Your Friends The Titanosaurs, part 33.3: Alamosaurus of Texas and Mexico?

Having visited New Mexico and Utah (with side trips to Wyoming and Montana), we come now to more southerly Alamosaurus, concentrated in the Big Bend region of Texas. After a slow start Big Bend titanosaurs have attracted a lot of study, with numerous publications over the past quarter-century. There are also some less well-documents reports from elsewhere in Texas and across the border in Chihuahua, Mexico. We also have a little more on the question of whether or not we're dealing with just one species, and Tyrannosaurus makes yet another cameo.

This post marks the end of the main part of "Your Friends The Titanosaurs" (I can hardly believe it!) although we do have a few things to follow up with before calling it closed.

A view of the Perot Museum of Natural History mount (Dallas, Texas) emphasizing the neck. The original neck vertebrae are visible near floor level behind the torso of the mount. In the lower right a Tyrannosaurus rex has second thoughts. Photo by Louis Tanner from Garland, TX, USA, CC-BY-2.0, via Wikimedia Commons.

Big Bend Alamosaurus

Alamosaurus of the Big Bend area is more complicated than what we've covered before: the history is more cluttered due to most of it being packed into the past 30 years; the stratigraphy is more complicated; there's a lot of material; and it's here we get rumblings of actual morphological differences. Let's get the stratigraphy out of the way up front. When the first Big Bend titanosaur bones were found in the late 1930s, they were found in what was called the Tornillo Formation. In the mid-1960s, the Tornillo Formation was raised to a group and divided into, in ascending order, the Aguja Formation, the Javelina Formation, the Black Peaks Formation, and the Hannold Hill Formation (see Maxwell et al. 1967). Over the years it became apparent that the contacts between the formations didn't quite work, and various proposals were put forward to resolve the issues. Recently, proposed changes were formalized (Lehman et al. 2018), with the upshot for our purposes being: 1) the base of the Javelina Formation was moved to the bottom of a specific sandstone bed, rather than the top; and 2) the former upper mudstone interval of the Javelina was moved to the Black Peaks Formation. The Cretaceous–Paleocene boundary occurs low in the mudstone interval, meaning the Black Peaks Formation straddles the two rather than being entirely Paleocene (Lehman et al. 2018). Most of the publications of the past 20–30 years were implicitly using something similar, although you need to pay attention as you go back in time. (See for example Lehman 1989, which has the old "K–T" in the upper Javelina.) Big Bend titanosaur material potentially belonging to Alamosaurus is found in both the Javelina Formation and lower Black Peaks Formation.

This chart excerpted from Figure 2 in Lehman et al. (2018) might help. CC-BY-NC.

These two rock formations are interpreted as a fluvial/floodplain system, with channel sandstones surrounded by finer overbank deposits, and the occasional limestone bed on the order of 30 to 50 cm thick (12 to 20 in) representing flood basin ponds (Lehman 1989). Montgomery and Barnes (2012) described examples of lake deposition in the upper Javelina, coincidentally associated with Alamosaurus fossils. The sinuous and meandering streams could attain respectable size, up to 1 km (0.6 mi) wide (Lehman 1989). The sedimentation rate was relatively low, estimated at about 2 cm/1000 years (0.8 in/1000 years) (Lehman et al. 2006). Paleosols ("fossil soils") resemble those of subhumid to semiarid regions, with warm temperatures and seasonal precipitation. Angiosperms grew near streams, and conifers grew farther away (Lehman 1989). Stumps from ancient forests made up of trees on the order of 23 to 48 m tall (75 to 160 ft) have been found in the Javelina, indicating open woodlands (Lehman and Shiller 2020). There is a tuff bed in the middle of the Javelina Formation dated to 69.0 ± 0.9 million years ago (Lehman et al. 2006), and the Cretaceous expired 66 million years ago, bracketing most of the titanosaur fossils. Lehman et al. (2018) noted that although titanosaur specimens are found throughout the Javelina Formation, there appears to be differences between those from the lower Javelina and those in the upper Javelina, which they attributed to Alamosaurus sanjuanensis. Ceratopsids and hadrosaurs are rather rare in the Javelina, but may also have different species in the lower and upper parts of the formation. The famous Quetzalcoatlus may show this pattern as well (Lehman et al. 2018). A tyrannosaurid, likely Tyrannosaurus rex, has been reported from the Javelina and lower Black Peaks, and titanosaur fossils have been found at or near every Big Bend tyrannosaur locality (Wick 2014). With that in mind, after three weeks of this mini-series I give you what everyone has been secretly hoping for: Tyrannosaurus fighting Alamosaurus.

