Journal of Vertebrate Paleontology 20(2):400–403, June 2000 䉷 2000 by the Society of Vertebrate Paleontology
ALAMOSAURUS (DINOSAURIA: SAUROPODA) FROM THE LATE CAMPANIAN OF NEW MEXICO AND ITS SIGNIFICANCE
ROBERT M. SULLIVAN1 and SPENCER G. LUCAS2, 1Section of Paleontology and Geology, The State Museum of Pennsylvania, Third and North Streets, Harrisburg, Pennsylvania 17108-1026; 2New Mexico Museum of Natural History and Science, 1801 Mountain Rd. NW, Albuquerque, New Mexico 87104
The titanosaurid Alamosaurus sanjuanensis Gilmore, 1922, has long been known as the last North American sauropod; its fossils from Wyoming, Utah, New Mexico and Texas have been generally agreed to be of Lancian (late Maastrichtian) age (Hunt and Lucas, 1992, 1993; Lehman, 1980; Lucas, 1980; Lucas et al. 1987; Lucas and Hunt, 1989). Lucas and Hunt (1989) coined the term ‘‘sauropod hiatus’’ to refer to the Cenomanian–late Maastrichtian absence of sauropods in North America, arguing that it resulted from the extirpation of sauropods in North America at about the end of the Albian, followed by their immigration (probably from South America) during the late Maastrichtian. During the summer of 1998, one of us (RMS) collected an incomplete sauropod tooth from the De-na-zin Member of the Kirtland Formation (Fig. 1). We refer this tooth to Alamosaurus sanjuanensis based on the earlier report of Kues et al. (1980), although we note that the type specimen of A. sanjuanensis lacks cranial material (Gilmore, 1922). In addition, we recently have identified uncataloged, and heretofore undocumented, sauropod vertebrae, collected by a University of New Mexico (UNM) field party during 1984 from the same stratigraphic interval. We recognize that Alamosaurus is a form genus, to which we provisionally refer all Late Cretaceous sauropod material from the San Juan Basin, pending the discovery of diagnostic material. Here, we document this important new material of late Campanian age, thus shortening the ‘‘sauropod hiatus.’’ Moreover, these new records undermine the use of Alamosaurus (or sauropods in general) as an index fossil of Lancian time and thus may call into question the age of the Naashiobito Member of the Kirtland Formation. Institutional Abbreviations: NMMNH, New Mexico Museum of Natural History and Science, Albuquerque; SMP, The State Museum of Pennsylvania, Harrisburg; USNM, United States National Museum of Natural History, Washington, D.C. DESCRIPTION AND COMPARISON OF NEW MATERIAL Tooth—The tooth, SMP VP-1097 (Fig. 2A), is incomplete, having been split nearly in half and broken at its base. However, it is the most complete single tooth known of Alamosaurus (see those described by Kues et al., 1980) and thus provides additional information regarding its dental morphology. It is relatively large (maximum height 37 mm) and is a typical sauropod, peg-like tooth, similar to those of Diplodocus, Nemegtosaurus and Alamosaurus (Nowinski, 1971; Kues et al., 1980). There are two areas that have been chipped on the lingual side; one near the top of the tooth and the other at the bottom (Fig. 2A, top). The preserved tooth represents the inner lingual side (see the distinct curvature described in Nemegtosaurus: Nowinski, 1971). The crown is worn sub-perpendicular to the tooth shaft and measures 10 mm at its maximum width. The top of the crown is worn, revealing a distinct central enameloid zone appromimately 3 mm in diameter (Fig. 2A, top). Although incomplete, it is apparent that this worn surface is elliptical. The lateral enamel surface is characterized by fine longitudinal ridges superimposed on a still finer ornamentation like that illustrated by Kues
