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Mar del Plata, Argentina

Chevarrfa J.E.,Division Paleozoologia Invertebrados | Damborenea S.E.,Division Paleozoologia Invertebrados | Manceftldo M.O.,Division Paleozoologia Invertebrados
Ameghiniana | Year: 2012

The study of palaeodemecological features requires some particular taphonomic conditions. These conditions were met in the Mulichinco Formation (Valanginian), where burrowing bivalve trace fossils are widespread and often appear in cross section on bedding surfaces. Two groups of such beds were analyzed, measuring population density, spatial distribution, size distribution and horizontal orientation of the burrows. The palaeoenvironment was established by means of a detailed sedimentological analysis, and the bivalve fauna present was checked, in order to attempt identifying their potential producers. High population densities were found in the two groups, indicating favourable physical conditions and good food supply, while differences in both spatial and size distributions were noticed between them; on most surfaces there was no preferred orientation. The first group (group A) showed a uniform pattern of spatial distribution and larger traces, with a remarkable absence of small sizes. In the second group (group B), the spatial distribution pattern is indistinguishable from a random distribution (except one case in which the pattern appears to be aggregated). Group A is interpreted as a set of escape traces made by deep burrowers in response to storm deposition, while group B is considered as resting/escape traces made by shallow burrowers in tide-dominated environments. Palaeodemecological studies of this kind are potentially useful tools for sedimentary and basin analyses.

Pereza L.M.,Division Paleozoologia Invertebrados | Pereza L.M.,CONICET | Iturreria S.F.G.,Division Paleozoologia Invertebrados | Griffini M.,Division Paleozoologia Invertebrados | Griffini M.,CONICET
Malacologia | Year: 2010

Two new species are described based on material from the Late Miocene Paraná Formation, Entre Ríos, Argentina: Polymesoda muravchiki, nov. sp., and Erodona doellojuradoi, nov. sp. The bearing rocks were deposited during the Entrerriense ingression that covered part of central-northern Argentina, reaching as far north as Bolivia, Paraguay, and southern Brazil. The exposure from where the material was collected represents a rich parautochtonous assemblage - also including marine taxa - that has been assigned to the Late Miocene. Extant representatives of Polymesoda and Erodona are known to inhabit mostly brackish environments in the Caribbean region, the eastern Pacific coast of America, and Southeast Asia (in the case of Polymesoda), and the Atlantic coast of southern South America (Erodona). The presence of these bivalves in the Paraná Formation suggests that at least a marginal connection may have existed between a southern arm of the Amazonian Sea and the Paraná Sea during the Miocene. It is highly unlikely that these taxa could have migrated along the Atlantic coast of South America - contrarily to the case of the fully marine taxa - in view of their peculiar ecological requirements.

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