Historical Biology, 2004, 1–13 Giants and Bizarres: Body Size of Some Southern South American Cretaceous Dinosaurs GERARDO V. MAZZETTAa,*, PER CHRISTIANSENb,† and RICHARD A. FARIN˜ Aa,‡ aDepartamento de Paleontologı´a, Facultad de Ciencias, Universidad de la Repu´blica, Igua´ 4225, 11400 Montevideo, Uruguay; bDepartment of Vertebrates, Zoological Museum, Universitetsparken 15, DK-2100 Copenhagen Ø, Denmark Body masses of some South American dinosaurs are sauropods from southern South America appear to estimated. The sauropod Argentinosaurus huinculensis have rivalled or even exceeded the hitherto largest reached 73 tonnes, and therefore, is the largest of all land known Laurasian forms. animals whose mass has been rigorously obtained. Another sauropod, Antarctosaurus giganteus, was the It is difficult to say to what extent the second largest, at nearly 69 tonnes, while Antarctosaurus known southern South American taxa might wichmannianus reached 34 tonnes. A third sauropod, the have characterised the Cretaceous continental bizarre-looking Amargasaurus cazaui, was much smaller, assemblages of all of South America. However, it with a body mass of only 2.5 tonnes. Among theropods, the body mass of the strangely looking, horned seems reasonable to suppose that most of the Carnotaurus sastrei, was volumetrically estimated at 1.5 dinosaur groups recorded in southern South America tonnes, while allometric equations on limb measure- were present in other regions of the continent as well, ments yielded overestimations. Moreover, the holotype where relatively less palaeontological excavations specimen of Giganotosaurus carolinii (MUCPv-CH-1) have been made and where preservation conditions was about as large as the average-sized Tyrannosaurus rex, and only marginally smaller than “Sue”, the largest are not as favourable as in Patagonia. Bonaparte specimen. However, a new dentary of Giganotosaurus (1986) has pointed out that most of the dinosaurian (MUCPv-95) is 8% longer than that of the holotype. taxa of the South American Cretaceous were Assuming geometric similarity, that individual must indicative of the endemism observed throughout have had a body mass above 8 tonnes and hence must have been the largest theropod ever found. most of the southern supercontinent of Gondwana. This process was likely brought about by the relative geographic isolation of Gondwana and Laurasia, Keywords: South American dinosaurs; Argentinosaurus; Antarctosaurus; Amargasaurus; Giganotosaurus; Carnotaurus which probably began in the Middle Jurassic and lasted until the Campanian (Bonaparte, 1986, 1996a). However, this palaeobiogeographic issue still needs INTRODUCTION further study to be fully understood. The current knowledge of the dinosaur fauna of the South Dinosaurs were the dominant forms of the Creta- American Cretaceous indicates that it shows ceous tetrapod faunas of southern South America, as reasonably good affinities with the Cretaceous they were in general throughout the Mesozoic faunas of other Gondwanan landmasses: Africa, ecosystems (Wing et al., 1992). Among the Indo-Madagascar, and Australia (Bonaparte, 1986; dinosaurs, sauropods are particularly interesting Sampson et al., 1998), and more substantial differ- from a biological point of view since some of them ences with the Laurasian fossil record (Bonaparte and constituted the largest terrestrial vertebrates by a Kielan-Jaworowska, 1987). wide margin (Benton, 1990; Christiansen, 1997; Several of the southern South American dinosaurs 2002). The large-bodied Laurasian sauropods are appear different from their Laurasian relatives. generally the more well known, and some of these The dicraeosaurine sauropod Amargasaurus cazaui were of truly gigantic proportions, but some appears to have taken the elongated, bifurcate neural *E-mail: [email protected] †E-mail: [email protected] ‡Corresponding author. E-mail: fari,[email protected] ISSN 0891-2963 print/ISSN 1029-2381 online q 2004 Taylor & Francis Ltd DOI: 10.1080/08912960410001715132 2 G.V. MAZZETTA et al. spines, common in the neck and anterior dorsal the holotype of another titanosaurian sauropod, regions of diplodocoids, to extremes, as they A. giganteus (MLP 23-316), were used for this study.