Manuscript title: A Fossil Razorback Sucker (Pisces: Catostomidae, Xyrauchen texanus) from Southeastern California Authors: Geoffrey M. Hoetker and Kenneth W. Gobalet Address: Department of Biology, California State University, Bakersfield, CA 93311 Phone: (805) 664-3038 Fax: (805) 665-6956 Email: [email protected] Key words: fossil razorback sucker, Xyrauchen texanus Running head: Fossil razorback sucker Manuscript category: Major article Suggested section for review: General ichthyology Number of text pages: 11 Number of line-art figures: 3 Number of halftone figures: 0 Number of tables: 1 Abstract Analysis of a nearly complete fossil sucker (SBCM A768-1) from the Anza Borrego desert indicates that it is a razorback sucker, Xyrauchen texanus (Abbott). The age of the fossil is probably Pliocene making it the oldest Xyrauchen sample known and extends the lineage back more than five million years. Meristic counts are as follows: 16 dorsal fin rays and pterygiophores; more than 9 pectoral fin rays; at least 8 pelvic fin rays; 7 anal pterygiophores; and about 42 post-Weberian vertebrae with a total of 46. The total length is approximately 440 mm and the standard length is 365 mm. Sixteen dorsal fin rays is at the upper end of the range for the species and the vertebral count is one below comparative materials. Measurements of the diagnostic predorsal hump suggest that this prominent feature is somewhat exaggerated in contrast with comparative materials, and indicates that the specimen may be a male. Introduction The monotypic genus Xyrauchen, including only the razorback sucker Xyrauchen texanus (Abbott), has been viewed as one of the more primitive members of the catostomid subfamily Catostominae, tribe Catostomini (Smith 1992a). Xyrauchen texanus and the Lost River sucker, Deltistes luxatus, are believed to represent sister groups of Chasmistes spp. (Miller and Smith 1981; Smith 1992a). All belong to large-bodied genera inhabiting extensive river systems of the American west: Chasmistes brevirostris and Deltistes in Klamath Lake and River; Ch. liorus in Utah Lake, Ch. cujus in Pyramid Lake and Truckee River, Ch. murei (extinct) in the Snake River, and Xyrauchen in the Colorado River basin. Razorback suckers are one of the larger native freshwater fishes of the Colorado system (Sigler and Miller 1963), approaching a meter in total length (TL). Its name derives from a distinctive, pronounced nuchal keel in adults. A fossil Xyrauchen (San Bernardino County Museum A768-1) purportedly of early Pliocene age was collected from the Anza-Borrego Desert "on the north side of the San Felipe Hills near the western boundary of Imperial County [California] and north of State Highway 78" (Stewart and Roeder 1993). Though it is reported to be from the early Pleiocene Diablo Formation (Stewart and Roeder 1993), it is doubtful if more precise locality and information on specimen age will become available. The Diablo Formation (Stewart and Roeder 1993), is a unit of the Palm Spring Group (Winker and Kidwell 1996). The nearly complete fossil (Fig. 1) is inside an elliptical, arenitic concretion, fractured lengthwise, with one half containing most skeletal material and the other with mostly imprints. The sandstone matrix is uniform in grain-size and fine, suggesting deltaic or floodplain deposition from the ancestral Colorado River and transport north and perhaps west by continual drift to the current location (Acosta et al. 1998). The fossil is herein compared with skeletons prepared from modern lower Colorado River X. texanus, other related catostomids (D. luxatus and Ch. cujus), and Catostomus latipinnis, which is a member of the sister genus to the Xyrauchen group (Xyrauchen + Chasmistes + Deltistes) (Smith 1992a). Description and Comparisons Estimates of 435-440 mm TL and 365 mm standard length (SL) place the fossil at the lower limit of adult size for X. texanus (McAda and Wydoski 1980; Minckley 1983, Tyus 1987; Marsh 1987). A vertebral count of -42 post-Weberian vertebrae (46 total) is one below the range in seven modem specimens (43-44 post-Weberian vertebrae, 47-48 total). Chasmistes cujus have 46-47 total vertebrae and a single Deltistes luxatus had 49. Smith (1992a) listed 44-45 total vertebrae for C. latipinnis. The fossil has 16 dorsal-fin rays and pterygiophores compared with a typical count of 14-15 and variation of 13-16 for X. texanus (Fig. 2) (Hubbs and Miller 1953; Minckley 1973). Deltistes, Chasmistes spp., and C. latipinnis all have 12-13 dorsal-fin rays (Smith 1992a). Two unbranched anterior elements in the fossil are consistent with comparative material of X. texanus. The fossil has seven anal pterygiophores more than nine pectoral fin rays, and at least eight pelvic fin rays. A distinctive anterodorsal keel becomes evident externally in X. texanus at 10-12.5 cm TL, and fully developed in young adults -300-350 cm long (W. L. Minckley, pers. comm.). The keel is by far the most conspicuous feature of the fossil. Even with the anterodorsal