Arcoida
In New Zealand, Arcoida first appeared in the Devonian, from which a single taxon has been identified. They survived through the Permian (7 taxa recognised) into the Triassic (6 staxa), and began to flourish in the Cretaceous when at least 18 taxa existed. Since then, their diversity has increased greatly, and about 100 taxa have been recognised from the Tertiary and Quaternary eras. Notable genera in New Zealand's geology include Glycimeris (image, right), which appeared in the Cretaceous and continued into the Tertiary, and provides some beautiful specimens in the Nelson area. They have a solid shell, which is typically elongate or circular-oval, often heavily ribbed. |
The Tertiary bivalve Glycimeris, from the Wairoa Valley, Nelson
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The enigmatic Mytiloid bivalve, Manticula problematica, from the Triassic near Wakefield
The Cretaceous Mytiloid bivalve, Inoceramus
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Mytiloida
The Mytiloida are relatively cosmopolitan bivalves that inhabit marine, estuarine and occasionally freshwater habitats. They anchor themselves not to the surface but by a byssus (thread-like extrusions) to a burrow. They are thus called endobyssate. In New Zealand, Mytiloids first appeared in the Permian (2 taxa), continued through the Triassic (6 taxa) and Jurassic (7 taxa), then flourished in the Cretaceous, from which about 35 taxa have been identified. Since then, they have declined somewhat in diversity, with only 25 or so taxa recognised from the Tertiary to Recent. They include some important fossil taxa, such as Manticula problematica - a somewhat enigmatic Triassic bivalve - and the genus Inoceramus which is an important marker for various Tertiary stages (see images, left). The shells of Mytiloids are equivalve, rounded, elongate or triangular depending on habits. The hinge is toothed and the shell is composed of an outer layer of fibrous calcitic prisms and a pearly (nacreous) inner layer. |
Pterioida
The Pterioida include some of the most important marine bivalves in New Zealand's geological record. They are marine organisms, with either an endo- or epibyssdate habit - i.e. they anchour themselves either to a burrow or to the surface of the substrate. Their shells are equivalve, variably shaped; and are composed of an outer layer of simple calcitic prisms and an inner pearly (nacreous) layer. The shell is often deeply folded, creating a wavy (i.e. plicate) commissure. In New Zealand, they first appeared in the Devonian, from which 11 taxa have been identified, and flourished in the Permian, which has produced over 50 taxa, including the genus of Atomodesma which is a crucial marker for the Permian. They seemed to find Triassic conditions even more to their liking, and over 50 taxa emerged, including genera such as Halobia and Monotis (see image, upper right), which are characteristic of the period. During the Jurassic, the number of taxa more than doubled, but thereafter their fortunes seemed to wane and they are presented by only 40 or so taxa in the Cretaceous and fewer than ten from the Tertiary. Amongst the Pterioida, however, are the Pectinidae - a group that now survives in the form of the Pecten shells, often found on modern beaches. They were common during the early Tertiary, and can be found as beautiful fossils in the Oligocene limestones of Golden Bay and the West Coast. Right: Pectinoid fossils exposed in weathered limestone near Pohara, Golden Bay
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A cluster of the Pterioid bivalve Monotis from the Triassic rocks near Richmond
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Pholadomyoida
The Pholadomyoida first emerged in New Zealand during the Devonian, from which 11 taxa including the important bivalve Glossites have been recorded. They expanded suring the Permian to over 30 taxa, but were apparently severely dimnished by the Permian extinction, for only 10 or so taxa have been identified from the Triassisc. These include Unionites (which desite its name does not belong to the Unionida!). In the Jurassic they recovered to over 30 taxa, but then disappear from the fossil record in this country. They are characterised by being more or less equivalve but have a widely divergent form. Their shell is composed of aragonitic prisms and nacre. They lived in a range of habitats and displayed varying habits: shallow-water forms are burrowing, nestling, epibyssate, or cemented suspension feeders; deepwater forms are often predatory. Left: Natural cast and moulld of Glossites cf.lingaulis, a Devonian Pholadomyoid bivalve. |
We need to remember this when we consider the record of Trinogioida in New Zealand. Their first appearance is in the form of one uncertain taxon during the Devonian, and only two taxa have been recorded from the Permian. In the Triassic, they suddenly expanded and over twenty taxa have been reported, including numerous species of the genus Trigonia and others of Maoritrigonia (see image, right). By the Cretaceous, however, they were already in decline, with only 13 taxa recorded, and none have been found since. Maybe they one day will. They are certainly distinctive bivalves. Marine and infaunal (i.e. living within the substrate sediments), they have an unusual triangular form comprising equal valves, arranged somewhat like a pyramid. They usually have strong ribs. The shell has an outer aragonitic prismatic layer and inner nacre layers. It displays strong hinge teeth, transversely grooved. The mantle is unfused.
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A natural mould and latex cast of Maoritrigonia sp.
- a Trigonioid bivalve from the Triassic, near Nelson. |