2682-2683, 2686-2687, 2689-2693, 2695-2701, 2703-2706, 2708, 2710); Level 23/24 SW (1216, 2537-2539); Level 23/24 NW (1214-1215, 2542-2547, 2549, 2551-2555); Level 24/25 SW (2560, 2562, 2565-2566); Level 24/25 NW (2572-2581, 2583-2589); Level 25/26 SW (2591-2592); Level 25/26 NW (2594-2598, 2600-2604); Level 26/27 NW (2605, 2607-2610, 2671); Level 27/28 SW (2613); Level 28/29 (2630); Level 28/29 SW (1227-1228, 2616, 2619-2620, 2622-2624, 2626-2628); Level 29/30 (2651); Level 29/30 SW (2633-2635, 2637-2648); Level 31.5/32 (2653-2655); Level 31.5/32 W (2656); Level 32/33 W (2657-2658); Level 34/35 SW (2659-2660); Level 36/37 SW (2663-2665); Level 38/39 SW (2666-2667); Level 39/40 SW (2669-2670). Lemmiscus, dentary fragment with 5T morphotype m1 (some specimens with additional teeth) (n = 11): Level 22/23 (2535); Level 22/23 W (2497); Level 22/23 NW (2501); Level 23/24 NW (2540); Level 24/25 SW (2559); Level 24/25 NW (2568-2571); Level 29/30 SW (2632); Level 36/ 37 SW (2662). Lemmiscus m1, P6 morphotype (n = 11): Level 22/23 NW (2522, 2530); Level 23/24 SW (2536); Level 23/24 (2681, 2707, 2711); Level 24/25 SW (2561, 2564); Level 24/25 NW (2582); Level 29/30 SW (2649); Level 35/36 SW (2661). Lemmiscus m1, I6 morphotype (n = 2): Level 23/ 24 (2685); Level 25/26 SW (2590). Lemmiscus m1, 6T morphotype (n = 5): Level 22/23 W (2499); Level 23/24 (2702); Level 24/25 SW (2563); Level 26/27 NW (2606); Level 27/28 SW (2611). Lemmiscus M3, Type 1 (n = 94): Level 21 NW (2712-2713, 2715-2716); Level 22/23 NW (2717-2722, 2724-2726, 2728-2732); Level 23/24 (2735, 2737-2741, 2744, 2746-2752); Level 23/24 SW (1206, 2766-2769); Level 23/24 NW (1185 [partial skull with left and right M3], 1205, 2753, 2755, 2757-2758, 2759 [partial skull with right M3], 2760, 2763, 2765); Level 24/25 SW (2774-2778); Level 24/25 NW (2770, 2773); Level 25/26 SW (2792-2795); Level 25-26 NW (2781-2786, 2789-2791); Level 26/27 NW (2796-2798); Level 28-29 (2800); Level 28-29 SW (2805, 2808); Level 2829 NW (2802-2804); Level 29/30 (2809-2812); Level 29/30 SW (2814-2822); Level 30/31 W (2823-2824); Level 31.5/32 W (2825); Level 39/40 SW (2830). Lemmiscus M3, T3 moderately or well developed (n = 18): Level 21 NW (2714); Level 22/23 NW (2723, 2727); Level 23/24 (2736, 2742-2743, 2745); Level 23/34 SW (1204); Level 23/24 NW (2754, 2756); Level 24/25 NW (2771); Level 24/25 SW (2779); Level 25/26 (2780); Level 25/26 NW (2787); Level 28/29 SW (2806); Level 37/ 38 SW (2826); Level 38/39 SW (2828); Level 39/40 SW (2831). Lemmiscus M3, 2T, T2 confluent with posterior loop (n = 10): Level 22/23 W (2734); Level 23/24 SW (1203); Level 23/24 NW (2761, 2764); Level 24/25 NW (2772, T1 confluent with anterior loop); Level 25/26 NW (2788); Level 26/ 27 NW (2799); Level 29/30 W (2813); Level 38/39 SW (2827 [T1 confluent with anterior loop], 2829). Lemmiscus M3, 2T, T1 and T2 confluent (n = 3): Level 24/25 NW (2772, T1 also confluent with anterior loop); Level 28/29 SW (2807); Level 28/29 NW (2801). Lemmiscus M3, 2T, T1 confluent with anterior loop (n = 1): Level 23/24 NW (2762). Microtus m1, 5T morphotype (n = 8): Level 22/ 23 NW (2500); Level 23/24 (2672); Level 23/24 SW (1184); Level 24/25 SW (2558); Level 24/25 NW (2567); Level 25/26 NW (2593); Level 29 (2631); Level 31.5/32 (2652). Microtus M3 (n = 9): Level 22/23 NW (2832); Level 23/24 (2833); Level 23/24 SW (1184); Level 23/24 NW (2834); Level 24/25 SW (2835); Level 25/26 NW (2836-2838); Level 30/31 W (2839).

Christopher J. Bell and Christopher N. Jass

Department of Geological Sciences, The University of Texas at Austin, Austin, Texas, 78712
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Author:Bell, Christopher J.; Jass, Christopher N.
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Date:Apr 1, 2004
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