Today the Chubut Monster, possibly the largest dinosaur known, has been published.
There are apparently six specimens from the same site. The largest of them may have exceeded 120 feet in length, and though the paper proposed a maximum mass of 82 tons, I suspect that when restored with the correct rib curvature and soft tissue levels, this animal may have exceeded 115 tons.
The AMNH mount, which is entirely made of fiberglass replicas, appears to be based on the holotype and a few similar-sized specimens. These are still smaller than the individual represented by the gigantic femur on the forklift pallets, which is a bit more eroded than the smaller femurs at the site.
Patagotitan mayorum is build like something between
Argentinosaurus and a lognkosaur, with traces of
Malawisaurus-like features as well. Though the cladistic analysis in the paper is odd to say the least (taking
Rinconsaurus and most other aeolosaurs out of
Saltasauridae and making them sister-group to lognkosauria, and throwing
Argentinosaurus into lognkosauria, while throwing
Malawisaurus into a derived lithostrotian cluster!) it does make two very interesting points; not only is the new Chubut Monster
Patagotitan related to
Argentinosaurus, but
Ruyangosaurus is classed as a titanosaur more derived than
Andesaurus.
It seems ironic that the paper does not draw a closer relation between
Patagotitan and
Ruyangosaurus, because the morphology of their posterior dorsal vertebrae appears almost identical. The oddly leaf-shaped neural spines, the many shallow and thin laminae, and V-shaped prezygapophyses so recently having lost the hypantrum, are very similar to those of
Patagotitan, which still retains a small hypantrum in a few of the mid-dorsals. The slender neural spines of the caudal vertebrae recall
Mendozasaurus. The centra of the dorsals resemble
Malawisaurus to some extent, and
Argentinosaurus from certain angles. It's looking like
Patagotitan occupies a node on the titanosaur family tree somewhat more derived than
Argentinosaurus, a direct descendant of the fork in the road between the "Malawisaurs" (
Malawisaurus and
Savannasaurus) and more derived groups such as the true lognkosaurs.
The biggest specimen of
Patagotitan is known from a femur that both has the unusual proximal curvature and lateral bulge of that of
Ruyangosaurus, and a size exceeding the (reconstructed) length of the
Argentinosaurus femur.
One final note: Just because I'm guesstimating that Patagotitan's largest individuals could have outclassed Argentinosaurus, Puertasaurus, etc. in overall mass, don't take that as meaning that "Patagotitan was always larger than any other titanosaur in all aspects". We have six individuals for Patagotitan and only one for Puertasaurus, one or possibly two for Argentinosaurus, all of which are far less complete than Patagotitan. These animals were all proportioned differently, and we know from experience that one can have a longer femur but be less massive overall, or that one species can have a longer body but less volume due to its rib cage shape, etc. One of these animals may be the longest, but a different one could be the tallest, or the heaviest. They are so close it's hard to call, so any attempt to do so (like mine) should take into account their proportions, or at least their likely proportions based on more complete relatives. For example lognkosaurs and lithostrotians appear to be wider than "andesaurs" or "argentinosaurs" for any given body length. More derived titanosaurs tend to outmass more basal ones of similar length, both due to wider rib cages and more robust limbs. And so on.
Argentinosaurus is known from two femurs, one which is an incomplete shaft, estimated at 2.5m when complete, the other being more complete and roughly 2.25m long. The largest Patagotitan femur (pictured above) is about 2.38m long, so smaller than the larger of the two Argentinosaurus femurs would have been when complete. That said, a 2.5m femur is only 12 cm larger than a 2.38m femur - literally just under five inches difference, no more. If Patagotitan had the same proportions as Argentinosaurus, then Argentinosaurus would be bigger - but given that Patagotitan's femur and many other elements look more derived, this was not necessarily the case. Its transverse processes are longer, and likely supported a proportionally wider rib cage, and if that's true then Patagotitan was probably more massive, to where the 5-inch femur length difference was rendered moot. And remember that 2.5m for the Argentinosaurus femur is an estimate for an incomplete midshaft. We don't know for sure if it was that long when complete. I've seen pictures of the second, smaller femur, but none from a good enough angle to guess its proportions, and none of them of good quality. The larger femur may end up being less than 2.5m long when scaled to the smaller one. There is no limb material at all for Puertasaurus, and only a fibula from the largest Alamosaurus, so again comparisons of limb bones are murky at best, and often don't tell you much about how the rest of the body was built.
At the end of the day, who is "the biggest" is a bit subjective, since (a) few people actually build a scale model of the largest specimens using actual measurements, (b) most giant sauropods are known from so few bones, so their proportions are sometimes poorly understood to begin with, and (c) most species are known from one or two specimens, a horrible sample size that gives no hints as to how large the biggest individuals of that species actually did get. So it's more accurate to say "the biggest known Patagotitan was probably bigger than the only known Puertasaurus or the only published Argentinosaurus." Not the "final verdict" people obsessively demand, but then again if paleontology was as well-funded as most other sciences, you might have a lot more specimens and a better idea of the upper limits.