{ resemble large spikes. Among the most interesting The mentioned material is housed at the exhibition of forms of titanosaurian sauropods known to date are the Museo de La Plata, Argentina, except for the Argentinosaurus huinculensis, widely regarded as the holotype of A. huinculensis, which is exhibited at the largest dinosaur hitherto known (Bonaparte and Museo Municipal “Carmen Funes” of Plaza Huincul, Coria, 1993; Appenzeller, 1994; Paul, 1994, 1997), and Neuque´n, Argentina. Additionally, the principal Antarctosaurus giganteus, which appears to have been hindlimb bones of Antarctosaurus wichmannianus nearly as large. Aspects of the palaeobiology of these were included. The latter material was collected in giants have not been investigated, nor have rigorous Chubut, Argentina (Huene, 1929), but presently is body mass estimates been attempted. Two recent housed at the Field Museum of Natural History theropod discoveries on Gondwana, although in Chicago. At present it appears uncertain incomplete or fragmentary, appear to have equalled if A. wichmannianus is a different species from or even exceeded in size to the largest and complete A. giganteus, but in this paper they are treated as two Tyrannosaurus rex, the specimen FMNH PR2081 species (the impressive size difference may be informally known as “Sue” (Horner and Lessem, considered another indication that they represent 1993). One of them is the predatory dinosaur two species). The femur (FMNH P13019) and Carcharodontosaurus saharicus,aspeciesfirst associated tibia (FMNH P13020) are not from the recognised in 1927 and represented by new material same hindlimb (the femur is a right side bone, unlike (a skull) discovered in the earliest Late Cretaceous the tibia) but were found together, indicating that (Cenomanian) of the Kem Kem region of Morocco they must have belonged to the same animal. This is (Sereno et al., 1996). The other, and probably further corroborated by the fact that the size and the largest terrestrial flesh-eater ever found, is proportions appear right for one individual, Giganotosaurus carolinii, from the mid-Cretaceous and since the texture and colour of the two bones is (Albian-Cenomanian) of north-western Patagonia, identical. Although the bones are of gigantic Argentina (Coria and Salgado, 1995; Calvo and dimensions they clearly belong to an individual Coria, 1998). distinctly smaller than the colossal A. giganteus Palaeobiological studies of South American dino- (the length of the femur in MLP 23-316 is 2350 mm saurs are scarce (Casamiquela, 1978; Mazzetta et al., compared to 1855 mm in FMNH P13019). 1998; Blanco and Mazzetta, 2001; Mazzetta and The skeletal cast of A. cazaui, mounted at the Blanco, 2001). From a biological point of view, many exhibition of the Museo Argentino de Ciencias dinosaurian taxa of southern South America are Naturales “Bernardino Rivadavia”, Buenos Aires, interesting and in need of palaeobiological analysis, Argentina, and additional material housed at the particularly the above-mentioned huge titanosaurian same institution, labelled MACN-N 15 (holotype), sauropods and the large-bodied theropod, as they were also used in the analyses. Additionally, limb appear to represent size maxima in dinosaur bone measurements of the holotype specimen of evolution. In this paper, we address the estimation C. sastrei (MACN-CH 894) published by Bonaparte of their body size to provide a basis for further et al. (1990), and measurements on appendicular palaeobiological studies related to locomotion and bones of the disarticulated holotype specimen of other palaeoecological issues. Additionally, some G. carolinii (MUCPv-CH-1), were also considered. bizarre Patagonian dinosaurs like the small sauro- All the species studied here were found in pod A. cazaui and the moderately-sized theropod Patagonia, Argentina. A. giganteus is from the Rı´o Carnotaurus sastrei are also included as they are Neuque´n Formation, Early Cenomanian, Neuque´n specimens constituted by nearly complete skeletons. Province (north-western Patagonia) (Huene, 1929), and A. cazaui (Fig. 1) is from the La Amarga Formation, Late Neocomian, Neuque´n Province MATERIALS AND METHODS (Salgado and Bonaparte, 1991). A. huinculensis and G. carolinii are both from the Rı´o Limay Formation, Included Taxa mid-Cretaceous (Albian-Cenomanian), Neuque´n An incomplete femur attributed to A. huinculensis Province (Bonaparte and Coria, 1993; Coria (labelled MLP-DP 46-VIII-21-3), and skeletal material and Salgado, 1995), while the other theropod belonging to its holotype (PVPH-1) and also to included in the study, C. sastrei (Fig. 2), was collected {Institutional abbreviations: CN, Zoology Museum, Copenhagen University; FMNH, Field Museum of Natural History, Chicago, USA; HMN, Museum fu¨ r Naturkunde der Humboldt Universita¨t, Berlin, Germany; MACN, Museo Argentino de Ciencias Naturales “Bernardino Rivadavia”, Buenos Aires, Argentina; MLP-DP (and also MLP), Departamento Cientı´fico de Paleontologı´a de Vertebradosof the Museo de La Plata, La Plata, Argentina; MUCPv-CH, Museo de Geologı´a y Paleontologı´a de la Universidad Nacional del Comahue, Paleontologı´ade Vertebrados, El Choco´n collection, Neuque´n, Argentina; PVPH, Museo Municipal “Carmen Funes”, Plaza Huincul, Neuque´n, Argentina. SOUTH AMERICAN DINOSAURS