The time has come to deal with the one and only named North American titanosaur, the redoubtable Alamosaurus sanjuanensis. You may remember that in the very first post of this series, I mentioned I was skipping Alamosaurus for the moment. I was waiting on it because I was concerned that it might suffer a taxonomic detonation at any time, so I thought I'd hold off as long as possible. Such has not happened (yet).
I originally thought I could get away with one post. This was before I actually sat down to the substantial body of literature on A. sanjuanensis. I ended up going through around 60 references, with another few I haven't been able to find yet due to them being dissertations or conference ephemera or some such. This number does not count the many, many cameo performances for our guest, which turns up in almost every paper dealing with titanosaurs, being cited for anatomical comparisons or included in a phylogeny. This is what comes of 1) a century of publications; 2) being the only named titanosaur in the paleontological hotbed of North America; 3) shouldering the burden of breaking the Great North American Sauropod Hiatus™; 4) being (seemingly) represented by lots and lots of specimens; and 5) having a wide geographic distribution.
In the interests of sanity (both yours and my own dwindling reserves) and length, I'm therefore splitting the material over multiple posts for February, going by geography. There are three natural geographic divisions (New Mexico, Utah, and Texas), plus some miscellaneous records of interest that should fit under that format (Wyoming and Montana with Utah, and Mexico with Texas), so I think three posts will take care of things. We'll start off with New Mexico, because chronologically that's where the story begins.
(Of course, in March someone will publish a paper that will undo everything.)
San Juan Basin Alamosaurus
Let's get this out of the way up front: Alamosaurus sanjuanensis is not named for the Alamo in San Antonio, Texas. Although there is indeed material referred to A. sanjuanensis from Texas (heaps of it, in fact), it is all from the Big Bend region, which is nowhere near the fort, and it wasn't discovered until nearly 20 years after the genus had been named from New Mexico. However, the Spanish root word is the same: álamo, which means "poplar" and in both cases ultimately goes back to cottonwood trees, within the genus Populus.
|USNM 10486, where it all starts: the holotype left scapula of Alamosaurus sanjuanensis (Plate 1 in Gilmore 1922).|
Alamosaurus sanjuanensis is based on fossils discovered by John B. Reeside of the U.S. Geological Survey in June 1921 (Gilmore 1922). The holotype, a mostly complete left scapula measuring a healthy 155 cm long (61 in) as preserved (Gilmore 1922), was briefly mentioned a year before its formal description (Gilmore 1921). For Charles W. Gilmore's formal description in 1922, the scapula was joined by a right ischium found about 60 m away (200 ft). The holotype scapula is catalogued as USNM 10486 (National Museum of Natural History, Washington, D.C.), and the ischium, designated the paratype, is USNM 10487. Gilmore thought it was possible that the two bones came from the same individual, but conceded there was no way to be sure. To be honest, his placing the two in the same species was a hunch, given that neither the scapula nor the ischium were particularly similar to the Morrison sauropods he was familiar with. Later discoveries would prove he'd been right on the money.
|And the somewhat less acclaimed paratype right ischium, USNM 10487 (Plate 2 in Gilmore 1922).|
The location and stratigraphy were described as "Barrel Spring Arroyo [=De-na-zin Wash], south of Ojo Alamo [Trading Post], San Juan County, New Mexico", in the Ojo Alamo Formation (Gilmore 1922). Gilmore did not provide an etymology, but we can guess that the genus and species were intended as something like "Ojo Alamo lizard from San Juan County"; I considered "San Juan Basin" as an alternative possibility for the species name, but Gilmore did not mention this feature in the publication, so it doesn't seem to have been on his mind. Those of you who have some familiarity with the San Juan Basin rocks will know that there has been a long back-and-forth about whether the rocks in question, now identified as the Naashoibito Member, are better classified with the Ojo Alamo Formation or the underlying Kirtland Formation (or Shale). The pendulum is currently on the Ojo Alamo side, so we'll stick with that.
