Art. XLIV.—On the Occurrence of Lower Ordovician Graptolites in Western Otago.
Communicated by Professor P. Marshall, M.A., D.Sc., Otago University.
[Read before the Otago Institute, 5th December, 1911.]
Through the kindness of Professor P. Marshall, of Dunedin, I have had the opportunity of examining half a dozen small slabs of graptolite-bearing shales collected by Mr. J. S. Nicol, of Gore, at Preservation Inlet, at the extreme south-west corner of New Zealand.
The rocks containing the fossils are of two kinds. Both are dark blue in colour. One is slightly micaceous, and has small white spots of some decomposed mineral in it. It has a somewhat irregular fracture. The other rock is of a darker tint, being almost black, and splits more freely along the bedding-plane. Both rocks are silicified. The graptolites are mostly very distinctly shown, being preserved in a silvery-white mineral which is generally spoken of as gumbelite. Further examination should result in the discovery of a larger supply of better material than has been submitted to me.
The fossils clearly belong to the series known as Lancefieldian in Victoria, which is very low down in the Ordovician, and contains some forms which elsewhere are of Cambrian age. One of the most striking features is the exact resemblance of the darker rock to that which occurs at Lancefield itself. The slabs containing the specimens shown in figs. 1 and 2 may be matched both lithologically and palaeontologically with examples from the Victorian locality, twelve hundred miles away. That both should be similarly silicified is very remarkable.
I have ventured on two specific identifications only, with a separate variety in one case. But besides these several examples are shown of another genus, Bryograptus, which may be new, and probably are, but as the thecal characters are not clearly shown I have not named them.
The figures show the following forms:—
Clonograptus tenellus Linnarson.
Clonograptus tenellus var. callavei Lapworth.
Clonograptus sp. n.
Tetragraptus decipiens T. S. Hall.
In addition, there are many fragments, suggestive but unidentifiable. None of these seem to belong to genera not represented at Lancefield. Clonograptus tenellus and its numerous varieties is, outside Australasia, a typical Cambrian species.* In Victoria it is, as has been shown, Lower Ordovician.†
The new species of Clonograptus is a fairly large one, and a perfect hydrosome would probably be 6 in. or 8 in. in diameter. But there is not enough of it preserved to justify description. Bryograptus is represented
[Footnote] * See especially A. H. Westergard, Studier öfver Dictyograptusskiffern, &c. Lunds Universitets Ansshrift. N.F., Afd. 2, bd. 5, nr. 3, 1909.
[Footnote] † T. S. Hall, Proc. Roy. Soc. Victoria, n.s., vol. ii, 1899, p. 164.
by numerous examples which may belong to two species, but they are closely allied. The angle of divergence of the primary branches is smaller than in B. victoriae T. S. H., but it is not far removed from that species. Tetragraptus decipiens T. S. H. is common, and shown in several of the typical positions which it assumes at Lancefield. Two large specimens are present, but I have not figured them.
Bryograptus, long considered to be confined to Cambrian rocks, has been shown by Ruedemann to range into the Ordovician in New York, just as it does in Australia, while Tetragraptus is unknown in the Cambrian.
The age of the graptolites dealt with is much older than that of any of the Lower Ordovician species yet recorded from New Zealand, and the missing zone—the Bendigonian—will probably be found on further search.
Explanation of Plate VIII.
Fig.1. Clonograptus tenellus. x 2.
Fig.2. Clonograptus tenellus. x 1.
Fig.3. Clonograptus tenellus var. callavei. x 1.
Fig.4. Clonograptus sp. n. x 1.
Fig.5. Bryograptus sp. x 2.
Fig.6. Bryograptus sp. x 2.
Fig.7. Tetragraptus decipiens. x 2.