The Braincase of the Basal Sauropod Dinosaur Spinophorosaurus and 3D Reconstructions of the Cranial Endocast and Inner Ear

Abstract

BackgroundSauropod dinosaurs were the largest animals ever to walk on land, and, as a result, the evolution of their remarkable adaptations has been of great interest. The braincase is of particular interest because it houses the brain and inner ear. However, only a few studies of these structures in sauropods are available to date. Because of the phylogenetic position of Spinophorosaurus nigerensis as a basal eusauropod, the braincase has the potential to provide key evidence on the evolutionary transition relative to other dinosaurs.Methodology/Principal FindingsThe only known braincase of Spinophorosaurus (‘Argiles de l'Irhazer’, Irhazer Group; Agadez region, Niger) differs significantly from those of the Jurassic sauropods examined, except potentially for Atlasaurus imelakei (Tilougguit Formation, Morocco). The basisphenoids of Spinophorosaurus and Atlasaurus bear basipterygoid processes that are comparable in being directed strongly caudally. The Spinophorosaurus specimen was CT scanned, and 3D renderings of the cranial endocast and inner-ear system were generated. The endocast resembles that of most other sauropods in having well-marked pontine and cerebral flexures, a large and oblong pituitary fossa, and in having the brain structure obscured by the former existence of relatively thick meninges and dural venous sinuses. The labyrinth is characterized by long and proportionally slender semicircular canals. This condition recalls, in particular, that of the basal non-sauropod sauropodomorph Massospondylus and the basal titanosauriform Giraffatitan.Conclusions/Significance Spinophorosaurus has a moderately derived paleoneuroanatomical pattern. In contrast to what might be expected early within a lineage leading to plant-eating graviportal quadrupeds, Spinophorosaurus and other (but not all) sauropodomorphs show no reduction of the vestibular apparatus of the inner ear. This character-state is possibly a primitive retention in Spinophorosaurus, but due the scarcity of data it remains unclear whether it is also the case in the various later sauropods in which it is present or whether it has developed homoplastically in these taxa. Any interpretations remain tentative pending the more comprehensive quantitative analysis underway, but the size and morphology of the labyrinth of sauropodomorphs may be related to neck length and mobility, among other factors.

Reptiles Extintos - Volumen en Homenaje a Zulma Gasparini

“…En algunos saurópodos, como diplodocoideos y dicraeosáu-ridos, se observan en el techo craneano una o dos aperturas denominadas apertura parietal (=fenestra frontoparietal) y postparietal respectivamente (e.g., Janensch, 1935Janensch, -1936Balanoff et al, 2010;Knoll et al, 2012), las cuales están ausentes en los titanosáuridos ( La más caudal, denominada crista tuberalis por numerosos autores, es un reborde corto y se extiende ventralmente contactando el basioccipital. En los saurópodos, la crista tuberalis se desarrolla por detrás del foramen metótico ( Fig.10; ver definición de la estructura en reptiles actuales realizada por Säve-Söderbergh, 1947).…”

“…XIII, 7) y del titanosauriforme Giraffatitan ("Brachiosaurus" en Janensch, 1935-1936Knoll y Schwarz-Wings, 2009). En todos los grupos de saurópodos estudiados mediante basicráneos fracturados o moldes endocraneanos artificiales, las carótidas internas ingresan a la fosa pituitaria a través de forámenes separados (e.g., Janensch, 1935Janensch, -1936Knoll et al, 2012;Paulina Carabajal et al, 2014). El foramen externo de las carótidas se ubica sobre la cara lateral del basiesfenoides, en la base del proceso basipterigoideo, en todas las formas excepto en titanosaurios (Paulina Carabajal, 2012) y en el diplodocoideo Apatosaurus (YPM 1860), cuyo foramen carotideo abre medialmente al proceso basipterigoideo y, por lo tanto, no se observa en vista lateral del neurocráneo.…”

“…Tres forámenes separados para las ramas V 1 , V 2 y V 3 han sido documentados solamente en terópodos therizinosaurios (Lautenschlager et al, 2012). En los saurópodos en general se observa una única abertura para todas las ramas del Trigémino, incluyendo titanosaurios, dicraeosáuridos, Apatosaurus, Diplodocus y Spinophorosaurus (e.g., Janensch, 1935Janensch, -1936Knoll et al, , 2012Balanoff et al, 2010;Paulina Carabajal et al, 2014), mientras que una rama oftálmica separada se observa en Camarasaurus (Zheng, 1996; GMNH-PV 101, obs. pers.…”

“…Esta estructura, probablemente vascular, ha sido identificada como arteria basilar (?) en el prosaurópodo Plateosaurus MB.R.4396), Giraffatitan (Janensch, 1935(Janensch, -1936Knoll y Schwarz-Wings, 2009), Spinophorosaurus (Knoll et al, 2012) y los titanosaurios MML-194 (García et al, 2008) y Bonatitan (Paulina Carabajal, 2012) (Figs. 8.4, 15).…”

“…8.4,11.5 (Raath, 1977), Carcharodontosaurus idiguensis (Brusatte y Sereno, 2007), Bambiraptor (Burnham, 2004), Velociraptor , Dromaeosaurus (Chure y Madsen, 1998), rostenotes, ROM 43250) y en terópodos avianos (Witmer et al, 2003). El receso flocular está reducido en prosaurópo-dos (e.g., Plateosaurus, SMNS F.65) y fue descrito en algunos pocos saurópodos basales (e.g., Janensch, 1935Janensch, -1936Knoll y Schwarz-Wings, 2009;Knoll et al, 2012;Paulina Carabajal et al, 2013).…”