Bajadasaurus

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Bajadasaurus
Temporal range: Late Berriasian-Valanginian
~140–134 Ma
Skeletal elements of Bajadasaurus.png
Skeletal elements
Scientific classification edit
Kingdom: Animalia
Phylum: Chordata
Clade: Dinosauria
Order: Saurischia
Suborder: Sauropodomorpha
Clade: Sauropoda
Family: Dicraeosauridae
Genus: Bajadasaurus
Gallina et al., 2019
Species:
B. pronuspinax
Binomial name
Bajadasaurus pronuspinax
Gallina et al., 2019

Bajadasaurus is a genus of sauropod dinosaur from the Early Cretaceous epoch (late Berriasian to Valanginian stages) of northern Patagonia. It was first described in 2019 based on a single specimen found in 2010 that includes a largely complete skull and parts of the neck. The only species is Bajadasaurus pronuspinax. The genus is classified as a member of the Dicraeosauridae, a group of comparatively small and short-necked sauropods that lived from the Early or Middle Jurassic to the end of the Early Cretaceous. Bajadasaurus sported bifurcated, extremely elongated neural spines extending from the neck vertebrae. Similar elongated spines are known from the closely related and more completely known Amargasaurus. Various possible functions have been proposed for these spines in Amargasaurus, with the 2019 description of Bajadasaurus suggesting that they could have served as passive defense against predators in both genera. The eye openings of Bajadasaurus were exposed in top view of the skull, possibly allowing the animal to look forward while feeding. Bajadasaurus was discovered in sedimentary rocks of the Bajada Colorada Formation, and its environment resembled a braided river system. It shared its environment with other dinosaurs including the sauropod Leinkupal and different theropods.

Discovery and naming[edit]

Location and quarry map of holotype

The only specimen was excavated in 2010 by paleontologists of the CONICET, the science agency of the Argentinian government. The site of discovery, the Bajada Colorada locality, is located 40 kilometres (25 mi) south the town of Picún Leufú, near the western banks of Limay River, in Patagonia. The specimen, of which initially only some teeth were exposed, was found by Argentinian paleontologist Pablo Gallina. As fossils in this area are often fragile, the specimen was not excavated bone-by-bone in the field but rescued as a single large block of rock and bone wrapped in plaster. Preparation of the block in the laboratory then revealed most of the skull as well as the first two and probably the fifth neck vertebra of a new genus of dinosaur.[1][2] The site of discovery is part of the Bajada Colorada Formation, a succession of sedimentary rocks in the Neuquén Basin that is dated to the late Berriasian to Valanginian stages of the Early Cretaceous. The specimen is now curated by the Museo Municipal Ernesto Bachmann in Villa El Chocón, Neuquén Province, under the specimen number MMCh-PV 75.[2]

The specimen was formally described as the holotype of a new genus and species, Bajadasaurus pronuspinax, by Gallina and colleagues in 2019. The generic name is derived from the Spanish word Bajada ("downhill") in reference to the Bajada Colorada locality, and the Greek saurus ("lizard"). The specific name is derived from the Latin pronus ("bent over forward") and the Greek spinax ("spine"), referring to the long and anteriorly curved neural spines (spinous processes) of the neck.[2]

Description[edit]

Bajadasaurus is classified as a member of the sauropod family Dicraeosauridae. As all sauropods, dicraeosaurids were large, four-legged herbivores with a long neck and tail and proportionally very small head. They were, however, small in comparison with most other sauropods, roughly reaching sizes of today's Asian Elephants, and their neck was comparatively short.[3][4] Long bifurcated neural spines were a common feature of the group, although only in Bajadasaurus and the closely related Amargasaurus they are extremely elongated.[2]

Skull[edit]

Skull in side view (top) with interpretive diagram (bottom)

The skull includes most of the skull roof and braincase, the pterygoid bones of the palate, as well as the lower and parts of the upper jaws, and is therefore the most complete of a dicraeosaurid known to date. Not preserved are parts of the middle section of the skull. Its overall built was gracile.[2]

