Saturday, January 19, 2019

Your Friends The Titanosaurs, part 8: Bonitasaura, Borealosaurus, and Brasilotitan

And so we reach Part 8, otherwise to be known as "Bonitasaura and these other two." Bonitasaura is one of the best-known sauropods, even if it isn't quite a household word, and also has some unusual jaw features going for it. Brasilotitan has some of the same jaw features, but is not known from nearly as much material and came along several years later. Borealosaurus has an uncertain grip on Titanosauria, and gripping things takes some creativity when you have either no fingers or just nubs.

Bonitasaura salgadoi

Bonitasaura salgadoi gets the honor of being perhaps the best-documented titanosaur. Yes, there's not a lot of competition at this time (there are many papers on Alamosaurus, but it doesn't feel right giving it too much credit when the taxonomy is still unsettled), but it's probably within the top five percent of classic dinosaurs as a whole. There are no fewer than seven publications and a dissertation (Gallina 2011a) devoted to B. salgadoi. If you're looking to build up a library of journal articles for this species, you'll need these, not counting all of the times it's been mentioned in passing:
  • The original description (Apesteguía 2004)
  • Cranial anatomy (Gallina and Apesteguía 2011)
  • Postcranial anatomy (Gallina and Apesteguía 2015)
  • Features of the axial skeleton (Gallina 2011b)
  • Paleopathologies (Gonzalez et al. 2017)
  • Histology (Gallina 2012)
  • Taphonomy of the site (Pérez et al. 2009)
B. salgadoi is based on a partial skeleton found in the Bajo de la Carpa Formation northwest of Cerro Policía in Río Negro Province, Argentina. The discovery locality, a hill called "La Bonita", was the inspiration for the genus name (similar to Baalsaurus), with the feminine ending -saura presumably chosen to go with the feminine "bonita". (I was a bit surprised about the meaning, which I guess makes me kind of like the people who misinterpret Torosaurus as "bull lizard".) The species name honors Argentine paleontologist Leonardo Salgado (Apesteguía 2004), giving us something like "Leonardo Salgado's lizard from La Bonita". The type specimen was originally given as MPCA 300 (Museo Provincial "Carlos Ameghino", Cipolletti, Río Negro) (Apesteguía 2004), but apparently some kind of wire was crossed concerning the number, and it's known as MPCA 460 in all of the later publications. Preparation of the material and additional work at the site revealed more bones (Gallina and Apesteguía 2011), and the most recent account of the type material (Gallina and Apesteguía 2015) lists the following:
  • For the skull, a left frontal, left parietal, right lacrimal, left quadrate, right dentary with 15 replacement teeth, and an isolated tooth;
  • For the axial skeleton, the axis, three other cervicals, five dorsals, 25 caudals, some cervical and dorsal ribs, and five chevrons;
  • For the forelimbs, a sternal plate fragment, partial right humerus, and two metacarpals from the left hand;
  • For the hindlimbs, the left pubis, ischium, and femur, right tibia, fragmentary left fibula, both astragali, eight of the ten metatarsals, and three phalanges
In addition, bones from two other individuals of B. salgadoi were found about 20 m (66 ft) away: MPCA 467, represented by a couple of caudal centra, a left ulna and fragment of the left radius, two metacarpal fragments, the head of the right femur, a fragment of the left tibia, and a complete left fibula from an animal about the size of MPCA 460; and MPCA 468, represented by a right tibia from a smaller individual (Gallina and Apesteguía 2015). Altogether, most of the skeleton is represented, except for the sacrum, ilium, shoulder blade, braincase, and front end of the upper jaw.

MPCA 460 comes from fluvial rocks near the top of the Bajo de la Carpa Formation, of middle Late Cretaceous (Santonian) age. The bones were found in an area of roughly 16 square meters (about 170 square feet), associated but disarticulated. They are well preserved, without surficial erosion or filling of internal cavities, indicating rapid burial and minimal exposure to the elements. The only articulated bones are apparently three caudals, which make a small part of a classic dinosaur opisthotonus arch (the thing where articulated dinosaur necks and tails are preserved curling back over the body). Transportation of bones appears to have been minimal, based on the types of bones present (Pérez et al. 2009).

