Saturday, February 13, 2021

Your Friends The Titanosaurs, part 33.2: Alamosaurus of Utah and points north?

We head north into Utah this time for the next major discoveries of Alamosaurus. Unlike Alamosaurus of the San Juan Basin or the Big Bend area of Texas (as we'll see next time), Alamosaurus sanjuanensis from the North Horn Formation of Utah has been more or less represented by one specimen, although others are known. Fortunately, that one specimen has been very informative in its own right. In addition to Utah, we have sketchy references to finds in Wyoming and Montana to deal with, plus a couple of incomplete threads from the San Juan Basin to tie up.

North Horn Formation Alamosaurus

For Alamosaurus of Utah, we start off again with Charles Gilmore. Oddly enough, the initial reportage of the Utah material is the same as the New Mexico material: a short piece in Science (Gilmore 1938) predating the final description (Gilmore 1946). (I don't know if this means anything, but the front matter of the USGS volume including Gilmore's report describes his contribution as having been published in January 1947, not 1946. I don't think anyone has ever cited it with a 1947 date.) This time, though, he had more to work with, so there was a longer gap between the two publications. In fact, he died (in 1945) before the second report was issued. Gilmore did not immediately assign the Utah material to A. sanjuanensis, but thought it would turn out to be something close (Gilmore 1938). Barnum Brown (1941) briefly mentioned it as an "undescribed genus".

The discovery of Alamosaurus sanjuanensis in the North Horn Formation goes back to a Smithsonian expedition in 1937. One of the members, George B. Pearce, discovered what proved to be a partial skeleton on June 15. The location was on the southwest toe of North Horn Mountain in Manti-La Sal National Forest, Emery County, Utah, and the stratigraphic position was in what Gilmore (1946) described as "Unit 4" of the lower North Horn Formation. (Coincidentally, the Alamosaurus-producing Ojo Alamo Formation, North Horn Formation, and Black Peaks Formation of the Big Bend area all cross the Cretaceous–Paleogene boundary, at least under the stratigraphic definitions used here.) The Smithsonian crew excavated the partial skeleton, catalogued as USNM 15560 (sometimes given as 15660). As reported in Gilmore (1946), it included 30 articulated caudal vertebrae beginning with the first, 25 chevrons, both ischia with attached fragments of the other pelvic bones, the left scapula and coracoid, articulated right arm and hand (no wrist bones or unguals; radius now missing per USNM online database), both sternal plates, and three partial ribs. Later, it would turn out that osteoderm fragments had also been collected (Carrano and D'Emic 2015). A few other bones were left in the field. Gilmore (1946) reported that dorsal vertebrae were present but too badly weathered to collect. A sacrum with five vertebrae was also present but left behind. Jim Jensen relocated the quarry in 1977 and attempted to collect the sacrum in 1980, but by then it had completely disintegrated (Curtice 2017). The carcass had been partially associated and partially articulated when buried, lying on its back (Gilmore 1946).

Excavating the arm of USNM 15560; part A of plate 4, Gilmore (1946). Note the columnar bundle made by the articulated metacarpals.

USNM 15560 has one of the better caudal series and arms among titanosaurs. As in some other titanosaurs, the first caudal is biconvex. Caudals 5 and 6 are fused. The neural arches and spines of the caudals are relatively low and the arches are positioned at the anterior end of the centra. The 25 chevrons probably represent the entire series of chevrons (Gilmore 1946). The scapula shows the same distinctive features that the San Juan Basin holotype has, indicating the two specimens represent the same species (D'Emic et al. 2011). This is important because USNM 15560 is much more complete than the holotype and can be used for comparison with more specimens. For example, because USNM 15560 also has ischia, it can be compared to the paratype ischium, which turns out to match and therefore belongs to Alamosaurus as well (D'Emic et al. 2011). (Provided, of course, that there aren't multiple species with rather similar scapulae and ischia.) The articulated forelimb is about 2.75 m (9 ft) long, divided between a 1360 mm (53.5 in) humerus, 885 mm (34.8 in) ulna, and 410 mm (16.1 in) metacarpus; the forearm is proportionally short compared to other sauropods, and the articulated metacarpus is columnar (Gilmore 1946). Carrano and D'Emic (2015) described three fragments that Gilmore overlooked as pieces of osteoderms. The largest and most complete is a bulb-type piece 23.9 cm long (9.41 in) in greatest dimension; it has been crushed, but it is not clear if it was hollow. It was described as most similar to an isolated Brazilian osteoderm (Carrano and D'Emic 2015).

