A Neogene Moon

Back when I was a young lad, some time not so long after the end-Cretaceous extinction, we often spent part of the Christmas holidays camped at the estuary beach-front below my great-grandparents' house. Among the things I recall doing there was going out at low tide with my great-grandmother to dig up cockles for lunch. The New Zealand cockle Austrovenus stutchburyi is not an immediate relative of the bivalves of the family Cardiidae known as cockles in Europe but a member of a different bivalve family, the Veneridae. Venerids are shallowly burrowing bivalves that generally live buried below the sand or mud just shallowly enough to extend their short siphons to the surface for filter-feeding.

Dorsal (left) and lateral views of Marama hurupiensis, from Beu & Maxwell (1990).

Because they live pre-buried in this manner in fairly low-energy habitats, venerids have an excellent fossil record. Marama is a fossil genus of a dozen species of venerids known only from New Zealand and Tasmania (Beu & Maxwell 1990; Beu 2012). The genus was first recognised by Marwick (1927) who divided it between two subgenera, Marama sensu stricto and Hina. Both names derive from Maori names for the moon, presumably in reference to the clams' appearance. Marama species are similar in overall appearance to the modern New Zealand cockle, the primary defining characters of the genus reflecting features of the shell hinge. These include the presence of a moderate anterior lateral tooth or tubercle in the left valve. The size of the species varies from the small M. tumida, a bit less than two centimetres in length, to the relatively large M. hurupiensis which reaches six centimetres in length. The shells are sculpted with concentric lamellae, varying from fine and very dense in M. tumida to strong and widely spaced in M. pristina to weak and sparse in M. ovata.

Marama species are known from the Kaiatan to the Nukumaruan stages in the New Zealand stratigraphic system, corresponding to the ealy Late Eocene to the late Pliocene/earliest Pleistocene in the international stratigraphic divisions. Many regions of the world have their own local stratigraphic divisions that may be used in preference to the glocal system for various reasons. In some cases, this may be because of difficulties in correlating the local geological record to global events. There may not be suitable resources preserved for calculating a deposit's absolute age, or a geographically isolated region may lack fossils of cosmopolitan index species. As a result, it may be possible to recognise temporally successive biotas in a region's palaeontological record without being able to tell for sure whether a given biota is (for instance) Eocene or Oligocene. Alternatively, because stratigraphic divisions are commonly based on biotic turnovers such as mass extinctions, the major local biotic events may not exactly line up with the global average (for instance, the characteristic biota of a given geological period may have persisted longer in one region than it did in another). In the case of the New Zealand palaeontological record, Marama was one of a number of molluscan genera that became extinct towards the end of the Nukumaruan in relation to cooling temperatures representing the onset of the Pleistocene ice ages.

REFERENCES

Beu, A. G. 2012. Marine Mollusca of the last 2 million years in New Zealand. Part 5. Summary. Journal of the Royal Society of New Zealand 42 (1): 1–47.

Beu, A. G., & P. A. Maxwell. 1990. Cenozoic Mollusca of New Zealand. New Zealand Geological Survey Paleontological Bulletin 58: 1–518.

Marwick, J. 1927. The Veneridae of New Zealand. Transactions and Proceedings of the New Zealand Institute 57: 567-636.