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)
This page last updated March 29th, 2002.