Genuine San Juan Basin Alamosaurus has only been found in the Naashoibito Member of the Ojo Alamo Formation (Jasinski et al. 2011), in which it is a charter member of the Alamo Wash local fauna (recent assessment in Jasinski et al. 2011). Most of the Alamo Wash dinosaur fossils reported to date are not diagnostic to genus or species, but aside from Alamosaurus they represent a typical Late Cretaceous North American assemblage. Of those that have been given a name, Alamosaurus sanjuanensis is best-represented, followed distantly by the nodosaur Glyptodontopelta mimus and the ceratopsid Ojoceratops fowleri. The Naashoibito Member dates to the Maastrichtian (Fowler and Sullivan 2011; Jasinksi et al. 2011). It is at most 30 m thick (100 ft) (Jasinski et al. 2011) and is thought to represent a geologically short span of deposition, on the order of 160,000 to 193,000 years (Jasinski et al. 2011), or perhaps 500,000 years (Fowler and Sullivan 2011). At times, fossils from the underlying De-na-zin Member of the Kirtland Formation have been attributed to Alamosaurus, which would extend the age of the genus several million years into the Campanian (Lucas and Sullivan 2000; Sullivan and Lucas 2000). Restudy of the specimens and their localities established that all such records were either incorrectly identified as Alamosaurus or had incorrect geologic determinations (Williamson and Weil 2008).
Returning to Gilmore in 1922, he had one scapula and one ischium of sauropod affiliation, and that doesn't give a lot of material for anatomical discussion. (The meager holotype has long been the ghost haunting Alamosaurus, as we'll see.) What was particularly interesting for Gilmore was the definite existence of a Late Cretaceous sauropod, in genuine Upper Cretaceous rocks with ankylosaurs, ceratopsians similar to Triceratops, theropods as large as Tyrannosaurus, and hadrosaurs. "But wait!" you interject. "What about the Late Cretaceous titanosaurs that were named before 1922, like Titanosaurus indicus and Titanosaurus australis and Argyrosaurus superbus, etc.? And why did you switch from Late to Upper?" (Second one's easy: it's Late or Early when you're talking about age, and Lower or Upper when you're talking about the stratigraphic position of the rocks. Sometimes you capitalize, sometimes you don't; don't ask.) I hesitate to generalize, but there seems to have been a strong reaction against the possibility of Late Cretaceous sauropods by American paleontologists, perhaps because they were so used to them only being in the Jurassic. For example, Barnum Brown did not come around to the idea of Cretaceous sauropods until being face to fossil with the Glen Rose tracks in the late 1930s and Texas Alamosaurus in 1940, and even into the 1950s found it hard to accept that there were sauropods at the end of the Mesozoic despite having excavated examples (Langston et al. 1989). This is why Gilmore felt it necessary to close the description of Alamosaurus sanjuanensis with a review other records of Late Cretaceous sauropods and note that it was "very probable" that at least some of them would turn out to be correct. (Spoiler: he was right.)
Although the holotype and paratype of Alamosaurus sanjuanensis amounted to just two bones, they have been joined by many others from the San Juan Basin. The oldest collected specimen predates the holotype by a few years, but was not reported until Gilmore (1946). It is a caudal centrum and neural spine (USNM 15658) collected by Reeside in 1916 from low in the Ojo Alamo Formation in the same area as the type material. At about the same time that Reeside was collecting the type material, Charles H. Sternberg was also in the area, and procured a small number of sauropod specimens that he sold to Carl Wiman of Uppsala University, Uppsala, Sweden. Mateer (1976) treated the specimens as topotypes (additional specimens from the holotype's locality), but other authors have regarded this with caution, because there is no guarantee they indeed came from the same locality based on the existing provenance information (Lucas and Sullivan 2000; D'Emic et al. 2011). Poropat (2013) reevaluated these fossils and found them to include a partial left ilium (PMU 24805; Museum of Evolution, Uppsala University), two-and-a-half sacrals (PMU 24803), and an elongate but dorsoventrally compressed middle cervical broadly similar to those of Futalognkosaurus (PMU 24804). Just a few years later, the 1924 Amherst College expedition to the San Juan Basin picked up additional fossils, including a nearly complete humerus of A. sanjuanensis that later ended up at the Springfield Science Museum in Springfield, Massachusetts (Dalman and Lucas 2016).
This is kind of the history of San Juan Basin Alamosaurus in a nutshell: lots and lots of isolated remains, not a whole lot to date articulated or associated (although Kues et al. 1980 did suggest that the specimens collected by Reeside and Sternberg may have belonged to the same individual). There are individual bones representing practically the entire skeleton, but it's difficult to say anything that relies on proportions within an individual skeleton. There is also the question of whether or not it's all really A. sanjuanensis. To date, no one has reported any features indicating more than one species of sauropod at the end of the Cretaceous in the San Juan Basin (Jasinski et al. 2011), but there's always that little spark of doubt that comes of dealing with isolated specimens. Lists of specimens have been published several times (Lucas et al. 1987; Lucas and Sullivan 2000; Jasinski et al. 2011). The most recent list, in Jasinski et al. (2011), included 48 catalog numbers, which in the interest of space I will simply summarize as follows: teeth, possible skull material, cervicals, dorsals, sacrals, caudals, ribs, a possible chevron, scapulae, a coracoid, ilia, ischia, pubic bones, humeri, a radius, femora, tibiae, fibulae, most of a foot, and fragments. They are primarily at the New Mexico Museum of Natural History and Science (NMMNH, Albuquerque; including former University of New Mexico collections and specimens collected from the San Juan Basin by University of Arizona personnel) or the State Museum of Pennsylvania (SMP, Harrisburg), with a few at the Smithsonian or Uppsala, as we've previously encountered.
