Anchisaurus is known from a handful of partial skeletons discovered in the Portland Formation of the Hartford Basin, the exact size of the handful depending on your opinion of Ammosaurus (mine being they're the same thing). The Portland Formation is one of a number of rock units deposited in rift basins that formed when North America was parting ways with Europe and Africa during the Triassic and Early Jurassic. If you're interested in the nitty-gritty concerning the various basins, numerous publications can be found here, provided by Paul Olsen. The Portland Formation (and its various alter-egos over time, such as the Longmeadow Sandstone) has historically been coy when it comes to dating. References before the 1980s usually slot it in the Late Triassic (for example Lull 1915 or Krynine 1950), but many references from the 1980s and 1990s describe it as significantly younger. Here's where we run into some terminology: the Early Jurassic is divided into four ages, like so:
Toarcian: 182.7 ± 0.7 million years ago (Ma) to 174.1 ± 1.0 Ma
Pliensbachian: 190.8 ± 1.0 Ma to 182.7 ± 0.7 Ma
Sinemurian: 199.3 ± 0.3 Ma to 190.8 ± 1.0 Ma
Hettangian: 201.3 ± 0.2 Ma to 199.3 ± 0.3 Ma
These are based on marine invertebrate fossils from Europe, to which radiometric dates have been added over time. As such, refinements in radiometric dates and reanalysis of the various invertebrates used to define the stages lead to adjustments in the timing of the boundaries. This is why you may remember the end of the Cretaceous being at 65 Ma, when it is now put at 66 Ma.
Anyway, the Portland Formation, being a terrestrial formation deposited in rift lakes, rivers, floodplains, alluvial fans, and so on, is not noted for its marine invertebrates. However, it does have some palynomorphs, organic microfossils like pollen and spores, and these can be used to tie into the marine record because palynomorphs get blown or washed into marine basins. Studies of palynomorphs in the 1970s and 1980s indicated that the Portland Formation could actually be as young as the Toarcian (see for example Cornet and Olsen ). From this, the dinosaur books wrote up the Portland Formation as "Pliensbachian–Toarcian". This age range has proven quite durable in the dinosaur literature despite some more recent developments that show the rocks to be somewhat older.
The Portland Formation is the uppermost of several rock units in the Hartford Basin. It is parked on top of igneous rocks produced during the split of North and South America from Europe and Africa, part of the Central Atlantic Magmatic Province, or CAMP. Rocks of the CAMP have been dated extensively since the 1990s; it helps to be both part of a major tectonic event and a suspect in an important extinction event. The most recent dates put the pulses of volcanic activity as occurring between about 201.6 to 200.9 million years ago (Blackburn et al. 2013). This means that the Portland Formation is about 200.9 million years old at its base, about as Early Jurassic as you can get without being tossed out for jumping the line. If you accept the presence of ancient Milankovitch cycles in the Hartford Basin rocks, you can extrapolate that the lower Portland Formation (which had substantial rift valley lake deposition) wrapped up at about 199 million years ago (Schaller et al. 2012). Milankovitch cycles are climate cycles that respond to various aspects of Earth's orbit which vary over regular time frames; you may have run into them in discussions of ice ages. Even if you don't think they are recorded in the Hartford Basin, you've still got ~200.9 million years for the base of the Portland, which is not exactly Pliensbachian–Toarcian (even if you play nice and note that in the 1980s, those two were dated closer to 195 to 190 million years ago, not 190 to 175 million years ago). The interesting thing is the age of the volcanic rocks has been studied and published for years, as I mentioned, but the "Pliensbachian–Toarcian" label has hung on. Well, that's inertia for you.
So, Anchisaurus (and fellow Portland Formation dinosaur Podokesaurus) are from the beginning of the Early Jurassic, not the end of it. In a side note, the Navajo Sandstone, found in the Southwest, is often considered comparable in age to the Portland Formation, in part because it was once thought that both formations had Ammosaurus (Galton 1971). It is now known that the Arizona "Ammosaurus" is something else (Irmis 2005). We can say that the two formations have similar vertebrate tracks, though.
|O. C. Marsh's vision of Anchisaurus; your memory has not been deceiving you - it has been passing itself off as 39 all these years.|
Blackburn, T. J., P. E. Olsen, S. A. Bowring, N. M. McLean, D. V. Kent, J. Puffer, G. McHone, E. T. Rasbury, and M. Et-Touhami. 2013. Zircon U-Pb geochronology links the end-Triassic extinction withthe Central Atlantic Magmatic Province. Science 340(6135):941–945.
Cornet, B., and P. E. Olsen. 1985. A summary of the biostratigraphy of the Newark Supergroup of eastern North America, with comments on early Mesozoic provinciality. Pages 67-81 in Weber, R., editor. Symposio Sobre Flores del Triasico Tardio st Fitografia y Paleoecologia, Memoria. Proc. II. Latin-American Congress on Paleontology (1984), Instituto de Geologia Universidad Nacional Autonoma de Mexico.
Galton, P. M. 1971. The prosauropod dinosaur Ammosaurus, the crocodile Protosuchus, and their bearing on the age of the Navajo Sandstone of northeastern Arizona. Journal of Paleontology 45(5):781-795.
Irmis, R. 2005. A review of the vertebrate fauna of the Lower Jurassic Navajo Sandstone. Pages 55-71 in McCord, R. D., editor. Vertebrate paleontology of Arizona. Mesa Southwest Museum, Mesa, AZ. Bulletin 11.
Krynine, P. D. 1950. Petrology, stratigraphy, and origin of the Triassic sedimentary rocks of Connecticut. State Geological and Natural History Survey of Connecticut, Hartford, CT. Bulletin 73.
Lull, R. S. 1915. Triassic life of the Connecticut Valley. Connecticut Geological and Natural History Survey, Hartford, CT. Bulletin 24.
Schaller, M. F., J. D. Wright, D. V. Kent, and P. E. Olsen. 2012. Rapid emplacement of the Central Atlantic Magmatic Province as a netsink for CO2. Earth and Planetary Science Letters 323-324:27–39.