South American Dinosaurs

Fernando E Novas. Encyclopedia of Dinosaurs. Editor: Philip J Currie & Kevin Padian. Amsterdam: Academic Press, 1997.

Dinosaurs from South America are known from Triassic, Jurassic, and Cretaceous rocks. Currently, nearly 50 dinosaur species (diagnosable on the basis of derived osteological features) have been identified in this continent. Representatives of Saurischia and Ornithischia were documented in South America as well as immediate dinosaur forerunners. Saurischians are better known than ornithischians, the latter currently being represented by only 10 species. Chronologically, dinosaurs are known from Carnian, Norian, Bajocian, Callovian, Hauterivian, and Aptian through Maastrichtian times.

Dinosaur remains have been found in several countries of South America (Weishampel, 1990), although discoveries in Argentina have shed the most relevant and informative fossil evidence for the understanding of many important aspects in dinosaur evolution (e.g., their origins in Triassic times and the effects of the fragmentation of Gondwana over the evolution of dinosaur communities).

Triassic

The Ischigualasto-Villa Unión Basin of northwest Argentina (Stipanicic and Bonaparte, 1979) is filled by an almost continuous sequence of sediments of Early, Middle, and Late Triassic ages (Fig. 1). The formations identified in this basin yielded remains of several archosaurs of principal interest in the early evolution of dinosaurs. Especially relevant are the Los Chañares (Ladinian) and Ischigualasto (Carnian) formations (Bonaparte, 1982). From Los Chañares Formation were discovered skeletons of Lagosuchus talampeyensis, Marasuchus lilloensis, and Pseudolagosuchus major (Bonaparte, 1975; Arcucci, 1987), the proximate sister taxa of Dinosauria (Fig. 2; Novas, 1992a, 1997a; Sereno and Arcucci, 1994). These slender archosaurs are among the first tetrapods that acquired bipedal and digitigrade postures. The earliest known saurischians and ornithischians have been recorded in the Ischigualasto Formation of northwest Argentina and Santa Maria Formation of southeast Brazil (Bonaparte, 1982). The basal radiation of the Theropoda is represented by Eoraptor lunensis and Herrerasauridae (= Staurikosaurus pricei +Herrerasaurus ischigualastensis), which constitute successive sister taxa of the Ceratosauria + Tetanurae clade (Novas, 1992a, 1994a; Sereno and Novas, 1992, 1994; Sereno et al., 1993). As noted elsewhere, the systematic position of these animals is still controversial. Also present is Pisanosaurus mertii(Casamiquela, 1967), a basal ornithischian with a mixture of derived and plesiomorphic features (Bonaparte, 1976; Novas, 1989; Sereno, 1991; Weishampel and Witmer, 1990).

The documented morphological diversity of the South American Carnian dinosaurs suggests that an extensive evolutionary radiation occurred before the Late Triassic. Following the method described by Norell (1993), the minimum age for the origin of the Dinosauria can be predicted as early as the Ladinian on the basis of the presence of immediate sister taxa of Dinosauria in the Ladinian Los Chañares Formation.

The evolutionary novelties evolved in the common dinosaurian ancestor (mainly involving pelvic girdle and hindlimb anatomy) apparently did not trigger an immediate numerical dominance but probably promoted extensive morphological disparity (Novas, 1997a). The early radiation of dinosauriforms (the clade formed by dinosaurs and their immediate forerunners; Novas, 1992a) was characterized by a sustained increase in body size, from 0.50 m in Mesotriassic dinosauriforms (e.g., M. lilloensis)upto 6 min Carnian dinosaurs (e.g., H. ischigualastensis; Novas, 1994a,b). This increase in body size also involved a shifting from insectivorous toward megapredatory feeding habits and was accompanied by an increase in numerical abundance in Carnian times. In the early Late Triassic, herrerasaurids entered the terrestrial biotas as large, highly predatory forms, sharing with rauisuchid archosaurs the role of superpredators. In the Ischigualasto Formation herrerasaurids are numerically more abundant than other contemporary terrestrial carnivores (e.g., Saurosuchus galilei, Venaticosuchus rusconii, and Chiniquodon theotonicus). However, there are no clear patterns of competitive exclusion among these lineages, mainly because herrerasaurids, ornithosuchids, and rauisuchids survived until the Norian (Bonaparte, 1972, 1982; Sereno et al., 1993).

