New tetrapod bone assemblage near Triassic-Jurassic boundary, southeastern PA (Late Rhaetian, Newark Basin)


Paul E. Olsen, Hans-Dieter Sues, Emma C. Rainforth, Brian Hartline, and Mike Szajna.
 
 

A fundamental problem in understanding mass extinctions in terrestrial settings has been the dearth of assemblages of vertebrates in temporally calibrated boundary sections. This has been especially true for the Triassic-Jurassic boundary, where this lack of dated assemblages of latest Triassic age has resulted in very divergent views on even the existence of a terrestrial mass extinction event (Olsen et al., 1987; Benton, 1991). Very recently a new locality producing fairly abundant tetrapod bones has been found in a  Triassic-Jurassic boundary section in southeastern PA in the Passaic Formation of the Newark rift basin. This section has been studied for over 10 years but it has only been recently that excavations have brought bone material to light. The section consists of cyclical, mostly lacustrine strata with minor fluvial interbeds, and includes the palynologically-dated Triassic-Jurassic boundary as well as the overlying Early Jurassic basalt flows. Numerous shallow lacustrine intervals produce abundant footprints throughout the section including above and below the boundary. The cyclical lake beds have a well-preserved and well-documented Milankovitch-type cyclostratigraphy (Olsen and Kent, 1996), magnetostratigraphy (Kent et al., 1995), and pollen and spore biostratigraphy (Fowell and Olsen, 1993) and hence provide a well tested link to the rest of the basin stratigraphy as well as temporal calibration for the vertebrate finds.
 
Because the bone material is newly discovered, it is mostly unidentified and not yet prepared. However, one partial, although well preserved skull is clearly of the procolophonid parareptile Hypsognathus, probably H. fenneri, known from other parts of the Passaic Formation. The units that have produced the most bone, including the Hypsognathus skull, were deposited about 500,000 years prior to the Triassic-Jurassic boundary, based on the Milankovitch stratigraphy. Thus, this Hypsognathus specimens represents the youngest known procolophonid and significantly narrows the interval between the last occurrence of this group and the Triassic-Jurassic boundary. Many other bones from the site are definitely not procolophonid and include jaws, teeth, and postcranial material, some of which comprises associated fragmentary skeletons. Dinosaur footprint-bearing beds are intimately interbedded with bone-producing layers, as well. We expect that many of the bones a this site will be the youngest-known representatives of their respective tetrapod groups.
 
We suspect that this sort of bone occurrence is, in fact, not unusual for the Passaic Formation and the eastern US Triassic-Jurassic in general. However, because of the highly indurated nature of the deposits, it is necessary to have fresh, clean rock under suitable illumination to find the bone. It is the rarity if this event in outcrop, rather than an intrinsic paucity of bone material itself that has led to the general opinion of these rocks as being poor in skeletal remains. We thus anticipate more bone discoveries and eventually enough data to test whether the Triassic-Jurassic boundary represented a catastrophic mass extinction of skeletal taxa as already suggested by reptile footprint taxa (Olsen et al., 1998).
 
 

In D.L. Wolberg, K. Gittis, S. Miller, L. Carey, and A. Raynore (eds.), The Dinofest Symposium, Abstracts, addendum (1998)


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