et al. (1980:868, text-fig. 5). Horizontal banding is evident for the entire length of the tooth. In addition, there are three prominent horizontal bands: one slightly proximal to the occlusal surface of the crown, a second (the most pronounced) located approximately 0.8 cm proximal to the first, and a third located approximately 1.0 cm proximal to that (Fig. 2A, top). The broken surface reveals details of the internal microstructure. There are distinct layers that are subparallel to the central pulp cavity that may represent growth pulses. In addition, there is an inverted conical zone that lies above the central pulp cavity and below the distinct central enameloid zone noted above. A detailed histological description of this tooth is beyond the scope of this paper. J. Kirkland (pers. comm., 1999) suggests that this tooth may belong to a crocodilian. However, close inspection and comparison to known crocodilian teeth indicate little similarity. Indeed, SMP VP-1097 has the wrinkled enamel texture, albeit faint, that is characteristic of eusauropods (Wilson and Sereno, 1998). Furthermore, the elliptical wear facet of the tooth indicates a crown-to-crown relationship between the upper and lower dentition, that is characteristic of diplodocid and titanosaur eusauropods (Wilson and Sereno, 1998), and not known in any crocodilian. An additional, slightly water-worn, specimen (SMP VP-1271), recently discovered from the De-na-zin Member, consists of an elongated, quasi-tubular root, measuring 48 mm long. It lacks the enamel portion, but the inner pulp core is visible dorsally. This long, narrow tooth root is consistent with that of a sauropod (eusauropod) and therefore we provisionally refer this specimen to Alamosaurus. Caudal Vertebrae—Three sauropod caudal vertebrae were found associated at a single site (NMMNH loc. 4224) by UNM student Jess Hunley in the summer of 1984. Although they are probably from the same individual, they have been given separate catalog numbers because of uncertainty regarding their association. All three caudal vertebrae are incomplete, largely missing the neural arches and spines, and lack chevrons. They are procoelous, lack pleurocoels on the sides of the centra and readily correspond to those of Alamosaurus sanjuanensis illustrated by Gilmore (1946:pl. 6) from the North Horn Formation of Utah (USNM 15560). The largest of the three, NMMNH P-29723 (Fig. 2B), probably represents either caudal vertebra 3 or 4 based on comparison with USNM 15560 (see Gilmore 1946: plate 6). The NMMNH vertebra is weathered and is slightly crushed, and fractured, with part of the right side eroded. Also, the right part of the neural arch is eroded, and the right prezygapophysis, both transverse processes, and the neural spine are missing. Centrum height is 24 cm (proximal end), and it is approximately 19 cm wide (distal end); the latter is close to the measurements of caudal vertebra 4 in Gilmore (1946:32). The base of the neural arch rises abruptly from the posterior part of the centrum. The maximum length of the centrum is approximately 15 cm on the left side, which is much shorter than vertebra 4 (28 cm) in Gilmore (1946:32), but this is probably because the posterior ball of NMMNH P-29723 is largely eroded away. The second caudal vertebra, NMMNH P-29722 (Fig. 2C), also lacks the neural arch and spine and may represent caudal vertebra 13 or 14.
The centrum is nearly complete, with only a section of bone broken on the posterior right lateral side. Maximum anterior and posterior centrum height are 15 cm and 16 cm, respectively. The maximum centrum length is 24 cm measured from the base of the anterior edge to the end of the ball. The third caudal vertebra, NMMNH P-28741 (Fig. 2D) may represent either vertebra 14 or 15 (Gilmore, 1946:plate 6). It is too incomplete to be certain because most of the right side missing. However, it is the most complete of the three caudal vertebrae with respect to the neural arch and spine. The base of the neural arch is intact along the left side of the centrum together with the left prezygapophysis. The neural arch and spine above the left prezygapophysis are missing. Maximum centrum height is approximately 12.5 cm measured anteriorly. Posterior centrum height cannot be measured due to breakage on the dorsal portion of the posterior ball. DISCUSSION
FIGURE 1. Composite stratigraphic section of the upper Kirtland Formation, San Juan Basin, New Mexico indicating the sites of the Alamosaurus specimens discussed in this paper. 1, relative stratigraphic position of SMP VP-1097, NMMNH nos. P-28741, P-29722 and P29723; 2, 3, relative stratigraphic position of Alamosaurus teeth and bones cited in Kues et al. (1980), Lehman (1980), and Lucas and Hunt (1989).
