Saturday, April 30, 2016

[Paleontology • 2016] Sarmientosaurus musacchioi • A Basal Lithostrotian Titanosaur (Dinosauria: Sauropoda) with a Complete Skull: Implications for the Evolution and Paleobiology of Titanosauria


Sarmientosaurus musacchioi
Martínez, Lamanna, Novas, Ridgely, Casal, Martínez, Vita & Witmer, 2016
 Life reconstruction of two individuals of the new titanosaurian dinosaur species Sarmientosaurus musacchioi in their ~95 million-year-old habitat in southern Chubut Province, central Patagonia, Argentina, with a digital rendering of the skull in the same position as the head of the foreground individual.

life reconstruction & skull by Mark A. Klingler, Carnegie Museum of Natural History and WitmerLab, Ohio University   DOI: 10.1371/journal.pone.0151661

Abstract

We describe Sarmientosaurus musacchioi gen. et sp. nov., a titanosaurian sauropod dinosaur from the Upper Cretaceous (Cenomanian—Turonian) Lower Member of the Bajo Barreal Formation of southern Chubut Province in central Patagonia, Argentina. The holotypic and only known specimen consists of an articulated, virtually complete skull and part of the cranial and middle cervical series. Sarmientosaurus exhibits the following distinctive features that we interpret as autapomorphies: (1) maximum diameter of orbit nearly 40% rostrocaudal length of cranium; (2) complex maxilla—lacrimal articulation, in which the lacrimal clasps the ascending ramus of the maxilla; (3) medial edge of caudal sector of maxillary ascending ramus bordering bony nasal aperture with low but distinct ridge; (4) ‘tongue-like’ ventral process of quadratojugal that overlaps quadrate caudally; (5) separate foramina for all three branches of the trigeminal nerve; (6) absence of median venous canal connecting infundibular region to ventral part of brainstem; (7) subvertical premaxillary, procumbent maxillary, and recumbent dentary teeth; (8) cervical vertebrae with ‘strut-like’ centroprezygapophyseal laminae; (9) extremely elongate and slender ossified tendon positioned ventrolateral to cervical vertebrae and ribs. The cranial endocast of Sarmientosaurus preserves some of the most complete information obtained to date regarding the brain and sensory systems of sauropods. Phylogenetic analysis recovers the new taxon as a basal member of Lithostrotia, as the most plesiomorphic titanosaurian to be preserved with a complete skull. Sarmientosaurus provides a wealth of new cranial evidence that reaffirms the close relationship of titanosaurs to Brachiosauridae. Moreover, the presence of the relatively derived lithostrotian Tapuiasaurus in Aptian deposits indicates that the new Patagonian genus represents a ‘ghost lineage’ with a comparatively plesiomorphic craniodental form, the evolutionary history of which is missing for at least 13 million years of the Cretaceous. The skull anatomy of Sarmientosaurus suggests that multiple titanosaurian species with dissimilar cranial structures coexisted in the early Late Cretaceous of southern South America. Furthermore, the new taxon possesses a number of distinctive morphologies—such as the ossified cervical tendon, extremely pneumatized cervical vertebrae, and a habitually downward-facing snout—that have rarely, if ever, been documented in other titanosaurs, thus broadening our understanding of the anatomical diversity of this remarkable sauropod clade. The latter two features were convergently acquired by at least one penecontemporaneous diplodocoid, and may represent mutual specializations for consuming low-growing vegetation.

Systematic Paleontology

Saurischia Seeley 1887 
Sauropodomorpha Huene 1932 

Sauropoda Marsh 1878 
Titanosauriformes Salgado, Coria, and Calvo 1997 

Titanosauria Bonaparte and Coria 1993 
Lithostrotia Upchurch, Barrett, and Dodson 2004 

Sarmientosaurus gen. nov.
urn:lsid:zoobank.org:act:537DFE26-54EC-4978-AC86-E83A04FA74DE

Sarmientosaurus musacchioi sp. nov.
urn:lsid:zoobank.org:act:C1090B8D-D051-44F3-B869-8B4A0C802176


Sarmientosaurus head posture, brain & eye: Digital renderings of the skull and reconstructed brain endocast and eye of the new titanosaurian dinosaur species Sarmientosaurus musacchioi.
At left is the skull rendered semi-transparent in left side view, showing the relative size and position of the brain endocast (in blue, pink, yellow, and red) and the inferred habitual head posture. At center is the isolated brain endocast in left side view, and at right is a left/front view of the skull showing the reconstructed eyeball and its associated musculature. Scale bar equals five centimeters.
Credit: WitmerLab, Ohio University.  

Holotype. MDT-PV 2, an originally articulated cranial and cervical skeleton consisting of the nearly complete skull, the partial axis associated with its rib from the right side and articulated with the cranial part of the third cervical vertebra, a fragment of the fifth cervical vertebra, the nearly complete sixth cervical vertebra and its right rib, the partial seventh cervical vertebra, and a section of ossified cervical tendon.