This is like what you'd get if you asked a seven-year-old for a great tattoo idea, and I mean that in the best possible sense. However, I do get a sense of indignation rather than terror or rage from the expression on the sauropod, as if telling the illustrator "Hey! You didn't say anything about getting mauled when I agreed to do this!", while the tyrannosaur mutters "Shut up! I need this gig!" Illustration from McCord 1977 (Bob Hersey); I prefer not to step on "Love in the Time of Chasmosaurs"'s territory, but I couldn't resist this time.

The discovery of sauropod bones in the Big Bend area can be traced at least to the late 1930s. Wann Langston, Jr. observed a sauropod femur at Chilicotal Mountain in 1938 and collected it for Texas Tech in 1947 (Langston et al. 1989), where it is cataloged as TTU 542 (Wick and Lehman 2014). TTU 542 happens to be the specimen that convinced Barnum Brown of Late Cretaceous sauropods when he saw it in the field (Langston et al. 1989). Works Progress Administration workers collected another femur in 1939 (Wick and Lehman 2014). Somewhat more famous are the specimens Brown and Roland T. Bird collected in 1940 for the American Museum of Natural History (AMNH; New York). Although Brown (1941) is sometimes cited as the first published report of sauropod bones from Big Bend, it's actually predated a few months by Anonymous (1941), which noted that he had brought back fossils including a cervical 0.9 m wide by 1.2 m long (36 in by 46 in). [Update, 2021/02/22: A couple of salient points noted by Matt Wedel over at SV-POW!: On the face of it, a cervical vertebra with these dimensions would be half again as long as the longest vert in the BIBE 45854 neck, discussed below, and comfortably in the top 10 longest cervical vertebrae of all. However, it's not clear from the wording in Anonymous (1941) if the measurements are actually the biological length and width of the vertebra, or just the two longest dimensions of a jacketed specimen. In that case, Alamosaurus cervicals are taller than long, and if we reorient the measurements, this mystery specimen is comparable to some of the vertebrae in BIBE 45854.] Accounts of the AMNH collecting at Big Bend focus on the adventures of Brown and Bird in their truck. For the record, at least in the beginning they were joined by frequent Brown collaborator Erich Schlaikjer, who'd driven the truck to Texas from Wyoming and had scouted the area the year before, which seems at least worth a mention in an obscure paleontological blog 80 years later. Anyway, Brown and Bird found abundant fragmentary fossils, including sauropod specimens. Per Langston et al. (1989), the very last dinosaur fossil Brown collected for the AMNH was a sauropod cervical from the Javelina Formation. The AMNH collecting ended early when their permit was revoked for reasons no longer known (Langston et al. 1989; this predates the 1944 establishment of Big Bend National Park, so a different landowner was involved).

Although we know that titanosaur fossils were being discovered and collected from the Big Bend area between the early 1940s and the early 1970s, none of these discoveries were reported in any major way. Even Maxwell et al. (1967), a major summary of Big Bend geology and paleontology, did little more than reiterate Brown's brief statements. Douglas Lawson's 1972 thesis provided additional information. 1972 is also notable as the year Langston found the partial skeleton now cataloged as TMM 41541-1 (Texas Memorial Museum, Austin, Texas). Collected in 1973 but not fully prepared until 2011, TMM 41541-1 was used as the model for a large part of the body of the Perot Museum mount. It came from the upper Javelina Formation and includes two posterior cervicals, a nearly complete series of dorsal vertebrae with ribs, the sacrum, the first caudal, parts of all of the pelvic bones, a humerus, and a femur (Wick and Corrick 2015). This individual was of respectable but not monstrous size: the humerus is 1503 mm long (59.17 in) (Lehman and Coulson 2002), and the femur is 1730 mm long (68.11 in) (Wick and Lehman 2014). The entire specimen has yet to be described in any widely available publication, although there *has* been a paper on an apparent vertebral ligament found with the bones (Woodruff et al. 2016).