The first six caudals of USNM 15560. #1, in the upper right, nicely shows what a biconvex caudal looks like. As mentioned, 5 and 6 are fused. Plate 5 in Gilmore (1946).

If you'd like to compare the holotype scapula to USNM 15560's scapula, here you go (coracoid thrown in at no additional cost). Figure 6 from Gilmore (1946).

With a bigger chunk of the skeleton and two decades of more sauropods, Gilmore could make more fruitful comparisons this time around. Interestingly, considering how brachiosaurs and titanosaurs are now placed together in Titanosauriformes, Gilmore decided that Brachiosaurus was the most similar North American sauropod to Alamosaurus. Von Huene by this time had included Alamosaurus in Titanosauridae (Huene 1929), and Gilmore regarded USNM 15560 as proving von Huene's assessment. He also stated that Alamosaurus and Indian Titanosaurus were seemingly "generically identical" but opted to keep them separate because Alamosaurus was not very similar to Titanosaurus australis (=Neuquensaurus).

A comparison of the arms of Alamosaurus sanjuanensis (left) and Apatosaurus louisae (right), at the same scale (Gilmore 1946).

To date, USNM 15560 is the only Alamosaurus specimen from the North Horn Formation that has attracted much attention. It continues to serve as an important reference specimen for Alamosaurus. For example, it is the basis for the tail and forelimbs of the skeletal mount at the Perot Museum of Natural History (Dallas, Texas) (May et al. 2012; Tykoski and Fiorillo 2017). A few other specimens have made cameos in the literature. D'Emic et al. (2011) referred a couple of caudals in the Brigham Young University collections (BYU 11392 and 11393; Provo, Utah) to the species, based on comparison with USNM 15560. Curtice (2017) mentioned BYU 9087, consisting of a caudal and the proximal ends of a large humerus and femur collected by Jim Jensen near the original USNM 15560 locality in 1966. BYU 9087 and USNM 15560 represent different individuals because the BYU caudal doesn't fit in the USNM caudal sequence and the BYU humerus is 20 cm wider (8 in) than the USNM humerus. The BYU material still appears to be Alamosaurus sanjuanensis, though (Curtice 2017). Curtice also noted a fibula on the order of 1.4 m long (4.5 ft) from a different locality, regrettably no longer extant (lost to the elements in the field after being jacketed but not collected).

Since we're here, why not have some more caudals? Plate 6 in Gilmore (1946).

The North Horn Formation is obscure among North American dinosaur-producing formations, although it *is* noted as a eggshell producer (Jensen 1966; Bray 1999). None of the eggshells have been confirmed as sauropod at this time, but you never know. The most abundant dinosaur appears to have been the ceratopsian Gilmore (1946) named Arrhinoceratops utahensis, usually placed in Torosaurus these days. The North Horn Formation is also another unit where we have both Alamosaurus sanjuanensis and Tyrannosaurus rex; in this case, there is tyrannosaur material known from a horizon between two Alamosaurus-bearing levels (Sampson and Loewen 2005).

Wyoming Alamosaurus?

The potential presence of Alamosaurus appears to have first been mentioned in Lehman (1981) and, slightly more obliquely in the same volume, Lucas (1981). Their combined accounts amount to the potential presence of Alamosaurus (or a nameless sauropod) in the Maastrichtian-age Evanston Formation of southwestern Wyoming, based on a written communication from Douglas Lawson to Spencer Lucas in 1978. Lucas and Hunt (1989) included a couple more bits of information: the record consists of three articulated caudals found in the Evanston Formation at Round Mountain, southwestern Wyoming and held at the University of California Museum of Paleontology (UCMP). The UCMP online database has no obvious hits, which doesn't necessarily mean anything; the specimens may simply not be on the online database, for example. It should be noted that it is possible to mistake hadrosaur caudals for titanosaur caudals (e.g., Arkharavia, Fort Crittenden Formation "titanosaur", Parrosaurus, some of the Mexican specimens that will be covered in the next Alamosaurus post).

Stray references to Alamosaurus possibly in the Lance Formation based on an Amherst specimen (Sloan 1970; Wolberg et al. 1986) are actually reporting what would come to be named Dyslocosaurus polyonychius.