Bonitasaura salgadoi quarry map, Figure 3 in Pérez et al. 2009. CC BY-NC 4.0.

Most of the original description (Apesteguía 2004) and an entire additional paper (Gallina and Apesteguía 2011) deal with the skull, particularly the dentary. We've already had some dealings with the phenomenon of the square-jawed titanosaur, seeing the early controversy with Antarctosaurus wichmannianus and the recent description of even more squared-off Baalsaurus mansillai. The dentary of B. salgadoi is broadly L-shaped when viewed from above or below, but there is a definite curve to it. Maybe J-shaped would be more apt? Ten tooth positions are preserved, although there may have been more depending on how much of the symphyseal region (the part where the two halves of the jaw meet) is missing. The tooth positions are generally larger in the front of the jaw than the side, and are crowded to the front of the jaw. Two or three peg-like teeth are visible in several of the tooth positions; the teeth aren't perfect pegs, having hexagonal cross sections, but you get the idea. The lower jaw of B. salgadoi also has a thin sharp ridge rising from the shaft beyond the teeth. This ridge has been interpreted as covered by a keratinous sheath, giving the animal a cheek "guillotine" opposed by something similar on the as-yet undiscovered upper jaw. Less-developed versions of this ridge have been identified on Antarctosaurus wichmannianus, Nemegtosaurus mongoliensis, Quaesitosaurus orientalis, and Rapetosaurus krausei. The rest of the skull, what there is of it, is not particularly unusual among titanosaurs (what there is of theirs), with what is interpreted as a fairly steep face (Apesteguía 2004; Gallina and Apesteguía 2011).

The famous right dentary of MPCA 460, photographed and illustrated in Gallina and Apesteguía (2011; Figure 5). Row A is dorsal view, Row B is medial view, Row C is ventral view, and Row D is lateral view. CC BY 4.0.

Line drawings of the restored skull of MPCA 460, known bones shaded gray, in lateral (A1), dorsal (A2), and posterior (A3) views, from Gallina and Apesteguía (2011; Figure 7). CC BY 4.0.

Postcranially, the vertebrae of B. salgadoi are much more distinctive than the limbs, which is generally par for the course in titanosaurs. They include several diagnostic features, most of which are characteristics of the laminae, or features that give B. salgadoi relatively robust neural arches just part where the neck joins the body (Gallina and Apesteguía 2015). Some but not all of the neural arches of MPCA 460 are fused to the centra (Gallina 2011b), evidence that it was only partially grown. The orientation of the neural spines from the base of the neck going into the back wander a bit, switching from leaning anteriorly to leaning posteriorly to standing vertically near the hips (Gallina 2011b).

Vertebrae of Bonitasaura salgadoi (row A) compared to the "reference vertebrae" of Trigonosaurus pricei (row B), not to scale (Gallina 2011b; Figure 5). CC BY-NC 4.0.

To date the only unique feature of the limbs is an "anterior, longitudinal ridge on the tibia with a marked prominence just over the anterior process of its distal end" (Gallina and Apesteguía 2015); in other words, there's an unusual ridge on the shin. B. salgadoi also had relatively gracile limbs, unlike the chunky builds of the professional small titanosaurs of South America, the saltasaurs (Gallina and Apesteguía 2015). The femur of MPCA 460 is 118.5 cm long (46.65 in) (Gallina and Apesteguía 2015), which is not large for a sauropod, but as mentioned it was only partially grown. One slightly odd feature is that there is not much of a bony "elbow" on the ulna of MPCA 467 (olecranon process), but this could be due to the youth of this individual (Gallina and Apesteguía 2015).

It is no secret that MPCA 460 was a small sauropod, but it is also recognized that it was not fully grown. Gallina (2012) performed histological analyses on several limb bones, vertebrae, and ribs, which all grew slightly differently. Gallina interpreted the animal as being at histologic ontogenetic stage 9 (see Klein and Sander 2008 for more information on this concept in sauropods; for the short version, there are 13 stages, with sexual maturity but not maximum size at HOS-8). Based on growth curves for other sauropods, Gallina (2012) estimated that the femur of an adult Bonitasaura salgadoi was more like 1.8 to 1.9 m long (5.9 to 6.2 ft), making for a respectably sized sauropod. It is a reasonable bet that an adult B. salgadoi did not look like this, though.

