5 Robinson’s pilchard: the importance of earbones

Osteichthyes, Clupeiformes: Sardinops robinsoni

Some species have parts to their skeletons that look almost identical to that same part from a different species. For example, it can be difficult to identify which species of dolphin a rib might belong to or which species of bird a toe bone might be from: many species of fish have similar looking fin rays. Often the shapes of these bones are the same between species and only the sizes of their bones varies. This means that if we find a fossil rib separated from the rest of the skeleton then we might be able to say that rib is from an ancient dolphin, but we would probably never know which species of dolphin it came from.

In contrast, sometimes a small body part is so distinctive that it identifies a species all by itself. This is the case with otoliths in fish.

Otoliths (ear stones) are mineral structures in the inner ear that have an important role in helping an animal to sense movement and maintain balance. When a fish moves its head the otoliths within also move, applying a gentle force to the surrounding tissue and giving the fish its sense of motion. Otoliths aren’t technically part of the skeleton however. Instead, otoliths are accumulations of calcium carbonate, the same material that molluscs use to construct their shells, and not calcium phosphate, the material that bones and teeth are built from. Regardless, otoliths are important to the sensory systems of all vertebrate species including humans and have particularly distinctive shapes in ray-finned fish (Actinopterygii).

In 2017 Werner Schwarzhans and his coauthors recognised 32 species of fossil fish by studying otoliths from New Zealand, with 14 of these species being new to science. These species were from six locations of late Oligocene age across the southern South Island of New Zealand. The number of otoliths collected from each location varied with one site producing several hundred specimens. It was at this location that Robinson’s pilchard was discovered (Sardinops robinsoni).

An otolith from Sardinops robinsoni is approximately 2 mm long and without a microscope might be mistaken for a coarse grain of sand. On closer inspection an otolith from this species “…is easily recognised by the characteristic shape of the rostrum and the deep excisura and moderately large antirostrum…” In simpler terms, this otolith has a smaller upper curve that is separated by a deep groove from a larger and pointed lower curve. Which is nearly exactly what the otolith in a living sardine looks like.

Sardines today (Sardinops sagax) have a global distribution which includes New Zealand where they are known locally as mōhimōhi, mōhi, and pilchards. Sardines are most commonly found in warmer coastal waters and along the continental margin in large embayments like Tīkapa Moana (the Hauraki Gulf). Slightly unusually, Robinson’s pilchard was collected from sediments deposited in a rocky shore to shallow subtidal environment, which might indicate that sardine ecology has changed over time. However, as the evolutionary relationship between living sardines and Robinson’s pilchard is not yet clear we don’t  know how New Zealand connects with the global sardine story. Given how important pilchards are to local marine food webs though, the discovery of Robinson’s pilchard provides important insight into the marine food webs of Zealandia during the late Oligocene.

—Written by Daniel B Thomas