Devonian trilobites from the Amazonas Basin: systematic diversity, taphonomy, and biogeographic significance

Luiza Corral Martins De Oliveira Ponciano,Maria Da Gloria Pires De Carvalho

doi:10.46357/bcnaturais.v10i1.492

Abstract

Although the Devonian invertebrate biota of the Amazonas Basin is incompletely known, its unusual mixture of supposedly Malvinokaffric and Appalachian taxa is of considerable biogeographical interest. The history of research on Devonian trilobites from the Amazonas Basin is summarized. These fossils occur in the Maecuru (latest Emsian – early Eifelian) and Ererê (latest Eifelian – early Givetian) formations, representing the families Homalonotidae, Dalmanitidae, and Calmoniidae. Trilobite diversity is higher in the Maecuru (nine named plus six indeterminate taxa) than the Ererê Formation (with only two named taxa), which displays faunal impoverishment generally. None of the Devonian Amazonas Basin trilobite species is known from other Malvinokaffric sites, suggesting very high endemicity within this basin. A taxonomic revision of “Homalonotus” derbyi (Clarke, 1890) is presented and a lectotype is designated. A lectotype and paralectotype are also designated for Eldredgeia paituna (Hartt & Rathbun, 1875). Some taphonomic observations of fossils from the Maecuru Formation are made, and a brief biogeographic synopsis of the Devonian Amazonas biota is also given.

Highlights

Of all the Paleozoic sedimentary basins in Brazil, the remoteness of Amazonas Basin, coupled with the paucity of exposures, has imposed severe limitations upon research on its Devonian fossils
Despite the paucity of Devonian fossils from the Amazonas Basin, increased systematic diversity among its trilobites has been recognized over the past century, in the Maecuru Formation and especially at genus level
This increase has resulted only from revisionary studies of existing collections, rather than from new collecting, suggesting that the true diversity of Amazonas trilobites may be seriously underrepresented and that many new forms could be revealed with renewed efforts to collect and describe fossils from this intriguing region, especially from the Maecuru Formation

Summary

Introduction

Of all the Paleozoic sedimentary basins in Brazil, the remoteness of Amazonas Basin, coupled with the paucity of exposures, has imposed severe limitations upon research on its Devonian fossils. Past investigations suggest that the Amazonas Basin was extremely important biogeographically during the Devonian, as a transitional “biogeographic boundary mixing area” (Boucot & Racheboeuf, 1993) characterized by the co-occurrence of Eastern Americas brachiopods and Malvinokaffric trilobites. The literature dealing with these fossils (especially trilobites) is widely scattered. In an attempt to rectify this situation, the present paper reviews the history of research on, and the systematic diversity of Devonian trilobites from the Amazonas Basin. Taphonomic observations are provided in order to improve understanding of the depositional environments in which these fossils occur, especially regarding the Maecuru Formation. A brief biogeographic synopsis of the Amazonas Devonian biota is presented, with emphasis on similarities and differences from Andean and Appalachian biotas

Methods

Discussion

Conclusion