30 A feather star buried alive

Echinodermata, Crinoidea: Rautangaroa aotearoa

Quite a few marine animals have been given fanciful common names that mostly belong to land plants. Examples include sea anemones (cnidarians), sea-gooseberries (a kind of jellyfish), sea tulips (ascidians), sea cucumbers (holothurians), and sea lilies (crinoids). Of these, sea lilies are the only group with a fossil record in New Zealand, although this is patchy at best.

Feather stars have a skeleton of thousands of tiny ossicles held together by ligaments and muscles. The ossicles have distinctive shapes that show what part of the animal they are from. The brachial and pinnule ossicles are from the arms (used to catch tiny food particles and to swim), the cirral ossicles are from the ‘legs’ or cirri (used to climb and hold onto the substrate), the centrodorsal ossicle is where the arms and cirri attach and contains the mouth and digestive tract. The feather star Rautangaroa aotearoa is one of the most complete Cenozoic feather stars known. Three of the five arms are still at least partly articulated, and some cirri ossicles are still lined-up in ‘life-position’, although they are folded up with the arms rather than pointing downwards. The cirri attach to the round sockets on the lower side of the centrodorsal. The star shaped-centrodorsal is very handsome. The illustration shows where the various ossicles are located in the animal. Image credit: JH Robinson.

Crinoid species are placed into two groups – stalked isocrinoids which remain attached to the sea floor throughout life – and feather stars, which lose the post-larval stalk which attached them to the seafloor. They become free-living as adults, able to move across the seabed, or even, in some cases, to swim.

The skeletons of crinoids are composed of a range of small calcareous elements called ossicles, held together by ligaments and muscles, allowing crinoids to be very flexible. Within a few days of death, the skeletons disarticulate into individual ossicles. Large feather stars may have a skeleton of more than half a million ossicles! Once they have disarticulated there is only one ossicle that is distinctive enough to allow identification of the genus and species – the largest ossicle, the centrodorsal.

The University of Otago Geology Museum has many fossil specimens of isolated but distinctive round or star-shaped disarticulated plates of isocrinoids, many from the Middle Triassic strata of the Murihiku Supergroup in Southland. Most specimens however, come from the mid-Cenozoic limestones and greensands of southern Zealandia.

Complete or semi-complete fossil feather stars are very rare, especially in the Cenozoic. So, the discovery of an almost complete feather star from Waipati in North Otago is of great importance. It seems likely that the animal was buried very rapidly, possibly while still alive. The specimen, collected by Ewan Fordyce in 1993 is catalogued as OU 44680. It was designated as the name-bearer for a new crinoid genus, Rautangaroa. In Te Māori, rau is feather and Tangaroa refers to the atua (i.e. ancestor with continuing influence, supernatural being) of the sea and fish.

The other remarkable feature of this fossil is the presence of a regenerating arm.  All echinoderms have the ability to regenerate body parts – think of starfish in rockpools with partly regrown arms. This feature is also characteristic of living and fossil stalked crinoids but the Geology Museum specimen is the only known example of such regeneration in a fossil feather star.

Crinoid limestones of Paleozoic age, containing a high percentage of crinoid ossicles, are fairly common in other parts of the world, as extinct groups of crinoids were extremely abundant in the distant past. Such limestones are much rarer in the Cenozoic. However, one site near Oamaru where the limestone comprises 30% crinoid ossicles (rather than the typical 1-2% seen at other nearby sites), displays the remains of what must have been a longstanding crinoid ‘meadow’ on the seafloor.

—Written by Jeffrey H Robinson and Daphne E Lee