Your Friends The Titanosaurs, part 3: Andesaurus, Antarctosaurus, and Argentinosaurus

Plenty of name recognition as far as titanosaurs go in this post of "Your Friends The Titanosaurs", which features Argentinosaurus huinculensis as leading contender for the dinosaur heavyweight crown, Antarctosaurus wichmannianus as one of the most historically important and troublesome titanosaurs, and the somewhat less well-known but supremely steady Andesaurus delgadoi. Three questionable species of Antarctosaurus have been held over for next time.

Andesaurus delgadoi

Like most titanosaurs, Andesaurus delgadoi is known from a single specimen, although some authors have occasionally placed other specimens in the genus (discussed at the end of the section). A. delgadoi was named by Calvo and Bonaparte in 1991 for MUCPv 132 (Museo de Geología y Paleontología de la Universidad Nacional del Comahue, Neuquén, Argentina), a partially articulated skeleton including four dorsal vertebrae, two sacral verts, 25 caudal verts, ribs and chevrons, the left pubis, both ischia, the right humerus and first metacarpal, the left fifth metacarpal, and the left femur (Calvo and Bonaparte 1991; amended by Mannion and Calvo 2011). The specimen was found near Villa El Chocón, eastern Neuquén Province, west-central Argentina (Calvo and Bonaparte 1991). At the time, the stratigraphy was given as the lower Candeleros Member of the Río Limay Formation (Calvo and Bonaparte 1991), but the Candeleros has since been upgraded to a formation in its own right, which I currently have listed as Early Cenomanian in age, at the onset of the Late Cretaceous.

Thirty-one vertebrae and a few limb and girdle bones is not a bad haul for a titanosaur, and show some distinctive features. The dorsal verts have unusually tall neural arches for macronarians, with the whole arch making up three quarters of the total height of the most complete dorsal and the neural spine being more than twice the height of the centrum (Calvo and Bonaparte 1991; Mannion and Calvo 2011). We're not into "fin-back" territory here, but it's something different for a titanosaur. The dorsals also have what is known as hyposphene–hypantrum articulation (you can switch the hypo–hypa around if you so desire). This is a kind of accessory vertebral articulation in which a projection on the posterior surface of a neural spine above the neural canal (the hyposphene) fits into a matching recess on the anterior surface of the following neural spine (the hypantrum). SV-POW has some discussion of this articulation system here and here, featuring Argentinosaurus. Why this is important is because this system is found in most other sauropods except rebbachisaurids and, importantly for our purposes, almost all titanosaurs, which came up with other ways of doing something similar (Apesteguía 2005). Unlike most titanosaurs, the anterior caudals are only ever-so-slightly procoelous (socket in front, ball in back), grading to slightly amphicoelous (sockets on both ends) farther along the tail (Mannion and Calvo 2011). The limb bones and girdle bones are rather less distinctive and more typically titanosaurian. Calvo and Bonaparte (1991) regarded the type individual as of large size, with the femur estimated at 1.55 m (5.09 ft) long. Mannion and Calvo (2011) did not attempt to estimate the length of the femur or humerus, both of which are only partially complete and represented in part by natural molds. Both Calvo and Bonaparte (1991) and Mannion and Calvo (2011) noted that the humerus was gracile.

