Systematics, taphonomy and paleoecology of the Trilobita Subclass, Phacopida (Homalonotidae, Calmoniidae), Ponta Grossa Formation (Devonian), Apucarana sub-basin, Paraná State, Brazil

Author(s)

Sabrina Pereira Soares

• Document type: Master's Dissertation
• Press: São Paulo.
• Institution: Universidade de São Paulo (USP). Instituto de Geociências (IG/BT)
• Defense date: 2007-12-13

Examining board members

Marcello Guimarães Simões; Luiz Eduardo Anelli; Ismar de Souza Carvalho

• Advisor: Marcello Guimarães Simões

Abstract

In the present study the Homalonotidae trilobites (Phacopida) from the Ponta Grossa Formation (Devonian), state of Paraná are revised. A total of 156 specimens, found in rocks of Jaguariaíva Member (=Sequence B), cropping-out at the Ponta Grossa, Tibagi and Jaguariaíva counties, were examined. Gathered data indicate that at least two species are present, Digonus noticus and Burmeisteria herschelii. Digonus noticus is a conspicuous element of the Devonian rocks of South Africa (Bokkeveld Group), Argentina (Lipéon Formation), Bolivia (Icla and Bélen formations), and Brazil (Pimenteiras and Ponta Grossa formations). Burmeisteria herschelii is common on Devonian rocks of Malvinas Island (Fox Bay Formation), South Africa (Bokkeveld Group), and Bolivia (Icla and Tarija formations), and were reported for the first time in the trilobitan fauna of the Ponta Grossa Formation, Apucarana Sub-basin. The results indicate that the Homalonotidae fauna of the Ponta Grossa Formation is not monospecific, that is, consisted only of D. noticus. Finally, homalonotids are not randomlly distributed along the Ponta Grossa Formation. Some species, (Digonus noticus) lived and/or were preserved in sedimentary deposits of sandy facies, generated just in and/or above the fair weather wave base, until muddy facies deposited below storm wave base. Others, such as Burmeisteria herschelii show a more restrict bathimetric distribution, that occurred preferentially at the fair weather wave base. In all these cases, homalonotids are most abundant in shallow water facies, deposited above storm wave base, being potentially an important paleoenviromental (neritic facies) indicator. In addition, based on the taphonomic analysis of 412 specimens of calmoniid, besides the homalonotid trilobites from the Ponta Grossa Formation, it is now clear that some species are artificial, since the morphological features that diagnose them are the product of fossilization and weathering. This is particularly true for those \"species\" that are preserved in different taphofacies, since they are submitted to distinct diagenetic processes (e.g., degree of compactation). For the studied calmoniid trilobites, three main morphological characters are susceptible to taphonomic modifications: a- variation in inflation of glabelar lobe, b- variation in lobe shape, depth of axial and glabelar furrows, and cpresence/absence of external ornamentation. For the homalonotids, the following morphological characters are susceptible to modifications: a- variation in glabelar lobe shape, and b- presence/absence of external ornamentation. Data presented here reinforce the idea that the erection of new trilobite species should be based on large scientific collections, enables the detection of taphonomic modifications. This procedure avoids the erection of new invalid taxa. Finally, the analyses of calmoniid trilobites from the Ponta Grossa Formation, cropping-out at the Tibagi and Jaguariaíva counties, indicate the presence of calmoniid specimens that are referable to Metacryphaeus rotundatus (Kozlowski, 1923). This is the first record of Metacryphaeus rotundatus in the Devonian of Brazil. Metacryphaeus rotundatus is a conspicuous species of the emsian rocks of the Icla Formation, Bolivia. Although there are similarities between the trilobitan fauna of Devonian Paraná Basin and others of Brazilian province and South African one, this finding is in accordance with new evidences (e.g., conulariids, homalonotids trilobites), indicating the presence of cosmopolitan species with Andean affinities. Metacryphaeus rotundatus lived in a broad paleoclimatic range, from temperate, cold temperate to subpolar climate. Finally, in the Devonian of Paraná Basin, Metacryphaeus rotundatus lived and were preserved in muddy, organic rich bottoms, deposited in offshore waters, below the storm wave base, associated to marine flooding surfaces, which may favor their geographic dispersion. (AU)