Two of the remaining four described middle Miocene species are some of the weirdest of all crown-group cowries (I say crown group because there were some truly bizarre looking stem-group cowries, e.g. Gisortia).
U. siphonata (above) is one of these. It is a very large cowry, attaining a length of almost 17 cm, which is not far behind the biggest specimens of Macrocypraea cervus, the largest extant cowry, which used to reach sizes of 19 cm in length. However U. siphonata is cheating a little since the anterior and posterior rostra of this species are produced into great upwardly curving ‘horns’. The rudimentary flanges that support the bases of each rostrum of U. eximia are much better developed in this species.
Even stranger is U. gastroplax, the ‘flanged cowry’ (specimen on the left is from Darragh 2002). This species also has elongated horn like rostra, although they are not as long as in U. siphonata. However the flanges have expanded outwards, merging together and making a continuous brim that encircles the entire base of the shell. The result looks much like a snow-shoe. Indeed it has been suggested that this is exactly what its function was and that it was an adaptation to living on soft, ‘soupy’ bottoms. One wonders then if U. siphonata was adapted to the same conditions but simply didn’t bother to keep itself on top of the sediment surface, and used its long rostra to carry its siphons up into clear water. The fourth middle Miocene Umbilia we will look at is little U. leptorhyncha (below). Although common and widespread, good specimens are rare due to the thin-shelled fragility of this species. This species is a departure from the other mid Miocene species in its small size, globose shape and poorly developed rostra. In these respects it most closely resembles U. prosila and may be closely related to it.
All of these species can be found in both South Australia and Victoria (U. gastroplax has not been officially recorded from South Australia but I have personally collected two specimens from the Cadell Formation on the banks of the River Murray)
Darragh recorded the extant U. hesitata as a fifth middle Miocene species, albeit one that only appears at the end of the stage, with little to no time overlap with the previously mentioned species. The taxonomy of these rare later middle Miocene Umbilia is a complicated issue. Two species have been named Umbilia tatei and Umbilia cera. Both are short , with weakly developed beaks and heavily calluses surrounding the basal margins and probably represent the same species whatever they are. Problematically if these really are small specimens of the extant ‘wonder cowry’ (as U. hesitata is sometimes called) then we have the problem that U. tatei would have priority over U. hesitata. I’m sure all the avid cowry collectors would object to replacing the entrenched U. hesitata with U. tatei. Fortunately I don’t think they have to. Although U. hesitata does display a range of adult sizes which overlaps with the small shells of U. tatei, small modern U. hesitata resemble typical large specimens more than they do U. tatei. In particular no modern U. hesitata has such thick marginal calluses as U. tatei, nor do they develop the elongate coarse dentition seen on the holotype of U. cera (these are not present in the types of U. tatei but the dentition of these specimens appears to be underdeveloped due to immaturity).
A late Miocene U. 'hesitata', probably belonging to U. tatei.
From the late Miocene and Pliocene (there is no Pleistocene record of Umbilia at all) there is just a single species, the extant U. hesitata (although some of these are a little different from modern U. hesitata while others probably belong to U. tatei). Then in our modern seas we find five species: U. hesitata, U. armeniaca, U. capricornica, U. orriettae and U. petillirostris.
However unlike the middle Miocene where you can find up to four species at the same locality almost all of the modern species have mutually exclusive ranges (only U. capricornica and U. petillirostris overlap in the deep Capricorn Channel of the Great Barrier Reef. Moving anticlockwise around the Australian coast we find U. armeniaca (Western Australia to Kangaroo Island, South Australia), U. hesitata (south eastern South Australia to Southern Queensland), U. orriettae (Moreton Bay, Queensland) and U. capricornica/U. petillirostris on the Great Barrier Reef. It is interesting to note that this pattern matches the phylogenetic pattern recovered in a comprehensive molecular phylogenetic analysis of modern cowries (Meyer 2004). In this analysis U. armeniaca was the sister group to all other living species and U. hesitata was the sister group to U. capricornica + U. petillirostris (U. orriettae was not included but morphologically it appears to be intermediate between U. hesitata and U. capricornica). Thus the modern forms appear to be the result of a radiation that proceeded from west to east. All of these living species are rather similar to one another and have a rather generalised shell structure compared to the excesses of the middle Miocene.
Umbilia hesitata, the most abundant extant species.
Most specimens display a moderately well-developed posterior rostrum and have highly reduced anterior tubercles that are separated by an oblique sulcus. These characters suggest that the modern taxa are more closely related to U. eximia and U. tatei than any of the other extinct species. However the type specimens of U. petillirostris stand out as something unusual. Unlike other living Umbilia the types of U. petillirostris are globose and thin-shelled with a very short posterior rostrum. In these respects U. petillirostris resembles the smaller middle Miocene species, U. leptorhyncha and the late Oligocene U. prosila. Darragh (2002) suggested that these three species represented a lineage that had been separate since the Oligocene. But using the molecular phylogeny this would suggest that all of the modern species have been separate since at least the late Oligocene, despite sharing a similar hesitata-like morphology that does not show up in the fossil record until the late Miocene. However I strongly doubt that U. petillirostris is closely related to U. prosila and U. leptorhyncha. Despite its globose shape and thin shell it displays characters typical of the modern clade such as large size, a moderately produced posterior beak, and weak anterior tubercles. A greater sample of specimens shows that U. petillirostris and U. capricornica are quite variable and that individuals of each species can be found that approach the other in morphology. Indeed some have suggested that the two species are not distinct at all (Wilson and Clarkson 2004). Nevertheless limited genetic sampling does indicate that U. petillirostris does maintain a distinct haplotype (Meyer 2004). Excellent photographs of all the living forms can be seen here.
Next week: the origin of Umbilia.
References
Darragh, TA (2002) A revision of the Australian genus Umbilia (Gastropoda: Cypraeidae). Memoirs of the Museum of Victoria 59: 355-392.
Meyer CP (2004) Toward comprehensiveness: increased molecular sampling within Cypraeidae and its phylogenetic implications. Malacologia 46: 127-156.
Wilson B, Clarkson P (2004) Australia's Spectacular Cowries: A Review and Field Study of Two Endemic Genera-Zoila and Umbilia. Odyssey: El Cajon, 369 pp.
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