All bones that surround the orbit (eye opening) are preserved, except for the jugal, which would have formed the floor of the opening. The lacrimal, which formed the front margin of the orbit, had a straight ridge on its upper half that is similar to that of Dicraeosaurus. It was pierced by a small foramen (opening), unlike the larger foramen seen in Dicraeosaurus. The upper-front corner of the orbit was formed by the prefrontal. The contribution of the prefrontal to the orbit was, however, smaller than in Dicraeosaurus and Amargasaurus; the bone was also smaller and less robust than in the latter genera. The upper rim of the orbit was formed by the frontal, which was fused to the parietal behind; together, these bones formed most of the rear part of the skull roof. In top view, the side margin of the frontal was S-shaped and narrowed from back to front. As a result, the eye openings were visible in top view, unlike in related genera except Lingwulong. At the back, the frontal also formed a small part of the supratemporal fenestra, a major opening in the rear part of the skull roof. The rear margin of the orbit was formed by the postorbital. Typically in dinosaurs, this bone featured a rearwards extending process, the posterior process. In Bajadasaurus, Dicraeosaurus, and Amargasaurus, this process was reduced and indistinct. The downward projecting process of the squamosal, a bone forming the upper rear corner of the skull, was well developed. This suggests that it was connected to the quadratojugal at the lower rear edge of the skull, although the articulation itself is not preserved. This probable articulation is absent in diplodocids, and has not previously been documented in dicraeosaurids. Behind the orbit and framed by the squamosal, postorbital, quadratojugal, and jugal was the lateral temporal fenestra, another major skull opening. In Bajadasaurus, this opening was narrow and obliquely oriented. The quadratojugal formed an obtuse angle that forms the lower rear corner of the lateral temporal fenestra, different to the condition seen in diplodocids.[2]

Skull roof and braincase in top (left) and rear views (right)

The braincase is, in a complete skull, only exposed with its occipital region in rear view. The uppermost bone of the occipital region is the supraoccipital, which in Bajadasaurus was completely fused to the exoccipital-opisthotic bone below and featured a distinct and narrow longitudinal ridge, the saggital nuchal crest. The posttemporal fenestrae, a pair of openings between the parietal and the occipital region, were medially extended (reach closer to the skull midline), which is an autapomorphy of Bajadasaurus (a unique feature not found in closely related genera). The occipital condyle, which articulated with the first vertebra of the neck, was wider than high. Its rear surface was not wider than its neck, different to Amargasaurus and Dicraeosaurus. The basisphenoid, which formed part of the floor of the braincase, had a pair of gracile bony extensions, the basipterygoid processes, which extended forwards and downwards to articulate with the pterygoid of the palate, bracing the braincase against the latter. An autapomorphy of the genus, these processes were longer and slenderer than in Dicraeosaurus and Amargasaurus, being more than six times longer than wide. The left and right pterygoids, the only elements preserved of the palate, featured a smooth crest that received the basipterygoid processes.[2]

Lower jaw in side (A), top (B), inner (c), and front (D) views

The teeth were restricted to the front parts of the jaws and were pencil-shaped, with their narrow crowns nearly straight or slightly curved inwards. Of the upper jaw, only the front section of the left maxilla (the largest bone of the upper jaw) is preserved. It preserves eight alveoli (tooth positions), a count similar to Suuwassea, but less than in Dicraeosaurus, which had 12 teeth in each maxilla. A seemingly complete tooth row of 24 teeth was found close to, but separated from, the left maxilla. This count corresponds to the tooth count of the lower jaw, where the teeth are still anchored within the left and right dentaries (the only tooth-bearing bones of the lower jaw). Bajadasaurus thus likely had 44 teeth in total. The dentary was slender, similar to Suuwassea but unlike the deep dentary of Dicraeosaurus. In top view, the dentaries do not form the box-shaped snout seen in diplodocids, but are more rounded with a J-shaped curvature, as typical for dicraeosaurids. The front of the dentary had a hook-like "chin" projecting downwards, as seen in other flagellicaudatans. The angular bone of the hind part of the lower jaw was very elongated and longer than the surangular bone, unlike in diplodocids.[2]

Cervical vertebrae[edit]