Montana Alamosaurus?

Dinosaur-field-guide-type books sometimes include Montana in their lists of Alamosaurus localities. This seems to go back to a brief reference in West Texas Geological Society (1965 and 1972 reprint) and Maxwell et al. (1967), containing the tantalizing but frustratingly incomplete information that Alamosaurus had also been found in the Hell Creek Formation of Petroleum County, Montana. (All three have the same information, except for a "Hill Creek" typo in the 1965/1972 work.) There isn't an obvious mistake in the provenance information, such as a similar county name or formation, and the specificity of county, state, and formation makes it seem like there should be *something* in back of it. While such an occurrence is certainly not impossible, there's simply no way to check the information as it stands.

(Did you know that the Normanpedia reports that Jack Horner had sauropod-like eggs from the Upper Cretaceous Willow Creek Formation of Montana? It's true, right there on page 97 of Norman [1985]. I'm not sure what to make of this; the Willow Creek Formation is an egg producer, but it's in Alberta, Canada, not Montana, and Zelenitzky et al. [2016] reported no sauropod eggs in the genuine Willow Creek Formation.)

Addendum to New Mexico Alamosaurus

A couple of things I missed from the previous post:

As mentioned in this comment by Michael Habib, there are some recent abstracts concerning San Juan Basin Alamosaurus material reposited at the Natural History Museum of Los Angeles County (LACM)  (Sander et al. 2014; Bansal and Habib 2017; Habib 2018). Sander et al. (2014) noted the first record of a potential pneumatic cavity in a sauropodomorph pubic bone (LACM 156591). Bansal and Habib (2017) and Habib (2018) discussed a series of cervicals with ossified tendons (LACM 157750 in the second abstract). They interpreted the neck as being elevated, perhaps by 35°, which would give this large individual a head height of 7.6 m (25 ft). The structure of the neck also suggests that the animal was relatively long-limbed for a sauropod (Habib 2018).

Finally, in the interest of completeness, I should mention that there has been some discussion of Paleocene Alamosaurus in the San Juan Basin (e.g., Fassett et al. 2011). This has not been widely accepted, to put it mildly (if you want, you can follow the back and forth on that article). Quite apart from any of the technical arguments, I find the premise to be unlikely on the grounds of titanosaur reproductive biology. If there was a breeding population of titanosaurs in the San Juan Basin at 64.8 ± 0.9 million years ago, between approximately three hundred thousand and 2.1 million years after the end-Cretaceous event of 66.0 million years ago, what is supposed to have kept them from being back in business altogether? A pitiful remnant hanging on for a few decades or centuries in marginal refugia is one thing, but we're talking about a lineage of egg-laying machines. (Furthermore, if they'd stuck around that long in the San Juan Basin, they should have had an easier time in places farther from Chicxulub, such as the entire Eastern and Southern Hemispheres.)


Bansal, S., and M. Habib. 2017. Constructing the colossus: new titanosaur material from New Mexico reveals novel mechanisms for accommodating gigantism and neck elevation. Journal of Vertebrate Paleontology, Program and Abstracts, 2017:77–78.

Bray, E. S. 1999. Eggs and eggshell from the Upper Cretaceous North Horn Formation, central Utah. Pages 361–375 in D. D. Gillette, editor. Vertebrate paleontology in Utah. Utah Geological Survey, Salt Lake City, Utah. Miscellaneous Publication 99-1.

Brown, B. 1941. The age of sauropod dinosaurs. Science 93(2425):594–595.

Carrano, M. T., and M. D. D'Emic. 2015. Osteoderms of the titanosaur sauropod dinosaur Alamosaurus sanjuanensis Gilmore, 1922. Journal of Vertebrate Paleontology 35(1):e901334. doi:10.1080/02724634.2014.901334.

Curtice, B. 2017. Remembering the Alamosaurus: Jensen relocates Gilmore's Alamosaurus quarry, USNM 15560, North Horn Formation, Emery County, Utah, and discovers a second individual Alamosaurus. Journal of the Arizona-Nevada Academy of Science 47(1):1–5. doi:10.2181/036.047.0101.