Despite its relative youth, MPCA 460 was not the healthiest animal. Gonzalez et al. (2017) recorded three different pathologies, making it the first titanosaur known to have had multiple pathologies: a tumor in the left femur, creating an ovoid growth; a bone spur on the left metatarsal III; and an infection in a caudal vert creating a drainage sinus. The presence of the tumor on the femur may have affected the movement of the left leg, leading to the bone spur in the foot (Gonzalez et al. 2017).

Material from the Rancho de Ávila site, in the same general area and from approximately the same horizon as B. salgadoi, was collected for the Museo de La Plata during 1921 and 1922 (Gallina and Otero 2015). Huene (1929) figured some but not all of the specimens, which represent multiple individuals of mid-sized sauropods and include two dorsal centra and a dorsal neural arch, a dorsal rib, 16 caudals, a caudal neural spine and a caudal neural arch, a left scapula, parts of four humeri, two radii, an ulna, parts of five metacarpals, an ilium fragment, parts of two femora, parts of two tibiae, parts of two fibulae, three metatarsals, and four phalanges (Gallina and Otero 2015; unfortunately, some are missing). Previously considered fossils of Laplatasaurus (Huene 1929), they compare well with B. salgadoi in several anatomical details, and so Gallina and Otero (2015) reassigned them to cf. Bonitasaura sp.

Borealosaurus wimani

Borealosaurus wimani seems kind of like your basic obscure, poorly represented titanosaur, although it is most notable for not having the classic titanosaurian feature of procoelous caudal vertebrae. Also, for a bit of variety it's from China. B. wimani was described by You et al. in 2004 from a handful of isolated sauropod fossils. The specimens were collected between 1999 and 2002 from near the Shuangmiao village of Beipiao in Liaoning Province. The stratigraphic unit is the Sunjiawan Formation, which at the time was dated to the early Late Cretaceous (Cenomanian–Turonian) (You et al. 2004), but which is now considered late Early Cretaceous (Albian) based on microfossils (Wang et al. 2015). The Sunjiawan Formation is not one of the famous Liaoning formations with feathered things, but it does also have ankylosaurs (Crichtonpelta benxiensis, dubious Crichtonsaurus bohlini) and big iguanodonts (Shuangmiaosaurus gilmorei), which are almost as nice to have as untold numbers of complete articulated birds and dromaeosaurs and pterosaurs and so forth (he said with a straight face).

The holotype of B. wimani is LPM0167 (Liaoning Paleontological Museum, Beipiao), a mid-distal caudal vertebra. You et al. (2004) also tentatively assigned a peg-like tooth crown, another middle caudal, and a mostly complete robust right humerus to the species, although noting that it was not possible to determine how or even if these specimens were associated with the holotype or each other. The genus name "Borealosaurus", meaning "northern lizard", is a reference to the find being in northern China, and the species name "wimani" honors Swedish paleontologist Carl Wiman, "who named the first Chinese dinosaur (Euhelopus) in 1929" (You et al. 2004). Technically, Wiman named it Helopus, but that name proved preoccupied and so was later replaced with Euhelopus, but he also named Tanius in the same paper. Therefore, if it bothers you that "Helopus" had to renamed, you've also got Tanius, which may not be the most exciting thing but it's certainly a dinosaur and it's doing the best that it can. The combination of genus and species names gives us something like "Carl Wiman's northern [Chinese] lizard".

