Tag Archives: orthalicoidea

Did Neotropical snails occur in Africa and Europe?

Fossils allow us a peek into the past, and although a lot remains uncertain and sometimes highly speculative (no molecules but only shell morphology to start with), they offer sometimes challenging views on worlds that have gone.

What we now call the Neotropics possibly once had a wider extension, and a team of Algerian and European scientists have made a study of Algerian shells that possibly give a clue. Hammouda et al. (2017) present a review of Eocene “Bulimes” and reached interesting but still tentative results.

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Their abstract reads “Terrestrial gastropods occur in many North African localities in Eocene continental deposits. Here we analyse the faunal assemblage from the Hamada de Meridja Formation in southwestern Algeria, dated as Early to Middle Eocene on the basis of charophytes. The assemblage consists of three closely related species that to date have been classied either in the extant Madagascan genus Leucotaenius v. Martens, 1860, or in the SW European Eocene genera Romanella Jodot, 1957 and Vicentinia Jodot, 1957. This is rejected for shell morphological and phylogeographical reasons, and a new classication as Maghrebiola gen. nov. is proposed. Maghrebiola is tentatively placed in the South American family Strophocheilidae, as species from the Early Eocene Itaboraı Basin of Brazil, currently placed in the genus Eoborus Klappenbach and Olazarri, 1970 in the family Strophocheilidae, superfamily Acavoidea, have a very similar shell habitus. This record possibly extends the known geographical range of the Strophocheilidae into the African continent during the Eocene. Immigration of this stock into North Africa during the Cretaceous via a still existing plate connection is assumed. An attribution of Maghrebiola to the African family Achatinidae is unlikely for shell morphological reasons despite certain habitus similarities, although the Priabonian genera Arabicolaria and Pacaudiella from Oman most likely belong into this family, and not to the Vidaliellidae as originally proposed. Possible causes for the very low diversity of the assemblage are mainly unfavourable living conditions, i.e. a relatively dry climate resulting in sparse vegetation and only occasional presence of water bodies, which may have had increased salinities, accounting for the lack of freshwater mollusks. The absence of any competing large gastropods may possibly have facilitated high intraspecic variability leading to sympatric occurrence of three closely related species, due to the animals occupying a wide range of available ecological niches. As the species discussed here have also been attributed to the genera Romanella and Vicentinia in the Vidaliellidae, we provide an appendix with annotated characterisations of most genera of the Vidaliellidae and list the nominal species assigned to them. This family is tentatively placed in the South American superfamily Orthalicoidea; its stock would have similarly immigrated from South America, but have successfully colonized mainly SW Europe, with only one Eocene species [Romanella kantarensis (Jodot, 1936)] recognized in Algeria”.

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The most recent, total overview of fossil and Recent Gastropoda was by Zilch, who classified the genera Romanella and Vidaliella with others in the “?Familia Anadromidae” within the Bulimulacea [currently Orthalicoidea] (Zilch, 1960); these genera are now placed in a family on their own, but based on their morphology there might be a resemblance with the Megaspiridae (e.g. Thaumastus). The link between the Mediterranean area and South America is further shown in the presence of the Clausiliid Neniinae, which the authors use as one of their arguments for their grouping of Maghrebiola with the Strophocheilidae. All this is an interesting but not implausible hypothesis.

References:
Hammouda, S.A., Kadolsky, D., Adaci, M., Mebrouk, F., Bensalah, M., Mahbouhi, M. & Tabuce, R., 2017. Taxonomic review of the “Bulimes”, terrestrial gastropods from the continental Eocene of the Hamada de Méridja (northwestern Sahara, Algeria) (Mollusca: Stylommatophora: Strophocheilidae?), with a discussion of the genera of the family Vidaliellidae. — Paläontologische Zeitschrift, (advance online) doi:10.1007/s12542-016-0333-5
Zilch, A., 1960. Gastropoda 2. Euthyneura. In: Schindewolf, O. (ed.) Handbuch der Paläozoologie, 6 (3–4): 401–834. Berlin: Borntraeger.