By the modern definition of Titanosauria as all sauropods within the clade formed by Andesaurus delgadoi and Saltasaurus loricatus, A. delgadoi has to be a titanosaurian, and fortunately it has turned out to be an ideal anchor. Even though the relative positions of every titanosaurian are extremely unstable, Andesaurus somehow keeps more or less the same sauropods within Titanosauria in almost every phylogenetic analysis. [Note added 2018/08/19: well, yeah, there's the Haestesaurus business, but we're none of us perfect.] It seems to have gotten this position through accidents of history. When first described, it was regarded by Calvo and Bonaparte (1991) as an unusual primitive titanosaurid, worthy of its own subfamily Andesaurinae. Bonaparte and Coria (1993) upgraded this to Andesauridae within Titanosauria for Andesaurus, fellow post subject Argentinosaurus, and Epachthosaurus. Andesauridae has dwindled to nothing but Andesaurus, but with Andesaurus well-established in the minds of researchers as a prime example of a basal titanosaur, it's a reasonably natural progression to define Titanosauria with Andesaurus as an anchor. There was some weirdness in the late 1990s into the early 2000s as certain clades became favored and others were abandoned or redefined. The most unusual for Andesaurus was Sanz et al. (1999), which found it to clade with Haplocanthosaurus in "Titanosauroidea" outside of Titanosauria. (Titanosauroidea is basically abandoned now. The original definition is the same as Somphospondyli, and the redefined version [Salgado 2003], which includes all titanosaurs more closely related to Saltasaurus than Andesaurus, is more or less redundant with Titanosauria when Andesaurus is the only titanosaur it does not cover. What did Andesaurus ever do to anyone to deserve being excluded by itself?)

Finally, a few specimens have been assigned to Andesaurus, but are not currently considered to represent the genus. These include: a partial pelvis and some caudals from the same area as the type specimen (Calvo 1999), now considered to belong to an undetermined titanosauriform (Mannion and Calvo 2011); and a chunk of tail (nine caudals plus seven and a half chevrons; MMCH-Pv 47, Museo "Ernesto Bachmann" Villa El Chocón) from near El Chocón (Otero et al. 2006), now considered to belong to an undetermined titanosaur (Mannion and Calvo 2011; Otero et al. 2011).

Antarctosaurus wichmannianus

Being a child of the 1980s, Antarctosaurus was literally the second titanosaur I knew of. (It was in one of those "100 Dinosaurs from A to Z"-type books, and came after Alamosaurus. That's how I can be so exact.) It was present in all the best pop dinosaur books, almost invariably represented by a reproduction or knock-off of Friedrich von Huene's skull restoration. This was really kind of a humbug because all there was to go on was part of the mandible and the braincase, and von Huene had done it circa 1929 when there was something like a grand total of four other kinds of sauropod skulls for models. Antarctosaurus is an outstanding example of a dinosaur that was once widely known, but has fallen into obscurity because it was supplanted by more complete finds; kind of a "placeholder", like Hadrosaurus but without the legacy.

Antarctosaurus wichmannianus was for many years both one of the most completely known titanosaurs (which in hindsight isn't saying much) and essentially the only one with skull remains. The visibility of the genus Antarctosaurus was further boosted by the assignment of several more species from South America and Asia, which we'll be encountering later. The genus and species are founded on bones collected Dr. Ricardo Wichmann circa 1912–1916 from the south bank of the Río Negro near the city of General Roca in Río Negro Province, Argentina. Today they are catalogued as MACN 6904 (Museo Argentino de Ciencias Naturales Bernardino Rivadavia, Buenos Aires). They include the braincase and partial lower jaw as well as a partial cervical, rib fragments, left scapula, right humerus, radius and ulna fragments, metacarpals, fragments of the ilia and pubis, right ischium, left femur, tibia, and fibula, metatarsals, and phalanges (Huene 1929; McIntosh 1990; Powell 2003), plus maybe a caudal that von Huene (1929) tentatively attributed to Laplatasaurus (Novas 2009). "Ankle bones" mentioned by von Huene (1929) are apparently not ankle bones (McIntosh 1990; Powell 2003). The stratigraphy is not nailed down, but the fossils are generally thought to have come from the Anacleto Formation, a Campanian-aged unit.