D'Emic, M. D., J. A. Wilson, and T. E. Williamson. 2011. A sauropod dinosaur pes from the latest Cretaceous of North America and the validity of Alamosaurus sanjuanensis (Sauropoda, Titanosauria). Journal of Vertebrate Paleontology 31(5):1072–1079.

Fassett, J. E., L. M. Heaman, and A. Simonetti. 2011. Direct U-Pb dating of Cretaceous and Paleocene dinosaur bones, San Juan Basin, New Mexico. Geology 39(2):159–162.

Gilmore, C. W. 1938. Sauropod dinosaur remains in the Upper Cretaceous. Science 87(2257):299–300.

Gilmore, C. W. 1946. Reptilian fauna of the North Horn Formation of central Utah. U.S. Geological Survey, Washington, D.C. Professional Paper 210-C. (line illustrations are better in the USGS pdf, but photos are better in this Google Books scan.)

Habib, M. 2018. Standing tall: new titanosaur material from New Mexico provides insights into the posture and gait of giant Late Cretaceous sauropods. Journal of Vertebrate Paleontology, Program and Abstracts, 2018:139.

Huene, F. von. 1929. Los Saurisquios y Ornitisquios del Cretáceo Argentino. Anales del Museo de La Plata 3:1–196.

Jensen, J. 1966. Dinosaur eggs from the Upper Cretaceous North Horn Formation of central Utah. Brigham Young University Geology Studies 13:55–67.

Lehman, T. H. 1981. The Alamo Wash local fauna: a new look at the old Ojo Alamo fauna. Pages 189–221 in S. G. Lucas, J. K. Rigby, Jr., and B. S. Kues, editors. Advances in San Juan Basin paleontology. University of New Mexico Press, Albuquerque, New Mexico.

Lehman, T. M. 1987. Late Maastrichtian paleoenvironments and dinosaur biogeography in the western interior of North America. Palaeogeography, Palaeoclimatology, Palaeoecology 60:189–217.

Lucas, S. G. 1981. Dinosaur communities of the San Juan Basin; a case for lateral variations in the composition of Late Cretaceous dinosaur communities. Pages 337–393 in S. G. Lucas, J. K. Rigby, Jr., and B. S. Kues, editors. Advances in San Juan Basin paleontology. University of New Mexico Press, Albuquerque, New Mexico.

Lucas, S. G., and A. P. Hunt. 1989. Alamosaurus and the sauropod hiatus in the Cretaceous of the North American western interior. Pages 75–85 in J. O. Farlow, editor. Paleobiology of the dinosaurs. Geological Society of America, Boulder, Colorado. Special Paper 238.

Maxwell, R. A., J. T. Lonsdale, R. T. Hazzard, and J. A. Wilson. 1967. Geology of Big Bend National Park, Brewster County, Texas [enormous file!]. University of Texas Bureau of Economic Geology Publication 6711.

May, P., M. Fair, B. Crawford, A. May, and M. Macleod. 2012. Digital development and mounting of an Alamosaurus skeleton for the Perot Museum of Nature and Science. Journal of Vertebrate Paleontology 32(Supplement):138.

Norman, D. 1985. The illustrated encyclopedia of dinosaurs. Crescent Books, New York, New York.

Sampson, S. D., and M. A. Loewen. 2005. Tyrannosaurus rex from the Upper Cretaceous (Maastrichtian) North Horn Formation of Utah: biogeographic and paleoecologic implications. Journal of Vertebrate Paleontology 25(2):469–472.

Sander, P. M., J. Hall, J. Soler, M. Wedel, and L. Chiappe. 2014. A pneumatic cavity in an Alamosaurus pubis: the first evidence of pubic pneumaticity in sauropodomorphs and the implications of pelvic pneumaticity in neosauropods. Journal of Vertebrate Paleontology, Program and Abstracts, 2014:220.

Sloan, R. E. 1970. Cretaceous and Paleocene terrestrial communities of western North America. North American Paleontological Convention. Proceedings, part E:427–453.

Tykoski, R. S., and A. R. Fiorillo. 2017. An articulated cervical series of Alamosaurus sanjuanensis Gilmore, 1922 (Dinosauria, Sauropoda) from Texas: new perspective on the relationships of North America’s last giant sauropod. Journal of Systematic Palaeontology 15(5):339–364. doi:10.1080/14772019.2016.1183150.