Cervical vertebra, thought to be the fifth, with long neural spines

Both proatlases—small, triangular bones located between the first cervical vertebra and the skull—were preserved in articulation with the skull. Of the first cervical vertebra, the atlas, only the upper elements, the atlantal neurapophyses, are preserved. These were triangular and wing-like in Bajadasaurus. The second cervical vertebra, the axis, is nearly complete. As in Dicraeosaurus, it is twice as high as long, while its centrum (or vertebral body) is twice as long as high. The diapophyses (sidewards projecting processes) were small and directed backwards as in Suuwassea rather than downwards as in Dicraeosaurus and Amargasaurus. The neural spine of the axis is narrow and not bifurcated; it differs from other sauropods in being vertically oriented (an autapomorphy of the genus); triangular in cross-section; and tapering towards its apex.[2]

Only a single vertebra is known from the remainder of the neck. This vertebra sports the most prominent feature of the genus, a deeply bifurcated and extremely elongated neural spine that makes the vertebra four times taller than long. This pair of rod-like elements measures 58 cm (23 in) in length and are only comparable to those of the related Amargasaurus. Unlike in the latter, the neural spine of Bajadasaurus is curved toward the front. Their base was triangular and compressed sideways; their cross-section along most of their length was egg-shaped. Their tips broadened slightly, unlike the acute tips in Amargasaurus. In Amargasaurus, the spines show striations on their surface that indicate that a horn sheath was present in life. Although similar striations cannot be observed on the spines of Bajadasaurus due to poor preservation, Gallina and colleagues in 2019 found it likely that they where covered by a horny sheath as well. The exact position of the vertebra in the neck is unclear, however. Its morphology is comparable to the fifth cervical of Dicraeosaurus, the probably sixth of Brachytrachelopan, and the seventh of Amargasaurus; based on these comparisons, it was tentatively described as the fifth cervical. The centrum of this vertebra is twice as long as tall and narrows into a ventral keel (a longitudinal ridge at the underside); this keel was broader and concave in other dicraeosaurids.[2]

Classification[edit]

Related genus Amargasaurus, whose similarly elongate neural spines were compared to those of Bajadasaurus

Dicraeosaurids are one of the three principal families comprising the Diplodocoidea, a major subdivision of sauropod dinosaurs. Within Diplodocoidea, dicraeosaurids form the sister group of the Diplodocidae, while the third family, the Rebbachisauridae, is more distantly related. Dicraeosaurids and diplodocids are united within the group Flagellicaudata, which is named after the whip-like tail characteristic for the group.[5] In their 2019 description of Bajadasaurus, Gallina and colleagues recognized seven additional dicraeosaurid genera. The earliest is Lingwulong from the late Early to early Middle Jurassic of China,[6] while three genera are known from the Late JurassicBrachytrachelopan from Argentina; Suuwassea from the US; and the eponymous Dicraeosaurus from Tanzania. Early Cretaceous dicraeosaurids include Bajadasaurus as well as Amargatitanis, Pilmatueia, and Amargasaurus, all from Argentina. An unnamed specimen from the Itapecuru Formation of Brazil indicates that the group persisted at least until the end of the Early Cretaceous.[7]

Isolated tooth row showing the premaxillary (pm1–4) and maxillary teeth (m1–8)

In their phylogenetic analysis, Gallina and colleagues (2019) recovered Bajadasaurus as an intermediate member of Dicraeosauridae, more derived than Suuwassea and Lingwulong, but less so than Pilmatueia, Amargasaurus, Dicraeosaurus, and Brachytrachelopan. Amargatitanis was removed from the analysis due to its unstable position. The referral of Bajadasaurus to the Dicraeosauridae was supported by six synapomorphies (anatomical features shared with other members of the group). Bajadasaurus itself can be differentiated from other dicraeosaurids by a unique combination of features, which includes four autapomorphies (a medially extended posttemporal fenestra; slender and long basipterygoid processes; vertically oriented neural spine of second cervical vertebra; and elongated, forward curved neural spines in the neck).[2]

The following cladogram by Gallina and colleagues (2019)[2] shows the possible relationships between members of the Dicraeosauridae:

Diplodocoidea

Rebbachisauridae

Flagellicaudata

Diplodocidae

Dicraeosauridae

Suuwassea

Lingwulong

Bajadasaurus

Pilmatueia

Amargasaurus

Dicraeosaurus

Brachytrachelopan

Paleobiology[edit]

Function of neural spines[edit]

Interpretation of the soft part anatomy of a neck vertebra of the related Amargasaurus, showing the bifurcated neural spines, supravertebral diverticulum, and horn sheaths in cross section. A similar anatomy has been suggested for Bajadasaurus.

