The Neomiodontids: Brackish-Water Bivalves of the Mesozoic

Cast from internal mould of Myrene tetoriensis, from here.

During the early part of the Cretaceous, an area corresponding to the modern Sea of Japan was occupied by a massive brackish-water lake that has been called Lake Tetori, after the geological Tetori Formation that it left behind it. My impression is that Lake Tetori was not an overly hospitable place: warm, shallow, and probably low in oxygen, it was home to a fairly depauperate fauna dominated by only two species of clam, known as Tetoria yokoyamai and Myrene tetoriensis (Kondo et al. 2006). Tetoria I shall leave for another time; Myrene (and its ilk) is the one I want to look at today.

Myrene tetoriensis belonged to a now-extinct family of bivalves known as the Neomiodontidae that lived during the Jurassic and Cretaceous periods. Neomiodontids were most diverse in the northern continents, though species have also been assigned to this family from India and Australia (Moore 1969). Not dissimilar in appearance to a modern pipi, though generally smaller in size, neomiodontids are primarily known from brackish-water or freshwater deposits. The late Triassic/early Jurassic saw something of a flush of bivalve lineages in low salinity environments: the separation of the ex-Pangaean continents resulted in an increase in continental margins, while high carbon dioxide concentrations in the atmosphere stymied the growth of calcium-heavy marine forms (Kondo & Sano 2009).

A rock full of fossils of Neomiodon, from here.

Most neomiodontids were found in sandy habitats, though Myrene tetoriensis lived in mud. They were shallow burrowers, living buried in the sand with the tip of the shell at surface level. Deep-burrowing bivalves possess elongate tubular siphons through which they breath and feed; the mantle boundary inside the shell of such species has a cavity called the pallial sinus into which the siphons can be retracted. In neomiodontids, the pallial sinus is undeveloped, indicating a proportional lack of development of any siphons. Neomiodontids would have mostly been suspension feeders, capturing food particles floating in the water; because of its muddier habitat, Myrene may have been a deposit feeder (Nishida et al. 2013). The narrow, relatively slim shape of neomiodontid shells suggests that they could probably burrow into their substrate rapidly if they became exposed by water action or potential predators.

When the Neomiodontidae was first established as a distinct family, Casey (1955) suggested that it could include the ancestors of the Sphaeriidae, a living group of small freshwater clams known as the pea clams. If this was the case, then neomiodontids did not truly go extinct during the Cretaceous but live on in their descendants. However, more recent authors do not seem to support this relationship. Kondo et al. (2006) note that the decline of brackish-water shallow burrowers such as neomiodontids correlated with the diversification of deeper-burrowing families and suggest a causal connection between the two, but it is worth noting that Tetoria, mentioned above as peaceful cohabitant of Myrene, was a deep-burrower. Nishida et al. (2013) see the extinction of neomiodontids somewhat differently. Citing the often transient nature of the habitats preferred by neomiodontids and other non-marine bivalves, they suggest that the neomiodontids were not a single lineage but represented numerous independent colonisations of non-marine habitats by members of the related marine family Arcticidae, with the 'neomiodontid' habitus the result of convergent evolution.In this view, the reasons for 'neomiodontid' extinction should be sought not with the neomiodontids themselves, but with the extinction of their arcticid progenitors.

REFERENCES

Casey, R. 1955. The Neomiodontidae, a new family of the Arcticacea (Pelecypoda). Proceedings of the Malacological Society 31: 208–222.

Kondo, Y., T. Kozai, N. Kikuchi & K. Sugawara. 2006. Ecologic and taxonomic diversification in the Mesozoic brackish-water bivalve faunas in Japan, with emphasis on infaunalization of heterodonts. Gondwana Research 10: 316–327.

Kondo, Y., & S. Sano. 2009. Origination of extant heteroconch families: ecological and environmental patterns in post-Paleozoic bivalve diversification. Paleontological Research 13 (1): 39–44.

Moore, R. C. (ed.) 1969. Treatise on Invertebrate Paleontology pt N. Mollusca 6. Bivalvia vol. 2. The Geological Society of America, Inc., and The University of Kansas.

Nishida, N., A. Shirai, K. Koarai, K. Nakada & M. Matsukawa. 2013. Paleoecology and evolution of Jurassic–Cretaceous corbiculoids from Japan. Palaeogeography, Palaeoclimatology, Palaeoecology 369: 239–252.