Humerus of a basal abelisauroid theropod from the Late Cretaceous of Patagonia

Here we describe a partial humerus of a medium-sized theropod dinosaur from the Upper Cretaceous (Late Turonian Early Coniacian) Portezuelo Formation, Neuquén Province, Argentina. The humerus shares with Abelisauroidea and Elaphrosaurus an articular head proximally oriented, and deltopectoral crest strongly reduced. The bone is referred to Abelisauroidea on the basis of the following derived features: proximal tuberosity on caudal surface of humeral shaft, and greater tubercle at level with the medial tuberosity. The gracile morphology of the humerus, as well as the poor development of its medial tuberosity, resemble more Masiakasaurus than the condition present in abelisaurids (e.g., Carnotaurus, Aucasaurus), in which the humerus is more robust and the medial tuber-osity is prominent. Available information demonstrates that during deposition of the Portezuelo Formation large and bulky abelisaurids (e.g., Ekrixinatosaurus) lived together with gracile and medium-sized abelisauroids.

Locality and horizon.The material was collected at Sierra del Portezuelo, 20 km West from Plaza Huincul, Neuquén Province, Argentina, from levels corresponding to the Portezuelo Formation.
guishes MCF-PVPH 53 (together with the four taxa cited before) from the remaining theropods (e.g., Syntarsus, Ceratosaurus, Allosaurus, Deinonychus), in which the humeral head is kidned-shaped (in proximal view) and flattened (in cranial aspect).However, MCF-PVPH 53 has a humeral head that looks more plesiomorphic than Carnotaurus and Masiakasaurus, in being craniocaudally more compressed than in the later o taxa (Fig. 4).

D escription
The preserved portion of humerus (Fig. 1) closely matches the morphology of the Malagasy abelisauroid Masiakasaurus knopfleri (Carrano et al., 2002), thus suggesting that the bone, as in this dinosaur, was long and slender when complete.On the basis of the maximum proximal width of humerus (5.1 cm), we estimate that the hole length of the bone was roughly 30 cm.
tw In MCF-PVPH 53 the greater tubercle is located on the proximolateral corner of humerus; it is reduced and located at level with the inter-nal tuberosity, as it occurs in other abelisauroids (e.g., Masiakasaurus, Aucasaurus, Carnotaurus).This condition differs from that of basal theropods (e.g., Syntarsus) , basal ceratosaurians e.g., Ceratosaurus) and most tetanurans (e.g., w The humeral head is rounded and proximally inflated, resembling the basal ceratosaurian Elaphrosaurus (Galton, 1982) and the abelisauroids Carnotaurus, Aucasaurus, and Masiakasaurus (Figs. 2-4).This condition distin-( Allosaurus, Deinonychus), in which the greater tubercle is more prominent and more proximally located with respect to the internal tuberosity.In Elaphrosaurus the greater tubercle is weakly developed (as in abelisauroids), but it is located ore proximally as in Theropoda ancestrally.
ous tuberosity.On the caudolateral edge of the bone, a smaller and lesser developed second tuberosity is present.Thus, the caudal surface of the humerus exhibits a double prominence located almost at level with the proximal end of the deltopectoral crest.This caudal tuberosity is documented also in Aucasaurus and Carnotaurus.In Masiakasaurus a prominence is visible, but its double condition can not be discerned (Carrano et al., 2002;fig. 11).The condition in Ceratosaurus is inconclusive (Madsen &Welles, 2000), but in coelophysoids (e.g., Syntarsus, iliensternus) and basal tetanurans (e.g., m In MCF-PVPH 53 the internal tuberosity is highly reduced, as it occurs in Masiakasaurus and probably also in Elaphrosaurus.However, this condition is not uniformly present among belisauroids because in abelisaurids (e.g., a Aucasaurus, Carnotaurus) the internal tuberos-ity is prominent and conical-shaped (Fig. 2).A similar condition to that of Abelisauridae is present in most theropods, including Herresaurus (Sereno, 1993), coelophysoids (e.g.,

