Smithsonian National Museum of Natural History

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Carboniferous

Early Mesozoic tetrapods of the Newark Supergroup of eastern North America

 

I am particularly interested in the evolutionary history of terrestrial vertebrates during the early Mesozoic, especially the Triassic Period. Most of the major groups of present-day land vertebrates or their closest relatives first appeared during the Triassic. Furthermore, the Triassic follows the end-Permian mass extinction and itself terminates with a major extinction event. Much of my work in the field and laboratory has focused on Late Triassic and Early Jurassic tetrapods from eastern North America.


The breakup of Pangaea commenced along the junction between present-day North America and Africa during the Middle Triassic. Strong extensional forces caused continental crust to stretch and thin along the rift axis, resulting in basin formation. These basins were filled by continental sedimentary strata, which were capped by or interbedded with extensive basalts and frequently intruded by diabase dikes or sills. The sedimentary and igneous fills of the basins are collectively known as the Newark Supergroup. Today a long chain of rift basins extends along the East Coast of North America, from the Canadian Maritimes in the north to the Carolinas in the south. The remnants of some thirty basins are exposed on land while others lie buried under the younger sediments of the Atlantic Coastal Plain or offshore. Most of what appears to be the largest rift basin, the Fundy basin, now lies under the waters of the Bay of Fundy in Nova Scotia and New Brunswick (Canada), but some of its thick fill of sedimentary and igneous rocks is exposed along the bay’s shorelines. Additional rift basins occur across the Atlantic Ocean, on the Iberian Peninsula and in Morocco. The entire rift province, known as the central Atlantic margin system, spans a broad range of paleolatitudes and range in time from the Middle Triassic to the Early Jurassic, representing some 30 million years.


Given the temporal range of the strata of the Newark Supergroup, they would appear to be an ideal setting for studying the succession of early Mesozoic continental ecosystems. Following the first reported discovery of dinosaurian footprints in the early nineteenth century, however, fossil vertebrate remains other than tetrapod trackways and fish were found only infrequently, and the rocks of the Newark Supergroup soon acquired a reputation as being largely devoid of vertebrate fossils. Starting in the 1970s, fieldwork by Paul E. Olsen (Lamont-Doherty Earth Observatory, Columbia University), Nick Fraser (now National Museums of Scotland), and me has initiated a new, highly productive phase of paleontological exploration that continues to the present day.


Much of my research is concerned with the evolutionary morphology, phylogenetic relationships, and paleoecology of the Triassic and Early Jurassic tetrapods from the Newark Supergroup, and have extensively published on this subject, often in collaboration with Paul Olsen and Don Baird (Pittsburgh). This work continues, and important new discoveries are still made every year. Far from being devoid of vertebrate fossils, the Newark Supergroup is yielding one of the most extensive and best dated sequences of early Mesozoic continental tetrapod assemblages. Thus, it offers important new insights into the major biotic changes during the early Mesozoic, especially the end-Triassic extinction event.

 

Map of the rift basins of the Newark Supergroup in eastern North America. Black areas denote exposed basins; light grey areas denote inferred basins. (Courtesy of P. E. Olsen.)

Maxilla (holotype) of the traversodont cynodont Boreogomphodon jeffersoni Sues and Olsen, 1990 from the Vinita Formation (Carnian) of the Richmond basin in Chesterfield County, Virginia. Traversodont cynodonts are very common in and were once thought to be restricted to the Triassic of Gondwana. However, several formations of the Newark Supergroup have yielded an abundance of well-preserved traversodont remains in recent years.

Skeleton (holotype) of the crocodylomorph reptile Dromicosuchus grallator Sues et al., 2003 from the "Lithofacies Association II" (Norian) of the Deep River basin in Durham County, North Carolina. This exquisitely preserved specimen is part of a remarkable recent discovery of Norian-age tetrapods from the Durham sub-basin of the Deep River basin.

Coastal exposure of the Triassic-Jurassic boundary at Five Islands, Nova Scotia. The cliffs are formed by the North Mountain Basalt, which overlies the red lake sediments of the Upper Triassic Blomidon Formation. The North Mountain Basalt is part of the Large Igneous Province known as the Central Atlantic Magmatic Province (CAMP) and is latest Triassic or earliest Jurassic in age. The eruption of the vast CAMP flood basalts is considered a leading candidate for causing the global end-Triassic extinction event.

Partial skull (holotype) of the sphenodontian reptile Clevosaurus bairdi Sues et al., 1994 from the Lower Jurassic McCoy Brook Formation near Parrsboro, Nova Scotia. This find was part of a remarkable discovery in the 1980s of thousands of excellently preserved skeletal remains of small tetrapods, which, based on the latest studies, may straddle the Triassic-Jurassic boundary.

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