Tuesday, 19 February 2019

Malawisaurus dixeyi: The braincase and inner ear of a Southern African Titanosaur.

Titanosaurs were a group if exceptionally large Sauropod Dinosaurs that dominated many faunas in the Southern Hemisphere during the Cretaceous. The group includes the largest known Dinosaurs, with species such as Argentinosaurus and Puertasaurus thought to have weighed close to 90 tonnes. However the group was quite diverse, and also contained many smaller species, as well as both long and short-necked forms, suggesting a wide range of ecological specialisations.

Titanosaur remains from Malawi, Southern Africa, were first described under the name Gigantosaurus dixeyi in 1928. These fossils underwent several name-changes as our understanding of Dinosaur taxonomy changed and grew during the twentieth century, eventually gaining the current name, Malawisaurus dixeyi, in 1993. The beds which produced these fossils are of Cretaceous age, though a more precise date has eluded geologists to date; biostratigraphic studies using Ostracods (small Crustaceans with distinctive shells and high species turnover), and the beds are closely related to carbonates that have been dated to between 123 and 111 million years old, based upon potassium-argon geochronology (argon is a noble gas, and cannot be incorporated into rocks, but unstable isotopes of potassium, which decay into argon can, and will then remain in the rock; since this happens steadily at a known rate, geochemists can date rocks by establishing the ratio of radioactive potassium to argon within them), but studies of the Vertebrate fauna preserved within these rocks has suggested a Late Cretaceous origin.

In a paper published in the journal PLoS One on 13 February 2019, Kate Andrzejewski, Michael Polcyn and Dale Winkler of the Roy M. Huffington Department of Earth Sciences at Southern Methodist University, Elizabeth Gomani Chindebvu of Culture and Community Development at the Ministry of Civic Education in Malawi, and Louis Jacobs, also of the Roy M. Huffington Department of Earth Sciences at Southern Methodist University, describe new Malawisaurus dixeyi material from Malawi, including a reconstruction of the inner ear and endocast (cast of the inside of the braincase, this is not the same as the brain, but can give information about it), and draw conclusions from this.

The described specimen, Mal-202-1, comprises a nearly complete basicranium and associated parietals, ectopterygoid, quadrate, cervical vertebrae, and post cranial elements, all assigned to Malawisaurus dixeyi, recovered from near Mwakasyunguti in Karonga District, northern Malawi, by the Malawi Dinosaur Project in the 1980s and 90s.

The specimen was scanned at the University of Texas High Resolution X-ray CT facility, enabling Andrzejewski et al. to construct a three dimensional computer model of the brain endocast and inner ear. The bones of the braincase are well preserved, and all show fully ossified sutures, suggesting that the specimen was a mature adult at time of death. An estimate of its size based upon the circumference of the humerus, suggests a living weight of 4.73 tonnes.

Braincase of Malawisaurus dixeyi. (A) lateral view; (B) lateral view with endocast; (C) posterior view. Abbreviations: BO, basioccipital; BP, basipterygoid process; BT, basal tuber; CAR, canal for cerebral carotid artery; FO, fenestra ovalis; LABYR, labyrinth; LS, laterosphenoid; OC, occipital condyle; PFO, pituitary fossa; PP, paroccipital process; SO, supraoccipital; SPHA, canal for sphenopalatine artery; III, oculomotor nerve; IV, trochlear nerve; V, trigeminal nerve; VI, abducens nerve; VII, facial nerve; IX-XI, shared canal for glossopharyngeal, vagus, and spinal accessory nerves; XII, hypoglossal nerve. Scale bar equals 10cm. Andrzejewski et al. (2019).

Andrzejewski et al.’s reconstruction lacks the olfactory and cerebral regions, or caudal dural expansion, a prominent venous feature of Sauropods. Like most Sauropod endocasts it shows a lack of distinction of gross regions of the brain, presumably obscured by the presence of overlying thick meninges and extensive venous sinuses in life, but does show typical Sauropod features such as a large pituitary fossa. The reconstruction does show the connections of the veinous canals (canals through which the veins pass) and cranial nerves, the general pattern of which are consistent with the interpretation of Malawisaurus as a Titanosaur.

Cranial endocast and vestibular labyrinth of Malawisaurus dixeyi. (A) left lateral view; (B) caudal view; (C) ventral view; (D) dorsal view; dashed line represents reconstruction of full endocast based on the endocast of Sarmientosaurus. Endocast represented by purple colouring; cranial nerves by yellow colouring; vestibular labyrinth by pink colouring; carotid artery by red colouring. Scale bar equals 5cm. Andrzejewski et al. (2019).

The inner ears of several Sauropods have been described previously, with a general pattern observed of larger vestibular labyrinths in early members of the group, and smaller in more derived Titanosaurs; that of Malawisaurus dixeyi appears to be intermediate in size, which is roughly what would be expected based upon current interpretations of its phylogenetic position as a Titanosaur that split from the group early in their history. It also shows uneven size of the semicircular canals, something which is found in earlier Sauropods, but not more derived Titanosaurs, again consistent with the current interpretations of the phylogenetic position of the species.

Left vestibular labyrinth of Malawisaurus dixeyi. (A) lateral view; (B) posterior view; (C) dorsal view. Abbreviations: C, cochlea; CRC, crus commune; CSC caudal (posterior) semicircular canal; FP, fenestra perilymphatica; FV fenestra vestibuli; LSC, lateral semicircular canal; RSC, rostral (anterior) semicircular canal; VE, vestibule of inner ear. Scale bar equals 2cm. Andrzejewski et al. (2019).

See also...

https://sciencythoughts.blogspot.com/2016/10/savannasaurus-elliottorum.htmlhttps://sciencythoughts.blogspot.com/2016/04/notocolossus-gonzalezparejasi-new.html
https://sciencythoughts.blogspot.com/2014/10/a-new-titanosaur-from-middle-cretaceous.htmlhttps://sciencythoughts.blogspot.com/2014/09/the-nearly-complete-skeleton-of-two.html
https://sciencythoughts.blogspot.com/2014/07/a-new-species-of-titanosaur-from-late.htmlhttps://sciencythoughts.blogspot.com/2013/12/a-new-species-of-titanosaur-from-early.html
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