Wichmann gave notice of his find in 1916, but it was not formally described until von Huene included it in his monster 1929 monograph on dinosaurs from the Cretaceous of Argentina. Although Andesaurus and Argentinosaurus have fairly self-explanatory names, Antarctosaurus is a bit different. It doesn't mean "dinosaur from Antarctica", but basically "not northern dinosaur". Von Huene honored Wichmann in the species name, although whenever anyone chooses the legitimate but rather unfortunate -ianus ending I have the nagging suspicion that perhaps there's some sarcasm or passive-aggressive feelings involved. (Maybe I'm just juvenile. Maybe it's different in German.)

For much of the 1990s and early 2000s, A. wichmannianus was the subject of speculation that either it wasn't a titanosaur at all, or that the type material was actually composed of mixed material from at least two sauropods, one a titanosaur and the other a diplodocid/oid (or nemegtosaurid, given that the world was not ready at that time for Nemegtosaurus to be a titanosaur). Matters were not helped by the absence of any kind of quarry map to show the original positions of the bones. The controversy began in the wake of challenges to the very simple pre-cladistics conceptions of sauropod classification (McIntosh 1990), and received a lot of fuel from the description of Malawisaurus (Jacobs et al. 1993), because the putative A. wichmannianus jaw and teeth were quite a bit different from those of Malawisaurus. The A. wichmannianus material includes pencil-like teeth limited to the front end of the jaw, which has a squared-off front. This is quite like the jaws of diplodocoids in general and, as it turned out, rebbachisaurids in particular.

Jacobs et al. (1993) proposed that A. wichmannianus was in fact a diplodocid. From then on through the next ten years, it was widely thought that A. wichmannianus was not a plain old titanosaur. At the same time, it was also a frequent participant in the controversy over whether Nemegtosaurus and Quaesitosaurus were titanosaurs or diplodocoids, thanks to its nemegtosaur-like jaws. A couple of highlights from these years include: Salgado and Calvo (1997), which kept it with the titanosaurs, but revamped von Huene's restoration to give it a brachiosaur-like look, including a prominent nasal arch; and Upchurch (1999)'s support of the chimera hypothesis. In 2004, though, Apesteguía described Bonitasaura salgadoi, a new small titanosaur that featured, among other things, a squared-off lower jaw with pencil-like teeth limited to the front end. This was followed in 2013 by Brasilotitan nemophagus, which also had a squared-off lower jaw with pencil-like teeth limited to the front end (Machado et al. 2013). The three species may be related (Machado et al. 2013).

In hindsight, people were handicapped by the limited comparative sample, the misfortune that Nemegtosaurus and Quaesitosaurus were disembodied heads, and the fact that von Huene's questionable reputation where bonebeds are concerned makes it easy to throw darts at his assignments of fossils (and we will see this come up again). We can regret a certain lack of imagination that titanosaurs might have had many different kinds of heads, but it's unfair to make an argument out of it; people are supposed to approach questions like this on the basis of what is known, not what might be possible. What is more regrettable is that paleontologists seem to have only been interested in A. wichmannianus when it was an apparent chimera and non-titanosaur nemegtosaurs were still in play; there seems to have been a collective "Oh" with the discovery of Bonitasaura, followed by radio silence. It doesn't even get invited to phylogenetic analyses. Gallina and Apesteguía (2011) did provide a newer skull restoration which follows a genuine titanosaur, Rapetosaurus. [Note added 2018/08/19: Ornithopsis in the comments pointed out Carabajal 2012, which looks at the neuroanatomy of titanosaurs including A. wichmannianus. PDF's free, too.]

On the left is Gallina and Apesteguía (2011)'s restoration; on the right the known elements are highlighted in red (by me, after Upchurch 1999).

