The Ballad of Atlantosaurus

"Come gather 'round me, people, and listen to the tale

Of Marsh's first big sauropod, its bones now rest at Yale

Atlantosaurus montanus, from Morrison, C-O,

Known from but a sacrum, it was famous long ago"

The majestic Atlantosaurus, next to an elephant that strangely seems to have arrived via 19th century Photoshop. Shamelessly appropriated from Wikipedia, appropriated in turn from De Wereld vóór de schepping van den mensch, Nicolas Camille Flammarion (1886), original title "Le Monde avant la création de l’homme" (1886).

More gone than Morosaurus. More disappeared than Dystrophaeus. About the only Morrison sauropods with less press these days are Caulodon, one of Cope's innumerable tooth taxa; Dystylosaurus, the Gummo Marx of '80-'90s Morrison dinosaurs; and Uintasaurus, a Camarasaurus from Dinosaur National Monument that got to be special for a while. Yet Atlantosaurus was considered a viable genus for years, receiving at least lip service as the pater familias of Atlantosauridae/Atlantosaurinae into the early 1980s. Go ahead, check your copy of Glut 1982; I'll wait. (While you're there, ponder that as recently as the early 1980s it was still a going thing to classify sauropods in two groups based on having peg-like or spoon-like teeth, shades of coelurosaurs and carnosaurs.) So, what is Atlantosaurus and what happened to it?

The first notice of Atlantosaurus comes from O. C. Marsh, who in July 1877 issued a brief notice to inform the world of a "new and gigantic dinosaur" from the "Cretaceous" of Colorado (Arthur Lakes' Quarry 1 at Morrison, Colorado [Ostrom and McIntosh 1966], relocated in 2009). (The publication also informed the world he had just renamed the preoccupied Laelaps to Dryptosaurus, which he accomplished with a footnote.) The new gigantic dinosaur, which he named Titanosaurus montanus, was known from remains including a partial sacrum and a femur (Marsh 1877a). The reader gets the impression that Marsh didn't know much of anything about his Titanosaurus, given that he considered it a possible "distant ally" of Hadrosaurus agilis (later known as Claosaurus, but still decidedly on the hadrosaur side of things). Unbeknownst to Marsh, he'd been pipped at the post by Richard Lydekker, who had just named a sauropod from India Titanosaurus indicus (Lydekker 1877). Having surely been taught a lesson on the dangers of not publishing fast enough, and with admirable speed for the 1870s, Marsh published the replacement name Atlantosaurus before the end of the year (Marsh 1877b). This article, incidentally, also coins Apatosaurus ajax, Apatosaurus grandis (now Camarasaurus grandis), Allosaurus fragilis, and, er, Nanosaurus rex, the ex-Othnielia; three hits out of four ain't bad. If you're looking for piercing insight, thorough description, or even illustrations, you'll have to go into hibernation for a couple of decades' worth of papers: he described the four new genera and species within three pages. There are a few points of interest for our topic, though: Marsh now recognizes the rocks as Jurassic in age; he is now calling the rocks the Atlantosaurus beds, due to the abundance of Atlantosaurus; he coins the family Atlantosauridae; and he considers Atlantosaurus the "largest of land animals", reaching "at least eighty feet long" if proportioned like a crocodile. Although we know now that sauropods did not have the proportions of crocodiles, 80 ft (24 m) is not actually a bad guess for a large Morrison sauropod, so give him credit for being right for the wrong reason. After Marsh renamed Titanosaurus montanus, Titanosaurus was never again involved in any taxonomic controversies. [whistles innocently]

The type sacrum of Atlantosaurus montanus (YPM 1835) in ventral view (underside). Plate XVII, Marsh (1896) (pay attention, because #2 is actually above #1 on this plate).

Where were we? Oh, yes. Marsh was not satisfied with there being only one species of Atlantosaurus, so in 1878 he gave the world A. immanis (Marsh 1878a), from Lakes Quarry 10 at Morrison (Ostrom and McIntosh 1966). This article, incidentally, is not one of his more memorable works. Creosaurus atrox is probably the most famous taxon it names, because it has been adopted by Allosaurus fragilis skeptics since the 1980s as the symbol of their alternate allosaur. Otherwise, you're looking at the luminaries Morosaurus impar, Allosaurus lucaris, Laosaurus gracilis, and L. celer. A. immanis was represented by at least a femur and caudal vertebrae. The femur caused Marsh to once again produce his remarkable crocodile in order to estimate a length of 115 ft (35 m). If you aren't impressed by comparisons that turned out to be irrelevant, the femur is of a decent size (98 in/2,500 mm long). Within a few months, A. immanis had shrunk to a more manageable 80 ft (24 m), and Atlantosauridae had lost the limelight to his shiny new suborder Sauropoda (Marsh 1878b), although he did keep using the family, which had expanded to embrace Apatosaurus and Brontosaurus before he died (Marsh 1898). There Atlantosaurus rested for a time.

Atlantosaurus immanis: a pubis and ischium, and four views of the left femur. Plate XVI, Marsh (1896).

By 1896, Atlantosaurus montanus was just a sacrum (and maybe a femur; YPM 1835), and A. immanis included a femur (now shrunk to "over 6 feet"), pubis, and ischium. The type material for these old species was always changing, it seems. Marsh also had a braincase that the text implies is from the Morrison (Colorado) sites as a whole, but which a plate caption assigns to A. montanus. Go figure. The braincase is a bit of a sticking point: someone forgot to record which Morrison quarry it came from, but it's diplodocid in origin (Berman and McIntosh 1978). To make things just a bit more interesting, the types of Apatosaurus ajax (YPM 1860) and Atlantosaurus immanis (YPM 1840), both from Quarry 10, were both accidentally labeled under the catalog number for ajax; they can fortunately be distinguished by color (YPM 1860 is from a dark clay layer and has dark bones, and YPM 1840 is from a light-colored sandstone and has light bones) (Berman and McIntosh 1978). Berman and McIntosh found the ajax option for the braincase to be more likely than the immanis option, and far more likely than the montanus option. (You're supposed to use the initial of the genus with the species, i.e. A. ajax, but I'm dropping it for brevity and clarity here, with two "A." genera.)