CONCLUSIONS C
The humerus of abelisaurid theropods is well represented by Carnotaurus and Aucasaurus.The same bone, however, remains poorly known in nonabelisaurid abelisauroids, the main source of information being currently restricted to the noasaurid Masiakasaurus (Carrano et al., 2002; due to the highly fragmentary condition of the holotype specimen of Deltadromeus agilis Sereno et al., 1996, and the few available descriptions and illustrations of the forelimb bones, we did not include this taxon in the present analysis).The humerus here de-scribed adds some new data on forelimb morphol-ogy of this roup of ceratosaurian theropods.
On the cranial surface of humeral shaft, and immediately distal to the humeral head, a shallow sulcus is present, presumably for attachment of the acrocoracohumeral ligament (Baumel & Raikow, 1993).The shallow condition of this sulcus contrasts with the deep ligament groove present in Carnotaurus and Masiakasaurus.Congruently, the caudal surface of MCF-PVPH 53 is devoid of a sulcus below the humeral head, reembling Masiakasaurus but contrasting with s Carnotaurus and Aucasaurus, in which a deep roove is present.g g In MCF-PVPH 53 the deltopectoral crest is strongly reduced and triangular in side view, being transversely thick and cranially projected.This peculiar condition is shared with other abelisauroids (e.g., Masiakasaurus, Carnotaurus), albeit at least some of these features (i.e., deltopectoral crest reduced and triangular shaped) are also present in Elaphrosaurus.MCF-PVPH 53 differs from Carnotaurus in that in the latter the deltopectoral crest is even more reuced.
MCF-PVPH 53 is interpreted as belonging to an abelisauroid theropod, as supported by the presence of a proximally inflated humeral head, a reduced deltopectoral crest, greater tubercle located at level with medial tuberosity, and tuberosity on caudal surface of humeral shaft (all these features are present in Elaphrosaurus; O. Rauhut, pers. comm.).Additionally, the humerus described here is not assigned to Abelisauridae because it retained a strongly reduced medial tuberosity that contrasts with the well developed one of Carnotaurus and Aucasaurus.The condi-d On the caudal surface of the humeral shaft, MCF-PVPH 53 bears a longitudinal and conspicu-tion of the humeral head in MCF-PVPH 53 looks more primitive than in Elaphrosaurus, Masiakasaurus, Aucasaurus and Carnotaurus, because the first one lacks a rounded humeral head (in proximal aspect), a greater tubercle that is clearly offset from the humeral head (O.Rauhut, pers. comm.), and a deep cranial ligament sulcus.In this context, MCF-PVPH 53 seems to be one of the most basal abelisauroids, even more basal than the Jurassic Elaphrosaurus.MCF-PVPH 53 opens a new panorama regarding to the radiations of basal members of Abelisauroidea, where stemabelisauroids survived until late Turonian-early Coniacian times, thus depicting a currently cov-ered radiation of basal abelisauroids from Middle Jurassic times, at least (as it is indicated by Elaphrosaurus), p to the Late Cretaceous.u Among Abelisauroidea two differerent kinds of humeri can be recognized: a plesiomorphic, gracile and elongate morphotype represented by Masiakasaurus and MCF-PVPH 53, and a shorter and stouter kind of humerus as present in abelisaurids (e.g., Aucasaurus and Carnotaurus).Curiously, they are reminiscent of the humerus of ornithomimosaurs and tyrannosaurids, respectively, in which the deltopectoral crest is proximodistally reduced and subtriangular in side view (e.g., Gauthier, 1986;Rauhut, 2003).Moreover, in Tyrannosaurus (Brochu, 2002) the humeral head is also proximally inflated, resembling the ondition present in abelisauroids.c The non-abelisaurid abelisauroid condition of MCF-PVPH 53 indicates that during the deposition of the Portezuelo Formation two kinds of abelisauroid theropods co-existed.On the one hand, the large-sized and bulky abelisaurid Ekrixinatosaurus ; on the other hand, a smaller and gracile basal abelisauroid (MCF-PVPH 53).Thus, the abelisauroid faunistic component of Portezuelo Formation resembles to that of other Cretaceous Gondwanan outcrops (e.g., India, Madagascar), which shows the simultaneous presence of large-sized abelisaurids (Indosuchus, India; Majungatholus, Madagascar) and smaller non-abelisaurid abelisauroid theropods (Laevisuchus, India; Masiakasaurus, Madagascar).