The rest of the 1970s into the 1980s were also quiet on Big Bend titanosaurs. There was another dissertation that included them (Lehman 1985), and a chapter in a Society of Vertebrate Paleontology field trip guidebook provided some historical information (Langston et al. 1989; updated in Lehman and Busbey 2007), but things didn't really get going until the mid-1990s. Now it seems like there's at least one paper on Big Bend titanosaur material every year.

The iconic Big Bend Alamosaurus specimen is a series of enormous articulated cervicals, discovered in the lower Black Peaks Formation in 1997 (or 1999) during a joint project of the Perot Museum of Nature and Science (Dallas Museum of Nature and Science at the time) and University of Dallas to excavate a juvenile titanosaur bonebed (see below) (Tykoski and Fiorillo 2017). These cervicals provided the basis for the neck of the Perot Museum mount (May et al. 2012; Tykoski and Fiorillo 2017). The specimen, cataloged as BIBE 45854 (Big Bend National Park*), includes nine cervicals thought to represent cervicals 6 through 14, plus fused cervical ribs. The longest centrum is approximately 810 mm long (32 in), the tallest vertebra is 1240 mm tall (48.8 in), and the widest (vertebra plus ribs) is 984 mm across (38.7 in). (Tykoski and Fiorillo 2017). As noted over at SV-POW!, the cervicals all look rather similar. One of their most obvious features is the enormous triangular neural spines, as if each cervical has its own sail. Tykoski and Fiorillo (2017) did not observe this feature in an immature specimen (TMM 43621-1), but could not be sure if this means the neural spine shape is growth-related or the absence was due to post-burial deformation.

(*all NPS units have four-letter acronyms usually based on the first four letters of a park's name if just one word, or the first two letters of the first two words if the name is longer than one word. BIBE, PEFO, DINO, GRCA, YELL, DEVA, CHIS—can you guess what these are?)

NPS photo of the cervicals in the field, from here.

Other specimens have provided additional anatomical details. As described by Wick and Lehman (2014), the femur is notably different from other titanosaur femora, with a combination of features that would have given the animal a narrower stance than the well-known titanosaurian wide stance. Five titanosaurian femora from the Big Bend region, with a wide range of sizes, all share these features (Wick and Lehman 2014). (Regrettably, another femur has been stolen from the field; Wick and Corrick 2015.) Big Bend titanosaurs also had osteoderms, with one large oval and unkeeled specimen from the upper Javelina described in Fronimos (2020). This tall, symmetrical, 19.9 cm long (7.83 in) osteoderm is not hollow, not differentiated into a bulb and root, and does not have a cingulum. It is similar to the North Horn Formation Alamosaurus osteoderm, but Fronimos (2020) refrained from assigning it to Alamosaurus because there were no diagnostic features. It also resembles the osteoderms of Mendozasaurus and unnamed South American forms (Fronimos 2020). Fronimos (2020) regarded the most likely functions as mineral storage, local defense, and display.

It's hard to be certain from the photos, but the mount doesn't *seem* to have any additional splay at the hips. (Also, there's those great honking cervical neural spines.) Rodney, CC-BY-2.0, via Wikimedia Commons.