Elongated and deeply bifurcated neural spines were common in dicraeosaurids. In Dicraeosaurus and Brachytrachelopan, they were inclined toward the front but remained much shorter than in Bajadasaurus. Only the spines of Amargasaurus, which where directed backwards, were elongated to a similar degree than those of Bajadasaurus. The spines of Amargasaurus led to much speculation about their possible life appearance and function. As hypothesized by separate authors, they could have supported a sail or horny sheaths, and could have been used for display, defense, or thermoregulation.[2] Daniela Schwarz and colleagues, in 2007, found that the bifurcated neural spines of dicraeosaurids enclosed an air sac, the so-called supravertebral diverticulum, that was connected to the lungs. In Dicraeosaurus, this air sac would have occupied the entire space between the left and right parts of the spines, while it would have been restricted to the lower third of the spines in Amargasaurus. The upper two thirds would likely have been covered by a horny sheath, as is indicated by longitudinal striations on their surface.[8][2]

Gallina and colleagues, in 2019, considered this the most reasonable interpretation that may likewise be applied to Bajadasaurus. These researchers further argued that horn is more resistant to impact-related fractures than bone, and that a horny sheath would therefore have protected the delicate spines from damage. Fracturing of the spines might have been a critical threat, as the bases of the spines roofed the spinal chord. The protection of the sheath would have been further enhanced if it would have extended past its bony core. Schwarz and colleagues reconstructed Amargasaurus with horny sheaths that did not reach far beyond their bony core; the same is true for most modern reptiles. In some modern even-toed ungulates, however, the horny sheath can be double the length of the horn core, and the exquisitely preserved ankylosaur Borealopelta was found with horny sheaths that extended the length of its spines by 25%, demonstrating that substantial horny extensions may occur in dinosaurs as well. Gallina and colleagues suggested that the spines of Amargasaurus and Bajadasaurus might have been 50% longer than indicated by their bony core.[2] Their bending would have further increased their resistance, as is the case in modern bighorn sheep.[2][9]

Speculative skeletal reconstruction by Gallina and colleagues, 2019,[2] showing known elements in white. The total number of neck vertebrae and length of the spines is based on Amargasaurus

Gallina and colleagues further speculated that the spines in both Amargasaurus and Bajadasaurus might have been used for defense. Due to its forward bent, the bifurcated neural spine of the supposed fifth neck vertebra would have reached past the head, acting as what was compared to a fence to deter predators. Similar, even larger spikes were postulated for the following neck vertebrae. Moderate damages would result in the break-off of the horny tips, leaving the bony spine intact. Amargasaurus lived around 15 million years later than Bajadasaurus, indicating that elongated neural spines were a long-lasting defense strategy.[2]

Senses[edit]

The orientation of the semicircular canals in the related Amargasaurus indicate that the head was habitually held with its snout facing downwards.[10] Assuming a similar head orientation in Bajadasaurus, Gallina and colleagues in 2019 hypothesized that the exposure of the eye openings in top view might have allow the animal to look forward while feeding, while the sight of most other sauropods is limited to the sides. These researchers furthermore speculated that this feature could even have allowed for stereoscopic vision.[2]

Paleoenvironment[edit]

Speculative life restoration based on Amargasaurus

Bajadasaurus was recovered from the Bajada Colorada Formation, a geological formation of the Mendoza Group that is exposed in northern Patagonia. The formation is composed of red and green-brown sandstones and conglomerates of fine to coarse grain size together with bands of reddish claystones and light brown siltstones. These sediments were mostly deposited by braided rivers, as is evident by well-preserved river channels with cross bedding. Paleosols are present in the formation. The Bajada Colorada Formation overlies the Quintuco and Picún Leufú Formations and is overlain by the Agrio Formation.[11][2][12] The upper age of the formation is restricted by the timing of an unconformity, dated at 134 mya.[13] Bajadasaurus stems from the Bajada Colorada locality, the type locality of the formation. The locality yielded the remains of another sauropod, the diplodocid Leinkupal laticauda, as well as of several species of theropod that can be ascribed to basal tetanurans and possibly to abelisauroids and deinonychosaurians.[11][2]

