The largest individual (at just under 27 meters and about 74 tonnes) is based solely on an incomplete tibia that would have been about 170 cm long, discribed by Rivera-Sylva et al. in 2006. The smallest individual (at under 8 meters and less than 2 tonnes) is based on a partially complete specimen described by Lehman and Coulson in 2002.
The second largest specimen to second smallest specimen pictured are based off of, in order, incomplete individuals described by Fowler and Sullivan (2011), undescribed specimen housed at the Perot Museum of Nature & Science, a partial sacrum mentioned by Mateer (1976) and the specimen described by Charles Gilmore (1946). All sizes are approximate due to the fragmentary nature of the specimens, and are subject to revision upon discovery of more complete specimens.
Gilmore, C.W. 1946. Reptilian fauna of the North Horn Formation of central Utah. U.S. Geological Survey Professional Paper 210−C: 29–51.
RIVERA-SYLVA, GUZMAN-GUTIÉRREZ, & PALOMINO-SÁNCHEZ. Preliminary Report on a vertebrate fossil assemblage from the Late Cretaceous of Chihuahua, Mexico. HANTKENIANA 5 (2006)
Fowler, D. W., and R. M. Sullivan. 2011. The first giant titanosaurian sauropod from the Upper Cretaceous of North America. Acta Palaeontologica Polonica 56 (4).
Lehman, T.M. and Coulson, A.B. 2002. A juvenile specimen of the sauropod dinosaur Alamosaurus sanjuanensis from the Upper Cretaceous of Big Bend National Park, Texas. Journal of Paleontology 76: 156–172.
Mateer, N. 1976. New topotypes of Alamosaurus sanjuanensis Gilmore (Reptilia: Sauropoda). Bulletin of the Geological Institutions of the University of Uppsala, New Series 6:93–95.
I have 2 questions with regard to Alamosaurus biogeography and ecology:
1) Did Alamosaurus make it into eastern North America (Appalachia) in the latest Cretaceous? This one may be hard or impossible to answer at this moment. AFAIK no sauropod remains have ever been found of the latest Cretaceous in Appalachia. All sauropods fossils there predate the (25-30 million year) sauropod hiatus, i.e. they are early-mid Cretaceous, not Campanian or Maastrichtian.
Since the Western Interior Seaway (WIS) closed around mid-Maastrichtian, and Alamosaurus appeared around that time or even just before, one would expect them to have migrated eastward as well.
2) I have seen mentioning of an ecological analysis stating that at any one given time there would have been a population of at least 350,000 (adult and sub-adult) Alamosaurus in (western) Texas. What is the source of that information, i.e. does anyone know the article/paper?
1) Since much of the fossil-bearing formations of Appalachia in the Maastricthian is poorly studied (comparatively-speaking), can't give you a real answer on this other than to state that Wikipedia has a list of dinosaur-bearing formations of that time, and only Alamosaurus is listed for N. America. https://en.wikipedia.org/wiki/List_of_vertebrate_fauna_of_the_Maastrichtian_stage#%E2%80%A0Sauropods
AFAIK, remains referable to Alamosaurus are only known from Utah, New Mexico, Texas, Northern Mexico and possibly Wyoming.
2) Don't know of a paper that discusses that number specifically. Best I can find study that could give rough numbers is: https://royalsocietypublishing.org/doi/pdf/10.1098/rspb.2010.2497?mobile=0&. They estimate Alamosaurus was about 0.2% of the population and use an equation to estimate number of herbivores/square kilometer as: N_h = c * M^d. They estimated the constants c = 100 and d = -0.75, based on modern herbivores. Using an estimated mass of 25,000kg for Alamosaurus would mean 0.05 individuals/km^2. The area of UT, NM, TX and WY is roughly ~1.5 million Km^2, which would be roughly equivalent to ~75,000 individuals, assuming average mass of 25,000 kg/individual. Most of the papers I have seen suggest that juveniles/sub-adults dominate sauropod populations, so if that were true, maybe they only averaged 5,000 kg and then population size could be close to ~250,000 individuals.
Of course, coeval formations that span that time don't all have records of Alamosaurus, which suggests it had preferences to certain types of habitat. On the other had, taphonomic biases can play into which taxa are represented, so it is very difficult to get a good handle on the extent of its range and thus population size (African elephants had a population over the entire continent of Africa ~25 million during the early 19th century - if Alamosaurus had a range of most of North America, possibly the overall population (juveniles, subadults and adults) could have been a similar size).
Thank you for your elaborate and very interesting reply!
With regard to your point 2, I just read the paper linked by you. Although it is indeed not the paper I was looking for, it is very interesting and I am no position to question their main conclusions. However, with regard to their Alamosaurus population estimates, I have a few objections (which BTW do in no way invalidate the rest of their paper, not its conclusions);
1) Their estimate of Alamosaurus population as a fraction of total large animal population (0.2%) in in my opinion way too low. The understandable mistake that they make is the assumption that large animals are always much rarer than smaller ones, which is not necessarily the case, especially if the large animal species is a rather opportunistic feeder.
