Stokesosaurus clevelandi Madsen, 1974

Brusatte, Stephen L. & Benson, Roger B. J., 2013, The systematics of Late Jurassic tyrannosauroid theropods from Europe and North America, Acta Palaeontologica Polonica 58 (1), pp. 47-54 : 51-52

Stokesosaurus clevelandi Madsen, 1974

Figs. 1B , 3C .

Holotype: UMNH 2938 (formerly UUVP 2938), a left ilium.

Type horizon: Brushy Basin Member of the Morrison Formation, lower Tithonian, Upper Jurassic.

Type locality: Cleveland−Lloyd Dinosaur Quarry, Utah, USA.

Emended diagnosis.—Tyrannosauroid theropod with a single autapomorphy: a swollen rim around the articular surface of the pubic peduncle, which is especially prominent on the medial surface ( Benson 2008). Furthermore, S. clevelandi can be differentiated from other phylogenetically proximal tyrannosauroids by a unique combination of characters: an anteroposteriorly thick ridge on the lateral surface of the ilium which projects posterodorsally and extends to the dorsal margin of the iliac blade.

Remarks.—The sole autapomorphy of Stokesosaurus clevelandi is absent in “ Stokesosaurus ” langhami ( Benson 2008) , Aviatyrannis ( Rauhut 2003a) , Guanlong ( Xu et al. 2006; IVPP V14531), Raptorex ( Sereno et al. 2009; LH PV18), and tyrannosaurids (e.g., Gorgosaurus, Lambe 1917 ; Tyrannosaurus, Brochu 2003 ). It appears to be absent in the type and only known specimen of Xiongguanlong (FRDC−GS JB16−2−1), but there is some breakage in this region. This character cannot be assessed in Dilong (IVPP V14243) and Eotyrannus (MIWG 1997.550) because the pubic peduncle is damaged in all known specimens.

The unique combination of characters differentiates S. clevelandi from “ S. ” langhami and all other tyrannosauroids. A thickened lateral ridge, defined here (and used in the phylogenetic analysis) as a ridge with an anteroposterior width greater than 20% of its dorsoventral height, is also present in Guanlong ( Xu et al. 2006) , Sinotyrannus ( Ji et al. 2009) , and tyrannosaurids (e.g., Brochu 2003). A thin ridge, however, is present in “ S. ” langhami ( Benson 2008) , Dilong (IVPP V14243), Eotyrannus (MIWG 1997.550), Aviatyrannis ( Rauhut 2003a) , and Xiongguanlong ( Li et al. 2010) . The lateral ridge extends

http://dx.doi.org/10.4202/app.2011.0141

to the dorsal margin of the ilium in Guanlong , Sinotyrannus , and Aviatyrannis , whereas it stops short of the dorsal margin in “ S. ” langhami , Eotyrannus , Xiongguanlong , and tyrannosaurids. Finally, the posterodorsally oriented ridge is present in “ S. ” langhami and Eotyrannus (see above). S. clevelandi , therefore, is the only tyrannosauroid that possesses a combination of a thick, posterodorsally−trending ridge that extends to the dorsal margin of the ilium.

Figures

Fig. 1. Ilia of basal non−tyrannosaurid tyrannosauroids with a posterodorsally inclined ridge on the lateral surface of the ilium. A. Right ilium (reversed) of Juratyrant langhami Benson, 2008 (OUMNH J.3311−21), Kimmeridge Clay, Dorset England, Late Jurassic (early Tithonian). B. Left ilium of Stokesosaurus clevelandi, Madsen 1974 (UMNH VP 7473), Morrison Formation, Utah, USA, Late Jurassic (early Tithonian). C. Left ilium of Eotyrannus lengi Hutt, Naish, Martill, Barker, and Newberry, 2001 (MIWG 1997.550), Wessex Formation, Isle of Wight, England, Early Cretaceous (Barremian). All in lateral view. Arrows denote the lateral ridge.

Fig. 3. Autapomorphies of tyrannosauroids Juratyrant langhami Benson, 2008, Kimmeridge Clay, Dorset England, Late Jurassic (early Tithonian) (A, B) and Stokesosaurus clevelandi Madsen, 1974, Morrison Formation, Utah, USA, Late Jurassic (early Tithonian) (C). A. Right pubis (OUMNH J.3311−22) in lateral view, with the autapomorphic lateral fossa denoted. B. Left ischium (OUMNH J.3311−25) in lateral (B1) and anterior (B2) views, with an inset close up (2.5× magnification) of the autapomorphic folded proximal region of the ischial apron (B3). The autapomorphic convex ischial tubercle is also denoted. C. Pubic peduncle of the left ilium (UMNH 2938) in medial view, with the autapomorphic swollen rim indicated.