References[edit]

  1. ^ Yong, Ed (2019-02-05). "This Dinosaur Had a Mohawk of Horns". The Atlantic. Retrieved 2019-02-09.
  2. ^ a b c d e f g h i j k l m n o p q r s t u Gallina, P.A.; Apesteguía, S.; Canale, J.I.; Haluza, A. (2019). "A new long-spined dinosaur from Patagonia sheds light on sauropod defense system". Scientific Reports. 9: 1392. doi:10.1038/s41598-018-37943-3.
  3. ^ Hallett, M.; Wedel, M. (2016). The Sauropod Dinosaurs: Life in the Age of Giants. Baltimore: Johns Hopkins University Press. p. 45. ISBN 978-1421420288.
  4. ^ Paul, G. S. (1994). "Dinosaur art & restoration notes: Dicraeosaurs" (PDF). The Dinosaur Report. 8.
  5. ^ Harris, JD; Dodson, P (2004). "A new diplodocoid sauropod dinosaur from the Upper Jurassic Morrison Formation of Montana, USA". Acta Palaeontologica Polonica. 49 (2): 197–210.
  6. ^ Xing Xu; Paul Upchurch; Philip D. Mannion; Paul M. Barrett; Omar R. Regalado-Fernandez; Jinyou Mo; Jinfu Ma; Hongan Liu (2018). "A new Middle Jurassic diplodocoid suggests an earlier dispersal and diversification of sauropod dinosaurs". Nature Communications. 9: Article number 2700. doi:10.1038/s41467-018-05128-1.
  7. ^ Novas, F.E. (2009). The age of dinosaurs in South America. Bloomington: Indiana University Press. pp. 172–174. ISBN 978-0-253-35289-7.
  8. ^ Schwarz, D.; Frey, E.; Meyer, C. A. (2007). "Pneumaticity and soft-tissue reconstructions in the neck of diplodocid and dicraeosaurid sauropods" (PDF). Acta Palaeontologica Polonica. 52 (1).
  9. ^ Drake, Aaron; Haut Donahue, Tammy L.; Stansloski, Mitchel; Fox, Karen; Wheatley, Benjamin B.; Donahue, Seth W. (2016-10-15). "Horn and horn core trabecular bone of bighorn sheep rams absorbs impact energy and reduces brain cavity accelerations during high impact ramming of the skull". Acta Biomaterialia. 44: 41–50. doi:10.1016/j.actbio.2016.08.019. ISSN 1742-7061.
  10. ^ Paulina Carabajal, A.; Carballido, J.L.; Currie, P.J. (2014). "Braincase, neuroanatomy, and neck posture of Amargasaurus cazaui (Sauropoda, Dicraeosauridae) and its implications for understanding head posture in sauropods". Journal of Vertebrate Paleontology. 34 (4): 870–882. doi:10.1080/02724634.2014.838174.
  11. ^ a b Gallina, P.A.; Apesteguía, S.Z.; Haluza, A.; Canale, J.I. (2014). "A diplodocid sauropod survivor from the Early Cretaceous of South America". PLoS ONE. 9 (5): e97128. Bibcode:2014PLoSO...997128G. doi:10.1371/journal.pone.0097128.
  12. ^ Leanza, Héctor A.; Hugo, Carlos A. (2001). "Cretaceous red beds from southern Neuquén Basin (Argentina): age, distribution and stratigraphic discontinuities". Publicación Electrónica de la Asociación Paleontológica Argentina. 7 (1): 116–122.
  13. ^ Leanza, Héctor A. (2005). "Las principales discordancias del Jurásico Superior y el Cretácico de la Cuenca Neuquina". Anales de la Academia Nacional de Ciencias Exactas, Físicas y Naturales. 57: 147–155.