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?Latest paper on the extinction of dinosaurs in North America



I've just found out that I have full online access to all journals by the
Geological Society of America. Yay! :-)

Dean A. Pearson, Terry Schaefer, Kirk R. Johnson, Douglas J. Nichols:
Palynologically calibrated vertebrate record from North Dakota consistent
with abrupt dinosaur extinction at the Cretaceous-Tertiary boundary, Geology
29(1), 39 -- 42 (January 2001)

Abstract: "New data from 17 Cretaceous-Tertiary (K-T) boundary sections and
53 vertebrate sites in the Hell Creek and Fort Union Formations in
southwestern North Dakota document a 1.76 m barren interval between the
highest Cretaceous vertebrate fossils and the palynologically recognized K-T
boundary. The boundary is above the formational contact at 15 localities and
coincident with it at two, demonstrating that the formational contact is
diachronous. Dinosaurs are common in the highest Cretaceous vertebrate
samples and a partial dinosaur skeleton in the Fort Union Formation is the
highest recorded Cretaceous vertebrate fossil in this area."

A few more excerpts:

"The palynological database includes 110 taxa for which occurrence data were
recorded in more than 230 samples. As many as 30 palynomorph taxa disappear
from the stratigraphic record at the K-T boundary."

"The presence of Cretaceous basal Fort Union beds indicates that
paleoenvironmental conditions leading to extensive formation of coal swamps
began in southwestern North Dakota prior to the end of Cretaceous time.
Typical Cretaceous plants persisted in these changing environments until the
terminal Cretaceous impact and consequent extinctions.
        Physical evidence for the K-T boundary impact is present both where
the boundary is above the formational contact and where the two are
coincident. The Pyramid Butte section has anomalous iridium and shocked
minerals ;1 m above the formational contact (Johnson et al., 1989), and we
have discovered shocked minerals where the K-T boundary and formational
contact are coincident at Mud Buttes (Nichols et al., 2000)."

"For this analysis we only address the 53 localities within 15 m of the K-T
boundary. This subset of sites yielded 3273 specimens representing 46 kinds
of vertebrates (7 fish, 5 amphibians, 9 turtles, 4 crocodilians, 1 lizard, 1
possible pterosaur, 12 dinosaurs, 2 birds, and 5 mammals) (Fig. 2). Dinosaur
specimens were numerically abundant in these samples and were present in all
but one of the localities.
        In northern Slope County, we found a partial associated skeleton of
a ceratopsian dinosaur (the collection includes a rib fragment, nasal horn,
brow horn, right maxillary, and two sections of frill) in laminated mudstone
84 cm above the base of the Fort Union Formation, which is marked by a 10 cm
lignite bed (Fig. 1). Palynological analysis of this section places the K-T
boundary 260.5 +- 2.5 cm above the formational contact (1.76 m above the
level of the ceratopsian bones). Occurrences of 19 taxa of angiosperm pollen
that are unknown above the K-T boundary show that the dinosaur is within
Cretaceous strata.
        When calibrated to the K-T boundary, occurrences of vertebrate
fossils indicate an uppermost Cretaceous barren zone (Fig. 2). Where the
basal Fort Union is Cretaceous, this gap is 1.76 m. At Mud Buttes, where the
K-T boundary and the formational contact are coincident, the gap is 2.37 m.
These results differ from Sheehan et al. (2000), who listed a minimum of 10
dinosaur specimens in the uppermost 5 ft (1.52 m) of the formation. We
checked their unpublished data (Sheehan, 2000, written commun.) and
ascertained that all their vertebrate sites in this interval were near Mud
Buttes, implying that their survey recorded dinosaur fossils where we found
none."

"Our data show that there is a gap below the K-T boundary in our study area,
but it is an interval in which we found no vertebrate fossils of any kind,
even those of vertebrate groups known to have survived the K-T boundary
event (e.g., fish, amphibians, turtles, crocodilians). It is possible that
this gap is a function of depositional conditions associated with the
formational transition rather than a reflection of true terminal Cretaceous
faunal diversity. In contrast, Sheehan et al. (2000) reported numerous
dinosaurs within 1.52 m of the K-T boundary from the southern part of our
study area. The reasons for these differences are unclear. Unfortunately,
they did not collect voucher specimens and have not published precise
locality and stratigraphic data.
        Our vertebrate data and those of Lillegraven and Eberle (1999) both
show a diverse, dinosaur-rich fauna in the highest Cretaceous vertebrate
sites, a pattern that is consistent with abrupt extinction of dinosaurs.
Combined with high levels of plant and insect extinction (Johnson et al.,
2000), these data support the model of a terminal Cretaceous biotic
catastrophe."

Have I already mentioned the youngest hadrosaur tracks, those dinosaur
fossils that are not going to be dissolved by acid rain, in this thread?
They are 37 cm below the boundary, writes Lockley.

Long live the youngest ammonite, found 10 (ten) cm below the boundary. I
mean, it's dead, but you get the idea. :o)