Christian A. Sidor

Predatory Dinosaurs from the Sahara and Late Cretaceous Faunal Differentiation

Late Cretaceous (Cenomanian) fossils discovered in the Kem Kem region of Morocco include large predatory dinosaurs that inhabited Africa as it drifted into geographic isolation. One, represented by a skull approximately 1.6 meters in length, is an advanced allosauroid referable to the African genus Carcharodontosaurus. Another, represented by a partial skeleton with slender proportions, is a new basal coelurosaur closely resembling the Egyptian genus Bahariasaurus. Comparisons with Cretaceous theropods from other continents reveal a previously unrecognized global radiation of carcharodontosaurid predators. Substantial geographic differentiation of dinosaurian faunas in response to continental drift appears to have arisen abruptly at the beginning of the Late Cretaceous.

Ghost lineages and "mammalness": assessing the temporal pattern of character acquisition in the Synapsida

The origin of mammals has been characterized as a gradual process, a claim based pri- marily on a well-preserved series of extinct nonmammalian synapsids (mammal-like reptiles) that span some 200 million years. In contrast to the origin of many other higher taxa, the origin of mammals from within cynodont-grade therapsids is not considered to coincide with a major mor- phological change, but rather to be simply the culmination of a series of more and more mammal- like transitional forms. To test these assertions, an asymmetrical cladogram extending from prim- itive pelycosaurs to morganucodontid mammaliaforms was created. Three different methodol- ogies were then used to compare the amount of morphological change between nodes on this clado- gram with the minimum missing time interval between each node, as inferred from sister taxon- based ghost lineages. In general, a statistically significant positive relationship was found, indi- cating that greater numbers of derived features tend to be correlated with longer ghost lineages. A significant correlation between the number of accumulated apomorphies and branching events was also found. Although the rate of character change was variable, in no case was a long ghost lineage associated with few apomorphies. These correlations are consistent with the hypothesis that rapid accumulation of derived features occurred relatively infrequently within the synapsid lineage leading toward mammals and that gradual character evolution predominated.

A NEW NOTOSUCHIAN FROM THE EARLY CRETACEOUS OF NIGER

P. C. SERENO1, C. A. SIDOR2, H. C. E. LARSSON3, and B. GADO4 1Department of Organismal Biology and Anatomy, University of Chicago, Chicago, Illinois 60615, dinosaur@uchicago.edu; 2Department of Anatomy, New York College of Osteopathic Medicine, Old Westbury, New York 11568; 3Redpath Museum, McGill University, 859 Sherbrooke Street West, Montreal, Quebec H3A 2K6; 4Institut de Recherches en Sciences Humaines, Niamey, Niger Republic

THE VERTEBRATE FAUNA OF THE UPPER PERMIAN OF NIGER. IV. NIGERPETON RICQLESI (TEMNOSPONDYLI: COCHLEOSAURIDAE), AND THE EDOPOID COLONIZATION OF GONDWANA

Abstract We describe the edopoid temnospondyl Nigerpeton ricqlesi from the Upper Permian Moradi Formation of northern Niger on the basis of two partial skulls and tentatively associated postcranial material. This crocodile-like taxon displays several edopoid characters states such as a long prenarial region with enlarged premaxillae, elongated vomers, large, posteriorly tapering choanae, and a jugal that broadens anteriorly. Nigerpeton possesses a unique carnivo rous dentition. It is autapomorphic in its possession of an extremely elongate snout bearing a maxillary bulge that accommodates three hypertrophied caniniform teeth, inner premaxillary tusks, and anterior paired fenestrae, which pierce the skull roof. In addition, both the maxilla and dentary tooth rows show the sporadic appearance of ‘doubled’ tooth positions. The lateral-line system is present at the adult stage, which suggests an aquatic habitat for this taxon. A phylogenetic analysis of Edopoidea and its relatives places Nigerpeton as the sister taxon to the Permo-Carboniferous genus Chenoprosopus from the U.S.A. As with other members of the Moradi tetrapod fauna, the discovery of Nigerpeton strengthens support for the hypothesis that West Africa hosted an endemic fauna at the close of the Paleozoic Era. Biogeographically, we propose that Late Carboniferous and Permian edopoids were geographically widespread and that they twice crossed the Central Pangean mountain chain (between Laurussia and Gondwana) during their evolution. This distribution was later fragmented with the onset of Late Permian climatic warming.

