322 
Figure 8 Similarity in percentage variation in 
gonopore number in the living species 
Schizaster (Schizaster) compact us and Schizaster 
(Ova) myorensis (data from McNamara and 
Philip 1980a), and the fossil species Schizaster 
(Schizaster) aff. compactus from the Middle 
Miocene Trealla Limestone, Gnargoo Range, 
Western Australia. 
of four, three and two gonopores. This 
developmental flexibility in S. (Schizaster) compactus 
highlights the impracticality of using gonopore 
number as a generic or subgeneric diagnostic 
character. It may, however, serve as a useful 
character for the delineation of species, as the very 
flexible nature of the gonopore production may in 
itself be a species character. 
The shape of the test shows a high degree of 
intraspecific variability. At mean test length, the 
test can vary in height between 64 and 72%TL 
(n=22). Similarly, the test width varies between 88 
and 97%TL (n=24). S. (Ova) myorensis shows similar 
degrees of intraspecific variation in mature 
specimens (McNamara and Philip 1980a, figure 
6A,B), but the ontogenetic variation that it displays 
does not occur in S. (Schizaster) compactus. On the 
aboral surface of the test the only character to show 
appreciable intraspecific variation is the position of 
the apical system. Specimens of mean test size 
show a variation in the distance of the apical 
system from the anterior ambitus of between 56 
and 64%TL. 
On the adoral surface the position of the 
peristome in relation to the anterior ambitus is 
highly variable, varying in mean specimens 
between 17 and 24%TL from the anterior ambitus. 
The maximum width of the plastron also shows a 
degree of variability, varying between 38 and 
46%TL. 
CONCLUSIONS 
The ontogenetic development of Schizaster 
(Schizaster) compactus shows some similarities and 
some dissimilarities to the ontogeny of Schizaster 
K.J. McNamara 
(Ova) myorensis. Both species share a narrowing of 
ambulacrum III adorally, an increase in number of 
pore pairs in the aboral petals and ambulacrum III, 
and relative decrease in size of tire peristome and 
periproct. Other changes seen in S. (Ova) myorensis, 
but not observed in S. (Schizaster) compactus, are 
changes in shape of the test, the posterior migration 
of the apical system and increase in petal sinuosity. 
The greater extent of morphological development 
during ontogeny in S. (Ova) myorensis occurs 
because, compared with S. (Schizaster) compactus, it 
is a more peramorphic species, lying further along 
the peramorphocline between the “Paraster" 
morphotype and "Schizaster" morphotype 
(McNamara and Philip 1980b). The morphological 
changes that are common in the ontogenies of the 
species reflect changes during growth associated 
with feeding (peristome and periproct size), funnel 
construction (ambulacrum III size and pore pair 
number) and respiration (pore pair number in the 
petals). 
The presence of variable pore pairs numbers in 
these two Australian species suggests the 
possibility that if large populations of other species 
of Schizaster from other regions are examined, the 
same effect will be seen. It is unlikely to be a purely 
Australian phenomenon. Neither is it likely to be a 
feature restricted to extant species. Indeed, an 
examination of fourteen specimens of an 
undescribed species of Schizaster from the Middle 
Miocene Trealla Limestone of the Gnargoo Range 
in the Carnarvon Basin, Western Australia, 
referred to as S. (Schizaster) aff. compactus by 
McNamara and Kendrick (1994), shows a similar 
variation of individuals with either two, three or 
four genital pores. Moreover, there is a startlingly 
similar ratio between the two species, with the 
Miocene species having 28.6% of specimens with 
two gonopores (compared with 29.6% in S. 
compactus), 14.3% with three (compared with 
14.6%) and 57.1% with four (compared with 
55.6%). 
It has been demonstrated elsewhere (McNamara 
1988, 1989, 1990) how important heterochrony has 
been to the evolution of spatangoid echinoids, and 
to Schizaster in particular (McNamara and Philip 
1980b). Heterochrony has been shown to be more 
important as an evolutionary mechanism in those 
species that show both high degrees of ontogenetic 
and adult morphological variation. Many 
characters in S. (Schizaster) compactus fulfil these 
two criteria. If, as seems likely from other studies 
of phenotypic variation in species of Schizaster 
(McNamara and Philip 1980a), suites of characters 
covary, then perturbations to the characters that 
undergo morphological change during ontogeny 
are likely to have been an important factor in the 
evolution of the large number of species of 
Schizaster during the Tertiary. 