The specimens of Alamosaurus sanjuanensis from the De-na-zin Member of the Kirtland Formation documented here are of late Campanian age. Radioisotopic ages (Ar/Ar method) of two air-fall ash beds in the De-na-zin Member are 73.37 ⫾ 0.18 Ma and 73.04 ⫾ 0.25 Ma (Fassett and Steiner, 1997), and thus indicate a late Campanian age (Obradovich, 1993). The reversed magnetization of sediments of the De-na-zin Member is readily interpreted as Chron 32r, also indicating a late Campanian age (Fassett and Steiner, 1997). Several dinosaur genera from the De-na-zin Member, Parasaurolophus, Pentaceratops, and Albertosaurus, are also found in the Campanian Fruitland Formation, and this is consistent with assigning a late Campanian age to the De-na-zin Member (Sullivan, 1999; Sullivan and Williamson, 1999). The late Campanian Alamosaurus specimens documented here are not, however, the first report of a late Campanian sauropod from North America. McCord (1997) documented a titanosaurid caudal vertebra from the Fort Crittenden Formation in the Santa Rita Mountains, Arizona. The Fort Crittenden Formation is a correlative of (if not the same lithostome as) the Ringbone Formation in southwestern New Mexico (Lucas et al., 1995). Both units contain a sparse dinosaur fauna, mostly of indeterminate hadrosaurs and theropods (cf. Albertosaurus) (Lucas et al., 1990; McCord, 1997). An Ar/Ar age of an ash bed at the base of the Hidalgo Formation, which immediately overlies the Ringbone Formation in the Little Hatchett Mountains of southwestern New Mexico, is 71.44 ⫾ 0.19 Ma (Lawton et al., 1993). This suggests that the underlying Ringbone Formation is of late Campanian age, as is its Arizona equivalent, the Fort Crittenden Formation. Therefore, the titanosaurid vertebra from Arizona reported by McCord (1997) is of late Campanian age. The late Campanian sauropod records from Arizona and New Mexico thus shorten the ‘‘sauropod hiatus,’’ first proposed by Lucas and Hunt (1989) as Cenomanian–late Maastrichtian, to Cenomanian–late Campanian. Thus, if indeed sauropods were extirpated in North America at the end of the Early Cretaceous, they reappeared (presumably as immigrants from South America or Asia) by late Campanian time. Alamosaurus has come to be regarded as an index taxon of Lancian (late Maastrichtian) time (Hunt and Lucas, 1992; Lehman, 1980; Lucas, 1992; Lucas and Hunt, 1989; Lucas et al., 1987). However, the late Campanian sauropod fossils documented here indicate the genus is now known from much older strata. Thus, fossils of Alamosaurus are not necessarily indicators of the Lancian, and some previously proposed correlations based on Alamosaurus may need to be reconsidered. Indeed, the age of the Naashoibito Member of the Kirtland Formation in the San Juan Basin, may need to be reassessed. Acknowledgments—We thank James Fassett (United States Geological Survey, Denver), Jess Hunley, and the late John Estep for their help in the field. RMS thanks Robert Purdy (United States National Museum, Washington, D.C.) for access to the Alamosaurus material (USNM 15560) from the North Horn Formation. We also thank James E. Kirkland (Utah Geological Survey, Salt Lake City) for giving his opinion on the taxonomic identification of SMP VP1097 and to John McIntosh, David Gillette (Museum of Northern Arizona, Flagstaff) and an anonymous reviewer for their comments on the manuscript.
JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 20, NO. 2, 2000
FIGURE 2. Alamosaurus sanjuanensis, new referred material. A, SMP VP-1097, incomplete tooth in lingual view (top) and lateral view (bottom); B, NMMNH P-29723, caudal vertebra 3 or 4, in left lateral view (left) and anterior axial view (right); C, NMMNH P-29722, caudal vertebra 13 or 14, in left lateral view (left) and anterior axial view (right); D, NMMNH P-28741, caudal vertebra 14 or 15, in left lateral view (left) and anterior axial view (right). Scale bar equals 1 cm for A, and equals 5 cm for Bâ€“D.
