Box Semionotid Species Flocks

During the Late Triassic and Early to Mid-Jurassic, a time span of 45Myr, there were 20 or more large lakes on the eastern seaboard of North America along a great rift valley that was formed by the initial phases of opening of the North Atlantic. The sediments deposited in these lakes, the Newark Supergroup, record in detail the histories of filling and drying of the lakes, and in places annual varves have allowed geologists to reconstruct the histories and time-scales in astonishing detail.

The lake cycles in the Newark Basin, like those in the Old Red Sandstone of northern Scotland (see pages 63-4), include dark mudstones laid down during anoxic phases and that are especially rich in fish fossils. Semionotids are particularly abundant, and excavations of a single lake cycle of the Towaco Formation of the Newark Basin in New Jersey, representing a single cycle of lake formation, deepening, shallowing and evaporation, lasting perhaps 21,000-23,500 years, showed their true diversity. The excavation yielded 2000 specimens belonging to 21 species (McCune, 1996), which are distinguished on the basis of body shape and the nature of the scales that run along the midline of the back (see illustration).

This is only a small sample of the total diversity of semionotids in the Newark Supergroup and many hundreds or thousands of species must have existed in the lakes, and during the repeated lake cycles. Whole faunas were wiped out by catastrophic drying episodes and replaced by new species flocks that evolved rapidly when the lakes became re-established. Modern parallels exist today in central African lakes where the cichlid teleosts have achieved great diversity by rapid speciation.

Read more at http://www.monmouth.com/~bcornefblufhead.htm.

A semionotid species flock from the Towaco Lake cycle of the Newark Basin, New Jersey. In a short interval of time, 21 species coexisted in these lakes and they are distinguished by body shape and by the patterns of dorsal scales. Seven species groups have been identified, each with a different type of dorsal scale sequence. (Modified from McCune, 1996.)

A semionotid species flock from the Towaco Lake cycle of the Newark Basin, New Jersey. In a short interval of time, 21 species coexisted in these lakes and they are distinguished by body shape and by the patterns of dorsal scales. Seven species groups have been identified, each with a different type of dorsal scale sequence. (Modified from McCune, 1996.)

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