There are also notable finds of juvenile titanosaurs at Big Bend. Straight (1996), Coulson (1998), and Lehman and Coulson (2002) described a disarticulated partial juvenile specimen (TMM 43621-1) from the lower Black Peaks Formation. As of 2002, this was the stratigraphically highest dinosaur specimen from Big Bend, being 2 m (7 ft) or less below Paleocene vertebrates (Lehman and Coulson 2002). It's fairly well represented, with rod-like tooth fragments, portions of at least seven or eight cervicals and six or seven dorsals (with the usual unfused neural arches), a sacral rib, the left coracoid, humerus, and ulna, the left ischium and pelvic fragments, both tibiae, left fibula, and fragmentary metatarsal (Lehman and Coulson 2002). For comparison, the humerus of this specimen is 60 cm long (24), versus 136 cm (53.5 in) for USNM 15560 from the North Horn Formation (Coulson 1998). Titanosaur humeri seem to a specialty of Big Bend: Fronimos and Lehman (2014) knew of eight ranging in length from 455 mm to an estimated 1570+ mm (17.9 in to 61.8 in), comfortably exceeding USNM 15560.

A bonebed with the remains of at least three juveniles has also been found, in the upper Javelina Formation (Fiorillo 1998; Montgomery and Fiorillo 2001; Myers and Fiorillo 2009; Montgomery and Barnes 2012). I don't know that there has been a bone-by-bone breakdown in the literature, but at least three individuals are present based on the number of left femora (Fiorillo 1998; Myers and Fiorillo 2009). By "juvenile", something approximately half-grown can be envisioned, because the femora are about 50–60% the size of an adult femur (Myers and Fiorillo 2009). The site appears to have been a lake (Montgomery and Barnes 2012), and the sediments show evidence of sauropod trampling (Fiorillo 1998; Myers and Fiorillo 2009; Montgomery and Barnes 2012). Montgomery and Barnes (2012) proposed that the three sauropods died after becoming mired, which tallies with the suggestion in Myers and Fiorillo (2009) that the site shows potentially gregarious animals that died at the same time. Myers and Fiorillo (2009) also noted that adult Big Bend titanosaurs had only been found as single individuals, suggesting that juveniles and adults had different social habits. As a side note, Main and Hammon (2003) discussed the use of ground penetrating radar at the juvenile bonebed and the BIBE 45854 site, where concretions resulted in more false positives (5) than actual bones (2).

The relatively large sample size of specimens, with a great variety of sizes and presumably growth stages, allows testing of questions that are not feasible for the great majority of titanosaur species. Woodward (2005) did histology on a variety of bones of Big Bend Alamosaurus, results of which were later formally published in Lehman and Woodward (2008) and Woodward and Lehman (2009). Lehman and Woodward (2008) estimated the average maximum growth rate of Big Bend Alamosaurus as 1090 kg/year (2040 lbs/year), with individuals reaching 90% of adult size around 45 years old. Woodward and Lehman (2009), with specimens from at least 22 individuals, observed that specimens with an "adult" size did not necessarily have mature bones, and specimens with mature bones were not necessarily at "adult" size (and so I ought to reconsider some of my statements in the last paragraph of the Garrigatitan post). For those of you interested in pneumatic features, Woodward and Lehman (2009) also noted that the ribs are extremely pneumatic. Pneumatic features are apparently very variable overall in the Big Bend sample (Fronimos 2011).

Does the Big Bend titanosaur material belong to Alamosaurus sanjuanensis? The answer has been elusive. D'Emic et al. (2011), who identified diagnostic features in the holotype and linked the North Horn Formation skeleton USNM 15560 to the species, did not compare the Big Bend material to the San Juan Basin and North Horn Formation material. In an abstract and Society of Vertebrate Paleontology conference presentation, Fronimos and Lamanna (2018) discussed vertebral features they regard as differentiating Texas titanosaur material (including TMM 41541-1) from Alamosaurus sanjuanensis. (More discussion may be in Fronimos's 2010 dissertation [Fronimos 2010b], but I haven't been able to see this to date.) Tykoski and Fiorillo (2017) and Lehman et al. (2018), meanwhile, considered the same specimen to be referable to A. sanjuanensis, and Tykoski and Fiorillo (2017) also mentioned a Big Bend scapula with the same features as the holotype. Further complicating matters, there may be two different species with distinct stratigraphy, one in the lower Javelina and another in the upper Javelina and lower Black Peaks (Fronimos and Lehman 2014; Lehman et al. 2018, which accepts the upper species as A. sanjuanensis). It certainly would not be surprising for there to be multiple closely related species or even genera. Fronimos and Lehman (2014) go over some of the differences between Big Bend Alamosaurus and specimens from other areas, and within the Big Bend sample.

