Honours Thesis Abstract
TAXONOMY AND PALAEOECOLOGY OF THE
EUCLA BASIN MIOCENE ECHINOID FAUNAS
Sarah Martin
University of Melbourne, Victoria
2003 Hellman & Schofield – AIG Honours Bursary Winner
This study aims to partially rectify the paucity of information on the Cainozoic echinoid
assemblages of Western Australia, by assessing the taxonomy and palaeoecology of three Eucla
Basin echinoid faunas. These assemblages are the Late-Oligocene-Early Miocene Abrakurrie
Limestone, and the Mid Miocene Nullarbor Limestone and Colville Sandstone; with this latter
unit representing the onshore lateral equivalent of the Nullarbor Limestone. While the
Abrakurrie Limestone was deposited in a bryozoan-rich, deep-shelf, cool-water environment
(James & Bone 1994), the Nullarbor Limestone and Colville Sandstone were deposited in a
calcareous algae-rich, shallow-shelf, warmer water environment (Lowry 1970). Due to shallow
burial and relative lack of cementation during the Abrakurrie Limestone’s post-depositional
history, the taphonomic influence on these specimens is low; while in the Mid Miocene units
taphonomic influence on specimens is high due to exposure, intense cementation and
karstification.
Seventeen species of echinoids occur in the Abrakurrie Limestone, including two new species
of Echinolampas. The assemblage is dominated numerically by holasteroids (Corystus
dysasteroides) and cassiduloids (four species of Echinolampas); the dominance of these sandtolerant groups suggests a sandy, but possibly variable, sediment. Cidaroids are also important
in the assemblage, with three species identified from test fragments, and ten spine morphologies
described. This group prefers firm substrates, and although sandy, the Abrakurrie Limestone
sediment must have been stable. The presence of marsupiate forms (Fossulaster halli and
Willungaster scutellaris) in the assemblage indicates a seasonal environment, such as those seen
in modern temperate to polar regions; while the richness of the fauna, both in species and
specimens, indicates a nutrient-rich environment. Evidence for interactions between the
echinoids and other members of the benthic community is ubiquitous, with a total of 13
different morphologies, representing seven symbiotic groups. Evidence for gastropod predation
is high – overall, the predation rate for the Abrakurrie Limestone is 21%. The Abrakurrie
Limestone preserves similar species to other Late-Oligocene assemblages of southeastern
Australia, suggesting that during the Late Oligocene-Early Miocene, a strong current, probably
the newly created circum-Antarctic current, connected faunas across the southern margin. This
current would have supplied the rich nutrient-conditions to southern Australian shelves through
upwelling. This, plus the seasonality induced by Australia’s position at high latitudes, created
the rich echinoid fauna of the Abrakurrie Limestone.
Twelve species occur in the Nullarbor Limestone, with eight species comprising the Colville
Sandstone assemblage, including one new species of Goniosigma and possibly a new genus of
temnopleurid. The Nullarbor Limestone assemblage is dominated by clypeasteroids, although
temnopleurids are exceptionally diverse; in the Colville Sandstone, temnopleurids and
spatangoids are most common. Again, the dominance of sand-tolerant species indicates a sandy
substrate for both units, although the Colville sediment appears to have been coarser-grained.
Cidaroids are completely absent from both assemblages, with the exception of one spine from
the Colville Sandstone, suggesting that the change from bryozoan to calcareous algae-
dominated conditions excluded this group in preference of the temnopleurids, probably due to a
decrease in substrate firmness. No marsupiate forms occur in either assemblage, indicating
lowered seasonality; although the lower species and specimen richness of the Mid Miocene
units indicates this was a nutrient poor environment in comparison to the Abrakurrie Limestone.
Evidence for interactions is also considerably less than in the Late Oligocene, especially in the
Nullarbor Limestone; gastropod predation rates are also lower (Nullarbor 14%), although the
low numbers of specimens in the Colville Sandstone (30% predated) skew the figures for this
unit. The echinoids of the Mid Miocene Eucla Basin units are quite different from other Mid
Miocene echinoid assemblages. Australia’s drift into lower latitudes, coupled with the effects of
the Miocene Climatic Optimum, probably switched on the Leeuwin current, reducing both
connectivity with south-eastern Australia and the influx of nutrients, resulting in the
considerably poorer echinoid assemblages in the Eucla Basin at this time.
The transition from the Early to Mid Miocene in the Eucla Basin is therefore characterised by a
decrease in echinoid diversity, decrease in predation and interactions, a change from
holasteroid-cassiduloid to clypeasteroid-spatangoid rich faunas, and the disappearance of
marsupiate forms. Comparing the Nullarbor and Colville assemblages, however, it can be seen
that the transition from shallow to slightly deeper water caused selection for less robust forms,
selection for forms more tolerant of finer / more variable sediment, selection for infaunal as
opposed to epifaunal forms, increase in species diversity, and decrease in predation and
interactions.