Good example are elephants: before human-caused decimations, and still in some African reserves, sustainable African elephant densities can be rather high, in a woodland savannah easily 1 – 2 per km2 (and in very good habitats even much higher). For instance, in the huge Selous Game Reserve in southern Tanzania, before the great slaughter, the population density was about 2 per km2. Elephants constituted about 10% of all larger mammals and even more spectacular: over 50% of all larger mammal biomass (!, some estimates for some areas go as high as 75%).
2) If we assume, as quoted, an average Alamosaurus weight of 25,000 kg and for elephants of 2,500 kg (mind: this is an average for the entire herd, probably slightly on the high side), that means that the average Alamosaurus was about 10 x as heavy as the average elephant.
As known, food requirement is not linear with weight, but increases to the power of 0.75 for warm-blooded animals (mammals, birds) and about to the power of 0.67 for cold-blooded animals. This is so-called metabolic weight. So, depending on metabolism, a 10x larger animal would have a 4.7 – 5.6 times greater food requirement. Let’s just assume a rounded figure of 5.
That means, that, in a similar environment as a tropical woodland savannah, and assuming the above elephant density and relative food requirement for Alamosaurus, this would result in an Alamosaurus density of 2/5 = 0.4 per km2.
For a 1.5 million km2 area, this would result in a standing population of 600,000 subadult and adult Alamosaurus. Even if we assume the lower elephant density of 1 per km2, for instance because of parts of the total distribution area being less suitable, this would equate to a population of 300,000 subadult and adult Alamosaurus (0.2 per km2).
3) However, there are two other aspects to consider:
a) The much greater vertical feeding reach of Alamosaurus (the whole idea of having a long neck). Giraffes routinely browse a vertical zone between 1.5 and 4.5 meters, sometimes higher and lower. That is a 3 meter wide zone. Elephants have something similar.
If we take the average (subadult) Alamosaurus and assume regular browsing between (just below) shoulder height and a 60 degree neck angle, the routine vertical feeding zone for an average (subadult) Alamosaurus would easily be roughly 5 – 6 meters wide, which is easily 1.5 – 2 times that of the average elephant. Hence, the available amount of food in a similar habitat would also be 1.5 – 2 times as great.
As a side-note, it seems unlikely that low-browsing ceratopsian (e.g. Torosaurus) population density would have been greater than that of Alamosaurus.
b) Contrary to what is often described, more recent and comprehensive research has shown that Alamosaurus habitat was not semi-arid, but rather a sub-humid subtropical woodland or open forest. See in particular the paper that I refer to in www.deviantart.com/ravepaleoar…
This describes the upper Aguja and entire Javelina formations, typical Alamosaurus habitat, and very wooded, i.e. full of Alamosaurus food. Not so surprising for an ecosystem dominated by a large sauropod.
Note: this excludes babies and juveniles, which typically would have been produced continuously and in great numbers but the vast majority of which would have perished. As is the case for crocodilians and large turtles/tortoises.
So, summarizing, I think it is indeed safe to assume that (sub)adult Alamosaurus density could easily have been anywhere between 0.2 and 0.8 per km2, and therefore living (sub)adult population size in the region mentioned (1.5 million km2) in the hundreds of thousands, if not more.
The by now famous and mysterious Mexican tibia is at least 1.7 meters long and hence 15-20% longer than the largest Alamosaurus skeletal reconstructions, based on SMP-VP 1850 (and about 10% longer than the largest Argentinosaurus tibia btw).
And: the giant cervical SMP-VP 1850 has a preserved centrum of 63 cm diameter, reconstructed centrum estimated at 70 cm (!), which is easily Argentinosaurus/Puertasaurus territory.
And if the Mexican bone isn't a tibia but a fibula, as Nima, the Paleo-king, claims, then the whole animal was even bigger.
Note that most of the larger specimens are very incomplete, known from only a few bones or in some cases only one bone.
Because fighting off waves of zombie dinosaurs is bad enough, but huge, sauropod zombies? Talkin' bout sweet, beautiful overkill.
Thanks again for your help. :3
Great work!. We have check the Rivera-Sylva et al. in 2006 paper, and there is no mention to the gigantic tibia. Could you give us the correct reference please?
The width of the posterior end of the centrum decreases substantially as you go anteriorly in the cervical series of Malawisaurus for instance. There the posterior end of the centrum is about 16.5 cm in the 13th cervical and 12.5 cm in the 11th cervical.
Unfortunately, I do not have posterior view photos of all the Perot cervicals, so I can't tell whether this substantial decrease holds true for Alamosaurus or not. As reconstructed in my skeletal, the posterior most cervical is about 37.7 cm wide, so interpolating from Malawisaurus the 11th cervical might be roughly 28.6 cm wide. If the Fowler and Sullivan cervical turned out to be the 11th instead of the 13th one, than maybe that individual would have been just under 30 meters long and roughly 100 tonnes. This is why I said at the end of my description above that "All sizes are approximate due to the fragmentary nature of the specimens, and are subject to revision upon discovery of more complete specimens."