West Texas Geological Society. 1965 (reissued 1972). Geology of the Big Bend area, Texas: field trip guidebook with road log and papers on natural history of the area. West Texas Geological Society, Midland, Texas. Publication 65-51 reissued as 72-59.

Wolberg, D. L., R. P. Lozinsky, and A. P. Hunt. 1986. Late Cretaceous (Maastrichtian–Lancian) vertebrate paleontology of the McRae Formation, Elephant Butte area, Sierra County, New Mexico. New Mexico Geological Society Guidebook 37:227–334.

Zelenitzky, D. K., F. Therrein, K. Tanaka, P. J. Currie, and C. L. DeBuhr. 2016. Latest Cretaceous eggshell assemblage from the Willow Creek Formation (upper Maastrichtian–lower Paleocene) of Alberta, Canada, reveals higher dinosaur diversity than represented by skeletal remains. Canadian Journal of Earth Sciences 54(2):134–140. doi:10.1139/cjes-2016-0080.


  1. The thought of titanosaurs and *just* titanosaurs surviving the Cretaceous among non-avian dinosaurs does make me wonder: would sauropod survival have seriously changed the course of mammal evolution? I can imagine that any post-KT environment that would have included sauropods -let alone ones on the scale of *Alamosaurus*- would have evolved along a very different trajectory than it did in real life. Would large herbivorous animals have had evolved in such an environment, you think?

  2. ...meaning *large herbivorous MAMMALS*, obviously.

    1. That's been the general thought, that there wasn't really an obvious way to get large herbivorous mammals with large herbivorous dinosaurs around. It does depend on the definition of "large", though. We can get different answers if we play around with the post-KT situation: say the large herbivorous dinosaurs only survive the immediate event in one area, such as the San Juan Basin, and then spread out; or some lineages go but others stay (let's say just titanosaurs make it). Now you've got a situation where large mammals can evolve in other areas and only later encounter large dinosaurs. I'm sure someone has used this scenario, but an obvious choice is to have dinosaurs survive only on India, and then spread across the other continents after a period of isolation. You would have abelisaurs and titanosaurs radiating from one area while early Cenozoic mammals are diversifying. (As for the titanosaurs themselves, they seem to have been great at speciating. Make them the only herbivores larger than a metric ton, and they probably would have taken full advantage in a geologically short period of time.)

  3. Could it be remotely possible that a small population could survive a short time after the extinction event in an isolated refugia, then due to a lack of predators devour the refugia before the rest of the continent had time to recover? Maybe their rapid reproduction and ability to wander far and wide worked against them in a situation with limited resources and a short amount of time to adapt with a smaller size and slower reproductive rate?

    1. But then shouldn't it have taken far less time than several hundred thousand to a couple of million years to eat through the refugia? Also, if it's a case of an ecological squeeze, we know that titanosaurs can go down to Magyarosaurus-sized in a pinch. It's not known how long that would take (might be quite quick given large dinosaurs began reproducing at much less than maximum size), but elephants on islands can dwarf in a very brief time span. Columbian mammoth to Channel Islands pygmy mammoth would easily fit, perhaps several times over, in the San Juan Basin time frame, and mammoths reproduced much more slowly than titanosaurs. They wouldn't fill up the landscape as quickly, but they also would not have been able to respond to changing conditions as rapidly.

    2. Logically, there would have been a point in time after the Chicxulub impact when, if Alamosaurus sanjuanensis had persisted that long, the worst would have been over and its lineage would have continued into the future until felled by some unrelated event. There's no way to know when this date would have been, but my feeling is that it likely would have occurred before 65.7 Ma and definitely before 63.9 Ma (the error bars on Fassett et al.'s date).

    3. I agree completely. The one caveat is that I'd be cautious about how well we understand how extinction works. Especially in terms of genetic bottlenecks and what kind of demands on an ecosystem a (very) large animal exerts and the community it takes for them to prosper. We still don't really understand an extinction event 10000 years BP let alone 65 million BP. Unfortunately I think over the next 1000 years humans are going to get some really great data about the effects of inbreeding and climate change on extinction rates.

      As an 'outsider' I don't have a good grasp of how robust these claims about fossils found above the impact layer are. I've heard there's hadrosaur remains found and Hell Creek deposits up to 30,000 years after the event but they're usually written off as reburials. Makes sense especially if they're scattered eroded individual bones, I imagine it would take at least a partially complete animal to provide overwhelming evidence.