Trigoniidae

Neotrigonia margaritacea, Victoria, Australia

The Trigoniidae, less common and cockles, are a family of the order Trigonioida within the palaeoheterodonten shells. The first representative of the family appear already in the Middle Triassic. Today comes before a genus ( Neotrigonia ) with about seven species.

Features

The housing of the Trigoniidae are usually highly ornamented. This ornament is made of ribs or nodes that are arranged in rows. The flaps are uniform and relatively thick. The front edge is rounded, the posterior margin may be somewhat extended. The vortex is far above forward. The ligament is external. The left door has a deeply recessed triangular tooth and a front and rear cardinal numbers. On the right flap there are two V- shaped arrangement cardinal teeth showing cross notches. The sphincter impressions are clear and the mantle margin is entire, integripalliat. The foot is relatively large and muscular. The shell can thus "jump"; usually the foot is, however, used for digging or for locomotion in the sediment.

Way of life

Today's Trigonien live buried in sandy sediments in water depths up to 400 m. Most fossil Trigonien were found in sedimentary rocks that were originally deposited in a few tens of water depths in the sea.

Evolution

The Trigoniidae family probably descended from the Triassic Myophoriidae and went through the Jura rapid diversification, which reached a high point in the Lower Cretaceous. In the Upper Cretaceous, the number of species declined sharply and there was an almost extinction at the Cretaceous - Tertiary boundary. From the Paleogene ( " tertiary " ) are only a few forms are known. Today, the family is still represented only a genre ( Neotrigonia ), who lives in the coastal waters off South Australia.

System

The Trigoniidae family is divided by individual scientists in very different subfamilies. Currently, up to 20 sub-families ( with up to more than 100 species ) are distinguished. Other authors do not go because of the large systematic uncertainties on a division into subfamilies and listen instead only on the genera. Some of the previous sub-families are in turn considered by some authors as a separate family ( eg ( sub) family Neotrigoniidae ).

Some fossil genera:

• Trigonia (Middle Triassic to Upper Cretaceous )
• Agonisca ( Middle Triassic )
• Geratrigonia ( Lower Jurassic )
• Iotrigonia (chalk)
• Maoritrigonia ( Upper Triassic )
• Sphenotrigonia ( Upper Cretaceous )
• Yaadia (chalk)