 

New taxa (28): Thaumastus

In a recent issue of Zoologia (formerly Revista Brasileira de Zoologia) a paper appeared with new taxa of Orthalicidae (Pena et al., 2011).

Two new species of Thaumastus (Thaumastus) from the state of Minas Gerais, southeastern Brazil, are described. They are diagnosed and characterized by the morphology of the shell and soft parts and compared with Brazilian species of the subgenus Thaumastus s.s. Martens, 1860. 

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Thaumastus (T.) parvus sp. nov. is similar to T (T.) baixoguanduensis Pena, Coelho & Salgado, 1996 but can be distinguished by the smaller size, smaller number of plates in the jaw, different number of follicle groups in the ovotestis and form of the fertilization complex. 
Type locality: Brazil, Edo. Minas Gerais, Belo Horizonte, Serra do Curral, Mangabeiras Park. Holotype: MNRJ 8107.

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Thaumastus (T.) caetensis sp. nov. is similar to T. (T.) largillierti (Philippi, 1845) but differs by the width and the contour of the parietal side of the shell aperture. T. (T.) caetensis is also closer to Thaumastus (T.) baixoguanduensis but can be distinguished by the smaller dimensions (height, width and number of protoconch whorls), the lack of a transversal light band on the body whorl, the jaw with smaller number of plates, and the radula with 35 teeth. In the soft parts, this new species differs also in the number of follicle gatherings in ovotestis, fertilization complex with globose shape, and penian retractor muscle terminally and laterally attached to flagellum.
Type locality: Brazil, Edo. Minas Gerais, Caet??, Serra da Piedade. Holotype MNRJ 8097.

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Reference:
Pena, M., Salgado, N. & Coelho, A.C. dos Santos, 2011. Two new species of Thaumastus (Gastropoda: Pulmonata: Orthalicidae: Bulimulinae) from the state of Minas Gerais, Brazil. – Zoologia 28(4):531-537. 

The magical number of seven

Neubert & Janssen (2004) made an interesting observation. They write (p. 242): “Although the system of the Orthalicoidea was intensively reviewed by Breure, it still remains in an unsatisfactory state. On the one hand, the enormous number of species level taxa were affiliated to a quite low number of genera, on the other hand the taxonomic rank and nomenclatural status of many of these names remained unsolved”.

Recently, Richard Zander, remarked on the Taxacom list that “the issue of how many species per genus is perhaps more a psychological or cognitive issue in so far as genera are man-made constructs. Eight is (today) the number of bits in a byte. Eight is the lucky number in many countries. Even more compelling, perhaps, is that the human mind on average has the capacity to operate simultaneously with seven words, concepts, sounds or the like (Miller 1956). So, perhaps the human mind has an innate urge to break up genera when they exceed that number?”.

The Orthalicoidea is a very diversified group, with over 1750 available names at the species level. Given the number of species in the catalogues by Richardson (1993, 1995 in Tryonia), the number of species per genus varies from 1 to 100+. 
Still, Neubert & Janssen have a point when they say that some genera seem to be too undifferentiated in my 1979 thesis. Bostryx, Naesiotus, and Drymaeus are the obvious candidates. If I would have had the opportunity to continue my research during the 1980s and beyond, I’m sure they wouldn’t have made this remark. Ongoing research involving these groups already indicate that they should be divided.

At the same time one could wonder about the number of genera per family. Also a man-made construct, although there is a useful ‘rule of thumb’ that a family should be monophyletic. Starting with one large family, Bulimulidae, at the time that Pilsbry wrote his monumental Manual of Conchology (1895-1902), my latest analysis counts no less than seven families. That sounds like a magical number…

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BTW: Zander’s remark was made in a discussion that started with this link: http://www.wired.com/wiredscience/2011/01/extinction-of-taxonomists/

References:
Miller, G.A., 1956. The magical number seven, plus or minus two. – Psychological Review 63: 81-87.
Neubert, E. & Janssen, R., 2004. Die Typen und Typoide der Natur-Museums Senckenberg, 84: Mollusca: Gastropoda: Pulmonata: Orthalicoidea: Bulimulidae (2), Orthalicidae, Placostylidae. – Archiv für Molluskenkunde 133: 193-297.