The August History of Filter-Feeding Ostracods

Today's post subject, the Cavellinidae, were a family of ostracods that were around from the Middle Silurian period to the Middle Triassic (Adamczak 2003a). And for those of you unfamiliar with ostracods: you lucky, lucky bastards. They're horrible.

I exaggerate slightly. Ostracods are a group of crustaceans that spend their lives enclosed in a pair of shells, superficially a bit like a bivalve, However, what they primarily are is very, very small (often less than a millimetre in total length), which makes them very difficult to work with as identification often requires dissecting out the (even smaller, needless to say) appendages hidden within the shells. Fortunately, I've personally managed so far to avoid being caught in ostracod purgatory, but many of my acquaintances have not been so fortunate. In the case of fossil ostracods like today's subjects, it is generally only the valves themselves and not any of the internal parts that are preserved to be of concern, but they're still small enough overall to hardly be counted as simple to work with.

External dorsal and lateral views of the carapace of each sex of the Silurian Gotlandella martinssoni, from Adamczak (2003a) (adr = admarginal ridge, mr = marginal ridge).

Cavellinids belong to a group of ostracods called the Platycopina, so-called because of their relatively flat sides. Among the modern fauna, platycopines are represented only by the genus Cytherella, whose distant ancestors were almost certainly among the species assigned to the Cavellinidae (Adamczak 2003a), so the 'extinction' of the cavellinids in the Triassic is really a pseudo-extinction as they were replaced by the descendant cytherellids. As befits its phylogenetic isolation from other living ostracods, Cytherella is an oddity in the modern fauna, being one of the few ostracod lineages to make a living as filter-feeders. The rear part of the carapace is expanded on the inside to form a brood chamber in which the eggs are nursed. It has been suggested that the evolution of filter-feeding and of the brood chamber were connected (Adamczak 2003b): as water is drawn in by the process of filter-feeding, it circulates around the brood chamber to keep the contents, whether eggs or newly hatched larvae, oxygenated. The constant flow of water also brings more oxygen to the adult's own gills than it would receive passively, so Cytherella are able to live in places with less dissolved oxygen than other ostracods (Lethiers & Whatley 1994).

Internal view of right valve of the Middle Devonian Birdsallella eifeliensis, from Adamczak (2003a). Abbreviations: cg = contact groove (where the left valve is nestled); li = limen (the inner partition separating off the probable brood chamber).

Though preserved appendages have not yet been recorded from any cavellinid, their valve morphology is very similar to that of Cytherella: closely sized valves (the right is only slightly larger and slightly overlapping the left), with the line of contact between the valves is fairly straight along the underside, and the valves gaping open slightly at the front but tightly closed towards the back. A constriction on the inside of the valve also indicates the presence of a Cytherella-like brood chamber, and like Cytherella the outer surface of the carapace is fairly smooth. In fact, the only really marked difference between cavellinids and cytherellids is the arrangement of the muscle scars indicating where the valves where held together: in Cytherella and fossil members of the Cytherellidae, the scars are arranged in a double row, while members of the Cavellinidae have the scars in a random cluster. Because of the similarities between cavellinids and Cytherella, it is inferred that cavellinids were also filter feeders. Filter-feeding ostracods seem to have been more diverse in the Palaeozoic than in the present, leading Lethiers & Whatley (1994) to suggest that the Palaeozoic marine environment may have contained lower oxygen levels in many places than the modern environment. However, this line of reasoning was dismissed by Becker (2005), who felt that there was no reason to assume that fossil filter-feeders would necessarily show the same preference for low-oxygen environments as modern Cytherella. Instead, Becker has argued that strongly calcified ostracods like cavellinids are indicative of relatively high energy environments in shallow coastal waters (Adamczak 2003a).

REFERENCES

Adamczak, F. J. 2003a. The platycopine dynasty 2. Family Cavellinidae Egorov, 1950. Authentic platycopines. N. Jb. Geol. Paläont. Abh. 229 (3): 375-391.

Adamczak, F. J. 2003b. The early platycopine dynasty (Ostracoda; Palaeozoic). Senckenbergiana Lethaea 83 (1-2): 53-59.

Becker, G. 2005. Functional morphology of Palaeozoic ostracods: phylogenetic implications. Hydrobiologia 538: 23-53.

Lethiers, F., & R. Whatley. 1994. The use of Ostracoda to reconstruct the oxygen levels of Late Palaeozoic oceans. Marine Micropaleontology 24 (1): 57-69.