Age Rank/Clade Rank Metrics—Sampling, Taxonomy, and the Meaning of "Stratigraphic Consistency"

Paleontologists frequently contrast clade rank (i.e., nodal or patristic distance from the base of a cladogram) with age rank (i.e., relative first known appearances of the analyzed taxa) to measure the degree of congruence between the estimated phylogeny and the fossil record. Although some potential biases of these methods have been examined (e.g., the effect of tree imbalance), other properties of age rank/clade rank (ARCR) comparisons have not been studied in detail. A basic premise of ARCR metrics is that outgroup taxa diverged earlier than ingroups and thus should first appear in older strata. For example, given phylogeny (A,(B,C)), then taxon A should be sampled before either taxon B or taxon C. We examine this premise in the context of (1) phylogenetic theory, (2) taxonomic practice, (3) sampling intensity (R), and (4) factors other than sampling intensity (including cladogram accuracy). Simulations combining clade evolution and sampling over time indicate a poor relationship between ARCR metrics and R when all taxa are apomorphy-based monophyletic groups. However, a good relationship exists when taxa are either stem-based monophyletic groups or if workers include taxa without a priori decisions about monophyly or paraphyly. These results are not surprising because cladograms predict the order in which lineages diverged (which applies to stem-based monophyletic taxa) and the order in which morphologic grades appeared (which applies to paraphyletic taxa relative to derived monophyletic groups). Other factors that increase ARCR metrics when the average R stays the same include high temporal variation in R, budding instead of bifurcating speciation patterns, low extinction rates, cladogram inaccuracy, and (to a much lesser extent) large clade size. These results suggest several plausible explanations for patterned differences in ARCR metrics among clades, thereby compromising their validity as measures of the quality of the fossil record.

A SECOND SPECIMEN OF LEMUROSAURUS PRICEI (THERAPSIDA: BURNETIAMORPHA)

Abstract A second specimen of the basal therapsid Lemurosaurus pricei preserves aspects of this taxon’s cranial anatomy, such as the structure of the temporal region, palate, and lower jaw, that were either unknown or poorly preserved in the holotype. Although typically considered a dinocephalian feature, intermeshing of the incisors is confirmed for Lemurosaurus. Autapomorphies of the genus include a nubbin-like boss at the apex of the lateral temporal fenestra, the presence of very coarse serrations on the posterior margins of the upper postcanine teeth, and the low, ridge-like form of the median frontal crest. The anatomical data provided by the new Lemurosaurus specimen permits the first cladistic appraisal of its phylogenetic position among biarmosuchian therapsids. A cladistic analysis of 10 therapsids, two outgroups, and 38 characters indicates that Lemurosaurus is the sister taxon to a clade including Bullacephalus, Proburnetia, and Burnetia. Taxonomically, this position implies the reclassification of Lemurosaurus as a basal burnetiamorph, rather than as an ictidorhinid. Biogeographically, the position of Lemurosaurus makes it most parsimonious to infer that burnetiamorphs originated in southern Africa, although this hypothesis ignores potential sampling biases.