In consequence, the available Triassic record of Argentina does not entirely support the interpretation that dinosaur dominance was reached in an empty ecospace after several nondinosaurian groups (e.g., rhynchosaurs; dicynodont, traversodont, and chiniquodont synapsids) became extinct (contra Benton, 1988). On the contrary, dinosauriforms coevolved during the Middle to early Late Triassic with the previously mentioned nondinosauriform tetrapods, amplifying the range of body size, species diversity, and feeding habits. In summary, the ecological diversification of the dinosauriforms documented in the Carnian can be seen as part of an uninterrupted coevolutionary process inaugurated in the Ladinian (Novas, 1994b).

The small sized (~1 m long) ornithischian P. mertii is known from only one specimen, suggesting that it constituted a minor component in the Triassic biotas of West Gondwana (Bonaparte, 1976, 1982). The Ischigualasto Formation has not currently yielded any sauropodomorph bones, in sharp contrast to the overlaying Los Colorados Formation (Norian), in which sauropodomorphs are numerically important (Bonaparte, 1972, 1982). However, the minimum age for the origin of the Sauropodomorpha can be predicted as early as the Carnian on the basis of the presence of their sister taxon (e.g., Theropoda) in the Ischigualasto Formation (Sereno and Novas, 1992; Fig. 3). Late Triassic dinosaurs are represented in South America by the melanorosauridRiojasaurus incertus and the plateosaurids Coloradisaurus brevis and Mussaurus patagonicus (Bonaparte, 1972, 1979, 1982; Bonaparte and Vince, 1979).

The “explosive evolution” (e.g., rapid increase in numerical abundance and body size) manifested by “prosauropod” dinosaurs during the Norian was interpreted as the result of opportunistic evolution after the extinction of several nondinosauriform tetrapods (e.g., rhynchosaurs and traversodonts; Benton, 1988). However, a tacit competitive scenario is entailed in this interpretation: If sauropodomorphs were present in the Carnian, as predicted previously, their rarity or virtual absence in the Ischigualasto Formation can be explained as a consequence of the presence of successful competitors (rhynchosaurs and traversodonts).

Jurassic

The record of Jurassic dinosaurs in South America is scarce. The early evolution of Sauropoda is represented by the Bajocian Amygdalodon patagonicus and the Callovian Volkheimeria chubutensis and Patagosaurus fariasi, from Patagonia (Bonaparte, 1986a). These taxa are interpreted as successive sister taxa of more derived sauropods (Bonaparte, 1986a). Theropoda is represented by the Callovian basal tetanurine Piatnitzkysaurus floresi, which resembles the European Eustreptospondylus cuvieri (Bonaparte, 1986a). Scarce remains of a presumed ornithischian have been documented in the Jurassic La Quinta Formation of Venezuela (Russell et al., 1992). Probably the maker of the footprints named Delatorrichnus goyenechei (Casamiquela, 1964) was a small quadrupedal ornithischian. During the Jurassic, Patagonia had positive hydrologic balance which favored the development of lake systems (Uliana and Biddle, 1988), around which large sauropods and theropod dinosaurs prospered. Dinosaur taxa from Patagonia (Bonaparte, 1986a) resemble nearly contemporary relatives from other parts of the world (e.g., England and India), suggesting the presence of a relatively uniform dinosaur fauna throughout the world. However, local environmental conditions in South America are represented by the Oxfordian La Matilde Formation, in which small dinosaurs (e.g., D. goyenechei,Sarmientichnus scagliai, and Wildeichnus navesi) were documented (Casamiquela, 1964). A similar dinosaur assemblage comes from the Late Jurassic–Early Cretaceous Botucatu Formation, southeast Brazil (Leonardi, 1989), deposited under desert conditions and also disturbed by widespread volcanic activity (Soares, 1981; Uliana and Biddle, 1988). These disturbed environmental conditions in central South America prevailed from Kimmeridgian through Neocomian times, approximately 27 ma (Uliana and Biddle, 1988). This paleodesert probably controlled dinosaurian distributions, creating a filter for intracontinental dispersion of animals and plants.

