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Lesothosaurus, "Fabrosaurids," and the Early Evolution of Ornithischia Author(S): Paul C

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Lesothosaurus, Lesothosaurus, "Fabrosaurids," and the Early Evolution of Ornithischia Author(s): Paul C. Sereno Source: Journal of Vertebrate Paleontology, Vol. 11, No. 2 (Jun. 20, 1991), pp. 168-197 Published by: Taylor & Francis, Ltd. on behalf of The Society of Vertebrate Paleontology Stable URL: https://www.jstor.org/stable/4523370 Accessed: 28-01-2019 13:42 UTC JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at https://about.jstor.org/terms The Society of Vertebrate Paleontology, Taylor & Francis, Ltd. are collaborating with JSTOR to digitize, preserve and extend access to Journal of Vertebrate Paleontology This content downloaded from 69.80.65.67 on Mon, 28 Jan 2019 13:42:40 UTC All use subject to https://about.jstor.org/terms Journal of Vertebrate Paleontology 11(2): 168-197, June 1991 © 1991 by the Society of Vertebrate Paleontology LESOTHOSAURUS, "FABROSAURIDS," AND THE EARLY EVOLUTION OF ORNITHISCHIA PAUL C. SERENO Department of Organismal Biology and Anatomy, University of Chicago, 1025 E. 57th Street, Chicago, Illinois 60637 ABSTRACT -New materials of Lesothosaurus diagnosticus permit a detailed understanding of one of the earliest and most primitive ornithischians. Skull proportions and sutural relations can be discerned from several articulated and disarticulated skulls. The snout is proportionately long with a vascularized, horn-covered tip. The premaxillary palate is broad and vomers are long and fused anteriorly. Unlike many later ornithischians, the postpalatine vacuities are broadly open. The basal tubera are short and gently depressed, and the epiotic contributes to the sidewall of the braincase. The mandibular symphysis is spout-shaped, and the dentition is marked by oblique wear facets, in contrast to earlier reports. The tooth-to-tooth wear facets and form of the predentary-dentary articulation suggest long-axis rotation of the mandibular rami during mastication. The forelimb is proportionately very short, with a partially opposable pollex. The ischium lacks an obturator process. The reduced hallux is held well above the substrate during locomotion. L. diagnosticus is diagnosed below on the basis of apomorphic features. Other "fabrosaurids" con- stitute a heterogeneous assemblage of poorly known ornithischians and hatchling prosauropods that do not share any apomorphic features with L. diagnosticus. Fabrosaurus australis is a nomen dubium, and, as a consequence, the family Fabrosauridae is invalid. Echinodon becklesii may represent a primitive heterodontosaur, and Tawasaurus, Fulengia, and portions of the holotype of Technosaurus represent hatchling prosauropods. Pisanosaurus mertii may be the most primitive ornithischian, as indicated by the form of the distal crus and astragalus, but a more precise phylogenetic assessment will require additional remains. INTRODUCTION understanding of the basal ornithischian Lesothosau- Ornithischian dinosaurs have been construed as a rus and to review the materials of other supposed prim- itive ornithischians. monophyletic, or "natural," group as early as the mid- dle of the nineteenth century, when known dinosaur ABBREVIATIONS remains were few and fragmentary (Cope, 1866). Con- spicuous ornithischian synapomorphies, such as the Institutional- AMNH, American Museum of Nat- horn-covered predentary bone that caps the anterior ural History, New York; BGM, Beijing Geological Mu- end of the lower jaw, were identified as soon as betterseum, Beijing; BMNH, British Museum (Natural His- material came to light (Baur, 1891; Marsh, 1895). tory), London; CUP, Catholic University of Peking Despite longstanding consensus regarding ornithis- (housed in the Field Museum of Natural History, Chi- chian monophyly, there has been no historical schemecago); IVPP, Institute of Vertebrate Paleontology and of relationships for the various distinctive subgroups Paleoanthropology, Beijing; MCZ, Museum of Com- of ornithischians, and until recently there were no fossilparative Zoology, Harvard University, Cambridge; ornithischians that approached the hypothetical MNA,an- Museum of Northern Arizona, Flagstaff; cestor. In the late sixties, abundant remains of the MNHN, basal Museum National d'Histoire Naturelle, Par- ornithischian Lesothosaurus diagnosticus were discov- is; PVL, Instituto Miguel Lillo, Tucuman; SAM, South ered in the Lower Jurassic of southern Africa (Cromp- African Museum, Cape Town; TTU, Texas Tech Uni- ton, 1968; Thulborn, 1971 la). Other early ornithischi- versity. ans have recently been reported from North