Many of the specimens have not been described in great detail, although see Lucas and Sullivan (2000) for a review of those specimens known by that point. Aside from the type and paratype, exceptions include the potential topotypes held in Uppsala and mentioned above (Poropat 2013), teeth reported by Kues et al. (1980), a foot described by D'Emic et al. (2011), and some notably large specimens reported by Fowler and Sullivan (2011). The teeth described by Kues et al. are all incomplete, but are clearly cylindrical. They have round cross-sections with diameters of 8 to 11 mm (0.3 to 0.43 in) and resemble Nemegtosaurus teeth in several features of the crown. The foot, part of NMMNH P-49967, is mostly complete, only lacking a couple of phalanges, making it one of the best titanosaur feet known. Overall it is broad, with long claws flattened side-to-side present on the first three toes, and apparently no more than two phalanges per toe. Unlike a titanosaur's hand, in which the metacarpals are arranged in a circle to form a columnar structure, the metatarsals articulate in a slight curve (D'Emic et al. 2011).
|Here's the foot, on display at the New Mexico Museum of Natural History and Science (photographed at the 2018 Society of Vertebrate Paleontology annual conference).|
D'Emic et al. (2011) hedged by assigning the specimen to ?Alamosaurus sanjuanensis, on the grounds that it wasn't directly comparable to the type specimen or other specimens that *are* comparable (a position that makes their Alamosaurus sanjuanensis more conservatively defined than Jasinski et al. 2011, who regarded all of the San Juan Basin material as one species). This seems like a logical place to bring up the validity of the genus and species. Alamosaurus sanjuanensis hasn't always been regarded favorably; for example, it is written off as a dubious titanosaur in the Normanpedia (Norman 1985). Lucas and Sullivan (2000) regarded the holotype scapula as non-diagnostic and A. sanjuanensis as a nomen vanum (meaning that the type is not diagnostic; sorry for the tautology). However, they recommended keeping the name around as a form taxon "for all North American Late Cretaceous sauropods" (p. 150). D'Emic et al. (2011) instead concluded that the type scapula is diagnostic based on several features, and found it to match the scapula of a more complete Utah specimen we'll meet next time, allowing them to assign several other New Mexico and Utah specimens to the taxon (with the potential for more based on further review).
A few of the New Mexico specimens represent very large individuals, including a cervical (SMP VP-1850) comparable in size to a Puertasaurus cervical (Fowler and Sullivan 2011); the distal third of a femur (SMP VP-1625) estimated at perhaps 185 cm long (72.8 in) if complete (Fowler and Sullivan 2011), and used in Paul (2019) to estimate a 27 metric ton (30 US ton) animal; and an anterior caudal (SMP VP-2104) comparable to a Futalognkosaurus caudal (Fowler and Sullivan 2011), which Paul (2019) regarded as suitable for an animal 15% larger than the one he modeled. The foot described in D'Emic et al. (2011) is also quite large. If A. sanjuanensis followed typical titanosaurian proportions, it belonged to an animal with a femur about 2 m long (7 ft) (D'Emic et al. 2011).
|The partial cervical was the basis for this reconstruction at the Badlands Dinosaur Museum in Dickinson, North Dakota.|
McRae Formation Alamosaurus
In addition to the San Juan Basin of northwestern New Mexico, a few bones described as Alamosaurus sp. or likely Alamosaurus have been recovered from the McRae Formation of Sierra County, southwestern New Mexico. These include humerus TKM007 and a 168-cm-long (66 in) femur, TKM009 (Lozinski et al. 1984) ("TKM" is a locality description; it's not clear from the publications where the specimens are reposited). The McRae Formation is notable as one of the formations where both Alamosaurus and Tyrannosaurus rex bones have been found (Lozinsky et al. 1984; Wolberg et al. 1986), allowing the more bloodthirsty among us to imagine dramatic encounters of T. rex and industrial-sized sauropods. ("Hey, kids! T. rex couldn't fight Brontosaurus, but here's the next best thing!") Stratigraphically, the specimens are from the Hall Lake Member (Lucas and Hunt 1989).
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