Year: 1998
Type: article
Abstract: Eight species of illaenimorph trilobites belonging to five genera of the Styginidae are described from limestones of the mid-late Wenlock to Ludlow Mirrabooka Formation and its stratigraphical equivalents in the Orange district.New South Wales.The morphology of illaenimorph ( = effaced) stygimds is discussed; the term 'omphalus' is introduced for the socketed, tubercle-like projection present in some genera on the interior of the cranidium at or in front of the anterior end of the axial furrow.Amongst other characters, the gross convexity of the exoskeleton, the form of the rostral plate, the presence of the omphalus, the form of the thorax, and possibly the form of the hypostome are deemed most useful for generic diagnosis; characters used for discrimination at a lower taxonomic level include the proportions of the exoskeleton, the degree of effacement, the pattern of cranidial muscle scars, the size and position of the eye, and the character and disposition of sculpture.New taxa are Excetra iotops gen.et sp.nov., Lalax olibros gen.et sp.nov., L. lens gen.et sp.nov., Rhaxeros synaimon sp.nov.and R. trogodes sp.nov.Bumastus (Bumastella) Kobayashi and Hamada is raised to generic status and its diagnosis emended; specimens from New South Wales are assigned to the type species B. spicula , which is considered to be synonymous with five other Japanese species assigned to three different genera by Kobayashi and Hamada.Bumastus is tentatively recorded on the basis of a single rostral plate; the genus is otherwise known with certainty only from Laurentia and eastern Avalonia.Meraspid transitory pygidia of Bumastella and Lalax from New South Wales are up to eight times larger than those of other styginids with well documented ontogenies; transitory pygidia of large size are known also in some other Silurian effaced styginids, and it is suggested that the phenomenon may result from neoteny.The assumption that sexual maturity in trilobites coincided with the meraspid-holaspid transition is refuted.The effaced styginids from New South Wales show strong faunal affinity with those from the Upper Wenlock or Lower Ludlow of Japan.HOLLOWAY AND LANE: EFFACED SILURIAN TRILOBITES 855 volcanics.The Boree Creek Formation was divided by Sherwin (1971a) into lower and upper limestone units, informally named Limestones A and B respectively, separated by a calcareous tuffaceous sandstone referred to as the 'tuffaceous trilobite bed'.A possible disconformity between Limestone A and the 'tuffaceous trilobite bed' was considered by Sherwin and Pickett (in Pickett 1982, p. 140) to be probably of only very local significance, and not to represent a significant break.These authors also expressed doubts about the lateral extent of the lithological subdivisions of the Boree Creek Formation; observations by DJH suggest that the subdivisions can be recognized only locally, that the lithology of the 'tuffaceous trilobite bed' is only the result of decalcification of the limestone, and that trilobites are not restricted to the middle part of the formation.Conodonts from the Boree Creek Formation were correlated by Bischoflf (1986, p. 36; text-fig.8) with the latest Llandovery to early Wenlock amorphognathoides and ranuliformis biozones.Disconformably overlying the Boree Creek Formation is the Mirrabooka Formation.This consists of 500 m of predominantly fine sandstones and siltstones but also includes several lenticular limestone bodies that were referred to by Sherwin (1971a) as Limestones C, H and I.The following graptolite species from near the base of the Mirrabooka Formation were recorded by Pickett (1982, p. 147): Pristiograptus meneghini, Monoclimacis cf.flemingi, M. cf.flumendosae, Dendrograptus sp. and Dictyonema sp.-which are considered to represent a mid to late (but not latest) Wenlock age.The age indicated by an assemblage (Bohemograptus bohemicus , Lobograptus ' scanicus ', Linograptus posthumus and Dictyonema sp.) from about the horizon of Limestone I (Text-fig.2), in the upper part of the formation is equivocal; correlation with the Ludlow scanicus and leintwardinensis biozones was suggested (Pickett 1982, p. 147), but Dr D. Loydell (pers.comm.)suggests that although the Ludlow is indicated, the named species are not all known to occur stratigraphically together elsewhere, and re-examination of the fauna should be undertaken.Overlying the Mirrabooka Formation with apparent conformity are up to 400 m of shales and siltstones that are commonly olive green or red; they were assigned by Sherwin (1971a, p. 219) to the Wallace Shale, the type locality of which lies some 25 km to the south.In the 'Mirrabooka' area the formation also contains pods of limestone up to 250 m long, and a boulder bed and exotic blocks of Ordovician sediments and volcanics.The graptolites Monograptus cf.ultimus and M. bouceki , indicative of the Upper Ludlow, occur in the lower part of the formation (Byrnes, in Pickett 1982, p. 154), and the upper part is considered to extend into the Devonian.In the eastern part of the area, just west of Borenore, the Rosyth Limestone (Walker 1959, p. 42) is equivalent to at least part of the Boree Creek Formation.Problems with the definition of the Rosyth Limestone were discussed by Pickett (1982, p. 161), who restricted the name to the lowermost part of the sequence, which consists of richly fossiliferous limestones and calcareous shales with a thickness of 50 m to more than 100 m.Overlying these strata, apparently disconformably, are 100 m of unnamed lithic arenites, shales and bedded limestones.These are in turn overlain disconformably by the 600 m thick massive white, grey and red limestones of the Borenore Limestone, which is equivalent to the Mirrabooka Formation and probably the lowermost Wallace Shale.Bischoff (1986, p. 38 ; text-fig.8) reported conodonts of the early Wenlock ranuliformis Biozone in the lower part of the Borenore Limestone; it is possible, however, that the specimens did not come from this formation but from the underlying unnamed limestone unit mentioned above.To the north of 'Mirrabooka', towards Molong, the Mirrabooka Formation grades into the Molong Limestone (Adrian 1971, p. 193), which directly overlies late Ordovician andesitic volcanics with unconformity, and is unconformably overlain by Late Devonian sandstones.Several discrete limestone bodies occurring to the south of the main outcrop area of the Molong Limestone in the vicinity of ' Mirrabooka' homestead were referred to by Sherwin (1971a) as Limestones D-G and J; these were considered by Sherwin (1971a, p. 212) to be southern extremities of the Molong Limestone, but Pickett (1982, p. 147) assigned them to the Mirrabooka Formation.The stratigraphical range of the Molong Limestone is uncertain, but the upper part is believed to be equivalent to part of the Wallace Shale and the lower part may contain equivalents of the Rosyth Limestone (Pickett 1982, p. 148).
Authors: Holloway, D J, Lane, Philip David
Language: English
Subfield: Paleontology
Field: Earth and Planetary Sciences
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W2965633133