Diagnosis.  Basal lithostrotian titanosaurian sauropod diagnosed by the following autapomorphies: (1) maximum (rostroventral—caudodorsal) diameter of orbit nearly 40% rostrocaudal length of cranium (as measured from tip of snout to occipital condyle); (2) complex maxilla—lacrimal articulation, with ascending ramus of maxilla embedded in and bordered laterally and medially by lacrimal dorsal process; (3) medial edge of caudal sector of maxillary ascending ramus bordering bony nasal aperture with low but well-defined ridge; (4) ‘tongue-like’ ventral process of quadratojugal that overlaps quadrate caudally; (5) separate foramina for all three branches of the trigeminal nerve; (6) absence of median venous canal connecting infundibular region to ventral part of brainstem; (7) premaxillary teeth subvertical, maxillary teeth procumbent, and dentary teeth recumbent; (8) middle cervical vertebrae with ‘strut-like’ (as opposed to ‘sheet-like’) centroprezygapophyseal laminae; (9) extremely elongate and slender ossified tendon extending along cervical series ventrolateral to vertebrae and ribs.

Etymology. Sarmiento, for the Patagonian town and the administrative department in which it is located, the latter of which has yielded numerous Cretaceous dinosaur fossils; saurus, Greek, ‘lizard.’ The specific name honors the late Dr. Eduardo Musacchio, a model scientist and educator at the Universidad Nacional de la Patagonia San Juan Bosco in Comodoro Rivadavia, Argentina.

Locality and horizon. Estancia Laguna Palacios (44°54'11.6'' S, 69°22'56.7'' W), Sierra Nevada Anticline, Golfo San Jorge Basin, south-central Chubut Province, central Patagonia, Argentina (Fig 1). Uppermost section of the Lower Member of the Upper Cretaceous Bajo Barreal Formation, Chubut Group. The specimen was found in situ in a tuffaceous sandstone that is regarded as Cenomanian—Turonian in age.



Fig 33. Comparison of titanosauriform sauropod dinosaur skulls in right lateral view.
(A) Giraffatitan brancai (redrawn and modified from Wilson and Sereno [103]). (B) Abydosaurus mcintoshi (redrawn and modified from Chure et al. [98]). (C) Sarmientosaurus musacchioi gen. et sp. nov. (D) Nemegtosaurus mongoliensis (redrawn and modified from Wilson [11]). (E) Rapetosaurus krausei (redrawn from Curry Rogers and Forster [13]). (F) Tapuiasaurus macedoi (redrawn from Zaher et al. [14]). Not to scale.

Conclusions
Sarmientosaurus musacchioi is the first titanosaurian sauropod from southern South America for which an articulated, virtually complete adult skull has been discovered. Phylogenetic analyses demonstrate that the new taxon is an archaic member of the titanosaurian subclade Lithostrotia, occupying a position more derived than Malawisaurus but more basal than taxa frequently regarded as nemegtosaurids (Nemegtosaurus, Rapetosaurus, and Tapuiasaurus) and saltasaurid titanosaurs such as Alamosaurus, Neuquensaurus, and Saltasaurus. As such, Sarmientosaurus is the most basal known titanosaur to be represented by a well-preserved skull. The new taxon exhibits a previously-undocumented cranial form that consists of an amalgam of plesiomorphic titanosauriform features such as a comparatively broad snout with a large narial fossa and a deep mandibular adductor chamber with more derived morphologies such as an elongate rostral process of the prefrontal (Figs 33 and 34). These characters offer novel cranial support for the phylogenetic hypothesis that titanosaurians are closely related to Brachiosauridae and other titanosauriforms—a hypothesis that, although now well-established, had previously been based primarily on evidence from the postcranial skeleton. Furthermore, the occurrence of the more derived lithostrotian Tapuiasaurus in the Aptian of Brazil raises the possibility that the new Patagonian taxon represents a titanosaurian ‘ghost lineage,’ the evolutionary history of which remains undocumented for almost all of the mid-Cretaceous.


Rubén D. F. Martínez , Matthew C. Lamanna, Fernando E. Novas, Ryan C. Ridgely, Gabriel A. Casal, Javier E. Martínez, Javier R. Vita and Lawrence M. Witmer. 2016. A Basal Lithostrotian Titanosaur (Dinosauria: Sauropoda) with a Complete Skull: Implications for the Evolution and Paleobiology of Titanosauria.  PLoS ONE. 11(4): e0151661. DOI: 10.1371/journal.pone.0151661

Newly discovered titanosaurian dinosaur from Argentina, Sarmientosaurus http://phy.so/380898167 via @physorg_com
Intact skull sheds light on Sarmientosaurus | The London Free Press http://www.lfpress.com/2016/04/26/heads-up-intact-skull-sheds-light-on-big-long-necked-dinosaurs

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