congresos y reuniones científicas
The tip of the iceberg: Fragments of the lepidosaur evolutionary history in South America
Congreso; III Congreso Latinoamericano de Paleontología de Vertebrados; 2008
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Although wide areas of the evolutionary history of lepidosaurs remain obscure, most of the available information comes from the northern hemisphere, resulting in biased conclusions. The information of the southern half of the world is patchier but the findings in Pakisagudem (India), Annoual (Morocco), Santana (Brazil) and La Buitrera (Argentina) demonstrated that a very rich and highly informative history was yet to be told. The available evidence allows a possible description of the lepidosaur evolutionary history in South America as follows: The Pangean stage includes Late Triassic clevosaurs (Ferigolo, 2000); considering the information from India, we could assume that derived acrodontan iguanians (Datta & Ray, 2006) could have been already present also in South America. A Jurassic Early Gondwanan stage, estimated for the split of major lizard clades, bear small and endemic Jurassic sphenodontines, whereas India shows more basal forms (Evans et al., 2001). Lizards are poorly known by the Patagonian Protolacerta and abundant material from India, like acrodont iguanians (Evans et al., 2002) and pleurodont lizards (Yadagiri, 1986). By the same time African localities show scincomorph paramacellodids (Zils et al., 1995). Probably these taxa were also present in South America. The Cretaceous is the richest source of evidence. A Late Gondwanan stage shows an Early Cretaceous radiation of squamates, close to the iguanian-scleroglossan split that evidences a continental isolation. This includes basal squamatans from Brazil (e.g., Evans & Yabumoto, 1992); and the differentiation of South American teiids and gymnophtalmids from their northern vicariants (Nydam et al., 2007). No sphenodontids were recorded. However, South American lepidosaur faunas were probably integrated by the following forms, which actually appear as fossils by the Late Cretaceous: sphenodontine, eilenodontine and sapheosaur sphenodontids; scolecophidian, anilioid and basal macrostomatan snakes; pleurodont iguanian; scincomorphan (teioids and paramacellodids) and gekkonid lizards. Early Late Cretaceous rocks of Patagonia provided abundant eilenodontine sphenodontians (Apesteguía, 2002; Simón & Kellner, 2003; Apesteguía & Novas, 2003; Apesteguía & Carballido, this congress) as well as squamatans: a possible iguanid (Apesteguía et al., 2005) and abundant remains of the basal snake Najash (Apesteguía & Zaher, 2006). The younger Santonian-Campanian rocks of the Río Colorado Subgroup provided the basal alethinophidian snake Dinilysia (Smith Woodward, 1901; Caldwell & Albino, 2002), and scincomorphs (Albino, 2002), but no sphenodontids. Eilenodontines were probably disappeared by that time. Uppermost Cretaceous beds from Baurú provided the lizard Pristiguana brasiliensis (Estes & Price, 1973) and, at North Patagonia, alethinophidian snakes (Albino, 1987; Gómez & Báez, 2006) and different sphenodontids (Apesteguía, 2005; Apesteguía & Rougier, 2007), both gigantic and small-sized (Martinelli & Forasiepi, 2004). Except for mosasauroids from brackish environments, no definitive lizards have been recorded yet. Although teiids and several other lizards were considered to have entered South America by the latest Cretaceous, prompting the extinction of sphenodontids, the arising panorama shows a balance between scarce, small and insectivorous native lizards and, on the other side, large, abundant and diverse herbivorous and animalivorous sphenodontids. South American sphenodontids became reduced or extinct by the K/P event, and only later squamates slowly increased their abundance. During the Paleogene, large madtsoid snakes became part of the top predators in South American terrestrial ecosystems (Simpson, 1933) and squamatan diversity increased in the widely distributed tropical environments. The Neogene experienced an increasing in lizard diversity, especially teiids and iguanids.