The wayward braincase, ventral and posterior views. Plate XV, Marsh (1896).

Following Marsh, Atlantosaurus just seems to have been there after its brief shining moment in the late 1870s. It looks to me like Marsh moved on pretty quickly once he had better and more interesting material. Did even he find it to be a less-than-optimal taxon? Just going by the blunt instrument that is number of species named, his favorite sauropods were Brontosaurus and Morosaurus, and outside of those he got to name Allosaurus, Ceratosaurus, Stegosaurus, Triceratops, etc., all of which he lavished more attention on. Atlantosaurus was more of a one-hit wonder, but by the rules of priority paleontologists had to keep paying it lip service, so Atlantosauridae and Atlantosaurus itself chugged on until they were gradually set aside or forgotten. The "Atlantosaurus beds" were still cropping up into the first decade of the 20th century (Hatcher 1903; Williston 1905) before being fully replaced by the Morrison Formation. Atlantosauridae was periodically revived, although it is unclear if anyone was really taking sauropod classification seriously between about 1920 and 1975. (It took until the 1970s for Apatosaurus to be firmly recognized as a diplodocid. See Berman and McIntosh 1978; that's how badly Apatosaurus, Brontosaurus, Camarasaurus, and Diplodocus had become screwed-up.) Eventually, Atlantosaurus was little more than a footnote; it gets about a page in Glut's encyclopedia series, and a line or two in the editions of The Dinosauria, which label the type species Titanosaurus montanus according to their house rules (dubious species revert to the oldest genus name).

Serious discussion of either Atlantosaurus species has been limited to a few publications in the past few decades. Berman and McIntosh (1978) found the type sacrum of Atlantosaurus montanus to be too fragmentary to distinguish from Apatosaurus, Camarasaurus, or Diplodocus, also reported from Morrison, and interpreted A. immanis to be another individual of Apatosaurus ajax. McIntosh (1995) got another species from Quarry 10 into the act, Apatosaurus laticollis, regarding the type of that species (YPM 1861) as the same individual as the type of immanis. Upchurch et al. (2004) accepted the arguments about ajax including immanis (including laticollis). Tschopp et al. (2015) instead found immanis to be an indeterminate basal apatosaurine, and laticollis to be more closely related to Apatosaurus louisae, possibly even the senior synonym. Unfortunately, they did not include Atlantosaurus montanus. Marsh's first Morrison sauropod remains a little-remembered footnote, and the second species is some kind of apatosaurine, by all accounts.

"So now you've heard the story, now you've heard the song

About a mighty sauropod, some twenty meters long

And how its fame spread widely, but faded soon away

And left it in obscurity down to this very day"

References:

Berman, D. S., and J. S. McIntosh. 1978. Skull and relationships of the Upper Jurassic sauropod Apatosaurus (Reptilia, Saurischia). Bulletin of the Carnegie Museum of Natural History 8:1–32.

Glut, D. F. 1982. The new dinosaur dictionary. Citadel Press, Secaucus, New Jersey.

Hatcher, J. B. 1903. Osteology of Haplocanthosaurus, with description of a new species and remarks on the probable habits of the Sauropoda and the age and origin of the Atlantosaurus beds: additional remarks on Diplodocus. Memoirs of the Carnegie Museum 2:1–72.

Lydekker, R. 1877. Notices of new and other Vertebrata from Indian Tertiary and Secondary rocks. Records of the Geological Survey of India 10(1):30–43.

Marsh, O. C. 1877a. Notice of a new and gigantic dinosaur. American Journal of Science, series 3, 14:87–88.

Marsh, O. C. 1877b. Notice of new dinosaurian reptiles from the Jurassic formation. American Journal of Science, series 3, 14:514–516.

Marsh, O. C. 1878a. Notice of new dinosaurian reptiles. American Journal of Science, series 3, 15:241–244.

Marsh, O. C. 1878b. Principle characters of American Jurassic dinosaurs, part 1. American Journal of Science, series 3, 15:416–416, plates IV–X.

Marsh, O. C. 1896. The dinosaurs of North America. U.S. Geological Survey, Washington, D.C. Annual Report 16:142–230.

Marsh, O. C. 1898. On the families of sauropodous Dinosauria. American Journal of Science, series 4, 6:487–488.

McIntosh, J. S. 1995. Remarks on the North American sauropod Apatosaurus Marsh. Sixth Symposium on Mesozoic Terrestrial Ecosystems and Biota Short Papers 119–123.

Ostrom, J. H., and J. S. McIntosh. 1966. Marsh's dinosaurs: the collections from Como Bluff. Yale University Press, New Haven, Connecticut.

Tschopp, E., O. V. Mateus, and R. B. J. Benson. 2015. A specimen-level phylogenetic analysis and taxonomic revision of Diplodocidae (Dinosauria, Sauropoda). PeerJ 3:e857.

Upchurch, P., Y. Tomida, and P. M. Barrett. 2004. A new specimen of Apatosaurus ajax (Sauropoda: Diplodocidae) from the Morrison Formation (Upper Jurassic) of Wyoming, USA. National Science Museum Monographs 26:1–118.

Williston, S. W. 1905. The Hallopus, Baptanodon, and Atlantosaurus Beds of Marsh. The Journal of Geology 13(4):338–350.