America MATERIAL (Galton, 1978; Chatterjee, 1984; Hunt, 1989), Europe (Thulborn, 1973), and China (Young, 1982). These The type dentary of Fabrosaurus australis was dis- early ornithischians, collectively referred to as "fabro- covered in 1959 in the Red Beds of the Stormberg saurs," have been regarded as ornithischian ancestors Series near Mapheteng in Lesotho, southern Africa (Fig. (Thulborn, 1970, 1971a, 1972) or as ornithischian sis- 1; MNHN LES9; Ginsburg, 1964). More complete ma- ter-groups (Galton, 1978; Norman, 1984; Milner and terial was discovered in 1963-1964 in Lesotho by an Norman, 1984; Cooper, 1985; Hunt, 1989). expedition from University College (London) and was The purpose of this paper is to augment our current referred to Lesothosaurus diagnosticus (BMNH RUB 17, 168 This content downloaded from 69.80.65.67 on Mon, 28 Jan 2019 13:42:40 UTC All use subject to https://about.jstor.org/terms SERENO - BASAL ORNITHISCHIANS 169 A F 'v~ B t`iir ' i + irr c 5mm FIGURE 1. Stereopairs of posterior dentary teeth of the holotype of Fabrosaurus australis (MNHN LES9) in lateral (A) and medial (B) views. RUB23; Galton, 1972). LESOTHOSAURUS Current DIAGNOSTICUS Galton,concepts 1978 of the cranial and postcranial morphology (Figs. 2-13) of primitive ornithischi- ans has been based almost exclusively on this material Syntypes-BMNH RUB 17 (Figs. 6, 7, 8B-F, 9A, B, (Thulborn, 1970, 1972). E, F); (Thulborn, 1970:figs. 2-7; Thulborn, 197 lb:figs. Further well-preserved cranial and postcranial re- 1-3, 8; Thulborn, 1972:figs. 2-12; Galton, 1978:fig. 2), mains of L. diagnosticus were collected in 1967-1968 mostly disarticulated remains of at least two individ- in Lesotho by a joint expedition from the British Mu- uals, one larger than the other, including most of one seum (Natural History), London University, Yale Uni- articulated skull with stapes, ceratohyals, and the an- versity and the South African Museum (Crompton, terior cervical vertebrae; BMNH RUB23 (Thulborn, 1968). Santa Luca (1984) described some of the post- 1970:fig. 1; Thulborn, 1971b:fig. 7); BMNH R11956, crania in this collection; the well preserved cranial ma- partial skull. terial (Crompton and Attridge, 1986; Weishampel, 1984) is considered below (Figs. 2-5; BMNH R8501, Locality and Horizon --Likhoele, near Mapheteng, Lesotho, southern Africa; "Red Beds," upper Elliot R11004). Formation. SYSTEMATIC PALEONTOLOGY Revised Diagnosis --Small-bodied basal ornithis- chian with cranium distinguished by anterior premax- Systematic hierarchy: illary foramen and maxilla-lacrimal articulation with Dinosauria Owen, 1842 lacrimal inserting into a slot in the apex of the maxilla. Postcranium distinguished by proportionately short Ornithischia Seeley, 1888 forelimbs (less than 40 percent hind-limb length), brev- is surface on postacetabular process of ilium broadly Genus LESOTHOSAURUS Galton, 1978 exposed in lateral view, groove on the dorsal margin Type Species -Lesothosaurus diagnosticus Galton, of the proximal ischial shaft, and reduced pedal digit 1978. I with the proximal end of metatarsal 1 splint-shaped Diagnosis-As for Lesothosaurus diagnosticus, di- and the distal end of the claw extending just beyond agnosed below. the second metatarsal. This content downloaded from 69.80.65.67 on Mon, 28 Jan 2019 13:42:40 UTC All use subject to https://about.jstor.org/terms 170 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 11, NO. 2, 1991 A / V A i A . ,L 'VY j· . - B a •~ ,. :t Jx : ~ ~ 'A qiT~· :.- 24 4. B.o, r_ i FIGURE 2. Stereopair of skull of Lesothosaurus diagnosticus (BMNH R8501) in dorsal (A) and ventral (B) views. See Fig. 3 for scale. This content downloaded from 69.80.65.67 on Mon, 28 Jan 2019 13:42:40 UTC All use subject to https://about.jstor.org/terms SERENO -BASAL ORNITHISCHIANS 171 pm Compared to other ornithischians, L. diagnosticus appears to be plesiomorphic in all other respects. L. B pd diagnosticus differs from Pisanosaurus by its advanced transversely expanded distal tibia and shallow ascend- ing process. Unlike Scutellosaurus, dorsal body armor is absent. In contrast to Echinodon, maxillary cheek emargination, maxillary-premaxillary diastema, and A dentary caniniform tooth are absent. Referred Specimens -BMNH R8501 (Figs. 2, 3; Crompton, 1968:fig. 18; Weishampel, 1984:fig. 2), nearly complete, disarticulated skull; BMNH R11002 (Fig. 9C, D), right ilium; BMNH R11003, left ilium; BMNH R11004 (Fig. 8A), partially disarticulated pos- m pa terior skull and anterior neck, including the braincase, parietals, right squamosal, right quadrate, right pos- ec pa terior lower jaw, axis, and third cervical; SAM unnum- bered (Figs. 4, 5; Crompton and
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