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. texanus. We assign it to that species. Prior to the considerable extirpations experienced by this species, X. texanus seemed to vary little morphologically throughout its range (Hubbs and Miller 1953), however meristic counts of the fossil and additional examination revealed greater prehistoric character variation.
Much speculation surrounds the function of the predorsal keel or "hump" (La
Rivers 1962; Minckley 1973; Moyle 1976) which may potentially serve as a stabilizer in the swift and deep waters of the Colorado River. Although swimming tests utilizing Colorado
River fishes with similar hump morphology have been conducted (Berry and Pimentel
1985), the specific effect of the presence of the hump on swimming performance has not been empirically tested. Minckley and Meffe (1987) found that several native fishes with the pronounced hump survived flooding in Arizona and New Mexico, while nonnative, introduced fishes lacking the hump did not. These differential responses reflect differences in evolutionary histories (Minckley and Meffe 1987), and selection for the development of humps in native fishes of the Colorado River may have been an evolutionary response to past extreme hydrological conditions. No fossil catostomids have been identified from the late Miocene Bidahochi
Formation, Arizona (Uyeno and Miller 1965), which was formed before the connection of the upper and lower Colorado River. Until the Anza-Borrego Xyrauchen specimen was discovered, fossil catostomids older than Pleistocene age were unknown (Minckley et al.
1986) from the Colorado River system. If the Xyrauchen specimen is of early Pliocene age
(Stewart and Roeder 1993), the X. texanus lineage would date back more than five million years. Late Miocene (8.5 to 5.5 million years old) Chasmistes and Deltistes from the Chalk
Hills Formation of Idaho are morphologically similar to modern forms (Miller and Smith
1981; Smith et al. 1982) and indicate an even earlier divergence for the groups (Smith
1992a) and likely Xyrauchen as well.
It is unfortunate that the specimen cannot be located more accurately in space and time. It is on a part of southern California being carried northward, extended, and otherwise influenced (e.g., in elevation) by southern extension of the San Andreas Fault zone. Its point of fossilization may thus be elsewhere than the point of discovery, or not.
If tectonic movement did occur after death, the fossil should have been carried north and perhaps west from the original point of interment. The Salton Sea Basin has enjoyed a broad, incipient, and sporadically direct connection with the lower Colorado River across extensive deltaic deposits south and west of Yuma, Arizona, in southernmost California, and in Sonora and northernmost Baja California, Mexico. Incursions of freshwater from the Colorado River have been accompanied by razorback invasions in both historic and prehistoric times (Wilkie 1978; Waters 1983; Minckley et al. 1991), and almost certainly before. Xyrauchen may have evolved in an obscured southwestern drainage that flowed south on the western escarpment of the Colorado Plateau prior to the connection of the upper Colorado River through Grand Canyon (W. L. Minckley pers. comm.).
Though the fossil is reported to be from the Pliocene Diablo Formation (Stewart and Roeder 1993), there is no precise geographic locale, or stratigraphic reference.
Connections between what are now upper and lower parts of the Colorado River formed in the Pliocene, initiating delta formation, but relations of the Salton Basin and delta are
complex (Crowell 1981). Fossils of other organisms, i.e, wood and other plant remains
from the general area are commonly reported by word-of-mouth as "Pleistocene" in age,
and other Xyrauchen materials from the Ocotillo Formation of the Anza Borrego Desert are
Pleistocene (Stewart and Roeder 1993), the fossil may be from that Pleistocene formation as well. The specimen is nonetheless well premineralized.
Materials Examined
Institutional abbreviations are as listed in Leviton et al. (1985) except KWG are the authors comparative materials stored at California State University, Bakersfield, SBCM is the San
Bernardino County Museum, and UCMP is the University of California, Museum of
Paleontology. Catostomus latipinnis: ASU 13844; Chasmistes cujus: KWG 346, 351,
359, 399, 488; Deltistes luxatus: CAS 97835, 97836, 135092, UCMP, 122582;
Xyrauchen texanus: ASU 13760, 13905, 13906, 14622, 14623, 14624, 14625, 14626,
14881, 14882, 13905-6, 14621-6, 14880-2, and three unnumbered specimens, CAS
66230.
Acknowledgments
We would like to thank M. A. Roeder for supplying the fossil for study and for information concerning the retrieval and aging of the fossil material. R. Coyle found the specimen which was prepared by M. Moore. G. Jefferson, W. L. Minckley, and G. R.
Smith contributed meristic information, and locality data, and other valuable comments.
The illustrations were completed by C. Wioch whose work was funded by a James
Brainerd Memorial Grant of the Anza-Borrego Desert Natural History Association and E.
Pedroza typed the manuscript.
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Department of Biology, California State University, Bakersfield, California 93311. Send
reprint requests to KWG. Table 1. Standard Lengths (SL) and predorsal bone (hump) measurements (both in mm) for the fossil specimen and for five specimens of Xyrauchen texanus. A description of each are as follows: 1) anterior height (from the anterior-most edge of the first predorsal bone, rising from the neurocranium, perpendicular to the apex of the hump); 2) maximum height (from the second vertebra of the Weberian apparatus to the apex of the hump); and
3) total hump length (from the anterior-most edge to the posterior-most edge of hump).
The anterior hump height, maximum height, and total hump length for all specimens were also calculated as percentages of the standard length, except for a single specimen which was incomplete.
hump hump hump ant. ht. max. ht. hump I. specimen SL ant.ht. max. ht. length as % SL as % SL as % SL
SBCM A768-1 365 37 47 87 10.1% 12.9% 23.8%
ASU 13760 360 n/a 37 n/a n/a 10.3% n/a
ASU 14881 540 41 50 144 7.6% 9.3% 26.7%
ASU 14882 520 36 49 118 6.9% 9.4% 22.7%
ASU no # 435 28 35 72 6.4% 8.0% 16.6%
CAS 66230 332 27 30 69 8.1% 9.0% 20.8% Figure 1. Early Pliocene fossil Xyrauchen texanus (SBCM A768-1) from Anza Borrego desert southern California.
Figure 2. Dorsal fin rays and pterygiophores of Xyrauchen texanus (ASU 14881).
Figure 3. Enlarged internuerals and proximal postcranial skeleton of Xyrauchen texanus
(ASU 14881).
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