As with Andesaurus delgadoi, there are a few specimens that have gotten attached to Antarctosaurus wichmannianus. In light of our very incomplete understanding of South American titanosaurs, Antarctosaurus and A. wichmannianus should probably be limited to MACN 6904 for the time being. A few of these other specimens have figured in the literature. A femur and tibia (FMNH [Field Museum of Natural History, Chicago] 13019 and 13120) attributed to Antarctosaurus by von Huene do not belong to this genus (Mannion and Otero 2012; the femur was used by Mazzetta et al. 2004 to calculate the weight of A. wichmannianus). A specimen (PVL 4628 [Colección de Paleontología de Vertebrados de la Fundación Instituto Miguel Lillo, Tucumán] and MACN-CH 217) initially attributed to Antarctosaurus (Bonaparte and Gasparini 1979) and later regarded as Argyrosaurus became the new genus and species Elaltitan lilloi in Mannion and Otero (2012).

Argentinosaurus huinculensis

There is no point in burying the lede: since its description in 1993, Argentinosaurus huinculensis has been the sauropod that all giant sauropods are measured against (at least those that can be directly observed, which leaves out "Amphicoelias fragillimus" and Bruhathkayosaurus), and every new giant sauropod turns out to be close to the same size or a little smaller. (This is practically a running joke at SV-POW.)  I've given my own feelings on the phenomenon of giant dinosaur measurement before (summary: people take this stuff way too seriously given the variables in play). One of the things working in its favor, oddly enough, is its incompleteness: there's enough known to get a few solid measurements, but not enough known to actually pin it down. For example, we know almost nothing about its overall proportions. At the moment of writing (2018/08/17), the "dinosaur size" article on Wikipedia cites sources for a range of about 50 to 96 metric tons (55 to 106 short tons) in weight and about 25 to 40 m (82 to 130 ft) in length.

As mentioned A. huinculensis is incompletely known. In fact, despite the giant shadow it casts, it has only gotten a couple of publications to itself (Bonaparte and Coria 1993; Sellers et al. 2013), and could really stand a modern diagnosis. It is based on PVPH-1 (Paleontologia de Vertebrados, Museo “Carmen Funes”, Plaza Huincul), a specimen including three anterior dorsals, three posterior dorsals, five sacrals, a rib, and a fibula (Bonaparte and Coria 1993; the "tibia" of the original report is a fibula per Mazzetta et al. 2004). This specimen was recovered from the Huincul Formation (then the Huincul Member of the Río Limay Formation) about 8 km (5 mi) east of Plaza Huincul in Neuquén Province, Argentina (Bonaparte and Coria 1993); at the time, this unit was dated to the Albian–Cenomanian, and has now been refined to the late Cenomanian. Since the initial description, a couple of femora have also been attributed to Argentinosaurus (Bonaparte 1996; Mazzetta et al. 2004).

Because of the apparent hyposphene–hypantrum articulation, as in Andesaurus above, Bonaparte and Coria (1993) placed it with Andesaurus in Andesauridae, but this feature instead appears to have been something similar but distinct ("hyposphenal bars" of Apesteguía 2005). A. huinculensis may be a member of Lognkosauria, with other super-titanosaurs (Carballido et al. 2017; Gonzàlez Riga et al. 2018), but this is an emerging result; A. huinculensis more typically ends up as a basal titanosaur, not too far from Andesaurus.

With only eleven vertebrae, a rib, a fibula, and two femora to work with, it can be difficult to come up with things to say about A. huinculensis once we've admired its great bigness for a suitable period of time. Sellers et al. (2013) used a skeletal mount of this species to model its locomotion, which seems like a bit of an odd choice because most of the mount is composed of modeled bones made via comparison to other titanosaurs. Therefore, the results shouldn't represent A. huinculensis per se but a sort of gestalt giant titanosaur, which is also of interest and could be compared to the more complete giant titanosaur Patagotitan. Sellers et al. (2013) could get their model to walk at low speeds (approximately 2 m/s, or 5 mph) in an appropriately ponderous fashion, and considered that much larger terrestrial vertebrates would probably need a different anatomy to handle weight greater than the estimated 83.2 metric ton (91.7 short ton) live weight of the skeletal mount.

The skeletal mount of Argentinosaurus huinculensis used by Sellers et al. (2013) (their Figure 1).

References

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