Alamosaurus

Credit: Alamotitan on the Wikimedia Commons.

*Alamosaurus* is an extinct genus of titanosaurian dinosaur that lived in North America during the Late Cretaceous.

## History

The holotype was discovered by Charles Gilmore, Charles Sternberg and John Bernard Reeside in June 1921 in the Barrell Springs Arroyo in the Naashoibito Member of the Ojo Alamo Formation (which was then defined as the Kirtland Formation), the Maastrichtian of New Mexico. Remains frim the Black Peaks, El Picacho and Javelina Formations are also assigned. The Evanston bears unidentified sauropods associated with individuals of this genus, with additional material found above Hams Fork. Gilmore described the holotype, USNM 10486 (a left scapula) and a paratype, USNM 10487 (a right ischium). The name *Alamosaurus* is typically associated with Alamo in San Antonio, Texas and the battle fought there, though this is untrue, as it had not yet been known from Texas and was originally found in New Mexico. "Alamo" translates to "poplar", and the rocks bearing the holotype have been debated as to whether they belong to the Kirtland or to the Ojo Alamo. The specific epithet is after San Juan County. Gilmore (1946) posthumously described a more complete specimen, USMN 15660, discovered on the North Horn Mountain of Utah on June 15, 1937 by George B. Pearce. It is a complete tail, right forelimb (complete, minus the manus) and both ischia. Since, hundreds of specimens from New Mexico, Texas and Utah have been discovered and referred, most often without heavy description. Though fragmentary, this taxon represented much of the known titanosaur material during the second half of the 1900s. TMM 43621-1, a juvenile skeleton from Texas, is one of the most complete and is known from Texas. Several blocks cataloged as USMN 15660 were found in close association were investigated by Michael Brett Surman in 2009, to which their proximity was estimated using bone impressions. In 2015, his research reported osteoderms, which is the first occurrence of such in this taxon. The only known crania are teeth.

 File:Alamosaurus tkm007.png
*Alamosaurus sp.* are known from the Hall Lake Formation, oriented higher in the formation than *Sierraceratops*, living with triceratopsinins and cf. *Tyrannosaurus*. The only evidence for *A. sp.* in Hall Lake is TKM007, a humerus, which has an unknown repository location, as "TKM" refers to the locality it was found at. This large humerus is 96 centimeters long and 45 in width, belonging to the right size. The specimen was very fragmented upon discovery. The massive deltopectoral crest rises above the humeral head and arcs downwards in a rough parallel to the external margin of the shaft, probably somewhat deflecting medially, though this area is extremely eroded. The margin itself is somewhat straight, with a concavity formed by the sharply-sloped internal margin contrasting. The deltopectoral crest is not as large and not as medially deflected as *A. sanjuanensis*; the observed medial deflection in some *Alamosaurus* may be due to crushing, however, so it may not be an effective descriptive autapomorphy.

## Description

It's body is typical of titanosaurs, with a body covered in osteoderms. Holtz (2012) estimated a total length of 30 meters (98 feet) or more, with a weight of about 72.5-80 tonnes (80-88 short tons) or more. Though the most complete fossils are not mature, SMP VP-1625, SMP VP-1850 and SMP VP-2104 suggest possible enormous sizes comparable to those such as *Argentinosaurus*. Based on a huge incomplete tibia, Hartman suggests a size slightly shorter than 38-30 meters (92-98 feet) and about equal in weight to *Argentinosaurus* and *Puertasaurus*. However, he is unsure if this tibia can be referable to this genus or to a novel taxon. G.S. Paul (2019) estimates SMP VP-1625 to be 27 tonnes (30 short tons) heavy and mentions a large partial anterior caudal to be 15% dimensionally larger with a similar mass to his *Dreadmoughtus* estimation (31 tonnes (34 short tons)). Molina-Perez and Larramendi (2020) suggest the largest individual is 26 meters (85.3 feet) tall and 38 tones (42 short tons) heavy. Teeth are rod-shaped and found associated with partial skeletons. The mid-caudals have elongate centra. Vertebral lateral fossae resemble shallow depressions, which is also found in *Saltasaurus*, *Malawisaurus*, *Aeolosaurus* and *Gondwanatitan*, also bearing similarity to the "depressions" in *Venenosaurus*, though these are deeper, extended farther into the column and divide into two chambers. It has more robust radii than the latter genus.

## Classification

Gilmore (1922) was uncertain of its affinities, assigning it to Sauropoda. Friedrich von Huen (1927) placed it into Titanosauridae. It is, for sure, a derived titanosaur, though determining further classification is difficult. This is amplified with some workers suggesting the use of Saltasauridae instead of Titanosauridae. One major phylogenetic study finds a close relationship with *Opisthocoelicaudia* within Opisthocoelicaudiinae, Saltasauridae. Competing opinion suggest sister relations with *Pellegrinisaurus* just outside of Saltasauridae. Some note similarity with *Neuquensaurus* and *Trigonosaurus*, which is used in many titanosaur studies. Fiorillo and Tykoski (2016) suggest sister relations with Lognkosauria outside of Saltasauridae, possibly bearing close relations or even ancestry with such based on cervical vertebral synapomorphies. This same study suggests ancestry with South America rather than with Asia.

## Paleoecology

Fossils are most often seen in the Naashoibito Member, Ojo Alamo Formation, which is 68-69 million years old and in the Javelina Formation, though this age is harder to determine. A specimen from the Black Peaks Formation overlies the Javelina and straddles the Chicxulub impactor event, whicht the specimen hailing from 66 million year-old sediments just meters below this boundary. Only one section of the Javelina is available for radiometric dating, providing an age of 69 million years ago. Using the underlying Aguja Formation, this suggests an *Alamosaurus* fauna lasting 66-70 million years, with occurrences near the base of the Javelina and just below the K-Pg boundary in the Black Peaks. The *Alamosaurus* fauna provides skeletal components of this taxon so frequently it is used to define faunal relations across the United States. The Edmontonian-Lancian transition was described by Lehman to be very dramatic in the south, introducing typical "Jurassic"-like animals. It is commonly found with *Quetzalcoatlus*, with this combination likely suggesting a semi-arid inland plains environment. This taxon may have immigrated in from South America during this time, which is supported by a supposed dominance occurring quickly. Some suggest ancestry with Asia, though this is not supported by its upland nature suggests it to be endemic rather than coastal species and tend to be less adept at crossing bodies of water. Additionally, Early Cretaceous titanosaurs are already known, which may indicate potential ancestors in North America. It coexisted with "Alamotyrannus", *Ojoraptorosaurus*. *Gryptosaurus alsatei*, cf. *Kritosaurus*, *Glyptodontopelta*, cf. *Torosaurus utahensis*, *Bravoceratops*, *Ojoceratops*, *Quetzalcoatlus*, rays, fish, amphibians, lizards, turtles and multituberculates in the American southwest.

## References

1. ↑ 1.0 1.1 1.2 1.3 1.4 https://en.wikipedia.org/wiki/Alamosaurus
2. ↑ https://www.sciencedirect.com/science/article/abs/pii/S0195667121002822?via%3Dihub
3. ↑ https://geoinfo.nmt.edu/publications/periodicals/nmg/6/n4/nmg\_v6\_n4\_p72.pdf
4. ↑ https://equatorialminnesota.blogspot.com/2021/01/your-friends-titanosaurs-331-alamosaurus-new-mexico.html