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Discovery of Earliest Jurassic Reptile Assemblages from Nova Scotia

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Discovery of Earliest Jurassic Reptile Assemblages from Nova Scotia Olsen, P. E., 1986, Discovery of earliest Jurassic reptile assemblage from Nova Scotia implies catastrophic end to Triassic: Lamont (Newsletter), v. 12, p. 1-3. Discovery of Earliest Jurassic Reptile Assemblages from Nova Scotia: Imply Catastrophic End to the Triassic Late Triassic and Early Jurassic sediments record group (Fig. 1). These are the remnants of the bones. Long-legged,fully terrestrial crocodiles and a crucial period in the evolution of the Earth's basin fill of the rifting episode of the present crocodile relatives are by farthe most common ani- biota. Not only does this interval witness the origi- Atlantic passive margin. Fossils, especially those mals, with at least 50 skulls and jaws uncovered. nation of the dinosaurs, pterosaurs, mammals, of terrestrial vertebrates, have traditionally been Second in abundance are small lizard-like sphen- turtles, lizards, crocodiles, and teleost fishes, but regarded as very rare in the Newark Supergroup. odontid rhynchocephalians represented by about the Triassic-Jurassic boundary itself is the third Last summer, however, in collaboration with a 30 jaws and one nearly complete skull (Fig. 5). most significant of the thirteen or so mass extinc- team headed by developmental biologist Neil H. Third, and perhaps the most precious of all, are tions that punctuate the Phanerozoic record. Shubin of Harvard and funded by the National remains of highly advanced mammal-like reptiles. Unfortunately, despite its crucial position, the Geographic Society, Lamont paleontologist Paul These are the trithelodonts, the reptile group Triassic-Jurassic transition has remained one of E. Olsen discovered a series of rich assemblages thought to most closely relate to mammals. Over 13 the most poorly documented successions of the of well-preserved reptiles in the earliest Jurassic trithelodont skulls and jaws have been recovered, Phanerozoic, especially for terrestrial vertebrates. part of the Fundy Group of the Newark Super- with several forms being represented - including In particular, the time-stratigraphic framework for group. These assemblages apparently just post- the Pachygenelus and Diarfhrognathus. This is continental rocks of Late Triassic and Early Jur- date the mass extinction event, and thus they the first definitive record of these genera outside assic age has been especially problematic, with shed considerable light on what may prove to be South Africa, and the first record of the reptile well dated earliest Jurassic terrestrial skeletal one of the great biological catastrophes that group closest to mammals in North America. Least assemblages being essentially unknown. A direct helped shape the modern world. common is a suite of ornithischian, theropod, and consequence of these stratigraphic difficulties is The animal remains occur within the upper part prosauropod dinosaur remains. This includes a a proliferation of conflicting hypotheses on the of the early Mesozoic Fundy Group of the Newark partial prosauropod dinosaur skeleton (collected nature of the faunal transition: from those suggest- Supergroup, a 1000-m-thickcontinental sequence in collaboration with Donald Baird of Princeton). ing a dramatic, catastrophic turnover, to those consisting of red clastics, minor carbonates, and There is a massive residuum of unidentified mate- suggesting none. extrusive tholeiitic basalts (Fig. 3). Five forma- rial that remains to be sorted through. A faunal Late Triassic and Early Jurassic continental tions spanning from the Anisian-Ladinian (Middle list is given in Table II. sediments are well represented in eastern North Triassic) to Hettangian (earliest Jurassic) are rec- The faunal assemblages occur in three distinct America, where they comprise the Newark Super- ognized (Table I). The new assemblages occur facies of the McCoy Brook Formation: 1) a brown in basal beds of the youngest formation of the fluvial sandstone dominated by sphenodontids, Fundy Group, the McCoy Brook Formation at and crocodilomorphs (this is also the unit which Wasson's Bluff near Parrsboro, Nova Scotia. The produced the prosauropod dinosaur skeleton); 2) upper flows of the underlying extrusive North a basalt-bearing basalt agglomerate, a probable Mountain Basalt are discontinuous in this area lahar, dominated by crocodilomorphs and the and the basal, bone-producing portions of the trithelodont mammal-like reptiles: and 3) a lacus- McCoy Brook Formation fill the highly irregular trine muddy limestone and interbedded basalt basalt topography where the two formations inter- agglomerate dominated by semionotid fishes and tongue (Fig. 4). ornithischian dinosaurs. SO far, roughly three tons of bone-bearing matrix