The land snails from Itaboraí Basin

Itaboraí Basin is located in Rio de Janeiro state, Brazil, dating from the Late Paleocene or Early Eocene. Despite being one
of the smallest basins in the country, Itaboraí houses an astounding fossil record in its limestones.


The snails

The first snail species found in Itaboraí. A. Brasilennea arethusae (shell height = 21 mm). B. Bulimulus fazendicus (16.5 mm). C. Eoborus sanctijosephi (44.5 mm). D. Itaborahia lamegoi (30 mm).
A. Brasilennea arethusae (shell height = 21 mm). B. Bulimulus fazendicus (16.5 mm). C. Eoborus sanctijosephi (44.5 mm). D. Itaborahia lamegoi (30 mm).

There are 20 species in the Itaborahian fauna (including four entirely new species; marked in bold on the list below), which is a fair amount of diversity. Almost all of them are land snails, with the exception of the freshwater Biomphalaria species. The genera BrasilenneaCortana and Itaborahia are endemic to the basin.

Land snails tend to be more abundant and diverse when their typical appropriate environmental conditions are present: high humidity, abundant vegetation, high calcium availability and finally, few or no predators. Clearly, Itaboraí was a calcareous basin, something that alone could attract many snails, but a fair amount of the other conditions should have also been present.

The basin’s flora is known to have been diverse and abundant, but little is known about its paleoenvironment. In any case, Itaboraí seemingly had a humid and hot climate, perhaps even similar to the present. Still, Bergqvist et al. (2006) argue for a milder climate in the beginning (where most snails occur), becoming more arid towards the end of the basin’s fossil record (with fewer or even no snails).


Biogeographical oddities

Itaboraí Basin’s early Cenozoic age and geographical location in South America makes its fossil snails very important to understand the evolution and biogeographic history of New World land snails. Besides having some of the most ancient records known for some families of snails, Itaboraí is notable for the occurrence of fossils of the families Cerionidae, Clausiliidae and Urocoptidae, which do not have present representatives in Brazil. As can be seen on the map, Itaboraí is far removed from these families’ recent distribution.

Map showing the Recent distribution of Cerionidae (blue), Clausiliidae/Neniinae (green) and Urocoptidae (red). The oldest fossils known for Cerionidae and Urocoptidae are indicated by X's in the USA and Canada, respectively.
Map showing the Recent distribution of Cerionidae
(blue), Clausiliidae/Neniinae (green) and Urocoptidae (red).
The oldest fossils known for Cerionidae and Urocoptidae are indicated by X’s in the USA and Canada, respectively.

Cerionidae: Itaboraí’s three Brasilennea species consist on the second oldest record of Cerionidae. The oldest is a probable Cerion, named C. acherontis, from the Late Cretaceous of Montana, USA. Despite the absence of recent cerionids in South America, it can be seen that the family’s distribution included this area during the first Cenozoic stages. It is even possible that, at that time, Cerionidae had a more ample distribution, occurring from northwestern USA (C. acherontis) to Rio de Janeiro (Brasilennea).

Clausiliidae: There is only one clausiliid in Itaboraí, tentatively identified as Temesa magalhaesi, of the subfamily Neniinae. The family is known since the Cretaceous of Europe and has a vast record in this continent; however the Itaborahian species is the oldest record of the family in South America.

Urocoptidae: The single urocoptid in Itaboraí is tentatively identified as “Brachypodella britoi. The oldest record for the family comes from the Late Cretaceous of Canada and Mexico. The presence of this family in Itaboraí, together with the North-American records, shows that the family’s distribution was much vaster in the past. It is interesting that the Cerionidae, which are closely related to the urocoptids, show a similar pattern.


Bibliography

The preceding text is a summary of the following papers, where this research was published:

  • SALVADOR, R.B. & SIMONE, L.R.L. 2011. The importance of Itaboraí Basin (Paleocene) as the home to early records of many pulmonate snail families. In: Atas do XXII Congresso Brasileiro de Paleontologia. Natal. Pp. 521-524.  [PDF]
  • SALVADOR, R.B. & SIMONE, L.R.L. 2012. New fossil pulmonate snails from the Paleocene of Itaboraí Basin, Brazil (Pulmonata: Cerionidae, Strophocheilidae, Orthalicidae). Archiv fur Molluskenkunde 141: 43-50.  [PDF]
  • SALVADOR, R.B. & SIMONE, L.R.L. 2013. A malacofauna fóssil da Bacia de Itaboraí, Rio de Janeiro: histórico dos estudos e perspectivas para o futuro. Revista da Biologia 11: 1-6.  [PDF]
  • SALVADOR, R.B. & SIMONE, L.R.L. 2013. Taxonomic revision of the fossil pulmonate mollusks of Itaboraí Basin (Paleocene), Brazil. Papéis Avulsos de Zoologia 53: 4-56.  [PDF]
  • SALVADOR, R.B.; ROWSON, B.; SIMONE, L.R.L. 2011. Re-writing the fossil history of Cerionidae (Gastropoda, Pulmonata): new family assignment of the Brazilian Paleocene genus Brasilennea Maury, 1935. Journal of Molluscan Studies 77: 445-447.  [PDF]