You et al. (2004) described a single distinguishing characteristic for B. wimani: "opisthocoelous mid-distal caudal vertebrae" (centra with convex articulation at the front, concave at the back). Knowing that titanosaurs are famous for procoelous caudals, B. wimani ends up among them in a roundabout way: at the time of its description, the only other sauropod widely known to have opisthocoelous caudals was the aptly named Opisthocoelicaudia (which is probably Nemegtosaurus, but that's for another day), which despite its opisthocoelous caudals has long been well accepted as an innovative titanosaur rather than something else. You et al. (2004) suggested that B. wimani and Opisthocoelicaudia may have been closely related. Several other titanosaur and titanosaur-ish sauropods are now known to have have at least some degree of opisthocoely, including an unnamed form from Kazakhstan (Averianov and Sues 2017), Fukuititan (Azuma and Shibata 2010), Nemegtosaurus (Currie et al. 2018), Rinconsaurus* (Calvo and Gonzalez Riga 2003), and Sonidosaurus (Xu et al. 2006). Further review of B. wimani has not been a high priority to date. Mannion et al. (2013) were not entirely convinced it was even a titanosaur, while Averianov and Sues (2017) regarded it as a basal titanosaurian. Given the age, it wouldn't be entirely surprising if B. wimani turned out to be just outside of Titanosauria, but we're going to need more more material.

*Rinconsaurus is neat because its tail contains just about every kind of vertebral articulation known to sauropods. It was published the year before B. wimani, but it takes time for information to filter through.

Brasilotitan nemophagus

By an odd coincidence, we get two of the few square-jawed titanosaurs in the same post. Brasilotitan nemophagus is even more square-jawed than Bonitasaura salgadoi (Calvo and Gonzalez Riga 2018), as discussed in the Baalsaurus mansillai post. It is not nearly as well-known, though. B. nemophagus is based on a small group of fossils collected from reddish fine-grained sandstone by William Nava in 2000, along the Raposo Tavares state road near Presidente Prudente in São Paulo, Brazil. The horizon was originally given as the Adamantina Formation (Machado et al. 2013), but this has since been revised to the Presidente Prudente Formation (Bandeira et al. 2016). The holotype is MPM 125R (Museu de Paleontologia de Marília), consisting of "a right dentary, two cervical vertebrae, three incomplete sacral vertebrae, fragment of an ilium, fragments of an ischium, one ungual and fragmentary elements". The bones are broken or incomplete, but have suffered little distortion. An isolated tooth (MPM 126R) has also been assigned to the species (Machado et al. 2013). The genus name is not too difficult to interpret, while the species name uses a combination of two Ancient Greek words to refer to its herbivorous habits: némos, for "pasture" or "wood", and phagos, "to eat" (Machado et al. 2013). This gives us something like "herbivorous Brazilian titan" or "herbivorous titan from Brazil".

The holotype of Brasilotitan nemophagus, as figured in Brusatte et al. (2017). Unfortunately, as with Austroposeidon magnificus, the caption doesn't describe this part of the figure. CC BY-NC 4.0.

The most notable part of the holotype is the squared-off dentary, which makes more of a right angle than the jaw of B. salgadoi. The tooth line is twisted inwardly somewhat at the symphysis and there is a slight "chin", with the jaw being deepest at this joint. There are fourteen tooth positions in the jaw, seven along the front and seven on the lateral shaft of the jaw, without any being obviously larger than the others (unlike Baalsaurus mansillai). Functional teeth were not present, but fragments and CT scanning show that the tooth positions would have had at least two replacements in line for functional teeth. Beyond the teeth, the shaft of the jaw shows the same kind of "guillotine" as seen in Antarctosaurus wichmannianus and Bonitasaura salgadoi. The loose tooth MPM 126R is a cylindrical peg with a sort of wrinkly surface. It possibly came from the holotype jaw, but isolated teeth of a similar morphology are not uncommon (Machado et al. 2013).

Of the other bones of the holotype, one of the cervical vertebrae is from the anterior part of the neck and is strongly arched or skewed. The other is from farther back and is more typical in form. Both have proportionally low neural spines and include small-chambered internal structuring ("camellate"), which is typical for titanosaurs. Despite the jaw similarities with Bonitasaura salgadoi, the cervicals are notably different. The rest of the elements are not particularly unusual. Some puncture marks in the hip fragments are interpreted as evidence of scavenging, and theropod teeth were found with the specimen. The incompleteness of the holotype makes it difficult to place, but it doesn't seem to be particularly close to most titanosaurs known from overlapping specimens (Machado et al. 2013). In a common story by now, B. nemophagus is represented by a handful of tantalizing elements not complete enough to thoroughly compare with all of the other titanosaurs represented by a handful of tantalizing elements.


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