sábado, novembro 03, 2012

Idade dos dinossauros e outros vertebrados do Jurássico Superior de Portugal

O Jurássico Superior (163.5 a 145.0 Milhões de anos) é claramente o período de tempo sobre o qual temos mais vestígios de dinossauros em Portugal. Mas esta época representa 17.5 milhões de anos pelo é importante melhorar a resolução destas idades e compreender a correlação estratigráfica dos achados. Esse trabalho, desenvolvido por Vasco Ribeiro e por mim, foi apresentado no congresso Society of Vertebrate Paleontology Meeting.

Este é o resumo:

Chronology of the Late Jurassic dinosaur faunas, and other reptilian faunas, from Portugal

The chronostratigraphy of Late Jurassic vertebrates from Portugal, including those from the Lourinhã Formation, which is known for its rich vertebrate fauna, is poorly understood due to the continental nature of the sediments and the diachrony of the lithostratigraphic units. 
Recent results using Sr87/86 isotopes confirmed the position of the Kimmeridgian-Tithonian boundary (150.8 Ma) in the Lusitanian Basin central sector. This boundary, within a marly layer representative of the more southernly limestone Farta Pão Formation, lies within the siliciclastic Lourinhã Formation and is assumed to be the transgressive upper Kimmeridgian -lower Tithonian event. The most productive vertebrate-bearing Upper Jurassic formations in Portugal are: the Alcobaça Formation, Lourinhã Formation (divided into the Amoreira-Porto Novo, Sobral, Bombarral, and Freixial (pars) members), and the Porto da Calada Formation.
The chronological range (given by biostratigraphy, eustatic curves, general regional context, and calibrated by strontium isotope curves) for important Portuguese specimens of chelonians, pterosaurs, dinosaurs, crocodylomorphs, and other reptilians is as follows: 
Early (to late?) Kimmeridgian (Alcobaça Beds Formation): Theriosuchus guimarotae, Machimosaurus hugii, Goniopholis baryglyphaeus, Lusitanisuchus mitrocostatus, Phyllodon henkeli, Parviraptor estesi, Marmoretta sp., Aviatyrannis jurassica.
Late Kimmeridgian (Lourinhã Formation, Amoreira-Porto Novo Member): Selenemys lusitanica, Plesiochelys sp., Cteniogenys reedi, Lusitanisuchus mitrocostatus, Rhamphorhynchus sp., Dracopelta zbyszewskii, Miragaia longicollum, Trimucrodon cuneatus, Camptosaurus aphanoecetes, Dinheirosaurus lourinhanensis, Turiasaurus sp., Ceratosaurus nasicornis, Torvosaurus aff. tanneri.
Around the Kimmeridgian/Tithonian boundary (Sobral Member): Selenemys lusitanica, Plesiochelys sp., Machimosaurus hugii, Rhamphorhynchus sp., Lourinhanosaurus antunesi, Lusotitan atalaiensis, Lourinhasaurus alenquerensis, Dryosaurus sp., cryptoclidid plesiosaur.
Early Tithonian (Sobral Member): Plesiochelys sp., Miragaia longicollum.
Upper early Tithonian (Bombarral Member): Plesiochelys sp., Allosaurus europaeus, Draconyx loureiroi, Stegosaurus sp.
Late Tithonian (Freixial Member): Plesiochelys sp., Theriosuchus sp. B, Ornithopoda sp. B. 
Despite the fragmentary occurrences of certain taxa, the chronology of some vertebrates seem to be age-restricted, and can thus be used for biostratigraphy. There is a peak of vertebrate fossil diversity and abundance near the Kimmeridgian/Tithonian boundary and a decline towards the end of the Tithonian. Is not yet understood if such trend represents true
diversity/abundance in the Jurassic or if it is caused by any geologic and taphonomic bias.

Ribeiro, V. Mateus O. (2012).  Chronology of the Late Jurassic dinosaur faunas, and other reptilian faunas, from Portugal. Journal of Vertebrate Paleontology, Program and Abstracts, 2012, p. 161 ISSN 1937-2809.

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