Cretaceous

During the Cretaceous South America was inhabited by a wide variety of dinosaurian clades. Ornithischians are recorded in this continent mostly from Upper Cretaceous rocks, although their presence in the Early Cretaceous is confirmed by ornithopod footprints from Brazil and Chile (Leonardi, 1989) and skeletal remains in Patagonia (Bonaparte, 1994). Ornithischians appear to be less diversified than those from the Cretaceous of North America, although the record of representatives of Stegosauria, Ankylosauria, Ornithopoda, and probably Ceratopsia suggests that a hidden diversity of ornithischians remains to be discovered. Ornithischians more frequently discovered in South America are basal iguanodontians (e.g., Dryomorpha) of small to medium size (Coria and Salgado, 1993a, 1997a; R. Martĺnez, personal communication). Theropoda is represented by the neoceratosaurian Abelisauria (Novas, 1992b), as well as tetanurans of different pedigree, including early birds (Chiappe, 1995). Abelisaurs in particular underwent a significant evolutionary radiation during the Cretaceous in South America (Bonaparte, 1991a). Abelisaurs were active predators, some of them no larger than a chicken (e.g., Velocisaurus unicus and Ligabueino andesi; Bonaparte, 1991b, 1994), but other taxa reached 10 m in length (e.g., Abelisauridae; Bonaparte and Novas, 1985; Bonaparte et al., 1990). Sauropod dinosaurs constituted the dominant group of megaherbivores in the South American Cretaceous landscape (Bonaparte and Kielan-Jaworowska, 1987). However, Leonardi (1989) pointed out that ichnological associations yield a sauropod to ornithischian ratio of 1:2, in contrast to the ratio of skeletal findings on the continent. Sauropods attained a high diversity of forms not recorded in the Cretaceous of other continents. Two main sauropod clades evolved in South America during the Cretaceous (Calvo and Salgado, 1997): the Hauterivian through Cenomanian diplodocid-related forms (including Amargasaurus cazaui and Limaysaurus tessonei; Salgado and Bonaparte, 1991; Calvo and Salgado, 1997), and Titanosauridae and their kin (Bonaparte and Coria, 1993; Salgado and Calvo, 1997) recorded from Albian (e.g., Chubutisaurus insignis; Salgado, 1993) through Maastrichtian times (Powell, 1986).

The following major intervals can be recognized for the Cretaceous of South America on the basis of different faunal assemblages:

1. Hauterivian interval: The local fauna of La Amarga Formation (northwest Patagonia) yielded remains of the oldest known abelisaur (L. andesi; Bonaparte, 1994, 1997), a stegosaur (Bonaparte 1994, 1997), and the dicraeosaurid sauropod A. cazaui (Salgado and Bonaparte, 1991; Salgado and Calvo, 1992).

2. Aptian–Albian interval: The Aptian–Albian interval is characterized by the presence of very large sauropods (e.g.,Chubutisaurus insignis from the Gorro Frigio Formation and indeterminate sauropod from the Matasiete Formation; Salgado, 1993; Martıánez et al., 1989a). The presence of large theropods in this interval is documented by fragmentary teeth (Del Corro, 1974), nearly half the size of those of Tyrannosaurus rex. During the Aptian– Albian, abelisaurs evolved toward bizarre forms such as the horned Carnotaurus sastrei (it must be noted, however, that the age of the La Colonia Formation, which yielded C. sastrei, was variously considered to be Albian and recently as Maastrichtian; Bonaparte et al., 1990; Ardolino and Delpino, 1987). Fragmentary evidence from Brazil suggests the possible presence of spinosaurid tetanurines (Rauhut, 1994) in South America during the Albian (Kellner, 1997).