A NEW BURNETIAMORPH THERAPSID FROM THE TEEKLOOF FORMATION, PERMIAN, OF SOUTH AFRICA

Abstract A new biarmosuchian therapsid, Lobalopex mordax gen. and sp. nov., from the Permian Teekloof Formation (Tropidostoma Assemblage Zone) of the Victoria West District, South Africa, is described on the basis of a partial skull with lower jaws and the first four cervical vertebrae. The genus is diagnosed by the following autapomorphies: median nasal eminence low and ridge-like; supraorbital boss small and unpachyostosed; posteriorly projecting supratemporal ‘horns’ present but relatively small; ventral surface of transverse flange of pterygoid edentulous and sharply ridged; and laterally-directed knob present lateral to contact between pterygoid and palatine dentigerous bosses. As in some other biarmosuchians, the cervical vertebrae are elongated, being roughly twice as long as they are wide or deep. A cladistic analysis including ten biarmosuchian taxa indicates that Lobalopex is the sister taxon to Burnetiidae and that Lemurosaurus is the most primitive burnetiamorph. These results fail to provide evidence for geographic endemism in these groups of Middle to Late Permian therapsids.

THE VERTEBRATE FAUNA OF THE UPPER PERMIAN OF NIGER—III, MORPHOLOGY AND ONTOGENY OF THE HINDLIMB OF MORADISAURUS GRANDIS (REPTILIA, CAPTORHINIDAE)

Abstract We describe hindlimb elements of the large captorhinid Moradisaurus grandis (Reptilia: Captorhinidae) recently collected from the Upper Permian Moradi Formation of north-central Niger. This description is based primarily on an associated partial skeleton comprising a hemimandible, vertebral column, and partial left and nearly complete right hindlimb. Additionally, we report on a well-preserved, articulated, and essentially complete juvenile pes that provides important information on the ontogeny of the captorhinid tarsus. The hindlimb of Moradisaurus is stout and more massively built than in any other known captorhinid. The femur displays several features interpreted as adaptations to the demands of large body size, and the tibia and fibula have short, wide shafts and hypertrophied condyles and epicondyles. The astragalus is very derived, possessing two accessory ossifications and a relatively horizontal tibial articulation, indicating that the tibia was held more vertically than in other captorhinids. The calcaneum is co-ossified with distal tarsal five. The juvenile pes indicates that the captorhinid centrale arose from the fusion of two ossification centers, and that the captorhinid astragalus arose from the fusion of at least three ossification centers.

THE NARIS AND PALATE OF LYCAENODON LONGICEPS (THERAPSIDA: BIARMOSUCHIA), WITH COMMENTS ON THEIR EARLY EVOLUTION IN THE THERAPSIDA

Abstract The anatomy of the external naris and anterior palate is described in detail for Lycaenodon longiceps, a morphologically conservative member of the primitive therapsid clade Biarmosuchia. Therapsids are distinguished from pelycosaur-grade synapsids by numerous features of the naris and palate. As in most early therapsids, the septomaxilla in Lycaenodon consists of a short intranarial portion and a long, posterodorsal, facial process. The septomaxillary foramen is reduced to a small, longitudinal slit between the septomaxilla and maxilla. On the palate, the unpaired vomer is clasped anteriorly by long palatal processes of the premaxillae. The choanae are elongate structures whose anterior portion housed the lower canine when the jaws were closed. The palatine and maxilla bear a small shelf (crista choanalis) on their medial surface that has been suggested to represent an early stage in the development of a secondary palate. The parallel acquisition of a crista choanalis and a bony secondary palate in several therapsid groups supports the hypothesis that respiration-related changes were an important aspect of early therapsid evolution. The genus Lycaenodon may be diagnosed by at least three autapomorphies: the premaxilla bears a scroll-like choanal process that roofs the anterior portion of the internal nares, the vomer is widest at the level of the upper canines with its downturned edges beginning more posteriorly, and the dorsal surface of the postfrontal is convex. Another specimen referred to Lycaenodon preserves additional features that might be autapomorphic, but are not preserved in the holotype.

Theropod hind limb disparity revisited: comments on Gatesy and Middleton (1997)

Theropod hind limb disparity revisited: comments on Gatesy and Middleton (1997) Matthew T. Carrano a b & Christian A. Sidor a a Department of Organismal Biology and Anatomy , University of Chicago , 1027 East 57th Street, Chicago, Illinois, 60637 b Department of Anatomical Sciences , Health Sciences Center T-8, State University of New York at Stony Brook , Stony Brook, New York, 11794 Published online: 24 Aug 2010.