How about the eternal question of "Where did North American titanosaurs come from, if we assume that the North American sauropod hiatus is real?" Most phylogenetic analyses to date show Alamosaurus in the company of Opisthocoelicaudia. The Big Bend titanosaur researchers tend to link it to South American titanosaurs. Lehman and Coulson (2002) suggested it was closer to Neuquensaurus and the Brazilian sauropods that would later be named Baurutitan and Trigonosaurus. Fronimos (2010a) and Fronimos and Lehman (2014) added Uberabatitan to the Brazilian contingent. Tykoski and Fiorillo (2017), adding Big Bend specimens attributed to Alamosaurus to different analyses, also did not find Alamosaurus near Opisthocoelicaudia, but to South American titanosaurs, including in one case the lognkosaurs.

El Picacho Formation Alamosaurus

For the El Picacho Formation, to the northwest of Big Bend in Texas, about all we have to go on is the presence of "fragmentary titanosaurid sauropod remains, probably also referable to [Alamosaurus sanjuanensis]" (Lehman 1987); the material was apparently first mentioned in Lehman's 1985 dissertation.

Alamosaurus in Mexico?

Geologic units do not follow national boundaries, unless the countries put the line on the crest of a mountain range or something like that, so it's not surprising that Big Bend National Park and adjacent areas on the other side of the US–Mexico border share geologic units. The rocks haven't been prospected as heavily for fossils on the Mexican side, but there are a few reports of Late Cretaceous titanosaurs (see Ramírez-Velasco and Hernández-Rivera 2015 for the most recent summary). Montellano Ballesteros (2003) was the first report, describing a handful of vertebrae in the collections of the Museo del Instituto de Geología, Universidad Nacional Autónoma de México: four posterior caudal centra (IGM 6080) and a dorsal or sacral vertebra (IGM 6080-5). They were collected from either the lower Javelina Formation or the upper part of the underlying Aguja Formation at Altares, Chihuahua, and assigned simply to Titanosauridae (Montellano Ballesteros 2003). D'Emic et al. (2010) could not be certain if the vertebrae were titanosaurian or hadrosaurian, noting that IGM 6080-5 in particular resembled sacrals of Gryposaurus.

By far the most famous specimen is a fragmentary tibia from the Aguja Formation of Chihuahua measuring 1.35 m long (4.43 ft) (Rivera-Sylva et al. 2006). This bone gets trotted out whenever people are wishcasting Alamosaurus sanjuanensis size, although it hasn't been firmly connected to the genus or species. Rivera-Sylva et al. (2006) described it as cf. A. sanjuanensis out of traditional usage, and Ramírez-Velasco and Hernández-Rivera (2015) regarded *all* Mexican titanosaurian specimens as only assignable to Titanosauria until further evaluation could be made. Interestingly enough, all but one of the handful of Mexican titanosaur specimens reported to date come from the Aguja Formation, not the overlying Javelina Formation (Ramírez-Velasco and Hernández-Rivera 2015), although I suppose there's a possibility that a different stratigraphic definition is in use. Anyway, aside from some limb fragments, the only other reported titanosaur from Chihuahua is a privately owned partial skeleton from the Javelina Formation on display at the Museo de Paleontología de Delicias in Delicias, Chihuahua (Rivera-Sylva and Carpenter 2014). Bones from this specimen include vertebrae, a femur, a proximal tibia, and an ulna from a respectably sized titanosaur (on the order of 22 m long, or 72 ft). The skeletal mount appears to have been augmented with hadrosaur caudals (Rivera-Sylva and Carpenter 2014). Rivera-Sylva and Carpenter (2014) suggested that it could be the first record of Alamosaurus sanjuanensis from Mexico.

References

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