NOTES LITERATURE CITED Fassett, J. E., and M. B. Steiner. 1997. Precise age of C33N-C32R magnetic-polarity reversal, San Juan Basin, New Mexico and Colorado; pp. 239–247 in O. J. Anderson, B. S. Kues and S. G. Lucas (eds.), Mesozoic Geology and Paleontology of the Four Corners Region, New Mexico Geological Society Guidebook, 48th Field Conference, Socorro. Gilmore, C. W. 1922. A new sauropod dinosaur from the Ojo Alamo Formation of New Mexico. Smithsonian Miscellaneous Collections, 72(2):1–9. 1946. Reptilian fauna from the North Horn Formation of central Utah. United States Geological Survey Professional Paper 210-C: 29–52. Hunt, A. P., and S. G. Lucas. 1992. Stratigraphy, paleontology and age of the Fruitland and Kirtland formations (Upper Cretaceous), San Juan Basin, New Mexico; pp. 217–239 in S. G. Lucas, B. S. Kues, T. E. Williamson and A. P. Hunt (eds.), New Mexico Geological Society Guidebook, 43rd Field Conference, San Juan Basin IV, Socorro. , and 1993. Cretaceous vertebrates; pp. 77–91 in S. G. Lucas and J. Zidek (eds.), Vertebrate Paleontology in New Mexico. New Mexico Museum of Natural History and Science Bulletin, No. 2, Albuquerque. Kues, B. S., T. Lehman, and J. K. Rigby, Jr. 1980. The teeth of Alamosaurus sanjuanensis, a Late Cretaceous sauropod. Journal of Paleontology 54:864–869. Lawton, T. F., G. T. Basabilvazo, S. A. Hodgson, D. A. Wilson, G. H. Mack, W. C. McIntosh, S. G. Lucas, and K. K. Kietzke. 1993. Stratigraphy of the Little Hatchet Mountains, southwestern New Mexico. New Mexico Geology 15:9–15. Lehman, T. M. 1980. The Alamo Wash local fauna: A new look at the old Ojo Alamo fauna; pp. 189–221 in S. G. Lucas, J. K. Rigby, Jr., and B. S. Kues (eds.), Advances in San Juan Basin Paleontology. University of New Mexico Press, Albuquerque. Lucas, S. G. 1980. Dinosaur communities of the San Juan Basin: A case for lateral variations in the composition of Late Cretaceous dinosaur communities; pp. 337–393 in S. G. Lucas, J. K. Rigby, Jr., and B. S. Kues (eds.), Advances in San Juan Basin Paleontology. University of New Mexico Press, Albuquerque. , N. J. Mateer, A. P. Hunt, and F. M. O’Neill. 1987. Dinosaurs, the age of the Fruitland and Kirtland formations and the Cretaceous–Tertiary boundary in the San Juan Basin, New Mexico; pp.
35–50 in J. E. Fassett and J. K. Rigby, Jr. (eds.), The Cretaceous– Tertiary Boundary in the San Juan and Raton Basins, New Mexico and Colorado. Geological Society of America Special Paper 209, Boulder. , and A. P. Hunt. 1989. Alamosaurus and the sauropod hiatus in the Cretaceous of North American Western Interior; pp. 75–85 in J. O. Farlow (ed.), Paleobiology of the Dinosaurs. Geological Society of America Special Paper 238, Boulder. , G. T. Basabilvazo, and T. F. Lawton. 1990. Late Cretaceous dinosaurs from the Ringbone Formation, southwestern New Mexico, U.S.A. Cretaceous Research 11:343–349. , B. S. Kues, and C. M. Gonza´ lez-Leo´ n. 1995. Paleontology of the Upper Cretaceous Cabullona Group, northeastern Sonora; pp. 143–165 in C. Jacques-Ayala, C. M. Gonza´ lez-Leo´ n, and J. Rolda´ n-Quintana (eds.), Studies on the Mesozoic of Sonora and Adjacent Areas. Geological Society of America Special Paper 301, Boulder. McCord, R. 1997. An Arizona titanosaurid sauropod and revision of the Late Cretaceous Adobe fauna. Journal of Vertebrate Paleontology 17:620–622. Nowinski, A. 1971. Nemegtosaurus mongoliensis n. gen., n. sp. (Sauropoda) from the uppermost Cretaceous of Mongolia. Results of the Polish-Mongolian Palaeontological Expeditions, Part III, Palaeontologica Polonica 25:57–85. Obradovich, J. D. 1993. A Cretaceous time scale; pp. 379–396 in W. G. E. Caldwell and E. G. Kauffman (eds.), Evolution of the Western Interior Basin. Geological Association of Canada Special Paper 39, St. John’s, Newfoundland. Sullivan, R. M. 1999. Nodocephalosaurus kirtlandensis, gen. et sp. nov., a new ankylosaurid dinosaur (Ornithischia: Ankylosauria) from the Upper Cretaceous Kirtland Formation (upper Campanian), San Juan Basin, New Mexico. Journal of Vertebrate Paleontology 19: 126–139. , and T. E. Williamson. 1999. A new skull of Parasaurolophus (Dinosauria: Hadrosauridae) and a revision of the genus. New Mexico Museum of Natural History and Science Bulletin, No. 15, 52 pp. Wilson, J. A., and P. C. Sereno. 1998. Early evolution and higher-level phylogeny of sauropod dinosaurs. Society of Vertebrate Paleontology Memoir 5, Journal of Vertebrate Paleontology 18, Supplement to Number 2, 68 pp. Received 21 July 1999; accepted 18 January 2000.