3. Cenomanian through Santonian interval: This interval of the Cretaceous, spanning +14 ma, involves the time of deposition of the Neuquén Group (Legarreta and Gulisano, 1989) and presumed equivalent formations of other sedimentary basins (e.g., San Jorge Basin; Barcat et al., 1989; Fig. 1). It is characterized by the explosive evolution of the titanosaur clade, including truly gigantic forms (e.g., Argentinosaurus huinculensis, “Antarctosaurus” giganteus,Argyrosaurus superbus; von Huene, 1928; Powell, 1986; Bonaparte and Coria, 1993), along with diplodocid-related sauropods (Sciutto and Martĺnez, 1994), some of them closely related to the African Rebbachisaurus garasbae(Bonaparte, 1997; Calvo and Salgado, 1997).

The “Neuquenian fauna” also includes small to medium-sized basal iguanodontians such as the Santonian Loncosaurus argentinus, Gasparinisaura cincosaltensis, an indeterminate tiny dryosaurid from the Turonian (Novas, 1997b), as well as yet undescribed Cenomanian ornithopods from the Bajo Barreal Formation (R. Martĺnez, personal communication). Footprints from the Cenomanian Rĺo Limay Formation reveal the presence of several ornithischian (e.g., ornithopod) taxa (Calvo, 1991).

The evolution of the Theropoda was also prolific at this time. Ceratosaurians are known by the turkey-sized Velocisaurus unicus (Bonaparte, 1991b) and by larger indeterminate taxa resembling Carnotaurus sastrei (Coria et al., 1991; Coria and Salgado, 1993). The Cenomanian basal tetanurine Giganotosaurus carolini (Coria and Salgado, 1995) is one of the largest known theropods (6–8 tons), which attained macropredaceous habits independently from the Laurasian T. rex. The other large group of tetanurines known from South America are Late Cretaceous birds (Avialae sensu Gauthier, 1986). Avialans are represented on this continent by several taxa more derived than Archaeopteryx lithographica (e.g., Alvarezsauridae, Enantiornithes, and Patagopteryx deferriasi; Bonaparte, 1991b; Chiappe, 1995; Novas, 1997b,c). They were small to medium-sized animals (0.60–2.00 m long) that fed on small food items, probably insects and fruits. Interestingly, the coeval alvarezsaurids and P. deferriasi secondarily reversed to nonvolant, cursorial habits. Enantiornithine birds are also recorded in the Santonian beds of the Neuque Group (Chiappe, 1995).

Maastrichtian interval: On the basis of the available record, an extinction event seems to have occurred during the Campanian, coincident with a severe orogenic phase in South America (Zambrano, 1981). After that, a major Maastrichtian transgressive phase took place (Zambrano, 1981; Uliana and Biddle, 1988), and wide lacustrine deposits developed in Patagonia. A new faunistic assemblage came on the scene, corresponding to the latest Campanian to early Maastrichtian Alamitian fauna (Bonaparte, 1987). During the Alamitian, basal iguanodontians as well as the gigantic sauropods characteristic of the previous “Neuquenian fauna” became extinct, as did the diplodocidrelated forms. New ornithischian clades are recorded during the Alamitian (e.g., “Kritosaurus” australis, Lambeosaurine indet., Ankylosauridae, and Ceratopsidae?; von Huene, 1928; Bonaparte, 1987; Powell, 1987; Salgado and Coria, 1997), which probably arrived from North America (Bonaparte, 1986b). Derived titanosaurs (e.g., Neuquensaurus australis and“Titanosaurus” araukanikus; Powell, 1986) were smaller in size than those of earlier times, although titanosaurs of large size are known from supposedly Maastrichtian beds of southern Patagonia (Chorrillo Formation; Powell, 1986). Interestingly, the previously mentioned replacement of herbivorous dinosaurs is coincident with the diversification of the angiosperm tree Nothofagus, which occurred during late Campanian– Maastrichtian in the southern South America– Antarctic Peninsula region (Romero, 1993). Titanosaur–hadrosaur communities of Alamitian age lived in lacustrine environments, related to a major Maastrichtian transgressive phase over most of South America (Uliana and Biddle, 1988). Alamitian age theropods of large size (nearly 10 m long) belong to the ceratosaurian subclade Abelisauridae (Abelisaurus comahuensis; Bonaparte and Novas, 1985), although Maastrichtian abelisaurs also include the turkey-sized Noasaurus leali (Bonaparte, 1991a), which evolved a raptorial sickle claw in pedal digit II, independent of dromaeosaurs and troodontids (Novas, 1992b).