The real value of these assemblages is that it have been collected, containing perhaps 100,000 (continued on page 2) Fig. 1: Just prior to the Middle Jurassic separation of the North American and African continental plate, a series of elongate riff basins formed around the incipient riffing axis. In eastern North America, these basins parallel the grain of the Appalachian Orogen and were filled with thousands of meters of continental sediments and minor tholeiitic igneous rocks. The faulted, tilted, and deeply eroded remnants are expose6 from Nova Scotia to South L Carolina and are termed the Newark Supergroup. The Fig. 2: Skull of the common primitive reptile Hypsog- years) below the Triassic-Jurassic boundary. Hypsog- Manicouagan impact structure, which dates from 210 nathus found in latest Triassic Newark Supergroup red nathus represents one of the 13 families of terrestrial ?- 4 ma, very close to the Triassic-Jurassic boundary, and sandstones from the Fundy Basin in Nova Scotia, reptiles that went extinct at or near the boundary. according to impact theory may have been responsible Hartford Basin, and Newark Basin. In Clifton, New (Drawing by Paul Olsen, twice actual size). for a mass extinction event at the boundary. Jersey, a skeleton has been found 100 m (400,000 lova Scotia Discovery North Mountain Basalt, by pollen and spore data, thie longer-term eccentricity cycles, the entire and by lacustrine cycles. According to the regionaI Niiwark extrusive episode seems to involve less mntinued from page 1) . magma evolution model of John Puffer of Rutger: j thian 500,000 years. This agrees with times foi possible to place the bone-bearing units in a and Tony Philpotts of University of Connecticut at igiieous cooling and fractionation of the basalts sveloping, high-resolution litho- and chronostrati- Storrs, the North Mountain Basalt correlates witt 1 W( Irked out by Philpotts. Because the North Moun- 'aphy in a way that has not been possible for the oldest of the high Fe20i-high TiOs tholeiites5 taiin Basalt seems to correlate with the oldest o arly Jurassic terrestrial vertebrate assemblages of the more southern Newark basins. The Triassic- thie flows of the southern basins, and the Triassic utside the Newark Supergroup. The age of the Jurassic boundary is palynologically fixed at about Juirassic boundary seems to be 100,000 year; wer McCoy Brook faunules is constrained by 30 m below the oldest basalt (Orange Mountain) oilder than the oldest basalts (again based or iree separate lines of age-correlativedata: First, in the Newark Basin and all of the interbeds of climate cycles), the new reptile assemblage: ~llenand spore assemblages from the upper 5 the southern extrusive units bear Hettangiar1 st~ould be less than 200,000-300,000 year: I of the Blomidon Formation, the Scots Bay For- aspect microfloras. The hierarchy of lacustrini? yc iunger than the boundary. lation and the middle McCoy Brook Formation, cycles that occurs interbedded with and underly Late Triassic continental faunas were dominatec re dominated by Comllina meyeriana, strongly ing the basalt flows in the Newark Basin can bi? by diverse quadrupedal archosauromorph reptiles Jggesting a maximum of an Hettangian age. used to estimate the total length of the Newarl< large labyrinthodont amphibians, and various econd, reptile footprint faunules from the basal extrusive episode and to infer the duration of thi3 mammal-like reptiles, and remains of these arc IcCoy Brook Formation and Scots Bay Forma- interval between the new reptile assemblages an(j fo und in Nova Scotia in formations below thf 3n are of "Connecticut Valley" aspect, which indi- the Triassic-Jurassic boundary. The lacustrini? 10 cally-defined Triassic-Jurassic boundary and it ates an Early Jurassic age. Third, conventional cycles have been interpreted as responses to cli- cc wrelative strata in more southern Newark basins -Ar, and K-Ar isochron dates from the North mate changes governed by the precession cycli3B' y the Early Jurassic, dinosaurs, diversecrocodilo lountain Basalt are consistent with an Early of 21,000 years and the eccentricity cycles oif morphs, mammals, and essentially modern smal urassic age. roughly 100,000 and 400,000 years following thi3 reptiles (including sphenodontids) and amphibi Detailed correlation with the rest of the Newark Milankovitch theory of climate change. By countin!3 a1is were dominant and the terrestrial fauna attain upergroup is afforded by the whole rock and the lake-level cycles caused by the precessioiI ec j the basic composition it would retain for thi ace element geochemistry of the underlying cycles and looking at the phase relationships oif ni?xt 140 million years. The Late Triassic-Earlv Jurassic aoe assem - blages from the Newark ~u~er~rou~,especiall! thie new McCoy Brook assemblages shed consid ei¥abl light on two previous hypotheses
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