Taxobox | name = Trigoniidae
image caption = "Trigonia" sp. (
Cretaceous) near Austin, Texas. Scale bar is 10 mm.
familia = Trigoniidae
familia_authority = Lamarck
subdivision_ranks = Common Genera
The Trigoniidae represent a large and morphologically interesting Family of highly ornamented non-siphonate
bivalves. They originated from the Myophoriidaein the Triassicand underwent an explosion of diversity in the Jurassic, reaching an acme in the Cretaceous, although most genera became extinct at the end of this period. The most striking feature of the Trigoniidae, which has attracted attention for centuries, is their striking external ornament. This is usually present as ribsor costaeor rows of aligned tubercles. Although they were abundant in the Mesozoicera, they are today represented by only one living genus, "Neotrigonia", which inhabits waters off the coast of south Australia.
Neotrigonia - the discovery of a living genus
Before the beginning of the nineteenth century, no trigoniid had been described from rocks younger than the Cretaceous Period. In 1802, however,
Francois Perondiscovered a living species in waters off the coast of Tasmania. Lamarcknamed it "Trigonia margaritacea" in 1804 with Cossmann renaming the genus "Neotrigonia" in 1912. Today, five living species have been identified, and are all found off the coast of Australia. "Neotrigonia" probably evolved from "Eotrigonia" ( Eoceneto Miocene) in the Miocene.
Previous research on the Trigoniidae
Because of their large size and pronounced ornament, trigoniid bivalves have long attracted interest.
Jean Guillaume Bruguierewas the first person to describe an example of "Trigonia" in 1789. Lamarcklater figured specimens from the Oxfordian of France. In Englandthe physician James Parkinson(the discoverer of Parkinson's disease) described examples of "Trigonia" and "Myophorella". Later, James Sowerbyand James De Carle Sowerbybegan to catalogue British examples in earnest. Etheldred Benettadded several Upper Jurassic species, although her work was not primarily recognised due to the academic status of women at that time.
In Europe the great
Louis Agassizpublished, in 1840, a large volume entitled "Memoire sur les Trigonies" which recognised the large variation encountered within the family, dividing it into eight sections, which was a precursor to the generic classification that occurred some fifty years later. Other notable workers that described and figured trigoniids include Friedrich August von Quenstedt, Alcide d'Orbignyand Georg August Goldfuss.
The major worker on the Trigoniidae in the nineteenth century was
John Lycett, a physician from Gloucestershirewho published a significant text entitled "A Monograph of British Fossil Trigoniae".
Later research - the twentieth century
Work on the Trigoniidae has generally been sparse in the twentieth century and has mainly concentrated upon the development towards a workable
taxonomy. Today, knowledge is sufficient to divide the Family into five Subfamilies(see below), which together contain more than sixteen genera, the most abundant being "Trigonia", "Myophorella", "Laevitrigonia", and "Orthotrigonia".
Higher level taxonomy
Family characteristics - trigoniid hinge
Members of the Trigoniidae are identified by the large and complex
dentitionthat joins the two valves together and allows articulation. The teethand supporting area can take up almost a third of the volume of the shell. The hingestructure is amongst the most complex of all bivalves, namely that the teeth are numerous and ridge-like with strong transverse striations. It is these striations which distinguishes the Trigoniidae from the more primitive Myophoriidae. The Trigoniidae almost certainly evolved by a monophyleticmodification of a Triassic myophoriid, with three genera appearing in the Middle Triassic.
Commonly found genera
Trigoniids are commonly found in both
limestone, mudstoneand sandstonein Jurassic and Cretaceous rocks all over the world. In Britain, examples are numerous in the Upper Jurassicrocks of the Dorsetcoast, particularly around the village of Osmington Mills. Other Jurassic rocks that yield specimens include the Cornbrashin Yorkshireand the Middle Jurassicsequence in the Cotswolds, particularly around Cleeve Hill, near Cheltenham.
The genus "Trigonia" is the most readily identifiable member of the family, having a series of strong ribs or costae along the
anteriorpart of the shell exterior. They are the first representatives of the family to appear in the Middle Triassic( Anisian) of Chileand New Zealand. The first European examples ("Trigonia costata" Parkinson) turn up in the Lower Jurassic ( Toarcian) of Sherborne, Dorset and Gundershofen, Switzerland.
Source: Francis, A.O. 2000. "The Palaeobiology of the European Jurassic Trigoniidae". Ph.D. thesis,
University of Birmingham, 323pp.
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