The earliest rhynchonellid brachiopods were small-shelled and preferred cool-water conditions in both siliciclastic and carbonate environments. Relatively large shells of Rhynchotrema evolved in mid-Caradoc time and rapidly became widespread in Laurentia. Hiscobeccus probably evolved from Rhynchotrema during the late Caradoc by developing a large, globular, strongly lamellose shell. The earliest form of Hiscobeccus shows transitional characteristics between Rhynchotrema and typical Hiscobeccus, most notably in its nonglobular shell that is wrinkled by growth lamellae only on the anterior two thirds of the shell. By early Ashgill time, Hiscobeccus became widespread in North American inland basins, although it remained common in marginal carbonate platforms and basins. During the Ashgill, Hiscobeccus, Lepidocyclus, and Hypsiptycha represented a distinct North American fauna, characterized by large, globular, and completely wrinkled shells. These morphological features were adaptations to the shallow, well-circulated, epicontinental, tropical seas with a soft muddy substrate. The gigantism exhibited by the Hiscobeccus Fauna is also shown by orthid and strophomenid brachiopods, gastropods, nautiloids, and trilobites. Extinction of the Hiscobeccus Fauna was probably related to its narrow range of environmental tolerance, especially during the Gondwana continental glaciation, which brought a major global sea-level drawdown and lowered ocean temperature. The cool-water Rostricellula and Rhynchotrema were uncommon in inland seas during the Ashgill and were represented by several opportunistic species in the Anticosti Basin during the Hirnantian and Llandovery, and most of these preferred siliciclastic-rich or deep-water shelf carbonate environments.
The role of pre-magmatic rifting in shaping a volcanic continental margin: An example from the Eastern North American Margin
