Introducing Bajadasaurus pronuspinax.

Bajadasaurus reconstruction (Museo Municipal Ernesto Bachmann, Villa El Chocón, Neuquén).

Dicraeosauridae is a family of mid-sized sauropod dinosaurs characterized by a distinctive vertebral column with paired, long, neural spines. The group was first described in 1914 by Werner Janensch with the discovery of the nearly complete skeletons of Dicraeosaurus in the expeditions to the upper Jurassic beds of Tendaguru, Tanzania. Dicraeosauridae includes  Amargasaurus, Pilmatueia, Suuwassea, and Brachytrachelopan. Now, the description of Bajadasaurus pronuspinax gen. et sp. nov., from the Early Lower Cretaceous of Bajada Colorada Formation in Northern Patagonia, Argentina), shed new light on the function of its spines and the defense behavior in sauropod dinosaurs.

Bajadasaurus was discovered in 2013, by a team of paleontologists from CONICET, Fundación Félix de Azara, Universidad Maimónides, and Museo Paleontológico Ernesto Bachmann. The generic name derived from Bajada (Spanish, in reference to the locality Bajada Colorada) and saurus (Greek, lizard). The specific name derived from pronus (Latin, bent over forward) and spinax (Greek, spine), referring to the anteriorly pointed, curved, neural spines of the cervical vertebrae.

Skeletal elements of Bajadasaurus pronuspinax. From Gallina et al., 2019.

The holotype, MMCh-PV 75, includes a nearly complete skull (left maxilla, left lacrimal, both prefrontals, both frontals, both parietals, both postorbitals, both squamosals, left quadratojugal, both pterygoids, both quadrates, supraoccipital, exoccipital-opisthotic complex, basioccipital, basisphenoid, both prootics, both laterosphenoids, both orbitosphenoids, both dentaries, left surangular, both angulars, both splenials, left prearticular, left articular, isolated upper tooth row), both proatlases, atlantal neurapophyses, axis and the fifth cervical vertebra.

The skull of Bajadasaurus is gracile, with dorsally exposed orbits, dorsoventrally compressed occipital condyle, extremely narrow basipterygoid processes, elongate and slender anterior processes of the squamosals, medially extended post-temporal fenestrae, short lateral temporal fenestrae and a reduced dentition in the maxilla and dentary, that largely differs from other known taxa within Dicraeosauridae. But the most striking feature of Bajadasaurus is the presence of extremely long cervical neural spines that curve anteriorly. Amargasaurus exhibit the same development of cervical neural spine elongation as Bajadasaurus, but the spines of the former point backwards rather than forwards. Dicraeosaurus and Brachytrachelopan show anteriorly inclined neural spines in the cervical vertebrae, but the spines are much shorter than in Bajadasaurus.

A group of Bajadasaurus. Illustration: Jorge A. González.

The discovery of Amargasaurus cazaui in 1991, from the Early Cretaceous beds of La Amarga Formation of Northern Patagonia, renewed the discussion on the peculiar vertebral anatomy of these sauropod dinosaurs, including interpretations as a support structure for a thermoregulatory sail, a padded crest as a display and/or clattering structure, a dorsal hump, or as internal cores of dorsal horn. Those explanation, except the last one, require that these long and extremely gracile bone projections, now recognized in Bajadasaurus as well, can support enough physical stress to avoid fracturing. Bone is stronger and stiffer in passive situations, however, horns and other keratin-based materials are tougher and highly resistant to impact-related fractures. Therefore, the keratinous sheath in Amargasaurus and perhaps Bajadasaurus provides a better mechanical solution against a potential fracture.

 

References:

Gallina, Pablo A., Apesteguía, Sebastián, Canale, Juan I., Haluza, Alejandro (2019), A new long-spined dinosaur from Patagonia sheds light on sauropod defense system, Scientific Reports volume 9, Article number: 1392 DOI: https://doi.org/10.1038/s41598-018-37943-3

Janensch, W. Die Wirbelsäule der Gattung Dicraeosaurus. Palaeontographica Supplement 7, 37–133 (1929).

Salgado, L. & Bonaparte, J. F. Un nuevo saurópodo Dicraeosauridae, Amargasaurus cazaui gen et sp. nov., de la Formación La Amarga, Neocomiano de la provincia del Neuquén, Argentina. Ameghiniana 28, 333–346 (1991).

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Lingwulong shenqi, the “Amazing Dragon”, and the dispersal of Sauropods.

Skeletal reconstruction and exemplar skeletal remains of Lingwulong shenqi. Scale bars = 100 cm for a and 5 cm for b–o. From Xu et al., 2018

Sauropods were the largest terrestrial vertebrates. Their morphology is easy recognizable: a long, slender neck and a tail at the end of a large body supported by four columnar limbs. Sauropods dominated many Jurassic and Cretaceous terrestrial faunas. Although they were globally distributed, the absence of Diplodocoidea from East Asia has been interpreted as a biogeographic pattern caused by the Mesozoic fragmentation of Pangea. However, a newly discovered dinosaur from the Middle Jurassic of northern China suggests that Sauropods dispersed and diversified earlier than previously thought.

Lingwulong shenqi — literally the “amazing dragon from Lingwu” — is the first well-preserved confirmed diplodocoid from East Asia (23 synapomorphies support the placement of Lingwulong within Diplodocoidea with 10 of these being unequivocal). The holotype, (LM) V001a, is a partial skull comprising the braincase, skull roof, and occiput, and an associated set of dentary teeth. The paratype, (LGP) V001b, comprises a semi-articulated partial skeleton including a series of posterior dorsal vertebrae, complete sacrum, the first caudal vertebra, partial pelvis, and incomplete right hind limb.

An artist’s interpretation of what Lingwulong shenqi (Image: Zhang Zongda)

The Lingwulong specimens were found in the Yanan Formation at Ciyaopu, in northwest China. This formation has been divided in four or five members. Although, no radiometric constraints have been obtained for the Yanan Formation, its age has been estimated on the basis of biostratigraphy. The presence of a conchostracans assemblage (including Palaeoleptoestheria, Triglypta, and Euestheria) is indicative of a Middle Jurassic age.

The East Asian Isolation Hypothesis (EAIH) has become a well-established explanation of profound differences between Jurassic (and sometimes Early Cretaceous) Asian terrestrial faunas, that resulted in the evolution of endemic groups such as mamenchisaurid sauropods, and the early diverging lineage of tetanurans, oviraptorosaurs, therizinosaurs. In this model, the isolation ended in the Early Cretaceous when marine regressions allowed the invasion of groups from elsewhere in Pangaea, and the dispersal of Asian endemics (e.g., oviraptorosaurs, marginocephalians) into Europe and North America. However, it was claimed that diplodocoids never took part in these dispersals because the end-Jurassic extinction that greatly reduced their diversity and geographic range in the Early Cretaceous. The discovery of Lingwulong undermines the EAIH, forcing a significant revision of hypotheses concerning the origins and early radiation of Neosauropoda.

 

References:

Xing Xu, Paul Upchurch, Philip D. Mannion, Paul M. Barrett, Omar R. Regalado-Fernandez, Jinyou Mo, Jinfu Ma and Hongan Liu. 2018. A New Middle Jurassic Diplodocoid Suggests An Earlier Dispersal and Diversification of Sauropod Dinosaurs. Nature Communications.9, 2700.  DOI:  10.1038/s41467-018-05128-1