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A large Monotis from 88 Valley. Note the presence of larger (primary) ribs and smaller (secondary) ribs.
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Monotis sp.
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When the German geologist and explorer Hochstetter visited the Nelson area in 1859, he took back samples of a distinctive bivalve which he regarded as Monotis-like. Back in Europe, Zittel classified them as a variety of Monotis salinaria, and called them M. salinaria var. richmondiana, after the Richmond area, where they had been found. Since then, the taxa has been closely associated with the Nelson and Tasman area, where it provides a distinct marker for the Warepan stage of the Trias. The specimens that can be found in the Top of the South are variable, yet relatively distinctive (see gallery of specimens from the collection, below). They comprise rather delicate, fan-shaped bivalves, slim in profile, with slightly curving ribs and a few faint and widely spaced growth-rings. The ribs (plicae) are slender and typically alternate between stronger primary and finer secondary or lower oprder ribs, as can be seen in the image above. The total number of ribs, and the number of different levels (e.g. secondary, tertiary or lower) varies. The beak likewise shows variations, from rather prominent and slightly dome-shaped (see image top right), to small and pointed (see above). Specimens also vary in size from 1 cm or less to 10-15 cm in breadth. Some are almost symmerical in outline (see above); others are clearly skewed (see top right). This variation suggests that we are dealing with more than one species of Monotis in the Top of the South. Identifying them, however, poses a serious problem that can severely tax the brain and the patience of any amateur geologist. Since Hochstetter visited, the number, status and definition of the different taxa included within label Monotis has been subject to repeated revision and review. The most recent review, by Grant Mackie (2015), suggests that five different genera and sub-genera of Monotis can be recognised in the Triassic of New Zealand: Entomonotis, Eomonotis, Inflatomonotis, Maorimonotis and Pacimonotis. Entomonotis richmondiana acutecosta?
Further Reading: Trechmann, C.T. 1917 The Trias of New Zealand. Quarterly Journal of the Geological Society 291, 165-246 (+ 10 plates). Grant-Mackie, J.A. 1978 Subgenera of the Upper Triassic bivalve Monotis. New Zealand Journal of Geology and Geophysics, 21(1), 97-111. Grant Mackie, J.A. 1978 Systematics of New Zealand Monotis (Upper Triassic Bivalvia) — subgenus Entomonotis. New Zealand Journal of Geology and Geophysics 21(4), 483-500. Grant Mackie, J.A. 1978 Systematics of New Zealand Monotis (Upper Triassic Bivalvia) — subgenus Maorimonotis. Journal of the Royal Society of New Zealand 8(3), 293-322. Grant Mackie, J.A. 2015 Taxonomy of the Late Triassic bivalve Monotis. New Zealand Journal of Geology and Geophysics 58(3), 244-251. |
Entomonotis richmondiana?
A dense bed of Monotis sp. (zoom in to see the detail)
Monotis sp. in a dense shell bed
Within these, the original taxa described by Zittel is assigned to the species Entomonotis richmondiana. According to the New Zealand Inventory of Biodiversity, however, a further ten species and sub-species of Entomonotis have been identified, along with four Maorimonotis, two Inflatomonotis, two Pacimonotis and Eomonotis. Distinguishing between all these taxa requires a specialist knowledge that is beyond most amateurs.
For those who want to try to take the classification further, three papers by Grant Mackie (1978) give useful information on the features that distinguish different species and sub-species, along with detailed descriptions of some of the taxa (see Further Reading, below). A summary of the descriptions can be downloaded here. For the most part, however, I refer specimens found simply to Monotis sp., with the expectation that many (though far from all) are Entomonotis richmondiana. In a few cases, however, a firmer distinction can be made, primarily on the basis of the overall shape of the specimen, the number and size of the ribs and ther shape of the ear. The image to the left, for example, shows a specimen terntatively referred to Entomonotis richmondiana acutecosta. Note the heavy, rounded ribs and the lack of secondary ribbing. Whichever species or subspecies of Monotis we are concerned with, all seem to be found solely within the Warepan stage deposits of the Triassic. Here, they often occur in abundance, as dense fossil clusters or beds (see images above). In this form they are some of the most striking finds you can make. |
Specimens of Monotis sp. from the collection
(All speciemns from Highfield Farm)
(All speciemns from Highfield Farm)