Unfortunately, to date dinosaur remains from the latest Maastrichtian have not been recorded.

Paleobiogeographic Evolution of South American Dinosaurs during the Cretaceous

South American dinosaur faunas are in contrast with those from Asia and North America mainly because in the latter two continents both tetanurans and ornithischians are quite diverse, and sauropods constitute a minor component (Bonaparte and Kielan-Jaworowska, 1987). These sharp differences in faunal composition can be interpreted as the result of the breakup of Laurasia and Gondwana in the course of the Cretaceous, resulting in the evolution of endemic dinosaur taxa for each continental mass (Bonaparte, 1986b).

Although most of the evidence supporting the isolation of Gondwanan from Laurasian tetrapod faunas comes from Late Cretaceous rocks, the presence in the Early Cretaceous of South America of several taxa not currently recorded in Neocomian formations from the Northern Hemisphere (e.g., Cloverly Formation; Russell, 1993) suggests that the isolation between Laurasia and Gondwana was already complete during the Neocomian. Such taxa include the previously mentioned dinosaurs from the Hauterivian La Amarga Formation (e.g., dicraeosaurids, basal abelisaurs, and stegosaurs; Bonaparte, 1994; Salgado and Bonaparte, 1991). However, the differences between southern South America (e.g., Patagonia) and Laurasian continents are probably due not only to the physical separation mentioned previously but also to more or less effective barriers, intrinsic from West Gondwana (e.g., extensive deserts in the Paran ˜ a and Chaco basins in northeast South America; Soares, 1981; Zambrano, 1981; Uliana and Biddle, 1988).

The Early and “middle” Cretaceous dinosaur faunas from South America correspond to the interval (e.g., Neocomian through Albian) during which this continent was geographically connected with Africa. As a result, some dinosaurian taxa are shared in common by both continents (e.g., stegosaurs, dicraeosaurids, and basal diplodocids closely related toR. garasbae; Salgado and Bonaparte, 1991; Salgado and Calvo, 1992; Calvo and Salgado, 1997; McIntosh, 1990). From Aptian through Santonian times (an interval of nearly of 33 my; Haq and Van Eysinga, 1994), South America progressively increased its geographic isolation from other continents (Scotese and Golonka, 1992). Complete isolation occurred at some point between 106 my (Albian) and 84 my (Campanian), with the full development of a marine barrier between the two continents (Pitman et al., 1993). Bizarre tetanurans from the Cenomanian of northern Africa (e.g.,Carcharodontosaurus saharicus, Bahariasaurus ingens, and Spinosaurus aegyptiacus; Rauhaut, 1994) suggest that Patagonia and that region of Africa were partially isolated during the beginning of the Late Cretaceous. However, spinosaurid-like teeth recently found in the Albian of Brazil (Kellner, 1997) suggest that the north portions of both South America and Africa retained faunistic affinities, corresponding to the south–north direction of opening of the South Atlantic (Uliana and Biddle, 1988; Pitman et al., 1993).

On the basis of the available vertebrate fossil record and according to current paleogeographic reconstructions (Scotese and Golonka, 1992), South America renewed contact with North America presumably during Campanian to Maastrichtian times, when intense orogeny caused continentalization of Caribbean regions (Bonaparte, 1986b; Zambrano, 1981; Pitman et al., 1993). Some clades interpreted to have evolved in South America, or more generally Gondwana, reached Asia during the Late Cretaceous (e.g., titanosaurids and alvarezsaurids; Bonaparte, 1986b; Novas, 1997b). The presence of indisputable titanosaurids in the Maastrichtian of North America (e.g., Alamosaurus sanjuanensis), as well as the record of Laurasian taxa in the Late Campanian–Maastrichtian of South America (e.g., Hadrosaurinae and Lambeosaurinae; Bonaparte, 1987; Powell, 1987), can be explained as the result of faunal interchange (Bonaparte, 1986b).