part missing (Fig. 1), it is noticeably more pronounced and the lead edge is more vertically oriented than in X. texanus skeletons available (Fig. 3). At its apex, the fossil's keel rises 37 mm upward from a long axis of the body extended from the top of the neurocranium, and it measures 87 mm long from its anterior edge at the posterior end of the neurocranium to the anterior edge of the first pterygiophore, respectively 10.1% and 23.8% of SL (Table 1). Averages of 7.6% (n=12) and 21.7% (n=4) were computed for comparative material. The maximum height of the hump is 12.9% of the standard length compared with a average of 9.2% for comparative material. Six interneurals are found anterior to the dorsal fin of the fossil and in comparative material of X. texanus (Figs. 1 and 3). The fossil clearly shows a prominent vertical recess in an impression of the lead intemeural of the keel, which is consistent with comparative material. Three enlarged intemeurals comprise the fossil's keel. All comparative specimens had 3-4 enlarged elements, consistent with Smith's (1992a) observations. C. latipinnis usually has 0-2 somewhat enlarged intemeural bones (Smith 1992a), and our single skeleton has a single, antero-posteriorly, elongated element. Five comparative Ch. cujus each had only one. The interneural elements in all but X. texanus are thin and in line with the dorsal edge of the neural spines. The dentaries in members of the Xyrauchen group are relatively long compared to other catostomids (Smith 1992a) and show little or none of the ventral deflection and arching of the anterior gnathic ramus that occurs in C. latipinnis. The well preserved left dentary of the fossil is consistent in shape and proportion of comparative specimens of X. texanus. Greatest length of the shaft is 21 mm and the distance from the ventral edge of the quadrate recess to the dorsal-most tip of the coronoid process is 22 mm. Position of the prominent foramen transversing the dentary below and near the posterior end of the gnathal ridge is also similar to that of X. texanus and Chasmistes. In these last two the opening is oriented medio-laterally, whereas in C. latipinnis the orientation is more anterio-posterior. The long axis of the maxillary body measures 27 mm in the fossil. The dentary process, its posterior-most portion, is missing. In the Xyrauchen group, the premaxillary (or anteromedian) process comes off of the narrow neck of the maxilla that is characteristic of catostomid fishes (Miller and Smith 1967), so that its tip is oriented anterior to the anterodorsal process. Although the neck of the left maxilla in the fossil is shattered and the anterodorsal process is the only one visible, there is no suggestion of it differing from that of the Xyrauchen group. In the Xyrauchen group, the anterior edge of the dermethmoid just posterior to the median, anteriorly directed dermethmoid spine, flares laterially and slightly posteriad. Although the fossil's skull has been titled dorsolaterally, perhaps as a result of torque and compression before fossilization (W. L. Minckley pers. comm.), it likewise exhibits the "doomed rectangle" shape described in Smith (1992a: fig. 4a) and is similar to Chasmistes. This contrasts with the shorter, wider, and more fully rectangular shape in C. latipinnis, and the elongate form of Deltistes laxatus. Although Smith (1992a) reported a long lower limb of the preopercle in both Xyrauchen and Deltistes, and a relatively shorter lower limb in Ch. cujus and C. latipinnis, we find this to be an equivocal character. The anterior tip of the preopercle ends 5.0 mm caudad of the mandibular condyle of the quadrate in the fossil. Adult X. texanus exhibit several sexually dimorphic characters, including pelvic- and anal-fin lengths, size (Minckley 1983), height of predorsal keel (Minckley et al. 1991), and curvature of the last anal fine-ray in males (McAda and Wydoski 1980). Minckley (pers. comm.) suggested the fossil might be a male based on flared pelvic fins and pronounced height and size of the predorsal keel. Unfortunately, much of its anal fin, including the last ray, has been lost. Gobalet (1992) was concerned that hybridization, known for many Colorado River fishes, might influence the identifications of archaeological material, and this clearly also applies to fossils. X. texanus has been shown to hybridize with C. latipinnis (Hubbs and Miller 1953; Tyus and Karp 1990; Minckley et al. 1991), and has the potential for introgression (Buth et al. 1987, Smith 1992b). The dorsal keel of hybrids, however, tends to be far less pronounced than that of X. texanus (Hubbs and Miller 1953), or even undetectable. The fossil's exaggerated keel minimizes the likelihood of hybrid influence. Discussion and Conclusion The Anza-Borrego fossil is clearly a species of Xyrauchen, and with exception of one fewer vertebrae, one greater number of dorsal-fin rays than typical, and an even more pronounced predorsal keel than modern specimens of comparable size, resembles X.